The effective electron mass in core-collapse supernovae
  Finite temperature field theory is used to calculate the correction to the mass of the electron in plasma with finite temperature and arbitrary chemical potential, and the results are applied to the core regions of type II supernovae (SNe). It is shown that the effective electron mass varies between 1 MeV at the edge of the SN core up to 11 MeV near the center. This changed electron mass affects the rates of the electroweak processes which involve electrons and positrons. Due to the high electron chemical potential, the total emissivities and absorptivities of interactions involving electrons are only reduced a fraction of a percent. However, for interactions involving positrons, the emissivities and absorptivities are reduced by up to 20 percent. This is of particular significance for the reaction antineutrino + proton <-> positron + neutron which is a source of opacity for antineutrinos in the cores of type II SNe. 
Stephen J. Hardy