Classical and Quantum Many-Body Description of Bremsstrahlung in Dense Matter: Landau–Pomeranchuk–Migdal Effect

Abstract

Some considerations about the importance of coherence effects for bremsstrahlung processes in nonequilibrium dense matter (Landau–Pomeranchuk–Migdal effect) are presented. They are of particular relevance for the application to photon and di-lepton production from high energy nuclear collisions, to gluon radiation in QCD transport, or parton kinetics and to neutrino and axion radiation from supernova explosion and from hot neutron stars. The soft behavior of the bremsstrahlung from a source described by classical transport models is discussed and pocket correction formulas for the in-matter radiation cross sections are suggested in terms of standard transport coefficients. The radiation rates are also discussed within a nonequilibrium quantum field theory (Schwinger–Kadanoff–Baym–Keldysh) formulation. A classification of diagrams and corresponding resummation in physically meaningful terms is proposed, which considers the finite damping width of all source particles in matter. This way each diagram in this expansion is already free from infra-red divergences. Both, the correct quasi–particle and quasi-classical limits are recovered from this subset of graphs. Explicit results are given for dense matter in thermal equilibrium. The diagrammatic description may suggest a formulation of a transport theory that includes the propagation of off-shell particles in nonequilibrium dense matter