# Chiral perturbation theory in the meson sector

## TL;DR

This paper reviews the current state of Chiral Perturbation Theory in the meson sector, highlighting recent theoretical developments, new relations, and preliminary fits of low-energy constants, with discussions on specific processes like eta to three pions.

## Contribution

It provides an overview of recent advances in ChPT, including new relations, fits of low-energy constants, and discussions on hard pion ChPT and specific meson decay processes.

## Key findings

- New relations as tests of ChPT
- Preliminary results of NLO low-energy-constants fit
- Arguments supporting hard pion ChPT

## Abstract

The present status of Chiral Perturbation Theory in the meson sector is discussed concentrating on recent developments. This write-up contains short discussions on a listing a few historical papers, the principles behind ChPT, two-flavour ChPT including some comments about the pion polarizability, three-flavour ChPT with a discussion of the recently found relations as tests of ChPT and preliminary results of a new fit of the NLO low-energy-constants (LECs). It discusses somewhat deeper $\eta\to3\pi$ and the arguments for the existence of a ``hard pion ChPT'' and its application to $K\to2\pi$.