Pressure to order $g^8*log(g)$ in $\phi^4$-theory at weak coupling

TL;DR

This paper computes the pressure of massless $^4$-theory up to order $g^8 \log(g)$ at weak coupling, employing effective field theory and dimensional reduction to separate and evaluate contributions from different momentum scales.

Contribution

It advances the calculation of thermodynamic pressure in $^4$-theory to higher order, including $g^8 \log(g)$, using novel multi-loop sum-integrals and renormalization group techniques.

Findings

Pressure calculated up to order $g^8 \\log(g)$ at weak coupling.

Effective field theory separates hard and soft contributions.

Results improve understanding of perturbative series convergence.

Abstract

We calculate the pressure of massless $\phi^4$-theory to order $g^8\log(g)$ at weak coupling. The contributions to the pressure arise from the hard momentum scale of order $T$ and the soft momentum scale of order $gT$. Effective field theory methods and dimensional reduction are used to separate the contributions from the two momentum scales: The hard contribution can be calculated as a power series in $g^2$ using naive perturbation theory with bare propagators. The soft contribution can be calculated using an effective theory in three dimensions, whose coefficients are power series in $g^2$. This contribution is a power series in $g$ starting at order $g^3$. The calculation of the hard part to order $g^6$ involves a complicated four-loop sum-integral that was recently calculated by Gynther, Laine, Schr\"oder, Torrero, and Vuorinen. The calculation of the soft part requires calculating the mass parameter in the effective theory to order $g^6$ and the evaluation of five-loop vacuum diagrams in three dimensions. This gives the free energy correct up to order $g^7$. The coefficients of the effective theory satisfy a set of renormalization group equations that can be used to sum up leading and subleading logarithms of $T/gT$. We use the solutions to these equations to obtain a result for the free energy which is correct to order $g^8\log(g)$. Finally, we investigate the convergence of the perturbative series.