Right handed neutrino production in dense and hot plasmas

For Dirac neutrinos with magnetic moment, we compute the production rate for right-handed neutrinos in a hot and dense QED plasma containing an initial population of left-handed neutrinos thermally distributed. The most important mechanisms for $\nu_L$ depolarization, or production of right-handed neutrinos, are the $\nu_L \to \nu_R$ chirality flip and the plasmon decay to $\bar{\nu}_L + \nu_R$. The rates for these processes are computed in terms of a resummed photon propagator which consistently incorporates the background effects to leading order. Applying the results to the cases of supernovae core collapse and the primordial nucleosynthesis in the early universe, we obtain upper limits on the neutrino magnetic moment.