rho meson broadening and dilepton production in heavy ion collisions

The modification of the width of the rho meson due to in-medium decays and collisions is evaluated. In high temperature and/or high density hadronic matter, the collision width is much larger than the one-loop decay width. The large width of the $\rho$ meson in matter seems to be consistent with some current interpretations of the $e^+ e^-$ mass spectra measured at the CERN/SPS.