Au-Au Collisions Research Articles

Common ways in the description of the “classical” plasma and quark-gluon plasma

Recent experimental results on the possible formation of the quark-gluon plasma in Au-Au collisions at the maximum energy of the Relativistic Heavy Ion Collider (RHIC) from the Brookhaven National Laboratory (BNL), USA, opened the discussions on the possibility to use notions, phenomena and specific parameters from the Plasma Physics in the description of the quark-gluon plasma, trying to exceed the differences between the different nature of the basic interactions in the two types of states of the matter. The present work is a continuation of our previous attempts in this direction. We discuss the possibility to describe the observed quark-gluon plasma at the RHIC maximum energy, supposed in liquid phase, using the parameters for dusty plasmas, strongly coupled plasmas, mainly Coulomb parameter and different wave lengths. The analogies lead at the idea that there is the common behaviours of the parameters sustaining the formation of the quark-gluon plasma in liquid state.

Percolation of color sources and the equation of state of QGP in central Au–Au collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV

The Color String Percolation Model (CSPM) is used to determine the equation of state (EOS) of the QGP produced in central Au-Au collisions at $\sqrt{s_{NN}}$ = 200 A GeV using STAR data at RHIC. When the initial density of interacting colored strings exceeds the 2D percolation threshold a cluster is formed, which defines the onset of color deconfinement. These interactions also produce fluctuations in the string tension which transforms the Schwinger particle (gluon) production mechanism into a maximum entropy thermal distribution. The single string tension is determined by identifying the known value of the universal hadron limiting temperature $T_{c}$ = 167.7 $\pm$ 2.6 MeV with the CSPM percolation temperature at the critical threshold $\xi_{c}$ =1.2. At mid-rapidity the initial Bjorken energy density and the initial temperature determine the number of degrees of freedom consistent with the formation of a $\sim$ 2+1 flavor QGP. An analytic expression for the equation of state, the sound velocity $C_{s}^{2}(\xi)$ is obtained in CSPM. The CSPM $C_{s}^{2}(\xi)$ and the bulk thermodynamic values $\varepsilon /T^{4}$ and $s /T^{3}$ are in excellent agreement in the phase transition region with recent lattice QCD simulations (LQCD) by the HotQCD Collaboration.

Azimuth Quadrupole Systematics in Au-Au Collisions

We have measured pt-dependent two-particle number correlations on azimuth and pseudorapidity for eleven centralities of = 62 and 200 GeV Au-Au collisions at STAR. 2D fits to these angular correlations isolate the azimuth quadrupole amplitude, denoted 2v22{2D}(pt), from localized same-side correlations. Event-plane V2(pt) measurements within the STAR TPC acceptance can be expressed as a sum of the azimuth quadrupole and the quadrupole component of the same-side peak. V2{2D}(pt) can be transformed to reveal quadrupole pt spectra which are approximately described by a fixed transverse boost and universal Levy form nearly independent of centrality. A parametrization of V2{2D}(pt) can be factored into centrality and pt-dependent pieces with a simple pt dependence above 0.75 GeV/c. Results from STAR are compared to published data and model predictions.

Elliptic Flow of Charged Particles in Pb-Pb Collisions atsNN=2.76 TeV

We report the first measurement of charged particle elliptic flow in Pb-Pb collisions at sqrt[S(NN)] =2.76 TeV with the ALICE detector at the CERN Large Hadron Collider. The measurement is performed in the central pseudorapidity region (|η|<0.8) and transverse momentum range 0.2<p t<5.0 GeV/c. The elliptic flow signal v₂, measured using the 4-particle correlation method, averaged over transverse momentum and pseudorapidity is 0.087 ± 0.002(stat) ± 0.003(syst) in the 40%-50% centrality class. The differential elliptic flow v₂ p t reaches a maximum of 0.2 near p t =3 GeV/c. Compared to RHIC Au-Au collisions at sqrt[S(NN)] 200 GeV, the elliptic flow increases by about 30%. Some hydrodynamic model predictions which include viscous corrections are in agreement with the observed increase.

Charged-Particle Multiplicity Density at Midrapidity in Central Pb-Pb Collisions atsNN=2.76 TeV

The first measurement of the charged-particle multiplicity density at mid-rapidity in Pb-Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}}$ = 2.76 TeV is presented. For an event sample corresponding to the most central 5% of the hadronic cross section the pseudo-rapidity density of primary charged particles at mid-rapidity is 1584 $\pm$ 4 (stat) $\pm$ 76 (sys.), which corresponds to 8.3 $\pm$ 0.4 (sys.) per participating nucleon pair. This represents an increase of about a factor 1.9 relative to pp collisions at similar collision energies, and about a factor 2.2 to central Au-Au collisions at $\sqrt{s_{\rm NN}}$ = 0.2 TeV. This measurement provides the first experimental constraint for models of nucleus-nucleus collisions at LHC energies.

Transverse mass distributions of protons produced in heavy-ion collisions at high energies

The transverse mass distributions of protons produced in Au-Au collisions at 8 A GeV and Pb-Pb collisions at 158 A GeV are calculated by using the Monte Carlo method in the framework of the multisource ideal gas model. It is found that our calculated results are in agreement with the experimental data in nucleus-nucleus collisions at high energies.

Lifetime of electric flux tubes near the QCD phase transition

Electric flux tubes are a well-known attribute of the quantum chromodynamic (QCD) vacuum in which they manifest confinement of electric color charges. Recently, experimental results appeared which suggest that not only do those objects persist at temperatures $T\ensuremath{\approx}{T}_{c}$ near the QCD phase transitions, but their decay is suppressed and the resulting clusters in Au-Au collisions are larger than in $\mathit{pp}$ collisions (i.e., in vacuum). This correlates well with recent theoretical scenarios that view the QCD matter in the $T\ensuremath{\approx}{T}_{c}$ region as a dual-magnetic plasma dominated by color-magnetic monopoles. In this view, the flux tubes are stabilized by dual-magnetic currents and are described by dual magnetohydrodynamics (DMHD). In this article, we calculate classically the dissipative effects in the flux tube. Such effects are associated with rescattering and finite conductivity of the matter. We derive the DMHD solution in the presence of dissipation and then estimate the lifetime of the electric flux tubes. The conclusion of this study is that a classical treatment leads to too short of a lifetime for the flux tubes.

Rayleigh-like distribution of particle transverse momenta in collisions at high energies

The transverse momentum distributions of final-state particles produced in collisions at high energies are studied by using a two-component Rayleigh-like distribution. This representation is based on Liu's multisource ideal gas model which describes protons and fragments in high energy nucleus-nucleus collisions. The calculated results are in good agreement with the experimental data of Au-Au, Cu-Cu, d-Au, and pp collisions at the relativistic heavy ion collider energies. The experimental particle momentum distributions of p-Be collisions at 6.4, 12.3, and 17.5 GeV/c, as well as Au-Au collisions at 1.5 AGeV are well described by a model based on a single Rayleigh-like distribution of particle transverse momenta.

Emission of Neutral Pions with High Transverse Momenta in A-A and p-p Collisions at Energies Available at the BNL Relativistic Heavy Ion Collider

Invariant neutral pion (π0) yields in Au-Au and Cu-Cu collisions and invariant π0 cross sections in p-p collisions are studied in the framework of a two-cylinder model. The considered distributions of neutral pions at the maximum energy of the relativistic heavy ion collider (RHIC) have a tail part in the region of high transverse momentum. A two-component distribution based upon the two-cylinder model is used to describe the experimental data of the PHENIX collaboration.

Cooper–Frye formula and non-extensive coalescence at RHIC energy

Transverse spectra are calculated for various types of hadrons stemming from AuAu collisions at s=200 GeV. We utilize a quark recombination model based on generalized Boltzmann–Gibbs thermodynamics for local hadron production at various break-up scenarios.

Nuclear shadowing and heavy-ion collisions at high energies

Nuclear shadowing effects for quarks and gluons are calculated using information on diffractive deep inelastic scattering on a nucleon. Role of these effects in interactions of hadrons and nuclei with nuclei at high energies is investigated. A decrease in particle densities for heavy-ion collisions in comparison with the Glauber model is predicted and nuclear modification factors are calculated. Distributions of gluons in nuclei are used to predict suppression of heavy quarkonia. The parameter-free calculation of J/ψ in DAu and AuAu collisions is in a good agreement with recent RHIC data. Predictions for heavy quarkonia suppression in heavy-ion collisions at LHC are formulated.

Why is [formula omitted] larger than predicted by hydrodynamics?

The second and fourth Fourier harmonics of the azimuthal distribution of particles, v 2 and v 4 , have been mesured in Au-Au collisions at the Relativistic Heavy Ion Collider (RHIC). The ratio v 4 / ( v 2 ) 2 is significantly larger than predicted by hydrodynamics. Effects of partial thermalization are estimated on the basis of a transport calculation, and are shown to increase the ratio by a small amount. We argue that the large value of v 4 / ( v 2 ) 2 seen experimentally is mostly due to elliptic flow fluctuations. However, the standard model of eccentricity fluctuations is unable to explain the large magnitude of v 4 / ( v 2 ) 2 in central collisions.

Erratum: Centrality dependence of strangeness enhancement in ultrarelativistic heavy ion collisions: A core-corona effect [Phys. Rev. C79, 064907 (2009)

In ultrarelativistic heavy ion collisions, the multiplicity of multi-strange baryons per participating nucleon increases with centrality in a different fashion for different systems and energies. At RHIC, for copper+copper (CuCu) collisions the increase is much steeper than for gold-gold (AuAu) collisions. We show that this system size dependence is due to a core-corona effect: the relative importance of the corona as compared to the core (thermalized matter) contribution varies and the contribution of a corona nucleon to the multiplicity differs from that of a core nucleon. $\phi$ mesons follow in principle the same trend, but the difference between core and corona multiplicity is relatively small, and therefore the CuCu and AuAu results are quite similar. This simple geometrical explanation makes also a strong case in favor of the validity of Glauber geometry in the peripheral regions of ultrarelativistic heavy ion collisions, which is crucial for understanding the early evolution of the system.

Hadron Production in a Thermal Model with Scaled Masses

We present an investigation of particle production yields measured in central Au-Au collision s=130GeV at RHIC via thermal model. In our approach we study the role played by the hadron masses in the measured hadronic ratio fittings. With this purpose we vary in an ad hoc way the baryonic masses of the octet and decuplet, as well as the pseudo-scalar and the vector mesonic nonets. We show how the quality of the hadronic ratio fittings is dependent on the scaling masses. In our study the hadron mass variation tries to mimic in a very crude way the in-medium mass effect present in dynamical models.

Photoproduction at the Relativistic Heavy Ion Collider with STAR

The strong electromagnetic fields of short duration associated with relativistic heavy ions make a heavy-ion collider a unique tool to study two-photon and photonuclear collisions. In this talk, we introduce the principles of photoproduction at hadron colliders, review recent results from RHIC on meson and e +e − production. At RHIC, STAR has studied exclusive ρ 0 vector meson production and ρ 0 production accompanied by electromagnetic dissociation of both nuclei in collisions of AuAu at 62, 130 and 200 GeV. Recent results suggest the validity of the Glauber calculations for the vector meson photoproduction and incosistency of the model based on the parton saturation phenomenon. The measurements are also sensitive to interference between production on the two nuclei: either ion can be the photon emitter or the target. The level of observed interference suggests that the final state wave function carries information about all possible decays long after the decay occurs. We also observe coherent photoproduction of a π + π − π + π − state which may be associated with ρ 0 ∗ ( 1450 ) .

Study of ultrarelativistic heavy-ion collisions in the PHENIX experiment

The latest physics results obtained in the PHENIX experiment by studying dAu, CuCu, and AuAu collisions at various interaction energies are reviewed. Attention is given primarily to the energy loss of hard-scattered partons and to the production of soft and high-transverse-momentum direct photons.

Non-photonic electron-hadron azimuthal correlation for AuAu, CuCu and pp collisions at [formula omitted

We present preliminary STAR results of azimuthal correlations between non-photonic electrons and hadrons in AuAu, CuCu and pp at s N N = 200 GeV . Comparison of the e-h correlations from these colliding systems allows one to study the system-size dependence of heavy quark jet-medium interactions. We also report on the relative charm and bottom contributions to non-photonic electrons extracted from correlations measured in pp collisions. Our results, when combined with R A A measurements of non-photonic electrons, constrain the charm and bottom energy loss in the dense medium.

Evolution of minimum-bias parton fragmentation in nuclear collisions

Minimum-bias fragment distributions (FDs) are calculated by folding a power-law parton energy spectrum with parametrized fragmentation functions (FFs) derived from ${e}^{+}$-${e}^{\ensuremath{-}}$ and $p$-$\overline{p}$ collisions. Substantial differences between measured ${e}^{+}$-${e}^{\ensuremath{-}}$ and $p$-$\overline{p}$ FFs suggest that FF ``universality'' may not be a valid assumption. The common parton spectrum is constrained by comparison with a $p$-$p$ ${p}_{t}$ spectrum hard component. Changes in FFs due to parton ``energy loss'' or ``medium modification'' are modeled by altering FF parametrizations consistent with rescaling QCD splitting functions. In-vacuum and in-medium FDs are compared with spectrum hard components from $200$-GeV Au-Au collisions for several centralities. The reference for all nuclear collisions is the FD derived from in-vacuum ${e}^{+}$-${e}^{\ensuremath{-}}$ FFs. The hard component for $p$-$p$ and peripheral Au-Au collisions is found to be strongly suppressed for smaller fragment momenta, consistent with the FD derived from in-vacuum $p$-$\overline{p}$ FFs. At a particular centrality the Au-Au hard component transitions to enhancement at smaller momenta and suppression at larger momenta, consistent with FDs derived from in-medium ${e}^{+}$-${e}^{\ensuremath{-}}$ FFs. Fragmentation systematics suggest that QCD color connections change dramatically in more-central $A$-$A$ collisions. Observed parton and hadron spectrum systematics are inconsistent with saturation-scale arguments used to support assumptions of parton thermalization.

Interplay of the emission from thermal and direct sources in relativistic heavy ion collisions

The separation of the source created in ultrarelativistic heavy ion collisions into a thermalized dense core and an outer mantle consisting of independent nucleon-nucleon collisions is discussed. Indications for such a two-component picture are found in the transverse mass spectra of kaons, protons, and antiprotons produced in Au-Au collisions at $\sqrt{s}=200$ GeV. Estimates of the sizes of the thermal and direct sources are compared with models separating the interaction zone into a core and a corona. New initial conditions corresponding to the dense core lead to a stronger azimuthal asymmetry of the fireball, and pressure gradients also increase. We find faster transverse hydrodynamic expansion and stronger elliptic flow for the dense core initial conditions.

Universal centrality and collision energy trends for v 2 measurements from 2D angular correlations

We have measured the $p_t$-integrated quadrupole component of two-particle azimuth correlations (related to quantity $v_2$, denoted in this case by $v_2 \{2D\}$) via two-dimensional (2D) angular autocorrelations on $(\eta, \phi)$ for unidentified hadrons in Au-Au collisions at 62 and 200 GeV. The 2D autocorrelation provides a method to remove non-quadrupole contributions to $v_2$ (conventionally termed ``nonflow'') under the assumption that such processes produce significant dependence on pair-wise relative $\eta$ within the detector acceptance. We hypothesize, based on empirical observations, that non-quadrupole contributions are dominated by minijets or minimum-bias jets. Using the optical Glauber eccentricity model for initial-state geometry we find simple and accurate universal energy and centrality trends for the quadrupole component. Centrality trends are determined only by the initial state (impact parameter $b$ and center-of-mass energy $\sqrt{s_{NN}}$). There is no apparent dependence on evolving system dynamics (e.g., equation of state or number of secondary collisions). Our measurements of the quadrupole and non-quadrupole components have implications for the contributions to $v_2$. They suggest that the main source of the difference between $v_2 \{2\}$ and $v_2 \{4\}$ (or $v_2 \{2D\}$) is measured properties of minijets.