Neutron decay anomaly, neutron stars, and dark matter

Abstract

The discrepancies in different measurements of the lifetime of isolated neutrons could be resolved by considering an extra neutron decay channel into dark matter, with a branching ratio of the order of Oð1%Þ. Although the decay channel into a dark fermion χ plus visible matter has already been experimentally excluded, a dark decay with either a scalar or dark photon in the final state still remains a possibility. In particular, a model with a fermion mass mχ ≈ 1 GeV and a scalar mϕ ≈ OðMeVÞ could provide not only the required branching ratio to explain the anomaly but also a good dark matter (DM) candidate with the right thermal abundance today. Although the interaction DM neutron will affect the formation of neutron stars, the combined effect of the dark matter self-interactions mediated by the light scalar and an effective repulsive interaction with the neutrons induced by the scalar-Higgs coupling would allow heavy enough neutron stars. Combining the constraints from neutron lifetime, dark matter abundance, neutron stars, Higgs physics, and big bang nucleosynthesis, we can restrict the light scalar mass to be in the range 2me < mϕ < 2me þ 0.0375 MeV.