Charged-Particle Decay at Finite Temperature
  Radiative corrections to the decay rate of charged fermions caused by the presence of a thermal bath of photons are calculated in the limit when temperatures are below the masses of all charged particles involved. The cancellation of finite-temperature infrared divergences in the decay rate is described in detail. Temperature-dependent radiative corrections to a two-body decay of a hypothetical charged fermion and to electroweak decays of a muon are given. We touch upon possible implications of these results for charged particles in the early Universe. 
Andrzej Czarnecki, Marc Kamionkowski, Samuel K. Lee, and Kirill   Melnikov
hep-ph astro-ph.CO