Session EJ: Mini-Symposium on Flow-Like Observables in Heavy Ion Collisions

Flow Measurements at the RHIC and LHC, What Have We Learned? What is Needed?

Higher-order flow coefficients, $v_n$, reflect the space-time evolution process of hot and dense medium formed in relativistic heavy ion collisions. In the low $p_T$ region, experimental $v_n$ data at the highest energy A+A collisions at the RHIC and LHC is successfully described by various hydrodynamic calculations that employ Glauber/CGC initial conditions for heavy ion collisions and a shear viscosity of the medium. Our goal is to determine a single combination of an initial state and a viscosity value which can describe the data. However, there are currently more than one such combination and further constraints from experiments and theories are of importance. Azimuthal anisotropy $v_n$ is also observed in small collisions systems such as p(d)+A collisions at RHIC and LHC. The CGC (initial state effect) and hydrodynamic expansion (final state effect) are suggested as a possible explanations. Understanding the primary causes of $v_n$ evolution is important for the understanding of small collision systems and might provide useful information to the understanding of the initial condition in A+A collisions. In this talk, we will (i) summarize what we have observed in p(d)-A and A+A collisions at the RHIC and LHC, and (ii) discuss what has to be done as next step towards more precise understanding of the properties of the medium.   

Charge-dependent elliptic flow from event-by-event anomalous hydrodynamic simulations

Macroscopic transport effects induced by the quantum anomaly have been attracting much attention. We develop an anomalous-hydrodynamic model for heavy-ion collisions in order to quantify the effects of anomalous transports in the quark-gluon plasma. We perform event-by-event simulations of the anomalous fluids, and we calculate the charge-dependent particle distributions from subsequent hadronizations. According to the distributions, we calculate observables like charge-dependent $v_2$ using the same methods as experimentalists use. We also take the initial fluctuations in chiral charge into account and see how they affect the final observables.   

Flow Measurements and selection of body-body and tip-tip enhanced samples in U+U collisions at STAR

The azimuthal anisotropy of particle production is commonly used in high-energy nuclear collisions to study the early evolution of the system. The prolate shape of uranium nuclei provides the possibility to study how the initial geometry of the nuclei affects the azimuthal distributions. It also provides a unique opportunity to understand the initial conditions for particle production at mid-rapidity in heavy ion collisions. In this talk, the two- and four- particle cumulant, $v_2\{2\}$ and $v_2\{4\}$, from U+U collisions at $\sqrt{s_{NN}}=$ 193 GeV and Au+Au collisions at $\sqrt{s_{NN}}=$200 GeV for inclusive charged hadrons will be presented. The STAR Zero Degree Calorimeter is used to subdivide the 0-1\% centrality bin into even finer centralities. Differences were observed between the multiplicity dependence of $v_2\{2\}$ for most central Au+Au and U+U collisions. Data were compared with both Monte Carlo Glauber and IP-Glasma models and it was seen that IP-Glasma model does a better job of describing data. It has also been demonstrated that ZDC and multiplicity in combination provide a way to select body-body or tip-tip enhanced samples of central U+U collisions. We will also present preliminary $v_3\{2\}$ results for inclusive charged hadrons from Au+Au and U+U collisions.   

Charged hadron flow in Cu+Au collisions at RHIC-PHENIX

Quark Gluon Plasma (QGP) was discovered in heavy ion collisions at RHIC. To investigate the characteristics of QGP, anisotropic flow plays an important role in study of QGP properties because it provides access to initial spacial anisotropy, density profile and shear viscosity over entropy ratio. In 2012, Cu+Au collisions, the first asymmetric collisions of heavy nuclei, were operated at RHIC. Measurement of flow in asymmetric collisions is a subject of special interest because they probe different density profiles, pressure gradients and initial triangularity comparing to symmetric collisions in mid-central collisions. In this talk we present current status of flow observables in Cu+Au 200 GeV collisions at PHENIX as a function of transverse momentum and (pseudo)rapidity.   

Production and Elliptic Flow of Dileptons and Photons in the semi-Quark Gluon Plasma

We consider the thermal production of dileptons and photons at temperatures above the critical temperature in QCD. In the semi Quark Gluon Plasma (QGP), color excitations are suppressed by a small value of the Polyakov loop. Comparing the semi-QGP to the usual, perturbative QGP, we find a mild enhancement of thermal dileptons. In contrast, to leading logarithmic order in weak coupling there are far fewer photons from the semi-QGP than the usual QGP. To illustrate the possible effects we use a hydrodynamic model. Dileptons uniformly exhibit a small elliptical flow, but the strong suppression of photons in the semi-QGP tends to bias the elliptical flow of photons to that generated in the hadronic phase.   

System-size and beam energy dependence of the space-time extent of the pion emission source

Two-pion interferometry measurements are used to extract the Gaussian source radii $R_{out}$, $R_{side}$ and $R_{long}$, of the pion emission sources produced in $d+$Au, Cu$+$Cu and Au$+$Au collisions for several beam collision energies at PHENIX experiment. The extracted radii, which are compared to recent STAR and ALICE data, show characteristic scaling patterns as a function of the initial transverse geometric size of the collision system, and the transverse mass of the emitted pion pairs. These scaling patterns indicate a linear dependence of $R_{side}$ on the initial transverse size, as well as a smaller freeze-out size for the $d+$Au system. Mathematical combinations of the extracted radii generally associated with the emission source duration and expansion rate exhibit non-monotonic behavior, suggesting a change in the expansion dynamics over this beam energy range.   

Forward and backward rapidity long-range correlation in p-Pb collisions at $\sqrt{s_{NN}}$=5.02 TeV with the ALICE detector

Measurements of the particle correlations are very useful to investigate the underlying mechanism and the dynamics of particle production in high-energy nucleus-nucleus collisions. Long range, near side angular correlations have been observed in high multiplicity p-p and p-Pb collisions at the LHC energy. Possible explanations of the long range correlations in high multiplicity p-p and p-Pb collisions are the collective behavior of the created medium and/or the remnants of the strong color fields created by the dense gluonic field (gluon saturation). The saturation effects are expected to be enhanced at forward rapidity region and the measurements of the particle productions with large rapidity gaps and the centrality dependence are important to quantify saturation and hydrodynamical final state effects. The two-particle correlations are measured with VZERO and TPC, where the ALICE VZERO detectors cover -3.7$ < |\eta| < $ -1.7 in backward and 2.8 $< |\eta| <$ 5.1 in forward. The status of the data analysis for p-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV will be presented.   

A parametric study of di-hadron azimuthal angular correlations in forward rapidity proton-nucleus collisions at RHIC and the LHC

We investigate the behavior of di-hadron azimuthal angular correlations in high energy proton-nucleus collisions at RHIC and the LHC in the forward rapidity region. We consider the correlations in different kinematics separated by the saturation scale of the nucleus and study the dependence of the correlations on the transverse momenta of the hadrons as compared to the nuclear saturation scale. We then investigate the effect of the recently proposed quartic terms in the action on the angular correlation.   

Transverse dynamics of multi-strange hadrons

We study the hadronic rescattering effects, in particular for multi-strange hadrons, on the final observables within an integrated dynamical approach in which a (3+1)D ideal hydrodynamic model is combined with hadronic cascade model, JAM. Since the measured observables contain all the information throughout the space-time evolution in relativistic heavy ion collisions, dissipation caused by hadronic rescattering in the late stage must be taken into account to investigate the properties of the quark gluon plasma. By comparing final observables to those obtained just after the QGP stage in this approach, we quantify the hadronic rescattering effects on particle ratio, azimuthal anisotropic flow $v_n$ and mean transverse momentum $\langle p_{\mathrm T} \rangle$. Furthermore we show that multi-strange hadrons can be direct probes of the QGP at hadronization due to a fact that multi-strange hadrons rescatter less than non-strange hadrons.   

Effects of causal hydrodynamic fluctuations on observables in relativistic heavy-ion collisions

In recent years, the flow harmonics $v_n$ are systematically observed in relativistic heavy-ion collision experiments at RHIC and LHC, and attract a lot of theoretical and experimental interests. Initial state fluctuations turned out to be important to explain these flow harmonics through event-by-event numerical calculations. While, there are other sources of fluctuations such as hydrodynamic fluctuations which are thermal fluctuations of hydrodynamics, and they have effects on the flow harmonics and other observables in the same matter as the initial fluctuations. We implement (3+1)-dimensional relativistic fluctuating hydrodynamics, which is viscous hydrodynamics with causal hydrodynamic fluctuations [1], in our dynamical models. We then investigate the effect of the hydrodynamic fluctuations on observables such as flow harmonics, and compare the effect with that of the initial fluctuations. \\[4pt] [1] Koichi Murase, Tetsufumi Hirano, arXiv:1304.3243 [nucl-th]   

Simulation of causal hydrodynamic fluctuation in Bjorken expansion

We investigate the effects of causal hydrodynamic fluctuation [1] on dynamics of the quark gluon plasma (QGP) in the Bjorken expansion model [2]. The QGP is created in relativistic heavy ion collisions and its time evolution can be described by relativistic hydrodynamics. So far the effects of event-by-event (e-by-e) initial fluctuation on final flow observables have been focused. In addition to these effects, fluctuation during hydrodynamic evolution should be also important on an e-by-e basis of hydrodynamic description of the QGP. We first introduce causal hydrodynamic fluctuation into the Bjorken expansion model. We then perform simulations of this Bjorken model with hydrodynamic fluctuation on an e-by-e basis. We find the final entropy fluctuates around the mean value. This indicates multiplicity also fluctuates due to hydrodynamic fluctuation and can contain transport properties of the QGP. This effect would be significant in small system such as p-A and peripheral A-A collisions. \\[4pt] [1] K. Murase and T. Hirano, arXiv:1304.3243.\\[0pt] [2] J.D. Bjorken, Phys. Rev. D 27, 140 (1983).