standard picture for the evolution of such matter consists in a pre-equilibrium stage dominated by strong chromoelectromagnetic fields, followed by the formation of the Quark Gluon Plasma (QGP) in local thermal equilibrium. The QGP thus formed expands and cools under its own pressure, going through the deconfinement transition in which hadrons are formed and later on detected (for reviews see [@revhydro1; @revhydro2; @reviewIANCU] and references therein).

Before the collision, the occupation number of gluons at high energy is so large that an approximation in terms of classical gauge fields obeying Yang-Mills equations becomes more suitable than a description in terms of on-shell particles [@reviewIANCU; @GLASMA]. The time scale for this highly nonlinear regime occurring at the earliest stage of a heavy ion collision is $Q_s^{-1}\sim 0.2$ fm/c at RHIC, where $Q_s$ is the saturation scale.

The dense system appearing between first impact and the formation of the equilibrated QGP develops Chromo-Weibel instabilities (see e.g. [@strick-Weibel; @arn-Weibel]): the non-equilibrium anisotropic distribution of the partons is responsible for the fast growth of the chromomagnetic plasma modes, which in turn isotropize the system and speed up the thermalization process to yield a thermalization time $\sim 1$ fm/c. This is roughly the value of 