200AGeVAu+AuCollisions Serve a Nearly Perfect Quark-Gluon Liquid

Using the newly developed hybrid model VISHNU which connects viscous hydrodynamics with a hadron cascade model, we study the differential and integrated elliptic flow v_2 at different centrality bins for 200 A GeV Au+Au collisions and 2.76 A TeV Pb+Pb collisions. We find that the average Quark Gluon Plasma (QGP) specific shear viscosity eta/s slightly increases from Relativistic Heavy Ion Collider (RHIC) to Large Hadron Collider (LHC) energies. However, a further study assuming different temperature dependencies for (eta/s)_QGP shows that one cannot uniquely constrain the form of (eta/s)_QGP(T) by fitting the spectra and v_2 alone. Based on our current understanding, the question on whether the QGP fluid is more viscous or more perfect in the temperature regime reached by LHC energies is still open.

Acoustic scaling of anisotropic flow in shape-engineered events: implications for extraction of the specific shear viscosity of the quark gluon plasma

In ultrarelativistic heavy-ion collisions at RHIC energies, charm quarks are predominantly produced in initial hard partonic scatterings. Therefore, they experience the entire evolution of the hot and dense medium produced in these collisions, known as the Quark-Gluon Plasma (QGP). The STAR experiment is capable of studying the production of charm quarks and their interactions with the QGP through the reconstruction of the hadronic decays of D$^0$, D$^\pm$, D$^\pm_\textrm{s}$ and $\Lambda^\pm_\textrm{c}$ hadrons. These measurements are possible thanks to the excellent track pointing resolution of the Heavy Flavor Tracker (HFT). In these proceedings, we present recent results on open-charm hadron measurements in Au+Au collisions at $\sqrt{s_\textrm{NN}}=200$ GeV. In particular, we discuss the nuclear modification factors of D$^\pm$ and D$^0$ mesons, which provide insights into the energy loss mechanism of charm quarks in the QGP, and the D$^0$ elliptic and triangular flow coefficients, that probe the charm quark transport in the QGP. We also present the D$^\pm_\textrm{s}$/D$^0$ and $\Lambda^\pm_\textrm{c}$/D$^0$ yield ratios as a function of transverse momentum and collision centrality that help us better understand the charm quark hadronization process in heavy-ion collisions. Finally, we show the rapidity-odd directed flow of D$^0$ mesons, which is sensitive to the initial tilt of the QGP bulk and can also probe the effects of the initial magnetic field in heavy-ion collisions.

In ultra-relativistic heavy-ion collisions, creation of a novel state of matter, the quark-gluon plasma (QGP), has been observed. Suppressed production of quarkonia, caused by the colour screening of the binding force, has been proposed as a direct evidence of the QGP formation. At RHIC energies, other phenomena such as the regeneration and co-mover absorption, are expected to have a small effect for the bottomonium family, which makes Υ a cleaner probe of the screening effect compared to the J / ψ meson. In these proceedings, the latest measurements of the Υ production suppression in Au + Au collisions at s NN = 200 GeV via the di-muon and di-electron decay channels by the STAR experiment at RHIC are presented and compared with data from the LHC and theoretical calculations. Moreover, Υ production measurements in p + p and p + Au collisions are also reported, providing a baseline and a quantification of the cold nuclear matter effects, respectively.

In this work, we report the thermal property of the matter created in relativistic heavy ion collisions by the systematic study of v₂ for charged hadrons. We study the dependence on collision energies, collision size and particle species, comparing the v₂ in Au+Au and Cu+Cu collisions for some centrality bins. We tested the scaling behavior of v₂ depending on the geometrical eccentricity at same N_part and depending on N_part at same geometrical eccentricity.

In this work we study the suppression of charged hadrons (π±, K±, p and ) in ultrarelativistic heavy ion (UHI) collisions, where the quark-gluon plasma (QGP) is believed to be produced. Jet energy loss in the QGP is the main explanation for jet quenching, which may suppress the number of hadrons produced while the jets traverse the dense and colorful medium created. By taking into account the energy loss of the partons traversing the medium and the nuclear shadowing of the parton distributions depending on the impact-parameter, we calculate the nuclear modification factor RAA for lead-lead collisions at the LHC. Finally, we estimate the opacity of the QGP in several centrality classes.

Heavy quarks are suitable probes to study the properties of Quark-Gluon Plasma (QGP), a strongly interacting medium, which can be created in ultrarelativistic heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC). Non-photonic electrons (NPE) that originate from semileptonic decays of D and B mesons can serve as a good proxy for heavy flavor quarks. Nuclear modification factor RAA of NPE is measured in heavy-ion collisions, which is sensitive to the effect of QGP on heavy quarks.Measurements of NPE RAA in Au+Au collisions at √sNN = 200 GeV reveal that NPE production is strongly suppressed. In year 2012 STAR collected data from U+U collisions at √sNN = 193 GeV. In most central collisions higher energy density can be achieved in comparison to collisions of gold nuclei. In this proceedings the preliminary results on NPE in 0-5% most central U+U collisions at a transverse momentum range 1.2 are presented. The nuclear modification factor in U+U collisions is compared to that in Au+Au collisions and theoretical models.

Charm quarks are an excellent probe to study properties of the Quark-Gluon Plasma (QGP) created in ultra-relativistic heavy-ion collisions. In particular, measurements of the $D_{s}^{\pm}$ meson production can provide valuable information on the charm quark hadronization mechanism in the QGP. We report the measurements from the STAR experiment on the invariant yields of $D_{s}^{\pm}$ mesons as a function of transverse momentum for different centrality classes of Au+Au collisions at $\sqrt{s_\mathrm{NN}}$ = 200 GeV. These measurements utilize the data with the Heavy Flavor Tracker detector from 2014 and 2016. The ratio between strange ($D_{s}^{\pm}$) and non-strange ($D^{0}$) open charm mesons will also be shown, and compared to PYTHIA and different model calculations. A clear enhancement relative to the PYTHIA calculation is seen in the ratio, while different model calculations incorporating strangeness enhancement and charm quark coalescence hadronization can describe the observed enhancement qualitatively. These results suggest that recombination of charm quarks with strange quarks in the QGP plays an important role in charm quark hadronization.

The specific shear viscosity (eta/s)_QGP of quark-gluon plasma (QGP) can be extracted from elliptic flow data in heavy-ion collisions by comparing them with the dynamical model VISHNU which couples a viscous fluid dynamic description of the QGP with a microscopic kinetic description of the late hadronic rescattering and freeze-out stage. A robust method for fixing (eta/s)_QGP from the collision centrality dependence of the eccentricity-scaled charged hadron elliptic flow is presented. The systematic uncertainties associated with this extraction method are discussed, with specific attention to our presently restricted knowledge of initial conditions. With the (eta/s)_QGP extracted in this way, VISHNU yields an excellent description of all soft-hadron data from Au+Au collisions at top RHIC energy. Extrapolations to Pb+Pb collisions at the LHC, using both a purely hydrodynamic approach and VISHNU, are presented and compared with recent experimental results from the ALICE Collaboration. The LHC data are again well described by VISHNU, with the same (eta/s)_QGP value as at RHIC energies.

Jet quenching is an important evidence of the quark-gluon plasma (QGP) formation. The effect of jet quenching is emerged in ultra-relativistic heavy-ion collisions (A+A) by suppressed production of hadron yields at high transverse momenta (pT > 5 GeV/c) when compared to one measured in elementary proton-proton collisions (p+p). Cu+Au collisions are characterised by the unique asymmetric nuclei overlap geometry different from one presented in symmetric collisions (Au+Au, Cu+Cu). It makes the Cu+Au collision system especially interesting for jet quenching studies. In this paper we present results of the KS meson production measurement in Cu+Au collisions at with respect to the meson pT and collision centrality obtained using the electromagnetic calorimeter of PHENIX experiment. KS mesons are reconstructed via the KS →π0(→ γγ) π0(→ γγ) decay channel. Obtained KSmeson nuclear modification factors are consistent with ones measured for π0 and η mesons and reconstructed jets in the same collision system and ones measured in Cu+Cu and Au+Au collisions at the same collision energy.

The transport properties of the strongly coupled quark-gluon plasma created in ultrarelativistic heavy-ion collisions are extracted by Bayesian parameter estimate methods with the latest collision beam energy data from the CERN Large Hadron Collider. This Bayesian analysis includes sophisticated flow harmonic observables for the first time. We found that the temperature dependence of specific shear viscosity appears weaker than in the previous studies. The results prefer a lower value of specific bulk viscosity and a higher switching temperature to reproduce additional observables. However, the improved statistical uncertainties both on the experimental data and hydrodynamic calculations with additional observables do not help to reduce the final credibility ranges much, indicating a need for improving the dynamical collision model before the hydrodynamic takes place. In addition, the sensitivities of experimental observables to the parameters in hydrodynamic model calculations are quantified. It is found that the analysis benefits most from the symmetric cumulants and nonlinear flow modes, which mostly reflect nonlinear hydrodynamic responses, in constraining the temperature dependence of the specific shear and bulk viscosities in addition to the previously used flow coefficients.

In the ultra-relativistic heavy-ion collisions a very hot and dense state of deconfined nuclear matter is created, known as Quark Gluon Plasma (QGP). It has been predicted theoretically, that some parity violating states may be created in heavy-ion collisions, which along with the presence of strong magnetic field results in the separation of charges along the axis of the magnetic field (Kharzeev et al, Prog Part Nucl Phys, 88:1–28, 2016, [1]), (Selyuzhenkov, [2]). We investigated the event-by-event charge separation in Au\(+\)Au collisions at \({\sqrt{s_{NN}}}\) =200 GeV using Sliding Dumbbell Method. The preliminary results of exploratory study are reported here.

Ultrarelativistic collisions of heavy atomic nuclei produce an extremely hot and dense phase of matter, known as quark–gluon plasma (QGP), which behaves like a near-perfect fluid with the smallest specific shear viscosity—the ratio of the shear viscosity to the entropy density—of any known substance1. Due to its transience (lifetime ~ 10−23 s) and microscopic size (10−14 m), the QGP cannot be observed directly, but only through the particles it emits; however, its characteristics can be inferred by matching the output of computational collision models to experimental observations. Previous work, using viscous relativistic hydrodynamics to simulate QGP, has achieved semiquantitative constraints on key physical properties, such as its specific shear and bulk viscosity, but with large, poorly defined uncertainties2–8. Here, we present the most precise estimates so far of QGP properties, including their quantitative uncertainties. By applying established Bayesian parameter estimation methods9 to a dynamical collision model and a wide variety of experimental data, we extract estimates of the temperature-dependent specific shear and bulk viscosity simultaneously with related initial-condition properties. The method is extensible to other collision models and experimental data and may be used to characterize additional aspects of high-energy nuclear collisions. As the quark–gluon plasma is a short-lived state of matter, its properties cannot be measured directly. A Bayesian parameter estimation method now provides a reliable estimation of the temperature-dependent specific shear and bulk viscosities.

The production of charmonium in heavy-ion collisions is investigated based on Boltzmann-type transport model for charmonium evolution and langevin equation for charm quark evolution. Charmonium suppression and regeneration in both quark-gluon plasma (QGP) and hadron phase are considered. Charm quarks are far from thermalization, and regeneration of charmonium in QGP and hadron gas is neglectable at SPS and FAIR. At peripheral collisions, charmonium suppression with hadron gas explains the experimental data well. But at central collisions, additional suppression from deconfined matter (QGP) is necessary for the data. This means there should be QGP produced at central collisions, and no QGP produced at peripheral collisions at SPS energy. Predictions are also made at FAIR $\sqrt{s_{NN}}=7.7$ GeV Au+Au collisions.

Experiment and lattice simulation show that the quark–gluon plasma (QGP) system displays strong interaction between constituents at temperature a few times the critical temperature Tc. This QGP picture can be explained by assuming that the QGP matter above Tc is rich in different kinds of bound states, namely resonance-like QGP (RQGP). The chemical composition of the QGP system produced in ultra-relativistic heavy-ion collisions can be investigated through a general charge balance function which describes two-wave quark production during expansion afterward. In this paper, we investigate the signals of this RQGP through general charge balance functions. We find that the quasiparticles in QGP contribute a little to the balance functions because of their heavy masses. The balance functions reduce to the situation discussed before where only one-wave charge production is involved if only the quasiparticles in QGP are considered. However, the baryonic bound states in QGP have a significant effect on the balance function [Formula: see text], causing a dip in the [Formula: see text] balance function at small Δy. The existence of the binary and baryonic bound states amplify the negative dip of the balance function BpK-(Δy) at Δy ∽ 1.