Near threshold radiative 3$π$ production in $e^+e^-$ annihilation

Abstract

We consider the $π^+π^-π_0γ$ final state in electron-positron annihilation at cms energies not far from the threshold. Both initial and final state radiations of the hard photon is considered but without interference between them. The amplitude for the final state radiation is obtained by using the effective Wess-Zumino-Witten Lagrangian for pion-photon interactions valid for low energies. In real experiments energies are never such small that $ρ$ and $ω$ mesons would have negligible effect. So a phenomenological Breit-Wigner factor is introduced in the final state radiation amplitude to account for the vector mesons influence. Using radiative 3$π$ production amplitudes, a Monte Carlo event generator was developed which could be useful in experimental studies.