Radial, sideward and elliptic flow at AGS energies

We study the baryon transverse in-plane (sideward) and elliptic flow from SIS to AGS energies for AuAu collisions in a relativistic dynamical simulation model that includes all baryon resonances up to a mass of 2 GeV as well as string degrees of freedom for the higher mass continuum. There are two factors which dominantly determine the baryon flow at these energies: the momentum dependence of the scalar and vector potentials and the resonance-string degrees of freedom. We fix the explicit momentum dependence of the nucleon-meson couplings of NL3(hard) equation of state (EoS) by the nucleon optical potential up to 1 GeV of kinetic energy. We simultaneously reproduce the sideward flow, the elliptic flow and the radial transverse mass distribution of protons data at AGS energies. In order to study the sensitivity of different mean-field EoS, we use NL2(soft) and NL23(medium) along with NL3(hard) momenta-dependent mean-field EoS. We find that to describe data on both sideward and elliptic flow, NL3 model is better at 2 A·GeV, while NL23 model is at 4–8 A·GeV.     

Elliptic flow in Au+Au collisions at square root(S)NN = 130 GeV

Elliptic flow from nuclear collisions is a hadronic observable sensitive to the early stages of system evolution. We report first results on elliptic flow of charged particles at midrapidity in Au+Au collisions at square root(S)NN = 130 GeV using the STAR Time Projection Chamber at the Relativistic Heavy Ion Collider. The elliptic flow signal, v2, averaged over transverse momentum, reaches values of about 6% for relatively peripheral collisions and decreases for the more central collisions. This can be interpreted as the observation of a higher degree of thermalization than at lower collision energies. Pseudorapidity and transverse momentum dependence of elliptic flow are also presented.     

在RHIC-PHENIX中(s_(NN))~(1/2)=200GeV Au Au碰撞中直接光子的椭圆流测量(英文)

Azimuthal anisotropy of direct photon is measured in S_(NN)~(1/2)=200GeV Au Au collisions at RHIC- PHENIX.Direct photon is one of the most effective probes to study properties of hot dence medium at initial state(also QGP state)of heavy ion collisions because photons almost do not interact strongly with any other particles caused by its long mean free path and they keep their conditions when they axe created.Within statistical and systematic errors,the elliptic flow parameter(v_2)of direct photon is consistent with zero. Direct photon v_2 is estimated by hadron decay photon contamination are subtracted from inclusive photon v_2 in intermediate to high transverse momentum(p_T)region(0 to 10GeV/c)for 3 centrality selections(20% steps)and minimum bias.     

Perturbative QCD calculations of elliptic flow and shear viscosity in Au+Au collisions at sqrt[s_{NN}]=200 GeV

The elliptic flow v_{2} and the ratio of the shear viscosity over the entropy density, eta/s, of gluon matter are calculated from the perturbative QCD (pQCD) based parton cascade Boltzmann approach of multiparton scatterings. For Au+Au collisions at sqrt[s]=200A GeV the gluon plasma generates large v_{2} values measured at the BNL Relativistic Heavy Ion Collider. Standard pQCD yields eta/s approximately 0.08-0.15 as small as the lower bound found from the anti-de Sitter/conformal field theory conjecture.     

Elliptic flow of transported and produced protons in Au+Au collisions with the UrQMD model

Within the framework of the UrQMD model, by tracing the number of initial quarks in protons, we study the elliptic flow of protons with 3, 2, 1, 0 initial quarks and anti-protons in Au+Au collisions at SNN~(1/2)= 7.7, 11.5, 39,200 GeV. The difference of elliptic flow between protons with 2, 1, 0 initial quarks and anti-protons is smaller than 0,or consistent with 0, respectively. The difference of elliptic flow between transported protons(with 3 initial quarks)and anti-protons is larger than 0 at 7.7, 11.5 and 39 GeV. This is in good agreement with the STAR results at 7.7 and 11.5 GeV, but overestimates the STAR results at 39 GeV. The yield of transported protons with 3 initial quarks is smaller than of protons with 2 and 1 initial quarks, and v_2 of all protons is much smaller than the STAR results. The observation of the v_2 difference of elliptic flow between transported protons and anti-protons in the UrQMD model partly explains the difference between protons and anti-protons observed in the Beam Energy Scan program at the Relativistic Heavy Ion Collider(RHIC).     

Elliptic flow for phi mesons and (anti)deuterons in Au+Au collisions at square root of sNN=200 GeV

Differential elliptic flow (v(2)) for phi mesons and (anti)deuterons (d)d is measured for Au+Au collisions at square root of sNN=200 GeV. The v(2) for phi mesons follows the trend of lighter pi+/- and K+/- mesons, suggesting that ordinary hadrons interacting with standard hadronic cross sections are not the primary driver for elliptic flow development. The v(2) values for (d)d suggest that elliptic flow is additive for composite particles. This further validation of the universal scaling of v(2) per constituent quark for baryons and mesons suggests that partonic collectivity dominates the transverse expansion dynamics.     

Φ Meson Production in Heavy Ion Collisions at RHIC

We present Φ meson production in Cu+Cu and Au+Au collisions measured by the STAR experiment at RHIC.The hadronic decay mode Φ→K~+K~- is used in the analysis.The yields for Φ meson in Cu+Cu and Au+Au collisions at a given beam energy are scaled by the number of participant.The N_(part) normalized Φ meson yields in heavy ion collisions over those from p+p collisions are larger than 1 and increase with collision energy.These results suggest that the source of enhancement of strange hadrons is related to the formation of a dense medium in high energy heavy ion collisions and can not be only due to canonical suppression of their production in smaller systems.We also present STAR results on the Φ meson elliptic flow υ_2 from 2~(1/SNN)=200 GeV Cu+Cu at RHIC.The elliptic flow in Cu+Cu system that has the similar relative magnitude and qualitative features as that in Au+Au system.The observations imply the hot and dense matter with partonic collectivity has been formed in heavy ion collisions at RHIC.However,eccentrality normalized υ_2,υ_2/(n_qε_(part)) is lower for Cu+Cu than for Au+Au collisions at 200 GeV.So this might indicate thermalization has not been reached in 200 GeV Cu+Cu collisions.     

Influence of shear viscosity of quark-gluon plasma on elliptic flow in ultrarelativistic heavy-ion collisions

We investigate the influence of a temperature-dependent shear viscosity over entropy density ratio η/s on the transverse momentum spectra and elliptic flow of hadrons in ultrarelativistic heavy-ion collisions. We find that the elliptic flow in √S(NN)=200 GeV Au+Au collisions at RHIC is dominated by the viscosity in the hadronic phase and in the phase transition region, but largely insensitive to the viscosity of the quark-gluon plasma (QGP). At the highest LHC energy, the elliptic flow becomes sensitive to the QGP viscosity and insensitive to the hadronic viscosity.     

Midrapidity Lambda and Lambda(macro) production in Au+Au collisions at the square root of [s(NN)]=130 GeV

We report the first measurement of strange (Lambda) and antistrange (Lambda macro) baryon production from square root of [s(NN)]=130 GeV Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC). Rapidity density and transverse mass distributions at midrapidity are presented as a function of centrality. The yield of Lambda and Lambda; hyperons is found to be approximately proportional to the number of negative hadrons. The production of Lambda; hyperons relative to negative hadrons increases very rapidly with transverse momentum. The magnitude of the increase cannot be described by existing hadronic string fragmentation models alone.     

An explanation of the elliptic flow difference between proton and anti-proton from the UrQMD model with hadron potentials

The time evolution of both proton and anti-proton v_2 flows from Au+Au collisions at (S_NN)~(1/2)=7.7 Ge V are examined by using both pure cascade and mean-field potential versions of the Ur QMD model. Due to a stronger repulsion at the early stage introduced by the repulsive potentials and hence much less annihilation probabilities, anti-protons are frozen out earlier with smaller v_2 values.Therefore, the experimental data of anti-proton v_2 as well as the flow difference between proton and anti-proton can be reasonably described with the potential version of UrQMD.     

Elliptic flow of thermal photons in relativistic nuclear collisions

We predict the transverse momentum (p(T)) dependence of elliptic flow of thermal photons for Au + Au collisions at the BNL Relativistic Heavy Ion Collider. We model the system hydrodynamically, with a thermalized quark-gluon plasma at early times followed by hadronization and decoupling. Photons are emitted throughout the expansion history. Contrary to hadron elliptic flow, which increases monotonically with p(T), the elliptic flow nu2(p(T)) of thermal photons is predicted to first rise and then fall again. Photon elliptic flow at high p(T) reflects the quark momentum anisotropy at early times when it is small, while at low p(T) it mirrors the large pion momentum anisotropy during the late hadronic emission stage. An interesting structure is predicted at intermediate p(T) approximately 0.4 GeV/c, where photon elliptic flow reflects the momenta and the (compared to pions) reduced nu2 of heavy vector mesons in the late hadronic phase.     

Anisotropic flow of Pb+Pb s_(NN)~(1/2)= 5.02 TeV from a multi-phase transport model

Anisotropic flow is an important observable in the study of the quark-gluon plasma that is expected to be formed in heavy-ion collisions. With a multiphase transport(AMPT) model we investigate the elliptic(v_2),triangular(v_3), and quadrangular(v_4) flow of charged particles in Pb+Pb collisions at s_(NN)~(1/2)= 5.02 TeV. We then compare our flow results with the published ALICE flow results. We find our AMPT simulated results are consistent with ALICE experimental data.     

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

A new robust method to extract the specific shear viscosity (η/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 v2/ε 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 v2/ε 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 ε. Comparing with measurements we find 1<4π(η/s)(QGP) < 2.5 where the uncertainty range is dominated by model uncertainties for the values of ε used to normalize the measured v2.     

Multistrange Baryon elliptic flow in Au+Au collisions at square root of sNN=200 GeV

We report on the first measurement of elliptic flow v2(pT) of multistrange baryons Xi- +Xi+ and Omega- + Omega+ in heavy-ion collisions. In minimum-bias Au+Au collisions at square root of s(NN)=200 GeV, a significant amount of elliptic flow, comparable to other nonstrange baryons, is observed for multistrange baryons which are expected to be particularly sensitive to the dynamics of the partonic stage of heavy-ion collisions. The pT dependence of v2 of the multistrange baryons confirms the number of constituent quark scaling previously observed for lighter hadrons. These results support the idea that a substantial fraction of the observed collective motion is developed at the early partonic stage in ultrarelativistic nuclear collisions at the Relativistic Heavy Ion Collider.     

超相对论Au+Au碰撞中椭圆流的时间依赖

用相对论量子分子动力学(RQMD)模型模拟了质心系束能量为s_NN=200GeV的Au+Au非对心碰撞, 研究了椭圆流对末态粒子冻出时间的关系. 研究了在不同的阶段, 椭圆流对末态粒子位置的关系. 结果表明椭圆流随冻出时间的单调递减, 椭圆流对横向半径的关系随冻出时间发生变化. 用压力梯度对所得结果进行了分析. 径向速度用来表征压力梯度.     

Hadron-quark phase coexistence in a hybrid MIT-Bag model

The phase transition of hadronic to quark matter and the boundaries of the hadron-quark coexistence phase are studied within the two Equation of State (EoS) models. The relativistic effective mean-field approach with constant and density-dependent meson-nucleon couplings is used to describe hadronic matter, and the MIT-Bag model is adopted to describe quark matter. The boundaries of the mixed phase for different Bag constants are obtained solving the Gibbs equations. We notice that the dependence on the Bag parameter of the critical temperatures (at zero chemical potential) can be well reproduced by a fermion ultrarelativistic quark gas model, without contribution from the hadron part. At variance, the critical chemical potentials (at zero temperature) are very sensitive to the EoS of the hadron sector. Hence, the contribution of the hadronic interaction is much more relevant for the determination of the transition to the quark-gluon plasma at finite baryon density and low T . Moreover, in the low-temperature and finite chemical potential region no solutions of the Gibbs conditions are existing for small Bag-constant values, B < (135 MeV)⁴ . Isospin effects in asymmetric matter appear important in the high chemical-potential regions at lower temperatures, of interest for the inner-core properties of neutron stars and for heavy-ion collisions at intermediate energies.     

RHIC能区Au+Au碰撞中椭圆流的横半径依赖

用RQMD模型模拟了质心系束能量为s_NN=200GeV的Au+Au非对心碰撞, 研究了椭圆流对末态粒子冻出位置的关系. 研究表明, 随着冻出半径的增加, 椭圆流逐渐增强, 在源的初始表面处达到最大值, 随着冻出半径的继续增加, 椭圆流逐渐减小. 椭圆流的这种变化趋势反映了随着半径的增加, 压力梯度在各个方向上不同的变化. 本文提出了利用平均径向速度分析压力梯度的方法.     

NeXSPheRIO results on elliptic flow and directed flow for Au+Au and Cu+Cu collisions at RHIC

Hydrodynamics has been rather successful at describing results obtained in relativistic nuclear collisions at RHIC. Here we show results obtained with NeXSPheRIO on Au+Au collisions and the less studied Cu+Cu collisions. We study elliptic flow and its connection with eccentricity suggested by PHOBOS, as well as present elliptic flow fluctuations. We also show results for directed flow and compare with PHOBOS and STAR data.     

Baryon flow at SIS energies and beyond

We calculate the baryon flow P_x/A(y) in the energy range from 0.25 to 4.0A GeV in a relativistic transport model for Ni + Ni and Au + Au collisions employing various models for the baryon self-energies. We find that to describe the flow data of the FOPI Collaboration above 1A GeV the strength of the vector potential has to be reduced at high relative momentum or at high density. The latter phenomenon leads to a softening of the nuclear equation of state at high density.     

The effect of Lorentz-like force on collective flows of K+in Au+Au collisions at 1.5 Ge V/nucleon

Producing kaon mesons in heavy-ion collisions at beam energies below their threshold energy is an important way to investigate the properties of dense nuclear matter. In this study, based on the newly updated version of the ultrarelativistic quantum molecular dynamics model, we introduce the kaon-nucleon(KN) potential, including both the scalar and vector(also dubbed Lorentz-like)aspects. We revisit the influence of the KN potential on the collective flow of K+mesons produced in Au+Au collisions at Elab= 1.5 Ge V/nucleon and find that the contribution of the newly included Lorentz-like force is very important, particulary for describing the directed flow of K+. Finally, the corresponding Kao S data of both directed and elliptic flows can be simultaneously reproduced well.