Dynamical Renormalization Group Approach to Quantum Kinetics in Scalar and Gauge Theories

TL;DR

This paper develops a dynamical renormalization group method to derive quantum kinetic equations from quantum field theory, enabling the study of relaxation processes in scalar and gauge theories with infrared divergences and non-exponential behavior.

Contribution

It introduces a real-time resummation technique using the dynamical renormalization group to obtain quantum kinetic equations directly from quantum field theory, addressing secular growth and pinch singularities.

Findings

Derived quantum kinetic equations using dynamical renormalization group.

Analyzed relaxation rates of pions and sigma mesons in chiral models.

Identified infrared divergences leading to crossover relaxation behaviors.

Abstract

We derive quantum kinetic equations from a quantum field theory implementing a diagrammatic perturbative expansion improved by a resummation via the dynamical renormalization group. The method begins by obtaining the equation of motion of the distribution function in perturbation theory. The solution of this equation of motion reveals secular terms that grow in time, the dynamical renormalization group resums these secular terms in real time and leads directly to the quantum kinetic equation. We used this method to study the relaxation in a cool gas of pions and sigma mesons in the O(4) chiral linear sigma model. We obtain in relaxation time approximation the pion and sigma meson relaxation rates. We also find that in large momentum limit emission and absorption of massless pions result in threshold infrared divergence in sigma meson relaxation rate and lead to a crossover behavior in relaxation. We then study the relaxation of charged quasiparticles in scalar electrodynamics (SQED). While longitudinal, Debye screened photons lead to purely exponential relaxation, transverse photons, only dynamically screened by Landau damping lead to anomalous relaxation, thus leading to a crossover between two different relaxational regimes. We emphasize that infrared divergent damping rates are indicative of non-exponential relaxation and the dynamical renormalization group reveals the correct relaxation directly in real time. Finally we also show that this method provides a natural framework to interpret and resolve the issue of pinch singularities out of equilibrium and establish a direct correspondence between pinch singularities and secular terms. We argue that this method is particularly well suited to study quantum kinetics and transport in gauge theories.