Extended non-chiral quark models confronting QCD

TL;DR

This paper explores how non-chiral quark models, especially quasilocal variants, can effectively approximate low-energy QCD dynamics, emphasizing the role of a light scalar meson in supporting the NJL mechanism.

Contribution

It demonstrates that non-chiral quasilocal quark models can serve as effective low-energy theories of QCD, highlighting the importance of a light scalar meson and deriving the scalar and pseudoscalar spectrum.

Findings

A light scalar meson ($m_\sigma = 500 ext{ to } 600$ MeV) supports the NJL mechanism.

The scalar and pseudoscalar spectra are compatible with QCD effective action truncation.

Matching with QCD sum rules enhances model predictability.

Abstract

We discuss the low energy effective action of QCD in the quark sector. When it is built at the CSB (chiral symmetry breaking) scale by means of perturbation theory it has the structure of a generalized Nambu-Jona-Lasinio (NJL) model with CSB due to attractive forces in the scalar channel. We show that if the lowest scalar meson state is sufficiently lighter than the heavy pseudoscalar $\pi'$ then QCD favors a low-energy effective theory in which higher dimensional operators (of the Nambu-Jona-Lasinio type) are dominated and relatively strong. A light scalar quarkonium ($m_\sigma = 500 \div 600$ MeV) would provide an evidence in favor to this NJL mechanism. Thus the non-chiral Quasilocal Quark Models (QQM) in the dynamical symmetry-breaking regime are considered as approximants for low-energy action of QCD. In the mean-field (large-N_c) approach the equation on critical coupling surface is derived. The mass spectrum of scalar and pseudoscalar excited states is calculated in leading-log approach which is compatible with the truncation of the QCD effective action with few higher-dimensional operators. The matching to QCD based on the Chiral Symmetry Restoration sum rules is performed and it helps to select out the relevant pattern of CSB as well as to enhance considerably the predictability of this approach.