Production ofγγpairs in quark-gluon plasma and hadron plasma

The ratio of the production rate of γγ pairs in the quark-gluon plasma to that in the hadron plasma is shown to be much larger than the ratio of the production rate of ${\mu }^{+}$ ${\mu }^{\mathrm{-}}$ pairs for the two plasma states. Hence the γγ-pair production is more adequate to discriminate the plasma states.