Production ofμ+μ−andγγpairs in ultrarelativistic heavy-ion collisions

The production of ${\mu }^{+}{\mu }^{-}$ and $\gamma \gamma$ pairs from expanding hot hadronic matter in ultrarelativistic heavy-ion collisions is computed in a model which exhibits a first-order phase transition between a hadron gas and a quark-gluon plasma. In this model the change of the equation of state with respect to the ideal-pion-gas approximation results in a steepening of the hadron-gas contribution to the invariant-mass distributions. In particular quark-gluon-plasma formation becomes more evident even in $\gamma \gamma$ pair production.