η→π+π−π0γ in chiral perturbation theory

We analyse the radiative decay η→π+π−π0γ in the low-energy expansion of the Standard Model. We employ the notion of “generalized bremsstrahlung” to take full advantage of the theoretical and experimental information on the corresponding non-radiative η→3π decay. The direct emission amplitude of O(p4) is due to one-loop diagrams with intermediate pions (isospin violating) and kaons (isospin conserving). The isospin conserving contributions to direct emission, including vector meson exchange appearing at O(p6), are suppressed.