Csatlos, M

Csige, L

De Boer, FWN

Gacsi, Z

Gulyas, J

Hunyadi, M

Ketel, TJ

Krasznahorkay, A

Van Klinken, J

Vitez, A

English

Electron-positron pairs have been observed in the 10.95 MeV 0(-) -&gt; 0(+) decay in O-16. This magnetic monopole (M0) transition cannot proceed by gamma-ray decay and is, to first order, forbidden for internal pair creation. However, the transition may also proceed by the emission of a light neutral 0(-) or 1(+) boson, which might play a role in the current quest for light dark matter in the Universe.</p

ARTS repository - University of Groningen

   University of GroningenLepton pairs from a forbidden M0 transitionKrasznahorkay, A; De Boer, FWN; Csatlos, M; Csige, L; Gacsi, Z; Gulyas, J; Hunyadi, M;Ketel, TJ; Van Klinken, J; Krasznahorkay, APublished in:Acta Physica Polonica BIMPORTANT 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:2006Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Krasznahorkay, A., De Boer, FWN., Csatlos, M., Csige, L., Gacsi, Z., Gulyas, J., Hunyadi, M., Ketel, TJ.,Van Klinken, J., Krasznahorkay, A., & Vitez, A. (2006). Lepton pairs from a forbidden M0 transition:Signaling an elusive light neutral boson? Acta Physica Polonica B, 37(1), 239-244.CopyrightOther 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: 21-06-2022Vol. 37 (2006) ACTA PHYSICA POLONICA B No 1LEPTON PAIRSFROM A FORBIDDEN M0 TRANSITION:SIGNALING AN ELUSIVE LIGHT NEUTRAL BOSON?∗A. Krasznahorkaya , F.W.N. de Boerb, M. Csatlósa, L. CsigeaZ. Gácsia, J. Gulyása, M. Hunyadia, T.J. Ketelc, J. van KlinkendA. Krasznahorkay Jra, A. VitézaaInstitute of Nuclear Research of the Hungarian Academy of SciencesP.O. Box 51, 4001 Debrecen, HungarybNational Instituut voor Kernfysica en Hoge-Energie Fysica1098 SJ Amsterdam, The NetherlandscDepartment of Physics and Astronomy of the Free Universityde Boelelaan 1081A, 1081 HV Amsterdam, The NetherlandsdKernfysisch Versneller Instituut (KVI), 9747 AA Groningen, The Netherlands(Received November 24, 2005)Electron–positron pairs have been observed in the 10.95 MeV 0− → 0+decay in 16O. This magnetic monopole (M0) transition cannot proceedby γ-ray decay and is, to first order, forbidden for internal pair creation.However, the transition may also proceed by the emission of a light neutral0− or 1+ boson, which might play a role in the current quest for light darkmatter in the Universe.PACS numbers: 23.20.Ra, 23.20.En, 14.70.Pw1. IntroductionWhen an excited nuclear level does not decay by γ radiation or by con-ventional internal pair creation (IPC) [1], an emission of a light neutralparticle which in turn decays into an e+e− pair is still a possible form of thedeexcitation. Such a light boson might be identified with a light and veryweakly coupled neutral spin-1 gauge boson U , introduced by Fayet [2] morethan two decades ago and revisited by Boehm and Fayet [3].∗ Presented at the XXIX Mazurian Lakes Conference on PhysicsAugust 30–September 6, 2005, Piaski, Poland.(239)240 A. Krasznahorkay et al.In 1997 de Boer et al. [4] observed some deviations between the mea-sured and predicted IPC angular correlations of e+e− in isoscalar magnetictransitions and explained it as a consequence of the formation and decay ofa short lived isoscalar neutral boson with spin-parity (pseudo scalar) 0− or(axial vector) 1+ and with a mass of about 9 MeV/c2.The possibility of the existence of such 9MeV/c2 boson provided themotivation to investigate the 10.95MeV 0− → 0+ g.s. electromagneticallyforbidden transition in 16O. Earlier, Eklund and Bent [5] and Alburger [6]searched for that transition by measuring the e+e− decay of the 10.95 MeVstate but a really sensitive search for those transitions remained to be made.2. Experimental methodsIn order to populate strongly and selectively the 0− state, we used the14N(3He, p)16O reaction at E(3He) = 2.4MeV [7], and performed the ex-periment in coincidence with the proton group populating the 0− state.TaN target produced at the KVI with an approximate thickness of 4mg/cm2on a 5µm thick Ta backing was used, allowing a beam current of 0.5 µA with-out considerable target damage during the course of the experiment. TheTa backing served also as an absorber for the 3He beam particles.A plastic scintillator disk with a diameter of 45mm and a thickness of6mm was used behind the target to get large solid angle and high count-rate capabilities for detecting the protons. A solid angle of about 2 sr andan energy resolution of about 1MeV was achieved for protons with sucha detector when populating the 0− state.For high-statistics measurements of the e+e− pairs, ∆E − E detectortelescopes from the IKF spectrometer constructed by Stiebing et al. [8] withlarge solid angles were used. The two largest telescopes (∆E detector:38 × 40 × 1mm3 and E detector: 80 × 60 × 70mm3) telescopes were setat 90 degrees. In order to increase our sensitivity for searching the boson,we improved the angular resolution by inserting a standard MWPC detectorbetween the ∆E and E detectors of the telescopes.The x, y position of the hits could be measured with an accuracy of5mm, which results in an angular resolution with FWHM = 8.5◦ betweenthe electrons and positrons in the 60–120◦ angular range. The actual angularresolution of the telescopes was somewhat worse because of the scatteringson the plastic ∆E detectors.The target was inclined by an angles of 45 degrees with respect to thebeam direction. The telescope detectors were placed outside the vacuumchamber. To minimize the amount of the material around the target, a 24 cmlong electrically conducting carbon fiber tube with a radius of 3.5 cm anda wall thickness of 0.8mm, as well as thin aluminum target frames held bythin rods were used.Lepton Pairs from a Forbidden M0 Transition: Signaling . . . 241The γ-rays were also measured in coincidence with the protons. A highlyefficient (120 %) clover type Ge detector equipped with a BGO anticoinci-dence shield [9] was used perpendicular to the beam and at a distance of25 cm from the target.3. ResultsFig. 1. shows a γ-ray and an e+ + e− sum spectrum measured in coin-cidence with the protons populating the 10.95MeV 0− state. The 3.84MeVγ-ray transition as well as a less intense e+ − e− line at 10.95MeV depop-ulating the 0− state can be clearly seen, while no γ-rays could be observedat 10.95MeV.ChannelsCounts/channel3.84 MeV6.057.1210.95 0- → 0+6.05 MeV gate10.95 MeV gate11010 210 3100 200 300 400 500 60011010 210 34 6 8 10 12Eγ (MeV)Counts/channel10.95 MeV gate4.439 MeV C123.840 MeV4.179 MeV4.914 MeV6.129 MeVSE6.915 MeVSE7.115 MeVSEO160+ 00+ 6.053- 6.132+ 6.921- 7.122- 8.871- 9.60  α2+ 9.85  α0- 10.95  5.5 fs4+ 11.094.914.183.849.582.698.877.126.926.136.05b)a)Fig. 1. γ-ray (a) and internal pair creation (b) spectra obtained from the14N(3He,p)16O reaction. See text for details.The energy calibration of the setup was performed with pairs of the6.05MeV transition and of the 4.44MeV and 15.1MeV transitions measuredin the 10B(3He, p)12C reaction with the same setup. The insert in the γspectrum of Fig. 1(a) shows a level scheme of 16O up to 11MeV, presentingthe relevant γ transitions, which are also indicated in the γ spectrum. Nogamma peak is observed above 9MeV.In the e++e− sum energy spectrum with an energy resolution of FWHM≈ 1.2MeV for the 6.05MeV transition a peak is observed at about 11MeV,which corresponds to the 0− → 0+10.95MeV transition in 16O.The γ-ray background in the E detectors originating from the targetis very well suppressed by requiring ∆E − E coincidences in addition tothe coincidence between two telescopes. In 16O the 3.84MeV (M1) and7.12MeV (E1) γ−γ cascade depopulating the 10.95MeV state may produce242 A. Krasznahorkay et al.high energy electrons by Compton scattering in both ∆E detectors. Theseevents will produce similar coincidence signals as the e+e− pairs. Theirenergy sums add up to a broad spectrum with a sharp edge at 10.95MeVand thus simulate the IPC process.In order to estimate the contribution of such kind of background, mea-surements were performed by using ∆E detectors with 3mm thicknesses in-stead of the 1-mm ones. With the thicker ∆E detectors we expected 9 timeslarger contributions from Compton–Compton coincidences. The results ofthese measurements confirmed our hypothesis that Compton–Compton co-incidences contribute to the background. This kind of background drops atabout 1 MeV above the e+e− peak allowing a reliable background estimation.As shown in Fig. 1, the e+e− spectrum was fitted with an exponentiallyfalling background and using 3 Gaussians corresponding to the 6.05MeVtransition, the peaks around 7MeV and the 10.95MeV e+e−-line. The inten-sity ratio obtained for the 10.95MeV and 7MeV lines is: Re = I(10.95MeV)/I(7MeV)= 0.045 ± 0.012.The 7MeV lines contain the 7.12MeV transition, which perfectly mea-sures the excitation of the 10.95-MeV state as it decays by 100 % branchingratio to that level, and also the 6.92MeV, 2+ → g.s. transition.The intensity ratio of the 6.92 and the 7.12MeV γ-lines was deducedfrom the γ-spectrum: R = I(7.12)/I(6.92) = 1.20. The branching ratio Bof the e+e− decay was obtained for the 0− state as:B =ReRα(6.92) + Re α(7.12) = (2 ± 0.5) × 10−4 , (1)where α(7.12) and α(6.92) denote the theoretical internal pair creation co-efficients [10] of 2.56 × 10−3 and 1.96 × 10−3 for the 7.12MeV E1 and the6.92MeV E2 transitions, respectively. As the lifetime of the 0− state isknown [11] to be 8 ± 5 fs, the partial lifetime of the e+e− transition will be(4 ± 2) × 10−11 s. This lifetime is comparable to the lifetime of a second-order electromagnetic process and in this sense agrees with the estimate fora pseudoscalar boson.Assume that a short-lived boson with mass mu is created in such a nu-clear transition with transition energy E and decays into an e+e− pair. Inthe center of mass system of the boson the e+ and e− are emitted under180◦. In the laboratory system the opening angle will be smaller than 180◦and depends on the e+ and e− energies. By assuming an isotropic emissionof the e+e− pairs in the c.m. system, the angular correlation of the pairscan be calculated. The correlation has always a sharp maximum (FWHM≤ 2◦) and the invariant mass mx is determined by the scattering angle Θ,measured in the laboratory system as follows [4]:Lepton Pairs from a Forbidden M0 Transition: Signaling . . . 243m2u≈ (1 − y2)E2 sin2(Θ2), (2)where the transition energy is calculated as E = E+ + E− + 1.022 MeVin which, E+(−) denotes the kinetic energy of the positron (electron) andy = (E+ − E−)/(E+ + E−) is the disparity, all is given in the laboratorysystem. As an example, for the 10.95MeV transition and a 9MeV/c2 bosonwe find a correlation angle Θ of about 111◦. By measuring the angularcorrelation pattern of the e+e− pairs the mass of the new boson can inprinciple be deduced.Figs. 2 (a) and (b) show the results of the angular correlation measure-ments obtained for the 6.05MeV and for the 10.95MeV transitions, respec-tively. As the angular correlation for the 6.05MeV E0 transition is varyingsmoothly (almost linearly) in the investigated angular range Fig. 2(a) showsthe effective solid angle of the telescopes as a function of the correlation an-gles. In Fig. 2(b) the counts have already been corrected for the effectivesolid angle of the telescopes determined from Fig. 2(a).As the angular correlation pattern expected from the decay of a bosonis sharper than the angular resolution of our telescopes, we expect a Gaus-sian like distribution with a width determined by the experimental angularresolution. Using a Gaussian with free parameters, we fit the experimentalangular correlation. The result is shown in Fig. 2(b) with solid curve. Weobtained Θ ≈ 103◦ from the fit, which gives mu ≈ 8.6MeV/c2 for the massof the boson.2000400060008000100001200075 80 85 90 95 100 105 110 115Ne+e-b)Θ (deg.)024681012141675 80 85 90 95 100 105 110 115Ne+e-Θ (deg.)a) b)Fig. 2. The angular correlation of the e+e− pairs obtained from the decay of the6.05 MeV (a) and from the 10.95 MeV (b) transitions in 16O.244 A. Krasznahorkay et al.However, as shown in Fig. 2(b) the angular correlation can also be fitted(with about the same χ2/f) with a linear function. According to a newexperiment performed very recently with 3 telescopes covering an angularrange of 60◦ < Θ < 170◦ we observed a more pronounced peak at Θ = 130◦on a slowly rising background (Fig. 2(b)), which might be explained byassuming a boson with a mass of mu ≈ 10MeV/c2.4. ConclusionWe observed for the first time the 10.95 MeV 0− → 0+ decay in 16O bymeasuring e+e− pairs. The energy sum of the pairs corresponds to the energyof the transition (10.95MeV), and the branching ratio B = (20 ± 5) × 10−5agrees with that of the expected boson. The angular correlation of the pairsmight be described by assuming the decay of a boson with a mass of mu ≈9–10MeV/c2. New experiments with better statistics are needed to settlethe question and to determine the mass of the boson more precisely.The authors acknowledge H. Fraiquin (KVI, Groningen) for making theTaN target and the Van de Graaff generator staff at Atomki for their supportduring the course of the experiment. This work has been supported by theHungarian OTKA Foundation No.T038404.REFERENCES[1] M.E. Rose, Phys. Rev. 76, 678 (1949).[2] P. Fayet, Nucl. Phys. B187, 184 (1981).[3] C. Boehm, P. Fayet, Nucl. Phys. B683, 219 (2004).[4] F.W.N. de Boer et al., J. Phys. G: Nucl. Part. Phys. 23, L85 (1997); Nucl.Phys. B72, 189 (1999); J. Phys. G: Nucl. Part. Phys. 27, L29 (2001).[5] K.E. Eklund, R.D. Bent, Phys. Rev. 112, 488 (1958).[6] D.A. Alburger, Phys. Rev. C18, 576 (1978).[7] D.A. Bromley et al., Phys. Rev. 114, 758 (1959).[8] K.E. Stiebing et al., J. Phys. G: Nucl. Part. Phys. 30, 165 (2004).[9] Z. Elekes et al., Nucl. Instrum. Methods 503, 580 (2003).[10] P. Schlüter, G. Soff, At. Data Nucl. Data Tables 24, 509 (1979).[11] M.C. Bertin, R.E. Pixley, Nucl. Phys. A150, 247 (1970).

Lepton pairs from a forbidden M0 transition:Signaling an elusive light neutral boson?

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Lepton pairs from a forbidden M0 transition:Signaling an elusive light neutral boson?

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