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.     

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 .     

Hadron production at forward rapidity in nuclear collisions at RHIC

The main experimental results obtained by the BRAHMS experiment at Relativistic Heavy Ion Collider (RHIC) for Au+Au collisions at \(\sqrt {s_{NN} } \) =62.4, 200 GeV and d+Au collisions at \(\sqrt {s_{NN} } \) = 200GeV are presented. The m _T spectra and the Gaussian-like rapidity densities of produced pions and kaons in Au+Au central collisions at \(\sqrt {s_{NN} } \) = 200 are GeV shown, and their rapidity densities are compared with results from models. The net-proton yield in the same system is compared with that from AGS and SPS energies to study the high energy collision scenario-transparency and stopping. The rapidity, energy and centrality dependence of the nuclear modification factors in both systems are compared with models to differentiate between the initial and final state effect.     

Particle production in nuclear collisions from pQCD

Particle production is calculated in high-energy proton-proton and nuclear collisions. Non-perturbative ingredients are reviewed. Fragmentation function uncertainties and the transverse momentum dependence of parton distributions are studied. Perturbative QCD results are compared to data.     

J/ψ rapidity density and its suppression in ultrarelativistic heavy-ion collisions

The production of charm quark-antiquark pairs and their hadronization are taken into account by the semi-local duality model. The thermalJ/ψ production rate is analysed in the framework of the Bjorken's scaling hadrodynamical picture and theJ/ψ yield as a function of the initial temperature of quark-gluon plasma is calculated in the case of the central region of untrarelativistic nucleus-nucleus collisions. In view of the ideal of the Debye screening an alternativeJ/ψ suppression mechanism is suggested and the thermalJ/ψ yield with and without the suppression mechanism are evaluated numerically, respectively.     

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.     

Direct hadron production in ultrarelativistic heavy-ion collisions

Hadrons emitted by the pre-surface layer of a quark-gluon plasma (QGP) before the phase transition into a hadronic gas are considered as possible sources of direct information about QGP. It is shown that if QGP is created in ultrarelativistic heavy-ion collisions, then these hadrons strongly contribute at soft p_t at SpS energy and dominate up to an order of magnitude at LHC energy.     

Significance of the fragmentation region in ultrarelativistic heavy-ion collisions

We present measurements of the pseudorapidity distribution of primary charged particles produced in Au+Au collisions at three energies, sqrt[s(NN)]=19.6, 130, and 200 GeV, for a range of collision centrali-ties. The distribution narrows for more central collisions and excess particles are produced at high pseudorapidity in peripheral collisions. For a given centrality, however, the distributions are found to scale with energy according to the "limiting fragmentation" hypothesis. The universal fragmentation region described by this scaling grows in pseudorapidity with increasing collision energy, extending well away from the beam rapidity and covering more than half of the pseudorapidity range over which particles are produced. This approach to a universal limiting curve appears to be a dominant feature of the pseudorapidity distribution and therefore of the total particle production in these collisions.     

Pion production in central high energy nuclear collisions

A model for pion production is studied in the context of a cascade calculation. The pions are produced through Δ-resonances which are allowed to decay. The emitted pions are assumed to interact with the other nucleons by forming new Δ -resonances. The time evolution of the pion and Δ-population is studied; it is found that Δs are always more numerous than pions during the sequence of baryon-baryon collisions. The spectrum of the pions is in considerably better agreement with experiment than the one obtained with frozen Δ-isobars. The presence of Δ-resonances appears to be important for the cooling of the pion system. The pion multiplicity is found to deviate from a Poisson distribution. The pion yield is overestimated by at least 25%; this result is discussed within the framework of conventional dynamics.     

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.     

Second-order dissipative fluid dynamics for ultrarelativistic nuclear collisions

The Müller-Israel-Stewart second-order theory of relativistic imperfect fluids based on Grad's moment method is used to study the expansion of hot matter produced in ultrarelativistic heavy-ion collisions. The temperature evolution is investigated in the framework of the Bjorken boost-invariant scaling limit. The results of these second-order theories are compared to those of first-order theories due to Eckart and to Landau and Lifshitz and those of zeroth order (perfect fluid) due to Euler.     

Regge vertex for quark production in the central rapidity region in the next-to-leading order

The effective vertex for quark production in the interaction of a Reggeized quark and a Reggeized gluon is calculated in the next-to-leading order (NLO). The resulting vertex is the missing component of the NLO multi-Regge amplitude featuring quark and gluon exchanges in the t channels. This calculation will make it possible to develop in future the bootstrap approach to proving quark Reggeization in the next-to-leading logarithmic approximation.     

The production of high-energy protons in central relativistic nuclear collisions

We extend our model of transport theory to be applicable to the inclusive production of protons with very high energy. We then consider the angular distribution of such protons, produced in a central collision of Ar on KC1 at 800 MeV per nucleon. The slight anisotropy observed in the data can be explained by a finite value of the friction constant which in turn determines the number of collisions needed for equilibrium to be reached. We also show that these data are quite sensitive to the reaction geometry and cannot be explained by the firestreak model.     

On the possibility of thermalization of heavy mesons in ultrarelativistic nuclear collisions

The phenomenological analysis and interpretation of experimental data from RHIC and LHC on the production of J/ψ and D mesons in heavy-ion collisions are performed within the two-component HYDJET++ model including the thermal and hard mechanisms of hadron production. It is shown that the thermal freeze-out of charmed mesons at RHIC energies occurs earlier than the thermal freeze-out of light hadrons (assumingly, simultaneously with chemical freeze-out), which indicates that J/ψ and D mesons are not in kinetic equilibrium with the formed hadronic matter. At the same time, a significant part of D mesons at LHC energies are in kinetic equilibrium with the formed thermalized matter, but J/ψ mesons are still characterized by early freeze-out.