Quark-gluon Plasma Formation Research Articles

J/ψ measurements in STAR Experiment at RHIC

During the last decade STAR experiment has studied the hot and dense nuclear matter produced in heavy ion collisions at RHIC. The study of quarkonia, such as J/ψ meson, production provides the information about properties of this nuclear matter. It is predicted that due to the Debye screening the quarkonia production is suppressed when Quark Gluon Plasma (QGP) is presented. In order to understand the properties of hot and dense nuclear matter, it is necessary to study the production of quarkonia in hadron-hadron, hadron-ion, and ion-ion collisions separately to distinguish cold nuclear matter effect, such as gluon shadowing, from the suppression due to the formation of QGP. Moreover, the J/ψ hadron azimuthal correlations allow to extract the fraction of J/ψ that originates from B-mesons. This proceedings presents recent J/ψ measurements at mid-rapidity in p+p, and Au+Au collisions at = 200 GeV at STAR, the fraction of J/ψ originates from B-mesons, and J/ψ eliptic flow v2 as a function of transverse momentum in Au+Au collisions in = 200 GeV at STAR.

Effects of nuclear absorption on theΛ¯/p¯ratio in relativistic heavy-ion collisions

An enhanced antiLambda/antiproton ratio in heavy-ion relative to p+p collisions has been proposed as one of the signatures for the Quark-Gluon Plasma (QGP) formation. A significantly large (antiLambda+antiSigma0+1.1*antiSigma-)/antiproton ratio of 3.5 has been observed in the mid-rapidity and low transverse momentum region in central Au+Au collisions at the nucleon-nucleon center-of-mass energy of 4.9 GeV at the Alternating Gradient Synchrotron (AGS). This is an order of magnitude larger than the values in peripheral Au+Au collisions and p+p collisions at the corresponding energy. By using the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) transport model, we demonstrate that the observed large ratio can be explained by strong absorption of antiprotons (~99.9%) and antiLambdas (~99%) in dense nuclear matter created in central collisions. We find within the model that the initial antiLambda/antiproton ratio, mainly from string fragmentation, does not depend on the collision centrality, and is consistent with that observed in p+p collisions. This suggests that the observed large (antiLambda+antiSigma0+1.1*antiSigma-)/antiproton ratio at the AGS does not necessarily imply the formation of the QGP. We further study the excitation function of the ratio in UrQMD, which may help in the search and study of the QGP.

Nonextensive statistical effects in the quark-gluon plasma formation at relativistic heavy-ion collisions energies

AbstractWe investigate the relativistic equation of state of hadronic matter and quark-gluon plasma at finite temperature and baryon density in the framework of the non-extensive statistical mechanics, characterized by power-law quantum distributions. We impose the Gibbs conditions on the global conservation of baryon number, electric charge and strangeness number. For the hadronic phase, we study an extended relativistic mean-field theoretical model with the inclusion of strange particles (hyperons and mesons). For the quark sector, we employ an extended MIT-Bag model. In this context we focus on the relevance of non-extensive effects in the presence of strange matter.

On QGP formation in pp collisions at 7 TeV

The mechanism of quark–gluon plasma (QGP) formation in pp collisions is discussed in the framework of the EPOS model. The enhancement of direct photons at low pt is predicted as a sensitive QGP signal in pp collisions at 7 TeV. Experimental measurements to test this prediction and to cross-check from other QGP signals are advocated.

Constituent quark scaling violation due to baryon number transport

In ultrarelativistic heavy-ion collisions at $\sqrt{{s}_{NN}}\ensuremath{\approx}200$ GeV, the azimuthal emission anisotropy of hadrons with low and intermediate transverse momentum (${p}_{T}\ensuremath{\lesssim}4$ GeV/c) displays an intriguing scaling. In particular, the baryon (meson) emission patterns are consistent with a scenario in which a bulk medium of flowing quarks coalesces into three-quark (two-quark) ``bags.'' While a full understanding of this number-of-constituent-quark (NCQ) scaling remains elusive, it is suggestive of a thermalized bulk system characterized by colored dynamical degrees of freedom---a quark-gluon plasma (QGP). In this scenario, one expects the scaling to break down as the central energy density is reduced below the QGP formation threshold; for this reason, NCQ-scaling violation searches are of interest in the energy scan program at the Relativistic Heavy Ion Collider. However, as $\sqrt{{s}_{NN}}$ is reduced, it is not only the initial energy density that changes; there is also an increase in the net baryon number at midrapidity, as stopping transports entrance-channel partons to midrapidity. This phenomenon can result in violations of simple NCQ scaling. Still in the context of the quark coalescence model, we describe a specific pattern for the breakdown of the scaling that includes different flow strengths for particles and their antipartners. Related complications in the search for recently suggested exotic phenomena are also discussed.

Evolution of collectivity as a signal of quark gluon plasma formation in heavy ion collisions

A measurement for studying the mass dependence of the dilepton interferometry in relativistic heavy-ion collision experiments as a tool to characterize the quark-gluon phase is proposed. In calculations involving dileptons, we show that the mass dependence of radii extracted from the virtual photon (dilepton) interferometry provide access to the development of collective flow with time. It is argued that the non-monotonic variation of HBT radii with invariant mass of the lepton pairs signals the formation of quark gluon plasma in heavy ion collisions. Our proposal of experimentally measuring the ratio, $R_{\mathrm out}/R_{\mathrm side}$ for dileptons can be used to estimate the average life times of the partonic as well as the hadronic phases.

Direct Photons at Low Transverse Momentum: A Signal for Quark-Gluon Plasma inppCollisions at LHC

We investigate photon production in a scenario of quark-gluon plasma formation in proton-proton scattering at 7 TeV. It is shown that thermal photon yields increase quadratically with the charged particle multiplicity. This gives an enhanced weight to high multiplicity events, and leads to an important photon production even in minimum-bias events, where the thermal photons largely dominate over the prompt ones at transverse momentum values smaller than 10 GeV/c.

Inclusive prompt photon production in nuclear collisions at RHIC and LHC

Nuclear modification factors of inclusive prompt photon production in d-Au collisions at RHIC and p-Pb collisions at the LHC are provided at different rapidities. The calculations are performed at NLO accuracy using the EPS09 NLO nuclear parton distribution functions (nPDFs) and their error sets. The results are compared to the ones obtained with the nDS and HKN07 NLO nPDFs, and to the corresponding nuclear modification factors of neutral pion production in these collisions. The sensitivity of these results to the scale choice is also investigated. Interestingly, the predictions using the different nPDF sets differ from each other to the extent that this observable can be expected to become very useful for probing nPDFs over a wide range of Bjorken-x. In order to obtain a perturbative QCD baseline in heavy-ion collisions, calculations are carried out for minimum bias Au-Au collisions at RHIC and Pb-Pb collisions at the LHC. We also estimate the maximal possible suppression which the produced QCD matter can be expected to have on inclusive prompt photon production due to the quenching of the fragmentation component. The nuclear modification factor for prompt photon production is thus suggested to be used for gauging both the cold and the hot nuclear matter effects on other hard processes which are expected to be affected by quark-gluon plasma formation, such as large-pT hadron and jet production.

Formula omitted] production and absorption in p + A and d+Au collisions

The level of “anomalous” charmonium suppression in high-energy heavy-ion collisions and its interpretation as a signal of quark-gluon plasma formation requires a robust understanding of charmonium production and absorption in proton-nucleus collisions. In a previous study we have shown that, contrary to common belief, the so-called J / ψ “absorption cross section”, σ a b s J / ψ , is not a “universal constant” but, rather, an effective parameter that depends very significantly on the charmonium rapidity and on the collision energy. Here we present ugraded Glauber calculations with the EPS09 parameterization of nuclear modifications of the parton densities. We confirm that the effective “absorption cross section” depends on the J / ψ kinematics and the collision energy. We also make further steps towards understanding the physics of the mechanisms behind the observed “cold nuclear matter” effects.

Model investigation on the probability of QGP formation at different centralities in relativistic heavy ion collisions

The quantitative dependence of quark-gluon plasma (QGP)-formation probability (${P}_{\mathrm{QGP}}$) on the centrality of $\mathrm{Au}\ensuremath{-}\mathrm{Au}$ collisions is studied using a bond percolation model. The ${P}_{\mathrm{QGP}}$ versus the maximum distance ${S}_{\mathrm{max}}$ for a bond to form is calculated from the model for various nuclei and the ${P}_{\mathrm{QGP}}$ at different centralities of $\mathrm{Au}\ensuremath{-}\mathrm{Au}$ collisions for the given ${S}_{\mathrm{max}}$ are obtained therefrom. The experimental data of the nuclear modification factor ${R}_{\mathit{AA}}({p}_{T})$ for the most central $\mathrm{Au}\ensuremath{-}\mathrm{Au}$ collisions at $\sqrt{{s}_{\mathit{NN}}}=200$ and $130$ GeV are utilized to transform ${S}_{\mathrm{max}}$ to $\sqrt{{s}_{\mathit{NN}}}$. The ${P}_{\mathrm{QGP}}$ for different centralities of $\mathrm{Au}\ensuremath{-}\mathrm{Au}$ collisions at these two energies are thus obtained, which is useful for correctly understanding the centrality dependence of the experimental data.

Renormalization group equation and QCD coupling constant in the presence of SU(3) chromo-electric field

We solve renormalization group equation in QCD in the presence of SU(3) constant chromo-electric field E^a with arbitrary color index a=1,2,...8 and find that the QCD coupling constant \alpha_s depends on two independent casimir/gauge invariants C_1=[E^aE^a] and C_2=[d_{abc}E^aE^bE^c]^2 instead of one gauge invariant C_1=[E^aE^a]. The \beta function is derived from the one-loop effective action. This coupling constant may be useful to study hadron formation from color flux tubes/strings at high energy colliders and to study quark-gluon plasma formation at RHIC and LHC.

Gauge-gravity duality and high energy collisions

We propose an overview on the Gauge/Gravity correspondence applied to the dynamics of high-energy collisions, such as the properties and formation of the quark-gluon plasma in heavy-ion collisions. Some recent applications to other high-energy scattering processes are also discussed.

Energy dependence of J/ψ absorption in proton-nucleus collisions

Charmonium states are expected to be considerably suppressed in the case of quark-gluon plasma formation in high-energy heavy-ion collisions. However, a robust identification of suppression patterns as signatures of a deconfined QCD medium requires a detailed understanding of the ``normal nuclear absorption'' already present in proton-nucleus collisions, where the charmonium production cross sections increase less than linearly with the number of target nucleons. We analyse the J/ψ production cross sections measured in proton-nucleus collisions in fixed target experiments, with proton beam energies from 200 to 920 GeV, and in d-Au collisions at RHIC, at (sNN)1/2 = 200 GeV, in the framework of the Glauber formalism, using several sets of parton distributions with and without nuclear modifications. The results reveal a significant dependence of the ``absorption cross section'' on the kinematics of the J/ψ and on the collision energy. Extrapolating the observed patterns we derive the level of absorption expected at Elab = 158 GeV, the energy at which the heavy-ion data sets were collected at the CERN SPS.

LHC capabilities for quarkonium

The measurement of the charmonium and bottomonium resonances in nucleus-nucleus collisions provides crucial information on high-density QCD matter. First, the suppression of quarkonia production is generally agreed to be one of the most direct probes of quark-gluon plasma formation. The observation of anomalous J/ψ suppression at the CERN-SPS and at RHIC is well established but the clarification of some important remaining questions requires equivalent studies of the ϒ family, will be possible at the LHC. Second, the production of heavy-quarks proceeds mainly via gluon-gluon fusion processes and, as such, is sensitive to saturation of the gluon density at low-x in the nucleus. Measured departures from the expected vacuum quarkonia cross-sections in Pb+Pb collisions at the LHC will thus provide valuable information not only on the thermodynamical state of the produced partonic medium, but also on the initial-state modifications of the nuclear parton distribution functions. The capabilities of the LHC detectors (ALICE, ATLAS and CMS) to study quarkonia production in Pb+Pb collisions at 5.5 TeV per nucleon pair are discussed.

Heavy ion event generator HYDJET++ (HYDrodynamics plus JETs)

Heavy ion event generator HYDJET++ (HYDrodynamics plus JETs)

Nuclear stopping and sideward-flow correlation from 0.35Ato 200AGeV

The correlation between the nuclear stopping and the scale invariant nucleon sideward flow at energies ranging from those available at the GSI heavy ion synchrotron (SIS) to those at the CERN Super Proton Synchrotron (SPS) is studied within ultrarelativistic quantum molecular dynamics (UrQMD). The universal behavior of the two experimental observables for various colliding systems and scale impact parameters are found to be highly correlated with each other. As there is no phase transition mechanism involved in the UrQMD, the correlation may be broken down by the sudden change of the bulk properties of the nuclear matter, such as the formation of quark-gluon plasma (QGP), which can be employed as a QGP phase transition signal in high-energy heavy ion collisions. Furthermore, we also point out that the appearance of a breakdown of the correlation may be a powerful tool for searching for the critical point on the QCD phase diagram.

Density matrix of a quantum field in a particle-creating background

We examine the time evolution of a quantized field in external backgrounds that violate the stability of vacuum (particle-creating backgrounds). Our purpose is to study the exact form of the final quantum state (the density operator at the final instant of time) that has emerged from a given arbitrary initial state (from a given arbitrary density operator at the initial time instant) in the course of evolution. We find a generating functional that allows one to obtain density operators for an arbitrary initial state. Averaging over states of the subsystem of antiparticles (particles), we obtain explicit forms of reduced density operators for the subsystem of particles (antiparticles). Analyzing one-particle correlation functions, we establish a one-to-one correspondence between these functions and the reduced density operators. It is shown that in the general case a presence of bosons (e.g., gluons) in the initial state increases the creation rate of the same type of bosons. We discuss the question (and its relation to the initial stage of quark–gluon plasma formation) whether a thermal form of one-particle distribution can appear even if the final state of the complete system is not in thermal equilibrium. In this respect, we discuss some cases when pair-creation by an electric-like field can mimic the one-particle thermal distribution. We apply our technics to some QFT problems in slowly varying electric-like backgrounds: electric, SU(3) chromoelectric, and metric. In particular, we analyze the time and temperature behavior of the mean numbers of created particles, provided that the effects of switching the external field on and off are negligible. It is demonstrated that at high temperatures and in slowly varying electric fields the rate of particle-creation is essentially time-dependent.

HEAVY-QUARKONIA IN THE STAR EXPERIMENT

Heavy Quarkonium states modifications in relativistic heavy ion collisions have been of great interest since the proposal by Matsui and Satz of J/ψ suppression as a signature of Quark-Gluon Plasma (QGP) formation. Recent studies suggest that the excited states χc, ψ(2 S ) and ϒ(3 S ) melt sequentially1,2 and the amount of observed suppression depends on the state and medium conditions. Therefore, this suppression pattern may be used as a probe of the medium temperature. In this work we present preliminary results on the charmonium and bottomnium measurements performed by the STAR experiment at RHIC for p + p and Cu + Cu collisions at [Formula: see text].

Nuclear effects in HERA-B

In this contribution we estimate the influence of the shadowing effect and initial state parton energy loss in the quarkonium production at HERA-B. We analyze the xF behavior of the effective exponent a(xF) and present a comparison with the preliminary HERA-B data for J=Y production. Moreover, we present our predictions for i production at HERA-B. The magnitude of these effects in the J=Y production at RHIC is also analyzed. The relativistic collider facilities RHIC and LHC will for the first time provide the opportunity to systematically study the physics of hot and ultradense matter in hadron-nucleus (pA) and nucleus-nucleus (AB) collisions at energies that are orders of magnitude larger than the energies of the current ac- celerators. The systematic study of pA collisions at the same energies is essential to gain insight into the structure of the dense medium effects. Such effects, as the energy loss and shadowing, are absent or small in pp collisions, but become increasingly prominent in pA collisions, and are of major im- portance in AA reactions. By comparing pA and AA reactions involving very heavy nuclei, one may be able to distinguish basic hadronic effects that dominate the dynamics in pA col- lisions, from a quark-gluon formation predicted to occur in heavy ion AA collisions. To gain insight into the underlying hadronic processes, one has to study collisions that are ex- pected to not lead to a QGP formation. Once the physics of at high densities is better understood, the mechanisms of quark-gluon plasma formation and related collective phe- nomena in heavy ion collisions could be disentangled from the basic hadronic effects. In this contribution we study the influence of the nuclear medium effects in the quarkonium production, particularly of the J=y, which is one of the proposed signatures of the QCD phase transition (1). In particular, we will consider the shad- owing effects in the parton distributions and the initial state parton energy loss, which modify the xF (· x1i x2) behavior of the cross section. We will focus our analyzes in the kine- matical range of HERA-B, which recently has obtained data for the quarkonium production at negative values of xF . As we will show, these data could be used to discriminate between the different models for the medium effects.

Photon and Dilepton Production at LHC

We review various production mechanisms of photons and small mass dileptons at large transverse momentum at the LHC. Their relevance as a signal for quark-gluon plasma formation is discussed.