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New experimental results on the 8Be anomaly
Rare nuclear decays are a promising search channel for new hidden particle species with masses near the 10 MeV scale. Recently, we measured the e+e− angular correlation in internal pair creation for the M1 transition depopulating the 18.15 MeV isoscalar 1+ state in 8Be, and observed a peak-like deviation (anomaly) from the predicted IPC [1]. To the best of our knowledge no nuclear physics related description of such anomaly can be made [2].
The deviation between the experimental and theoretical angular correlations is significant and can be described by assuming the creation and subsequent decay of a boson with mass of m0c2 16.7 MeV. The branching ratio of the e+e− decay of such a boson to the γ decay of the 18.15 MeV level of 8Be is found to be 6x10-6 [1]. We have also investigated the similar 17.65 MeV 1+ → 0+ M1 transition in 8Be, which has isovector character, but no deviation was observed. The anomaly can be explained by a 17 MeV particle, called X(17) in the literature, that is produced in the decay of the excited state to the ground state, and then decays to e+e− pairs.
In order to check the experimental results, recently we reinvestigated the anomaly by using a new Tandetron accelerator of our Institute. The multi-wire proportional counters were replaced with silicon DSSD detectors, as well as the complete electronics and data acquisition system was upgraded. We have re-measured the e+e− angular correlations for the M1 transitions depopulating the 17.64 MeV and 18.15 MeV 1+ states in 8Be, and within the error bars the previous data were reproduced.
I am going to speak about our most recent results and plans, as well as the possible interpretations of the results [3-5].

[1] A.J. Krasznahorkay et al., Phys. Rev. Lett. 116 042501 (2016)
[2] Xilin Zhang and Gerald A. Miller, Phys. Lett. B773 (2017) 159
[3] J. Feng et al., Phys. Rev. Lett. 1 17, 071803 (2016)
[4] Jonathan Kozaczuk, David E. Morrissey, and S. R. Stroberg, Phys. Rev. D 95, (2017) 115024.
[5] Daniele S. M. Alves and Neal Weiner, arXiv:1710.03764v1
Attila J. Krasznahorkay
Institute for Nuclear Research, Hungarian Academy of Sciences, MTA Atomki, Debrecen
24/11/2017 à 11:00
IPN Orsay, salle des Conseils, bâtiment 100

Notes de dernières minutes :

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Institut de Physique Nucléaire Orsay - 15 rue Georges CLEMENCEAU - 91406 ORSAY (FRANCE)
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