The rainbow modified-ladder approximation and degenerate pion

TL;DR

This paper explores a modified-ladder approximation in the Bethe-Salpeter equation framework, demonstrating its symmetry-preserving properties and applying it to pion properties, with results consistent with fundamental relations.

Contribution

It introduces and justifies a modified-ladder approximation as a symmetry-preserving truncation scheme for Bethe-Salpeter equations in QCD.

Findings

Pion mass and decay constant are degenerate in ladder and modified-ladder approximations.

The modified-ladder approximation maintains the GMOR relation.

The approach provides a consistent framework for studying correlation functions in QCD.

Abstract

Correlation functions can be described by the corresponding equations, $viz.$, gap equation for quark propagator and the inhomogeneous Bethe-Salpeter equation for vector dressed-fermion-Abelian-gauge-boson vertex in which specific truncations have to be implemented. The general vector and axial-vector Ward-Green-Takahashi identities require these correlation functions to be interconnected, in consequence of this, truncations made must be controlled consistently. It turns out that if the rainbow approximation is assumed in gap equation, the scattering kernel in Bethe-Salpeter equation can adopt the ladder approximation, which is one of the most basic attempts to truncate the scattering kernel. Additionally, a modified-ladder approximation is also found to be a possible symmetry-preserving truncation scheme. As an illustration of this approximation for application a treatment of pion is included. Pion mass and decay constant are found to be degenerate in ladder and modified-ladder approximations, even though the Bethe-Salpeter amplitude are with apparent distinction. The justification for the modified-ladder approximation is examined with the help of the Gell-Mann-Oakes-Renner (GMOR) relation.