Revisiting sterile neutrino dark matter in gauged U(1)B-L model

We reexamine sterile neutrino dark matter in gauged U(1)B-L model. Improvements have been made by tracing and careful evaluation of the evolution of the number densities of sterile neutrinos N and extra neutral gauge bosons Z'. As a result, the cosmologically-interesting gauge coupling of U(1)B-L for freeze -in sterile neutrinos turns out to be smaller than the values reported in the literature. This avoids the overproduction of Z' so that it is consistent with the big bang nucleosynthesis and the cosmic microwave background constraints on the effective number of neutrino species. Similarly, the free-streaming length constraints exclude a large parameter space derived in previous studies. In addition to known freeze-in pair production of N from the standard model fermion pairs, we find the case that N is dominantly produced from a pair of Z' at the temperature characterized by the B - L breaking scalar mass. Thus, the naive truncation of the U(1)B-L scalar contribution made in the literature is not valid.