Impact of the pre-equilibrium stage of ultra-relativistic heavy ion collisions: isotropization and photon production

We present a model in which gluon and quark pairs are produced by means of the Schwinger mechanism from the decay of color-electric flux tubes, which are expected to be produced in the early stage of ultra-relativistic heavy ion collisions. The evolution equations of the initial field are coupled to the relativistic transport equation which describes the dynamics of the many particle system and is formulated in terms of a fixed viscosity over entropy density ratio η/s. This self-consistent solution of the problem allows to take into account the backreaction of the color currents on the classical field. We study isotropization and thermalization of the plasma produced by the field decay for a expanding geometry in 1 + 1 D and in 3 + 1 D . We find that the initial color-electric field decays within 1fm/c ; in the case of large η/s oscillations of the field appear along the whole temporal evolution of the system, affecting also the ratio between longitudinal and transverse pressure. In the case of small viscosities (η/ s≲ 3 / 4 π) we find an equilibration time less than 1 fm/c, in agreement with the common lore of hydrodynamics. Including the pertinent scattering processes into the collision integral of the Boltzmann equation we investigate photon production within our model for the pre-equilibrium dynamics and with simulations starting with equilibrium initial conditions. Thus, we are able to identify the contribution of the early-stage to the photon spectrum in Au-Au collisions at RHIC sNN=200 GeV and in Pb-Pb collisions at LHC sNN=2.76 TeV. We find that there is no dark age in relativistic heavy ion collisions: early-stage photons enhance the direct photon spectrum in the intermediate transverse momentum region (pT≳ 1. 5 -2 GeV depending on the collision energy) and their abundance is comparable with that produced by a thermalized quark-gluon plasma.