Analysis of multi-strange hyperon production in p+p and Au+Au collisions at RHIC

The analysis of multi-strange hyperon production (Ξ^?, Ω^? and their anti-particles) at mid-rapidity is part of the program covered by the STAR experiment at RHIC. We report on preliminary results in Au+Au and p+p collisions at $\sqrt {s_{NN} } = 130$ and 200 GeV. Measurements of particle ratios are presented and production yields and spectra are discussed for both collision systems.     

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.     

Global observables at PHENIX

We present PHENIX recent results on charged particle and transverse energy densities measured at mid-rapidity in Au?Au collisions at $\sqrt {s_{NN} } = 130$ GeV and 200 GeV over a broad range of centralities. The mean transverse energy per charged particle is derived. The first PHENIX measurements at $\sqrt {s_{NN} } = 19.6$ GeV are also presented. A comparison with calculations from various theoretical models is performed.     

Resonance production in STAR

The recent results from resonance production in central Au+Au and p+p collisions at $\sqrt {s_{NN} } = 200$ GeV from the STAR experiment at RHIC are presented and discussed.     

Elliptic flow measurements from STAR

In these proceedings some of the highlights of the elliptic flow measurements from STAR at $\sqrt {s_{NN} } = 130$ and 200 GeV for Au + Au collisions are presented.     

Characteristics of charged particle production in relativistic heavy-ion collisions

Inclusive spectra of charged particles at midrapidity in Au+Au collisions at $\sqrt {s_{NN} } = 130$ GeV and 200 GeV were measured with the STAR detector at RHIC. The measured mean transverse momentum 〈p _T 〉 shows a characteristic dependence on charged particle multiplicity and beam energy in Au+Au collisions that is distinctly different from pp, $p\bar p$ and e⁺e^? collisions. A 32%±3%(syst) increase in 〈p _T 〉 from pp to Au+Au collisions was observed at 200 GeV. While the charged multiplicity was found to increase by 19%±5%(syst) from $\sqrt {s_{NN} } = 130$ GeV to 200 GeV, no significant difference in 〈p _T 〉 was found between the two energies. A comparison with model predictions is discussed.     

Extraction of kinetic freeze-out properties and effect of resonance decays

We present STAR results from identified particle spectra measured in $\sqrt {s_{NN} } = 62.4$ GEV Au-Au collisions. Particle production and system dynamics are compared to results at $\sqrt {s_{NN} } = 200$ GeV. We extract kinetic and chemical freeze-out parameters using blast wave model parametrization and statistical model. We discuss the effect of resonance decays on the extracted kinetic freeze-out parameters.     

K S 0 , Λ and ≡ production at intermediate to high pT from Au+Au collisions at \sqrt {s_{NN} } = 39, 11.5 and 7.7 GeV

We report on the p _T dependence of nuclear modification factors (R _CP) for K _S ⁰ , ??, ?? and the $\bar NK_S^0 $ ratios at mid-rapidity from Au+Au collisions at $\sqrt {s_{NN} } $ = 39, 11.5 and 7.7 GeV. At $\sqrt {s_{NN} } $ = 39 GeV, the R _CP data show a baryon/meson separation at intermediate p _T and a suppression for K _S ⁰ for p _T up to 4.5 GeV/c; the $\bar \Lambda K_S^0 $ shows baryon enhancement in the most central collisions. However, at $\sqrt {s_{NN} } $ = 11.5 and 7.7 GeV, R _CP shows less baryon/meson separation and $\bar NK_S^0 $ shows almost no baryon enhancement. These observations indicate that the matter created in Au+Au collisions at $\sqrt {s_{NN} } $ = 11.5 or 7.7 GeV might be distinct from that created at $\sqrt {s_{NN} } $ = 39 GeV.     

Open charm production at RHIC: Recent results from STAR

We report recent measurements on the open charm production in d+Au and p+p collisions at $\sqrt {s_{NN} } = 200$ GeV from the STAR detector at RHIC. The two independent measurements — direct open charm hadron (D⁰, D^* etc.) reconstruction and non-photonic single electron spectrum — provide consistent results. The mid-rapidity charm differential cross section per nucleon-nucleon collision from d+Au collisions at RHIC is $d\sigma _{c\bar c}^{NN} /dy = 0.30 \pm 0.04(stat.) \pm 0.09(syst.)$ mb, which is higher than predictions from most of the NLO pQCD calculations. Implications for charmonium production in Au+Au collisions will also be discussed.     

Antiproton and charged kaon production in \sqrt {s_{NN} } = 130 GeV Au+Au collisions at RHICGeV Au+Au collisions at RHIC

Preliminary results on antiproton and charged kaon spectra and the net-proton number at mid-rapidity are reported for Au+Au collisions at $\sqrt {s_{NN} } = 130$ GeV as measured by the STAR experiment at RHIC. Inverse slope parameters of the transverse mass distributions increase with the collision centrality, consistent with a strong radial flow. The antiproton and charged kaon extrapolated yields, normalized to the uncorrected negatively charged hadron yield, increase with the collision centrality. A finite but small number of net-baryons is found to be present at mid-rapidity.     

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.     

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.     

High transverse momentum correlations in STAR

Azimuthal correlations of charged hadrons with transverse momenta above 2 GeV/c are measured relative to ≥ GeV/c trigger particle in p+p and Au+Au at $\sqrt {s_{NN} } = 200$ GeV in STAR. The correlations exhibit small relative azimuthal angle correlations characteristic of jets. The p+p and peripheral Au+Au data exhibit a peak on the opposite side of the jet indicating the presence of hard scattering in the form of di-jets. The away-side jet disappears for the most central Au+Au collisions, suggesting jet quenching in a dense medium.     

Recent results from PHOBOS

The PHOBOS Collaboration has measured charged particles emitted over the range ?5.4<η<5.4 for Au+Au collisions at energies of $\sqrt {s_{NN} } = 19.6$ , 62.4, 130 and 200 GeV. In this contribution I will present recent results on the pseudorapidity distributions, total charged-particle production, and collective flow of particles obtained from these measurements.     

Charged hadron suppression in Au+Au collisions at high p T from STAR

We report high statistics measurements of inclusive charged hadron production in Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV and 130 GeV. A large, approximately constant hadron suppression is observed in central Au+Au collisions for 5 ‖p _T ‖ 12 GeV/c. The collision energy dependence of the yields and the centrality and p _T dependence of the suppression provide stringent constraints on theoretical models of suppression. Models incorporating initial-state gluon saturation or partonic energy loss in dense matter are largely consistent with observations. The p _T -dependent suppression expected from models incorporating jet attenuation in cold nuclear matter or absorption of fragmentation hadrons is not observed.     

Open heavy flavor production from light ion collisions at RHIC

PHENIX measures leptons at mid and forward rapidities and extracts leptons resulting from semi-leptonic decays of heavy quarks. We present PHENIX results related to heavy quark production, specifically the invariant cross section of the non-photonic single electrons produced at midrapidity in p+p and d+Au collisions at $\sqrt {s_{NN} } = 200$ GeV. The measured cross section for p+p collisions is compared with the NLO prediction, and a possible excess over the prediction is noted. The measured cross section for d+Au collisions scales with the number of colliding nucleon pairs from the p+p spectra.     

High p T identified particles in PHENIX: Data vs. theory

Two of the most interesting experimental results of heavy-ion reactions at RHIC collider energies are in the hard scattering sector where central Au+Au data show a very different behaviour compared to p+p and peripheral Au+Au collisions. The so-called “high p _T π ⁰ suppression” and the “anomalous” baryon/meson ratio observed by PHENIX in central Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV are reviewed and compared to various theoretical calculations based on different strongly interacting medium scenarios.     

Probing the QCD phase boundary with elliptic flow in relativistic heavy ion collisions at STAR

We present measurement of elliptic flow, v ₂, for charged and identified particles at midrapidity in Au+Au collisions at $\sqrt {s_{NN} } $ = 7.7?C39 GeV. We compare the inclusive charged hadron v ₂ to those from transport model calculations, such as the UrQMD model, AMPT default model and AMPT string-melting model. We discuss the energy dependence of the difference in v ₂ between particles and anti-particles. The v ₂ of ? meson is observed to be systematically lower than other particles in Au+Au collisions at $\sqrt {s_{NN} } $ = 11.5 GeV.     

Mid-rapidity ϕ meson production at \sqrt {s_{NN} } = 200 GeV Au+Au and pp collisions from STAR

We present the results for the measurement of ? meson production in $\sqrt {s_{NN} } = 200$ GeV Au+Au and pp collisions at the Relativistic Heavy Ion Collider (RHIC). Using the event mixing technique, spectra and yields are obrained from the ?→K⁺K^? decay channel for five centrality bins in Au+Au collisions and in pp collisions. We observe that the spectrum shape in Au+Au collisions depends weakly on the centrality and the shape of the spectrum in pp collisions is significantly different from that in Au+Au collisions.     

Resonance studies at STAR

Preliminary results from measurements of resonances (K ^*0(892), $\overline {K*^0 } (892)$ , Φ(1020), and ρ(770)) and weakly decaying particles (Λ(1116), $\bar \Lambda (1116)$ , and K _S ⁰ (498)) are presented. The measurements are performed at mid-rapidity by the STAR detector in $\sqrt {s_{NN} } = 130$ GeV Au?Au collisions at RHIC. The ratios K ^*0/h?, $\overline {K*^0 } /K$ , and $\bar \Lambda /\Lambda $ are compared to measurements at different energies and colliding systems. Estimates of thermal parameters, such as temperature and baryon chemical potential, are also presented.