LIFETIMES OF QUASI-PARTICLES AND COLLECTIVE EXCITATIONS IN HOT QED PLASMAS

The perturbative calculation of the lifetime of fermion excitations in a QED plasma at high temperature is plagued with infrared divergences which are not eliminated by the screening corrections. The physical p rocesses responsible for these divergences are the collisions involvin g the exchange of long wavelength, quasistatic, magnetic photons, whic h are not screened by plasma effects. The leading divergences can be r esummed in a nonperturbative treatment based on a generalization of th e Bloch-Nordsieck (BN) model at finite temperature. The resulting expr ession of the fermion propagator is free of infrared problems, and exh ibits a nonexponential damping at large times: S-R(t)similar to exp{-a lpha Tt In omega(p)t}, where omega(p)=eT/3 is the plasma frequency and alpha=e(2)/4 pi.