Search for a neutron dark decay in ⁶He

by Marius Le Joubioux, 3rd year PhD student at GANIL

The neutron lifetime discrepancy of 4σ between « beam » and « bottle » measurements could be interpreted as a sign of a new decay channel into dark particles [1]. If such a decay exists, it could also occur in unstable nuclei with a sufficiently low neutron binding energy S_n < 1.572 MeV [2]. The two-neutrons halo nucleus ⁶He with S_2n = 975.45 keV serves as a good candidate to investigate this model proposed by Fornal and Grinstein. Indeed, a dark decay in ⁶He would result in the emission of a free neutron with the following 3-body decay : ⁶He → ⁴He + n + χ where χ is a dark fermion. This decay mode would be a unique signature of dark matter creation since there is no neutron emission in the standard ⁶He decay scheme. In this presentation, we report on the measurement of an upper limit for this dark decay mode in ⁶He at Brᵪ ≤ 4e-10 with a 95 % confidence level. We will then discuss how this experimental value constrains the mixing parameter ε/(m_n – mᵪ) and how it compares to other experiments.

[1] Bartosz Fornal and Benjamin Grinstein. “Dark Matter Interpretation of the Neutron Decay Anomaly”. In: Phys.Rev. Lett. 120 (19 May 2018), p. 191801. doi: 10.1103/PhysRevLett.120.191801. url: https://link.aps.org/doi/10.1103/PhysRevLett.120.191801. 

[2] M. Pfutzner and K. Riisager. “Examining the possibility to observe neutron dark decay in nuclei”. In:Phys.Rev.C97 (4Apr.2018) ,p.042501.doi:10.1103/PhysRevC.97.042501.url: https://link.aps.org/doi/10.1103/PhysRevC.97.042501