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

Abstract

Final state radiation in the process $e^+e^-\toπ^+π^-$ 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 $ϕ$ meson intermediate state is taken into account and the corresponding contributions (the direct decay $ϕ\toπ^+π^-γ$ and the double resonance decay $ϕ\toρ^\pmπ^\mp\toπ^+π^-γ$) are added to FEVA. A method to test effects of non-pointlike behaviour of pions in a model-independent way is proposed.

16 pages, 6 figures