Title: Mixed Dark Matter from Sterile Neutrinos
Abstract: Sterile neutrinos are SU(2) singlets that mix with active neutrinos via a mass matrix, its diagonalization leads to mass eigenstates that couple via standard model vertices. We study the cosmological production of heavy neutrinos via standard model charged and neutral current vertices under a minimal set of assumptions: i) the mass basis contains a hierarchy of heavy neutrinos, ii) these have very small mixing angles with the active (flavor) neutrinos, iii) standard model particles, including light (active-like) neutrinos are in thermal equilibrium. The same weak interaction processes that produce active-like neutrinos also produce the heavier species. We introduce the kinetic equations that describe their production, freeze out and decay and discuss the various processes that lead to their production in a wide range of temperatures assessing their feasibility as dark matter candidates. We consider the production of heavy neutrinos from pion decay shortly after the QCD crossover including finite temperature corrections to the pion form factors and mass. We consider the different decay channels that allow for the production of heavy neutrinos showing that their frozen distribution functions exhibit effects from "kinematic entanglement" and argue for their viability as mixed dark matter candidates. We discuss abundance, phase space density and stability constraints and conjecture that those with short lifetimes may undergo cascade decay into lighter DM candidates and/or inject non-LTE neutrinos into the cosmic neutrino background. We provide a comparison with non-resonant production via active-sterile mixing. We also study production of sterile like neutrinos in the temperature range T ~ M_W from vector boson decay in the medium. While sterile neutrinos produced via vector boson decay do not simultaneously satisfy abundance, lifetime and cosmological constraints to be the sole dark matter component, massive sterile-like neutrinos are produced via this mechanism within the parameter range that recently was argued to solve the 7Li problem, although with substantial tension in the parameter space. This production mechanism yields a heavier sterile like neutrino in the MeV mass range, and could decay into lighter keV sterile neutrinos that could contribute to warm dark matter.