Relaxation for massive particles: transport and causality

TL;DR

This paper analytically computes all first-order transport coefficients for a massive particle gas in relaxation time approximation, revealing the correlators' complex structure and their relation to causality and lightcone dynamics.

Contribution

It provides the first complete analytic characterization of correlators for massive particles, linking their structure to causal lightcone properties.

Findings

Analytic expressions for all first-order transport coefficients.

Detailed analysis of correlator structure as a function of mass.

Criterion to determine effective lightcone velocity from correlator cuts.

Abstract

Correlators at finite density and temperature encode important information about a physical system, such as transport and causality. For a massive gas of particles in the relaxation time approximation of kinetic theory, we provide complete analytic results for all first order transport coefficients, including thermoelectric coefficients. We demonstrate the first complete picture of the complex structure of the correlators as a function of mass, providing an interpretation in terms of the causal structure of lightcones. We provide a simple criterion to extract the effective lightcone velocity from the cut of the retarded correlator.