Session E13: Ultrarelativistic Heavy Ion Collisions I: Elliptic Flow and Azimuthal Correlations

The effect of chemical freezeout on transverse dynamics in relativistic heavy ion collisions

We show that chemical freezeout plays a very important role not only in the particle ratios but also in the transverse expansion in relativistic heavy ion collisions. Mean transverse momentum $\langle p_T \rangle$ of pions increases with the proper time in chemical equilibrium. In hydrodynamic simulations with chemical equilibrium, one makes full use of the above behavior of $\langle p_T \rangle$ to reproduce $p_T$ spectra and $v_2(p_T)$ at the cost of particle yields. However, $\langle p_T \rangle$ decreases with proper time when one properly takes account of chemical freezeout in hydrodynamic simulations. Thus hydrodynamic results with chemical freezeout turn out to deviate from experimental data. This suggests the importance of viscosity in hadronic fluids.   

Scaling properties of azimuthal anisotropy of mesons and baryons

Detailed systematic differential measurements of the azimuthal anisotropy of identified particles can provide important insights on crucial questions related to issues of thermalization and the properties of the new phase of matter created at RHIC. In recent measurements, the PHENIX collaboration has performed a detailed set of differential measurements at $\sqrt{s}=$ 62.4 and 200 GeV. New results from these measurements taken together with current and earlier measurements provide the most comprehensive data set for azimuthal anisotropy to date. This data set will be used to demonstrate various scaling properties important to an understanding of the mechanistic origin of azimuthal anisotropy.   

$\phi$-meson $v_{2}$ from Au+Au collisions at $\sqrt{s_{NN}}$=62.4 GeV

In high-energy nuclear collisions, it may be possible to create a phase of matter called the Quark Gluon Plasma (QGP). An observable which is developed at early times in the system and which may provide information about the QGP phase, is the elliptic flow ($v_{2}$) of particles. Since the $\phi$-meson presumably has a small hadronic interaction cross-section, we expect it to carry $v_{2}$ information that is independent of hadronization and freeze-out processes i.e it will be a clean probe of the medium at early stages of the collision. We will present preliminary $v_{2}$ results for the $\phi$-meson from Au+Au collisions at $\sqrt{s_{NN}}$=62.4 GeV from the STAR experiment at RHIC. We will also compare the elliptic flow of the $\phi$-meson with that of other identified particles. In addition, comparison of the results with model expectations will be discussed.   

Elliptic Flow in BRAHMS

Elliptic flow is sensitive to the initial partonic dynamics of heavy-ion collisions and can be deduced from the observation of an anisotropic particle density with respect to an event plane defined by the collision geometry. Before the RHIC run in 2004 of Au-Au collisions at #sqrt{s$_{NN}$} = 200 GeV, the BRAHMS multiplicity array was reconfigured to allow for a more robust measure of the collision event plane. The dependence of the elliptic flow signature (v$_{2}$) for charged particles on centrality and pseudorapidity ( |#eta| $<$ 3 ) will be presented, showing that the event plane can be accurately determined by the BRAHMS experiment. A comparison will be made with the event plane resolutions reported from the other RHIC experiments. With the event plane determined from the multiplicity array, the elliptic flow of the identified particles detected in the BRAHMS spectrometers will also be discussed.   

Azimuthal Anisotropy at Intermediate Rapidity in $\sqrt{S_{NN}} =$ 200 GeV Au-Au Collisions in PHENIX at RHIC-BNL

In mid-central heavy ion collisions, the nuclear overlap region is almond shaped. This spatial anisotropy leads to a momentum space anisotropy, which has symmetry about the plane defined by the beam axis and the impact parameter. This reaction plane (or event plane) can be determined in experiment using the final particle azimuthal distribution. The reaction plane resolution depends on particle multiplicity, azimuthal angle resolution, azimuthal hermeticity, and the amount of actual asymmetry that exists in the collision. We will present the effect of these factors on the resolution of the reaction plane for Au-Au collisions in general and more specifically for the pad planes of the PHENIX Multiplicity Vertex Detector (MVD). These pad planes are in the pseudorapidity range 1.8 $< |eta| <$ 2.6 on either side of the vertex region for which PHOBOS data (nucl-ex/0403025) suggest a v$_{2}$ of about 4 percent for mid-central Au-Au collisions at $\sqrt{S_ {NN}}$ of 200 GeV.   

Search for Conical Flow in Ultra-Relativistic Heavy-Ion Collisions at RHIC

An extremely dense, strongly coupled medium, created in ultra-relativistic heavy-ion collisions at RHIC, is responsible for the suppression of the yield of high $p_T$ particles in central Au+Au collisions with respect to scaled p+p collisions. This suppression has been interpreted as due to the energy loss of hard scattered partons in the dense medium. Since these partons propagate through the created matter much faster than the expected speed of sound in the medium, the resulting disturbances created by the energy deposition may produce in-medium conical flow, similar to a Mach cone. The resulting shock wave may give rise to increased particle production at well-defined angles with respect to the parton direction, which can be experimentally assessed via two-particle correlations. We present STAR results on the measurements of azimuthal correlations of charged hadrons with high $p_T$ trigger particles and study the results as a function of the associated hadron $p_T$.   

Comparison of high-pt di-hadron correlations in 62 AGeV and 200 AGeV Au+Au collisions

One of the most striking observations at RHIC is the disappearance of the back-to-back correlation of medium to high pt particles (2-6 GeV) in central Au+Au collisions at 200 GeV. This can be attributed to energy-loss of the fragmenting partons due to interactions with the hot and dense medium that is formed in these collisions. New correlation measurements from the recent RHIC run at a lower energy of 62.4 GeV will be presented and compared to the existing results at 200 GeV. The results will also be compared to Pythia simulations to explore the connection to the underlying parton dynamics in more detail.   

Jet Properties from Di-hadron Azimuthal Correlations in d+Au Collision

Results from the different RHIC experiments reveal an enhancement of the high-$p_{T}$ single particle spectra from $d+Au$ collisions as compared to $p+p$ collisions. This enhancement is usually attributed to multiple scattering of the projectile parton in the target nucleus prior to the hard collision. Multiple scattering will also affect the acoplanarity of di-jets produced in these collisions, which is measured as an increase in $k_{T}$, the parton transverse momentum. In this talk we present high-$p_{T}$ azimuthal correlations using di-hadrons to identify di-jets in PHENIX. We present final results on the measurement of $k_{T}$ from these correlations and present the centrality dependence. These results are compared to results from $p+p$ collisions to establish the increase in $k_{T}$ due to the cold nuclear medium. We compare the results to theoretical models that reproduce the single particle sprectra from $d+Au$ collisions. These results serve as a baseline for modifications of jet structure in $Au+Au$ collisions.