Radiative four-meson amplitudes in chiral perturbation theory

We present a general discussion of radiative four-meson processes to O(p4) in chiral perturbation theory. We propose a definition of “generalized bremsstrahlung” that takes full advantage of experimental information on the corresponding non-radiative process. We also derive general formulae for one-loop amplitudes which can be applied, for instance, to η → 3πγ, ππ → ππγ and K → 3πγ.