Search for a neutron dark decay in $^6$He

TL;DR

This study searches for neutron dark decays in $^6$He, setting stringent upper limits on the decay branching ratio and providing new constraints that improve previous bounds by several orders of magnitude.

Contribution

The paper presents the first experimental search for neutron dark decays in halo nuclei using $^6$He, establishing significantly tighter upper limits on the decay branching ratio.

Findings

Upper limit on dark decay branching ratio: Br_χ ≤ 4.0×10^{-10} (95% C.L.)

Translated bounds on dark neutron branching ratio: O(10^{-5})

Improved constraints over previous global bounds by orders of magnitude

Abstract

Neutron dark decays have been suggested as a solution to the discrepancy between bottle and beam experiments, providing a dark matter candidate that can be searched for in halo nuclei. The free neutron in the final state following the decay of $^6$He into $^4$He $+$ $n$ + $\chi$ provides an exceptionally clean detection signature when combined with a high efficiency neutron detector. Using a high-intensity $^6$He$^+$ beam at GANIL, a search for a coincident neutron signal resulted in an upper limit on a dark decay branching ratio of Br$_\chi \leq 4.0\times10^{-10}$ (95\% C.L.). Using the dark neutron decay model proposed originally by Fornal and Grinstein, we translate this into an upper bound on a dark neutron branching ratio of $\mathcal{O}(10^{-5})$, improving over global constraints by one to several orders of magnitude depending on $m_\chi$.