arXiv:hep-ph/9609369v1 17 Sep 1996 DAMPING RATE OF QUASIPARTICLES IN DEGENERATE ULTRARELATIVISTIC PLASMAS Michel Le Bellac Institut Non Lin´ eaire de Nice UMR 129 1361 Route des Lucioles 06560 Valbonne (FRANCE) and, Cristina Manuel Dpt. Estructura i Constituents de la Mat` eria Facultat de F´ ısica, Universitat de Barcelona Diagonal 647, 08028 Barcelona (SPAIN) Abstract We compute the damping rate of a fermion in a dense relativistic plasma at zero temperature. Just above the Fermi sea, the damping rate is dominated by the exchange of soft magnetic photons (or gluons in QCD) and is proportional to (E − μ), where E is the fermion energy and μ the chemical potential. We also compute the contribution of soft electric photons and of hard photons. As in the nonrelativistic case, the contribution of longitudinal photons is proportional to (E − μ) 2 , and is thus non leading in the relativistic case. PACS No: 11.10.Wx, 12.20.-m, 12.20.Ds, 52.60.+h ECM-UB-PF-96/15, INLN 96/19 September/1996