2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/202660We examine the relation between the damping rate of a massless, chiral fermion that propagates in a medium, and the rate $Γ$ 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 $Γ$ is defined by using the appropriate wave functions of the mode.Version to appear in Phys. Rev. DHigh Energy Physics - Phenomenologytext