Lifetime of quasiparticles in hot gauge theories

The perturbative calculation of the lifetime of charged excitations in ultrarelativistic plasmas is plagued with infrared divergences which are not eliminated by the screening corrections. The physical processes responsible for these divergences are the collisions involving the exchange of longwavelength, quasistatic, magnetic gluons (or photons), which are not screened by plasma effects. In QED, the leading divergences can be resummed in a non-perturbative treatement based on a generalization of the BlochNordsieck model at finite temperature. The resulting expression of the fermion propagator is free of infrared problems, and exhibits a non-exponential damping at large times: SR(t) ∼ exp{−αT t lnωpt}, where ωp = eT/3 is the plasma frequency and α = e /4π. Contributed talk to “Quark Matter’96”, the 12th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions, May 20-24, 1996, Heidelberg, Germany. To appear in Nucl. Phys. A Laboratoire de la Direction des Sciences de la Matière du Commissariat à l’Energie Atomique