Csatlos, M.

Csige, L.

De Boer, F. W. N.

Gacsi, Z.

Gulyas, J.

Ketel, T. J.

Krasznahorkay, A.

Nyako, B. M.

Van Klinken, J.

Vitez, A.

English

In a measurement of the angular correlation of e(+)e(-) pairs in the isovector M1 decay from 1(+) level at 17.64 MeV in Be-8, a large deviation was found from quantum electrodynamics(QED)-prediction for internal pair conversion (IPC). By postulating the emission of a neutral particle with a mass of 12 (2.5) MeV/c(2) the structure of the angular correlation can be described.</p

Proceedings - University of Groningen

   University of GroningenAnomalous internal pair creation in Be-8 as a signature of the decay of a new particleVitez, A.; Krasznahorkay, A.; Gulyas, J.; Csatlos, M.; Csige, L.; Gacsi, Z.; Krasznahorkay, A.;Nyako, B. M.; De Boer, F. W. N.; Ketel, T. J.Published 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:2008Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Vitez, A., Krasznahorkay, A., Gulyas, J., Csatlos, M., Csige, L., Gacsi, Z., Krasznahorkay, A., Nyako, B. M.,De Boer, F. W. N., Ketel, T. J., & Van Klinken, J. (2008). Anomalous internal pair creation in Be-8 as asignature of the decay of a new particle. Acta Physica Polonica B, 39(2), 483-487.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: 11-10-2022Vol. 39 (2008) ACTA PHYSICA POLONICA B No 2ANOMALOUS INTERNAL PAIR CREATION IN 8BeAS A SIGNATURE OF THE DECAYOF A NEW PARTICLE∗A. Vitéz, A. Krasznahorkay, J. Gulyás, M. Csatlós, L. CsigeZ. Gácsi, A. Krasznahorkay Jr., B.M. NyakóInstitute of Nuclear Research of the Hungarian Academy of Sciences4028 Debrecen, HungaryF.W.N. de BoerNational Instituut voor Kernfysica en Hoge-Energie Fysica1098 SJ Amsterdam, The NetherlandsT.J. KetelFaculty of Sciences, Vrije Universiteit, 1081 HV Amsterdam, The NetherlandsJ. van KlinkenKernfysisch Versneller Instituut, 9747 AA Groningen, The Netherlands(Received January 14, 2008)In a measurement of the angular correlation of e+e− pairs in the isovec-tor M1 decay from 1+ level at 17.64 MeV in 8Be, a large deviation was foundfrom quantum electrodynamics(QED)-prediction for internal pair conver-sion (IPC). By postulating the emission of a neutral particle with a mass of12 (2.5) MeV/c2 the structure of the angular correlation can be described.PACS numbers: 23.20.Ra, 23.20.En, 14.70.Pw1. IntroductionIf a light exotic particle predicted by supersymmetric models of particlephysics exists, it may be emitted in nuclear transitions. A light and weaklycoupled neutral spin-1 gauge boson U was predicted by Fayet [1] and re-cently revisited by Boehm and Fayet [2]. It was argued by Boehm et al. [3]and recently by Fayet [4] and Beacom [5] that light dark-matter particlesdecaying through such bosons into e+e− pairs may be the source of theobserved 511 keV emission line in the galactic bulge [6].∗ Presented at the XXX Mazurian Lakes Conference on Physics, Piaski, Poland,September 2–9, 2007.(483)484 A. Vitéz et al.In a search for possible signals of short-lived neutral bosons de Boer et al.[7] has found significant deviation from the QED-prediction in the angularcorrelation of the e+e− pairs originated from the decay of the 17.64MeV M1transition in 8Be. The deviation was interpreted as a consequence of thecreation and decay of a neutral boson with a mass of about 9MeV/c2.QED predict [8] that the angular correlation of e+ and e− emitted in IPCdrops rapidly with the separation angle θ. In striking contrast, when thetransition takes place by emission of a short-lived neutral particle decayinginto an e+e− pair, the angular correlation becomes sharply peaked at largerangles. In the center-of-mass system the emission takes place back to back,but in the laboratory system the angle becomes smaller due to the Lorentzboost. Thus by measuring the angular correlation of the e+e− pairs, therelative branch of a particle to internal pairs can be determined.In our previous work [9] we have studied the angular correlations ofe+e− pairs emerging from the 10.96MeV 0− → 0+ and 8.87MeV 2− → 0+transitions in 16O and found some indications also for the 9MeV boson.The aim of the present work was to repeat the experiment performedpreviously by de Boer et al. [7] with better angular resolution, with moreefficient detectors, check the existence of the aforementioned 9MeV bosonand determine their mass and branching ratio more precisely.2. ExperimentIn order to populate the 17.64MeV 1+ state strongly and selectively, weused the reaction 7Li(p, γ)8Be at the Ep = 441 keV resonance. The experi-ments were performed at the 1MV Van de Graaff accelerator of ATOMKI.A 1mg/cm2 LiF target evaporated onto a 10µm thick Al backing was usedwith a beam current of 1 µA.The angular distribution of e+e− pairs was measured using a new pairspectrometer of better angular resolution. The e+e− pairs were detectedby 6 plastic ∆E–E telescopes. ∆E detectors 52 × 52 × 1mm3 are 3 timesthinner than the ones used in Ref. [10], which was expected to give about9 times better γ-ray suppression factor. The E detectors of 80×60×70mm3are similar to the large detectors in Ref. [10]. The telescope detectors wereplaced outside the vacuum chamber made of carbon fibre. The positions ofthe hits were measured by multiwire proportional counters (MWPC) [11]used in front of the ∆E and E detectors. The telescopes were set at 60◦relative to each other in a close geometry.The energy and efficiency calibration of the telescopes was made withrespect to the 6.05MeV transition in 16O excited in the 19F(p, α)16O re-action at the Ep = 840 keV resonance. Extensive Monte Carlo (MC) sim-ulations of the experiment were performed using the GEANT3 code withAnomalous Internal Pair Creation in 8Be as a Signature of . . . 485target chamber, target backing, windows, detector geometries included, inorder to model the detector response. The energy spectra and the efficiencycurve as a function of the separation angle were well reproduced by the MCsimulations.3. ResultsThe results obtained for the angular correlation of the sum of the 14.64and 17.64MeV M1 transitions in 8Be is shown in Fig. 1(a) together with thetheoretical prediction by Rose [8].0.811.21.41.61.8240 60 80 100 120 140Correlation angle Θ (deg.)Yields of pairs/IPC7Li(p,γ)8BeEp=441 keVF.W.N. de Boer (star)present results (full circle)10-210-140 60 80 100 120 140Correlation angle Θ (deg.)Yields of pairs (arb. u.)7Li(p,γ)8BeEp=441 keVRose (solid line)(a) (b)Fig. 1. (a) Measured angular correlation of the e+e− pairs compared with thetheoretical one [8]. (b) Normalized angular correlation compared with the previousexperimental data [7].One can observe excess intensities at small angles and also at large onesaround 110◦. Fig. 1(b) shows the ratio of the experimental/theoretical datatogether with the values published previusly by de Boer et al., [7]. Bothdata sets show similar deviations from unity at small angles, which may beexplained by the effect of the external pair creation (EPC) on the targetframe and on the wall of the target chamber. The deviations at large anglesmight be explained by creation and subsequent decay of a new particle,introduced in Ref. [7]. In order to explain the deviation at large angles wehave made extended MC simulations by assuming particles with differentmasses. Fig. 2(a) shows the difference of the measured and calculatedangular correlations together with the result of a combined MC simulationfor both transitions obtained by assuming a mass of 12 MeV/c2 for thehypotetyc particle. The results of the full χ2 analysis as a function of themass of the assumed particle is shown in Fig. 2(b). The experimental resultscan be explained best if the mass of the particle is 12.0 ± 2.5MeV/c2. The486 A. Vitéz et al.large error is due to the fact that no energy separation has been performedbetween the wide (1.5MeV) 14.64 and the narrow (9 keV) 17.64 sum-peaks.However, it enables us to a valid comparison between the new data with theolder ones, which were also unseparated.particle mass (MeV/c2)χ2/f0.511.522.539 10 11 12 13 14 15 16-1012345650 60 70 80 90 100 110 120 130 140Correlation angle Θ (deg.)Yields of pairs - IPC (arb. u.)7Li(p,γ)8BeEp=441 keV(+ mark)m0=12 MeV/c2(solid line)EPC0+−2(b)(a) m  = 12.0   2.5 MeV/cFig. 2. (a) Difference of the measured and predicted angular correlation comparedwith the MC results obtained for the decay of a new boson. (b) Determination ofthe new particle by the χ2/f method, by comparing the experimental data withthe results obtained for different masses.The obtained mass marginally differs from the previously published value.We are planning to repeat the experiment with thinner target to excite the441 keV resonance more cleanly without the disturbing effects of the nonres-onant capture transitions. We are also planning to modify the setup, to getmore flat efficiency curve as a function of the correlation angle.This work has been supported by the Hungarian OTKA FoundationNo. K72566.REFERENCES[1] P. Fayet, Nucl. Phys. B187, 184 (1981).[2] C. Boehm, P. Fayet, Nucl. Phys. B683, 219 (2004).[3] C. Boehm et al., Phys. Rev. Lett. 92, 101301 (2004).[4] P. Fayet, Phys. Rev. D70, 023514 (2004).[5] J.P. Beacom et al., astro-ph/0409405.[6] J. Knodlseder et al., astro.-ph/0309442.Anomalous Internal Pair Creation in 8Be as a Signature of . . . 487[7] F.W.N. de Boer et al., Phys. Lett. B388, 235 (1996); J. Phys. G 23, L85(1997); J. Phys. G 27 L29 (2001).[8] M.E. Rose, Phys. Rev. 76, 678 (1949).[9] A. Krasznahorkay et al., Acta Phys. Pol. B 37, 239 (2006).[10] K.E. Stiebing et al., J. Phys. G 30, 165 (2004).[11] G. Charpak, F. Sauli, Nucl. Instrum. Methods 162, 405 (1979).

Anomalous internal pair creation in Be-8 as a signature of the decay of a new particle

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Anomalous internal pair creation in Be-8 as a signature of the decay of a new particle

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