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

Abstract

Final-state radiation in the process e + e − → π+π− is considered for the cuts used in the analysis of KLOE data at large angles. By means of the Monte Carlo event generator FEVA, effects of nonpointlike behavior of pions are estimated in the framework of resonance perturbation theory. An additional complication related to the ϕ-meson intermediate state is taken into account and the corresponding contributions (the direct decay ϕ → π+π−γ and the double resonance decay ϕ → ρ±π∓ → π+π−γ) are added to FEVA. A method is proposed to test effects of nonpointlike behavior of pions in a model-independent way.