Relativistic shocks in viscous gluon matter

Considering the relativistic Riemann problem with a discontinuity of pressure in viscous gluon matter, we investigate the existence of relativistic shock waves at RHIC. Calculations employing the parton cascade BAMPS demonstrate for the first time the transition from viscous to ideal shocks by varying the shear viscosity to the entropy density ratio η/s from infinity towards zero. We show that if the η/s ratio of the medium is larger than 0.2, relativistic shocks will be hardly observed. Comparisons with viscous hydrodynamic calculations using vSHASTA confirm our findings. Moreover, on the contrary to the parton cascade, the recent version of vSHASTA fails to create shocks in a strong dissipative medium. The Knudsen number in the shock front is a proper quantity to understand the break down of hydrodynamics in an out of equilibrium state.