$e^+e^-$ pairs from $π^-$A reactions

We present a dynamical study of $e^+e^-$ production in $\pi^-$A reactions at 1.3 and 1.7 GeV on the basis of the Coupled-Channel-BUU approach. The contributions from vector mesons ($\rho,\omega$) are calculated taking into account the collisional broadening effect and are compared to background sources in the dilepton spectrum from the Dalitz decays of $\omega$ and $\eta$ 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 $\rho$ and $\omega$ and a momentum dependent $\rho$-meson spectral function that includes the polarization of the $\rho$-meson due to resonant $\rho -N$ scattering.