Calculation of the one loop box integral at Finite Temperature and Density

TL;DR

This paper derives and provides numerical formulas for the one-loop box integral at finite temperature and density, crucial for calculations in hadronization and scattering processes within NJL-like models.

Contribution

It introduces general formulas for the one-loop box integral at finite temperature and density, including cases with multiple fermion lines and collinear momenta, correcting previous results.

Findings

Explicit real and imaginary parts of the box integral are obtained.

Formulas are applicable for any temperature, mass, and chemical potential.

Constraints on particle momenta for integral existence are established.

Abstract

Calculation of hadronization, decay or scattering processes at non-zero temperatures and densities within the Nambu-Jona-Lasinio-like models requires some techniques for computation of Feynmann diagrams. Decomposition of Feynman diagrams at the one loop level leads to the appearance of elementary integrals with one, two, three, and four fermion lines. For example, evaluation of the $\pi\pi$ scattering amplitude requires calculating a box diagram with four fermion lines. In this work, the real and imaginary parts of the box integral at the one loop level are provided in the form suitable for numerical evaluation. The obtained expressions are applicable to any value of temperature, particle mass, and chemical potential. We pay special attention to the conditions for the existence of the appearing improper integrals and correct the results \cite{Klevansky} for the three fermion lines. As a result, we have obtained constraints on possible values of particle momenta. Among the expressions for the box integral, the general formulas for the integral with an arbitrary number of lines are derived for the case with zero or collinear fermion momenta.