Enhancement of photon production from nonequilibrium disoriented chiral condensates

We study single photon production during the nonequilibrium stages of the formation of chiral condensates within the “quench” scenario of the chiral phase transition. The dynamics is modeled with a gauged linear $\sigma$ model. We find that strong nonequilibrium fluctuations of the charged pions lead to an enhancement of photon production. We argue that this mechanism is nonperturbative and a novel, nonperturbative quantum kinetic approach to the description of photon production far off equilibrium is developed. We find that nonequilibrium spinodal instabilities of long wavelength pion fluctuations are responsible for an enhanced photon production rate for energies $<~80$ MeV at order $\alpha$. We follow the evolution of the dynamics throughout the phase transition, which in this scenario occurs on a time scale of about $2.5-3$ fm/$c$ and integrate the photon yield through its evolution. The spectrum of photons produced throughout the phase transition is a nonequilibrium one. For thermal initial conditions at the time of the quench it interpolates between a thermal distribution about $6\%$ above the initial temperature (at the time of the quench) for low-energy $<~80$ MeV photons, and a high-energy tail in thermal equilibrium at the initial temperature, with a smooth crossover at 100 MeV. The rate displays a peak at $\sim 35$ MeV which receives a larger enhancement the closer the initial temperature at the time of the quench is to the critical temperature. It is found that the enhancement of photon production at low energies is not an artifact caused by the initial distribution of the photons, but is due to the pionic instabilities. We suggest that these strong out of equilibrium effects may provide experimental signatures for the chiral phase transition and formation and relaxation of disoriented chiral condensates in heavy-ion collisions. The new kinetic approach to photoproduction described here is general and could also be implemented in a first order phase transition in which nucleating bubbles provide the nonequilibrium processes.