Searcharxiv⌕ Search

arXiv subjects

Tetsufumi Hirano

Publications and source records attributed to Tetsufumi Hirano.

At least 55 records · Page 3Linked to original sources
arXiv2014

Di-jet asymmetric momentum transported by QGP fluid

We study the collective flow of the {QGP}-fluid which transports the energy and momentum deposited from jets. Simulations of the propagation of jets together with expansion of the {QGP}-fluid are performed by solving relativistic hydrodynamic equations numerically in the fully (3+1)-dimensional space. Mach cones are induced by the energy-momentum deposition from jets and extended by the expansion of the {QGP}. As a result, a large fraction of the transverse momentum deposited from jets is compensated by low-$p_{T}$ particles at large angles from the jet axis. This result is consistent with the data from the {CMS} Collaboration and gives a novel interpretation of them.

nucl-th↗
arXiv2014

Momentum transport away from a jet in an expanding nuclear medium

We study the transport dynamics of momenta deposited from jets in ultrarelativistic heavy-ion collisions. Assuming that the high-energy partons traverse expanding quark-gluon fluids and are subject to lose their energy and momentum, we simulate dijet asymmetric events by solving relativistic hydrodynamic equations numerically without linearization in the fully (3+1)-dimensional coordinate. Mach cones are formed and strongly broadened by radial flow of the background medium. As a result, the yield of low-$p_{T}$ particles increases at large angles from the jet axis and compensates the dijet momentum imbalance inside the jet-cone. This provides an intimate link between the medium excitation by jets and results in dijet asymmetric events observed by the CMS Collaboration.

nucl-th↗
arXiv2014

Theory Summary

In this review, I show a personal overview of theoretical results shown in the International Conference on the Initial Stages in High-Energy Nuclear Collision, in Illa da Toxa, Galicia, Spain, Sept.~8-14, 2013.

nucl-th↗
arXiv2013

Relativistic fluctuating hydrodynamics with memory functions and colored noises

Relativistic dissipative hydrodynamics including hydrodynamic fluctuations is formulated by putting an emphasis on non-linearity and causality. As a consequence of causality, dissipative currents become dynamical variables and noises appeared in an integral form of constitutive equations should be colored ones from fluctuation-dissipation relations. Nevertheless noises turn out to be white ones in its differential form when noises are assumed to be Gaussian. The obtained ifferential equations are very useful in numerical implementation of relativistic fluctuating hydrodynamics.

nucl-th↗
arXiv2013

Integrated Dynamical Approach to Relativistic Heavy Ion Collisions

We review integrated dynamical approaches to describe heavy ion reaction as a whole at ultrarelativistic energies. Since final observables result from all the history of the reaction, it is important to describe all the stages of the reaction to obtain the properties of the quark gluon plasma from experimental data. As an example of these approaches, we develop an integrated dynamical model, which is composed of a fully (3+1) dimensional ideal hydrodynamic model with the state-of-the-art equation of state based on lattice QCD, and subsequent hadronic cascade in the late stage. Initial conditions are obtained employing Monte Carlo versions of the Kharzeev-Levin-Nardi model (MC-KLN) or the Glauber model (MC-Glauber). Using this integrated model, we first simulate relativistic heavy ion collisions at the RHIC and LHC energies starting from conventional smooth initial conditions. We next utilise each Monte Carlo samples of initial conditions on an event-by-event basis and perform event-by-event dynamical simulations to accumulate a large number of minimum bias events. A special attention is paid to performing the flow analysis as in experiments toward consistent comparison of theoretical results with experimental data.

nucl-th↗
arXiv2012

Emission of Low Momentum Particles at Large Angles from Jet

We study dynamics of a QGP fluid induced by energetic partons propagating through it. We construct a (3+1)-dimensional QGP-fluid+Jet model. When a jet traverses a uniform fluid, it induces a Mach cone structure of energy density distribution and a vortex ring surrounding a path of the jet. When a pair of jets travels through a radially expanding fluid, low momentum particles are dominantly induced at large angles from the quenched jet. This result is qualitatively consistent with observation of the CMS Collaboration at LHC.

nucl-th↗
arXiv2012

Dynamical modeling of high energy heavy ion collisions

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

nucl-th↗
arXiv2011

Low-mass dilepton production through transport process in quark-gluon plasma

We attempt to understand the low-mass dielectron enhancement observed by PHENIX Collaboration at Relativistic Heavy Ion Collider (RHIC) by transport peak in the spectral function. On the basis of the second-order formalism of relativistic dissipative hydrodynamics, we parameterize the spectral function in low-frequency and long-wavelength region by two transport coefficients, electric diffusion coefficient $D$ and relaxation time $τ_{\rm J}$, and compared our theoretical dielectron spectra with the experimental data. We study 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 $D\geq 2/T$, with $T$ being temperature. Our analysis shows that dielectrons emitted through transport process mainly come from high-temperature QGP phase.

nucl-th↗
arXiv2011

Muon pairs from In+In collision at SPS energy

NA60 collaboration has extracted the inverse slope parameters, T_{eff} of the dimuon spectra originating from the In+In collisions at root(s_NN)=17.3 GeV for various invariant mass region. They have observed that the inverse slope parameter as a function of invariant mass of the lepton pair drops beyond the rho-peak. In the present work, first we reproduce the observed invariant mass and transverse momentum spectra of the muon pairs. Then show that the slope parameters extracted from the transverse momentum distributions for various invariant mass region windows can be explained by assuming formation of a partonic phase initially which reverts to hadronic phase through a weak first order phase transition at a temperature T_c ~ 175 MeV. It is observed that a scenario without the formation of a partonic phase does not reproduce the non-monotonic behaviour of the inverse slope parameter non typical of radial flow.

nucl-th↗
arXiv2011

The QGP shear viscosity -- elusive goal or just around the corner?

With the new viscous hydrodynamic + hadron cascade hybrid code VISHNU, a rather precise (O(25%)) extraction of the 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, v_2 and v_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 v_2(p_T) for identified hadrons between RHIC and LHC energies provides additional tests of the evolution model.

nucl-th↗
arXiv2011

Can transport peak explain the low-mass enhancement of dileptons at RHIC?

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 $τ_{\rm J}$. 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.

nucl-th↗
arXiv2011

Viscous Hydrodynamic Evolution with Non-Boost Invariant Flow for the Color Glass Condensate

The Large Hadron Collider (LHC) 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 viscous hydrodynamic evolution with non-boost invariant flow for understanding the CGC in terms of the heavy ion collisions.

nucl-th↗
arXiv2011

200 A GeV Au+Au collisions serve a nearly perfect quark-gluon liquid

The specific shear viscosity (eta/s)_QGP of a Quark-Gluon-Plasma (QGP) at temperatures T_c < T < 2T_c is extracted from the centrality dependence of the eccentricity-scaled elliptic flow measured in ultra-relativistic heavy-ion collisions. Coupling viscous fluid dynamics for the QGP with a microscopic transport model for hadronic freeze-out we find that the eccentricity-scaled elliptic flow is a universal function of charged multiplicity per unit overlap area, (1/S)(dN_ch/dy), that depends only on the viscosity but not on the model used for computing the initial fireball eccentricity. Comparing with measurements we find 1 < (4pi)(eta/s)_QGP < 2.5 where the uncertainty range is dominated by model uncertainties for the eccentricity values used to normalize the measured elliptic flow.

nucl-th↗
arXiv2011

Hadron spectra and elliptic flow for 200 A GeV Au+Au collisions from viscous hydrodynamics coupled to a Boltzmann cascade

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, 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.

nucl-th↗
arXiv2011

Suppression of high $p_{T}$ hadrons in $Pb+Pb$ Collisions at LHC

Nuclear modification factor $R_{AA}(p_{T})$ for large transverse momentum pion spectra in $Pb+Pb$ collisions at $\sqrt{s}=2.76$ TeV is predicted within the NLO perturbative QCD parton model. 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 the RHIC data on suppression of large $p_{T}$ hadron spectra. Data on charged hadron multiplicity $dN_{ch}/dη=1584 \pm 80$ in central $Pb+Pb$ collisions from the ALICE Experiment at the LHC are used to constrain the initial parton density both for determining the jet transport parameter and the 3+1D ideal hydrodynamic evolution of the bulk matter that is employed for the calculation of $R_{PbPb}(p_{T})$ for neutral pions.

nucl-th↗
arXiv2011

Longitudinal Viscous Hydrodynamic Evolution for the Shattered Colour Glass Condensate

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 viscosity plays an important role in determining parameters for the initial distributions.

nucl-th↗
arXiv2010

Elliptic flow in Pb+Pb collisions at sqrt{s_{NN}} = 2.76 TeV: hybrid model assessment of the first data

We analyze the elliptic flow parameter v_2 in Pb+Pb collisions at sqrt{s_{NN}} = 2.76 TeV and in Au+Au collisions at sqrt{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 v_2(p_T) hardly changes when the collision energy increases, whereas the integrated v_2 increases due to the enhancement of mean transverse momentum. The amount of increase of both v_2 and mean p_T depends significantly on the model of initialization.

nucl-th↗