Collective expansion in central Au+Au collisions.

We have updated the parton and hadron cascade model PACIAE 2.0 for relativistic nuclear collisions (cf. Comput. Phys. Comm. 183(2012)333) to the new issue of PACIAE 2.1. The PACIAE model is based on PYTHIA. In the PYTHIA model, once the generated particle/parton transverse momentum $p_T$ is randomly selected, its component of $[p_x,p_y]$ was originally put on the circle with radius $p_T$ randomly. It is now replaced by put on the circumference of ellipse with half major and minor axes of $p_T(1+\delta_p)$ and $p_T(1-\delta_p)$, respectively, in order to investigate better the transverse momentum anisotropy in final hadronic state.

PHENIX has measured the e+e− pair continuum in sNN=200 GeV Au+Au and p+p collisions over a wide range of mass and transverse momenta. The e+e− yield is compared to the expectations from hadronic sources, based on PHENIX measurements. In the intermediate-mass region, between the masses of the ϕ and the J/ψ meson, the yield is consistent with expectations from correlated cc̄ production, although other mechanisms are not ruled out. In the low-mass region, below the ϕ, the p+p inclusive mass spectrum is well described by known contributions from light meson decays. In contrast, the Au+Au minimum bias inclusive mass spectrum in this region shows an enhancement by a factor of 4.7±0.4stat±1.5syst±0.9model. At low mass (mee<0.3 GeV/c2) and high pT (1<pT<5 GeV/c) an enhanced e+e− pair yield is observed that is consistent with production of virtual direct photons. This excess is used to infer the yield of real direct photons. In central Au+Au collisions, the excess of the direct photon yield over the p+p is exponential in pT, with inverse slope T=221±19stat±19syst MeV. Hydrodynamical models with initial temperatures ranging from Tinit≃300–600 MeV at times of 0.6–0.15 fm/c after the collision are in qualitative agreement with the direct photon data in Au+Au. For low pT<1 GeV/c the low-mass region shows a further significant enhancement that increases with centrality and has an inverse slope of T≃100 MeV. Theoretical models underpredict the low-mass, low-pT enhancement.43 MoreReceived 1 December 2009DOI:https://doi.org/10.1103/PhysRevC.81.034911©2010 American Physical Society

Momentum spectra of charged pions over nearly full rapidity coverage from target to beam rapidity have been measured in the 0-5% most central Au+Au collisions in the beam energy range from 2 to 8 AGeV by the E895 Experiment. Using a thermal parameterization to fit the transverse mass spectra, rapidity density distributions are extracted. The observed spectra are compared with predictions from the RQMD v2.3 cascade model and also to a thermal model including longitudinal flow. The total 4$\pi$ yields of the charged pions are used to infer an initial state entropy produced in the collisions.

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Transverse momentum spectra of pi+/-, p, and p up to 12 GeV/c at midrapidity in centrality selected Au + Au collisions at square root sNN=200 GeV are presented. In central Au + Au collisions, both pi +/- and p(p) show significant suppression with respect to binary scaling at pT approximately >4 GeV/c. Protons and antiprotons are less suppressed than pi+/-, in the range 1.5 approximately < pT approximately < 6 GeV/c. The pi-/pi+ and p/p ratios show at most a weak pT dependence and no significant centrality dependence. The p/pi ratios in central Au + Au collisions approach the values in p + p and d + Au collisions at pT approximately >5 GeV/c. The results at high pT indicate that the partonic sources of pi+/-, p, and p have similar energy loss when traversing the nuclear medium.

New results on antiparticle to particle ratios in Cu+Cu and Au+Au collisions at = 62.4 and 200 GeV from the PHOBOS experiment at RHIC are presented. Transverse momentum spectra of pions, kaons, protons and antiprotons from Au+Au collisions at = 62.4 GeV close to mid-rapidity are also discussed. Antiparticle to particle ratios are found to be remarkably independent of the collision centrality in both colliding systems. The collision energy dependence of the ratios is very significant in Cu+Cu collisions. Baryons are found to have substantially harder transverse momentum spectra than mesons. The pT region in which the proton to pion ratio reaches unity in central Au+Au collisions at = 62.4 GeV fits into a smooth trend as a function of collision energy. The observed particle yields at very low pT are comparable to extrapolations from higher pT for kaons, protons and antiprotons. The net proton yield at mid-rapidity is found to be proportional to the number of participant nucleons in Au+Au collisions at 62.4 and 200 GeV energies.

High- pT Physics in STAR

We investigate identical pion HBT intensity interferometry in central Au+Au collisions at 1.23A GeV. High-statistics π−π− and π+π+ data are measured with HADES at SIS18/GSI. The radius parameters, derived from the correlation function depending on relative momenta in the longitudinally comoving system and parametrized as three-dimensional Gaussian distribution, are studied as function of transverse momentum. A substantial charge-sign difference of the source radii is found, particularly pronounced at low transverse momentum. The extracted source parameters agree well with a smooth extrapolation of the center-of-mass energy dependence established at higher energies, extending the corresponding excitation functions down towards a very low energy.

Azimuthal correlations of charged hadrons with direct-photon ( γdir ) and neutral-pion ( π0 ) trigger particles are analyzed in central Au+Au and minimum-bias p+p collisions at sNN=200 GeV in the STAR experiment. The charged-hadron per-trigger yields at mid-rapidity from central Au+Au collisions are compared with p+p collisions to quantify the suppression in Au+Au collisions. The suppression of the away-side associated-particle yields per γdir trigger is independent of the transverse momentum of the trigger particle ( pTtrig ), whereas the suppression is smaller at low transverse momentum of the associated charged hadrons ( pTassoc ). Within uncertainty, similar levels of suppression are observed for γdir and π0 triggers as a function of zT ( ≡pTassoc/pTtrig ). The results are compared with energy-loss-inspired theoretical model predictions. Our studies support previous conclusions that the lost energy reappears predominantly at low transverse momentum, regardless of the trigger energy.

We present the first results on 3-particle Δη–Δη correlations in minimum bias d + Au, peripheral and central Au+Au collisions at = 200 GeV measured by the STAR experiment. The analysis technique is described in detail. The ridge particles, observed at large Δη in dihadron correlations in central Au+Au collisions, appear to be uniformly distributed over the measured Δη–Δη region in 3-particle correlation. The results, together with theoretical models, should help further our understanding of the underlying physics of the ridge.

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 pi^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.

Recent data on multifragmentation in central Au+Au collisions at beam energy 100 MeV/nucleon are analyzed within the statistical and molecular dynamics models. The data are consistent with statistical breakup of a thermalized source with radius of 14 fm, mass number around 250, and excitation energy about 7 meV/nucleon, in good agreement with molecular dynamics calculations. According to this model, fragment formation takes place within a time interval of 50 fm/c after the maximum compression. Intense preequilibrium emission of light particles and strong collective flow of fragments are predicted.

In jet quenching, a hard QCD parton, before fragmenting into a jet of hadrons, deposits a fraction of its energy in the medium, leading to suppressed production of high-${p}_{T}$ hadrons. The process can generate shock waves. We study the distortion of Mach shock waves due to jet quenching in central Au+Au collisions and its effect on particle production. Finite fluid velocity and inhomogeneity of the medium can distort the Mach shock front significantly such that the inside shock front disappear and the outside shock front is opened up. We also show that the STAR data on azimuthal distribution of background subtracted secondaries, associated with high ${p}_{T}$ trigger, are reasonably well explained by the excess pions produced due to partonic energy loss.

We present a study of three-particle correlations among a trigger particle and two associated particles in Au + Au collisions at s N N = 200 GeV using a multi-phase transport model (AMPT) with both partonic and hadronic interactions. We found that three-particle correlation densities in different angular directions with respect to the triggered particle (‘center’, ‘cone’, ‘deflected’, ‘near’ and ‘near-away’) increase with the number of participants. The ratio of ‘deflected’ to ‘cone’ density approaches to 1.0 with the increasing of number of participants, which indicates that partonic Mach-like shock waves can be produced by strong parton cascades in central Au + Au collisions.

RHIC experiments carry out a comprehensive physics program which studies open heavy flavor and quarkonium production in relativistic heavy-ion collisions. The discovery at RHIC of large high-pT suppression and flow of electrons from heavy quarks flavors have altered our view of the hot and dense matter formed in central Au + Au collisions at SNN=200 GeV. These results suggest a large energy loss and flow of heavy quarks in the hot, dense matter. In recent years, the RHIC experiments upgraded the detectors; (1) PHENIX Collaboration installed silicon vertex tracker (VTX) at mid-rapidity region and forward silicon vertex tracker (FVTX) at the forward rapidity region, and (2) STAR Collaboration installed the heavy flavor tracker (HFT) and the muon telescope detector (MTD) both at the mid-rapidity region. With these new upgrades, both experiments have collected large data samples. These new detectors enhance the capability of heavy flavor measurements via precision tracking. The PHENIX experiments established measurements of ψ (1S ) and ψ (2S ) production as a function of system size, p + p , p + Al, p + Au, and 3 He + Au collisions at SNN=200 GeV. In p/3 He + A collisions at forward rapidity, we observe no difference in the ψ (2S )/ψ (1S ) ratio relative to p + p collisions. At backward rapidity, where the comoving particle density is higher, we find that the ψ (2S ) is preferentially suppressed by a factor of two. STAR Collaboration presents the first J/ψ and ϒ measurements in the di-muon decay channel in Au + Au collisions at SNN=200 GeV at mid-rapidity at RHIC. We observe clear J/ψ RAA suppression and qualitatively well described by transport models simultaneously accounting for dissociation and regeneration processes.