Neutrino gravitational decay in a medium

Various decay processes that are forbidden in the vacuum by a combination of requirements such as angular momentum conservation and Lorentz invariance, can occur if the decaying particle is propagating in a medium, even if the medium is homogeneous and isotropic. Here we consider specifically the decay of a neutrino into another neutrino and a graviton, in a thermal background of electrons. The gravitational transition amplitude and the rate for the gravitational decay of the neutrino in such environments is determined. Although the results simulate Lorentz symmetry violating effects, in this case they arise from completely Lorentz-invariant Standard Model physics.