平野 哲文

We attempt to understand the low-mass dielectron enhancement observed by the PHENIX Collaboration at the Relativistic Heavy Ion Collider by transport peak in the spectral function. On the basis of the second-order formalism of relativistic dissipative hydrodynamics, we parametrize the spectral function in low-frequency and long-wavelength regions by two transport coefficients, electric diffusion coefficient D and relaxation time τJ, and compare our theoretical dielectron spectra with the experimental data. We study the spectrum of dielectrons produced in relativistic heavy-ion collisions by using the profile of matter evolution under full (3+1)-dimensional hydrodynamics. We find that the experimental data require the diffusion coefficient to be D2/T, with T being temperature. Our analysis shows that dielectrons emitted through the transport process mainly come from the high-temperature quark-gluon plasma phase. © 2012 American Physical Society.

We present detailed theoretical approaches to high energy nuclear collisions, putting special emphasis on the technical aspects of numerical simulations. Models include relativistic hydrodynamics, Monte-Carlo implementation of the k(T)-factorization formula, jet quenching in expanding fluids, a hadronic transport model, and the Vlasov equation for colored particles.

We analyze the elliptic flow parameter v(2) in Pb+Pb collisions at the LHC energy using a hybrid model in which the evolution of the quark-gluon plasma is described by ideal hydrodynamics and the subsequent hadronic stage by a hadron cascade model. For initial conditions, we employ Monte Carlo versions of the Glauber and the Kharzeev-Levin-Nardi models and compare results with each other. We demonstrate that the differential elliptic flow v(2)(p(T)) does not change so much when the collision energy increases, whereas the integrated v(2) increases due to the enhancement of mean transverse momentum.

With the new viscous hydrodynamic + hadron cascade hybrid code VISHNU, a rather precise (O(25%)) extraction of the quark gluon plasma (QGP) shear viscosity (eta/s)(QGP) from heavy-ion elliptic flow data is possible if the initial eccentricity of the collision fireball is known with <5% accuracy. At this point, eccentricities from initial state models differ by up to 20%, leading to an O(100%) uncertainty for (eta/s)(QGP). It is shown that a simultaneous comparison of elliptic and triangular flow, upsilon(2) and upsilon(3), puts strong constraints on initial state models and can largely eliminate the present uncertainty in (eta/s)(QGP). The variation of the differential elliptic flow upsilon(2)(p(T)) for identified hadrons between RHIC and LHC energies provides additional tests of the evolution model.

The nuclear modification factor R-AA(p(T)) for large p(T) hadrons in central Pb + Pb collisions at root s = 2.76 TeV/eta is calculated within the next-to-leading order perturbative QCD parton model with medium-modified fragmentation functions and agree well with the new data. The jet transport parameter that controls medium modification is assumed to be proportional to the initial parton density and the coefficient is fixed by the RHIC data. The charged hadron multiplicity dN(ch)/d eta = 1584 +/- 80 in central Pb + Pb collisions from the ALICE experiment at the LHC is used to determine both the jet transport parameter and the initial condition for (3+1)D ideal hydrodynamic evolution of the bulk matter that is employed for the calculation of R-AA (p(T)).

With the new viscous hydrodynamic + hadron cascade hybrid code VISHNU, a rather precise (25%) extraction of the quark gluon plasma (QGP) shear viscosity (η/s)QGP from heavy-ion elliptic flow data is possible if the initial eccentricity of the collision fireball is known with &lt 5% accuracy. At this point, eccentricities from initial state models differ by up to 20%, leading to an (100%) uncertainty for (η/s)QGP. It is shown that a simultaneous comparison of elliptic and triangular flow, v2 and v3, puts strong constraints on initial state models and can largely eliminate the present uncertainty in (η/s)QGP. The variation of the differential elliptic flow v2(pT) for identified hadrons between RHIC and LHC energies provides additional tests of the evolution model. © 2011 IOP Publishing Ltd.

We propose a novel relation between the low-mass enhancement of dielectrons observed at PHENIX and transport coefficients of QGP such as the charge diffusion constant D and the relaxation time τJ. We parameterize the transport peak in the spectral function using the second-order relativistic dissipative hydrodynamics by Israel and Stewart. Combining the spectral function and the full (3+1)-dimensional hydrodynamical evolution with the lattice EoS, theoretical dielectron spectra and the experimental data are compared. Detailed analysis suggests that the low-mass dilepton enhancement originates mainly from the high-temperature QGP phase where there is a large electric charge fluctuation as obtained from lattice QCD simulations. © 2011 IOP Publishing Ltd.

The Large Hadron Collider experiments have revealed that the predictions of the color glass condensate (CGC) tend to underestimate the multiplicity at mid-rapidity. We develop and estimate a full second-order viscous hydrodynamic model for the longitudinal expansion to find that the CGC rapidity distributions are visibly deformed during the hydrodynamic stage due to the interplay between the entropy production and the entropy flux to forward rapidity. The results indicate the importance of considering viscous hydrodynamic evolution with non-boost invariant flow for understanding the CGC in heavy-ion collisions. © 2011 IOP Publishing Ltd.

The nuclear modification factor RAA(pT) for large pT hadrons in central Pb + Pb collisions at TeV/n is calculated within the next-to-leading order perturbative QCD parton model with medium-modified fragmentation functions and agree well with the new data. The jet transport parameter that controls medium modification is assumed to be proportional to the initial parton density and the coefficient is fixed by the RHIC data. The charged hadron multiplicity dNch/dη = 1584 80 in central Pb + Pb collisions from the ALICE experiment at the LHC is used to determine both the jet transport parameter and the initial condition for (3+1)D ideal hydrodynamic evolution of the bulk matter that is employed for the calculation of RAA(pT). © 2011 IOP Publishing Ltd.

The nuclear modification factor RAA(pT) for large transverse momentum pion spectra in Pb+Pb collisions at √s=2.76 TeV is predicted within the next-to-leading order perturbative QCD parton model. The effect of jet quenching is incorporated through medium-modified fragmentation functions within the higher-twist approach. The jet transport parameter that controls medium modification is proportional to the initial parton density, and the coefficient is fixed by data on the suppression of large-pT hadron spectra obtained at the BNL Relativistic Heavy Ion Collider. Data on charged hadron multiplicity dNch/dη=1584±80 in central Pb+Pb collisions from the ALICE experiment at the CERN Large Hadron Collider are used to constrain the initial parton density both for determining the jet transport parameter and the 3 + 1 dimensional (3 + 1D) ideal hydrodynamic evolution of the bulk matter that is employed for the calculation of R PbPb(pT) for neutral pions. © 2011 American Physical Society.

We investigate hydrodynamic evolution of the quark-gluon plasma for the colour glass condensate type initial conditions. We solve full second-order viscous hydrodynamic equations in the longitudinal direction to find that non-boost invariant expansion leads to visible deformation on the initial rapidity distribution. The results indicate that hydrodynamic evolution with entropy production from viscosity plays an important role in determining parameters for the initial distributions. (C) 2011 Elsevier B.V. All rights reserved.

The nuclear modification factor R-AA (p(T)) for large transverse momentum pion spectra in Pb+Pb collisions at root s = 2.76 TeV is predicted within the next-to-leading order perturbative QCD parton model. The effect of jet quenching is incorporated through medium-modified fragmentation functions within the higher-twist approach. The jet transport parameter that controls medium modification is proportional to the initial parton density, and the coefficient is fixed by data on the suppression of large-pT hadron spectra obtained at the BNL Relativistic Heavy Ion Collider. Data on charged hadron multiplicity dN(ch)/d eta = 1584 +/- 80 in central Pb+Pb collisions from the ALICE experiment at the CERN Large Hadron Collider are used to constrain the initial parton density both for determining the jet transport parameter and the 3 + 1 dimensional (3 + 1D) ideal hydrodynamic evolution of the bulk matter that is employed for the calculation of R-PbPb(p(T)) for neutral pions.

We analyze the elliptic flow parameter v2 in Pb+Pb collisions at √sNN = 2.76 TeV and in Au+Au collisions at √s NN=200 GeV using a hybrid model in which the evolution of the quark gluon plasma is described by ideal hydrodynamics with a state-of-the-art lattice QCD equation of state, and the subsequent hadronic stage by a hadron cascade model. For initial conditions, we employ Monte Carlo versions of the Glauber and the Kharzeev-Levin-Nardi models and compare results with each other. We demonstrate that the differential elliptic flow v2(pT) hardly changes when the collision energy increases, whereas the integrated v2 increases due to the enhancement of mean transverse momentum. The amount of increase of both v2 and mean pT depends significantly on the model of initialization. © 2011 American Physical Society.

We analyze the elliptic flow parameter upsilon(2) in Pb+Pb collisions at root S-NN = 2.76 TeV and in Au+Au collisions at root S-NN = 200 GeV using a hybrid model in which the evolution of the quark gluon plasma is described by ideal hydrodynamics with a state-of-the-art lattice QCD equation of state, and the subsequent hadronic stage by a hadron cascade model. For initial conditions, we employ Monte Carlo versions of the Glauber and the Kharzeev-Levin-Nardi models and compare results with each other. We demonstrate that the differential elliptic flow upsilon(2)( p(T)) hardly changes when the collision energy increases, whereas the integrated upsilon(2) increases due to the enhancement of mean transverse momentum. The amount of increase of both upsilon(2) and mean p(T) depends significantly on the model of initialization.

A new robust method to extract the specific shear viscosity (eta/s)(QGP) of a quark-gluon plasma (QGP) at temperatures T-c < T <= 2T(c) from the centrality dependence of the eccentricity-scaled elliptic flow v(2)/epsilon measured in ultrarelativistic heavy-ion collisions is presented. Coupling viscous fluid dynamics for the QGP with a microscopic transport model for hadronic freeze-out we find for 200 A GeV Au + Au collisions that v(2)/epsilon is a universal function of multiplicity density (1/S)(dN(ch)/dy) that depends only on the viscosity but not on the model used for computing the initial fireball eccentricity epsilon. Comparing with measurements we find 1 < 4 pi(eta/s)(QGP) < 2.5 where the uncertainty range is dominated by model uncertainties for the values of epsilon used to normalize the measured v(2).

It is shown that the recently developed hybrid code VISHNU, which couples a relativistic viscous fluid dynamical description of the quark-gluon plasma (QGP) with a microscopic Boltzmann cascade for the late hadronic rescattering stage, yields an excellent description of charged and identified hadron spectra and elliptic flow measured in 200 A GeV Au + Au collisions at the Relativistic Heavy-Ion Collider (RHIC). Using initial conditions that incorporate event-by-event fluctuations in the initial shape and orientation of the collision fireball and values eta/s for the specific shear viscosity of the quark-gluon plasma that were recently extracted from the measured centrality dependence of the eccentricity-scaled, p(T)-integrated charged hadron elliptic flow v(2,ch)/epsilon, we obtain universally good agreement between theory and experiment for the p(T) spectra and differential elliptic flow v(2)(p(T)) for both pions and protons at all collision centralities.

We predict the elliptic flow parameter v(2) in U+U collisions at root s(NN) = 200 GeV and in Pb+Pb collisions at root s(NN) = 2.76 TeV using a hybrid model in which the evolution of the quark gluon plasma is described by ideal hydrodynamics with a state-of-the-art lattice QCD equation of state and the subsequent hadronic stage is described by a hadron cascade model.

We investigate shear and bulk viscous hydrodynamic deformation on the rapidity distributions of the color glass condensate (CGC) in the relativistic heavy ion collisions at the RHIC and the LHC with a novel numerical and theoretical scheme. The results show that the rapidity distributions of the CGC are clearly deformed due to the entropy flux to forward rapidity and the entropy production from viscosity. The hydrodynamic effects tend to make the distribution steeper at the LHC than at the RHIC. This could be a candidate explanation for the rather surprising excess of multiplicity in the LHC data compared with the bare CGC predictions. The hydrodynamic parameter dependences are also discussed for more quantitative analyses.

We would like to formulate relativistic dissipative hydrodynamics for multicomponent systems with multiple conserved currents. This is important for analyses of the hot matter created in relativistic heavy ion collisions because particle creations and annihilations of various particle species are frequently taking place there. We show that consistent formulation in such systems involves many non-trivialities, and derive constitutive equations that satisfy Onsager reciprocal relations and describe the systems without ambiguity.

We derive the second order hydrodynamic equations for the relativistic system of multi-components with multiple conserved currents by generalizing the Israel-Stewart theory and Grad's moment method. We find that, in addition to the conventional moment equations, extra moment equations associated with conserved currents should be introduced to consistently match the number of equations with that of unknowns and to satisfy the Onsager reciprocal relations. Consistent expansion of the entropy current leads to constitutive equations which involve the terms not appearing in the original Israel-Stewart theory even in the single component limit. We also find several terms which exhibit thermal diffusion such as Soret and Dufour effects. We finally compare our results with those of other existing formalisms. (C) 2010 Elsevier B.V. All rights reserved.

The rapidity dependence of the elliptic flow of direct photons in Au+Au collisions at root s(NN) = 200 GeV are predicted, based on a three-dimensional ideal hydrodynamic description of the hot and dense matter The rapidity dependence of the elliptic flow v(2)(y) of direct photons (mainly thermal photons) is very sensitive to the initial energy density distribution along longitudinal direction, which provides a useful tool to extract the realistic initial condition from measurements

We present a global fit to single-and double-inclusive suppression data of high-P-T particles in central Au+Au collisions at top RHIC energy. We also include in this analysis data on heavy quarks via their D and B meson semi-leptonic decays (i.e. non-photonic electrons). The analysis is based on the parton quenching weights for medium-induced gluon radiation computed in the BDMPS approximation then embedded in a hydrodynamical description of the bulk medium. Our results indicate that values of the transport coefficient (q) over cap more than four times larger than perturbative estimates are preferred by experimental data. This confirms previous calculations based on simpler implementations of the medium geometry or only the single-inclusive suppression. We also comment on the statistical compatibility of the heavy quark data within a radiative-only energy loss scenario, and on the sensitivity of the results to nuclear modification of the parton distribution functions (PDFs) and to assumptions on the energy loss during times in the collision prior to the hydrodynamical behavior.

We investigate particle spectra and elliptic flow coefficients in relativistic heavy-ion collisions by taking into account the distortion of phase space distributions by bulk viscosity at freezeout. We first calculate the distortion of phase space distributions in a multicomponent system with Grad's 14-moment method. We find some subtle issues when macroscopic variables are matched with microscopic momentum distributions in a multicomponent system, and we develop a consistent procedure to uniquely determine the corrections to the phase space distributions. Next, we calculate particle spectra by using the Cooper-Frye formula to see the effect of the bulk viscosity. Despite the relative smallness of the bulk viscosity, we find that it is likely to have a visible effect on particle spectra and elliptic flow coefficients. This indicates the importance of taking into account bulk viscosity together with shear viscosity to constrain the transport coefficients with better accuracy from comparison with experimental data. © 2009 The American Physical Society.

We investigate particle spectra and elliptic flow coefficients in relativistic heavy-ion collisions by taking into account the distortion of phase space distributions by bulk viscosity at freezeout. We first calculate the distortion of phase space distributions in a multicomponent system with Grad's 14-moment method. We find some subtle issues when macroscopic variables are matched with microscopic momentum distributions in a multicomponent system, and we develop a consistent procedure to uniquely determine the corrections to the phase space distributions. Next, we calculate particle spectra by using the Cooper-Frye formula to see the effect of the bulk viscosity. Despite the relative smallness of the bulk viscosity, we find that it is likely to have a visible effect on particle spectra and elliptic flow coefficients. This indicates the importance of taking into account bulk viscosity together with shear viscosity to constrain the transport coefficients with better accuracy from comparison with experimental data.

We estimate the effects of viscosity on the phase space distribution appearing in the Cooper-Frye formula within the framework of the Grad's fourteen moment method and find that there are non-trivialities in the discussion of a multi-component system. We calculate the viscous corrections of particle spectra and elliptic flow coefficients from the distortion of the distribution using the flow and the hypersurface taken from a (3+1)-dimensional ideal hydrodynamic simulation. We see that the bulk viscosity have visible effects on particle spectra. The results suggest we should treat both shear and bulk viscosity carefully when constraining the transport coefficients from experimental data.

We develop a relativistic Langevin dynamics under the background of strongly interacting quark-gluon fluid described by the (3+1)-dimensional hydrodynamics. The drag force acting on charm and bottom quarks is parametrized according to the formula obtained from the anti-de-Sitter space/conformal field theory (AdS/CFF) correspondence. In this setup, we calculate the nuclear modification factor R-AA for the single-electrons from the charm and bottom quarks to extract the magnitude of the drag force from the PHENIX and STAR data. The R-AA for single-electrons with p(T) >= 3 GeV indicates that the drag force is close to the AdS/CFT prediction. Effects of the drag force on the elliptic flow of single-electrons are also discussed. Moreover, we predict the electron-muon correlation which is closely related to the heavy-quark pair correlation in hot matter.

The elliptic flow v(2) of thermal photons at midrapidity in Au+Au collisions at root s(NN) = 200 GeV is predicted, based on three-dimensional ideal hydrodynamics. Because of the interplay between the asymmetry and the strength of the transverse flow, the thermal photon v(2) reaches a maximum at pT similar to 2GeV/c and the pT-integrated 2 reaches a maximum at about 50% centrality. The pT-integrated v(2) is very sensitive to the lower limit of the integral but not sensitive to the upper limit due to the rapid decrease in the spectrum of the transverse momentum.

The transverse momentum (p(t)) dependence, the centrality dependence, and the rapidity dependence of the elliptic flow of thermal photons in Au + Au collisions at root s(NN) = 200 GeV are predicted on the basis of a three-dimensional ideal hydrodynamic description of the hot and dense matter. The elliptic flow parameter nu(2), i.e., the second Fourier coefficient of azimuthal distribution, of thermal photons first increases with p(t) and then decreases for p(t) > 2 GeV/c, because of the weak transverse flow at the early stage. The p(t)-integrated nu(2) first increases with centrality, reaches a maximum at about 50% centrality, and then decreases. The rapidity dependence of the elliptic flow nu(2)(y) of direct photons ( mainly thermal photons) is very sensitive to the initial energy density distribution along the longitudinal direction, which provides a useful tool to extract the realistic initial condition from measurements.

As a new and clean probe to the strongly interacting quark-gluon plasma (sQGP), we propose an azimuthal correlation of an electron and a muon that originate from the semileptonic decay of charm and bottom quarks. By solving the Langevin equation for the heavy quarks under the hydrodynamic evolution of the hot plasma, we show that substantial quenching of the away-side peak in the electron-muon correlation can be seen if the sQGP drag force acting on heavy quarks is large enough as suggested from the gauge/gravity correspondence. The effect could be detected in high-energy heavy ion collisions at the Relativistic Heavy Ion Collider and the Large Hadron Collider.

We study effects of eccentricity fluctuations on the elliptic flow coefficient v2 at midrapidity in both Au+Au and Cu+Cu collisions at sNN=200 GeV by using a hybrid model that combines ideal hydrodynamics for space-time evolution of the quark gluon plasma phase and a hadronic transport model for the hadronic matter. For initial conditions in hydrodynamic simulations, both the Glauber model and the color glass condensate model are employed to demonstrate the effect of initial eccentricity fluctuations originating from the nucleon position inside a colliding nucleus. The effect of eccentricity fluctuations is modest in semicentral Au+Au collisions, but significantly enhances v2 in Cu+Cu collisions. © 2009 The American Physical Society.

The jet quenching effect has been investigated in the direct photon production, based on a realistic data-constrained (3+1)-dimensional hydrodynamic description of the expanding hot and dense matter, a reasonable treatment of the propagation of partons and their energy loss in the fluid and a systematic study of the main sources of direct photons. Our resultant p(t) spectra agree with recent PHENIX data in a broad p(t) range. Parton energy loss in the plasma eventually significantly affects direct photon production from fragmentation and jet-photon conversion, similar to hadron suppression in central heavy-ion collisions. But this only causes about 40% decrease in the total production of direct photons, due to the mixture with other direct photon sources.

The relativistic diffusion process of heavy quarks is formulated on the basis of the relativistic Langevin equation in Ito discretization scheme. The drag force inside the quark-gluon plasma (QGP) is parametrized according to the formula for the strongly coupled plasma obtained by the anti-de-Sitter space/conformal field theory (AdS/CFT) correspondence. The diffusion dynamics of charm and bottom quarks in QGP is described by combining the Langevin simulation under the background matter described by the relativistic hydrodynamics. Theoretical calculations of the nuclear modification factor R-AA and the elliptic flow v(2) for the single electrons from the charm and bottom decays are compared with the experimental data from the relativistic heavy-ion collisions. The R-AA for electrons with large transverse momentum (p(T)>3 GeV) indicates that the drag force from the QGP is as strong as the AdS/CFT prediction.

We calculate the centrality dependence of transverse momentum (pt) spectra for direct photons in Au+Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC) energy sNN=200 □ GeV, based on a realistic data-constrained (3+1)-dimensional hydrodynamic description of the expanding hot and dense matter, a reasonable treatment of the propagation of partons and their energy loss in the fluid, and a systematic study of the main sources of direct photons. The resultant pt spectra agree with recent PHENIX data in a broad pt range. The competition among the different direct photon sources is investigated at various centralities. Parton energy loss in the plasma is considered for photons from fragmentation and jet-photon conversion, which causes about 40% decrease in the total contribution. In the high pt region, the observed RAA of photons is centrality independent at the accuracy of 5% based on a realistic treatment of energy loss. We also link the different behavior of RAA for central and peripheral collisions, in the low pt region, to the fact that the plasma in central collisions is hotter than that in peripheral ones. © 2009 The American Physical Society.

We calculate the centrality dependence of transverse momentum (p(t)) spectra for direct photons in Au + Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC) energy root sNN = 200 GeV, based on a realistic data-constrained (3 + 1)-dimensional hydrodynamic description of the expanding hot and dense matter, a reasonable treatment of the propagation of partons and their energy loss in the fluid, and a systematic study of the main sources of direct photons. The resultant p(t) spectra agree with recent PHENIX data in a broad p(t) range. The competition among the different direct photon sources is investigated at various centralities. Parton energy loss in the plasma is considered for photons from fragmentation and jet-photon conversion, which causes about 40% decrease in the total contribution. In the high p(t) region, the observed R-AA of photons is centrality independent at the accuracy of 5% based on a realistic treatment of energy loss. We also link the different behavior of RAA for central and peripheral collisions, in the low p(t) region, to the fact that the plasma in central collisions is hotter than that in peripheral ones.

We have developed a hydro+J/psi model to study the sequential suppression of charmonia at RHIC energy. The (3+1)-dimensional relativistic ideal hydrodynamics is employed to describe the spacetime evolution of hot and dense matter, and charmonia are treated as impurity traversing through the matter. The observed J/psi suppression at mid-rapidity in Au+Au collisions at RHIC is reproduced well by the sequential melting of chi(c), psi' and J/psi in the medium. The melting temperature of directly produced J/psi is well determined to be similar to 2T(c). Azimuthal anisotropy of J/psi (v(2)) is calculated with this model. Survival probability and azimuthal anisotropy in the 'hot-wind scenario' are also presented.

We simulate the dynamics of Au+Au collisions at the relativistic heavy ion collider (RHIC) with a hybrid model that treats the quark-gluon plasma macroscopically as an ideal fluid, but models the hadron resonance gas microscopically using a hadronic cascade. We find that much of the mass-ordering pattern for nu(2)(p(T)) observed at RHIC is generated during the hadronic stage due to build-up of additional radial flow. We also find that the mass-ordering pattern is violated for phi mesons due to small interaction cross section in the hadron resonance gas.

We investigate the role of jet and shower parton broadening by the strong colour field in the Delta eta-Delta phi correlation of high p(T) particles. When anisotropic momentum broadening (Delta p(z) > Delta p(T)) is given to jet and shower partons in the initial stage, a ridge-like structure is found to appear in the two-hadron correlation. The ratio of the peak to the pedestal yield is overestimated.

It is quite important to describe space-time evolution of matter created in relativistic heavy ion collisions toward understanding bulk and transport properties of the quark gluon plasma (QGP). We discuss current status of dynamical modeling in relativistic heavy ion collisions based on fully three dimensional ideal hydrodynamics. We obtain local temperature of the QGP by numerically solving hydrodynamic equations. We also utilize hydrodynamic solutions to estimate other observables such as high p(T) spectra for hadrons, J/psi suppression, string formation between a jet parton and a fluid parton and electro-magnetic spectra.

We simulate the dynamics of Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) with a hybrid model that treats the dense early quark-gluon plasma (QGP) stage macroscopically as an ideal fluid but models the dilute late hadron resonance gas (HG) microscopically using a hadronic cascade. By comparing with a pure hydrodynamic approach we identify effects of hadronic viscosity on the transverse momentum spectra and differential elliptic flow v2(pT). We investigate the dynamical origins of the observed mass ordering of v2(pT) for identified hadrons, focusing on dissipative effects during the late hadronic stage. Within our approach, we find that, at RHIC energies, much of the finally observed mass splitting is generated during the hadronic stage, due to buildup of additional radial flow. The meson, having a small interaction cross section, does not fully participate in this additional flow. As a result, it violates the mass-ordering pattern for v2(pT) that is observed for other hadron species. We also show that the early decoupling of the meson from the hadronic rescattering dynamics leads to interesting and unambiguous features in the pT dependence of the nuclear suppression factor RAA and of the /p ratio. © 2008 The American Physical Society.

A strong $J/\psi$ suppression in central Au+Au collisions has been observed<br /> by the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC). We<br /> develop a hydro+$J/\psi$ model in which hot quark-gluon matter is described by<br /> the full (3+1)-dimensional relativistic hydrodynamics and $J/\psi$ is treated<br /> as an impurity traversing through the matter. The experimental $J/\psi$<br /> suppression pattern in mid-rapidity is reproduced well by the sequential<br /> melting of $\chi_{\rm c}$, $\psi&#039;$, and $J/\psi$ in dynamically expanding<br /> fluid. The melting temperature of directly produced $J/\psi$ is well<br /> constrained by the participant-number dependence of the $J/\psi$ suppression<br /> and is found to be about $2.T_{\rm c}$ with $T_{\rm c}$ being the<br /> pseudo-critical temperature.

We propose a new hadronization mechanism, jet-fluid string (JFS) formation and decay, to understand observables in intermediate to high-$p_{T}$ regions comprehensively. In the JFS model, hard partons produced in jet lose their energy in traversing the QGP fluid, which is described by fully three-dimensional hydrodynamic simulations. When a jet parton escapes from the QGP fluid, it picks up a partner parton from a fluid and forms a color singlet string, then it decays to hadrons. We find that high-$p_T$ $v_2$ values in JFS are about two times larger than in the independent fragmentation

Recent development of a hydrodynamic model is discussed by putting an emphasis on realistic treatment of the early and late stages in relativistic heavy ion collisions. The model, which incorporates a hydrodynamic description of the quark-gluon plasma with a kinetic approach of hadron cascades, is applied to analysis of elliptic flow data at the Relativistic Heavy Ion Collider energy. It is predicted that the elliptic flow parameter based on the hybrid model increases with the collision energy up to the Large Hadron Collider energy.

We can establish a new picture, the perfect fluid strongly coupled quark gluon plasma (sQGP) core and the dissipative hadronic corona, of the space-time evolution of produced matter in relativistic heavy ion collisions at RHIC. It is also shown that the picture works well also in the forward rapidity region through an analysis based on a new class of the hydro-kinetic model and is a manifestation of deconfinement.

Single particle spectra as well as elliptic flow in Cu+Cu collisions at root s(NN)=200 GeV are investigated within a hadronic cascade model and an ideal hydrodynamic model. Pseudorapidity distribution and transverse momentum spectra for charged hadrons are surprisingly comparable between these two models. However, a large deviation is predicted for the elliptic flow. The forthcoming experimental data will clarify the transport and thermalization aspects of matter produced in Cu+Cu collisions.