Electron-positron pairs from thermal resonances in ultrarelativistic nuclear collisions

We use a boost-invariant one-dimensional (cylindrically symmetric) fluid dynamics code to calculate $e^+e^-$ production from $\rho^0$ and $\omega$ decay in the central rapidity region of a central S+Au collision at $\sqrt{s}=20$ GeV/nucleon. We use equations of state with a first-order phase transition between a massless pion gas and quark gluon plasma, with transition temperatures in the range $150-200$ MeV. The production cross section at the $\rho$ mass loosely constrains the transition and freeze-out temperatures, and we find that the $m_T$ spectrum is a good thermometer for sufficiently high $T_c$.