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

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 $<sup>6$He into $<sup>4$He $+$ $n$ + $\chi$ provides an exceptionally clean detection signature when combined with a high efficiency neutron detector. Using a high-intensity $<sup>6$He$<sup>+$ beam at GANIL, a search for a coincident neutron signal resulted in an upper limit on a dark decay branching ratio of Br$<em>\chi \leq 4.0\times10<sup>{-10}$ (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 $\mathcal{O}(10<sup>{-5})$, improving over global constraints by one to several orders of magnitude depending on $m</em>\chi$.