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 6He nuclei through the
channel 6He→4He+n+χ. The experiment used a
25~keV high intensity 6He+ beam with a high efficiency neutron detector.
The search for a signal correlated with the 6He activity in the neutron
detection rate resulted in a branching ratio Brχ≤4.0×10−10 with a 95\% C.L. over the mass window 937.993<mχ<mn−0.975 MeV. This result is five orders of magnitude smaller than required
to solve the neutron lifetime discrepancy