Thermal e + e - Production in High-Energy Nuclear Collisions

We use a boost-invariant one-dimensional (cylindrically symmetric) fluid dynamics code to calculate e + e - production from the ρ-ω peak in the central rapidity region of S + Au and Pb + Pb collisions at SPS energy (√s = 20 GeV/nucleon). We assume that the hot matter is in thermal equilibrium throughout the expansion, but consider deviations from chemical equilibrium in the high temperature (deconfined) phase. 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 ≤ T c ≤ 200 MeV.