Off-diagonal kinetic theory in ultrasoft momentum region at high temperature

TL;DR

This paper derives a kinetic equation for ultrasoft fermionic excitations at high temperature in QED and Yukawa models, linking kinetic theory with resummed perturbation theory for ultrasoft momenta.

Contribution

It introduces a linearized kinetic equation for ultrasoft fermions derived from the Kadanoff-Baym equation, connecting it with resummed perturbation theory in high-temperature QED.

Findings

Derived a kinetic equation for ultrasoft fermions in high-temperature QED.

Showed equivalence between kinetic equation and resummed perturbation theory.

Formulated equations for n-point functions with ultrasoft external momenta.

Abstract

We derive a linearized kinetic equation for fermionic excitations with an ultrasoft momentum, g^2T, from the Kadanoff-Baym equation in a Yukawa model and quantum electrodynamics (QED) at extremely high T, where g is the coupling constant. We show that this equation is equivalent to the self-consistent equation in the resummed perturbation theory used in the analysis of the fermion spectrum with the ultrasoft momentum. Furthermore, we derive the equation that determines the n-point function with external lines for a pair of fermions and (n-2) bosons with ultrasoft momenta in QED.