Recent developments in the physics of light quarks

TL;DR

This paper reviews recent progress in understanding light quark physics, emphasizing improved determinations of $C0C0$ S-wave scattering lengths through experimental data and lattice QCD, confirming the effectiveness of chiral expansion and the significance of the quark condensate.

Contribution

It highlights advancements in calculating light quark interactions and validates the use of chiral perturbation theory and lattice QCD in low-energy QCD analysis.

Findings

Excellent agreement between experimental and lattice results.

Validation of chiral expansion as an effective analysis tool.

Confirmation of the quark condensate as the key order parameter.

Abstract

My talk was dedicated to the memory of Jan Stern. The brief account given below focuses on the progress achieved in the determination of the $\pi\pi$ S-wave scattering lengths, both experimentally and with light dynamical quarks on a lattice. In view of the excellent agreement, we can conclude that (a) the expansion in powers of the two lightest quark masses represents a very efficient tool for the analysis of the low energy structure of QCD and (b) the size of the energy gap of QCD is governed by the order parameter of lowest dimension, the quark condensate.