Neutron-Mirror-Neutron Oscillation and Neutron Star Cooling

It was pointed out in a recent paper that the observed cooling rate of old, cold neutron stars (NS) can provide an upper limit on the transition rate of neutron to mirror neutron (n - n'). This limit is so stringent that it would preclude any discovery of n -> n' oscillation in the current round of terrestrial searches for the process. Motivated by this crucially important conclusion, we critically analyze this suggestion and note an interesting new effect present in nearly exact mirror models for n -> n' oscillation, which significantly affects this bound. The new element is the beta decay n' -> p' + e' + (v) over bar'(e), which creates a cloud of mirror particles n', p', e', and D' inside the NS core. The e' can "rob" the energy generated by the n -> n' transition via e -> e' scattering enabled by the presence of a (minute) millicharge in mirror particles. This energy is emitted as unobserved mirror photons via fast mirror bremsstrahlung leading to a relaxation of this upper limit.