Elliptic Flow V2 Research Articles

Azimuthal anisotropy of ϕ meson in U+U and Au+Au collisions at RHIC

The measurements of the azimuthal anisotropy of φ meson in the U+U and Au+Au collisions at the Relativistic Heavy-Ion Collider (RHIC) are reported. The centrality dependence of the Fourier coefficients v2, v3, v4 and v5 is presented for φ meson at midrapidity (|ƞ| < 1.0), in U+U and Au+Au collisions at -√8NN = 193 and 200 GeV, respectively. The ƞ-sub event plane method is used with a n gap of 0.1 to suppress the non-flow effects. A strong centrality dependence is observed for the φ meson elliptic flow (v2), whereas no clear centrality dependence is observed for v3, v4 and v5. Ratios of the Fourier coefficients, v3/v2 and v4/v22 as a function of transverse momentum (pT) are also presented. A systematic comparison of the Fourier coefficients for the two systems U+U and Au+Au is discussed.

Intercomparison of flow measurements at RHIC experiments

The measurements of collective flow effects in particle production have provided invaluable insights on the transport properties of the strongly interacting matter produced in relativistic heavy-ion collisions at RHIC. The detailed comparison of flow measurements from PHENIX and STAR experiments at RHIC have been presented and discussed. For elliptic flow v2 of charged hadrons from Au+Au collisions at 200 GeV the two data sets overlap excellently for centralities > 20%, they increasingly diverge at small centralities, with a 30% difference between STAR an PHENIX in the 0-5% centrality bin. For v3 values the agreement is much worse and coming from the difference in STAR measurements. More investigations are needed to understand the reason for such differences.

Elliptic and triangular flow of identified particles from the AMPT model at RHIC energies

The elliptic flow (v2) at 11.5, 39 and 200 GeV and triangular flow (v3) at 200 GeV of identified particles ( and ) from 0–80% central Au+Au collisions are analyzed using a multiphase transport (AMPT) model. It is shown that the experimental results from the η-sub event plane method can be reproduced with a parton scattering cross-section between 1.5 and 3 mb. We also studied the differential and integrated ratios and conclude that they are anti-correlated with the parton scattering cross-section.

Boltzmann dynamics and temperature dependence of energy loss: Towards an understanding of the RAA and v2 puzzle for D-Mesons

The two key observables related to heavy quarks that have been measured in experiments are the nuclear suppression factor RAA and the elliptic flow v2. Reproducing these two observables simultaneously is a puzzle which have challenged all the existing models. We have studied two ingredients responsible to address a large part of such a puzzle: the temperature dependence of the energy loss and the full solution of the Boltzmann collision integral for the scattering between the heavy quarks and the particle of the bulk. We have considered four different models to evaluate the temperature dependence of drag and diffusion coefficients of the heavy quark. All these four different models are set to reproduce the same RAA(pT) measured in experiments at RHIC and LHC energy. We have found that for the same RAA(pT) one can generate 2-3 times more v2 depending on the temperature dependence of the heavy quarks drag coefficient. Moreover comparing the Fokker-Planck and the Boltzmann approach we have found that the latter seems more efficient into reproducing the elliptic flow for the same RAA. Even a larger difference between the two approaches emerges from the comparison of a more differential observable, the angular correlation.

Toward a solution to the RAA and v2 puzzle for heavy quarks

The heavy quarks constitute a unique probe of the quark–gluon plasma properties. A puzzling relation between the nuclear modification factor RAA(pT) and the elliptic flow v2(pT) has been observed both at RHIC and LHC energies. Predicting correctly both observables has been a challenge to all existing models, especially for D mesons. We discuss how the temperature dependence of the heavy quark drag coefficient is responsible for a large part of such a puzzle. In particular, we have considered four different models to evaluate the temperature dependence of drag and diffusion coefficients propagating through a quark gluon plasma (QGP). All the four different models are set to reproduce the same RAA(pT) observed in experiments at RHIC and LHC energy. We point out that for the same RAA(pT) one can generate 2–3 times more v2 depending on the temperature dependence of the heavy quark drag coefficient. A non-decreasing drag coefficient as T→Tc is a major ingredient for a simultaneous description of RAA(pT) and v2(pT).

Selecting specific initial configurations using spectator neutrons in U + U collisions

We present a method to select events with specific initial configuration, namely body-tip, in heavy-ion collisions using deformed Uranium nuclei. We propose to use asymmetry in spectator neutron numbers to filter out these body-tip events from the unbiased configurations in U+U collisions. We have used a variable S_eta to differentiate between the body-tip and unbiased configurations. We have calculated the second order azimuthal anisotropy, namely elliptic flow (v2), for this body-tip configuration in the framework of a transport model and found it to be consistently lower compared to that in unbiased configurations as we expected. The purity of selecting such events in a real experiment is also discussed.

Measurements of non-photonic electrons with the STAR experiment

Non-photonic electrons (NPE), produced by semileptonic decays of D and B mesons, are good probes to study the properties of hot and dense medium created in relativistic heavy ion collisions at RHIC. Studies of heavy quark production in p+p collisions can test the validity of perturbative QCD. They also provide a baseline to study the effects of nuclear matter on the production of heavy quarks in heavy ion collisions. In this paper, we present recent results of NPE spectra measured in p+p collisions at = 200 GeV in mid-rapidity. We also report NPE nuclear modification factor RAA and elliptic flow v2 in Au+Au collisions at = 200 GeV.

Effects of quark chemical equilibration on thermal photon elliptic flow

Large hadronic elliptic flow v2 is considered as an evidence for the existence of a strongly-coupled QGP fluid in high-energy heavy-ion collisions. On the other hand, direct photon v2 has recently been found to be much larger than that of hydrodynamic estimations, which is recognized as “photon v2 puzzle”. In this study, I discuss the implication of late production of quarks in an initially gluon-rich medium because photons are coupled to quarks. Numerical analyses imply that thermal photon v2 can be visibly enhanced. This indicates that interplay of equilibration processes and collective expansion would be important.

Heavy Quark Dynamics in QGP: Boltzmann vs Langevin

In this manuscript we review the basic concepts related to the study of the dynamics of the heavy quarks in quark-gluon plasma created in ultra-relativistic heavy-ion collisions. We discuss the relevant physical scale as well as the difficulties of the present theoretical approach with an aim to have a self-consistent description of the experimental data at both RHIC and LHC. In the second part we challenge the assumption of brownian motion for charm quarks and compare the dynamical evolution of charm and bottom quarks in the Fokker-Planck approach with the Boltzmann Transport calculationone. We show that while for bottom the motion appears quite close to a Brownian one, this does note seems to be the case for charms quarks. In particular the solution of the full two-body collision integral shows that the anisotropic flows are large with respect to those predicted by a Langevin dynamics. We show that using isotropic cross section one may describe the suppression RAA and elliptic flow v2 simultaneously.

Quarkonia production in the STAR experiment

In this paper, we highlight recent STAR J/ψ and ϒ results. J/ψ nuclear modification factors (RAA) in Au+Au collisions at √sNN = 200, 62.4 and 39 GeV and in U+U collisions at √sNN = 193 GeV, ϒ RAA in d+Au and Au+Au collisions at √sNN = 200 and in U+U collisions at √sNN = 193 GeV are measured and compared to different theoretical calculations. We also report J/ψ elliptic flow (v2) results in Au+Au collisions at √sNN = 200 GeV and the first ψ(2S) to J/ψ ratio measurement in p + p collisions at √s = 500 GeV.

A Review onϕMeson Production in Heavy-Ion Collision

The main aim of the relativistic heavy-ion experiment is to create extremely hot and dense matter and study the QCD phase structure. With this motivation, experimental program started in the early 1990s at the Brookhaven Alternating Gradient Synchrotron (AGS) and the CERN Super Proton Synchrotron (SPS) followed by Relativistic Heavy Ion Collider (RHIC) at Brookhaven and recently at Large Hadron Collider (LHC) at CERN. These experiments allowed us to study the QCD matter from center-of-mass energies (sNN) 4.75 GeV to 2.76 TeV. Theϕmeson, due to its unique properties, is considered as a good probe to study the QCD matter created in relativistic collisions. In this paper we present a review on the measurements ofϕmeson production in heavy-ion experiments. Mainly, we discuss the energy dependence ofϕmeson invariant yield and the production mechanism, strangeness enhancement, parton energy loss, and partonic collectivity in nucleus-nucleus collisions. Effect of later stage hadronic rescattering on elliptic flow (v2) of proton is also discussed relative to corresponding effect onϕmesonv2.

Electromagnetic probes of the QGP

We investigate the properties of the QCD matter across the deconfinement phase transition in the scope of the parton-hadron string dynamics (PHSD) transport approach. We present here in particular the results on the electromagnetic radiation, i.e. photon and dilepton production, in relativistic heavy-ion collisions. By comparing our calculations for the heavy-ion collisions to the available data, we determine the relative importance of the various production sources and address the possible origin of the observed strong elliptic flow v2 of direct photons. We argue that the different centrality dependence of the hadronic and partonic sources for direct photon production in nucleusnucleus collisions can be employed to shed some more light on the origin of the photon v2 “puzzle”. While the dilepton spectra at low invariant mass show in-medium effects like an enhancement from multiple baryonic resonance formation or a collisional broadening of the vector meson spectral functions, the dilepton yield at high invariant masses (above 1.1 GeV) is dominated by QGP contributions for central heavy-ion collisions at ultra-relativistic energies. This allows to have an independent view on the parton dynamics via their electromagnetic massive radiation.

Interplay between bulk medium evolution and (D)GLV energy loss

We study the consistency between high-pT nuclear suppression (R_AA) and elliptic flow (v2) using Gyulassy-Levai-Vitev (GLV) energy loss or a simpler power-law dE/dL formula, for a variety of bulk evolution models. The results generally confirm our earlier work [arXiv:1305.1046] that found suppressed elliptic flow for transversely expanding media. One exception is the set of hydrodynamic solutions used recently [arXiv:1305.6458] by Betz and Gyulassy, which give significantly higher v2 but unfortunately assume unrealistic bag-model equation of state. On the other hand, we show that covariant treatment of energy loss introduces an interplay between jet direction and hydrodynamic flow of the medium, which largely counteracts elliptic flow suppression caused by transverse expansion.

What the collective flow excitation function can tell about the quark–gluon plasma

Recent STAR data from the RHIC beam energy scan (BES) show that the midrapidity slope dv1/dy of the directed flow v1 of net-protons changes sign twice within the collision energy range 7.7 - 39 GeV. To investigate this phenomenon, we study the collision energy dependence of v1 utilizing a Boltzmann + hydrodynamics hybrid model. Calculations with dynamically evolved initial and final state show no qualitative difference between an equation of state with a cross-over and one with a first-order phase transition, in contrast to earlier pure fluid predictions. Furthermore, our analysis of the elliptic flow v2 shows that pre-equilibrium transport dynamics are partially compensating for the diminished elliptic flow production in the hydrodynamical phase at lower energies, which leads to a qualitative agreement with STAR BES results in midcentral collisions. No compensation from transport is found in our model for integrated v3, which decreases from approx 0.02 at sqrt(sNN) = 27 GeV to approx 0.005 at sqrt(sNN) = 7.7 GeV in midcentral collisions.

Anisotropic flows and the shear viscosity of the QGP within a transport approach

We study the build up of elliptic flow v2 and high order harmonics vn within a transport approach at fixed shear viscosity to entropy density ratio η/s and with initial state fluctuations. In particular we study the effect of a temperature dependent η/s for two different beam energies: RHIC for Au+Au at = 200 GeV and LHC for Pb + Pb at = 2.76 TeV. We find that for the two different beam energies considered the suppression of the elliptic flow and of higher harmonics v3(pT) and v4(pT) due to the viscosity of the medium have different contributions coming from the cross over or QGP phase. Moreover, we discuss the correlation between the initial spatial anisotropies ϵn and flow coefficients vn. We observe that the elliptic flow v2 is strongly correlated with initial eccentricity ϵ2. While higher harmonics v3 and v4 are weakly correlated to their asymmetry measure in coordinate space ϵ3 and ϵ4.

Flow harmonics in Pb + Pb collisions at SNN=2.76 TeV with the ATLAS detector

The measurement of the azimuthal anisotropy of charged particles produced in lead–lead collisions at SNN=2.76TeV is reported. The Fourier coefficients vn of the azimuthal angle distribution are obtained using multi-particle cumulants. The results on the transverse momentum and centrality dependence of the elliptic flow (v2) obtained from two-, four-, six- and eight-particle cumulants are presented. Higher-order harmonics, v3 and v4, are also derived using two- and four-particle cumulants and shown as a function of centrality and transverse momentum. A reduction of contributions from correlations not related to the collective flow are discussed. Event-by-event fluctuations of the flow harmonics are also presented as a function of transverse momentum and the collision centrality.

Probing the QCD phase diagram with measurements of ϕ-meson production and elliptic flow in heavy-ion collisions at STAR

We present measurements of ϕ-meson production and elliptic flow (v2) at mid-rapidity in Au + Au collisions at = 7.7 – 200 GeV. Energy dependence of nuclear modification factor (RCP) of ϕ meson is presented. The ϕ-meson RCP has a value ≥ 1.0 for ≤ 39 GeV. The Ω/ϕ ratios are also presented and show a different trend at the intermediate transverse momentum (pT) for = 11.5 GeV compared to higher beam energies. The number-of-constituent quark (NCQ) scaling of v2 has been studied at various beam energies. The NCQ scaling holds for particles and anti-particles separately including the ϕ meson for ≥ 19.6 GeV, which can be considered as an evidence of partonic collectivity. We observe that ϕ-meson v2 falls off the trend from the other hadrons at highest measured pT values by 1.8σ and 2.3σ at = 7.7 and 11.5 GeV, respectively.

Open Heavy Flavor Production in Heavy-ion Collisions from STAR

We report on the STAR measurements of open charm hadron and heavy flavor decay electron (NPE) production in Au+Au collisions at = 200 GeV. The centrality dependences of D-meson and NPE pT spectra, nuclear modification factors as well as the NPE elliptic flow v2 are presented. The model comparisons and interpreted physics are discussed.

Elliptic flow of Λ, Ξ and Ω in 2.76 A TeV Pb+Pb collisions

The elliptic flow v2 for strange and multi-strange bayons in 2.76 A TeV Pb+Pb collisions is investigated with VISHNU hybrid model that connects 2+1-d viscous hydrodynamics with a hadron cascade model. It is found that VISHNU nicely describes v2(pT) data for Λ, Ξ and Ω at various centralities. Comparing with the ALICE data, it roughly reproduces the mass-ordering of v2 among π, K, p, Ξ and Ω within current statistics, but gives an inverse v2 mass-ordering between p and Λ.

Strongly interacting parton-hadron matter in- and out-off equilibrium

We study the equilibrium properties of strongly-interacting infinite parton-hadron matter, characterized by the transport coefficients such as shear and bulk viscosity and electric conductivity, and the non-equilibrium dynamics of heavy-ion collisions within the Parton-Hadron-String Dynamics (PHSD) transport approach, which incorporates explicit partonic degrees of freedom in terms of strongly interacting quasiparticles (quarks and gluons) in line with an equation of state from lattice QCD as well as the dynamical hadronization and hadronic collision dynamics in the final reaction phase. We discuss in particular the possible origin for the strong elliptic flow v2 of direct photons observed at RHIC energies.