Top RHIC Research Articles

Limiting fragmentation in a thermal model with flow

The property of limiting fragmentation of various observables such as rapidity distributions ($dN/dy$), elliptic flow ($v_{2}$), average transverse momentum ($\langle p_{T} \rangle$) etc. of charged particles is observed when they are plotted as a function of rapidity ($y$) shifted by the beam rapidity ($y_{beam}$) for a wide range of energies from AGS to RHIC. Limiting fragmentation (LF) is a well studied phenomenon as observed in various collision energies and colliding systems experimentally. It is very interesting to verify this phenomenon theoretically. We study such a phenomenon for pion rapidity spectra using our hydrodynamic-like model where the collective flow is incorporated in a thermal model in the longitudinal direction. Our findings advocate the observation of extended longitudinal scaling in the rapidity spectra of pions from AGS to lower RHIC energies, while it is observed to be violated at top RHIC and LHC energies. Prediction of LF hypothesis for Pb+Pb collisions at $\sqrt{s_{NN}}$=5.02 TeV is given.

Measurement of hadron suppression and study of its connection with vanishing v3 at low [formula omitted] in Au + Au collisions with STAR

At top RHIC and LHC energies the suppression of high transverse momentum (pT) hadrons provides evidence for partonic energy loss in QGP. We study partonic energy loss in the RHIC Beam Energy Scan (BES) by investigating the centrality dependence of the binary-collision-scaled high-pT yields. Observing a decrease of the scaled yield in more central collisions is proposed as a possible signature for jet quenching. Even at energies and centralities where this signature is lost a QGP may still be formed since competing phenomena responsible for enhancements may overwhelm the suppression from energy loss. Measurements in several ranges of pT from sNN=7.7,11.5,14.5,19.6,27,39,62.4, and 200 GeV data show that relative hadron suppression persists at least down to 14.5 GeV.To further investigate both the possible formation of a QGP at these lower energies and whether the observed hadron suppression coincides with the onset of other QGP signatures, we examine the energy and centrality dependence of v3. Models have shown that the development of v3 requires the presence of a low viscosity phase early in the collision. We find that for collisions with Npart<50, v3 disappears for energies below 14.5 GeV, suggestive of a turn-off of the QGP. But for Npart<50, v3 persists down to the lowest energies. Together, these signatures provide possible evidence for the formation of a QGP in the lowest energy collisions at RHIC.

DRoplet and hAdron generator for nuclear collisions: An update

DRoplet and hAdron generator for nuclear collisions: An update

Testing of the coalescence mechanism in high energy heavy ion collisions using two-particle correlations with identified particle trigger

In central Au-Au collisions at top RHIC energy, two particle correlation measurements with identified hadron trigger have shown attenuation of near side proton triggered jet-like yield at intermediate transverse momentum ($p{_T}$), 2$< p{_T} <$ 6 GeV/$\it{c}$. The attenuation has been attributed to the anomalous baryon enhancement observed in the single inclusive measurements at the same $p{_T}$ range. The enhancement has been found to be in agreement with the models invoking coalescence of quarks as a mechanism of hadronization. Baryon enhancement has also been observed at LHC in the single inclusive spectra. We study the consequence of such an enhancement on two particle correlations at LHC energy within the framework of A Multi Phase Transport (AMPT) model that implements quark coalescence as a mode of hadronization. In this paper we have calculated the proton over pion ratio and the near side per trigger yield associated to pion and proton triggers at intermediate $p{_T}$ from String Melting (SM) version of AMPT. Results obtained are contrasted with the AMPT Default (Def.) which does not include coalescence. Baryon enhancement has been observed in AMPT SM at intermediate $p{_T}$. Near side jet-like correlated yield associated to baryon (proton) trigger in the momentum region where baryon generation is enhanced is found to be suppressed as compared to the corresponding yields for the meson (pion) trigger in most central Pb-Pb events. No such effect has been found in the Default version of AMPT.

Centrality and Transverse Momentum Dependence of Elliptic Flow of Multistrange Hadrons and ϕ Meson in Au+Au Collisions at √[sNN]=200 GeV.

We present high precision measurements of elliptic flow near midrapidity (|y|<1.0) for multistrange hadrons and ϕ meson as a function of centrality and transverse momentum in Au+Au collisions at center of mass energy √[sNN]=200 GeV. We observe that the transverse momentum dependence of ϕ and Ω v2 is similar to that of π and p, respectively, which may indicate that the heavier strange quark flows as strongly as the lighter up and down quarks. This observation constitutes a clear piece of evidence for the development of partonic collectivity in heavy-ion collisions at the top RHIC energy. Number of constituent quark scaling is found to hold within statistical uncertainty for both 0%-30% and 30%-80% collision centrality. There is an indication of the breakdown of previously observed mass ordering between ϕ and proton v2 at low transverse momentum in the 0%-30% centrality range, possibly indicating late hadronic interactions affecting the proton v2.

Investigating the scaling of higher-order flows in relativistic heavy-ion collisions

The modified number of constituent quark (NCQ) scaling $v_{n}/n_{q}^{n/2} \sim KE_{T}/n_{q}$ for mesons and baryons and the scaling relation $v_{n} \sim v_{2}^{n/2}$ for higher-order anisotropic flows, which were observed experimentally, have been investigated at top RHIC energy. It has been found that the modified NCQ scaling can not be obtained from the naive coalescence even by taking into account event-by-event fluctuations but may be due to hadronic afterburner or thermal freeze-out. In addition, we observed that the behavior of the $v_{n}/v_{2}^{n/2}$ ratio is sensitive to the partonic interaction, and it is different for mesons and baryons from the naive coalescence but is expected to be almost the same experimentally.

Rapidity distribution of protons from the potential version of UrQMD model and the traditional coalescence afterburner

Rapidity distributions of both E895 proton data at AGS energies and NA49 net proton data at SPS energies can be described reasonably well with a potential version of the UrQMD in which mean-field potentials for both pre-formed hadrons and confined baryons are considered, with the help of a traditional coalescence afterburner in which one parameter set for both relative distance $R_0$ and relative momentum $P_0$, (3.8 fm, 0.3 GeV$/$c), is used. Because of the large cancellation between the expansion in $R_0$ and the shrinkage in $P_0$ through the Lorentz transformation, the relativistic effect in clusters has little effect on the rapidity distribution of free (net) protons. Using a Woods-Saxon-like function instead of a pure logarithmic function as seen by FOPI collaboration at SIS energies, one can fit well both the data at SIS energies and the UrQMD calculation results at AGS and SPS energies. Further, it is found that for central Au+Au or Pb+Pb collisions at top SIS, SPS and RHIC energies, the proton fractions in clusters are about 33$\%$, 10$\%$, and 0.7$\%$, respectively.

Centrality dependence of elliptic flow of multi-strange hadrons in Au+Au collisions at √sNN = 200 GeV

We present recent results of the mid-rapidity elliptic flow (ν2) for multi-strange hadrons and the ϕ meson as a function of centrality in Au + Au collisions at the center of mass energy √sNN = 200 GeV. The transverse momentum dependence of ϕ and Ω ν2 is similar to that of pion and proton, indicating that the heavier strange (s) quark flows as strongly as the lighter up (u) and down (d) quarks. These observations constitute a clear piece of evidence for the development of partonic collectivity in heavy-ion collisions at the top RHIC energy. In addition, the mass ordering of ν2 breaks between the ϕ and proton at low transverse momenta in the 0-30% centrality bin, possibly due to the effect of late hadronic interactions on the proton ν2.

ExtractingpΛscattering lengths from heavy ion collisions

The source radii, previously extracted by STAR Collaboration from the $p-\Lambda \oplus \bar{p}-\bar{\Lambda}$ and $\bar{p}-\Lambda \oplus p-\bar{\Lambda}$ correlation functions measured in 10% most central Au+Au collisions at top RHIC energy $\sqrt{s_{NN}}=200$ GeV, differ by a factor of 2. The probable reason for this is the neglect of residual correlation effect in the STAR analysis. In the present paper we analyze baryon correlation functions within Lednicky and Lyuboshitz analytical model, extended to effectively account for the residual correlation contribution. Different analytical approximations for such a contribution are considered. We also use the averaged source radii extracted from the hydrokinetic model (HKM) simulations to fit the experimental data. In contrast to the STAR experimental study, the calculations in HKM show both $p\Lambda$ and $p\bar{\Lambda}$ radii to be quite close, as expected from theoretical considerations. Using the effective Gaussian parametrization of residual correlations we obtain a satisfactory fit to the measured baryon-antibaryon correlation function with the HKM source radius value 3.28 fm. The baryon-antibaryon spin-averaged strong interaction scattering length is also extracted from the fit to the experimental correlation function.

Experimental studies of the quantum chromodynamics phase diagram at the STAR experiment

We review the STAR experiment’s results to date from the Beam Energy Scan (BES) at Brookhaven’s Relativistic Heavy Ion Collider, and outline future plans and prospects in this area. BES Phase-I is based on Au + Au data taken in 2010 and 2011 at \(\sqrt {s_{NN}}= 7.7\), 11.5, 19.6, 27 and 39 GeV, and when interpreted in conjunction with the large datasets available at 62.4 and 200 GeV, permits an initial exploration of the phase diagram of quantum chromodynamics (QCD) matter. The three goals of BES Phase-I are as follows: (1) a search for turn-off of the promising signatures of quark gluon plasma (QGP) already reported at the top RHIC energies; (2) a search for evidence of a possible first-order phase transition such as a signature of softening of the QCD equation of state (EoS); (3) a search for a critical end point as expected in a scenario where there is a cross-over transition from hadronic matter to QGP at the highest RHIC energies, but a first-order phase transition at lower energies with finite net-baryon density. We summarize several analyses of BES data from 2010 and 2011 that are either published or submitted, as well as several more that have been reported at meetings in preliminary forms. The physics interpretation of BES Phase-I measurements is frequently limited by the increasing statistical error bars as the beam energy decreases, and the planned BES Phase-II will have much improved capabilities in this regard.

Overview of STAR results on correlations, jets and heavy-flavor production

Measurements of jets and heavy-flavor production play an important role in understanding properties of hot and dense nuclear matter created in high energy heavy-ion collisions. As direct measurements of jets are difficult due to large underlying background, jet properties can be also studied via di-hadron and multihadron correlations. In this contribution to these proceedings, an overview of recent STAR results on correlations, jet-hadron correlations, and heavy-flavor production in Au+Au collisions at top RHIC energy ( = 200 GeV) is given. In order to draw quantitative conclusions on properties of the hot and dense nuclear matter created in Au+Au collisions, reference measurements in elementary proton-proton collisions and d+Au collisions are essential to estimate contributions due to cold nuclear matter effects and are discussed as well.

Exploring nuclear matter at neutron star core densities

Substantial experimental and theoretical efforts worldwide are devoted to explore the phase diagram of strongly interacting matter. At top RHIC and LHC energies, the QCD phase diagram is studied at very high temperatures and very low net-baryon densities. These conditions presumably existed in the early universe about a microsecond after the big bang. For larger net-baryon densities and lower temperatures, it is expected that the QCD phase diagram exhibits a rich structure such as a critical point, a first order phase transition between hadronic and partonic matter, or new phases like quarkyonic matter. The experimental discovery of these prominent landmarks of the QCD phase diagram would be a major breakthrough in our understanding of the properties of nuclear matter.

Modifications of heavy-flavor spectra insNN=62.4GeV Au-Au collisions

We calculate open heavy-flavor (HF) production in Au+Au collisions at $\sqrt{s_{\rm NN}}$=62.4 GeV utilizing a nonperturbative transport approach as previously applied in nuclear collisions at top RHIC and LHC energies. The effects of hot QCD matter are treated in a strong-coupling framework, by implementing heavy-quark diffusion, hadronization and heavy-flavor meson diffusion within a hydrodynamic background evolution. Since in our approach the heavy-flavor coupling to the medium is strongest in the pseudo-critical region, it is of interest to test its consequences at lower collision energies where the sensitivity to this region should be enhanced relative to the hotter (early) fireball temperatures reached at top RHIC and LHC energies. We find that the suppression and flow pattern of the non-photonic electrons from heavy-flavor decays at 62.4 GeV emerges from an intricate interplay of thermalization and initial-state effects, in particular a Cronin enhancement which is known to become more pronounced toward lower collision energies.

Present and future of QGP research in high energy heavy ion collision experiments

from p-p and p-Pb collisions at LHC at different centre of mass energies have been used as references to the results from Pb-Pb collisions. These results are further compared with detailed and elaborate measurements performed with Au-Au collisions at p sNN = 200 GeV at RHIC. The basic properties of the matter created at LHC have been found to be similar to that at RHIC, e.g., the matter in Pb-Pb collisions at LHC also behaves like a strongly coupled partonic liquid as inferred for the Au-Au collisions at top RHIC energy. However, with higher collision energy at LHC, a new set of probes in heavy ion collisions like jets and higher states of quarkonia open up new avenues in detailed understanding of the properties of the matter. Recent findings of the collective behaviour at p-Pb collisions at LHC however questions the existing theoretical understanding of the medium formation in p-Pb and Pb-Pb collisions. RHIC and LHC explore the high temperature and nearly zero net-baryon density region of the phase diagram, another extremes are being explored at the beam energy scan (BES) programme at RHIC and the upcoming CBM experiment at FAIR. The BES-I results demonstrate that at lower p sNN , several findings that were interpreted as the signatures of the formation of de-confined partonic matter at top RHIC energy were found to be absent. The interpretation of these results require thorough understanding of the role of baryons in the matter created at lower p sNN . The CBM experiment at FAIR aims to explore the matter created at low collision energy by the use of bulk and rare probes.

Beam energy dependence of dielectron production in Au+Au collisions from STAR at RHIC

We present the energy-dependent study of dielectron production in 0-80% minimum-bias Au+Au collisions at $\sqrt{s_{NN}}$ energies of 19.6, 27, 39, and 62.4 GeV in STAR. Invariant mass ($M_{ee}$) and transverse momentum ($p_T$) differential measurements of dielectron yields are compared to cocktail simulations of known hadronic sources and semi-leptonic charmed decays. The enhancement (excess yield) prominent in the Low-Mass Region (LMR) over the cocktail at all energies, is further compared to calculations of $\rho$ in-medium modifications. Within statistical and systematic uncertainties, we find that the model consistently describes this enhancement from SPS up to top RHIC energies in its $M_{ee}$- as well as $p_T$-dependence. Dielectron measurements drive the statistics for the future BES Phase-II program, which promises to improve our understanding of the LMR enhancement's trend with total baryon densities.

Femtoscopic pair correlations of mesons and baryons at RHIC and LHC from hydrokinetic model

Abstract The femtoscopic scales for identical pions and kaons obtained in hydrokinetic model (HKM) are presented for the top energy RHIC and LHC A + A collisions. The source functions for p Λ ¯ ⊕ p ¯ Λ pairs in Au + Au collisions are found and used for extraction of the scattering length in these two-baryon systems by means of FSI correlation technique. The role of residual correlations in formation of the total baryon–antibaryon correlation function is analyzed.

Beam-energy dependence of charge separation along the magnetic field in Au+Au collisions at RHIC.

Local parity-odd domains are theorized to form inside a quark-gluon plasma which has been produced in high-energy heavy-ion collisions. The local parity-odd domains manifest themselves as charge separation along the magnetic field axis via the chiral magnetic effect. The experimental observation of charge separation has previously been reported for heavy-ion collisions at the top RHIC energies. In this Letter, we present the results of the beam-energy dependence of the charge correlations in Au+Au collisions at midrapidity for center-of-mass energies of 7.7, 11.5, 19.6, 27, 39, and 62.4GeV from the STAR experiment. After background subtraction, the signal gradually reduces with decreased beam energy and tends to vanish by 7.7GeV. This implies the dominance of hadronic interactions over partonic ones at lower collision energies.

Final state effects on charge asymmetry of pion elliptic flow in high-energy heavy-ion collisions

Within a multi-phase transport (AMPT) model with the string melting mechanism and imported initial electric quadrupole moment, the difference between the elliptic flow of positive and negative pions is calculated. The slope parameter r of the linear dependence of Δv2=v2(π−)−v2(π+) on Ach=(N+−N−)/(N++N−) is yielded owing to the parton cascade, which converts the initial electric quadrupole distribution into the final charge-dependent elliptic flow. The slope parameter r is found to be increased by the hadronization given by the coalescence, and decreased by the resonance decays. The slope parameter r is very sensitive to both the initial electric quadrupole percentage and centrality bin and consequently a helpful constraint on the quadrupole moment of the chiral magnetic wave is obtained for Au + Au collisions at the top RHIC energy.

Hadronic observables in hydrokinetic picture of A+A collisions at RHIC and LHC

The simultaneous description of the hadronic yields, pion, kaon and proton spectra, elliptic flows and femtoscopy scales in hydrokinetic model of A+A collisions is presented at different centralities for the top RHIC and LHC energies. The hydrokinetic model is used in its hybrid version that allows one to switch correctly to the UrQMD cascade at the isochronic hypersurface which separates the cascade stage and decaying hydrodynamic one. The results are compared with pure hybrid model where hydrody- namics and hadronic cascade are matching just at the non-space-like hypersurface of chemical freeze-out. The initial conditions are based on both Glauber- and KLN- Monte- Carlo simulations and results are compared. It seems that the observables, especially femtoscopy data, prefer the Glauber initial conditions. The modification of the particle number ratios caused, in particular, by the particle annihilations at the afterburn stage is analyzed. 1 Introduction and model description In this paper, we apply the hybrid hydrokinetic model (1) for the description of bulk matter dynamics in heavy ion collisions at top RHIC and 2.76 TeV LHC energies. The unique feature of the model is an advanced procedure of connection from hydrodynamic approach for hot and dense matter above chemical freezeout temperature to cascade model applied for rescatterings in the rarefied hadron gas. We start by discussing the ingredients of the model. In the present analysis we employ the two variants of initial conditions for the hydrodynamic stage of evolution: the Monte-Carlo Glauber (MC-Glauber) and MC-KLN models. In MC-Glauber (Monte-Carlo Glauber) model, one starts from sampling the individual nucleon po- sitions inside the nuclei according to an average nucleon density distribution. The nuclei are then arranged with a relative impact parameter b and projected onto the transverse plane. The collision cri- terion for a pair of nucleons in each event is based on the value of nucleon-nucleon cross-section σNN at the corresponding collision energy. The nucleon-nucleon collisions result in deposition of a certain amount of multiplicity (entropy) to different cells in transverse plane. The contributions to multiplicity (entropy) from the hard elementary collisions and from the soft ones are different, the former are proportional to the number of binary collisions while the later are associated with number of wounded

Elliptic flow parameter in Au+Au and p+Au collisions at top RHIC energy

The new issue of parton and hadron cascade model PACIAE 2.1 is utilized investigating the charged particle elliptic flow parameter v2 in Au + Au and p+ Au collisions at top RHIC energy. If the transverse momenta px and py of particles from string fragmentation are randomly sampled on the circumference of an ellipse instead of circle originally the PHENIX charged particle v2(η) and v2(pT) data in the 20-40% and 40-50% central Au + Au collisions at [Formula: see text] are fairly reproduced by the PACIAE 2.1 calculations, respectively. In addition, the charged particle v2(η) and v2(pT) in the p+ Au collisions at [Formula: see text] are predicted. It turned out that the charged particle elliptic flow parameter v2 may reach a measurable value of ~0.04 in the above p+ Au collisions.