Stage Of Relativistic Heavy-ion Collisions Research Articles

Quarkonium suppression as a probe of a saturated gluon plasma?

A dense parton system is expected to be formed in the early stage of relativistic heavy-ion collisions at RHIC energies and above. The probability of a quark–gluon plasma production and the resulting strength of its signatures depends strongly on the initial conditions associated to the distributions of partons in the nuclear wave functions. At very high energies, the growth of parton distributions should saturate, possibly forming a Color Glass Condensate, which is characterized by a bulk momentum scale Qs. As a direct consequence, the possible signatures of the QGP should be Qs-dependent if the saturation scenario is valid for RHIC and LHC. In this Letter we assume the saturation scenario for the QGP formation and estimate the saturation scale dependence of quarkonium suppression. We conclude that, if this scenario is valid, the ϒ suppression only occurs at large values of Qs.

Modeling the breakup stage of relativistic heavy-ion collisions

By using the output of a thermal model as the input to a hadronic cascade, we investigate the role of kinetic and chemical equilibrium in the breakup stage of $158A \mathrm{GeV}/c$ Pb+Pb collisions at CERN. Gross features of experimental pion and proton spectra are fit, as well as the transverse energy and baryons per unit rapidity. However, experimental source sizes extracted from pion correlations and the low ${p}_{t}$ behavior of pion spectra are not reproduced. We argue that pionic phase space has become overpopulated compared to equilibrium considerations. Other issues regarding chemical and kinetic equilibration are also explored.

COHERENCE AND INCOHERENCE IN THE PHOTON AND DILEPTON PRODUCTION BY BREMSSTRAHLUNG IN RELATIVISTIC HEAVY-ION COLLISIONS

Due to the strong collective deceleration during the initial stage of relativistic heavy-ion collisions, the nuclear matter irradiates real and virtual bremsstrahlung. We describe the process of bremsstrahlung emission in the framework of a semiclassical model in order to study coherence and incoherence effects in the production process. Guided by the intuitive notation of shock fronts being formed between the incident nuclei, we use a simple parametrization of the nuclear current density. The photon spectrum is studied up to photon energies of 300 MeV. In particular, a gradual transition from the coherent production process of low-energy photons to the incoherent one for hard photons is demonstrated. For heavy collision systems coherence effects in the photon spectra dominate, showing characteristic structures arising from shock fronts. The dilepton spectrum is described in first-order perturbation theory. Generally, dileptons are found to be produced incoherently. Only in the case of dielectron production with small invariant pair masses do moderate coherence effects survive.