Dissipation in the very early stage of the hydrodynamic evolution in relativistic heavy ion collisions

We propose a modification of the hydrodynamic model of the dynamics in ultrarelativistic nuclear collisions. A modification of the energymomentum tensor at the initial stage describes the lack of isotropization of the pressure. Subsequently, the pressure is relaxing towards the equilibrium isotropic form in the local comoving frame. Within the Bjorken scaling solution a bound is found on the decay time of the initial anisotropy of the energy-momentum tensor. For the strongest dissipative effect allowed, we find a relative entropy increase of about 30%, a significant hardening of the transverse momentum spectra, and no effect on the HBT radii.