Tetsufumi Hirano

Effects of bulk viscosity at freezeout

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 Grads 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.

Eccentricity fluctuation effects on elliptic flow in relativistic heavy ion collisions

We study effects of eccentricity fluctuations on the elliptic flow coefficient v{sub 2} at midrapidity in both Au+Au and Cu+Cu collisions at {radical}(s{sub NN})=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 v{sub 2} in Cu+Cu collisions.

Hydrodynamics and Flow

The main purpose of the lecture was to lead students and young postdocs to the frontier of the hydrodynamic description of relativistic heavy-ion collisions (H.I.C.) in order for them to understand talks and posters presented in the Quark Matter 2008 (QM08) conference in Jaipur, India [1]. So the most recent studies were not addressed in this lecture as they would be presented during the QM08 conference itself. Also, we try to give a very pedagogical lecture here. For the readers who may want to study relativistic hydrodynamics and its application to H.I.C. as an advanced course, we strongly recommend them to consult the references. This lecture note is divided into three parts. In the first part we give a brief introduction to relativistic hydrodynamics in the context of H.I.C. In the second part we present the formalism and some fundamental aspects of relativistic ideal and viscous hydrodynamics.

Relativistic Hydrodynamics at RHIC and LHC

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.

Eccentricity Fluctuation in Initial Conditions of Hydrodynamics

Abstract We study effects of eccentricity fluctuations on the elliptic flow coefficient ν 2 at mid-rapidity in both Au+Au and Cu+Cu collisions at s N N = 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. We find that the effect of eccentricity fluctuation is modest in semicentral Au+Au collisions but significantly enhances ν 2 in Cu+Cu collisions.

Onset of J/ psi Melting in Quark-Gluon Fluid at RHIC

A strong J/{psi} suppression in central Au+Au collisions has been observed by the PHENIX experiment at the Relativistic Heavy Ion Collider. We develop a hydro+J/{psi} model in which hot quark-gluon matter is described by the full (3+1)-dimensional relativistic hydrodynamics and J/{psi} is treated as an impurity traversing through the matter. The experimental J/{psi} suppression pattern in midrapidity is reproduced well by the sequential melting of {chi}{sub c},{psi}{sup }, and J/{psi} in dynamically expanding fluid. The melting temperature of directly produced J/{psi} is well constrained by the participant-number dependence of the J/{psi} suppression and is found to be about 2T{sub c} with T{sub c} being the pseudocritical temperature.

On the role of initial conditions and final state interactions in ultrarelativistic heavy ion collisions

We investigate the rapidity dependence of the elliptical flow in heavy ion collisions at 200 GeV (cms), by employing a three-dimensional hydrodynamic evolution, based on different initial conditions, and different freeze-out scenarios. It will be shown that the form of pseudo-rapidity (η) dependence of the elliptical flow is almost identical to spacetime-rapidity (ηs) dependence of the initial energy distribution, independent of the freeze-out prescriptions.

Jet quenching and direct photon production

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 pt spectra agree with recent PHENIX data in a broad pt 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.

Hadronic dissipative effects on transverse dynamics at RHIC

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 v2(pT) 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 mesons due to small interaction cross section in the hadron resonance gas.