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).     

Measurement of low mass dielectron continuum in $\sqrt{s_{NN}}$ =200 GeV Au+Au collisions in the PHENIX experiment at RHIC

The first measurement of the dielectron continuum at RHIC energies was performed by the PHENIX experiment for Au+Au collisions at  =200 GeV. Mass spectra for different centralities are presented and compared with the expectations from hadron decays.     

Heavy flavor production in PHENIX

The PHENIX experiment at RHIC measured single electron spectra in p + p, d + Au and Au + Au collisions at GeV, and in Au + Au collisions at GeV. In these spectra, electrons from semi-leptonic decays of charmed particles are the dominant contribution after subtraction of all 'photonic' sources (photon conversions, Dalitz decays, decays of light vector mesons). The p + p open charm production cross-section is found to be in good agreement with pQCD NLO calculations. The shape of the distributions obtained for p + p interactions is compared with those observed for nucleus-nucleus collisions. From p + p to d + Au and Au + Au interactions, open charm production is found to scale with the number of binary collisions . Au + Au data at GeV is compatible with the ISR p + p results scaled by . The elliptic flow parameter v₂ of heavy flavor electrons has also been measured, and is found to be non-zero in the intermediate p_T range. Arrival of the final proofs: 29 June 2005 PACS: . ^* Deceased Spokesperson     

Proton–proton,anti-proton–anti-proton,proton–anti-proton correlations in Au + Au collisions measured by STAR at RHIC

The analysis of two-particle correlations provides a powerful tool to study the properties of hot and dense matter created in heavy-ion collisions at ultra-relativistic energies. Applied to identical and non-identical hadron pairs, it makes the study of space-time evolution of the source in femtoscopic scale possible. Baryon femtoscopy allows extraction of the radii of produced sources which can be compared to those deduced from identical pion studies, providing complete information about the source characteristics. In this paper we present the correlation functions obtained for identical and non-identical baryon pairs of protons and anti-protons. The data were collected recently in Au+Au collisions at  =62 GeV and  =200 GeV by the STAR detector at the RHIC accelerator. We introduce corrections to the baryon–baryon correlations taking into account: residual correlations from weak decays, particle identification probability and the fraction of primary baryons. Finally we compare our results to theoretical predictions. PACS 25.75.-q; 25.75.Gz     

High transverse momentum suppression and surface effects in <Emphasis Type="Bold">Cu+Cu</Emphasis> and <Emphasis Type="Bold">Au+Au</Emphasis> collisions within the PQM model

We study parton suppression effects in heavy-ion collisions within the parton quenching model (PQM). After a brief summary of the main features of the model, we present comparisons of calculations for the nuclear modification and the away-side suppression factor to the data in Au+Au and Cu+Cu collisions at  =200 GeV. We discuss properties of light hadron probes and their sensitivity to the medium density within the PQM Monte Carlo framework.     

What can we learn from high-p<Subscript>T</Subscript> correlations of neutral strange baryons and mesons at RHIC?

We present results on two-particle azimuthal correlations of neutral strange baryons (Λ, Λ̄) and mesons (K_S ⁰) for p_T=2–6 GeV/c associated with non-identified charged particles in d+Au and Au+Au collisions at  =200 GeV measured by the STAR experiment. We investigate in detail the associated yield of charged particles as a function centrality of the collision and transverse momentum of trigger and associated particles to look for possible flavor, baryon/meson and particle/anti-particle differences. We compare our results to the proton and pion triggered correlations as well as to a fragmentation and recombination model. PACS 25.75.-q; 25.75.Gz     

Jet properties from two-particle azimuthal correlations in d+Au collisions at $\sqrt{s_{NN}}$ =200 GeV at STAR

We present measurements of azimuthal correlations between photons (from π⁰ decay) and charged hadrons in d+Au collisions at  =200 GeV. We use di-hadron correlations to study parton fragmentation in d+Au collisions at RHIC. Specifically, the near-side and away-side peaks of the azimuthal angular difference distribution are used to measure the root-mean-squared (RMS) fragmentation transverse momentum and the mean intrinsic parton transverse momentum . The measurements with leading photons are compared to results using leading charged particles. PACS  25.75.-q     

Longitudinal broadening of near-side jets due to parton cascade

Longitudinal broadening along the Δη direction on the near side in the two-dimensional (Δφ×Δη) di-hadron correlation distribution has been studied for central Au+Au collisions at  GeV, within a dynamical multi-phase transport model. It was found that longitudinal broadening is generated by a longitudinal flow induced by a strong parton cascade in central Au+Au collisions, to be compared with p+p collisions at  GeV. The longitudinal broadening may shed light on the strongly interacting partonic matter at RHIC.     

Direct photons in d+Au collisions at $\sqrt{s_{NN}}$ =200 GeV with STAR

Results are presented of an ongoing analysis of direct photon production in  =200 GeV deuteron-gold collisions (d+Au) with the STAR experiment at RHIC. A significant excess of direct photons is observed near mid-rapidity (0<y<1) and found to be consistent with next-to-leading order pQCD calculations including the contribution from fragmentation photons. PACS  25.75.-q     

Analysis of non-photonic electrons in Cu + Cu collisions at $\sqrt{s_{NN}}=200$ GeV

Heavy-flavor semileptonic decays are expected to dominate the spectrum of non-photonic electrons produced from collisions at the energies of the Relativistic Heavy Ion Collider. The non-photonic electron yield is suppressed by approximately a factor of 5 in central Au + Au events at  GeV relative to p+p events with the same collision energy. Most theoretical models predict less non-photonic-electron suppression than is observed experimentally. We present a preliminary measurement of the yield of non-photonic electrons in Cu + Cu events at  GeV, as well as the nuclear modification factor.     

High-p T results from PHOBOS

We present results from the PHOBOS experiment on charged hadron production over a large range in , from Data from d + Au collisions at GeV and Au + Au collisions at and 200 GeV are shown. The data indicate that in Au + Au collisions a strongly interacting state of matter is formed, that appears qualitatively different from the system formed in pp or d + Au collisions. The systematics of particle production in d + Au and Au + Au collisions exhibit remarkably simple scaling rules, including an apparent factorization of the energy and centrality dependence over a wide range of collision energies. Arrival of the final proofs: 29 July 2005 PACS: 25.75.-q, 25.75.Dw, 25.75.Gz     

Recent high-p_{\rm T} results from STAR

The STAR Collaboration has a broad range of recent results on intermediate and high- phenomena in Au + Au collisions at = 200 and 62 GeV and in d + Au at = 200 GeV. These include new measurements of spectra, azimuthal anisotropies and di-hadron correlations. The comparison of the 62 and 200 GeV Au + Au results indicates that jet quenching, elliptic flow and di-hadron correlation measurements are very similar at the two energies. Meson-baryon differences that have been seen at intermediate in 200 GeV Au + Au collisions are also present in 62 GeV Au + Au collisions and in 200 GeV d + Au collisions. Measurements of backward-forward inclusive hadron yield asymmetries and forward-midrapidity di-hadron correlations in d + Au collisions are consistent with the saturation picture. A brief review of these results is presented. Arrival of the final proofs: 30 June 2005 PACS: 25.75.Dw, 25.75.Ld, 25.75.Gz     

First direct measurement of jets in $\sqrt{s_{\mathit {NN}}}=200~\mbox{GeV}$ heavy ion collisions by STAR

We present the first measurement of reconstructed jets in ultra-relativistic heavy ion collisions. Utilizing the large coverage of the STAR Time Projection Chamber and Electromagnetic Calorimeter, we apply several modern jet reconstruction algorithms and background subtraction techniques and explore their systematic uncertainties in heavy ion events. The differential energy spectrum for inclusive jet production in central Au+Au collisions at is presented. In order to assess the jet reconstruction biases, this spectrum is compared with the jet cross section measured in  GeV p+p collisions scaled by the number of binary N–N collisions to account for nuclear geometric effects.     

Search for the onset of baryon anomaly at RHIC-PHENIX

The baryon production mechanism at the intermediate p_T (2–5 GeV/c) at RHIC is still not well understood. The beam energy scan data in Cu+Cu and Au+Au systems at RHIC may provide us a further insight on the origin of the baryon anomaly and its evolution as a function of . In 2005 RHIC physics program, the PHENIX experiment accumulated the first intensive low beam energy data in Cu+Cu collisions. We present the preliminary results of identified charged hadron spectra in Cu+Cu at and 62.4 GeV using the PHENIX detector. The centrality and beam energy dependences of (anti)proton to pion ratios and the nuclear modification factors for charged pions and (anti)protons are presented. PACS 25.75.Dw     

Test of quantum mechanics by neutron interferometry

Measurements of the π⁰ and direct photon nuclear modification factors in p+p and A+A collisions in the PHENIX experiment at RHIC will be reviewed and recent results at  GeV will be presented. Using for the first time the p+p reference measured in the same experiment instead of averaging world data the π⁰ suppression turns out to be almost as large at  GeV as at  GeV, implying gluon densities dN^g/dy>800. Possible origins of photon suppression at high p_T in  GeV are discussed in light of the new results on photon R_AA at  GeV.     

Direct-photon production in heavy-ion collisions from SPS to RHIC energies

Direct photons are an important tool for the detection of the quark-gluon plasma in ultra-relativistic nucleus-nucleus collisions. Direct-photon measurements were made in Pb + Pb collisions at GeV and in Au + Au collisions at GeV. These results are reviewed and compared with model calculations.Arrival of the final proofs: 9 May 2005PACS: 25.75.DwThis revised version was published online in July 2005 with a correction to the article category and incorporation of the revised date.     

Azimuthal anisotropy of electrons from heavy flavor decays in \sqrt{s_{NN}} =200 GeV Au-Au collisions at PHENIX

The PHENIX experiment has measured the azimuthal anisotropy parameter v₂, the second harmonic of the azimuthal distribution, for electrons at mid-rapidity (|η|<0.35) as a function of transverse momentum (0.5<p_T (GeV/c)<5.0) in Au+Au collisions at  =200 GeV. From the result we have calculated the non-photonic electron v₂, which is expected to reflect charm quark azimuthal anisotropy, by subtracting the v₂ of electrons from other sources such as photon conversions and Dalitz decays.     

J/ψ production in PHENIX

Heavy quarkonia production is expected to be sensitive to the formation of a quark gluon plasma (QGP). The PHENIX experiment has measured J/ψ production at  =200 GeV in Au+Au and Cu+Cu collisions, as well as in reference p+p and d+Au runs. J/ψ’s were measured both at mid (|y|<0.35) and forward (1.2<|y|<2.2) rapidity. In this letter, we present the A+A preliminary results and compare them to normal cold nuclear matter expectations derived from PHENIX d+Au and p+p measurements as well as to theoretical models including various effects (color screening, recombination, sequential melting...).     

Heavy quarks at RHIC from parton transport theory

There are several indications that an opaque partonic medium is created in energetic Au+Au collisions at the relativistic heavy ion collider (RHIC). At the extreme densities of ∼10–100 times normal nuclear density reached, even heavy-flavor hadrons are affected significantly. Heavy-quark observables are presented from the parton transport model MPC, focusing on the nuclear suppression pattern, azimuthal anisotropy (“elliptic flow”), and azimuthal correlations. Comparison with Au+Au data at top RHIC energy indicates significant heavy-quark rescattering, corresponding roughly five times higher opacities than estimates based on leading-order perturbative QCD. We propose measurements of charm–anticharm, e.g., D-meson azimuthal correlations as a sensitive, independent probe to corroborate these findings. PACS 25.75.-q; 25.75.Ld; 25.75.Gz     

Suppression of high-pT non-photonic electrons in Au+Au collisions at $\sqrt{s_{NN}}$ =200 GeV

The strong suppression of high p_T hadrons observed at RHIC has led to the interpretation that energetic partons lose their energy via induced gluon radiation in the hot and dense matter before fragmenting into hadrons. The study of heavy quark production can extend our understanding of this scenario. Due to the dead cone effect, the suppression of heavy quark mesons at high p_T is expected to be smaller than that observed for charged hadrons at the same energy. The measurement of non-photonic single electrons up to high p_T provides information on charm and beauty production. The semi-leptonic decays of D and B mesons are the dominant contribution to the non-photonic electron spectra. The preliminary spectra from p+p, d+Au and Au+Au collisions at  =200 GeV have been extracted for mid-rapidity non-photonic electrons in the range 1.5<p_T (GeV/c)<10. The corresponding nuclear modification factors (R_AA) are presented and show a large suppression in central Au+Au collisions, indicating an unexpectedly large energy loss for heavy quarks in the hot and dense matter created at RHIC. This observed suppression is compared to recent theoretical models. PACS  13.85.Qk; 13.20.Fc; 13.20.He; 25.75.Dw