N_f Dependence of the Quark Condensate from a Chiral Sum Rule

TL;DR

This paper investigates how the quark condensate varies with the number of flavors in QCD-like theories using chiral sum rules and perturbation theory, providing bounds and experimental input analysis.

Contribution

It derives a model-independent lower bound for the chiral coupling constant $L_6$ and explores its strong dependence on $N_f$, indicating proximity to a chiral phase transition.

Findings

Derived a sum rule for $L_6$ based on chiral perturbation theory.

Established a lower bound for $L_6$ independent of models.

Found $L_6$ suggests a strong $N_f$ dependence near a chiral phase transition.

Abstract

How fast does the quark condensate in QCD-like theories vary as a function of $N_f$ is inferred from real QCD using chiral perturbation theory at order one-loop. A sum rule is derived for the single relevant chiral coupling-constant, $L_6$. A model independent lower bound is obtained. The spectral function satisfies a Weinberg-type superconvergence relation. It is discussed how this, together with chiral constraints allows a solid evaluation of $L_6$, based on experimental $\pi\pi-K\bar K$ S-wave T-matrix input. The resulting value of $L_6$ is compatible with a strong $N_f$ dependence possibly suggestive of the proximity of a chiral phase transition