Collective dynamics in highly relativistic heavy-ion collisions

Abstract

Hydrodynamics with cylindrical symmetry in transverse direction and longitudinal scaling flow is employed to calculate the transverse momentum spectra of various hadrons and clusters (e.g. pi, K, N, Phi, Lambda, d, He) in central heavy-ion collisions at CERN-SPS and BNL-RHIC energies up to pT=4 GeV. We discuss the sensitivity of these spectra with respect to the initial transverse density profile as well as to the choice of ``freeze-out&#39;&#39; hypersurface. For SPS energy and pT<2 GeV, overall good agreement of the pT distributions with data is found when freeze-out occurs along the T=130 MeV isotherm. Even high-pT neutral pion data can be described for a particular choice of the initial transverse density profile. It is shown that the average transverse velocity <vT> of heavy hadrons and hadronic clusters is a good measure for the collective flow velocity. The latter is found to be rather similar for SPS and RHIC energies, due to the ``stall&#39;&#39; of the flow within the long-lived mixed phase at RHIC. In case of thermalization and hydrodynamical expansion, the mean transverse momentum <pT> increases linearly with the hadron mass. In contrast, the string model FRITIOF7.02, which does not account for rescattering of secondary hadrons, predicts a strong dependence of <pT> on the quark composition of the hadron. Due to the different sensitivity to hard processes, hadrons with charm (anti-)quarks acquire significantly more transverse momentum than hadrons without c (or even without s) quarks.