Properties of light mesons in ultrarelativistic nucleus-nucleus collisions

The PHENIX experiment at the Relativistic Heavy Ion Collider is designed for studying the new state of nuclear matter, which is presumably created in central ultrarelativistic collisions of heavy nuclei: the so-called quark-gluon plasma. Light mesons are an important tool in this research. This paper reviews the recent experimental results on the light meson invariant yields, nuclear modification factors, and the yield ratios for the vector and pseudoscalar mesons in the p + p, d + Au, and Au + Au collisions at the energy of √s _NN = 200 GeV.     

Chemical thermalization in relativistic heavy ion collisions

We compute by numerical integration of the Dirac equation the number of quark-antiquark pairs initially produced in the classical color fields of colliding ultrarelativistic nuclei. While the number of pairs is parametrically suppressed in the coupling constant, we find that in this classical field model their production rate is comparable to the thermal ratio of gluons/pairs=9Nf/32. After isotropization one thus would have a quark-gluon plasma in chemical equilibrium.     

Core-corona separation in ultrarelativistic heavy ion collisions

Simple geometrical considerations show that the collision zone in high energy nuclear collisions may be divided into a central part ("core"), with high energy densities, and a peripheral part ("corona"), with smaller energy densities, more like in pp or pA collisions. We present calculations that allow us to separate these two contributions, and which show that the corona contribution is quite small (but not negligible) for central collisions, but gets increasingly important with decreasing centrality. We will discuss consequences concerning results obtained in heavy ion collisions at the BNL Relativistic Heavy Ion Collider and CERN Super Proton Synchrotron.     

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.     

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.     

On the possibility of electronic compression waves in heavy ion collisions

In the collision of heavy atoms compression waves in the atomic electron clouds are predicted. The bulk properties of electronic matter are estimated. Reaction cross sections for electron emission are calculated in a schematic model.     

Three dimensional hydrodynamics of ultrarelativistic heavy ion collisions

We have utilized a 2+1 dimensional numerical code based on Flux Corrected Transport method to find a solution for 3+1 dimensional cylindrically symmetric hydrodynamic flow of hadronic matter which is assumed to be formed in extremely high energy heavy ion collisions. The hydrodynamics is supplemented with a decoupling calculation in order to produce measurable particle distributions. This numerical procedure is applied here to Landau type initial conditions which have been fixed using a simple geometrical picture for a central O+Pb collision at 200 GeV/nucleon. The bag equation of state for nonbaryonic matter is used to simulate the deconfinement phase transition to quark gluon plasma. The resulting pseudorapidity distributions of transverse energy for the final massless pions are calculated.     

Fragmentation of target spectators in ultrarelativistic heavy ion collisions

We demonstrate that the assumption that target spectators fragment isotropically in a gently moving coordinate system is in agreement with pseudorapidity distributions, measured in central ultrarelativistic heavy ion collisions of projectile energy from 14.5A GeV to 200A GeV. Target spectator remnants might be excluded from measurement by introducing a low energy cutoff for the accepted particles at approximately 200 MeV. An approximate scaling is presented for the pseudorapidity distribution of the target spectator fragments. Theoretical tools used for studying intermediate energy heavy ion collisions seem to be applicable in describing target spectator fragmentation in ultrarelativistic heavy ion collisions.     

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.     

Production and rescattering of heavy quarks in ultrarelativistic nucleus-nucleus collisions

Contributions of various mechanisms to the heavy-quark-production cross section in ultrarelativistic nucleus-nucleus collisions, including the direct pair production $b\bar b$ and $c\bar c$ as a result of hard primary collisions and pair production in showers induced by hard-produced partons in the initial and final states, are investigated. The sensitivity of the muon pair spectra with large invariant masses (from semileptonic $B\bar B$ decays) and spectra of the secondary J/ψ (from individual B decays) to the multiple scattering and the energy losses of b quarks in dense quark-gluon matter is studied. The formation of such matter is expected in heavy-ion collisions at the LHC.     

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.     

Inclusive meson production in peripheral collisions of ultrarelativistic heavy ions

There exist several proposals to use Weizsäcker-Williams photons generated by ultrarelativistic heavy ions to produce exotic particles in γγ fusion reactions. To estimate the background conditions for such reactions we analyze various mechanisms of meson production in very peripheral collisions of ultrarelativistic heavy ions at RHIC and LHC energies. Besides the γγ fusion they include also electromagnetic γA interactions and strong nucleon-nucleon interactions in grazing AA collisions. All these processes are characterised by low multiplicities of produced particles. The γA an d AA events are simulated by corresponding Monte Carlo codes, RELDIS and FRITIOF. In each of these processes a certain fraction of pions is produced close to the mid-rapidity regionthat gives a background for the γγ events. The possibility of selecting the mesons produced in the γγ fusion events via different p _t cut procedures is demonstrated.     

Influence of thermalization on the initial condition for heavy ion collisions

In this paper, we discuss the important role of the thermalization process in the initial distribution of QGP. We find that the negligible heat conduction inside QGP can be expressed as an effective Fourier law and we further analyse qualitatively the results caused by a thermalized initial condition. Based on this arguments, we construct a simple phenomenological model and work with the hydro code, and then we compare our results with the experimental data and the results of the standard initial model. It is found that, as we have argued, a thermalized initial condition suppresses the value of the elliptic flow.