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 lin ear sigma model. We find that strong nonequilibrium fluctuations of th e charged pions lead to an enhancement of photon production. We argue that this mechanism is nonperturbative and a novel, nonperturbative qu antum kinetic approach to the description of photon production far off equilibrium is developed, We find that nonequilibrium spinodal instab ilities of long wavelength pion fluctuations are responsible for an en hanced photon production rate for energies less than or equal to 80 Me V at order alpha. We follow the evolution of the dynamics throughout t he 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 les s than or equal to 80 MeV photons, and a high-energy tail in thermal e quilibrium at the initial temperature, with a smooth crossover at 100 MeV. The rate displays a peak at similar to 35 MeV which receives a la rger enhancement the closer the initial temperature at the time of the quench is to the critical temperature. It is found that the enhanceme nt of photon production at low energies is not an artifact caused by t he initial distribution of the photons, but is due to the pionic insta bilities. We suggest that these strong out of equilibrium effects may provide experimental signatures for the chiral phase transition and fo rmation 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 trans ition in which nucleating bubbles provide the nonequilibrium processes . [S0556-2821(97)01820-1].