WAVE-FUNCTION RENORMALIZATION AT FINITE-TEMPERATURE - ART. NO. 105023

We present a derivation of the medium dependent wave function renormal ization for a spinor field in the presence of a thermal bath. We show that, as already pointed out in the literature, projector operators ar e not multiplicatively renormalized and the effect involves a nontrivi al spinor dependence, which disappears in the zero temperature covaria nt limit. The results, which differ from what is already found in lite rature, are then applied to the decay of a massive scaler boson into t wo fermions and to the beta-decay and crossed related processes releva nt for primordial nucleosynthesis. [S0556-2821(98)05520-9].