Thermal effects on dilepton production from pi-pi annihilation

We study finite temperature effects on dilepton production from pion-pion annihilation in hot hadronic matter. The softening of the pion dispersion relation in a medium is found to enhance the production rate of dileptons with invariant masses in the region of 2m(pi)*(T) < M < m(rho). On the other hand, the reduction of the pion electromagnetic form factor at finite temperature leads to a suppression of the dilepton production rate. Including both effects, we have found that the dilepton yield is slightly enhanced in the invariant mass region M = 270 similar to 600 MeV but is suppressed around the vector meson resonance. We further discuss the relevance of our results to recent experimental data from the S+Au collisions at CERN/SPS energies by the CERES Collaboration.