VISCOSITY AND THE EQUATION OF STATE IN HIGH-ENERGY HEAVY-ION REACTIONS

Viscous hydrodynamic calculations of high energy heavy-ion collisions (Nb-Nb and Au-Au) from 200 to 800 MeV/nucleon are presented. The resul ting baryon rapidity distributions, the in-plane transverse momentum t ransfer (bounce-off), and the azimuthal dependence of the midrapidity particles (off-plane squeeze out) compare well with Plastic Ball data. We find that the considered observables are sensitive both to the nuc lear equation of state and to the nuclear shear viscosity eta. Transve rse momentum distributions indicate a high shear viscosity (eta almost -equal-to 60 MeV/fm2c) in the compression zone, in agreement with nucl ear matter estimates. The bulk viscosity zeta influences only the entr opy production during the expansion stage; collective observables like flow and dN/dY do not depend strongly on zeta. The recently observed off-plane (phi=90-degrees) squeeze-out, which is found in the triple-d ifferential rapidity distribution, exhibits the strongest sensitivity to the nuclear equation of state. It is demonstrated that for very cen tral collisions, b=1 fm, the squeeze-out is visible even in the double -differential cross section. This is experimentally accessible by stud ying azimuthally symmetric events, as confirmed recently by data of th e European 4pi detector collaboration at Gesellchaft fur Schwerionfors chung Darmstadt.