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.

• Received 20 March 1989

DOI:https://doi.org/10.1103/PhysRevD.40.2914