Damping Rate of a Fermion in a Medium

We examine the relation between the damping rate of a massless, chiral fermion that propagates in a medium, and the rate $\Gamma$ of approach to equilibrium. It is proven that these quantities are equal, by showing that they are given by the same formula in terms of the imaginary part of the self-energy evaluated at the energy of the propagating fermion mode. This result is valid provided $\Gamma$ is defined by using the appropriate wave functions of the mode.