Is there a dark decay of neutrons in $^6$He ?

M. Le Joubioux, H. Savajols, W. Mittig, X. Fléchard, Yu. E. Penionzhkevich, D. Ackermann, C. Borcea, L. Caceres, P. Delahaye, F. Didierjean, S. Franchoo, A. Grillet, B. Jacquot, M. Lebois, X. Ledoux, N. Lecesne, E. Liénard, S. Lukyanov, O. Naviliat-Cuncic, J. Piot, A. Singha, V. Smirnov, C. Stodel, D. Testov, J. C. Thomas, D. Verney
Nuclear Experiment, Nuclear Experiment (nucl-ex)
2023-08-30 16:00:00
Motivated by the four standard deviations discrepancy between the mean values for the neutron lifetime obtained from beam and bottle experiments, we have searched for a hypothetical neutron dark decay in $^6$He nuclei through the channel $^6{\rm He} \rightarrow ^4{\rm He}+n+\chi$. The experiment used a 25~keV high intensity $^6$He$^+$ beam with a high efficiency neutron detector. The search for a signal correlated with the $^6$He activity in the neutron detection rate resulted in a branching ratio ${\rm Br}_\chi \leq 4.0\times10^{-10}$ with a 95\% C.L. over the mass window $937.993 < m_\chi < m_n-0.975$ MeV. This result is five orders of magnitude smaller than required to solve the neutron lifetime discrepancy.
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