Axions, if existing, can cause a strong signal of positron-electron pairs in isoscalar M1 transitions in competition with gamma-ray emission. We searched for such a signal in the 3.59 MeV transition in 10B with a fourfold Mini-Orange spectrometer. No axion events were found within two percent of the prediction for the standard axion- to γ-ray branching ratio

Abrahams, K

Balanda, A

Bokemeyer, H

de Boer, FWN

de Voigt, MJA

Jansen, JFW

Kotlinski, B

van Dantzig, R

van Klinken, J

Physics Letters B

Axions, if existing, can cause a strong signal of positron-electron pairs in isoscalar M1 transitions in competition with gamma-ray emission. We searched for such a signal in the 3.59 MeV transition in 10B with a fourfold Mini-Orange spectrometer. No axion events were found within two percent of the prediction for the standard axion- to γ-ray branching ratio.<br/

ARTS repository - University of Groningen

   University of GroningenSearch for short-lived axions in a nuclear isoscalar transitionde Boer, FWN; Abrahams, K; Balanda, A; Bokemeyer, H; van Dantzig, R; Jansen, JFW;Kotlinski, B; de Voigt, MJA; van Klinken, JPublished in:Physics Letters BDOI:10.1016/0370-2693(86)90123-1IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1986Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):de Boer, FWN., Abrahams, K., Balanda, A., Bokemeyer, H., van Dantzig, R., Jansen, JFW., Kotlinski, B.,de Voigt, MJA., & van Klinken, J. (1986). Search for short-lived axions in a nuclear isoscalar transition.Physics Letters B, 180(1-2), 4-8. https://doi.org/10.1016/0370-2693(86)90123-1CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 09-09-2023Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 S E A R C H  F O R  S H O R T - L I V E D  A X I O N S  IN  A N U C L E A R  I S O S C A L A R  T R A N S I T I O N  F .W.N.  D E  BOER 1, K. A B R A H A M S  2, A. B A L A N D A  3,4, H. B O K E M E Y E R  5, R. V A N  D A N T Z I G  6, J .F .W.  J A N S E N ,  B. K O T L I N S K I ,  M.J.A. D E  V O I G T  and  J. V A N  K L I N K E N  Kernfysisch Versneller lnstituut, Universiteit van Groningen, NL-974 7 AA Groningen, The Netherlands Received 22 July 1986; revised manuscript received 10 September 1986 Axions, if existing, can cause a strong signal of positron-electron pairs in isoscalar M1 transitions in competition with gamma-ray emission. We searched for such a signal in the 3.59 MeV transition in I°B with a fourfold Mini-Orange spectro- meter. No axion events were found within two percent of the prediction for the standard axion- to ~,-ray branching ratio. A light pseudoscalar Goldstone boson, the axion, has been introduced by Weinberg [1] and Wilczek [2], as a consequence o f  the global U(1) symmetry pro- posed by Peccei and Quinn [3] to explain the ab- sence of  large P and CP violations in strong interac- tions. In a treatise of  possible axion search experi- ments, Donnelly et al. [4] discussed the probabil i ty for nuclear de-excitation by  axion emission in compe- t i t ion with magnetic ~/radiation. Subsequent axion searches provided such stringent upper bounds on mass and lifetime that  the nonexistence of  axions was anticipated [5]. However, the interest in an observable axion was revived recently [6] after the puzzling ob- servation in superheavy collisional systems at GSI o f  a sharp positron peak at approximately 330 keV, which was later [7] found to be in coincidence with a simi- lar electron peak. Could the observed (e+e - )  pairs t Present address: NIKHEF-K, P.O. Box 4395, NL-1009AJ Amsterdam, The Netherlands. 2 Energieonderzoek Centrum Nederland (ECN), Petten, The Netherlands. 3 Physics Laboratory of the Free University, NL-1081 HV Amsterdam, The Netherlands. 4 Jagiellonian University, PL-30059 Cracow 12, Poland. 5 Gesellschaft for Schwerionenforschung (GSI), Postfach 110541, D-6100 Darmstadt 11, Fed. Rep. Germany. 6 NIKHEF-K, P.O. Box 4395, NL-1009 AJ Amsterdam, The Netherlands. arise from a "visible" axion with a mass o f  about 1.68 MeV and a lifetime of~< 10 -11 s? The axion mass and lifetime can be related to one free parameter X, the ratio o f  vacuum expectat ion values for two Higgs field componen t s f s in  X and f c o s  X with f =  (G FV ~) - I / 2  = 250 GeV (G F being the Fermi coupling constant): m a = 25N(X + l /X)  (keV), (1) where N is the number of  active quark pairs (N = 3 in ref. [4]). For  m a > 2m e the axion will predominantly decay in a pos i t ron-e lec t ron  pair with lifetime r(a -~ e+e - )  = 8"tr f2X2me2(m 2 - 4me2) -1/2 (s). (2) Searches for axions decaying into two photons with radioactive sources, reactors and beam dumps constrain [5] the mass to < 1 MeV and the lifetime to < 10 - 6  s, whereas a search [8] for decay into posi- t ron -e l ec t ron  pairs limits the lifetime to < 2 X 10 -10 s. Searches with pseudoscalar- and vector mesons (K +, J/~k and Y) were sensitive to lifetimes of  10 -11 s due to time dilation of  the order of  104 . The anom- alous magnetic moment  o f  the electron constrains the lifetime to > 5 X 10 -13 s [ 9 - 1 2 ] .  In summary, a rather narrow window of  lifetimes between 10 -13 and 10 -10 s cannot be excluded on the basis of  exist- ing experimental evidence. If  the observed positron peak is due to an axion with mass 1.68 MeV, its X 0370-2693/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 value according to eq. (1) is either X = 0.045 or X = 22 leading to a lifetime of  about 5 × 10 -12 s (al- lowed) or 2 × 10 - 6  s (excluded).  Either of  these two X values would lead to contradictions for branching ratios of  meson decays in the standard axion model. Quite recently, however, Peccei et al. [13] have suc- ceeded in reconciling the previous unsuccessful searches with an axion of  1.68 MeV in a 'Wiable axion model"  which suppresses the coupling to the heavier quarks. Almost simultaneously, Krauss and Wilczek [14] pre- sented a "short-lived axion variant" coupled preferen- tially to light fermions and stressed with Brodsky et al. [15] the need of  new experiments.  In this letter we report  on a search for axions in the unexplored region with lifetimes T < 10 - 9  s. We looked for the e+e - decay of  such axions in competi- t ion with internal pair creation (IPC) of  a magnetic q-ray transition. Specifically for isoscalar transitions, the branching ratio for (e+e - )  pair creation from a pseudoscalar decay particle and M1 gamma-decay can be as large [4,16] as 50%. This is three orders o f  mag- nitude larger than IPC. As isoscalar transition the 3.59 MeV M1 + E2 transition in 10B was chosen. The 2 +, 0 state at 3.59 MeV in 10B decays to the 3 +, 0 groundstate with a 19% branching ratio. The transition has a mixing ratio [17] 5(E2/M1) of  1.5 + 0.6 while the parallel 2 +, 0 ~ 1 +, 0 transition of  2.87 MeV is [17] predominantly E2 with 5 - 1  < 0.2. Neglecting internal conversion by atomic electrons, two-photon emission and other higher-order processes, the IPC contributes at the level of  10 -3  compared with gamma rays in the de,excitation of  the 3.59 MeV state. Axion emission would imply a source of  (e+e - )  pairs in addit ion to the expected (e+e - )  pairs from IPC. The energy and angular correlations of  (e+e-)  pairs from the 3.59 MeV transition are ftxed kinemati- cally. In our experimental arrangement the M I and E2 IPC is detected with total  efficiency e of  typically 10 - 4  (see below). The IPC angular correlation be- tween the e ÷ and e -  is maximum at relative angle 0 = 0 °, while in case of  the axions the maximum will occur around 0 = 45 °. The angle between the axes o f  neighbouring Mini-Oranges is 60 ° (fig. 1), which causes an axion decay to be seen with somewhat higher efficiency than IPC; a numerical estimate gave e A ~ 1.5e, assuming that the axions are emitted iso- tropically. The nuclear axion-decay rate (in the case of  existing axions) is predicted in ref. [4] to be as large A e . . / ~ d e t .  target _ - ~ 4 5 v ~  .beam ~Ge detector Fig. 1. Fourfold Mini-Orange arrangement for the axion search. To the left a view of the horizontal plane and to the right a view upstream along the beam axis. The hypothetical axi0n ~ e + + e- decay is supposed to occur inside or closely (at gm distance) be- hind the target layer. In the left part of the figure an e- particle is indicated, while the corresponding e + particle will be detected by a neighbouring e + detector (out of plane). Volume 180, number  1,2 PHYSICS LETTERS B 6 N o v e m b e r  1986 as I a = 0.39 I3,(M 1). The number of  observed pairs can now be writ ten as: N(e+e-)/e = {0.39 X 1.5 + ~ ( M 1 ) )  Lr(M1 ) + otTr (E2)[~ (E2), (3) where e is the efficiency. Theoretical IPC coefficients o t  can be obtained from ref. [18]. I f  axions indeed exist then the first term (= 0.585) will dominate since the others are of  order 10 - 3  . In the experiment the 3.59 MeV state in 10B was excited in inelastic scattering of  7 MeV protons by a 10B target of  1 mg/cm 2. Using a fourfold Mini-Orange system [19] with two detector  systems for e + and two for e -  detection, we registered prompt  (e+e - )  signals and their energy sums. The four spectrometers were set at a backward angle of  45 ° with respect to the incident proton beam. This is shown by a view in the horizontal plane in fig. 1 (left). The axes of  oppo- site Mini-Oranges (with the same polari ty) made an angle of  90 ° with each other,  those of  neighbouring ones (with alternating polari ty)  one of  60 ° . For clarity fig. 1 (right) shows four equal Mini-Orange systems, each composed of  four magnets and the e ÷ and e -  sys- I I I I 1 l ° B ( p , p ' )  Ep = 7 M e V  e ÷ ..- e- SPECTRUM 3.35 MeV 120" 4OCa - 3;059 MeV 4.43 MeV 2.87 MeV I] B 12 C 80 - lo B I~ I l r '  . I 6.05 MeV 40-- ~ / ~ - ,  { u "LP ~ 0 //, I I I I - _ , ,  H 2 3 4 5 6 MeV Z moO2 :::) Ee. ÷Ee.÷ 2 0 U 106-- 8,000f ~lr G A M M A -  RAY SPECTRUM- ~o 5 6,OOOl- ~ A  ~ ~ . . ~  l t 'g'- ~'J l 8 13 MeV 16 0 10 3 I I i 102 -- I I I .-¢ I I I I I 10 2 3 4 5 6 MeV Ey Fig. 2. Sum of  four  e + + e -  pa i r  spec t ra  ( top)  c o m p a r e d  w i t h  a s imu l t aneous ly  r eco rded  ~/-ray s p e c t r u m  ( b o t t o m )  us ing a Ge de-  t ec to r ,  following the l°B (p, p') reaction at 7 MeV. In case of an existing axion with mass of 1.68 MeV and lifetime 5 X 10 -12 s, the prediction by DonneUy et al. [4] corresponds to a peak at 3.59 MeV with height of over 1000 counts. Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 tems being only different by inversion of their mag- netic fields. In reality the present experiment was lim- ited by the available magnets so that two four-gap and two five-gap systems had to be used. Since the latter magnets were relatively weaker, the overall features of the four magnet systems were similar, with maxi- mum transmissions around 1.3 MeV. The Mini-Orange spectrometers were equipped with 300 mm 2 Si(Li) detectors: two with a thickness of 3 mm and two with a thickness of 5 mm. With the KVI data acquisition system (VAX 750) the twofold coin- cidences were registered on magnetic tape for off-line analysis. Fig. 2 (top) displays the essential result: the sum of the (e+e-) pair spectra from the four neighbouring detector combinations in which we can look for a pos- sible axion signal in competition with expected IPC peaks. The (e÷e - )  spectrum is compared with a simul- taneously recorded spectrum of gamma rays (bottom) using a Ge detector. It is apparent that the internal pair peaks of the two isoscalar transitions at 2.87 MeV and 3.59 MeV in 10B are only weakly visible in the (e+e -)  spectrum. Interestingly, two rather prominent peaks at 3.35 and 6.05 MeV show up in the sum spectrum. They have no counterpart in the 3'-ray spectrum and belong to to- tally converted E0 transitions in 40Ca and 160 respec- tively, which are to our knowledge the only two exist- ing E0 transitions in nature depopulating first excited states between 3 and 7 MeV. The spectra of scattered protons, recorded separately, indeed showed a signifi- cant content of 160 (and 12C) impurity in the target. A tracer impurity causes in the pair spectra the ap- pearance of the E0 transition in 40Ca. The 6.05 MeV transition in 160 occurs just above energy threshold and also in the extreme upper transmission range of the Mini-Oranges. Furthermore, it is observed despite the fact that the two 3 mm Si(Li) detectors had insuf- ficient stopping power for most of the e + and e -  pulses. Evidently, the arrangement is hypersensitive for detection of totally converted E0 transitions. The background of the pair spectrum is complex, containing a continuum distribution following 10B(p, a) distributions from pairs created inside the vacuum walls, e + and e -  particles from e.g. the E0(160) transi- tions which pass through the detectors. Yet, the main part of the spectrum belongs to IPC coincidences and not to e.g. Compton scattered 3' rays since the yield of e + + e + and e-  + e -  type combinations is less by an or- der of magnitude than the e + + e -  pairs. In the 3'-ray spectrum all transitions of sufficiently short half-life are Doppler broadened and/or shifted. The 10B 2.87 and 3.59 MeV 3' rays show up strongly. The spectrum also shows a number of 27A1 3" rays produced in the walls of the Mini-Orange vacuum chamber, together with the 4.43 MeV E2 transition in 12C and the 6.13 MeV E3 transition in 160 (target impurities), visible by photo peaks, single- and double-escape peaks. Efficiency calibration for IPC detection was ob- tained as follows: at the low energy side the system was calibrated (i) internally at 1.85 MeV with the 2.87 MeV transition in 10B (> 98% E2) and (ii) externally at 1.733 MeV using the 2.755 MeV E2 transition in 24Na with theoretical [18] IPC's of 7.4 × 10 -4 re- spectively 7.0 X 10 -4. (iii) A comparison of the inten- sity of inelastically scattered protons from the 4.43 MeV level in 12C with that from the 3.59 MeV level in 10B, the ratio being 0.6, showed us that only a frac- tion of the corresponding 3'-ray intensity in 12C is produced in the target and apparently most of it orig- inates in the walls of the chamber. The ratio of 3'-ray intensities was determined to be 0.5 using experimen- tal data for differential and total cross sections for 10B (p,p ')  [20] and 12C (p, p') [21,22] inelastic scat- tering at 7 MeV under 135 °. The obtained gamma-ray intensity was then used together with the internal pair intensity and the theoretical [18] IPC of 1.3 × 10 -3 for an E2 transition of 4.43 MeV to internally determine the efficiency of the Mini-Orange system at 3.41 MeV. All three efficiency calibrations are equal to a value of e = 2.3 X 10 -4 within 35%. The isoscalar 3.59 MeV transition has in the (e+e - )  pair spectrum roughly the same intensity as the paral- lel 2.87 MeV transition used for internal calibration. The branching ratio of the 3.59 MeV pair peak was determined to be (2.1 + 1.1) X 10 -3 which is consis- tent with internal pair creation, %r being 1.0 X 10 -3 for a 28% M1 and 72% E2 transition. The full strength of axion decay with the Donnelly value o f I  a = 0.39 1~ (M1) for an axion of 1.68 MeV would have caused a peak height of over 1000 counts or two orders of mag- nitude !arger than as observed. With 90% confidence our result corresponds with a boundary being 54 times smaller than the value predicted by the standard model [4]. In recent more sophisticated axion models [13,14] the conjectured axion couples to one or two Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 quark pairs IN = 1 or 2 in eq. (1)]. In the case of  only two light quarks, no coupling is expected to a magnet- ic isoscalar transition. In the case of coupling to two quark pairs our experiment leads with 90% confidence to a boundary factor of  31 with respect to the theoret- ical prediction in the case of  existing axions. Conse- quently our search yields no evidence for the existence of  a standard (N = 3) or a viable (N = 2) axion. After evaluation of  our experimental  data, we be- came aware of  IPC measurements in the early de- cades of  nuclear physics, when axions were not yet  advocated, but  when (e+e - )  angular correlations were used as a probe for distinguishing multipolarit ies of  high-energy "/-rays. We were both  surprised and im- pressed by the angular correlations reported for the 3.59 MeV transition in the clean reaction 9Be(d, n)10B by Gorodetsky et al. [23]. They used scintillation counters with inherently two orders of  magnitude less energy resolution than our Si(Li) detectors ( though this resolution is partly spoiled by  Doppler broaden- ing), and could hardly separate a strong 3.37 MeV transition in 10B from the 3.59 MeV isoscalar t ransi-  tion in 10B. Yet, their angular distr ibution shows no enhancement near 45 ° where the standard axion would have given at least one order of  magnitude larger signal than measured. Also, a recent reanalysis by Calaprice et al. [24] of  IPC data from Warburton et al. [25] shows no evidence for axion competi t ion in either isoscalar or isovector M1 transitions. The authors wish to thank H.P. Blok, P.F.A. Goudsmit ,  R.H. Siemssen and A.H. Wapstra for valuable discussions. This investigation was sup- ported by the Netherlands Organization for Pure Scientific Research (ZWO) through the Foundat ion for Fundamental  Research on Matter (FOM). References [1] S. Weinberg, Phys. Rev. Lett. 40 (1978) 223. [2] F. Wilczek, Phys. Rev. Lett. 40 (1978) 279. [3] R~D. Peccei and H.R. Quinn, Phys. Rev. Lett. 38 (1977) 1440. [4] T.W. DonneUy et al., Phys. Rev. D 18 (1978) 1607. [5] A. Zehnder, in: Fundamental interactions in low-energy systems (Plenum, New York, 1985). [6] T. Cowa net  al., Phys. Rev. Lett. 54 (1985) 1761. [7] T. Cowan et al., Phys. Rev. Lett. 56 (1986) 444. [8] F.P. Calaprice et al., Phys. Rev. D 20 (1979) 2708. 56 0986) 302. [9] A. Schgfer et al., J. Phys. G 11 (1985) 169. [10] A.B. Balantekin et al., Phys. Rev. Lett. 55 (1985) 461. [11] N. Mukhopadhyay and A. Zehnder, Phys. Rev. Lett. 56 (1986) 206. [12] A. Chodos and L.R.C. Wijewardhama, Phys. Rev. Lett. 56 (1986) 302. [13] R.D. Peccei, Tai Tsun Wu and T. Yanagida, Phys. Lett. B 172 (1986) 435. [14] L.M. Krauss and F. Wilczek, Phys. Lett. B 173 (1986) 189. [15] S.J. Brodsky et al., Phys. Rev. Lett. 56 (1986) 1763. [16] K. Abrahams et al., KVI Annual Report 1985. [17] E.K. Warburton et al., Phys. Rev. 17 (1968) 1178. [18] P. Schliiter and G. Soff, Atom. Data Nucl. Data Tables 24 (1979) 509. [19] J. van Klinken, S.J. Feenstra and G. Dumont, Nucl. In- strum. Methods 151 (1978) 433. [20] B.A. Watson et al., Phys. Rev. 187 (1969) 1351. [21] J.B. Swint et al., Nucl. Phys. 86 (1966) 119. [22] A.C.L. Barnar d et al., Nucl. Phys. 86 (1966) 130. [23] S. Gorodetsky, P. Chevallier, R. Armbruster and G. Sutter, Nucl. Phys. 12 (1959) 349. [24] F.P. Calapriee et al., private communication, and to be published. [25] E.K. Warburton et al., Phys. Rev. 132 (1963) 776,133 (1964) B42, 

English

Search for short-lived axions in a nuclear isoscalar transition

   University of GroningenSearch for short-lived axions in a nuclear isoscalar transitionde Boer, FWN; Abrahams, K; Balanda, A; Bokemeyer, H; van Dantzig, R; Jansen, JFW;Kotlinski, B; de Voigt, MJA; van Klinken, JPublished in:Physics Letters BDOI:10.1016/0370-2693(86)90123-1IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1986Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):de Boer, FWN., Abrahams, K., Balanda, A., Bokemeyer, H., van Dantzig, R., Jansen, JFW., Kotlinski, B.,de Voigt, MJA., & van Klinken, J. (1986). Search for short-lived axions in a nuclear isoscalar transition.Physics Letters B, 180(1-2), 4-8. https://doi.org/10.1016/0370-2693(86)90123-1CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 20-06-2022Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 S E A R C H  F O R  S H O R T - L I V E D  A X I O N S  IN  A N U C L E A R  I S O S C A L A R  T R A N S I T I O N  F .W.N.  D E  BOER 1, K. A B R A H A M S  2, A. B A L A N D A  3,4, H. B O K E M E Y E R  5, R. V A N  D A N T Z I G  6, J .F .W.  J A N S E N ,  B. K O T L I N S K I ,  M.J.A. D E  V O I G T  and  J. V A N  K L I N K E N  Kernfysisch Versneller lnstituut, Universiteit van Groningen, NL-974 7 AA Groningen, The Netherlands Received 22 July 1986; revised manuscript received 10 September 1986 Axions, if existing, can cause a strong signal of positron-electron pairs in isoscalar M1 transitions in competition with gamma-ray emission. We searched for such a signal in the 3.59 MeV transition in I°B with a fourfold Mini-Orange spectro- meter. No axion events were found within two percent of the prediction for the standard axion- to ~,-ray branching ratio. A light pseudoscalar Goldstone boson, the axion, has been introduced by Weinberg [1] and Wilczek [2], as a consequence o f  the global U(1) symmetry pro- posed by Peccei and Quinn [3] to explain the ab- sence of  large P and CP violations in strong interac- tions. In a treatise of  possible axion search experi- ments, Donnelly et al. [4] discussed the probabil i ty for nuclear de-excitation by  axion emission in compe- t i t ion with magnetic ~/radiation. Subsequent axion searches provided such stringent upper bounds on mass and lifetime that  the nonexistence of  axions was anticipated [5]. However, the interest in an observable axion was revived recently [6] after the puzzling ob- servation in superheavy collisional systems at GSI o f  a sharp positron peak at approximately 330 keV, which was later [7] found to be in coincidence with a simi- lar electron peak. Could the observed (e+e - )  pairs t Present address: NIKHEF-K, P.O. Box 4395, NL-1009AJ Amsterdam, The Netherlands. 2 Energieonderzoek Centrum Nederland (ECN), Petten, The Netherlands. 3 Physics Laboratory of the Free University, NL-1081 HV Amsterdam, The Netherlands. 4 Jagiellonian University, PL-30059 Cracow 12, Poland. 5 Gesellschaft for Schwerionenforschung (GSI), Postfach 110541, D-6100 Darmstadt 11, Fed. Rep. Germany. 6 NIKHEF-K, P.O. Box 4395, NL-1009 AJ Amsterdam, The Netherlands. arise from a "visible" axion with a mass o f  about 1.68 MeV and a lifetime of~< 10 -11 s? The axion mass and lifetime can be related to one free parameter X, the ratio o f  vacuum expectat ion values for two Higgs field componen t s f s in  X and f c o s  X with f =  (G FV ~) - I / 2  = 250 GeV (G F being the Fermi coupling constant): m a = 25N(X + l /X)  (keV), (1) where N is the number of  active quark pairs (N = 3 in ref. [4]). For  m a > 2m e the axion will predominantly decay in a pos i t ron-e lec t ron  pair with lifetime r(a -~ e+e - )  = 8"tr f2X2me2(m 2 - 4me2) -1/2 (s). (2) Searches for axions decaying into two photons with radioactive sources, reactors and beam dumps constrain [5] the mass to < 1 MeV and the lifetime to < 10 - 6  s, whereas a search [8] for decay into posi- t ron -e l ec t ron  pairs limits the lifetime to < 2 X 10 -10 s. Searches with pseudoscalar- and vector mesons (K +, J/~k and Y) were sensitive to lifetimes of  10 -11 s due to time dilation of  the order of  104 . The anom- alous magnetic moment  o f  the electron constrains the lifetime to > 5 X 10 -13 s [ 9 - 1 2 ] .  In summary, a rather narrow window of  lifetimes between 10 -13 and 10 -10 s cannot be excluded on the basis of  exist- ing experimental evidence. If  the observed positron peak is due to an axion with mass 1.68 MeV, its X 0370-2693/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 value according to eq. (1) is either X = 0.045 or X = 22 leading to a lifetime of  about 5 × 10 -12 s (al- lowed) or 2 × 10 - 6  s (excluded).  Either of  these two X values would lead to contradictions for branching ratios of  meson decays in the standard axion model. Quite recently, however, Peccei et al. [13] have suc- ceeded in reconciling the previous unsuccessful searches with an axion of  1.68 MeV in a 'Wiable axion model"  which suppresses the coupling to the heavier quarks. Almost simultaneously, Krauss and Wilczek [14] pre- sented a "short-lived axion variant" coupled preferen- tially to light fermions and stressed with Brodsky et al. [15] the need of  new experiments.  In this letter we report  on a search for axions in the unexplored region with lifetimes T < 10 - 9  s. We looked for the e+e - decay of  such axions in competi- t ion with internal pair creation (IPC) of  a magnetic q-ray transition. Specifically for isoscalar transitions, the branching ratio for (e+e - )  pair creation from a pseudoscalar decay particle and M1 gamma-decay can be as large [4,16] as 50%. This is three orders o f  mag- nitude larger than IPC. As isoscalar transition the 3.59 MeV M1 + E2 transition in 10B was chosen. The 2 +, 0 state at 3.59 MeV in 10B decays to the 3 +, 0 groundstate with a 19% branching ratio. The transition has a mixing ratio [17] 5(E2/M1) of  1.5 + 0.6 while the parallel 2 +, 0 ~ 1 +, 0 transition of  2.87 MeV is [17] predominantly E2 with 5 - 1  < 0.2. Neglecting internal conversion by atomic electrons, two-photon emission and other higher-order processes, the IPC contributes at the level of  10 -3  compared with gamma rays in the de,excitation of  the 3.59 MeV state. Axion emission would imply a source of  (e+e - )  pairs in addit ion to the expected (e+e - )  pairs from IPC. The energy and angular correlations of  (e+e-)  pairs from the 3.59 MeV transition are ftxed kinemati- cally. In our experimental arrangement the M I and E2 IPC is detected with total  efficiency e of  typically 10 - 4  (see below). The IPC angular correlation be- tween the e ÷ and e -  is maximum at relative angle 0 = 0 °, while in case of  the axions the maximum will occur around 0 = 45 °. The angle between the axes o f  neighbouring Mini-Oranges is 60 ° (fig. 1), which causes an axion decay to be seen with somewhat higher efficiency than IPC; a numerical estimate gave e A ~ 1.5e, assuming that the axions are emitted iso- tropically. The nuclear axion-decay rate (in the case of  existing axions) is predicted in ref. [4] to be as large A e . . / ~ d e t .  target _ - ~ 4 5 v ~  .beam ~Ge detector Fig. 1. Fourfold Mini-Orange arrangement for the axion search. To the left a view of the horizontal plane and to the right a view upstream along the beam axis. The hypothetical axi0n ~ e + + e- decay is supposed to occur inside or closely (at gm distance) be- hind the target layer. In the left part of the figure an e- particle is indicated, while the corresponding e + particle will be detected by a neighbouring e + detector (out of plane). Volume 180, number  1,2 PHYSICS LETTERS B 6 N o v e m b e r  1986 as I a = 0.39 I3,(M 1). The number of  observed pairs can now be writ ten as: N(e+e-)/e = {0.39 X 1.5 + ~ ( M 1 ) )  Lr(M1 ) + otTr (E2)[~ (E2), (3) where e is the efficiency. Theoretical IPC coefficients o t  can be obtained from ref. [18]. I f  axions indeed exist then the first term (= 0.585) will dominate since the others are of  order 10 - 3  . In the experiment the 3.59 MeV state in 10B was excited in inelastic scattering of  7 MeV protons by a 10B target of  1 mg/cm 2. Using a fourfold Mini-Orange system [19] with two detector  systems for e + and two for e -  detection, we registered prompt  (e+e - )  signals and their energy sums. The four spectrometers were set at a backward angle of  45 ° with respect to the incident proton beam. This is shown by a view in the horizontal plane in fig. 1 (left). The axes of  oppo- site Mini-Oranges (with the same polari ty) made an angle of  90 ° with each other,  those of  neighbouring ones (with alternating polari ty)  one of  60 ° . For clarity fig. 1 (right) shows four equal Mini-Orange systems, each composed of  four magnets and the e ÷ and e -  sys- I I I I 1 l ° B ( p , p ' )  Ep = 7 M e V  e ÷ ..- e- SPECTRUM 3.35 MeV 120" 4OCa - 3;059 MeV 4.43 MeV 2.87 MeV I] B 12 C 80 - lo B I~ I l r '  . I 6.05 MeV 40-- ~ / ~ - ,  { u "LP ~ 0 //, I I I I - _ , ,  H 2 3 4 5 6 MeV Z moO2 :::) Ee. ÷Ee.÷ 2 0 U 106-- 8,000f ~lr G A M M A -  RAY SPECTRUM- ~o 5 6,OOOl- ~ A  ~ ~ . . ~  l t 'g'- ~'J l 8 13 MeV 16 0 10 3 I I i 102 -- I I I .-¢ I I I I I 10 2 3 4 5 6 MeV Ey Fig. 2. Sum of  four  e + + e -  pa i r  spec t ra  ( top)  c o m p a r e d  w i t h  a s imu l t aneous ly  r eco rded  ~/-ray s p e c t r u m  ( b o t t o m )  us ing a Ge de-  t ec to r ,  following the l°B (p, p') reaction at 7 MeV. In case of an existing axion with mass of 1.68 MeV and lifetime 5 X 10 -12 s, the prediction by DonneUy et al. [4] corresponds to a peak at 3.59 MeV with height of over 1000 counts. Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 tems being only different by inversion of their mag- netic fields. In reality the present experiment was lim- ited by the available magnets so that two four-gap and two five-gap systems had to be used. Since the latter magnets were relatively weaker, the overall features of the four magnet systems were similar, with maxi- mum transmissions around 1.3 MeV. The Mini-Orange spectrometers were equipped with 300 mm 2 Si(Li) detectors: two with a thickness of 3 mm and two with a thickness of 5 mm. With the KVI data acquisition system (VAX 750) the twofold coin- cidences were registered on magnetic tape for off-line analysis. Fig. 2 (top) displays the essential result: the sum of the (e+e-) pair spectra from the four neighbouring detector combinations in which we can look for a pos- sible axion signal in competition with expected IPC peaks. The (e÷e - )  spectrum is compared with a simul- taneously recorded spectrum of gamma rays (bottom) using a Ge detector. It is apparent that the internal pair peaks of the two isoscalar transitions at 2.87 MeV and 3.59 MeV in 10B are only weakly visible in the (e+e -)  spectrum. Interestingly, two rather prominent peaks at 3.35 and 6.05 MeV show up in the sum spectrum. They have no counterpart in the 3'-ray spectrum and belong to to- tally converted E0 transitions in 40Ca and 160 respec- tively, which are to our knowledge the only two exist- ing E0 transitions in nature depopulating first excited states between 3 and 7 MeV. The spectra of scattered protons, recorded separately, indeed showed a signifi- cant content of 160 (and 12C) impurity in the target. A tracer impurity causes in the pair spectra the ap- pearance of the E0 transition in 40Ca. The 6.05 MeV transition in 160 occurs just above energy threshold and also in the extreme upper transmission range of the Mini-Oranges. Furthermore, it is observed despite the fact that the two 3 mm Si(Li) detectors had insuf- ficient stopping power for most of the e + and e -  pulses. Evidently, the arrangement is hypersensitive for detection of totally converted E0 transitions. The background of the pair spectrum is complex, containing a continuum distribution following 10B(p, a) distributions from pairs created inside the vacuum walls, e + and e -  particles from e.g. the E0(160) transi- tions which pass through the detectors. Yet, the main part of the spectrum belongs to IPC coincidences and not to e.g. Compton scattered 3' rays since the yield of e + + e + and e-  + e -  type combinations is less by an or- der of magnitude than the e + + e -  pairs. In the 3'-ray spectrum all transitions of sufficiently short half-life are Doppler broadened and/or shifted. The 10B 2.87 and 3.59 MeV 3' rays show up strongly. The spectrum also shows a number of 27A1 3" rays produced in the walls of the Mini-Orange vacuum chamber, together with the 4.43 MeV E2 transition in 12C and the 6.13 MeV E3 transition in 160 (target impurities), visible by photo peaks, single- and double-escape peaks. Efficiency calibration for IPC detection was ob- tained as follows: at the low energy side the system was calibrated (i) internally at 1.85 MeV with the 2.87 MeV transition in 10B (> 98% E2) and (ii) externally at 1.733 MeV using the 2.755 MeV E2 transition in 24Na with theoretical [18] IPC's of 7.4 × 10 -4 re- spectively 7.0 X 10 -4. (iii) A comparison of the inten- sity of inelastically scattered protons from the 4.43 MeV level in 12C with that from the 3.59 MeV level in 10B, the ratio being 0.6, showed us that only a frac- tion of the corresponding 3'-ray intensity in 12C is produced in the target and apparently most of it orig- inates in the walls of the chamber. The ratio of 3'-ray intensities was determined to be 0.5 using experimen- tal data for differential and total cross sections for 10B (p,p ')  [20] and 12C (p, p') [21,22] inelastic scat- tering at 7 MeV under 135 °. The obtained gamma-ray intensity was then used together with the internal pair intensity and the theoretical [18] IPC of 1.3 × 10 -3 for an E2 transition of 4.43 MeV to internally determine the efficiency of the Mini-Orange system at 3.41 MeV. All three efficiency calibrations are equal to a value of e = 2.3 X 10 -4 within 35%. The isoscalar 3.59 MeV transition has in the (e+e - )  pair spectrum roughly the same intensity as the paral- lel 2.87 MeV transition used for internal calibration. The branching ratio of the 3.59 MeV pair peak was determined to be (2.1 + 1.1) X 10 -3 which is consis- tent with internal pair creation, %r being 1.0 X 10 -3 for a 28% M1 and 72% E2 transition. The full strength of axion decay with the Donnelly value o f I  a = 0.39 1~ (M1) for an axion of 1.68 MeV would have caused a peak height of over 1000 counts or two orders of mag- nitude !arger than as observed. With 90% confidence our result corresponds with a boundary being 54 times smaller than the value predicted by the standard model [4]. In recent more sophisticated axion models [13,14] the conjectured axion couples to one or two Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 quark pairs IN = 1 or 2 in eq. (1)]. In the case of  only two light quarks, no coupling is expected to a magnet- ic isoscalar transition. In the case of coupling to two quark pairs our experiment leads with 90% confidence to a boundary factor of  31 with respect to the theoret- ical prediction in the case of  existing axions. Conse- quently our search yields no evidence for the existence of  a standard (N = 3) or a viable (N = 2) axion. After evaluation of  our experimental  data, we be- came aware of  IPC measurements in the early de- cades of  nuclear physics, when axions were not yet  advocated, but  when (e+e - )  angular correlations were used as a probe for distinguishing multipolarit ies of  high-energy "/-rays. We were both  surprised and im- pressed by the angular correlations reported for the 3.59 MeV transition in the clean reaction 9Be(d, n)10B by Gorodetsky et al. [23]. They used scintillation counters with inherently two orders of  magnitude less energy resolution than our Si(Li) detectors ( though this resolution is partly spoiled by  Doppler broaden- ing), and could hardly separate a strong 3.37 MeV transition in 10B from the 3.59 MeV isoscalar t ransi-  tion in 10B. Yet, their angular distr ibution shows no enhancement near 45 ° where the standard axion would have given at least one order of  magnitude larger signal than measured. Also, a recent reanalysis by Calaprice et al. [24] of  IPC data from Warburton et al. [25] shows no evidence for axion competi t ion in either isoscalar or isovector M1 transitions. The authors wish to thank H.P. Blok, P.F.A. Goudsmit ,  R.H. Siemssen and A.H. Wapstra for valuable discussions. This investigation was sup- ported by the Netherlands Organization for Pure Scientific Research (ZWO) through the Foundat ion for Fundamental  Research on Matter (FOM). References [1] S. Weinberg, Phys. Rev. Lett. 40 (1978) 223. [2] F. Wilczek, Phys. Rev. Lett. 40 (1978) 279. [3] R~D. Peccei and H.R. Quinn, Phys. Rev. Lett. 38 (1977) 1440. [4] T.W. DonneUy et al., Phys. Rev. D 18 (1978) 1607. [5] A. Zehnder, in: Fundamental interactions in low-energy systems (Plenum, New York, 1985). [6] T. Cowa net  al., Phys. Rev. Lett. 54 (1985) 1761. [7] T. Cowan et al., Phys. Rev. Lett. 56 (1986) 444. [8] F.P. Calaprice et al., Phys. Rev. D 20 (1979) 2708. 56 0986) 302. [9] A. Schgfer et al., J. Phys. G 11 (1985) 169. [10] A.B. Balantekin et al., Phys. Rev. Lett. 55 (1985) 461. [11] N. Mukhopadhyay and A. Zehnder, Phys. Rev. Lett. 56 (1986) 206. [12] A. Chodos and L.R.C. Wijewardhama, Phys. Rev. Lett. 56 (1986) 302. [13] R.D. Peccei, Tai Tsun Wu and T. Yanagida, Phys. Lett. B 172 (1986) 435. [14] L.M. Krauss and F. Wilczek, Phys. Lett. B 173 (1986) 189. [15] S.J. Brodsky et al., Phys. Rev. Lett. 56 (1986) 1763. [16] K. Abrahams et al., KVI Annual Report 1985. [17] E.K. Warburton et al., Phys. Rev. 17 (1968) 1178. [18] P. Schliiter and G. Soff, Atom. Data Nucl. Data Tables 24 (1979) 509. [19] J. van Klinken, S.J. Feenstra and G. Dumont, Nucl. In- strum. Methods 151 (1978) 433. [20] B.A. Watson et al., Phys. Rev. 187 (1969) 1351. [21] J.B. Swint et al., Nucl. Phys. 86 (1966) 119. [22] A.C.L. Barnar d et al., Nucl. Phys. 86 (1966) 130. [23] S. Gorodetsky, P. Chevallier, R. Armbruster and G. Sutter, Nucl. Phys. 12 (1959) 349. [24] F.P. Calapriee et al., private communication, and to be published. [25] E.K. Warburton et al., Phys. Rev. 132 (1963) 776,133 (1964) B42, 

Proceedings - University of Groningen

   University of GroningenSearch for short-lived axions in a nuclear isoscalar transitionde Boer, FWN; Abrahams, K; Balanda, A; Bokemeyer, H; van Dantzig, R; Jansen, JFW;Kotlinski, B; de Voigt, MJA; van Klinken, JPublished in:Physics Letters BDOI:10.1016/0370-2693(86)90123-1IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1986Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):de Boer, FWN., Abrahams, K., Balanda, A., Bokemeyer, H., van Dantzig, R., Jansen, JFW., Kotlinski, B.,de Voigt, MJA., & van Klinken, J. (1986). Search for short-lived axions in a nuclear isoscalar transition.Physics Letters B, 180(1-2), 4-8. https://doi.org/10.1016/0370-2693(86)90123-1CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 03-06-2022Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 S E A R C H  F O R  S H O R T - L I V E D  A X I O N S  IN  A N U C L E A R  I S O S C A L A R  T R A N S I T I O N  F .W.N.  D E  BOER 1, K. A B R A H A M S  2, A. B A L A N D A  3,4, H. B O K E M E Y E R  5, R. V A N  D A N T Z I G  6, J .F .W.  J A N S E N ,  B. K O T L I N S K I ,  M.J.A. D E  V O I G T  and  J. V A N  K L I N K E N  Kernfysisch Versneller lnstituut, Universiteit van Groningen, NL-974 7 AA Groningen, The Netherlands Received 22 July 1986; revised manuscript received 10 September 1986 Axions, if existing, can cause a strong signal of positron-electron pairs in isoscalar M1 transitions in competition with gamma-ray emission. We searched for such a signal in the 3.59 MeV transition in I°B with a fourfold Mini-Orange spectro- meter. No axion events were found within two percent of the prediction for the standard axion- to ~,-ray branching ratio. A light pseudoscalar Goldstone boson, the axion, has been introduced by Weinberg [1] and Wilczek [2], as a consequence o f  the global U(1) symmetry pro- posed by Peccei and Quinn [3] to explain the ab- sence of  large P and CP violations in strong interac- tions. In a treatise of  possible axion search experi- ments, Donnelly et al. [4] discussed the probabil i ty for nuclear de-excitation by  axion emission in compe- t i t ion with magnetic ~/radiation. Subsequent axion searches provided such stringent upper bounds on mass and lifetime that  the nonexistence of  axions was anticipated [5]. However, the interest in an observable axion was revived recently [6] after the puzzling ob- servation in superheavy collisional systems at GSI o f  a sharp positron peak at approximately 330 keV, which was later [7] found to be in coincidence with a simi- lar electron peak. Could the observed (e+e - )  pairs t Present address: NIKHEF-K, P.O. Box 4395, NL-1009AJ Amsterdam, The Netherlands. 2 Energieonderzoek Centrum Nederland (ECN), Petten, The Netherlands. 3 Physics Laboratory of the Free University, NL-1081 HV Amsterdam, The Netherlands. 4 Jagiellonian University, PL-30059 Cracow 12, Poland. 5 Gesellschaft for Schwerionenforschung (GSI), Postfach 110541, D-6100 Darmstadt 11, Fed. Rep. Germany. 6 NIKHEF-K, P.O. Box 4395, NL-1009 AJ Amsterdam, The Netherlands. arise from a "visible" axion with a mass o f  about 1.68 MeV and a lifetime of~< 10 -11 s? The axion mass and lifetime can be related to one free parameter X, the ratio o f  vacuum expectat ion values for two Higgs field componen t s f s in  X and f c o s  X with f =  (G FV ~) - I / 2  = 250 GeV (G F being the Fermi coupling constant): m a = 25N(X + l /X)  (keV), (1) where N is the number of  active quark pairs (N = 3 in ref. [4]). For  m a > 2m e the axion will predominantly decay in a pos i t ron-e lec t ron  pair with lifetime r(a -~ e+e - )  = 8"tr f2X2me2(m 2 - 4me2) -1/2 (s). (2) Searches for axions decaying into two photons with radioactive sources, reactors and beam dumps constrain [5] the mass to < 1 MeV and the lifetime to < 10 - 6  s, whereas a search [8] for decay into posi- t ron -e l ec t ron  pairs limits the lifetime to < 2 X 10 -10 s. Searches with pseudoscalar- and vector mesons (K +, J/~k and Y) were sensitive to lifetimes of  10 -11 s due to time dilation of  the order of  104 . The anom- alous magnetic moment  o f  the electron constrains the lifetime to > 5 X 10 -13 s [ 9 - 1 2 ] .  In summary, a rather narrow window of  lifetimes between 10 -13 and 10 -10 s cannot be excluded on the basis of  exist- ing experimental evidence. If  the observed positron peak is due to an axion with mass 1.68 MeV, its X 0370-2693/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 value according to eq. (1) is either X = 0.045 or X = 22 leading to a lifetime of  about 5 × 10 -12 s (al- lowed) or 2 × 10 - 6  s (excluded).  Either of  these two X values would lead to contradictions for branching ratios of  meson decays in the standard axion model. Quite recently, however, Peccei et al. [13] have suc- ceeded in reconciling the previous unsuccessful searches with an axion of  1.68 MeV in a 'Wiable axion model"  which suppresses the coupling to the heavier quarks. Almost simultaneously, Krauss and Wilczek [14] pre- sented a "short-lived axion variant" coupled preferen- tially to light fermions and stressed with Brodsky et al. [15] the need of  new experiments.  In this letter we report  on a search for axions in the unexplored region with lifetimes T < 10 - 9  s. We looked for the e+e - decay of  such axions in competi- t ion with internal pair creation (IPC) of  a magnetic q-ray transition. Specifically for isoscalar transitions, the branching ratio for (e+e - )  pair creation from a pseudoscalar decay particle and M1 gamma-decay can be as large [4,16] as 50%. This is three orders o f  mag- nitude larger than IPC. As isoscalar transition the 3.59 MeV M1 + E2 transition in 10B was chosen. The 2 +, 0 state at 3.59 MeV in 10B decays to the 3 +, 0 groundstate with a 19% branching ratio. The transition has a mixing ratio [17] 5(E2/M1) of  1.5 + 0.6 while the parallel 2 +, 0 ~ 1 +, 0 transition of  2.87 MeV is [17] predominantly E2 with 5 - 1  < 0.2. Neglecting internal conversion by atomic electrons, two-photon emission and other higher-order processes, the IPC contributes at the level of  10 -3  compared with gamma rays in the de,excitation of  the 3.59 MeV state. Axion emission would imply a source of  (e+e - )  pairs in addit ion to the expected (e+e - )  pairs from IPC. The energy and angular correlations of  (e+e-)  pairs from the 3.59 MeV transition are ftxed kinemati- cally. In our experimental arrangement the M I and E2 IPC is detected with total  efficiency e of  typically 10 - 4  (see below). The IPC angular correlation be- tween the e ÷ and e -  is maximum at relative angle 0 = 0 °, while in case of  the axions the maximum will occur around 0 = 45 °. The angle between the axes o f  neighbouring Mini-Oranges is 60 ° (fig. 1), which causes an axion decay to be seen with somewhat higher efficiency than IPC; a numerical estimate gave e A ~ 1.5e, assuming that the axions are emitted iso- tropically. The nuclear axion-decay rate (in the case of  existing axions) is predicted in ref. [4] to be as large A e . . / ~ d e t .  target _ - ~ 4 5 v ~  .beam ~Ge detector Fig. 1. Fourfold Mini-Orange arrangement for the axion search. To the left a view of the horizontal plane and to the right a view upstream along the beam axis. The hypothetical axi0n ~ e + + e- decay is supposed to occur inside or closely (at gm distance) be- hind the target layer. In the left part of the figure an e- particle is indicated, while the corresponding e + particle will be detected by a neighbouring e + detector (out of plane). Volume 180, number  1,2 PHYSICS LETTERS B 6 N o v e m b e r  1986 as I a = 0.39 I3,(M 1). The number of  observed pairs can now be writ ten as: N(e+e-)/e = {0.39 X 1.5 + ~ ( M 1 ) )  Lr(M1 ) + otTr (E2)[~ (E2), (3) where e is the efficiency. Theoretical IPC coefficients o t  can be obtained from ref. [18]. I f  axions indeed exist then the first term (= 0.585) will dominate since the others are of  order 10 - 3  . In the experiment the 3.59 MeV state in 10B was excited in inelastic scattering of  7 MeV protons by a 10B target of  1 mg/cm 2. Using a fourfold Mini-Orange system [19] with two detector  systems for e + and two for e -  detection, we registered prompt  (e+e - )  signals and their energy sums. The four spectrometers were set at a backward angle of  45 ° with respect to the incident proton beam. This is shown by a view in the horizontal plane in fig. 1 (left). The axes of  oppo- site Mini-Oranges (with the same polari ty) made an angle of  90 ° with each other,  those of  neighbouring ones (with alternating polari ty)  one of  60 ° . For clarity fig. 1 (right) shows four equal Mini-Orange systems, each composed of  four magnets and the e ÷ and e -  sys- I I I I 1 l ° B ( p , p ' )  Ep = 7 M e V  e ÷ ..- e- SPECTRUM 3.35 MeV 120" 4OCa - 3;059 MeV 4.43 MeV 2.87 MeV I] B 12 C 80 - lo B I~ I l r '  . I 6.05 MeV 40-- ~ / ~ - ,  { u "LP ~ 0 //, I I I I - _ , ,  H 2 3 4 5 6 MeV Z moO2 :::) Ee. ÷Ee.÷ 2 0 U 106-- 8,000f ~lr G A M M A -  RAY SPECTRUM- ~o 5 6,OOOl- ~ A  ~ ~ . . ~  l t 'g'- ~'J l 8 13 MeV 16 0 10 3 I I i 102 -- I I I .-¢ I I I I I 10 2 3 4 5 6 MeV Ey Fig. 2. Sum of  four  e + + e -  pa i r  spec t ra  ( top)  c o m p a r e d  w i t h  a s imu l t aneous ly  r eco rded  ~/-ray s p e c t r u m  ( b o t t o m )  us ing a Ge de-  t ec to r ,  following the l°B (p, p') reaction at 7 MeV. In case of an existing axion with mass of 1.68 MeV and lifetime 5 X 10 -12 s, the prediction by DonneUy et al. [4] corresponds to a peak at 3.59 MeV with height of over 1000 counts. Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 tems being only different by inversion of their mag- netic fields. In reality the present experiment was lim- ited by the available magnets so that two four-gap and two five-gap systems had to be used. Since the latter magnets were relatively weaker, the overall features of the four magnet systems were similar, with maxi- mum transmissions around 1.3 MeV. The Mini-Orange spectrometers were equipped with 300 mm 2 Si(Li) detectors: two with a thickness of 3 mm and two with a thickness of 5 mm. With the KVI data acquisition system (VAX 750) the twofold coin- cidences were registered on magnetic tape for off-line analysis. Fig. 2 (top) displays the essential result: the sum of the (e+e-) pair spectra from the four neighbouring detector combinations in which we can look for a pos- sible axion signal in competition with expected IPC peaks. The (e÷e - )  spectrum is compared with a simul- taneously recorded spectrum of gamma rays (bottom) using a Ge detector. It is apparent that the internal pair peaks of the two isoscalar transitions at 2.87 MeV and 3.59 MeV in 10B are only weakly visible in the (e+e -)  spectrum. Interestingly, two rather prominent peaks at 3.35 and 6.05 MeV show up in the sum spectrum. They have no counterpart in the 3'-ray spectrum and belong to to- tally converted E0 transitions in 40Ca and 160 respec- tively, which are to our knowledge the only two exist- ing E0 transitions in nature depopulating first excited states between 3 and 7 MeV. The spectra of scattered protons, recorded separately, indeed showed a signifi- cant content of 160 (and 12C) impurity in the target. A tracer impurity causes in the pair spectra the ap- pearance of the E0 transition in 40Ca. The 6.05 MeV transition in 160 occurs just above energy threshold and also in the extreme upper transmission range of the Mini-Oranges. Furthermore, it is observed despite the fact that the two 3 mm Si(Li) detectors had insuf- ficient stopping power for most of the e + and e -  pulses. Evidently, the arrangement is hypersensitive for detection of totally converted E0 transitions. The background of the pair spectrum is complex, containing a continuum distribution following 10B(p, a) distributions from pairs created inside the vacuum walls, e + and e -  particles from e.g. the E0(160) transi- tions which pass through the detectors. Yet, the main part of the spectrum belongs to IPC coincidences and not to e.g. Compton scattered 3' rays since the yield of e + + e + and e-  + e -  type combinations is less by an or- der of magnitude than the e + + e -  pairs. In the 3'-ray spectrum all transitions of sufficiently short half-life are Doppler broadened and/or shifted. The 10B 2.87 and 3.59 MeV 3' rays show up strongly. The spectrum also shows a number of 27A1 3" rays produced in the walls of the Mini-Orange vacuum chamber, together with the 4.43 MeV E2 transition in 12C and the 6.13 MeV E3 transition in 160 (target impurities), visible by photo peaks, single- and double-escape peaks. Efficiency calibration for IPC detection was ob- tained as follows: at the low energy side the system was calibrated (i) internally at 1.85 MeV with the 2.87 MeV transition in 10B (> 98% E2) and (ii) externally at 1.733 MeV using the 2.755 MeV E2 transition in 24Na with theoretical [18] IPC's of 7.4 × 10 -4 re- spectively 7.0 X 10 -4. (iii) A comparison of the inten- sity of inelastically scattered protons from the 4.43 MeV level in 12C with that from the 3.59 MeV level in 10B, the ratio being 0.6, showed us that only a frac- tion of the corresponding 3'-ray intensity in 12C is produced in the target and apparently most of it orig- inates in the walls of the chamber. The ratio of 3'-ray intensities was determined to be 0.5 using experimen- tal data for differential and total cross sections for 10B (p,p ')  [20] and 12C (p, p') [21,22] inelastic scat- tering at 7 MeV under 135 °. The obtained gamma-ray intensity was then used together with the internal pair intensity and the theoretical [18] IPC of 1.3 × 10 -3 for an E2 transition of 4.43 MeV to internally determine the efficiency of the Mini-Orange system at 3.41 MeV. All three efficiency calibrations are equal to a value of e = 2.3 X 10 -4 within 35%. The isoscalar 3.59 MeV transition has in the (e+e - )  pair spectrum roughly the same intensity as the paral- lel 2.87 MeV transition used for internal calibration. The branching ratio of the 3.59 MeV pair peak was determined to be (2.1 + 1.1) X 10 -3 which is consis- tent with internal pair creation, %r being 1.0 X 10 -3 for a 28% M1 and 72% E2 transition. The full strength of axion decay with the Donnelly value o f I  a = 0.39 1~ (M1) for an axion of 1.68 MeV would have caused a peak height of over 1000 counts or two orders of mag- nitude !arger than as observed. With 90% confidence our result corresponds with a boundary being 54 times smaller than the value predicted by the standard model [4]. In recent more sophisticated axion models [13,14] the conjectured axion couples to one or two Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 quark pairs IN = 1 or 2 in eq. (1)]. In the case of  only two light quarks, no coupling is expected to a magnet- ic isoscalar transition. In the case of coupling to two quark pairs our experiment leads with 90% confidence to a boundary factor of  31 with respect to the theoret- ical prediction in the case of  existing axions. Conse- quently our search yields no evidence for the existence of  a standard (N = 3) or a viable (N = 2) axion. After evaluation of  our experimental  data, we be- came aware of  IPC measurements in the early de- cades of  nuclear physics, when axions were not yet  advocated, but  when (e+e - )  angular correlations were used as a probe for distinguishing multipolarit ies of  high-energy "/-rays. We were both  surprised and im- pressed by the angular correlations reported for the 3.59 MeV transition in the clean reaction 9Be(d, n)10B by Gorodetsky et al. [23]. They used scintillation counters with inherently two orders of  magnitude less energy resolution than our Si(Li) detectors ( though this resolution is partly spoiled by  Doppler broaden- ing), and could hardly separate a strong 3.37 MeV transition in 10B from the 3.59 MeV isoscalar t ransi-  tion in 10B. Yet, their angular distr ibution shows no enhancement near 45 ° where the standard axion would have given at least one order of  magnitude larger signal than measured. Also, a recent reanalysis by Calaprice et al. [24] of  IPC data from Warburton et al. [25] shows no evidence for axion competi t ion in either isoscalar or isovector M1 transitions. The authors wish to thank H.P. Blok, P.F.A. Goudsmit ,  R.H. Siemssen and A.H. Wapstra for valuable discussions. This investigation was sup- ported by the Netherlands Organization for Pure Scientific Research (ZWO) through the Foundat ion for Fundamental  Research on Matter (FOM). References [1] S. Weinberg, Phys. Rev. Lett. 40 (1978) 223. [2] F. Wilczek, Phys. Rev. Lett. 40 (1978) 279. [3] R~D. Peccei and H.R. Quinn, Phys. Rev. Lett. 38 (1977) 1440. [4] T.W. DonneUy et al., Phys. Rev. D 18 (1978) 1607. [5] A. Zehnder, in: Fundamental interactions in low-energy systems (Plenum, New York, 1985). [6] T. Cowa net  al., Phys. Rev. Lett. 54 (1985) 1761. [7] T. Cowan et al., Phys. Rev. Lett. 56 (1986) 444. [8] F.P. Calaprice et al., Phys. Rev. D 20 (1979) 2708. 56 0986) 302. [9] A. Schgfer et al., J. Phys. G 11 (1985) 169. [10] A.B. Balantekin et al., Phys. Rev. Lett. 55 (1985) 461. [11] N. Mukhopadhyay and A. Zehnder, Phys. Rev. Lett. 56 (1986) 206. [12] A. Chodos and L.R.C. Wijewardhama, Phys. Rev. Lett. 56 (1986) 302. [13] R.D. Peccei, Tai Tsun Wu and T. Yanagida, Phys. Lett. B 172 (1986) 435. [14] L.M. Krauss and F. Wilczek, Phys. Lett. B 173 (1986) 189. [15] S.J. Brodsky et al., Phys. Rev. Lett. 56 (1986) 1763. [16] K. Abrahams et al., KVI Annual Report 1985. [17] E.K. Warburton et al., Phys. Rev. 17 (1968) 1178. [18] P. Schliiter and G. Soff, Atom. Data Nucl. Data Tables 24 (1979) 509. [19] J. van Klinken, S.J. Feenstra and G. Dumont, Nucl. In- strum. Methods 151 (1978) 433. [20] B.A. Watson et al., Phys. Rev. 187 (1969) 1351. [21] J.B. Swint et al., Nucl. Phys. 86 (1966) 119. [22] A.C.L. Barnar d et al., Nucl. Phys. 86 (1966) 130. [23] S. Gorodetsky, P. Chevallier, R. Armbruster and G. Sutter, Nucl. Phys. 12 (1959) 349. [24] F.P. Calapriee et al., private communication, and to be published. [25] E.K. Warburton et al., Phys. Rev. 132 (1963) 776,133 (1964) B42, 

University of Groningen Research Database

  
 University of Groningen
Search for short-lived axions in a nuclear isoscalar transition
de Boer, FWN; Abrahams, K; Balanda, A; Bokemeyer, H; van Dantzig, R; Jansen, JFW;
Kotlinski, B; de Voigt, MJA; van Klinken, J
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Physics Letters B
DOI:
10.1016/0370-2693(86)90123-1
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de Boer, FWN., Abrahams, K., Balanda, A., Bokemeyer, H., van Dantzig, R., Jansen, JFW., ... van Klinken,
J. (1986). Search for short-lived axions in a nuclear isoscalar transition. Physics Letters B, 180(1-2), 4-8.
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Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 
SEARCH FOR SHORT-L IVED AX IONS 
IN A NUCLEAR ISOSCALAR TRANSIT ION 
F.W.N.  DE BOER 1, K. ABRAHAMS 2, A. BALANDA 3,4, H. BOKEMEYER 5, 
R. VAN DANTZIG 6, J .F.W. JANSEN,  B. KOTL INSK I ,  M.J.A. DE VOIGT and 
J. VAN KL INKEN 
Kernfysisch Versneller lnstituut, Universiteit van Groningen, NL-974 7 AA Groningen, The Netherlands 
Received 22 July 1986; revised manuscript received 10 September 1986 
Axions, if existing, can cause a strong signal of positron-electron pairs in isoscalar M1 transitions in competition with 
gamma-ray emission. We searched for such a signal in the 3.59 MeV transition i  I°B with a fourfold Mini-Orange spectro- 
meter. No axion events were found within two percent of the prediction for the standard axion- to ~,-ray branching ratio. 
A light pseudoscalar Goldstone boson, the axion, 
has been introduced by Weinberg [1] and Wilczek [2], 
as a consequence of  the global U(1) symmetry pro- 
posed by Peccei and Quinn [3] to explain the ab- 
sence of large P and CP violations in strong interac- 
tions. In a treatise of possible axion search experi- 
ments, Donnelly et al. [4] discussed the probability 
for nuclear de-excitation by axion emission in compe- 
tition with magnetic ~/radiation. Subsequent axion 
searches provided such stringent upper bounds on 
mass and lifetime that the nonexistence of axions was 
anticipated [5]. However, the interest in an observable 
axion was revived recently [6] after the puzzling ob- 
servation in superheavy collisional systems at GSI of  a 
sharp positron peak at approximately 330 keV, which 
was later [7] found to be in coincidence with a simi- 
lar electron peak. Could the observed (e+e - )  pairs 
t Present address: NIKHEF-K, P.O. Box 4395, NL-1009AJ 
Amsterdam, The Netherlands. 
2 Energieonderzoek Centrum Nederland (ECN), Petten, The 
Netherlands. 
3 Physics Laboratory of the Free University, NL-1081 HV 
Amsterdam, The Netherlands. 
4 Jagiellonian University, PL-30059 Cracow 12, Poland. 
5 Gesellschaft for Schwerionenforschung (GSI), Postfach 
110541, D-6100 Darmstadt 11, Fed. Rep. Germany. 
6 NIKHEF-K, P.O. Box 4395, NL-1009 AJ Amsterdam, The 
Netherlands. 
arise from a "visible" axion with a mass of  about 1.68 
MeV and a lifetime of~< 10 -11 s? 
The axion mass and lifetime can be related to one 
free parameter X, the ratio of  vacuum expectation 
values for two Higgs field componentsfs in X and 
fcos  X with f=  (GFV~)- I /2  = 250 GeV (G F being the 
Fermi coupling constant): 
m a = 25N(X + l /X) (keV), (1) 
where N is the number of active quark pairs (N = 3 in 
ref. [4]). For m a > 2m e the axion will predominantly 
decay in a positron-electron pair with lifetime 
r(a -~ e+e - )  = 8"trf2X2me2(m 2 - 4me2) -1/2 (s). (2) 
Searches for axions decaying into two photons 
with radioactive sources, reactors and beam dumps 
constrain [5] the mass to < 1 MeV and the lifetime 
to < 10 -6  s, whereas a search [8] for decay into posi- 
t ron-electron pairs limits the lifetime to < 2 X 10 -10 
s. Searches with pseudoscalar- and vector mesons 
(K +, J/~k and Y) were sensitive to lifetimes of 10 -11 s 
due to time dilation of the order of 104 . The anom- 
alous magnetic moment of  the electron constrains 
the lifetime to > 5 X 10 -13 s [9 -12] .  In summary, a
rather narrow window of lifetimes between 10 -13 
and 10 -10 s cannot be excluded on the basis of exist- 
ing experimental evidence. If the observed positron 
peak is due to an axion with mass 1.68 MeV, its X 
0370-2693/86/$ 03.50 © Elsevier Science Publishers B.V. 
(North-Holland Physics Publishing Division) 
Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 
value according to eq. (1) is either X = 0.045 or X = 
22 leading to a lifetime of about 5 × 10 -12 s (al- 
lowed) or 2 × 10 -6  s (excluded). Either of these two 
X values would lead to contradictions for branching 
ratios of meson decays in the standard axion model. 
Quite recently, however, Peccei et al. [13] have suc- 
ceeded in reconciling the previous unsuccessful searches 
with an axion of 1.68 MeV in a 'Wiable axion model" 
which suppresses the coupling to the heavier quarks. 
Almost simultaneously, Krauss and Wilczek [14] pre- 
sented a "short-lived axion variant" coupled preferen- 
tially to light fermions and stressed with Brodsky et al. 
[15] the need of new experiments. 
In this letter we report on a search for axions in the 
unexplored region with lifetimes T < 10 -9 s. We 
looked for the e+e - decay of such axions in competi- 
tion with internal pair creation (IPC) of a magnetic 
q-ray transition. Specifically for isoscalar transitions, 
the branching ratio for (e+e - )  pair creation from a 
pseudoscalar decay particle and M1 gamma-decay can 
be as large [4,16] as 50%. This is three orders of  mag- 
nitude larger than IPC. As isoscalar transition the 3.59 
MeV M1 + E2 transition in 10B was chosen. 
The 2 +, 0 state at 3.59 MeV in 10B decays to the 
3 +, 0 groundstate with a 19% branching ratio. The 
transition has a mixing ratio [17] 5(E2/M1) of 
1.5 + 0.6 while the parallel 2 +, 0 ~ 1 +, 0 transition of 
2.87 MeV is [17] predominantly E2 with 5-1 < 0.2. 
Neglecting internal conversion by atomic electrons, 
two-photon emission and other higher-order processes, 
the IPC contributes at the level of 10 -3 compared 
with gamma rays in the de,excitation of the 3.59 MeV 
state. Axion emission would imply a source of (e+e - )  
pairs in addition to the expected (e+e - )  pairs from 
IPC. The energy and angular correlations of (e+e-) 
pairs from the 3.59 MeV transition are ftxed kinemati- 
cally. In our experimental rrangement the M I and 
E2 IPC is detected with total efficiency e of typically 
10 -4  (see below). The IPC angular correlation be- 
tween the e ÷ and e -  is maximum at relative angle 
0 = 0 °, while in case of the axions the maximum will 
occur around 0 = 45 °. The angle between the axes of  
neighbouring Mini-Oranges i  60 ° (fig. 1), which 
causes an axion decay to be seen with somewhat 
higher efficiency than IPC; a numerical estimate gave 
e A ~ 1.5e, assuming that the axions are emitted iso- 
tropically. The nuclear axion-decay rate (in the case of 
existing axions) is predicted in ref. [4] to be as large 
A e . . /~det .  
ta rget  _ 
- ~45v~ .beam 
~Ge detector 
Fig. 1. Fourfold Mini-Orange arrangement for the axion search. To the left a view of the horizontal plane and to the right a view 
upstream along the beam axis. The hypothetical xi0n ~ e + + e- decay is supposed to occur inside or closely (at gm distance) be- 
hind the target layer. In the left part of the figure an e- particle is indicated, while the corresponding e +particle will be detected 
by a neighbouring e + detector (out of plane). 
Volume 180, number 1,2 PHYSICS LETTERS B 6 November  1986 
as I a = 0.39 I3,(M 1). The number of observed pairs 
can now be written as: 
N(e+e-)/e = {0.39 X 1.5 + ~(M1) )  Lr(M1 ) 
+ otTr (E2)[~ (E2), (3) 
where e is the efficiency. Theoretical IPC coefficients 
ot  can be obtained from ref. [18]. If  axions indeed 
exist then the first term (= 0.585) will dominate since 
the others are of order 10 -3  . 
In the experiment the 3.59 MeV state in 10B was 
excited in inelastic scattering of 7 MeV protons by a 
10B target of 1 mg/cm 2. Using a fourfold Mini-Orange 
system [19] with two detector systems for e + and 
two for e -  detection, we registered prompt (e+e - )  
signals and their energy sums. The four spectrometers 
were set at a backward angle of 45 ° with respect o 
the incident proton beam. This is shown by a view in 
the horizontal plane in fig. 1 (left). The axes of oppo- 
site Mini-Oranges (with the same polarity) made an 
angle of 90 ° with each other, those of neighbouring 
ones (with alternating polarity) one of 60 ° . For clarity 
fig. 1 (right) shows four equal Mini-Orange systems, 
each composed of four magnets and the e ÷ and e -  sys- 
I I I I 1 
l °B (p ,p ' )  Ep =7MeV 
e ÷ ..- e- SPECTRUM 3.35 MeV 
120" 4OCa - 
3;059 MeV 4.43 MeV 
2.87 MeV I] B 12 C 
80 - lo B I~ I l r' . I 6.05 MeV 
40-- ~ / ~ - ,  { u "LP 
~ 0 //, I I I I - _ , ,  
H 2 3 4 5 6 MeV 
Z moO2 :::) Ee. ÷Ee.÷ 2 0 U 
106-- 8,000f ~lr GAMMA-  RAY SPECTRUM- 
~o 5 6,OOOl- ~A 
~ ~ . . ~  l t 'g'- ~'J l 8 13 MeV 
16 0 
10 3 I I i 
102 -- I I I 
.-¢ I I I I I 10 2 3 4 5 6 MeV 
Ey 
Fig. 2. Sum of four e + + e-  pair spectra (top) compared wi th  a s imultaneously  recorded ~/-ray spectrum (bot tom)  using a Ge de- 
tector,  following the l°B (p, p') reaction at 7 MeV. In case of an existing axion with mass of 1.68 MeV and lifetime 5 X 10 -12 s, 
the prediction by DonneUy et al. [4] corresponds toa peak at 3.59 MeV with height of over 1000 counts. 
Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 
tems being only different by inversion of their mag- 
netic fields. In reality the present experiment was lim- 
ited by the available magnets so that two four-gap and 
two five-gap systems had to be used. Since the latter 
magnets were relatively weaker, the overall features 
of the four magnet systems were similar, with maxi- 
mum transmissions around 1.3 MeV. 
The Mini-Orange spectrometers were equipped with 
300 mm 2 Si(Li) detectors: two with a thickness of 3 
mm and two with a thickness of 5 mm. With the KVI 
data acquisition system (VAX 750) the twofold coin- 
cidences were registered on magnetic tape for off-line 
analysis. 
Fig. 2 (top) displays the essential result: the sum 
of the (e+e-) pair spectra from the four neighbouring 
detector combinations in which we can look for a pos- 
sible axion signal in competition with expected IPC 
peaks. The (e÷e -) spectrum is compared with a simul- 
taneously recorded spectrum of gamma rays (bottom) 
using a Ge detector. It is apparent that the internal 
pair peaks of the two isoscalar transitions at 2.87 MeV 
and 3.59 MeV in 10B are only weakly visible in the 
(e+e -) spectrum. 
Interestingly, two rather prominent peaks at 3.35 
and 6.05 MeV show up in the sum spectrum. They have 
no counterpart in the 3'-ray spectrum and belong to to- 
tally converted E0 transitions in 40Ca nd 160 respec- 
tively, which are to our knowledge the only two exist- 
ing E0 transitions in nature depopulating first excited 
states between 3 and 7 MeV. The spectra of scattered 
protons, recorded separately, indeed showed a signifi- 
cant content of 160 (and 12C) impurity in the target. 
A tracer impurity causes in the pair spectra the ap- 
pearance of the E0 transition in 40Ca. The 6.05 MeV 
transition in 160 occurs just above energy threshold 
and also in the extreme upper transmission range of 
the Mini-Oranges. Furthermore, it is observed espite 
the fact that the two 3 mm Si(Li) detectors had insuf- 
ficient stopping power for most of the e + and e- 
pulses. Evidently, the arrangement is hypersensitive 
for detection of totally converted E0 transitions. 
The background of the pair spectrum iscomplex, 
containing a continuum distribution following 10B(p, 
a) distributions from pairs created inside the vacuum 
walls, e + and e- particles from e.g. the E0(160) transi- 
tions which pass through the detectors. Yet, the main 
part of the spectrum belongs to IPC coincidences and 
not to e.g. Compton scattered 3'rays since the yield of 
e + + e + and e- + e- type combinations is less by an or- 
der of magnitude than the e + + e- pairs. In the 3'-ray 
spectrum all transitions of sufficiently short half-life 
are Doppler broadened and/or shifted. The 10B 2.87 
and 3.59 MeV 3' rays show up strongly. The spectrum 
also shows a number of 27A1 3" rays produced in the 
walls of the Mini-Orange vacuum chamber, together 
with the 4.43 MeV E2 transition in 12C and the 6.13 
MeV E3 transition in 160 (target impurities), visible 
by photo peaks, single- and double-escape aks. 
Efficiency calibration for IPC detection was ob- 
tained as follows: at the low energy side the system 
was calibrated (i) internally at 1.85 MeV with the 2.87 
MeV transition in 10B (> 98% E2) and (ii) externally 
at 1.733 MeV using the 2.755 MeV E2 transition in 
24Na with theoretical [18] IPC's of 7.4 × 10 -4 re- 
spectively 7.0 X 10 -4. (iii) A comparison of the inten- 
sity of inelastically scattered protons from the 4.43 
MeV level in 12C with that from the 3.59 MeV level 
in 10B, the ratio being 0.6, showed us that only a frac- 
tion of the corresponding 3'-ray intensity in 12C is 
produced in the target and apparently most of it orig- 
inates in the walls of the chamber. The ratio of 3'-ray 
intensities was determined to be 0.5 using experimen- 
tal data for differential and total cross sections for 
10B (p,p') [20] and 12C (p, p') [21,22] inelastic scat- 
tering at 7 MeV under 135 °. The obtained gamma-ray 
intensity was then used together with the internal 
pair intensity and the theoretical [18] IPC of 1.3 × 
10 -3 for an E2 transition of 4.43 MeV to internally 
determine the efficiency of the Mini-Orange system 
at 3.41 MeV. All three efficiency calibrations are equal 
to a value of e = 2.3 X 10 -4 within 35%. 
The isoscalar 3.59 MeV transition has in the (e+e -) 
pair spectrum roughly the same intensity as the paral- 
lel 2.87 MeV transition used for internal calibration. 
The branching ratio of the 3.59 MeV pair peak was 
determined to be (2.1 + 1.1) X 10 -3 which is consis- 
tent with internal pair creation, %r being 1.0 X 10 -3 
for a 28% M1 and 72% E2 transition. The full strength 
of axion decay with the Donnelly value o f I  a = 0.39 1~ 
(M1) for an axion of 1.68 MeV would have caused a
peak height of over 1000 counts or two orders of mag- 
nitude !arger than as observed. With 90% confidence 
our result corresponds with a boundary being 54 
times smaller than the value predicted by the standard 
model [4]. In recent more sophisticated axion models 
[13,14] the conjectured axion couples to one or two 
Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 
quark pairs IN = 1 or 2 in eq. (1)]. In the case of only 
two light quarks, no coupling is expected to a magnet- 
ic isoscalar transition. In the case of coupling to two 
quark pairs our experiment leads with 90% confidence 
to a boundary factor of 31 with respect o the theoret- 
ical prediction in the case of existing axions. Conse- 
quently our search yields no evidence for the existence 
of a standard (N = 3) or a viable (N = 2) axion. 
After evaluation of our experimental data, we be- 
came aware of IPC measurements in the early de- 
cades of nuclear physics, when axions were not yet 
advocated, but when (e+e - )  angular correlations were 
used as a probe for distinguishing multipolarities of
high-energy "/-rays. We were both surprised and im- 
pressed by the angular correlations reported for the 
3.59 MeV transition in the clean reaction 9Be(d, n)10B 
by Gorodetsky et al. [23]. They used scintillation 
counters with inherently two orders of magnitude less 
energy resolution than our Si(Li) detectors (though 
this resolution is partly spoiled by Doppler broaden- 
ing), and could hardly separate a strong 3.37 MeV 
transition in 10B from the 3.59 MeV isoscalar transi- 
tion in 10B. Yet, their angular distribution shows no 
enhancement ear 45 ° where the standard axion 
would have given at least one order of magnitude 
larger signal than measured. Also, a recent reanalysis 
by Calaprice t al. [24] of IPC data from Warburton 
et al. [25] shows no evidence for axion competition 
in either isoscalar or isovector M1 transitions. 
The authors wish to thank H.P. Blok, 
P.F.A. Goudsmit, R.H. Siemssen and A.H. Wapstra 
for valuable discussions. This investigation was sup- 
ported by the Netherlands Organization for Pure 
Scientific Research (ZWO) through the Foundation 
for Fundamental Research on Matter (FOM). 
References 
[1] S. Weinberg, Phys. Rev. Lett. 40 (1978) 223. 
[2] F. Wilczek, Phys. Rev. Lett. 40 (1978) 279. 
[3] R~D. Peccei and H.R. Quinn, Phys. Rev. Lett. 38 (1977) 
1440. 
[4] T.W. DonneUy et al., Phys. Rev. D 18 (1978) 1607. 
[5] A. Zehnder, in: Fundamental interactions in low-energy 
systems (Plenum, New York, 1985). 
[6] T. Cowa net al., Phys. Rev. Lett. 54 (1985) 1761. 
[7] T. Cowan et al., Phys. Rev. Lett. 56 (1986) 444. 
[8] F.P. Calaprice t al., Phys. Rev. D 20 (1979) 2708. 
56 0986) 302. 
[9] A. Schgfer et al., J. Phys. G 11 (1985) 169. 
[10] A.B. Balantekin et al., Phys. Rev. Lett. 55 (1985) 461. 
[11] N. Mukhopadhyay nd A. Zehnder, Phys. Rev. Lett. 
56 (1986) 206. 
[12] A. Chodos and L.R.C. Wijewardhama, Phys. Rev. Lett. 
56 (1986) 302. 
[13] R.D. Peccei, Tai Tsun Wu and T. Yanagida, Phys. Lett. 
B 172 (1986) 435. 
[14] L.M. Krauss and F. Wilczek, Phys. Lett. B 173 (1986) 
189. 
[15] S.J. Brodsky et al., Phys. Rev. Lett. 56 (1986) 1763. 
[16] K. Abrahams et al., KVI Annual Report 1985. 
[17] E.K. Warburton et al., Phys. Rev. 17 (1968) 1178. 
[18] P. Schliiter and G. Soff, Atom. Data Nucl. Data Tables 
24 (1979) 509. 
[19] J. van Klinken, S.J. Feenstra nd G. Dumont, Nucl. In- 
strum. Methods 151 (1978) 433. 
[20] B.A. Watson et al., Phys. Rev. 187 (1969) 1351. 
[21] J.B. Swint et al., Nucl. Phys. 86 (1966) 119. 
[22] A.C.L. Barnar d et al., Nucl. Phys. 86 (1966) 130. 
[23] S. Gorodetsky, P. Chevallier, R. Armbruster and 
G. Sutter, Nucl. Phys. 12 (1959) 349. 
[24] F.P. Calapriee t al., private communication, and to be 
published. 
[25] E.K. Warburton et al., Phys. Rev. 132 (1963) 776,133 
(1964) B42, 


Dissertations of the University of Groningen

   University of GroningenSearch for short-lived axions in a nuclear isoscalar transitionde Boer, FWN; Abrahams, K; Balanda, A; Bokemeyer, H; van Dantzig, R; Jansen, JFW;Kotlinski, B; de Voigt, MJA; van Klinken, JPublished in:Physics Letters BDOI:10.1016/0370-2693(86)90123-1IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1986Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):de Boer, FWN., Abrahams, K., Balanda, A., Bokemeyer, H., van Dantzig, R., Jansen, JFW., Kotlinski, B.,de Voigt, MJA., & van Klinken, J. (1986). Search for short-lived axions in a nuclear isoscalar transition.Physics Letters B, 180(1-2), 4-8. https://doi.org/10.1016/0370-2693(86)90123-1CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 04-06-2022Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 S E A R C H  F O R  S H O R T - L I V E D  A X I O N S  IN  A N U C L E A R  I S O S C A L A R  T R A N S I T I O N  F .W.N.  D E  BOER 1, K. A B R A H A M S  2, A. B A L A N D A  3,4, H. B O K E M E Y E R  5, R. V A N  D A N T Z I G  6, J .F .W.  J A N S E N ,  B. K O T L I N S K I ,  M.J.A. D E  V O I G T  and  J. V A N  K L I N K E N  Kernfysisch Versneller lnstituut, Universiteit van Groningen, NL-974 7 AA Groningen, The Netherlands Received 22 July 1986; revised manuscript received 10 September 1986 Axions, if existing, can cause a strong signal of positron-electron pairs in isoscalar M1 transitions in competition with gamma-ray emission. We searched for such a signal in the 3.59 MeV transition in I°B with a fourfold Mini-Orange spectro- meter. No axion events were found within two percent of the prediction for the standard axion- to ~,-ray branching ratio. A light pseudoscalar Goldstone boson, the axion, has been introduced by Weinberg [1] and Wilczek [2], as a consequence o f  the global U(1) symmetry pro- posed by Peccei and Quinn [3] to explain the ab- sence of  large P and CP violations in strong interac- tions. In a treatise of  possible axion search experi- ments, Donnelly et al. [4] discussed the probabil i ty for nuclear de-excitation by  axion emission in compe- t i t ion with magnetic ~/radiation. Subsequent axion searches provided such stringent upper bounds on mass and lifetime that  the nonexistence of  axions was anticipated [5]. However, the interest in an observable axion was revived recently [6] after the puzzling ob- servation in superheavy collisional systems at GSI o f  a sharp positron peak at approximately 330 keV, which was later [7] found to be in coincidence with a simi- lar electron peak. Could the observed (e+e - )  pairs t Present address: NIKHEF-K, P.O. Box 4395, NL-1009AJ Amsterdam, The Netherlands. 2 Energieonderzoek Centrum Nederland (ECN), Petten, The Netherlands. 3 Physics Laboratory of the Free University, NL-1081 HV Amsterdam, The Netherlands. 4 Jagiellonian University, PL-30059 Cracow 12, Poland. 5 Gesellschaft for Schwerionenforschung (GSI), Postfach 110541, D-6100 Darmstadt 11, Fed. Rep. Germany. 6 NIKHEF-K, P.O. Box 4395, NL-1009 AJ Amsterdam, The Netherlands. arise from a "visible" axion with a mass o f  about 1.68 MeV and a lifetime of~< 10 -11 s? The axion mass and lifetime can be related to one free parameter X, the ratio o f  vacuum expectat ion values for two Higgs field componen t s f s in  X and f c o s  X with f =  (G FV ~) - I / 2  = 250 GeV (G F being the Fermi coupling constant): m a = 25N(X + l /X)  (keV), (1) where N is the number of  active quark pairs (N = 3 in ref. [4]). For  m a > 2m e the axion will predominantly decay in a pos i t ron-e lec t ron  pair with lifetime r(a -~ e+e - )  = 8"tr f2X2me2(m 2 - 4me2) -1/2 (s). (2) Searches for axions decaying into two photons with radioactive sources, reactors and beam dumps constrain [5] the mass to < 1 MeV and the lifetime to < 10 - 6  s, whereas a search [8] for decay into posi- t ron -e l ec t ron  pairs limits the lifetime to < 2 X 10 -10 s. Searches with pseudoscalar- and vector mesons (K +, J/~k and Y) were sensitive to lifetimes of  10 -11 s due to time dilation of  the order of  104 . The anom- alous magnetic moment  o f  the electron constrains the lifetime to > 5 X 10 -13 s [ 9 - 1 2 ] .  In summary, a rather narrow window of  lifetimes between 10 -13 and 10 -10 s cannot be excluded on the basis of  exist- ing experimental evidence. If  the observed positron peak is due to an axion with mass 1.68 MeV, its X 0370-2693/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 value according to eq. (1) is either X = 0.045 or X = 22 leading to a lifetime of  about 5 × 10 -12 s (al- lowed) or 2 × 10 - 6  s (excluded).  Either of  these two X values would lead to contradictions for branching ratios of  meson decays in the standard axion model. Quite recently, however, Peccei et al. [13] have suc- ceeded in reconciling the previous unsuccessful searches with an axion of  1.68 MeV in a 'Wiable axion model"  which suppresses the coupling to the heavier quarks. Almost simultaneously, Krauss and Wilczek [14] pre- sented a "short-lived axion variant" coupled preferen- tially to light fermions and stressed with Brodsky et al. [15] the need of  new experiments.  In this letter we report  on a search for axions in the unexplored region with lifetimes T < 10 - 9  s. We looked for the e+e - decay of  such axions in competi- t ion with internal pair creation (IPC) of  a magnetic q-ray transition. Specifically for isoscalar transitions, the branching ratio for (e+e - )  pair creation from a pseudoscalar decay particle and M1 gamma-decay can be as large [4,16] as 50%. This is three orders o f  mag- nitude larger than IPC. As isoscalar transition the 3.59 MeV M1 + E2 transition in 10B was chosen. The 2 +, 0 state at 3.59 MeV in 10B decays to the 3 +, 0 groundstate with a 19% branching ratio. The transition has a mixing ratio [17] 5(E2/M1) of  1.5 + 0.6 while the parallel 2 +, 0 ~ 1 +, 0 transition of  2.87 MeV is [17] predominantly E2 with 5 - 1  < 0.2. Neglecting internal conversion by atomic electrons, two-photon emission and other higher-order processes, the IPC contributes at the level of  10 -3  compared with gamma rays in the de,excitation of  the 3.59 MeV state. Axion emission would imply a source of  (e+e - )  pairs in addit ion to the expected (e+e - )  pairs from IPC. The energy and angular correlations of  (e+e-)  pairs from the 3.59 MeV transition are ftxed kinemati- cally. In our experimental arrangement the M I and E2 IPC is detected with total  efficiency e of  typically 10 - 4  (see below). The IPC angular correlation be- tween the e ÷ and e -  is maximum at relative angle 0 = 0 °, while in case of  the axions the maximum will occur around 0 = 45 °. The angle between the axes o f  neighbouring Mini-Oranges is 60 ° (fig. 1), which causes an axion decay to be seen with somewhat higher efficiency than IPC; a numerical estimate gave e A ~ 1.5e, assuming that the axions are emitted iso- tropically. The nuclear axion-decay rate (in the case of  existing axions) is predicted in ref. [4] to be as large A e . . / ~ d e t .  target _ - ~ 4 5 v ~  .beam ~Ge detector Fig. 1. Fourfold Mini-Orange arrangement for the axion search. To the left a view of the horizontal plane and to the right a view upstream along the beam axis. The hypothetical axi0n ~ e + + e- decay is supposed to occur inside or closely (at gm distance) be- hind the target layer. In the left part of the figure an e- particle is indicated, while the corresponding e + particle will be detected by a neighbouring e + detector (out of plane). Volume 180, number  1,2 PHYSICS LETTERS B 6 N o v e m b e r  1986 as I a = 0.39 I3,(M 1). The number of  observed pairs can now be writ ten as: N(e+e-)/e = {0.39 X 1.5 + ~ ( M 1 ) )  Lr(M1 ) + otTr (E2)[~ (E2), (3) where e is the efficiency. Theoretical IPC coefficients o t  can be obtained from ref. [18]. I f  axions indeed exist then the first term (= 0.585) will dominate since the others are of  order 10 - 3  . In the experiment the 3.59 MeV state in 10B was excited in inelastic scattering of  7 MeV protons by a 10B target of  1 mg/cm 2. Using a fourfold Mini-Orange system [19] with two detector  systems for e + and two for e -  detection, we registered prompt  (e+e - )  signals and their energy sums. The four spectrometers were set at a backward angle of  45 ° with respect to the incident proton beam. This is shown by a view in the horizontal plane in fig. 1 (left). The axes of  oppo- site Mini-Oranges (with the same polari ty) made an angle of  90 ° with each other,  those of  neighbouring ones (with alternating polari ty)  one of  60 ° . For clarity fig. 1 (right) shows four equal Mini-Orange systems, each composed of  four magnets and the e ÷ and e -  sys- I I I I 1 l ° B ( p , p ' )  Ep = 7 M e V  e ÷ ..- e- SPECTRUM 3.35 MeV 120" 4OCa - 3;059 MeV 4.43 MeV 2.87 MeV I] B 12 C 80 - lo B I~ I l r '  . I 6.05 MeV 40-- ~ / ~ - ,  { u "LP ~ 0 //, I I I I - _ , ,  H 2 3 4 5 6 MeV Z moO2 :::) Ee. ÷Ee.÷ 2 0 U 106-- 8,000f ~lr G A M M A -  RAY SPECTRUM- ~o 5 6,OOOl- ~ A  ~ ~ . . ~  l t 'g'- ~'J l 8 13 MeV 16 0 10 3 I I i 102 -- I I I .-¢ I I I I I 10 2 3 4 5 6 MeV Ey Fig. 2. Sum of  four  e + + e -  pa i r  spec t ra  ( top)  c o m p a r e d  w i t h  a s imu l t aneous ly  r eco rded  ~/-ray s p e c t r u m  ( b o t t o m )  us ing a Ge de-  t ec to r ,  following the l°B (p, p') reaction at 7 MeV. In case of an existing axion with mass of 1.68 MeV and lifetime 5 X 10 -12 s, the prediction by DonneUy et al. [4] corresponds to a peak at 3.59 MeV with height of over 1000 counts. Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 tems being only different by inversion of their mag- netic fields. In reality the present experiment was lim- ited by the available magnets so that two four-gap and two five-gap systems had to be used. Since the latter magnets were relatively weaker, the overall features of the four magnet systems were similar, with maxi- mum transmissions around 1.3 MeV. The Mini-Orange spectrometers were equipped with 300 mm 2 Si(Li) detectors: two with a thickness of 3 mm and two with a thickness of 5 mm. With the KVI data acquisition system (VAX 750) the twofold coin- cidences were registered on magnetic tape for off-line analysis. Fig. 2 (top) displays the essential result: the sum of the (e+e-) pair spectra from the four neighbouring detector combinations in which we can look for a pos- sible axion signal in competition with expected IPC peaks. The (e÷e - )  spectrum is compared with a simul- taneously recorded spectrum of gamma rays (bottom) using a Ge detector. It is apparent that the internal pair peaks of the two isoscalar transitions at 2.87 MeV and 3.59 MeV in 10B are only weakly visible in the (e+e -)  spectrum. Interestingly, two rather prominent peaks at 3.35 and 6.05 MeV show up in the sum spectrum. They have no counterpart in the 3'-ray spectrum and belong to to- tally converted E0 transitions in 40Ca and 160 respec- tively, which are to our knowledge the only two exist- ing E0 transitions in nature depopulating first excited states between 3 and 7 MeV. The spectra of scattered protons, recorded separately, indeed showed a signifi- cant content of 160 (and 12C) impurity in the target. A tracer impurity causes in the pair spectra the ap- pearance of the E0 transition in 40Ca. The 6.05 MeV transition in 160 occurs just above energy threshold and also in the extreme upper transmission range of the Mini-Oranges. Furthermore, it is observed despite the fact that the two 3 mm Si(Li) detectors had insuf- ficient stopping power for most of the e + and e -  pulses. Evidently, the arrangement is hypersensitive for detection of totally converted E0 transitions. The background of the pair spectrum is complex, containing a continuum distribution following 10B(p, a) distributions from pairs created inside the vacuum walls, e + and e -  particles from e.g. the E0(160) transi- tions which pass through the detectors. Yet, the main part of the spectrum belongs to IPC coincidences and not to e.g. Compton scattered 3' rays since the yield of e + + e + and e-  + e -  type combinations is less by an or- der of magnitude than the e + + e -  pairs. In the 3'-ray spectrum all transitions of sufficiently short half-life are Doppler broadened and/or shifted. The 10B 2.87 and 3.59 MeV 3' rays show up strongly. The spectrum also shows a number of 27A1 3" rays produced in the walls of the Mini-Orange vacuum chamber, together with the 4.43 MeV E2 transition in 12C and the 6.13 MeV E3 transition in 160 (target impurities), visible by photo peaks, single- and double-escape peaks. Efficiency calibration for IPC detection was ob- tained as follows: at the low energy side the system was calibrated (i) internally at 1.85 MeV with the 2.87 MeV transition in 10B (> 98% E2) and (ii) externally at 1.733 MeV using the 2.755 MeV E2 transition in 24Na with theoretical [18] IPC's of 7.4 × 10 -4 re- spectively 7.0 X 10 -4. (iii) A comparison of the inten- sity of inelastically scattered protons from the 4.43 MeV level in 12C with that from the 3.59 MeV level in 10B, the ratio being 0.6, showed us that only a frac- tion of the corresponding 3'-ray intensity in 12C is produced in the target and apparently most of it orig- inates in the walls of the chamber. The ratio of 3'-ray intensities was determined to be 0.5 using experimen- tal data for differential and total cross sections for 10B (p,p ')  [20] and 12C (p, p') [21,22] inelastic scat- tering at 7 MeV under 135 °. The obtained gamma-ray intensity was then used together with the internal pair intensity and the theoretical [18] IPC of 1.3 × 10 -3 for an E2 transition of 4.43 MeV to internally determine the efficiency of the Mini-Orange system at 3.41 MeV. All three efficiency calibrations are equal to a value of e = 2.3 X 10 -4 within 35%. The isoscalar 3.59 MeV transition has in the (e+e - )  pair spectrum roughly the same intensity as the paral- lel 2.87 MeV transition used for internal calibration. The branching ratio of the 3.59 MeV pair peak was determined to be (2.1 + 1.1) X 10 -3 which is consis- tent with internal pair creation, %r being 1.0 X 10 -3 for a 28% M1 and 72% E2 transition. The full strength of axion decay with the Donnelly value o f I  a = 0.39 1~ (M1) for an axion of 1.68 MeV would have caused a peak height of over 1000 counts or two orders of mag- nitude !arger than as observed. With 90% confidence our result corresponds with a boundary being 54 times smaller than the value predicted by the standard model [4]. In recent more sophisticated axion models [13,14] the conjectured axion couples to one or two Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 quark pairs IN = 1 or 2 in eq. (1)]. In the case of  only two light quarks, no coupling is expected to a magnet- ic isoscalar transition. In the case of coupling to two quark pairs our experiment leads with 90% confidence to a boundary factor of  31 with respect to the theoret- ical prediction in the case of  existing axions. Conse- quently our search yields no evidence for the existence of  a standard (N = 3) or a viable (N = 2) axion. After evaluation of  our experimental  data, we be- came aware of  IPC measurements in the early de- cades of  nuclear physics, when axions were not yet  advocated, but  when (e+e - )  angular correlations were used as a probe for distinguishing multipolarit ies of  high-energy "/-rays. We were both  surprised and im- pressed by the angular correlations reported for the 3.59 MeV transition in the clean reaction 9Be(d, n)10B by Gorodetsky et al. [23]. They used scintillation counters with inherently two orders of  magnitude less energy resolution than our Si(Li) detectors ( though this resolution is partly spoiled by  Doppler broaden- ing), and could hardly separate a strong 3.37 MeV transition in 10B from the 3.59 MeV isoscalar t ransi-  tion in 10B. Yet, their angular distr ibution shows no enhancement near 45 ° where the standard axion would have given at least one order of  magnitude larger signal than measured. Also, a recent reanalysis by Calaprice et al. [24] of  IPC data from Warburton et al. [25] shows no evidence for axion competi t ion in either isoscalar or isovector M1 transitions. The authors wish to thank H.P. Blok, P.F.A. Goudsmit ,  R.H. Siemssen and A.H. Wapstra for valuable discussions. This investigation was sup- ported by the Netherlands Organization for Pure Scientific Research (ZWO) through the Foundat ion for Fundamental  Research on Matter (FOM). References [1] S. Weinberg, Phys. Rev. Lett. 40 (1978) 223. [2] F. Wilczek, Phys. Rev. Lett. 40 (1978) 279. [3] R~D. Peccei and H.R. Quinn, Phys. Rev. Lett. 38 (1977) 1440. [4] T.W. DonneUy et al., Phys. Rev. D 18 (1978) 1607. [5] A. Zehnder, in: Fundamental interactions in low-energy systems (Plenum, New York, 1985). [6] T. Cowa net  al., Phys. Rev. Lett. 54 (1985) 1761. [7] T. Cowan et al., Phys. Rev. Lett. 56 (1986) 444. [8] F.P. Calaprice et al., Phys. Rev. D 20 (1979) 2708. 56 0986) 302. [9] A. Schgfer et al., J. Phys. G 11 (1985) 169. [10] A.B. Balantekin et al., Phys. Rev. Lett. 55 (1985) 461. [11] N. Mukhopadhyay and A. Zehnder, Phys. Rev. Lett. 56 (1986) 206. [12] A. Chodos and L.R.C. Wijewardhama, Phys. Rev. Lett. 56 (1986) 302. [13] R.D. Peccei, Tai Tsun Wu and T. Yanagida, Phys. Lett. B 172 (1986) 435. [14] L.M. Krauss and F. Wilczek, Phys. Lett. B 173 (1986) 189. [15] S.J. Brodsky et al., Phys. Rev. Lett. 56 (1986) 1763. [16] K. Abrahams et al., KVI Annual Report 1985. [17] E.K. Warburton et al., Phys. Rev. 17 (1968) 1178. [18] P. Schliiter and G. Soff, Atom. Data Nucl. Data Tables 24 (1979) 509. [19] J. van Klinken, S.J. Feenstra and G. Dumont, Nucl. In- strum. Methods 151 (1978) 433. [20] B.A. Watson et al., Phys. Rev. 187 (1969) 1351. [21] J.B. Swint et al., Nucl. Phys. 86 (1966) 119. [22] A.C.L. Barnar d et al., Nucl. Phys. 86 (1966) 130. [23] S. Gorodetsky, P. Chevallier, R. Armbruster and G. Sutter, Nucl. Phys. 12 (1959) 349. [24] F.P. Calapriee et al., private communication, and to be published. [25] E.K. Warburton et al., Phys. Rev. 132 (1963) 776,133 (1964) B42, 

University of Groningen

   University of GroningenSearch for short-lived axions in a nuclear isoscalar transitionde Boer, FWN; Abrahams, K; Balanda, A; Bokemeyer, H; van Dantzig, R; Jansen, JFW;Kotlinski, B; de Voigt, MJA; van Klinken, JPublished in:Physics Letters BDOI:10.1016/0370-2693(86)90123-1IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1986Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):de Boer, FWN., Abrahams, K., Balanda, A., Bokemeyer, H., van Dantzig, R., Jansen, JFW., ... van Klinken,J. (1986). Search for short-lived axions in a nuclear isoscalar transition. Physics Letters B, 180(1-2), 4-8.https://doi.org/10.1016/0370-2693(86)90123-1CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 SEARCH FOR SHORT-L IVED AX IONS IN A NUCLEAR ISOSCALAR TRANSIT ION F.W.N.  DE BOER 1, K. ABRAHAMS 2, A. BALANDA 3,4, H. BOKEMEYER 5, R. VAN DANTZIG 6, J .F.W. JANSEN,  B. KOTL INSK I ,  M.J.A. DE VOIGT and J. VAN KL INKEN Kernfysisch Versneller lnstituut, Universiteit van Groningen, NL-974 7 AA Groningen, The Netherlands Received 22 July 1986; revised manuscript received 10 September 1986 Axions, if existing, can cause a strong signal of positron-electron pairs in isoscalar M1 transitions in competition with gamma-ray emission. We searched for such a signal in the 3.59 MeV transition i  I°B with a fourfold Mini-Orange spectro- meter. No axion events were found within two percent of the prediction for the standard axion- to ~,-ray branching ratio. A light pseudoscalar Goldstone boson, the axion, has been introduced by Weinberg [1] and Wilczek [2], as a consequence of  the global U(1) symmetry pro- posed by Peccei and Quinn [3] to explain the ab- sence of large P and CP violations in strong interac- tions. In a treatise of possible axion search experi- ments, Donnelly et al. [4] discussed the probability for nuclear de-excitation by axion emission in compe- tition with magnetic ~/radiation. Subsequent axion searches provided such stringent upper bounds on mass and lifetime that the nonexistence of axions was anticipated [5]. However, the interest in an observable axion was revived recently [6] after the puzzling ob- servation in superheavy collisional systems at GSI of  a sharp positron peak at approximately 330 keV, which was later [7] found to be in coincidence with a simi- lar electron peak. Could the observed (e+e - )  pairs t Present address: NIKHEF-K, P.O. Box 4395, NL-1009AJ Amsterdam, The Netherlands. 2 Energieonderzoek Centrum Nederland (ECN), Petten, The Netherlands. 3 Physics Laboratory of the Free University, NL-1081 HV Amsterdam, The Netherlands. 4 Jagiellonian University, PL-30059 Cracow 12, Poland. 5 Gesellschaft for Schwerionenforschung (GSI), Postfach 110541, D-6100 Darmstadt 11, Fed. Rep. Germany. 6 NIKHEF-K, P.O. Box 4395, NL-1009 AJ Amsterdam, The Netherlands. arise from a "visible" axion with a mass of  about 1.68 MeV and a lifetime of~< 10 -11 s? The axion mass and lifetime can be related to one free parameter X, the ratio of  vacuum expectation values for two Higgs field componentsfs in X and fcos  X with f=  (GFV~)- I /2  = 250 GeV (G F being the Fermi coupling constant): m a = 25N(X + l /X) (keV), (1) where N is the number of active quark pairs (N = 3 in ref. [4]). For m a > 2m e the axion will predominantly decay in a positron-electron pair with lifetime r(a -~ e+e - )  = 8"trf2X2me2(m 2 - 4me2) -1/2 (s). (2) Searches for axions decaying into two photons with radioactive sources, reactors and beam dumps constrain [5] the mass to < 1 MeV and the lifetime to < 10 -6  s, whereas a search [8] for decay into posi- t ron-electron pairs limits the lifetime to < 2 X 10 -10 s. Searches with pseudoscalar- and vector mesons (K +, J/~k and Y) were sensitive to lifetimes of 10 -11 s due to time dilation of the order of 104 . The anom- alous magnetic moment of  the electron constrains the lifetime to > 5 X 10 -13 s [9 -12] .  In summary, arather narrow window of lifetimes between 10 -13 and 10 -10 s cannot be excluded on the basis of exist- ing experimental evidence. If the observed positron peak is due to an axion with mass 1.68 MeV, its X 0370-2693/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 value according to eq. (1) is either X = 0.045 or X = 22 leading to a lifetime of about 5 × 10 -12 s (al- lowed) or 2 × 10 -6  s (excluded). Either of these two X values would lead to contradictions for branching ratios of meson decays in the standard axion model. Quite recently, however, Peccei et al. [13] have suc- ceeded in reconciling the previous unsuccessful searches with an axion of 1.68 MeV in a 'Wiable axion model" which suppresses the coupling to the heavier quarks. Almost simultaneously, Krauss and Wilczek [14] pre- sented a "short-lived axion variant" coupled preferen- tially to light fermions and stressed with Brodsky et al. [15] the need of new experiments. In this letter we report on a search for axions in the unexplored region with lifetimes T < 10 -9 s. We looked for the e+e - decay of such axions in competi- tion with internal pair creation (IPC) of a magnetic q-ray transition. Specifically for isoscalar transitions, the branching ratio for (e+e - )  pair creation from a pseudoscalar decay particle and M1 gamma-decay can be as large [4,16] as 50%. This is three orders of  mag- nitude larger than IPC. As isoscalar transition the 3.59 MeV M1 + E2 transition in 10B was chosen. The 2 +, 0 state at 3.59 MeV in 10B decays to the 3 +, 0 groundstate with a 19% branching ratio. The transition has a mixing ratio [17] 5(E2/M1) of 1.5 + 0.6 while the parallel 2 +, 0 ~ 1 +, 0 transition of 2.87 MeV is [17] predominantly E2 with 5-1 < 0.2. Neglecting internal conversion by atomic electrons, two-photon emission and other higher-order processes, the IPC contributes at the level of 10 -3 compared with gamma rays in the de,excitation of the 3.59 MeV state. Axion emission would imply a source of (e+e - )  pairs in addition to the expected (e+e - )  pairs from IPC. The energy and angular correlations of (e+e-) pairs from the 3.59 MeV transition are ftxed kinemati- cally. In our experimental rrangement the M I and E2 IPC is detected with total efficiency e of typically 10 -4  (see below). The IPC angular correlation be- tween the e ÷ and e -  is maximum at relative angle 0 = 0 °, while in case of the axions the maximum will occur around 0 = 45 °. The angle between the axes of  neighbouring Mini-Oranges i  60 ° (fig. 1), which causes an axion decay to be seen with somewhat higher efficiency than IPC; a numerical estimate gave e A ~ 1.5e, assuming that the axions are emitted iso- tropically. The nuclear axion-decay rate (in the case of existing axions) is predicted in ref. [4] to be as large A e . . /~det .  ta rget  _ - ~45v~ .beam ~Ge detector Fig. 1. Fourfold Mini-Orange arrangement for the axion search. To the left a view of the horizontal plane and to the right a view upstream along the beam axis. The hypothetical xi0n ~ e + + e- decay is supposed to occur inside or closely (at gm distance) be- hind the target layer. In the left part of the figure an e- particle is indicated, while the corresponding e +particle will be detected by a neighbouring e + detector (out of plane). Volume 180, number 1,2 PHYSICS LETTERS B 6 November  1986 as I a = 0.39 I3,(M 1). The number of observed pairs can now be written as: N(e+e-)/e = {0.39 X 1.5 + ~(M1) )  Lr(M1 ) + otTr (E2)[~ (E2), (3) where e is the efficiency. Theoretical IPC coefficients ot  can be obtained from ref. [18]. If  axions indeed exist then the first term (= 0.585) will dominate since the others are of order 10 -3  . In the experiment the 3.59 MeV state in 10B was excited in inelastic scattering of 7 MeV protons by a 10B target of 1 mg/cm 2. Using a fourfold Mini-Orange system [19] with two detector systems for e + and two for e -  detection, we registered prompt (e+e - )  signals and their energy sums. The four spectrometers were set at a backward angle of 45 ° with respect o the incident proton beam. This is shown by a view in the horizontal plane in fig. 1 (left). The axes of oppo- site Mini-Oranges (with the same polarity) made an angle of 90 ° with each other, those of neighbouring ones (with alternating polarity) one of 60 ° . For clarity fig. 1 (right) shows four equal Mini-Orange systems, each composed of four magnets and the e ÷ and e -  sys- I I I I 1 l °B (p ,p ' )  Ep =7MeV e ÷ ..- e- SPECTRUM 3.35 MeV 120" 4OCa - 3;059 MeV 4.43 MeV 2.87 MeV I] B 12 C 80 - lo B I~ I l r' . I 6.05 MeV 40-- ~ / ~ - ,  { u "LP ~ 0 //, I I I I - _ , ,  H 2 3 4 5 6 MeV Z moO2 :::) Ee. ÷Ee.÷ 2 0 U 106-- 8,000f ~lr GAMMA-  RAY SPECTRUM- ~o 5 6,OOOl- ~A ~ ~ . . ~  l t 'g'- ~'J l 8 13 MeV 16 0 10 3 I I i 102 -- I I I .-¢ I I I I I 10 2 3 4 5 6 MeV Ey Fig. 2. Sum of four e + + e-  pair spectra (top) compared wi th  a s imultaneously  recorded ~/-ray spectrum (bot tom)  using a Ge de- tector,  following the l°B (p, p') reaction at 7 MeV. In case of an existing axion with mass of 1.68 MeV and lifetime 5 X 10 -12 s, the prediction by DonneUy et al. [4] corresponds toa peak at 3.59 MeV with height of over 1000 counts. Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 tems being only different by inversion of their mag- netic fields. In reality the present experiment was lim- ited by the available magnets so that two four-gap and two five-gap systems had to be used. Since the latter magnets were relatively weaker, the overall features of the four magnet systems were similar, with maxi- mum transmissions around 1.3 MeV. The Mini-Orange spectrometers were equipped with 300 mm 2 Si(Li) detectors: two with a thickness of 3 mm and two with a thickness of 5 mm. With the KVI data acquisition system (VAX 750) the twofold coin- cidences were registered on magnetic tape for off-line analysis. Fig. 2 (top) displays the essential result: the sum of the (e+e-) pair spectra from the four neighbouring detector combinations in which we can look for a pos- sible axion signal in competition with expected IPC peaks. The (e÷e -) spectrum is compared with a simul- taneously recorded spectrum of gamma rays (bottom) using a Ge detector. It is apparent that the internal pair peaks of the two isoscalar transitions at 2.87 MeV and 3.59 MeV in 10B are only weakly visible in the (e+e -) spectrum. Interestingly, two rather prominent peaks at 3.35 and 6.05 MeV show up in the sum spectrum. They have no counterpart in the 3'-ray spectrum and belong to to- tally converted E0 transitions in 40Ca nd 160 respec- tively, which are to our knowledge the only two exist- ing E0 transitions in nature depopulating first excited states between 3 and 7 MeV. The spectra of scattered protons, recorded separately, indeed showed a signifi- cant content of 160 (and 12C) impurity in the target. A tracer impurity causes in the pair spectra the ap- pearance of the E0 transition in 40Ca. The 6.05 MeV transition in 160 occurs just above energy threshold and also in the extreme upper transmission range of the Mini-Oranges. Furthermore, it is observed espite the fact that the two 3 mm Si(Li) detectors had insuf- ficient stopping power for most of the e + and e- pulses. Evidently, the arrangement is hypersensitive for detection of totally converted E0 transitions. The background of the pair spectrum iscomplex, containing a continuum distribution following 10B(p, a) distributions from pairs created inside the vacuum walls, e + and e- particles from e.g. the E0(160) transi- tions which pass through the detectors. Yet, the main part of the spectrum belongs to IPC coincidences and not to e.g. Compton scattered 3'rays since the yield of e + + e + and e- + e- type combinations is less by an or- der of magnitude than the e + + e- pairs. In the 3'-ray spectrum all transitions of sufficiently short half-life are Doppler broadened and/or shifted. The 10B 2.87 and 3.59 MeV 3' rays show up strongly. The spectrum also shows a number of 27A1 3" rays produced in the walls of the Mini-Orange vacuum chamber, together with the 4.43 MeV E2 transition in 12C and the 6.13 MeV E3 transition in 160 (target impurities), visible by photo peaks, single- and double-escape aks. Efficiency calibration for IPC detection was ob- tained as follows: at the low energy side the system was calibrated (i) internally at 1.85 MeV with the 2.87 MeV transition in 10B (> 98% E2) and (ii) externally at 1.733 MeV using the 2.755 MeV E2 transition in 24Na with theoretical [18] IPC's of 7.4 × 10 -4 re- spectively 7.0 X 10 -4. (iii) A comparison of the inten- sity of inelastically scattered protons from the 4.43 MeV level in 12C with that from the 3.59 MeV level in 10B, the ratio being 0.6, showed us that only a frac- tion of the corresponding 3'-ray intensity in 12C is produced in the target and apparently most of it orig- inates in the walls of the chamber. The ratio of 3'-ray intensities was determined to be 0.5 using experimen- tal data for differential and total cross sections for 10B (p,p') [20] and 12C (p, p') [21,22] inelastic scat- tering at 7 MeV under 135 °. The obtained gamma-ray intensity was then used together with the internal pair intensity and the theoretical [18] IPC of 1.3 × 10 -3 for an E2 transition of 4.43 MeV to internally determine the efficiency of the Mini-Orange system at 3.41 MeV. All three efficiency calibrations are equal to a value of e = 2.3 X 10 -4 within 35%. The isoscalar 3.59 MeV transition has in the (e+e -) pair spectrum roughly the same intensity as the paral- lel 2.87 MeV transition used for internal calibration. The branching ratio of the 3.59 MeV pair peak was determined to be (2.1 + 1.1) X 10 -3 which is consis- tent with internal pair creation, %r being 1.0 X 10 -3 for a 28% M1 and 72% E2 transition. The full strength of axion decay with the Donnelly value o f I  a = 0.39 1~ (M1) for an axion of 1.68 MeV would have caused apeak height of over 1000 counts or two orders of mag- nitude !arger than as observed. With 90% confidence our result corresponds with a boundary being 54 times smaller than the value predicted by the standard model [4]. In recent more sophisticated axion models [13,14] the conjectured axion couples to one or two Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 quark pairs IN = 1 or 2 in eq. (1)]. In the case of only two light quarks, no coupling is expected to a magnet- ic isoscalar transition. In the case of coupling to two quark pairs our experiment leads with 90% confidence to a boundary factor of 31 with respect o the theoret- ical prediction in the case of existing axions. Conse- quently our search yields no evidence for the existence of a standard (N = 3) or a viable (N = 2) axion. After evaluation of our experimental data, we be- came aware of IPC measurements in the early de- cades of nuclear physics, when axions were not yet advocated, but when (e+e - )  angular correlations were used as a probe for distinguishing multipolarities ofhigh-energy "/-rays. We were both surprised and im- pressed by the angular correlations reported for the 3.59 MeV transition in the clean reaction 9Be(d, n)10B by Gorodetsky et al. [23]. They used scintillation counters with inherently two orders of magnitude less energy resolution than our Si(Li) detectors (though this resolution is partly spoiled by Doppler broaden- ing), and could hardly separate a strong 3.37 MeV transition in 10B from the 3.59 MeV isoscalar transi- tion in 10B. Yet, their angular distribution shows no enhancement ear 45 ° where the standard axion would have given at least one order of magnitude larger signal than measured. Also, a recent reanalysis by Calaprice t al. [24] of IPC data from Warburton et al. [25] shows no evidence for axion competition in either isoscalar or isovector M1 transitions. The authors wish to thank H.P. Blok, P.F.A. Goudsmit, R.H. Siemssen and A.H. Wapstra for valuable discussions. This investigation was sup- ported by the Netherlands Organization for Pure Scientific Research (ZWO) through the Foundation for Fundamental Research on Matter (FOM). References [1] S. Weinberg, Phys. Rev. Lett. 40 (1978) 223. [2] F. Wilczek, Phys. Rev. Lett. 40 (1978) 279. [3] R~D. Peccei and H.R. Quinn, Phys. Rev. Lett. 38 (1977) 1440. [4] T.W. DonneUy et al., Phys. Rev. D 18 (1978) 1607. [5] A. Zehnder, in: Fundamental interactions in low-energy systems (Plenum, New York, 1985). [6] T. Cowa net al., Phys. Rev. Lett. 54 (1985) 1761. [7] T. Cowan et al., Phys. Rev. Lett. 56 (1986) 444. [8] F.P. Calaprice t al., Phys. Rev. D 20 (1979) 2708. 56 0986) 302. [9] A. Schgfer et al., J. Phys. G 11 (1985) 169. [10] A.B. Balantekin et al., Phys. Rev. Lett. 55 (1985) 461. [11] N. Mukhopadhyay nd A. Zehnder, Phys. Rev. Lett. 56 (1986) 206. [12] A. Chodos and L.R.C. Wijewardhama, Phys. Rev. Lett. 56 (1986) 302. [13] R.D. Peccei, Tai Tsun Wu and T. Yanagida, Phys. Lett. B 172 (1986) 435. [14] L.M. Krauss and F. Wilczek, Phys. Lett. B 173 (1986) 189. [15] S.J. Brodsky et al., Phys. Rev. Lett. 56 (1986) 1763. [16] K. Abrahams et al., KVI Annual Report 1985. [17] E.K. Warburton et al., Phys. Rev. 17 (1968) 1178. [18] P. Schliiter and G. Soff, Atom. Data Nucl. Data Tables 24 (1979) 509. [19] J. van Klinken, S.J. Feenstra nd G. Dumont, Nucl. In- strum. Methods 151 (1978) 433. [20] B.A. Watson et al., Phys. Rev. 187 (1969) 1351. [21] J.B. Swint et al., Nucl. Phys. 86 (1966) 119. [22] A.C.L. Barnar d et al., Nucl. Phys. 86 (1966) 130. [23] S. Gorodetsky, P. Chevallier, R. Armbruster and G. Sutter, Nucl. Phys. 12 (1959) 349. [24] F.P. Calapriee t al., private communication, and to be published. [25] E.K. Warburton et al., Phys. Rev. 132 (1963) 776,133 (1964) B42, 

   University of GroningenSearch for short-lived axions in a nuclear isoscalar transitionde Boer, FWN; Abrahams, K; Balanda, A; Bokemeyer, H; van Dantzig, R; Jansen, JFW;Kotlinski, B; de Voigt, MJA; van Klinken, JPublished in:Physics Letters BDOI:10.1016/0370-2693(86)90123-1IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1986Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):de Boer, FWN., Abrahams, K., Balanda, A., Bokemeyer, H., van Dantzig, R., Jansen, JFW., Kotlinski, B.,de Voigt, MJA., & van Klinken, J. (1986). Search for short-lived axions in a nuclear isoscalar transition.Physics Letters B, 180(1-2), 4-8. https://doi.org/10.1016/0370-2693(86)90123-1CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 16-07-2023Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 S E A R C H  F O R  S H O R T - L I V E D  A X I O N S  IN  A N U C L E A R  I S O S C A L A R  T R A N S I T I O N  F .W.N.  D E  BOER 1, K. A B R A H A M S  2, A. B A L A N D A  3,4, H. B O K E M E Y E R  5, R. V A N  D A N T Z I G  6, J .F .W.  J A N S E N ,  B. K O T L I N S K I ,  M.J.A. D E  V O I G T  and  J. V A N  K L I N K E N  Kernfysisch Versneller lnstituut, Universiteit van Groningen, NL-974 7 AA Groningen, The Netherlands Received 22 July 1986; revised manuscript received 10 September 1986 Axions, if existing, can cause a strong signal of positron-electron pairs in isoscalar M1 transitions in competition with gamma-ray emission. We searched for such a signal in the 3.59 MeV transition in I°B with a fourfold Mini-Orange spectro- meter. No axion events were found within two percent of the prediction for the standard axion- to ~,-ray branching ratio. A light pseudoscalar Goldstone boson, the axion, has been introduced by Weinberg [1] and Wilczek [2], as a consequence o f  the global U(1) symmetry pro- posed by Peccei and Quinn [3] to explain the ab- sence of  large P and CP violations in strong interac- tions. In a treatise of  possible axion search experi- ments, Donnelly et al. [4] discussed the probabil i ty for nuclear de-excitation by  axion emission in compe- t i t ion with magnetic ~/radiation. Subsequent axion searches provided such stringent upper bounds on mass and lifetime that  the nonexistence of  axions was anticipated [5]. However, the interest in an observable axion was revived recently [6] after the puzzling ob- servation in superheavy collisional systems at GSI o f  a sharp positron peak at approximately 330 keV, which was later [7] found to be in coincidence with a simi- lar electron peak. Could the observed (e+e - )  pairs t Present address: NIKHEF-K, P.O. Box 4395, NL-1009AJ Amsterdam, The Netherlands. 2 Energieonderzoek Centrum Nederland (ECN), Petten, The Netherlands. 3 Physics Laboratory of the Free University, NL-1081 HV Amsterdam, The Netherlands. 4 Jagiellonian University, PL-30059 Cracow 12, Poland. 5 Gesellschaft for Schwerionenforschung (GSI), Postfach 110541, D-6100 Darmstadt 11, Fed. Rep. Germany. 6 NIKHEF-K, P.O. Box 4395, NL-1009 AJ Amsterdam, The Netherlands. arise from a "visible" axion with a mass o f  about 1.68 MeV and a lifetime of~< 10 -11 s? The axion mass and lifetime can be related to one free parameter X, the ratio o f  vacuum expectat ion values for two Higgs field componen t s f s in  X and f c o s  X with f =  (G FV ~) - I / 2  = 250 GeV (G F being the Fermi coupling constant): m a = 25N(X + l /X)  (keV), (1) where N is the number of  active quark pairs (N = 3 in ref. [4]). For  m a > 2m e the axion will predominantly decay in a pos i t ron-e lec t ron  pair with lifetime r(a -~ e+e - )  = 8"tr f2X2me2(m 2 - 4me2) -1/2 (s). (2) Searches for axions decaying into two photons with radioactive sources, reactors and beam dumps constrain [5] the mass to < 1 MeV and the lifetime to < 10 - 6  s, whereas a search [8] for decay into posi- t ron -e l ec t ron  pairs limits the lifetime to < 2 X 10 -10 s. Searches with pseudoscalar- and vector mesons (K +, J/~k and Y) were sensitive to lifetimes of  10 -11 s due to time dilation of  the order of  104 . The anom- alous magnetic moment  o f  the electron constrains the lifetime to > 5 X 10 -13 s [ 9 - 1 2 ] .  In summary, a rather narrow window of  lifetimes between 10 -13 and 10 -10 s cannot be excluded on the basis of  exist- ing experimental evidence. If  the observed positron peak is due to an axion with mass 1.68 MeV, its X 0370-2693/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 value according to eq. (1) is either X = 0.045 or X = 22 leading to a lifetime of  about 5 × 10 -12 s (al- lowed) or 2 × 10 - 6  s (excluded).  Either of  these two X values would lead to contradictions for branching ratios of  meson decays in the standard axion model. Quite recently, however, Peccei et al. [13] have suc- ceeded in reconciling the previous unsuccessful searches with an axion of  1.68 MeV in a 'Wiable axion model"  which suppresses the coupling to the heavier quarks. Almost simultaneously, Krauss and Wilczek [14] pre- sented a "short-lived axion variant" coupled preferen- tially to light fermions and stressed with Brodsky et al. [15] the need of  new experiments.  In this letter we report  on a search for axions in the unexplored region with lifetimes T < 10 - 9  s. We looked for the e+e - decay of  such axions in competi- t ion with internal pair creation (IPC) of  a magnetic q-ray transition. Specifically for isoscalar transitions, the branching ratio for (e+e - )  pair creation from a pseudoscalar decay particle and M1 gamma-decay can be as large [4,16] as 50%. This is three orders o f  mag- nitude larger than IPC. As isoscalar transition the 3.59 MeV M1 + E2 transition in 10B was chosen. The 2 +, 0 state at 3.59 MeV in 10B decays to the 3 +, 0 groundstate with a 19% branching ratio. The transition has a mixing ratio [17] 5(E2/M1) of  1.5 + 0.6 while the parallel 2 +, 0 ~ 1 +, 0 transition of  2.87 MeV is [17] predominantly E2 with 5 - 1  < 0.2. Neglecting internal conversion by atomic electrons, two-photon emission and other higher-order processes, the IPC contributes at the level of  10 -3  compared with gamma rays in the de,excitation of  the 3.59 MeV state. Axion emission would imply a source of  (e+e - )  pairs in addit ion to the expected (e+e - )  pairs from IPC. The energy and angular correlations of  (e+e-)  pairs from the 3.59 MeV transition are ftxed kinemati- cally. In our experimental arrangement the M I and E2 IPC is detected with total  efficiency e of  typically 10 - 4  (see below). The IPC angular correlation be- tween the e ÷ and e -  is maximum at relative angle 0 = 0 °, while in case of  the axions the maximum will occur around 0 = 45 °. The angle between the axes o f  neighbouring Mini-Oranges is 60 ° (fig. 1), which causes an axion decay to be seen with somewhat higher efficiency than IPC; a numerical estimate gave e A ~ 1.5e, assuming that the axions are emitted iso- tropically. The nuclear axion-decay rate (in the case of  existing axions) is predicted in ref. [4] to be as large A e . . / ~ d e t .  target _ - ~ 4 5 v ~  .beam ~Ge detector Fig. 1. Fourfold Mini-Orange arrangement for the axion search. To the left a view of the horizontal plane and to the right a view upstream along the beam axis. The hypothetical axi0n ~ e + + e- decay is supposed to occur inside or closely (at gm distance) be- hind the target layer. In the left part of the figure an e- particle is indicated, while the corresponding e + particle will be detected by a neighbouring e + detector (out of plane). Volume 180, number  1,2 PHYSICS LETTERS B 6 N o v e m b e r  1986 as I a = 0.39 I3,(M 1). The number of  observed pairs can now be writ ten as: N(e+e-)/e = {0.39 X 1.5 + ~ ( M 1 ) )  Lr(M1 ) + otTr (E2)[~ (E2), (3) where e is the efficiency. Theoretical IPC coefficients o t  can be obtained from ref. [18]. I f  axions indeed exist then the first term (= 0.585) will dominate since the others are of  order 10 - 3  . In the experiment the 3.59 MeV state in 10B was excited in inelastic scattering of  7 MeV protons by a 10B target of  1 mg/cm 2. Using a fourfold Mini-Orange system [19] with two detector  systems for e + and two for e -  detection, we registered prompt  (e+e - )  signals and their energy sums. The four spectrometers were set at a backward angle of  45 ° with respect to the incident proton beam. This is shown by a view in the horizontal plane in fig. 1 (left). The axes of  oppo- site Mini-Oranges (with the same polari ty) made an angle of  90 ° with each other,  those of  neighbouring ones (with alternating polari ty)  one of  60 ° . For clarity fig. 1 (right) shows four equal Mini-Orange systems, each composed of  four magnets and the e ÷ and e -  sys- I I I I 1 l ° B ( p , p ' )  Ep = 7 M e V  e ÷ ..- e- SPECTRUM 3.35 MeV 120" 4OCa - 3;059 MeV 4.43 MeV 2.87 MeV I] B 12 C 80 - lo B I~ I l r '  . I 6.05 MeV 40-- ~ / ~ - ,  { u "LP ~ 0 //, I I I I - _ , ,  H 2 3 4 5 6 MeV Z moO2 :::) Ee. ÷Ee.÷ 2 0 U 106-- 8,000f ~lr G A M M A -  RAY SPECTRUM- ~o 5 6,OOOl- ~ A  ~ ~ . . ~  l t 'g'- ~'J l 8 13 MeV 16 0 10 3 I I i 102 -- I I I .-¢ I I I I I 10 2 3 4 5 6 MeV Ey Fig. 2. Sum of  four  e + + e -  pa i r  spec t ra  ( top)  c o m p a r e d  w i t h  a s imu l t aneous ly  r eco rded  ~/-ray s p e c t r u m  ( b o t t o m )  us ing a Ge de-  t ec to r ,  following the l°B (p, p') reaction at 7 MeV. In case of an existing axion with mass of 1.68 MeV and lifetime 5 X 10 -12 s, the prediction by DonneUy et al. [4] corresponds to a peak at 3.59 MeV with height of over 1000 counts. Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 tems being only different by inversion of their mag- netic fields. In reality the present experiment was lim- ited by the available magnets so that two four-gap and two five-gap systems had to be used. Since the latter magnets were relatively weaker, the overall features of the four magnet systems were similar, with maxi- mum transmissions around 1.3 MeV. The Mini-Orange spectrometers were equipped with 300 mm 2 Si(Li) detectors: two with a thickness of 3 mm and two with a thickness of 5 mm. With the KVI data acquisition system (VAX 750) the twofold coin- cidences were registered on magnetic tape for off-line analysis. Fig. 2 (top) displays the essential result: the sum of the (e+e-) pair spectra from the four neighbouring detector combinations in which we can look for a pos- sible axion signal in competition with expected IPC peaks. The (e÷e - )  spectrum is compared with a simul- taneously recorded spectrum of gamma rays (bottom) using a Ge detector. It is apparent that the internal pair peaks of the two isoscalar transitions at 2.87 MeV and 3.59 MeV in 10B are only weakly visible in the (e+e -)  spectrum. Interestingly, two rather prominent peaks at 3.35 and 6.05 MeV show up in the sum spectrum. They have no counterpart in the 3'-ray spectrum and belong to to- tally converted E0 transitions in 40Ca and 160 respec- tively, which are to our knowledge the only two exist- ing E0 transitions in nature depopulating first excited states between 3 and 7 MeV. The spectra of scattered protons, recorded separately, indeed showed a signifi- cant content of 160 (and 12C) impurity in the target. A tracer impurity causes in the pair spectra the ap- pearance of the E0 transition in 40Ca. The 6.05 MeV transition in 160 occurs just above energy threshold and also in the extreme upper transmission range of the Mini-Oranges. Furthermore, it is observed despite the fact that the two 3 mm Si(Li) detectors had insuf- ficient stopping power for most of the e + and e -  pulses. Evidently, the arrangement is hypersensitive for detection of totally converted E0 transitions. The background of the pair spectrum is complex, containing a continuum distribution following 10B(p, a) distributions from pairs created inside the vacuum walls, e + and e -  particles from e.g. the E0(160) transi- tions which pass through the detectors. Yet, the main part of the spectrum belongs to IPC coincidences and not to e.g. Compton scattered 3' rays since the yield of e + + e + and e-  + e -  type combinations is less by an or- der of magnitude than the e + + e -  pairs. In the 3'-ray spectrum all transitions of sufficiently short half-life are Doppler broadened and/or shifted. The 10B 2.87 and 3.59 MeV 3' rays show up strongly. The spectrum also shows a number of 27A1 3" rays produced in the walls of the Mini-Orange vacuum chamber, together with the 4.43 MeV E2 transition in 12C and the 6.13 MeV E3 transition in 160 (target impurities), visible by photo peaks, single- and double-escape peaks. Efficiency calibration for IPC detection was ob- tained as follows: at the low energy side the system was calibrated (i) internally at 1.85 MeV with the 2.87 MeV transition in 10B (> 98% E2) and (ii) externally at 1.733 MeV using the 2.755 MeV E2 transition in 24Na with theoretical [18] IPC's of 7.4 × 10 -4 re- spectively 7.0 X 10 -4. (iii) A comparison of the inten- sity of inelastically scattered protons from the 4.43 MeV level in 12C with that from the 3.59 MeV level in 10B, the ratio being 0.6, showed us that only a frac- tion of the corresponding 3'-ray intensity in 12C is produced in the target and apparently most of it orig- inates in the walls of the chamber. The ratio of 3'-ray intensities was determined to be 0.5 using experimen- tal data for differential and total cross sections for 10B (p,p ')  [20] and 12C (p, p') [21,22] inelastic scat- tering at 7 MeV under 135 °. The obtained gamma-ray intensity was then used together with the internal pair intensity and the theoretical [18] IPC of 1.3 × 10 -3 for an E2 transition of 4.43 MeV to internally determine the efficiency of the Mini-Orange system at 3.41 MeV. All three efficiency calibrations are equal to a value of e = 2.3 X 10 -4 within 35%. The isoscalar 3.59 MeV transition has in the (e+e - )  pair spectrum roughly the same intensity as the paral- lel 2.87 MeV transition used for internal calibration. The branching ratio of the 3.59 MeV pair peak was determined to be (2.1 + 1.1) X 10 -3 which is consis- tent with internal pair creation, %r being 1.0 X 10 -3 for a 28% M1 and 72% E2 transition. The full strength of axion decay with the Donnelly value o f I  a = 0.39 1~ (M1) for an axion of 1.68 MeV would have caused a peak height of over 1000 counts or two orders of mag- nitude !arger than as observed. With 90% confidence our result corresponds with a boundary being 54 times smaller than the value predicted by the standard model [4]. In recent more sophisticated axion models [13,14] the conjectured axion couples to one or two Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 quark pairs IN = 1 or 2 in eq. (1)]. In the case of  only two light quarks, no coupling is expected to a magnet- ic isoscalar transition. In the case of coupling to two quark pairs our experiment leads with 90% confidence to a boundary factor of  31 with respect to the theoret- ical prediction in the case of  existing axions. Conse- quently our search yields no evidence for the existence of  a standard (N = 3) or a viable (N = 2) axion. After evaluation of  our experimental  data, we be- came aware of  IPC measurements in the early de- cades of  nuclear physics, when axions were not yet  advocated, but  when (e+e - )  angular correlations were used as a probe for distinguishing multipolarit ies of  high-energy "/-rays. We were both  surprised and im- pressed by the angular correlations reported for the 3.59 MeV transition in the clean reaction 9Be(d, n)10B by Gorodetsky et al. [23]. They used scintillation counters with inherently two orders of  magnitude less energy resolution than our Si(Li) detectors ( though this resolution is partly spoiled by  Doppler broaden- ing), and could hardly separate a strong 3.37 MeV transition in 10B from the 3.59 MeV isoscalar t ransi-  tion in 10B. Yet, their angular distr ibution shows no enhancement near 45 ° where the standard axion would have given at least one order of  magnitude larger signal than measured. Also, a recent reanalysis by Calaprice et al. [24] of  IPC data from Warburton et al. [25] shows no evidence for axion competi t ion in either isoscalar or isovector M1 transitions. The authors wish to thank H.P. Blok, P.F.A. Goudsmit ,  R.H. Siemssen and A.H. Wapstra for valuable discussions. This investigation was sup- ported by the Netherlands Organization for Pure Scientific Research (ZWO) through the Foundat ion for Fundamental  Research on Matter (FOM). References [1] S. Weinberg, Phys. Rev. Lett. 40 (1978) 223. [2] F. Wilczek, Phys. Rev. Lett. 40 (1978) 279. [3] R~D. Peccei and H.R. Quinn, Phys. Rev. Lett. 38 (1977) 1440. [4] T.W. DonneUy et al., Phys. Rev. D 18 (1978) 1607. [5] A. Zehnder, in: Fundamental interactions in low-energy systems (Plenum, New York, 1985). [6] T. Cowa net  al., Phys. Rev. Lett. 54 (1985) 1761. [7] T. Cowan et al., Phys. Rev. Lett. 56 (1986) 444. [8] F.P. Calaprice et al., Phys. Rev. D 20 (1979) 2708. 56 0986) 302. [9] A. Schgfer et al., J. Phys. G 11 (1985) 169. [10] A.B. Balantekin et al., Phys. Rev. Lett. 55 (1985) 461. [11] N. Mukhopadhyay and A. Zehnder, Phys. Rev. Lett. 56 (1986) 206. [12] A. Chodos and L.R.C. Wijewardhama, Phys. Rev. Lett. 56 (1986) 302. [13] R.D. Peccei, Tai Tsun Wu and T. Yanagida, Phys. Lett. B 172 (1986) 435. [14] L.M. Krauss and F. Wilczek, Phys. Lett. B 173 (1986) 189. [15] S.J. Brodsky et al., Phys. Rev. Lett. 56 (1986) 1763. [16] K. Abrahams et al., KVI Annual Report 1985. [17] E.K. Warburton et al., Phys. Rev. 17 (1968) 1178. [18] P. Schliiter and G. Soff, Atom. Data Nucl. Data Tables 24 (1979) 509. [19] J. van Klinken, S.J. Feenstra and G. Dumont, Nucl. In- strum. Methods 151 (1978) 433. [20] B.A. Watson et al., Phys. Rev. 187 (1969) 1351. [21] J.B. Swint et al., Nucl. Phys. 86 (1966) 119. [22] A.C.L. Barnar d et al., Nucl. Phys. 86 (1966) 130. [23] S. Gorodetsky, P. Chevallier, R. Armbruster and G. Sutter, Nucl. Phys. 12 (1959) 349. [24] F.P. Calapriee et al., private communication, and to be published. [25] E.K. Warburton et al., Phys. Rev. 132 (1963) 776,133 (1964) B42, 

   University of GroningenSearch for short-lived axions in a nuclear isoscalar transitionde Boer, FWN; Abrahams, K; Balanda, A; Bokemeyer, H; van Dantzig, R; Jansen, JFW;Kotlinski, B; de Voigt, MJA; van Klinken, JPublished in:Physics Letters BDOI:10.1016/0370-2693(86)90123-1IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1986Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):de Boer, FWN., Abrahams, K., Balanda, A., Bokemeyer, H., van Dantzig, R., Jansen, JFW., Kotlinski, B.,de Voigt, MJA., & van Klinken, J. (1986). Search for short-lived axions in a nuclear isoscalar transition.Physics Letters B, 180(1-2), 4-8. https://doi.org/10.1016/0370-2693(86)90123-1CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 27-04-2023Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 S E A R C H  F O R  S H O R T - L I V E D  A X I O N S  IN  A N U C L E A R  I S O S C A L A R  T R A N S I T I O N  F .W.N.  D E  BOER 1, K. A B R A H A M S  2, A. B A L A N D A  3,4, H. B O K E M E Y E R  5, R. V A N  D A N T Z I G  6, J .F .W.  J A N S E N ,  B. K O T L I N S K I ,  M.J.A. D E  V O I G T  and  J. V A N  K L I N K E N  Kernfysisch Versneller lnstituut, Universiteit van Groningen, NL-974 7 AA Groningen, The Netherlands Received 22 July 1986; revised manuscript received 10 September 1986 Axions, if existing, can cause a strong signal of positron-electron pairs in isoscalar M1 transitions in competition with gamma-ray emission. We searched for such a signal in the 3.59 MeV transition in I°B with a fourfold Mini-Orange spectro- meter. No axion events were found within two percent of the prediction for the standard axion- to ~,-ray branching ratio. A light pseudoscalar Goldstone boson, the axion, has been introduced by Weinberg [1] and Wilczek [2], as a consequence o f  the global U(1) symmetry pro- posed by Peccei and Quinn [3] to explain the ab- sence of  large P and CP violations in strong interac- tions. In a treatise of  possible axion search experi- ments, Donnelly et al. [4] discussed the probabil i ty for nuclear de-excitation by  axion emission in compe- t i t ion with magnetic ~/radiation. Subsequent axion searches provided such stringent upper bounds on mass and lifetime that  the nonexistence of  axions was anticipated [5]. However, the interest in an observable axion was revived recently [6] after the puzzling ob- servation in superheavy collisional systems at GSI o f  a sharp positron peak at approximately 330 keV, which was later [7] found to be in coincidence with a simi- lar electron peak. Could the observed (e+e - )  pairs t Present address: NIKHEF-K, P.O. Box 4395, NL-1009AJ Amsterdam, The Netherlands. 2 Energieonderzoek Centrum Nederland (ECN), Petten, The Netherlands. 3 Physics Laboratory of the Free University, NL-1081 HV Amsterdam, The Netherlands. 4 Jagiellonian University, PL-30059 Cracow 12, Poland. 5 Gesellschaft for Schwerionenforschung (GSI), Postfach 110541, D-6100 Darmstadt 11, Fed. Rep. Germany. 6 NIKHEF-K, P.O. Box 4395, NL-1009 AJ Amsterdam, The Netherlands. arise from a "visible" axion with a mass o f  about 1.68 MeV and a lifetime of~< 10 -11 s? The axion mass and lifetime can be related to one free parameter X, the ratio o f  vacuum expectat ion values for two Higgs field componen t s f s in  X and f c o s  X with f =  (G FV ~) - I / 2  = 250 GeV (G F being the Fermi coupling constant): m a = 25N(X + l /X)  (keV), (1) where N is the number of  active quark pairs (N = 3 in ref. [4]). For  m a > 2m e the axion will predominantly decay in a pos i t ron-e lec t ron  pair with lifetime r(a -~ e+e - )  = 8"tr f2X2me2(m 2 - 4me2) -1/2 (s). (2) Searches for axions decaying into two photons with radioactive sources, reactors and beam dumps constrain [5] the mass to < 1 MeV and the lifetime to < 10 - 6  s, whereas a search [8] for decay into posi- t ron -e l ec t ron  pairs limits the lifetime to < 2 X 10 -10 s. Searches with pseudoscalar- and vector mesons (K +, J/~k and Y) were sensitive to lifetimes of  10 -11 s due to time dilation of  the order of  104 . The anom- alous magnetic moment  o f  the electron constrains the lifetime to > 5 X 10 -13 s [ 9 - 1 2 ] .  In summary, a rather narrow window of  lifetimes between 10 -13 and 10 -10 s cannot be excluded on the basis of  exist- ing experimental evidence. If  the observed positron peak is due to an axion with mass 1.68 MeV, its X 0370-2693/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 value according to eq. (1) is either X = 0.045 or X = 22 leading to a lifetime of  about 5 × 10 -12 s (al- lowed) or 2 × 10 - 6  s (excluded).  Either of  these two X values would lead to contradictions for branching ratios of  meson decays in the standard axion model. Quite recently, however, Peccei et al. [13] have suc- ceeded in reconciling the previous unsuccessful searches with an axion of  1.68 MeV in a 'Wiable axion model"  which suppresses the coupling to the heavier quarks. Almost simultaneously, Krauss and Wilczek [14] pre- sented a "short-lived axion variant" coupled preferen- tially to light fermions and stressed with Brodsky et al. [15] the need of  new experiments.  In this letter we report  on a search for axions in the unexplored region with lifetimes T < 10 - 9  s. We looked for the e+e - decay of  such axions in competi- t ion with internal pair creation (IPC) of  a magnetic q-ray transition. Specifically for isoscalar transitions, the branching ratio for (e+e - )  pair creation from a pseudoscalar decay particle and M1 gamma-decay can be as large [4,16] as 50%. This is three orders o f  mag- nitude larger than IPC. As isoscalar transition the 3.59 MeV M1 + E2 transition in 10B was chosen. The 2 +, 0 state at 3.59 MeV in 10B decays to the 3 +, 0 groundstate with a 19% branching ratio. The transition has a mixing ratio [17] 5(E2/M1) of  1.5 + 0.6 while the parallel 2 +, 0 ~ 1 +, 0 transition of  2.87 MeV is [17] predominantly E2 with 5 - 1  < 0.2. Neglecting internal conversion by atomic electrons, two-photon emission and other higher-order processes, the IPC contributes at the level of  10 -3  compared with gamma rays in the de,excitation of  the 3.59 MeV state. Axion emission would imply a source of  (e+e - )  pairs in addit ion to the expected (e+e - )  pairs from IPC. The energy and angular correlations of  (e+e-)  pairs from the 3.59 MeV transition are ftxed kinemati- cally. In our experimental arrangement the M I and E2 IPC is detected with total  efficiency e of  typically 10 - 4  (see below). The IPC angular correlation be- tween the e ÷ and e -  is maximum at relative angle 0 = 0 °, while in case of  the axions the maximum will occur around 0 = 45 °. The angle between the axes o f  neighbouring Mini-Oranges is 60 ° (fig. 1), which causes an axion decay to be seen with somewhat higher efficiency than IPC; a numerical estimate gave e A ~ 1.5e, assuming that the axions are emitted iso- tropically. The nuclear axion-decay rate (in the case of  existing axions) is predicted in ref. [4] to be as large A e . . / ~ d e t .  target _ - ~ 4 5 v ~  .beam ~Ge detector Fig. 1. Fourfold Mini-Orange arrangement for the axion search. To the left a view of the horizontal plane and to the right a view upstream along the beam axis. The hypothetical axi0n ~ e + + e- decay is supposed to occur inside or closely (at gm distance) be- hind the target layer. In the left part of the figure an e- particle is indicated, while the corresponding e + particle will be detected by a neighbouring e + detector (out of plane). Volume 180, number  1,2 PHYSICS LETTERS B 6 N o v e m b e r  1986 as I a = 0.39 I3,(M 1). The number of  observed pairs can now be writ ten as: N(e+e-)/e = {0.39 X 1.5 + ~ ( M 1 ) )  Lr(M1 ) + otTr (E2)[~ (E2), (3) where e is the efficiency. Theoretical IPC coefficients o t  can be obtained from ref. [18]. I f  axions indeed exist then the first term (= 0.585) will dominate since the others are of  order 10 - 3  . In the experiment the 3.59 MeV state in 10B was excited in inelastic scattering of  7 MeV protons by a 10B target of  1 mg/cm 2. Using a fourfold Mini-Orange system [19] with two detector  systems for e + and two for e -  detection, we registered prompt  (e+e - )  signals and their energy sums. The four spectrometers were set at a backward angle of  45 ° with respect to the incident proton beam. This is shown by a view in the horizontal plane in fig. 1 (left). The axes of  oppo- site Mini-Oranges (with the same polari ty) made an angle of  90 ° with each other,  those of  neighbouring ones (with alternating polari ty)  one of  60 ° . For clarity fig. 1 (right) shows four equal Mini-Orange systems, each composed of  four magnets and the e ÷ and e -  sys- I I I I 1 l ° B ( p , p ' )  Ep = 7 M e V  e ÷ ..- e- SPECTRUM 3.35 MeV 120" 4OCa - 3;059 MeV 4.43 MeV 2.87 MeV I] B 12 C 80 - lo B I~ I l r '  . I 6.05 MeV 40-- ~ / ~ - ,  { u "LP ~ 0 //, I I I I - _ , ,  H 2 3 4 5 6 MeV Z moO2 :::) Ee. ÷Ee.÷ 2 0 U 106-- 8,000f ~lr G A M M A -  RAY SPECTRUM- ~o 5 6,OOOl- ~ A  ~ ~ . . ~  l t 'g'- ~'J l 8 13 MeV 16 0 10 3 I I i 102 -- I I I .-¢ I I I I I 10 2 3 4 5 6 MeV Ey Fig. 2. Sum of  four  e + + e -  pa i r  spec t ra  ( top)  c o m p a r e d  w i t h  a s imu l t aneous ly  r eco rded  ~/-ray s p e c t r u m  ( b o t t o m )  us ing a Ge de-  t ec to r ,  following the l°B (p, p') reaction at 7 MeV. In case of an existing axion with mass of 1.68 MeV and lifetime 5 X 10 -12 s, the prediction by DonneUy et al. [4] corresponds to a peak at 3.59 MeV with height of over 1000 counts. Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 tems being only different by inversion of their mag- netic fields. In reality the present experiment was lim- ited by the available magnets so that two four-gap and two five-gap systems had to be used. Since the latter magnets were relatively weaker, the overall features of the four magnet systems were similar, with maxi- mum transmissions around 1.3 MeV. The Mini-Orange spectrometers were equipped with 300 mm 2 Si(Li) detectors: two with a thickness of 3 mm and two with a thickness of 5 mm. With the KVI data acquisition system (VAX 750) the twofold coin- cidences were registered on magnetic tape for off-line analysis. Fig. 2 (top) displays the essential result: the sum of the (e+e-) pair spectra from the four neighbouring detector combinations in which we can look for a pos- sible axion signal in competition with expected IPC peaks. The (e÷e - )  spectrum is compared with a simul- taneously recorded spectrum of gamma rays (bottom) using a Ge detector. It is apparent that the internal pair peaks of the two isoscalar transitions at 2.87 MeV and 3.59 MeV in 10B are only weakly visible in the (e+e -)  spectrum. Interestingly, two rather prominent peaks at 3.35 and 6.05 MeV show up in the sum spectrum. They have no counterpart in the 3'-ray spectrum and belong to to- tally converted E0 transitions in 40Ca and 160 respec- tively, which are to our knowledge the only two exist- ing E0 transitions in nature depopulating first excited states between 3 and 7 MeV. The spectra of scattered protons, recorded separately, indeed showed a signifi- cant content of 160 (and 12C) impurity in the target. A tracer impurity causes in the pair spectra the ap- pearance of the E0 transition in 40Ca. The 6.05 MeV transition in 160 occurs just above energy threshold and also in the extreme upper transmission range of the Mini-Oranges. Furthermore, it is observed despite the fact that the two 3 mm Si(Li) detectors had insuf- ficient stopping power for most of the e + and e -  pulses. Evidently, the arrangement is hypersensitive for detection of totally converted E0 transitions. The background of the pair spectrum is complex, containing a continuum distribution following 10B(p, a) distributions from pairs created inside the vacuum walls, e + and e -  particles from e.g. the E0(160) transi- tions which pass through the detectors. Yet, the main part of the spectrum belongs to IPC coincidences and not to e.g. Compton scattered 3' rays since the yield of e + + e + and e-  + e -  type combinations is less by an or- der of magnitude than the e + + e -  pairs. In the 3'-ray spectrum all transitions of sufficiently short half-life are Doppler broadened and/or shifted. The 10B 2.87 and 3.59 MeV 3' rays show up strongly. The spectrum also shows a number of 27A1 3" rays produced in the walls of the Mini-Orange vacuum chamber, together with the 4.43 MeV E2 transition in 12C and the 6.13 MeV E3 transition in 160 (target impurities), visible by photo peaks, single- and double-escape peaks. Efficiency calibration for IPC detection was ob- tained as follows: at the low energy side the system was calibrated (i) internally at 1.85 MeV with the 2.87 MeV transition in 10B (> 98% E2) and (ii) externally at 1.733 MeV using the 2.755 MeV E2 transition in 24Na with theoretical [18] IPC's of 7.4 × 10 -4 re- spectively 7.0 X 10 -4. (iii) A comparison of the inten- sity of inelastically scattered protons from the 4.43 MeV level in 12C with that from the 3.59 MeV level in 10B, the ratio being 0.6, showed us that only a frac- tion of the corresponding 3'-ray intensity in 12C is produced in the target and apparently most of it orig- inates in the walls of the chamber. The ratio of 3'-ray intensities was determined to be 0.5 using experimen- tal data for differential and total cross sections for 10B (p,p ')  [20] and 12C (p, p') [21,22] inelastic scat- tering at 7 MeV under 135 °. The obtained gamma-ray intensity was then used together with the internal pair intensity and the theoretical [18] IPC of 1.3 × 10 -3 for an E2 transition of 4.43 MeV to internally determine the efficiency of the Mini-Orange system at 3.41 MeV. All three efficiency calibrations are equal to a value of e = 2.3 X 10 -4 within 35%. The isoscalar 3.59 MeV transition has in the (e+e - )  pair spectrum roughly the same intensity as the paral- lel 2.87 MeV transition used for internal calibration. The branching ratio of the 3.59 MeV pair peak was determined to be (2.1 + 1.1) X 10 -3 which is consis- tent with internal pair creation, %r being 1.0 X 10 -3 for a 28% M1 and 72% E2 transition. The full strength of axion decay with the Donnelly value o f I  a = 0.39 1~ (M1) for an axion of 1.68 MeV would have caused a peak height of over 1000 counts or two orders of mag- nitude !arger than as observed. With 90% confidence our result corresponds with a boundary being 54 times smaller than the value predicted by the standard model [4]. In recent more sophisticated axion models [13,14] the conjectured axion couples to one or two Volume 180, number 1,2 PHYSICS LETTERS B 6 November 1986 quark pairs IN = 1 or 2 in eq. (1)]. In the case of  only two light quarks, no coupling is expected to a magnet- ic isoscalar transition. In the case of coupling to two quark pairs our experiment leads with 90% confidence to a boundary factor of  31 with respect to the theoret- ical prediction in the case of  existing axions. Conse- quently our search yields no evidence for the existence of  a standard (N = 3) or a viable (N = 2) axion. After evaluation of  our experimental  data, we be- came aware of  IPC measurements in the early de- cades of  nuclear physics, when axions were not yet  advocated, but  when (e+e - )  angular correlations were used as a probe for distinguishing multipolarit ies of  high-energy "/-rays. We were both  surprised and im- pressed by the angular correlations reported for the 3.59 MeV transition in the clean reaction 9Be(d, n)10B by Gorodetsky et al. [23]. They used scintillation counters with inherently two orders of  magnitude less energy resolution than our Si(Li) detectors ( though this resolution is partly spoiled by  Doppler broaden- ing), and could hardly separate a strong 3.37 MeV transition in 10B from the 3.59 MeV isoscalar t ransi-  tion in 10B. Yet, their angular distr ibution shows no enhancement near 45 ° where the standard axion would have given at least one order of  magnitude larger signal than measured. Also, a recent reanalysis by Calaprice et al. [24] of  IPC data from Warburton et al. [25] shows no evidence for axion competi t ion in either isoscalar or isovector M1 transitions. The authors wish to thank H.P. Blok, P.F.A. Goudsmit ,  R.H. Siemssen and A.H. Wapstra for valuable discussions. This investigation was sup- ported by the Netherlands Organization for Pure Scientific Research (ZWO) through the Foundat ion for Fundamental  Research on Matter (FOM). References [1] S. Weinberg, Phys. Rev. Lett. 40 (1978) 223. [2] F. Wilczek, Phys. Rev. Lett. 40 (1978) 279. [3] R~D. Peccei and H.R. Quinn, Phys. Rev. Lett. 38 (1977) 1440. [4] T.W. DonneUy et al., Phys. Rev. D 18 (1978) 1607. [5] A. Zehnder, in: Fundamental interactions in low-energy systems (Plenum, New York, 1985). [6] T. Cowa net  al., Phys. Rev. Lett. 54 (1985) 1761. [7] T. Cowan et al., Phys. Rev. Lett. 56 (1986) 444. [8] F.P. Calaprice et al., Phys. Rev. D 20 (1979) 2708. 56 0986) 302. [9] A. Schgfer et al., J. Phys. G 11 (1985) 169. [10] A.B. Balantekin et al., Phys. Rev. Lett. 55 (1985) 461. [11] N. Mukhopadhyay and A. Zehnder, Phys. Rev. Lett. 56 (1986) 206. [12] A. Chodos and L.R.C. Wijewardhama, Phys. Rev. Lett. 56 (1986) 302. [13] R.D. Peccei, Tai Tsun Wu and T. Yanagida, Phys. Lett. B 172 (1986) 435. [14] L.M. Krauss and F. Wilczek, Phys. Lett. B 173 (1986) 189. [15] S.J. Brodsky et al., Phys. Rev. Lett. 56 (1986) 1763. [16] K. Abrahams et al., KVI Annual Report 1985. [17] E.K. Warburton et al., Phys. Rev. 17 (1968) 1178. [18] P. Schliiter and G. Soff, Atom. Data Nucl. Data Tables 24 (1979) 509. [19] J. van Klinken, S.J. Feenstra and G. Dumont, Nucl. In- strum. Methods 151 (1978) 433. [20] B.A. Watson et al., Phys. Rev. 187 (1969) 1351. [21] J.B. Swint et al., Nucl. Phys. 86 (1966) 119. [22] A.C.L. Barnar d et al., Nucl. Phys. 86 (1966) 130. [23] S. Gorodetsky, P. Chevallier, R. Armbruster and G. Sutter, Nucl. Phys. 12 (1959) 349. [24] F.P. Calapriee et al., private communication, and to be published. [25] E.K. Warburton et al., Phys. Rev. 132 (1963) 776,133 (1964) B42, 

Search for short-lived axions in a nuclear isoscalar transition

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