Photon pair production in expanding hot hadronic matter

The emission of γγ pairs from expanding hot hadronic matter produced in the central region of ultrarelativistic heavy-ion collisions is analyzed. We have argued that, looking only at the invariant-mass distribution, the production of the quark-gluon plasma could be more evident via observation of ${\mu }^{+}$ ${\mu }^{\mathrm{-}}$ than via γγ pair production. The ratio of γγ rapidity density to the square of the charged-pion rapidity density exhibits an abrupt change in slope when the phase transition in the system takes place. This property, which is qualitatively similar to ${\mu }^{+}$ ${\mu }^{\mathrm{-}}$ pair production, can provide an experimental test for quark-gluon-plasma formation in the system.