  Chiral perturbation theory (ChPT) is an effective field theory that describes
the properties of strongly-interacting systems at energies far below typical
hadron masses. The degrees of freedom are hadrons instead of the underlying
quarks and gluons. ChPT is a systematic and model-independent approximation
method based on an expansion of amplitudes in terms of light-quark masses and
momenta. The following is a brief overview of ChPT that is largely based on
Scherer, Schindler, Lect. Notes Phys. 830 (2012), which can be referred to for
a more detailed introduction.
