Photon production from a thermalized quark-gluon plasma: quantum kinetics and non-perturbative aspects

We study the production of photons from a quark-gluon plasma in local thermal equilibrium by introducing a non-perturbative formulation of the real time evolution of the density matrix. The main ingredient is the real time effective action for the electromagnetic field to O(αem) and to all orders in αs. The real time evolution is completely determined by the solution of a classical stochastic non-local Langevin equation which provides a Dyson-like resummation of the perturbative expansion. The Langevin equation is solved in closed form by Laplace transform in terms of the thermal photon polarization. A quantum kinetic description emerges directly from this formulation. We find that photons with k≲200MeVthermalize as plasmon quasiparticles in the plasma on time scales t∼10-20fm/c which is of the order of the lifetime of the QGP expected at RHIC and LHC. We then obtain the direct photon yield to lowest order in αem and to leading logarithmic order in αs in a uniform expansion valid at all time. The yield during a QGP lifetime t∼10fm/c is systematically larger than that obtained with the equilibrium formulation and the spectrum features a distinct flattening for k≳2.5GeV. We discuss the window of reliability of our results, the theoretical uncertainties in any treatment of photon emission from a QGP in LTE and the shortcomings of the customary S-matrix approach.