Dimuon production in ultrarelativistic nuclear collisions and QCD phase transitions

Dilepton production rates in nuclear collisions are calculated in order to study their sensitivity to the quark-gluon plasma and to the hadronic phases. This treatment differs from previous work on the subject in two respects: The width of the rho-meson, being exchanged in annihilation, is made temperature dependent, thus taking into account resonance melting as the critical deconfinement temperatureT _d is approached. Secondly, we study in addition to the standard scenario where chiral symmetry restoration and deconfinement occur at the same temperature (T _c =T _d ), an alternative possibility where deconfinement preceeds chiral symmetry restoration (T _d T _c ). Results differ substantially from those obtained assuming a temperature independent rho-meson width, andT _c =T _d .     

Centrality dependence of multiplicities in ultrarelativistic nuclear collisions

We compute the centrality dependence of multiplicities of particles produced in ultrarelativistic nuclear collisions at various energies and atomic numbers. The computation is carried out in perturbative QCD with saturated densities of produced gluons and by including effects of nuclear geometry. Numbers are given for Au+Au collisions at RHIC energies.     

Decoherence and energy loss in QCD cascades in nuclear collisions

The paper analyzes the consequences of the assumption that the effects of quantum coherence and the resulting angular ordering in QCD cascades are disrupted within the hot fireball created in ultrarelativistic heavy ion collisions. The effects of decoherence are taken into account by switching off angular ordering for the vertices of MC-generated cascade that lie within the fireball of some given size. We show that decoherence exerts a pronounced influence on the spatiotemporal pattern of a cascade. Related effects due to non-radiative losses of cascade gluons and those of final prehadrons are discussed. We show that decoherence leads to a substantial increase in the yield of cascade particles hadronizing inside the hot zone thus exerting a strong influence on the properties of final hadrons and amplifying the effects of collisional energy losses of cascade partons and final prehadrons that take place inside the fireball.     

Centrality and multiparticle production in ultrarelativistic nuclear collisions

A critical analysis of methods for selecting central events in high-energy proton–nucleus (pA) and nucleus–nucleus (AA) collisions is presented. A sample of event classes in which background fluctuations associated with the dispersion of the impact parameter of each event or the number of participant nucleons are minimal is examined. At the SPS and LHC energies, the numbers of nucleon–nucleon collisions are estimated with the aid of the Monte Carlo event generators HIJING and AMPT, which take into account energy–momentum conservation, and on the basis of a non-Glauber model involving string fusion and a modified Glauber model. The results obtained in this way demonstrate the need for revising the extensively used application of the Glauber model in normalizing multiplicity yields in experimental data on pA and AA collisions in the soft region of the spectrum.     

Mass dependence of hadron distributions in ultrarelativistic nucleus-nucleus collisions

(Anti-) baryon and kaon distributions in nucleus-nucleus reactions at 200 AGeV are studied in the framework of the transport model RQMD. Production mechanisms for strangeness and baryon pairs are tested by comparing their projectile and target mass dependence to available experimental data. RQMD contains two collective production processes, fusion of overlapping strings into highly charged chromoelectric ropes and hadronic rescattering. It turns out that both rope formation and hadronic rescattering are of importance for creation-and annihilation-of strangeness and antibaryons.     

Azimuthal asymmetry of jet production as a signal of parton energy losses in semicentral heavy-ion collisions

The question is investigated of whether an azimuthal asymmetry in the hadron-jet spectra can arise because of rescattering and energy losses of partons produced via hard processes in a dense quark-gluon matter formed in the region of the initial nuclear overlap in collisions characterized by a nonzero value of the impact parameter. Methods are discussed for determining the reaction-plane angle in ultrarelativistic heavy-ion collisions with the aid of the flux of semihard particles.     

One more hydrodynamic scenario of ultrarelativistic nuclear collisions

A new hydrodynamic scenario of ultrarelativistic nuclear collisions is discussed. It includes two stages: one-dimensional scaling expansion and break-up of a hydrodynamic system into separate spherical droplets. The correlation between the average transverse momentum and multiplicity (sensitive to the quark-hadron phase transition) is calculated.     

Evaporation of hadrons with largeP T and dilepton emission from the mixed phase of QCD matter in ultrarelativistic nuclear collisions

To probe the mixed phase of QCD matter we study thermal dilepton emission and evaporation of hadrons with largeP _T in the framework of hydrodynamical model for ultrarelativistic nuclear collisions.     

Chiral QCD phase transition in nuclear collisions

The characteristic aspects of multiparticle states generated at , as a result of chiral QCD phase transition, are studied in the framework of the O(4) theory. Predictions concerning critical events, in connection with current and future experiments with ultrarelativistic heavy ions, are presented. Received: 9 June 1997 / Revised version: 18 July 1997 / Published online: 26 February 1998     

Beam energy dependence of spontaneous positron creation in delayed nuclear collisions

We compute positron emission from U+Cm collisions within a quantum mechanical model for delayed nuclear collisions. We demonstrate a striking beam energy dependence of the strength of the spontaneous positron peak.     

Semi-hard parton rescatterings in nuclear collisions at very high energies

Representing the semi-hard partonic interactions by the exchange of Lipatov's perturbative Pomeron, we express the semi-hard nuclear cross section as a self shadowing cross section. With the help of a generating functional technique, we obtain average numbers of multiple semi-hard partonic collisions without any need of using explicit expressions for the multi-parton distributions. The average number of semi-hard interactions of a given projectile parton against a target nucleus is estimated quantitatively and it is shown to grow very rapidly above one with increasing the c.m. energy.     

Radiative parton energy loss and jet quenching in high-energy heavy-ion collisions

We study within the light-cone path integral approach [3] the effect of the induced gluon radiation on high-p _T hadrons in high-energy heavy-ion collisions. The induced gluon spectrum is represented in a new form which is convenient for numerical simulations. For the first time, computations are performed with a realistic parameterization of the dipole cross section. The results are in reasonable agreement with the suppression of high-p _T hadrons in Au + Au collisions at \(\sqrt s = 200\)GeV observed at RHIC.