Final state radiation and a possibility to test a pion-photon interaction model near two-pion threshold

Abstract

Final state radiation in the process $e^+e^-\to\pi^+\pi^-$ is considered for the cuts used in the analysis of KLOE data at large angles. By means of a Monte Carlo event generator FEVA, effects of non-pointlike behaviour of pions are estimated in the framework of Resonance Perturbation Theory. An additional complication related with the $\phi$ meson intermediate state is taken into account and the corresponding contributions (the direct decay $\phi\to\pi^+\pi^-\gamma$ and the double resonance decay $\phi\to\rho^\pm\pi^\mp\to\pi^+\pi^-\gamma$) are added to FEVA. A method to test effects of non-pointlike behaviour of pions in a model-independent way is proposed.