$e^+e^-$ pairs from $π^-$A reactions

Abstract

We present a dynamical study of $e^+e^-$ production in $π^-$A reactions at 1.3 and 1.7 GeV on the basis of the Coupled-Channel-BUU approach. The contributions from vector mesons ($ρ,ω$) are calculated taking into account the collisional broadening effect and are compared to background sources in the dilepton spectrum from the Dalitz decays of $ω$ and $η$ mesons produced in the reaction. Two possible scenarios for the medium modifications of the vector mesons are investigated, i.e. the 'dropping mass' scheme for the $ρ$ and $ω$ and a momentum dependent $ρ$-meson spectral function that includes the polarization of the $ρ$-meson due to resonant $ρ-N$ scattering.