Thermal dileptons from pi-p interactions in a hot pion gas

The production of low mass dileptons from pi-rho interactions in a hot medium is studied. Applying finite temperature perturbation theory the dilepton rate is computed up to order g(p)(2). For dilepton masses below the rho the two-body reactions pi pi-->rho gamma*, pi rho-->pi gamma*, and the decay process rho-->pi pi gamma* give significant contributions. Nonequilibrium contributions to the thermal rate are estimated, including the modification of the particle distribution function with a nonzero pion chemical potential. A comparison of the dilepton rate with the recent data measured in nucleus-nucleus collisions al CERN SPS energy by the CERES Collaboration is also performed. It is shown that the additional thermal dileptons from pi-rho interactions can partially account for the excess of the soft dilepton yield seen experimentally.