Plans for correlation studies for BES program at RHIC

We present an overview of the main concepts concerning correlation studies for the Beam Energy Scan Program at STAR. QGP signatures at higher RHIC energies are the most obvious example of the creation of a new state of matter. In order to learn more about the transition to this state and to find a possible location of a critical point between a first-order transition and cross-over area, a new program dedicated to such analyses is formulated. Correlation studies are one of the most important obervables of the scanning of unknown region of the phase diagram. Measurements of elliptic flow, local parity violation in strong interactions, azimuthally-sensitive correlations as well as two-proton femtoscopy and nonidentical particle correlations are discussed in this paper.     

Anisotropic flow at RHIC

STAR’s measurement of directed flow for pions,kaons（K 0 S ）,protons and anti-protons,for Au＋Au collisions at 200 GeV obtained in Run7 are presented,as well as elliptic flow for identified particles measured in Au＋Au（Run7） and Cu＋Cu（Run5） collisions.It is found that the slope of proton v 1 （y） at midrapidity is extremely small.Elliptic flow results are compared to Hydro calculation and the discrepancy is discussed.     

Heavy-ion collisions at RHIC

The European Physical Journal C - In the first three years of running, the four RHIC experiments have collected a rich set of high quality data on nuclear collisions over a wide range in collision...     

z Scaling at RHIC

The concept of z scaling reflecting the general regularities of high-p _T particle production is reviewed. Properties of data z presentation are discussed. New data on high-p _T particle spectra obtained at the RHIC are analyzed in the framework of z presentation. It was shown that these experimental data confirm z scaling. Predictions of strange particle spectra are considered to be useful for understanding of strangeness origin in mesons and baryons and search for new physics phenomena at the RHIC. The text was submitted by the author in English.     

Hydrodynamic flow at RHIC

We review the apparently hydrodynamic behaviour of low transverse momentum particles (pT ≤ 1.5 GeV/c) produced in central and semicentral (b ≤ 7 fm) heavy ion collisions at RHIC. We investigate the impact parameter dependence of various observables, elaborating on radial and elliptic flow and particle multiplicities. We also discuss possible ambiguities in the initialization of the hydrodynamic system and present observables that should allow for their resolution.     

Collective dynamics at RHIC

We discuss the strong collective flow effects, 〈v ₂〉 and 〈β _t 〉, observed at RHIC (√ ^s NN=130 GeV). We argue that part of the collectivity could have already developed at partonic stage. To ‘see’ the partonic effects directly, measurements of multi-strange baryons like Ξ, Ω and charmed particles like J/ψ transverse momentum distributions are important. They provide one of the possible keys to the discovery of collective modes with the partonic (quarks and gluons) degrees of freedom.     

Finding charm at RHIC

Dileptons produced by hard scatterings, such as heavy quark decays and the Drell-Yan mechanism, probe the gluon and sea quark distributions in hadrons. In nucleus-nucleus collisions, these processes are the thermal ‘back-ground’. To determine the magnitude and behavior of this background, we calculate the hard scattering contribution to next to leading order in perturbative QCD at RHIC. We compare these results to optimistic estimates of thermal dilepton production and find that initial charm production is dominant. However, experimental cuts can reduce the charm background, enhancing thermal information.     

Anisotropic flow at RHIC

STAR’s measurement of directed flow for pions,kaons(K 0 S ),protons and anti-protons,for Au+Au collisions at 200 GeV obtained in Run7 are presented,as well as elliptic flow for identified particles measured in Au+Au(Run7) and Cu+Cu(Run5) collisions.It is found that the slope of proton v 1 (y) at midrapidity is extremely small.Elliptic flow results are compared to Hydro calculation and the discrepancy is discussed.     

Clocking hadronization at RHIC

The RHIC accelerator collided Au on Au beams at $\sqrt {s_{NN} } = 130$ GeV in the summer of 2000. To study these collisions, the RHIC experiments have been using a number of observables. One new observable in the field of relativistic heavy ions is the balance function. The balance function can be used to measure the correlation of charged particle pairs in rapidity. The rapidity separation of a particle pair that is created at the same point indicates the time of hadronization for that pair. Preliminary balance function analyses of STAR data are reported. The calculation of a balance function for hadron gas simulations (HIJING) is also discussed in this paper. These preliminary results indicate that the balance function is a useful observable in heavy-ion collisions.     

Entropy production at RHIC

For central heavy ion collisions at the RHIC energy, the entropy per unit rapidity dS/dy at freeze-out is extracted with minimal model dependence from available experimental measurements of particle yields, spectra, and source sizes estimated from two-particle interferometry. The extracted entropy rapidity density is consistent with lattice gauge theory results for a thermalized quark–gluon plasma with an energy density estimated from transverse energy production at RHIC.     

Balance functions at RHIC

The balance function is based on the principle that charge is locally conserved when particles are pair produced. Balance functions have been measured for all charged pairs, identified pion pairs, and identified charged kaon pairs in Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV and p+p collisions at $\sqrt {s_{NN} } = 200$ GeV at the Relativistic Heavy Ion Collider using STAR. Balance functions for all charged particles from Au+Au scale smoothly with centrality to the p+p value. Balance functions for charged particles and pions are narrower in central collisions than in peripheral collisions consistent with trends predicted by models incorporating the concept of late hadronization. Balance functions for kaon pairs represent a strangeness balance. Balance functions for kaons are narrower than those for pion pairs and may show less dependence on centrality.     

Probing longitudinal dynamics at RHIC

Particle production of charged hadrons in Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV bas been studied as a function of rapidity by the BRAHMS Collaboration at RHIC. Selected recent results are presented with emphasis on longitudinal dynamics of particle production. The rapidity dependence of particle production imposes more stringent constraints on theoretical models describing dynamics of nuclear matter created by high-energy heavy ion collisions.     

Exploring superdense matter at RHIC

I report first results on Au + Au collisions at GeV per nucleon pair from all four experiments at the Relativistic Heavy Ion Collider.Received: 30 September 2002, Published online: 22 October 2003PACS: 25.75.Dw Particle and resonance production     

Net charge fluctuations at RHIC

We present measurements of dynamical net charge fluctuations in Au+Au collisions at $\sqrt {s_{NN} } = 20$ , 62, 130 and 200 GeV using the measure ν _+?,dyn. We observe the dynamical fluctuations are finite at all energies, and do not exhibit dependence on beam energy. We find net charge fluctuations violate the trivial 1/N scaling expected for nuclear collisions consisting of independent nucleon-nucleon interactions. We also find dynamical fluctuations exhibit sizable dependence of the pseudo-rapidity and azimuthal ranges of integration. We compare measured data with transport models and a toy model invoking radial flow, and show the bulk of the measured correlations can be accounted for by resonance production and radial collective flow.     

Highlights from the STAR experiment at RHIC

Ultra-relativistic heavy-ion collisions produced at RHIC differ significantly from a superposition of proton-proton collisions. Evidence of collective expansion has been gathered. The yield of high transverse momentum particles has been found to be lower in head-on Au?Au collisions than is expected by scaling p-p collisions. Di-jet processes, which are frequent in p-p collisions, are almost absent in head-on Au?Au collisions. The current results from RHIC indicate that Au?Au collisions at $\sqrt {S_{NN} } = 130$ GeV and $\sqrt {S_{NN} } = 200$ GeV yield an expanding system that is opaque to high momentum partons.     

Strange particle production at RHIC

We report STAR measurements of mid-rapidity yields for the Λ , , K _S ⁰ , Ξ ⁻, , Ω ⁻, particles in Cu + Cu and Au + Au  GeV collisions. We show that at a given number of participating nucleons, bulk strangeness production is higher in Cu + Cu collisions compared to Au + Au collisions at the same center of mass energy, counter to predictions from the Canonical formalism. We compare both the Cu + Cu and Au + Au yields to AMPT and EPOS predictions, and find they reproduce key qualitative aspects of the data. Finally, we investigate other scaling parameters and find bulk strangeness production for both the measured data and theoretical predictions, scales better with the number participants that undergo more than one collision.