CLASSICAL AND QUANTUM MANY-BODY DESCRIPTION OF BREMSSTRAHLUNG IN DENSE MATTER - LANDAU-POMERANCHUK-MIGDAL EFFECT

Some considerations about the importance of coherence effects for brem sstrahlung processes in nonequilibrium dense matter (Landau-Pomeranchu k-Migdal effect) are presented. They are of particular relevance for t he application to photon and di-lepton production from high energy nuc lear collisions, to gluon radiation in QCD transport, or parton kineti cs and to neutrino and axion radiation From supernova explosion and fr om hot neutron stars. The soft behavior of the bremsstrahlung from a s ource described by classical transport models is discussed and pocket correction formulas for the in-matter radiation cross sections are sug gested in terms of standard transport coefficients. The radiation rate s are also discussed within a nonequilibrium quantum field theory (Sch winger-Kadanoff-Baym-Keldysh) formulation. A classification of diagram s and corresponding resummation in physically meaningful terms is prop osed, which considers the finite damping width of all source particles in matter. This way each diagram in this expansion is already Gee fro m infra-red divergences. Both, the correct quasi-particle and quasi-cl assical limits are recovered from this subset of graphs. Explicit resu lts are given for dense matter in thermal equilibrium. The diagrammati c description may suggest a formulation of a transport theory that inc ludes the propagation of off-shell particles in nonequilibrium dense m atter. (C) 1996 Academic Press, Inc.