Scale-setting, flavour dependence and chiral symmetry restoration

TL;DR

This paper investigates how the number of active quark flavours affects the strong interaction in QCD, identifying a critical flavour number where chiral symmetry restoration occurs, using Dyson-Schwinger equations and lattice data.

Contribution

It introduces a physical scale setting method to accurately incorporate flavour dependence into the QCD interaction, estimating the critical quark flavour number for chiral symmetry breaking.

Findings

Critical number of active quarks for chiral symmetry breaking: approximately 9.

QCD may become conformal near this critical flavour number.

Provides a unified approach combining Dyson-Schwinger equations and lattice results.

Abstract

We determine the flavour dependence of the renormalisation-group-invariant running interaction through judicious use of both unquenched Dyson-Schwinger equation and lattice results for QCD's gauge-sector two-point functions. An important step is the introduction of a physical scale setting procedure that enables a realistic expression of the effect of different numbers of active quark flavours on the interaction. Using this running interaction in concert with a well constrained class of dressed--gluon-quark vertices, we estimate the critical number of active lighter-quarks above which dynamical chiral symmetry breaking becomes impossible: $n_f^{\rm cr}\approx 9$; and hence in whose neighbourhood QCD is plausibly a conformal theory.