Top RHIC Research Articles

Energy dependence of directed flow inAu+Aucollisions from a multiphase transport model

The directed flow of charged hadron and identified particles has been studied in the framework of a multi-phase transport (AMPT) model, for $^{197}$Au+$^{197}$Au collisions at $\sqrt{s_{NN}}=$200, 130, 62.4, 39, 17.2 and 9.2 GeV. The rapidity, centrality and energy dependence of directed flow for charged particles over a wide rapidity range are presented. AMPT model gives the correct $v_1(y)$ slope, as well as its trend as a function of energy, while it underestimates the magnitude. Within the AMPT model, the proton $v_1$ slope is found to change its sign when the energy increases to 130 GeV - a feature that is consistent with ``anti-flow''. Hadronic re-scattering is found having little effect on $v_1$ at top RHIC energies. These studies can help us to understand the collective dynamics at early times in relativistic heavy-ion collisions, and they can also be served as references for the RHIC Beam Energy Scan program.

Charmed hadron signals of partonic medium

We present a short review of our results on the collectivity and the suppression pattern of charmed mesons in heavy-ion collisions based on the microscopic Hadron-String-Dynamics (HSD) transport approach in different scenarios of charm interactions with the surrounding matter—the ‘comover’ dissociation by mesons with further recreation by channels and ‘pre-hadronic’ interaction scenarios. While at SPS energies the hadronic ‘comover’ absorption scenario is found to be compatible with the experimental data, the dynamics of quarks at RHIC are dominated by partonic or ‘pre-hadronic’ interactions in the strongly coupled quark–gluon plasma stage and cannot be modelled by pure ‘hadronic’ interactions. We find that the collective flow of charm in the purely hadronic scenario appears compatible with the data at SPS energies but underestimates the data at top RHIC energies. Thus, the large elliptic flow v2 of D mesons and the low RAA(pT) of J/Ψ seen experimentally at RHIC have to be attributed to the early interactions of non-hadronic degrees of freedom. Simultaneously, we observe that non-hadronic interactions are mandatory in order to describe the narrowing of the J/Ψ rapidity distribution from pp to central Au + Au collisions at the top RHIC energy. We demonstrate additionally that the strong quenching of low-pTJ/Ψ's in central Au + Au collisions indicates that a fraction of final J/Ψ mesons is created by a coalescence mechanism close to the phase boundary.

OPEN AND HIDDEN CHARM IN PROTON–NUCLEUS AND HEAVY-ION COLLISIONS

We review dynamical and thermal models for the collectivity and the suppression pattern of charmed mesons — produced in proton–nucleus and nucleus–nucleus collisions at SPS (~158 A GeV) and RHIC energies (~21 A TeV). In particular, we examine the charmonium "melting" and the "comover dissociation" scenarios — implemented in a microscopic transport approach — in comparison to the available data from the SPS and RHIC. The analysis shows that the dynamics of c and [Formula: see text] quarks. quarks at RHIC are dominated by partonic or "pre-hadronic" interactions in the strongly coupled plasma stage and can neither be modeled by "hadronic" interactions nor described appropriately by color screening alone. Both the "charmonium melting" and the hadronic "comover absorption and recreation model" are found, however, to be compatible with the experimental observation at SPS energies; the experimental ratio of Ψ′/J/Ψ versus centrality clearly favors the "hadronic comover" scenario. We find that the collective flow of charm in the purely hadronic Hadron-String Dynamics (HSD) transport appears compatible with the data at SPS energies, but the data at top RHIC energies are substantially underestimated. Thus, the large elliptic flow v2 of D-mesons and the low RAA(pT) of J/Ψ seen experimentally have to be attributed to early interactions of non-hadronic degrees of freedom. Simultaneously, we observe that non-hadronic interactions are mandatory in order to describe the narrowing of the J/Ψ rapidity distribution from pp to central Au + Au collisions at the top RHIC energy of [Formula: see text]. Additionally we demonstrate that the strong quenching of high-pTJ/Ψ's in central Au + Au collisions indicates that a large fraction of final J/Ψ mesons is created by a coalescence mechanism close to the phase boundary. Throughout this review we, furthermore, provide predictions for charm observables from Au + Au collisions at FAIR energies of 25–35 A GeV.

Evidence for non-hadronic interactions of charm degrees of freedom in heavy-ion collisions at relativistic energies

Within the Hadron-String Dynamics (HSD) transport approach we study the suppression pattern of charmonia at RHIC with respect to centrality and rapidity employing various model concepts, such as variants of the ‘comover absorption’ model or the ‘charmonium melting’ scenario. We find that especially the ratio of the forward to mid-rapidity nuclear modification factors of J / Ψ ( R A A forward ( J / Ψ ) / R A A mid ( J / Ψ ) ) cannot be explained by the interactions with ‘formed’ comoving mesons or by the ‘color screening mechanism’ alone. Only when incorporating interactions of the c or c ¯ quark with a pre-hadronic medium satisfactory results are obtained. A detailed comparison to the PHENIX data demonstrates that non-hadronic interactions are mandatory to describe the narrowing of the J / Ψ rapidity distribution from pp to central Au + Au collisions. The Ψ ′ to J / Ψ ratio is found to be crucial in disentangling the different charmonium absorption scenarios, especially in the RHIC energy range. Furthermore, a comparison of the transport calculations to the statistical model of Gorenstein and Gazdzicki as well as the statistical hadronization model of Andronic et al. shows differences in the energy dependence as well as centrality dependence of the J / Ψ to pion ratio which may be exploited experimentally to disentangle different concepts. We find additionally that the collective flow of charm in the HSD transport appears compatible with the data at SPS energies but substantially underestimates the data at top RHIC energies, such that the large elliptic flow v 2 of charm seen experimentally has to be attributed to early interactions of non-hadronic degrees of freedom.

Nuclear suppression at RHIC and LHC in Glauber–Gribov approach

The approach to problem of nuclear shadowing based on Gribov Reggeon calculus is presented. Here the total cross section of hA interaction is found in a parameter-free description, employing the new data on the gluon density of the Pomeron, measured with high precision at HERA, as input. The model is then applied for calculation of J/ψ production in dAu collisions at top RHIC energy. It is shown that the theoretical estimates are in a very good agreement with the PHENIX data, and further predictions for the J/ψ suppression in pPb collisions at energies of LHC are made.

A STATISTICAL MODEL ANALYSIS OF K/π FLUCTUATIONS IN HEAVY ION COLLISIONS

We briefly describe two statistical hadronization models, based respectively on the presence and absence of light quark chemical equilibrium, used to analyze particle yields in heavy ion collisions. We then try to distinguish between these models using K/π fluctuations data. We find that while the non-equilibrium model provides an acceptable description of fluctuations at top SPS and RHIC energies, both models considerably under-estimate fluctuations at low SPS energies.

Direct photon production in p + p and Au + Au collisions at in the PACIAE model

A parton and hadron cascade model, PACIAE, is used in investigating the PHENIX data of the direct photon and π0 invariant differential cross sections in p + p collisions and the direct photon and π0pT distributions in 0–10% of most central Au + Au collisions at top RHIC energy. It turns out that the above data are possible to be described by this microscopic transport model.

Elliptic flow and constituent quark scaling from hadron-string transport models

The elliptic flow (v2) excitation function and identified hadron v2 at top RHIC energy are investigated with the ultra-relativistic quantum molecular dynamics (UrQMD) model. The underestimation of the v2 data by the UrQMD model at RHIC energies can possibly be explained by assuming a phase transition from hadron gas to quark–gluon plasma at ∼30 A GeV. The missing pressure might be generated by strong interactions in the early pre-hadronic/partonic phase of central Au+Au (Pb+Pb) collisions. As for the v2 of identified particles at top RHIC energy, the multi-strange particles show the same amount of v2 as light baryons. Both the mass ordering at low pT and the number of constituent quark scaling of v2(pT) at intermediate pT are approximately reproduced.

Event-by-event analysis of baryon-strangeness correlations: Pinning down the critical temperature and volume of quark-gluon-plasma formation

The recently proposed baryon-strangeness correlation (C_BS) is studied with a string-hadronic transport model (UrQMD) for various energies from E_lab=4 AGeV to \sqrt s=200 AGeV. It is shown that rescattering among secondaries can not mimic the predicted correlation pattern expected for a Quark-Gluon-Plasma. However, we find a strong increase of the C_BS correlation function with decreasing collision energy both for pp and Au+Au/Pb+Pb reactions. For Au+Au reactions at the top RHIC energy (\sqrt s=200 AGeV), the C_BS correlation is constant for all centralities and compatible with the pp result. With increasing width of the rapidity window, C_BS follows roughly the shape of the baryon rapidity distribution. We suggest to study the energy and centrality dependence of C_BS which allow to gain information on the onset of the deconfinement transition in temperature and volume.

Modified string fragmentation function and particle production in nuclear collisions at top RHIC energy

A modified string fragmentation function is assumed in a hadron and string cascade model, LUCIAE (m), accounting for interactions among strings formed at the early stage of relativistic nucleus–nucleus collisions. It turns out that the experimental data of global properties such as the charged multiplicity and the ϕ meson rapidity density at mid-rapidity in p+p, d+Au and Au+Au collisions at GeV are reproduced well by LUCIAE (m) rather than by LUCIAE (d) (default LUCIAE). However, LUCIAE (m) fails to account for the suppression of a high pt transverse momentum spectrum observed in central Au+Au collisions at RHIC, even if final state rescatterings among produced hadrons are included in the calculations. This fact indicates that some novel mechanism has to be introduced to understand the suppression of hadron production at high pt.

Cronin Effect from Backward to Forward Rapidity: A Tale of Two Misteries

I discuss recent experimental data on the Cronin effect in deuteron-gold collisions at the top RHIC energy, in a pseudorapidity range η∈[−2, 3]. Two theoretical approaches are compared and contrasted: the pQCD-based Glauber-Eikonal model and Colour Glass Condensate models. Neither can describe the Cronin effect over the whole pseudorapidity interval up to now explored experimentally, its most mysterious and intriguing part being at negative rapidity.

Investigating multi-strange baryon production in Au+Au collisions at GeV and its excitation function in high-energy heavy-ion collisions

We present here preliminary results on multi-strange baryon (Ξ and Ω) production in Au+Au collisions at the collision energy of GeV as measured with the STAR detector at RHIC. Two different methods have been used on the data for selecting these particles: the ‘classical’ method and a linear discriminant analysis (LDA). We discuss the observed dynamical and chemical properties with respect to beam energy from SPS to top RHIC energies.

Multi-strange baryon production in Au+Au collisions at top RHIC energy as a probe of bulk properties

We report STAR preliminary results on multi-strange baryon production in Au+Au collisions at at RHIC. Its implication for the formation of a new state of matter is discussed. The system size dependence on the production of strange baryons is investigated to study the onset of strange quark equilibration in the medium. The nuclear modification factor of Λ, Ξ and Ω is also presented. Its suppression at pT > 3 GeV/c supports the formation of a dense interacting medium at RHIC. The spectra of multi-strange baryons reveal that within a hydro-inspired model, they may decouple earlier than lighter particles and that their flow may be mostly developed at a partonic level. This idea is emphasized by the measurement of the v2 of and whose behaviour is close to the baryon elliptic flow in the intermediate pT region where a constituent quark scaling of v2 is observed.

Strangelet search at RHIC

Two position sensitive shower maximum detectors (SMDs) for zero degree calorimeters (ZDCs) were installed by STAR before run 2004 at both upstream and downstream from the interaction point along the beam axis where particles with small rigidity are swept away by strong magnetic field. The ZDC–SMDs provides information about neutral energy deposition as a function of transverse position in ZDCs. We report the preliminary results of strangelet search from a triggered data-set sampling 100 million Au+Au collisions at top RHIC energy.

Probing bulk properties and partonic collectivity via multi-strange baryons in Au+Au collisions at top RHIC energy

The study of the multi-strange production in the final state of ultra-relativistic Au+Au collisions at top RHIC energy gives information on the freeze-out conditions and probably on earlier stages of the collision. The chemical freeze-out temperature as well as the strangeness phase space occupancy factor gamma_s extracted from statistical models are studied as a function of the collision centrality. Within the framework of a hydrodynamically-inspired model the transverse collectivity of the collision is investigated. In this view, multi-strange particles seem to develop significant radial transverse flow. Furthermore, they present an elliptic flow as stronger as previously measured baryons supporting the idea that a fraction of the final collective motion has been developed at an early partonic stage of the collision.

Statistical hadronization of charmed quarks at SPS and RHIC

Production of open and hidden charm hadrons in heavy ion collisions is considered within the statistical coalescence model. The charmed quark-antiquark pairs are assumed to be created at the initial stage of the reaction in hard parton collisions. The number of these pairs is conserved during the evolution of the system. At the hadronization stage, the charmed (anti)quarks are distributed among open and hidden charm hadrons in accordance with the laws of statistical mechanics. The model is in agreement with the experimental data on J/psi to Drell-Yan ratio in Pb+Pb collisions at SPS. This agreement can be reached only if a rather strong enhancement of the open charm production in central Pb+Pb collisions is assumed. A possible mechanism of the charm enhancement is discussed. At the top RHIC energy, the model predicts an increase of J/psi to Drell-Yan ratio in more central nucleus-nucleus collisions with respect to less central ones.