A study of vorticity formation in high energy nuclear collisions

We present a quantitative study of vorticity formation in peripheral ultrarelativistic heavy ion collisions at sqrt(s_NN) = 200 GeV by using the ECHO-QGP numerical code, implementing relativistic dissipative hydrodynamics in the causal Israel-Stewart framework in 3+1 dimensions with an initial Bjorken flow profile. We consider and discuss different definitions of vorticity which are relevant in relativistic hydrodynamics. After demonstrating the excellent capabilities of our code, we show that, with the initial conditions needed to reproduce the measured directed flow in peripheral collisions corresponding to an average impact parameter b=11.6 fm and with the Bjorken flow profile for a viscous Quark Gluon Plasma with eta/s=0.16 fixed, a vorticity of up to 0.05 c/fm can develop at freezeout. The ensuing polarization of Lambda baryons is at most of the order of 1% at midrapidity. We show that the amount of developed directed flow is sensitive to both the initial angular momentum of the plasma and its viscosity.