Dark decay of the neutron
  New decay channels for the neutron into dark matter plus other particles have
been suggested for explaining a long-standing discrepancy between the neutron
lifetime measured from trapped neutrons versus those decaying in flight. Many
such scenarios are already ruled out by their effects on neutron stars, and the
decay into dark matter plus photon has been experimentally excluded. Here we
explore the decay into a dark Dirac fermion $\chi$ and a dark photon $A'$,
which can be consistent with all constraints if $\chi$ is a subdominant
component of the dark matter. Neutron star constraints are evaded if the dark
photon mass to coupling ratio is $m_{A'}/g' \lesssim (45-60)\,$MeV, depending
upon the nuclear equation of state. $g'$ and the kinetic mixing between U(1)$'$
and electromagnetism are tightly constrained by direct and indirect dark matter
detection, supernova constraints, and cosmological limits.
