Thermal momentum distribution from path integrals with shifted boundary conditions

TL;DR

This paper introduces a method to compute the distribution of total momentum in thermal field theories using shifted boundary conditions in path integrals, enabling efficient Monte Carlo calculations of thermodynamic quantities.

Contribution

It establishes a novel connection between momentum cumulants and thermodynamic potentials via shifted boundary conditions, facilitating direct Monte Carlo evaluation.

Findings

Successfully computed entropy density in SU(3) Yang-Mills at three temperatures.

Demonstrated the method's effectiveness for extracting thermodynamic quantities.

Linked momentum fluctuations to fundamental thermodynamic properties.

Abstract

For a thermal field theory formulated in the grand canonical ensemble, the distribution of the total momentum is an observable characterizing the thermal state. We show that its cumulants are related to thermodynamic potentials. In a relativistic system for instance, the thermal variance of the total momentum is a direct measure of the enthalpy. We relate the generating function of the cumulants to the ratio of (a) a partition function expressed as a Matsubara path integral with shifted boundary conditions in the compact direction, and (b) the ordinary partition function. In this form the generating function is well suited for Monte-Carlo evaluation, and the cumulants can be extracted straightforwardly. We test the method in the SU(3) Yang-Mills theory and obtain the entropy density at three different temperatures.