– 1 – Form Factors for Radiative Pion and Kaon Decays

The radiative decays, π ± → l ± νγ and K ± → l ± νγ, with l standing for an e or a μ, and γ for a real or virtual photon (e + e − pair), provide a powerful tool to investigate the hadronic structure of pions and kaons. The structure-dependent part SD i of the amplitude describes the emission of photons from virtual hadronic states, and is parametrized in terms of form factors F i , with i = V, A (vector, axial vector), in the standard description [1,2]. Exotic, non-standard contributions like i = T, S (tensor, scalar) have also been considered, and we shall discuss them below. Apart from the SD terms, the decay amplitude depends also on Inner Bremsstrahlung IB from the weak decay π ± (K ±) → l ± ν accompanied by the photon radiated from the external charged particles. Naturally, experiments try to optimize their kinematics so as to minimize the " trivial " IB part of the amplitude.