Neutral meson production at high p<Subscript>T</Subscript> with the PHENIX experiment at RHIC

Over the first five years of operation the PHENIX experiment at RHIC has collected a wealth of data for various systems and collision energies that is providing valuable information for the understanding of the suppression pattern observed in central Au+Au collisions at . An overview on transverse-momentum (p_T) spectra of π⁰ and η in different collision energies and systems is presented.     

What more can be learned from high p<Subscript>T</Subscript> probes at RHIC?

The current status of the understanding of jet quenching in nuclear collisions at RHIC is reviewed. The experimentally large level of suppression of jets in Au+Au collisions at RHIC is a success, but also introduces a challenge in terms of quantitative understanding of the properties of the collision zone. The medium appears to be equally black to all interacting probes utilized to date, limiting the amount of tomographic information that can be obtained from quenching phenomena. In order to recover this information, a probe to which the medium is gray needs to be found. PACS 25.75.-q     

Parton rearrangement and fast thermalization in heavyion collisions at RHIC and LHC energies

The implications of parton rearrangement processes on the dynamics of ultra-relativistic heavyion collisions have been investigated. A microscopic transport approach, namely the quark gluon string model (QGSM) which has been extended for a locally density-dependent partonic rearrangement and fusion procedure served as the tool for this investigations. The model emulates effectively the dynamics of a strongly coupled quark plasma and final hadronic interactions. Main QGSM results on anisotropic flow components v ₁ and v ₂ at top RHIC energy are compiled. Predictions for the pseudorapidity dependence of directed and elliptic flow in Pb+Pb collisions under LHC conditions are presented.     

Investigation of the high-p<Subscript>T</Subscript> strange baryon anomalies at RHIC

Results from RHIC have shown that there is an enhanced baryon/meson ratio in the intermediate transverse momentum range (2<p_T<6 GeV/c) in Au+Au collisions at both  =130 and 200 GeV. This was initially demonstrated by measurements of the p̄/π^- ratio which was then extended in p_T by the Λ/K⁰ _S ratio. The data were successfully described by models utilising different hadronization mechanisms: those having recombination of quarks and others having an interplay between flow, jet quenching and incorporating baryon junction loops. The strange particle data from the first Au+Au run at  =200 GeV gave tantalising hints that the observed enhancement of baryons compared to mesons was diminished by a p_T of 6 GeV/c, but a lack of statistics in this range made a definitive statement impossible. Here we present an extended analysis of identified strange baryons and mesons in Au+Au collisions at  =200 GeV using data obtained by the STAR experiment from the 2004 running period. The increase in statistics extends the measurement of Λ hyperons out to at least 7 GeV/c and K⁰ _S mesons out to 9 GeV/c. This data allows us to place limits on the range where in-vacuum fragmentation functions are applicable and the effect of baryon dominance is reduced. We also discuss the prospects for making these measurements using multiply-strange baryons and mesons (Ω and ϕ).     

Midrapidity production of secondaries in pp collisions at RHIC and LHC energies in the quark–gluon string model

We consider the phenomenological implications of the assumption that baryons are systems of three quarks connected through a gluon string junction. The transfer of baryon number in rapidity space due to the string junction propagation is considered in detail. At high energies this process leads to a significant effect on the net baryon production in hN collisions at midrapidities. The numerical results for midrapidity inclusive densities of different secondaries in the framework of the quark–gluon string model are in reasonable agreement with the experimental data. One universal value, λ≃0.25, for the strangeness suppression parameter correctly describes the yield ratios of Λ/p, Ξ/Λ, and Ω/Ξ. The predictions for pp collisions at LHC energies are also presented. PACS 25.75.Dw     

Feynman scaling violation on baryon spectra in pp collisions at LHC and cosmic ray energies

A significant asymmetry in baryon/antibaryon yields in the central region of high energy collisions is observed when the initial state has nonzero baryon charge. This asymmetry is connected with the possibility of baryon charge diffusion in rapidity space. Such a diffusion should decrease the baryon charge in the fragmentation region and translate into the corresponding decrease of the multiplicity of leading baryons. As a result, a new mechanism for Feynman scaling violation in the fragmentation region is obtained. Another numerically more significant reason for the Feynman scaling violation comes from the fact that the average number of cut Pomerons increases with initial energy. We present the quantitative predictions of the Quark-Gluon String Model for the Feynman scaling violation at LHC energies and at even higher energies that can be important for cosmic ray physics.     

Low-<Emphasis Type="Italic">p</Emphasis><Subscript>T</Subscript> proton-proton physics at low luminosity at LHC

This review of low-p _T proton-proton physics at low luminosity at the large hadron collider (LHC) should cover all LHC experiments, but in practice, is mainly related to ALICE, for reasons which will be explained. However, the relevance to other LHC experiments is clear, as low-p_T. phenomena represent an important component of the background to their high-p_T. phenomena which needs to be calibrated. The ALICE collaboration will study proton-proton collisions as part of their heavy-ion programme, where most signals are relative to the proton-proton system. In addition, the ALICE detector’s unique acceptance at low p_T as well as its unique particle identification capability will make it possible to carry out a program of genuine proton-proton physics complementary to those of other LHC experiments.     

Scaling features of charged-hadron spectra in AA collisions at RHIC in z presentation

Results of analysis of experimental data on inclusive spectra of charged hadrons produced in heavy-ion collisions at RHIC are shown in z presentation. The data indicate similarity as a characteristic feature of mechanism of hadron production at high energies. It is argued that this property includes structure of the colliding objects, interaction of their constituents, and character of the fragmentation process. Relation of the scaling variable z with the entropy and “specific heat” of the interacting system is discussed. A microscopic scenario of the constituent interactions connected with the momentum distribution of hadrons in the final state is suggested. We estimate energy losses of particles in the produced matter characterized by the multiplicity density dN _ch/dη. The text was submitted by the authors in English.     

Self-similarity of pion production in AA collisions at RHIC

Experimental data on inclusive spectra of pions produced in heavy-ion collisions at RHIC are analyzed in the framework of z-scaling. The data indicate similarity as a characteristic feature of the mechanism of pion production at high energies. It is argued that this property includes structure of the colliding objects, interaction of their constituents, and character of the fragmentation process. A microscopic scenario of nucleus interactions at a constituent level in terms of momentum fractions is developed. The centrality dependence of the shape of the scaling function Ψ(z) and the fractal dimention ? _AA of the fragmentation process is studied. Energy losses of particles in the final state as a function of the collision energy, transverse momentum, and centrality are estimated. The scale dependence of the energy losses is discussed. A decreasing tendency of specific heat of the produced medium with the system size is established. The obtained results may be exploited to search for and study new physics phenomena in pion production in pp and AA collisions at high multiplicities.     

High-p<Subscript>T</Subscript> π<Superscript>0</Superscript> production with respect to the reaction plane using the PHENIX detector at RHIC

The origin of the azimuthal anisotropy in particle yields at high p_T (p_T>5 GeV/c) in RHIC collisions remains an intriguing puzzle. Traditional flow and parton energy loss models have failed to completely explain the large v₂ observed at high p_T. Measurement of this parameter at high p_T will help to gain an understanding of the interplay between flow, recombination and energy loss, and the role they play in the transition from soft to hard physics. Neutral mesons measured in the PHENIX experiment provide an ideal observable for such studies. We present recent measurements of π⁰ yields with respect to the reaction plane, and discuss the impact current models have on our understanding of these mechanisms. PACS 25.75.-q; 25.75.Dw     

Open and hidden charm production in dA collisions at RHIC and LHC

We discuss aspects of open and hidden charm production in deuterium-nucleus collisions at RHIC and LHC energies. We describe calculations of the total cross section and the charm quark transverse momentum distributions. We next explain how shadowing and moderate nuclear absorption can explain the PHENIX d Au/pp ratios and predict the combined effect of shadowing and absorption in 6.2 TeV d + Pb collisions.Arrival of the final proofs: 28 April 2005     

A first look at Au+Au collisions at RHIC energies using the PHOBOS detector

The PHOBOS detector has been used to study Au + Au collisions at√^sNN = 56,130, and 200 GeV Several global observables have been measured and the results are compared with theoretical models. These observables include the charged-particle multiplicity measured as a function of beam energy, pseudo-rapidity, and centrality of the collision. A unique feature of the PHOBOS detector is its almost complete angular coverage such that these quantities can be studied over a pseudo-rapidity interval of |η|≤5.4. This allows for an almost complete integration of the total charged particle yield, which is found to be about N _ch ^tot = 4200 ±470 at √^sNN = 130 GeV and N _ch ^tot = 5300 ±530 at √^sNN = 200 GeV. The ratio of anti-particles to particles emitted in the mid-rapidity region has also been measured using the PHOBOS magnetic spectrometer. Of particular interest is the ratio of anti-protons to protons in the mid-rapidity region, which was found to be (i.e.921-1) at √^sNN = 130 GeV. This high value suggests that an almost baryon-free region has been produced in the collisions.     

High<Emphasis Type="Italic">p</Emphasis><Subscript>T</Subscript> physics at STAR

We discuss the capabilities of STAR in exploring the physics at highp _T in ultrarelativistic heavy-ion colisions from RHIC at √^s _NN = 130 GeV Preliminary results show that the spectra of negatively charged particles get suppressed at largerp _T in comparison top-p data. A strong azimuthal anisotropy observed at large transverse momentum region. A preliminary ratio -p/p has been measured by STAR-RICH detector. Some ongoing studies and future plans are discussed.     

VISHNU hybrid model for the viscous QCD matter at RHIC and LHC energies

In this proceeding, we briefly describe the viscous hydrodynamics + hadron cascade hybrid model VISHNU for relativistic heavy ion collisions and report the current status on extracting the QGP viscosity from elliptic flow data.     

Strange particle production in relativistic nucleus-nucleus collisions at RHIC and LHC energy regions

PACIAE, a parton and hadron cascade model, is utilized to systematically investigate strange particle production and strangeness enhancement in Au＋Au collisions and in Pb＋Pb collisions with the √SNN = 200 GeV at the RHIC and 2.76 TeV at the LHC, respectively. The experimental results at different centralities, using data from the STAR collaboration and the ALICE collaboration, are well described by the PACIAE model. This may represent the importance of the parton and hadron rescatterings, as well as the reduction mechanism for strange quark suppression, that are implemented in the PACIAE model.     

Strange particle production in relativistic nucleus-nucleus collisions at RHIC and LHC energy regions

PACIAE, a parton and hadron cascade model, is utilized to systematically investigate strange particle production and strangeness enhancement in Au+Au collisions and in Pb+Pb collisions with the √s_NN=200 GeV at the RHIC and 2.76 TeV at the LHC, respectively. The experimental results at different centralities, using data from the STAR collaboration and the ALICE collaboration, are well described by the PACIAE model. This may represent the importance of the parton and hadron rescatterings, as well as the reduction mechanism for strange quark suppression, that are implemented in the PACIAE model.     

Photon-photon luminosities in relativistic heavy ion collisions at LHC energies

Effective – luminosities are calculated for various realistic hadron collider scenarios. The main characteristics of photon-photon processes at relativistic heavy-ion colliders are established and compared to the corresponding –-luminosities ate ⁺-e ^– — and future photon linear colliders (PLC). Higher order corrections as well as inelastic processes are discussed. It is concluded that feasible high luminosity Ca–Ca collisions at the Large Hadron Collider (LHC) are an interesting option for – physics up to about 100 GeV – CM energy.