e+e−pairs fromπ−Areactions

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.