Critical Scattering and Two Photon Spectra for a Quark/Meson Plasma

At the Mott transition of a quark/meson plasma, mesons become unbound. This leads to the effect of critical scattering, which is studied by investigating photon pair production due to the process $q\bar q\to\gamma\gamma$. This process can proceed either via direct annihilation or via the formation of a mesonic resonance. It is shown that the latter channel leads to an enhancement of photon pairs with invariant mass equal to the thermal pion mass. The size of this effect measures the time the temperature stays near the Mott temperature during the evolution. It is particularly pronounced for a first order phase transition. The $\pi^0\to\gamma\gamma$ decay gives a strong background contribution and may make the observation of critical scattering in two-photon spectra of present day heavy ion collision experiments difficult.