Fermion damping in a fermion-scalar plasma - art. no. 105001

In this article we study the dynamics of fermions in a fermion-scalar plasm a. We begin by obtaining the effective in-medium Dirac equation in real tim e which is fully renormalized and causal and leads to the initial value pro blem. For a heavy scalar we find the novel result that the decay of the sca lar into fermion pairs in the medium leads to damping of the fermionic exci tations and their in-medium propagation as quasiparticles. That is, the fer mions acquire a width due to the decay of the heavier scalar in the medium. We find the damping rate to lowest order in the Yukawa coupling for arbitr ary values of scalar and fermion masses, temperature and fermion momentum. An all-order expression for the damping rate in terms of the exact quasipar ticle wave functions is established. A kinetic Boltzmann approach to the re laxation of the fermionic distribution function confirms the damping of fer mionic excitations as a consequence of the induced decay of heavy scalars i n the medium. A linearization of the Boltzmann equation near equilibrium cl early displays the relationship between the damping rate of fermionic mean fields and the fermion interaction rate to lowest order in the Yukawa coupl ing directly in real time. [S0556-2821(99)07604-3].