Search for the Neutron Decay n→X+γ, Where X is a Dark Matter Particle.

Fornal and Grinstein recently proposed that the discrepancy between two different methods of neutron lifetime measurements, the beam and bottle methods, can be explained by a previously unobserved dark matter decay mode, n -> X + gamma We perform a search for this decay mode over the allowed range of energies of the monoenergetic gamma ray for X to be dark matter. ACompton-suppressed high-purity germanium detector is used to identify gamma rays from neutron decay in a nickel-phosphorous-coated stainless-steel bottle. A combination of Monte Carlo and radioactive source calibrations is used to determine the absolute efficiency for detecting gamma rays arising from the dark matter decay mode. We exclude the possibility of a sufficiently strong branch to explain the lifetime discrepancy with 97% confidence.