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

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 6He into 4He+n+χ provides an exceptionally clean detection signature when combined with a high efficiency neutron detector. Using a high-intensity 6He+ beam at Grand Accélérateur National d’Ions Lourds, a search for a coincident neutron signal resulted in an upper limit on a dark decay branching ratio of Brχ≤4.0×10−10 (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 O(10−5), improving over global constraints by one to several orders of magnitude depending on mχ.