Non perturbative renormalization group and momentum dependence of n-point functions (II)

TL;DR

This paper advances a non-perturbative renormalization group method to compute n-point functions with momentum dependence, improving accuracy at next-to-leading order and matching lattice data closely.

Contribution

It extends a previous approximation scheme to next-to-leading order, addressing exceptional momentum configurations and enhancing the calculation of self-energy and critical temperature shifts.

Findings

Next-to-leading order results significantly improve accuracy.

Self-energy calculations agree within 4% of large N results.

Method matches lattice data closely for Bose-Einstein condensation.

Abstract

In a companion paper (hep-th/0512317), we have presented an approximation scheme to solve the Non Perturbative Renormalization Group equations that allows the calculation of the $n$-point functions for arbitrary values of the external momenta. The method was applied in its leading order to the calculation of the self-energy of the O($N$) model in the critical regime. The purpose of the present paper is to extend this study to the next-to-leading order of the approximation scheme. This involves the calculation of the 4-point function at leading order, where new features arise, related to the occurrence of exceptional configurations of momenta in the flow equations. These require a special treatment, inviting us to improve the straightforward iteration scheme that we originally proposed. The final result for the self-energy at next-to-leading order exhibits a remarkable improvement as compared to the leading order calculation. This is demonstrated by the calculation of the shift $\Delta T_c$, caused by weak interactions, in the temperature of Bose-Einstein condensation. This quantity depends on the self-energy at all momentum scales and can be used as a benchmark of the approximation. The improved next-to-leading order calculation of the self-energy presented in this paper leads to excellent agreement with lattice data and is within 4% of the exact large $N$ result.