Identified particle production in p+p, d+Au and Au+Au collisions at RHIC

Measurements of identified particle production with the PHENIX experiment at RHIC have reached a mature state, where a multitude of nuclear systems at different colliding energies have been studied. The discovery configurations of $\sqrt {s_{NN} } = 130$ and 200 GeV Au+Au collisions have now been supplemented by additional Au+Au and Cu+Cu configurations at various energies, along with baseline p+p and d+Au runs at $\sqrt {s_{NN} } = 130$ GeV. In this work we present a systematic study of the Cronin effect in d+Au collisions and recent results from p+p collisions. We then proceed to make a critical comparison of pion, kaon and proton production in heavy ion and baseline systems, and discuss the observed nuclear effects on hadron production.     

Strange hadron production in heavy ion collisions from SPS to RHIC

Strange particles have been a very important observable in the search for a deconfined state of strongly interacting matter, the quark–gluon plasma (QGP), which is expected to be formed in ultra-relativistic heavy ion collisions. We review the main experimental observations made at the Super Proton Synchrotron (SPS) at CERN, Geneva, and at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL). The large amount of recently collected data allows for a comprehensive study of strangeness production as a function of energy and system size. We review results on yields, transverse mass and rapidity spectra, as well as elliptic flow. The measurements are interpreted in the context of various theoretical concepts and their implications are discussed. Of particular interest is the question whether strange particles are in any way sensitive to a partonic phase. Finally, a compilation of experimental data is provided.     

Forward a production and nuclear stopping power in d+Au collisions at RHIC

Using the forward time projection chambers of STAR we measure the centrality dependent A and ā yields in d+Au collisions at √s _NN =200 GeV at forward and backward rapidities. The contributions of different processes to particle production and baryon transport are probed exploiting the inherent asymmetry of the d+Au system. While the d side appears to be dominated by multiple independent nucleon-nucleon collisions, nuclear effects contribute significantly on the Au side. Using the constraint of baryon number conservation, the rapidity loss of baryons in the incoming deuteron can be estimated as a function of centrality. This is compared to a model and to similar measurements in Au+Au, which gives insights into the nuclear stopping power at relativistic energies.     

Charged-particle pseudorapidity distributions in Au+Au collisions at RHIC

Using the Glauber model, we present the formulas for calculating the numbers of participants,spectators and binary nucleon-nucleon collisions. Based on this work, we get the pseudorapidity distributions of charged particles as the function of the impact parameter in nucleus-nucleus collisions. The theoretical results agree well with the experimental observations made by the BRAHMS Collaboration in Au+Au collisions at √SNN=200 GeV in different centrality bins over the whole pseudorapidity range.     

Charged-particle pseudorapidity distributions in Au+Au collisions at RHIC

Using the Glauber model, we present the formulas for calculating the numbers of participants, spectators and binary nucleon-nucleon collisions. Based on this work, we get the pseudorapidity distributions of charged particles as the function of the impact parameter in nucleus-nucleus collisions. The theoretical results agree well with the experimental observations made by the BRAHMS Collaboration in Au＋Au collisions at √^SNN=200 GeV in different centrality bins over the whole pseudorapidity range.     

Charge fluctuations in Au+Au collisions at RHIC energy

A hadron and string cascade model, JPCIAE, together with the corresponding Monte Carlo event generator, has been employed in this paper to investigate further the charge fluctuations in Au+Au collisions at Snn = 130 GeV. The default JPCIAE calculations are in good agreement with PHENIX and STAR data. We found that the thermal predictions for the π gas, the resonance π gas and quark matter deviate, respectively, from the corresponding dynamical simulations from the JPCIAE model. The discrepancies were also found between the π charge fluctuations and the charge fluctuations of all species of hadrons. However the charge fluctuations for "π from ρ and ω decay" and for all the hadrons from resonance decay are close to each other, indicating the correlation between positively and negatively charged hadrons is not sensitive to the species of hadrons. This work shows further that it is questionable to use the charge fluctuations as a signature of QGP.     

Jet production at next-to-leading order in p+Au collisions at the RHIC

We calculate jet productions in p+Au collisions at the RHIC at next-to-leading order with perturbative QCD. Inclusive jet transverse energy spectrum, dijet invariant mass spectrum, dijet angular distribution, and corresponding nuclear modification factors for the three observables in p+Au collisions at √s=200 GeV are given, where the initial-state cold nuclear matter (CNM) effects are included by taking advantage of four parametrization sets of nuclear parton distribution functions (nPDFs) - EPS, nCTEQ, HKN and DS. We demonstrate that inclusive jet transverse energy (E_T) spectrum, dijet invariant mass (M_JJ) spectrum with all 4 nPDFs are increased at low E_T or M_JJ, whereas at high E_T or M_JJ large deviation of results with different nPDFs is observed. It is found that the dijet angular distributions in p+Au collisions do not vary relative to those in p+p collisions for all 4 nPDFs.     

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.     

Strange particle production mechanisms in proton—Proton collisions at RHIC

We present data on strange particle production in elementary proton-proton collisions at RHIC energies. Comparison to leading order and next-to-leading order (NLO) calculations shows that the fragmentation process is flavor dependent and that higher order corrections are needed to describe all spectra, in particular at these collision energies, which are modest compared to those at the Tevatron. A model (EPOS) which takes into account multiple scattering between projectile constituents seems to describe the data best.     

Quarkonium production from d + Au to Au + Au collisions

The PHENIX experiment measured J/ production in pp, d + Au and Au + Au reactions at = 200 GeV over a wide range of rapidity and transverse momentum. The nuclear modification factor obtained by comparing the d + Au and pp cross sections as a function of rapidity, is consistent with shadowing of the gluon distribution functions. J/ production in Au + Au collisions was compared to the production in pp collisions and it was found to be inconsistent with models that predict strong enhancement relative to binary collision scaling. Arrival of the final proofs: 29 June 2005 PACS: 25.75-q, 25.75-Dw, ^* Deceased Spokesperson     

Event-by-event hadron ratio fluctuations from Au+Au collisions at RHIC-STAR

We present measurements of event-by-event fluctuations on hadron multiplicity ratios (K/??, p/??, K/p) in Au+Au collisions at $\sqrt {s_{NN} } $ and 200 GeV using the STAR detector at RHIC. The magnitude of dynamical fluctuations ?? _dyn for p/?? and K/p ratios change smoothly from a large negative value at 7.7 GeV to a smaller negative value at 200 GeV while that for the K/?? ratios exhibits no significant beam energy dependence. The dynamical fluctuations related to pair production ?? _dyn ^pair dyn for the p/K, K/p, K ^?/K ⁺ and p/p ratios at 200 GeV all exhibit a maximum at the mid-central collisions and decrease at the most peripheral and most central collisions.     

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.     

J/Ψ measurement in p+p, d+Au and Au+Au collisions by the PHENIX experiment at RHIC

J/Ψ’s are produced mostly via interactions involving gluons, and are a sensitive probe of the gluon structure function and its modification in nuclei. They are also considered as a leading signal for studying the creation of hot and dense matter in relativistic heavy ion collision. Measurement of J/Ψ production in different colliding systems is important for understanding the nuclear modification factor, and setting a baseline for the study of J/Ψ suppression in heavy ion collisions. In this talk we report the latest results on J/Ψ measurements by the PHENIX experiment at RHIC in p+p; d+Au, and Au+Au collisions at backward, forward, and mid-rapidity. Nuclear effects are studied as a function of transverse momentum, rapidity and centrality.     

Directed flow of identified particles from Au+Au collisions at RHIC

We present the measurement of directed flow (v ₁) for the identified particles, namely, Λ, $ \bar \Lambda $ \bar \Lambda and K _s ⁰, as a function of rapidity and centrality in Au+Au collisions at $ \sqrt {s_{NN} } $ \sqrt {s_{NN} } = 200 GeV and 62.4 GeV. The measurement is based on the run IV data obtained by the STAR experiment at RHIC. In order to enhance event plane resolution, we use tracks reconstructed from the Forward Time Projection Chambers (FTPCs), together with the sideward deflection of spectator neutrons measured by the STAR’s Shower Maximum Detector at Zero Degree Calorimeters (ZDC-SMDs). We find that for 200 GeV, proton and antiproton v ₁ is less than 1%, the K _s ⁰ Λ, $ \bar \Lambda $ \bar \Lambda v ₁ is less than 2%; for 62 GeV, proton v ₁ is less than 1% and antiproton is less than 2%, v ₁ for K _s ⁰, Λ, $ \bar \Lambda $ \bar \Lambda is less than 2% in Au+Au collisions at 200 GeV.     

Evidence from d+Au measurements for final-state suppression of high-p(T) hadrons in Au+Au collisions at RHIC

We report measurements of single-particle inclusive spectra and two-particle azimuthal distributions of charged hadrons at high transverse momentum (high p(T)) in minimum bias and central d+Au collisions at sqrt[s(NN)]=200 GeV. The inclusive yield is enhanced in d+Au collisions relative to binary-scaled p+p collisions, while the two-particle azimuthal distributions are very similar to those observed in p+p collisions. These results demonstrate that the strong suppression of the inclusive yield and back-to-back correlations at high p(T) previously observed in central Au+Au collisions are due to final-state interactions with the dense medium generated in such collisions.     

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.     

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.     

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.