Dynamics of strange, charm and high momentum hadrons in relativistic nucleus-nucleus collisions

Abstract

We investigate hadron production and attenuation of hadrons with strange and charm quarks (or antiquarks) as well as high transverse momentum hadrons in relativistic nucleus-nucleus collisions from 2 AGeV to 21.3 ATeV within two independent transport approaches (UrQMD and HSD). Both transport models are based on quark, diquark, string and hadronic degrees of freedom, but do not include any explicit phase transition to a quark-gluon plasma. From our dynamical calculations we find that both models do not describe the maximum in the $K^+/\pi^+$ ratio at 20 - 30 AGeV in central Au+Au collisions found experimentally, though the excitation functions of strange mesons are reproduced well in HSD and UrQMD. Furthermore, the transport calculations show that the charmonium recreation by $D+\bar{D} \to J/\Psi + meson$ reactions is comparable to the dissociation by 'comoving' mesons at RHIC energies contrary to SPS energies. This leads to the final result that the total $J/\Psi$ suppression as a function of centrality at RHIC should be less than the suppression seen at SPS energies where the 'comover' dissociation is substantial and the backward channels play no role. Furthermore, our transport calculations -- in comparison to experimental data on transverse momentum spectra from $pp$, d+Au and Au+Au reactions -- show that pre-hadronic effects are responsible for both the hardening of the hadron spectra for low transverse momenta (Cronin effect) as well as the suppression of high $p_T$ hadrons. The mutual interactions of formed hadrons are found to be negligible in central Au+Au collisions at $\sqrt{s}$ = 200 GeV for $p_T \geq$ 6 GeV/c and the sizeable suppression seen experimentally is attributed to a large extent to the interactions of 'leading' pre-hadrons with the dense environment.Comment: 10 pages, incl. 7 postscript figures, invited talk presented at the 25th Course "Heavy Ion Reactions from Nuclear to Quark Matter", Erice, Sept. 16-24, 2003, to be publ. in Prog. Part. Nucl. Phy