Mass corrections to the hard thermal/dense loops

TL;DR

This paper calculates how a small fermion mass affects the photon polarization tensor in high-temperature or dense electromagnetic plasmas, showing the importance of mass corrections in certain regimes.

Contribution

It introduces a method to compute mass corrections to hard thermal/dense loops using effective field theory and transport theory, demonstrating their significance relative to other corrections.

Findings

Mass corrections are comparable to two-loop corrections for soft masses.

Infrared divergencies cancel out when using dimensional regularization.

Mass corrections dominate when the fermion mass is between eT and T.

Abstract

We compute corrections to the hard thermal (or dense) loop photon polarization tensor associated to a small mass $m$ of the fermions of an electromagnetic plasma at high temperature $T$ (or chemical potential $\mu$). To this aim we use the on-shell effective field theory, amended with mass corrections. We also carry out the computation using transport theory, reaching to the same result. Interemediate steps in the computations reveal the presence of potential infrared divergencies. We use dimensional regularization, as it is respectful with the gauge symmetry, and then show that all infrared divergencies cancel in the final result. We compare the mass corrections with both the power and two-loop corrections, and claim that they are equally important if the mass is soft, that is, of order $e T$ (or $e \mu$), where $e$ is the gauge coupling constant, but are dominat if the mass obeys $ e T < m \ll T $ (or $e \mu < m \ll \mu)$.