Chiral perturbation theory predictions for η → π+π−π0γ

The η → π+π−π0γ decay is discussed in the general context of Chiral Perturbation Theory (ChPT), assuming that the low-energy constants (counterterms) are saturated by vector-meson resonances. The η → π+π−π0γ amplitude can be separated in two distinct pieces: the inner bremsstrahlung, A(IB), and the structure dependent (or direct emission), A(SD), amplitudes. The former—which essentially contains the same physics as A(η → π+π−π0)—is found to dominate over the second one—which looks more interesting from the ChPT point of view.