Bulletin of the American Physical Society
2011 Fall Meeting of the APS Division of Nuclear Physics
Volume 56, Number 12
Wednesday–Saturday, October 26–29, 2011; East Lansing, Michigan
Session HA: Ultrarelativistic Heavy Ions II |
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Chair: Sean Gavin, Wayne State University Room: 62 |
Friday, October 28, 2011 10:30AM - 10:42AM |
HA.00001: Two-particle correlations on transverse rapidity and the momentum dependence of angular correlation features in Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV at STAR Elizabeth Oldag Two-particle 2D correlations on transverse rapidity $y_t$ from Au+Au collisions at STAR will be presented. The correlations are formed from charged particles with $p_t \geq 0.15$ GeV/c, $|\eta| \leq 1$, and $2\pi$ azimuth. A peak in transverse rapidity correlations around ($y_t,y_t$) = (3,3) ($p_t$ = 1.4 GeV/c) is observed and remains at approximately the same position from peripheral to most central collisions. Pairs distinguished by charge combination and small versus large relative azimuth angles will also be presented. A peak in ``back-to-back'' pairs around ($y_t,y_t$)=(3,3) persists even in more central collisions remaining at approximately the same $y_t$. To study how pairs correlated in the ($y_t,y_t$) peak contribute to correlation features in relative pseudorapidity $\eta_{\Delta}$ and azimuth $\phi_{\Delta}$, a new cut scheme was implemented consisting of 28 distinct momentum ranges. The ($y_t,y_t$) distribution of particles producing the angular correlation structures associated with minimum bias jets will be presented. Interestingly, these results show that the extended correlation on relative $\eta$ commonly referred to as the ``ridge'' is not softer relative to the minijet peak near the origin and is harder than the inclusive spectra. [Preview Abstract] |
Friday, October 28, 2011 10:42AM - 10:54AM |
HA.00002: Two particle number and momentum correlations for $\sqrt{s_{NN}} = $200 GeV Au$+$Au collisions at STAR Larry Tarini Recent two-particle correlation measurements of Au$+$Au collisions at $\sqrt{s_{NN}} = $200 GeV at STAR differ from previous p$+$p correlation results due to the appearance of strong long-range correlations in relative pseudorapidity, a phenomenon known as ``the ridge.'' Using three different number and transverse momentum pair correlation observables, we investigate the ridge and related phenomena as a function of collision centrality and charge dependence. Parameters for a Gaussian fit in $\Delta\eta$ and harmonic coefficients in $\Delta\phi$ are extracted for number correlations. [Preview Abstract] |
Friday, October 28, 2011 10:54AM - 11:06AM |
HA.00003: Incident Energy Dependence of $p_{t}$ Correlations at RHIC John Novak We present results for two-particle transverse momentum correlations $\left\langle {\Delta {p_{t,i}}\Delta {p_{t,j}}} \right\rangle$ as a function of event centrality for Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 7.7, 11.5, 39, and 200 GeV at RHIC. Correlations were observed to decrease with centrality. The correlations multiplied by the multiplicity density increase with incident energy, and the centrality dependence may be indicative of physics. The square root of the correlations divided by the event-wise average transverse momentum per event shows little beam energy dependence for beam energies from 39 GeV up to LHC energies, but decreases at lower beam energies. This result differs with previous results from CERES [1], but is replicated by UrQMD.\\[4pt] [1] D. Adamova et al. [CERES Collaboration], Nucl. Phys. {\bf A727}, 97 (2003). [Preview Abstract] |
Friday, October 28, 2011 11:06AM - 11:18AM |
HA.00004: Comparing correlation predictions from a glasma flux-tube model with measurements Lanny Ray, Thomas Trainor A Glasma flux-tube model has been proposed to explain strong elongation on relative pseudorapidity ($\eta$) of the same-side 2D peak in minimum-bias angular correlations from $\sqrt{s_{NN}}$ = 200 GeV Au-Au collisions. In this model the same-side peak, or ``soft ridge,'' is said to arise from coupling of the flux tubes to radial flow. Gluons radiated transversely from the flux tubes are boosted by radial flow to form a narrow structure or ridge on azimuth. We have tested this conjecture by comparing predictions for particle production, spectra and transverse momentum correlations [1] from the Glasma model and conventional fragmentation processes with measurements. We conclude that the Glasma model is not relevant for understanding the same-side $\eta$-elongated correlations in Au-Au collisions at RHIC. A two-component model (soft plus hard) of hadron production, including minimum-bias parton fragmentation, provides a quantitative description of most data although the $\eta$ elongation remains unexplained. \\[4pt] [1] T. Lappi, arXiv:1011.0821 [hep-ph] (2010). [Preview Abstract] |
Friday, October 28, 2011 11:18AM - 11:30AM |
HA.00005: Viscous Diffusion and Transverse Momentum Correlations at RHIC and LHC Rajendra Pokharel, Sean Gavin, George Moschelli RHIC experiments indicated that the matter formed in relativistic nuclear collisions flows like a nearly perfect fluid, with a small viscosity to entropy density ratio. Various estimates from elliptic flow measurements give 1-2 times the Kovtun-Son-Starinets bound. However, there is theoretical uncertainty resulting from our incomplete knowledge of initial conditions, equation of state and the role of the mini-jets. Recently, the STAR collaboration reported measurements of using transverse momentum correlations. The idea behind this method is that shear viscosity diffuses the distribution of transverse momentum fluctuations reducing the peak value of the correlation function while broadening its rapidity distributions and thus relating the viscosity to evolution of the width. This approach to viscosity is different from those involving elliptic flow. Here we present the theory using the second order viscous hydrodynamics and compare the calculations with recent STAR data. [Preview Abstract] |
Friday, October 28, 2011 11:30AM - 11:42AM |
HA.00006: Estimates of the non-flow contribution in Pb-Pb flow analysis using two-particle correlations in $pp$ with the ALICE detector at the LHC Vera Loggins Two-particle azimuthal correlations are statistically the most precise method for measuring anisotropic flow in heavy-ion collisions. The main drawback of this method is its strong sensitivity to non-flow correlations which, unlike real flow, do not have a geometrical origin. Non-flow contributions can be estimated from two-particle azimuthal correlations using $pp$ data. We report measurements using the $$ method applied to $pp$ collisions at $\sqrt s=$ 2.76 TeV and $\sqrt s=$ 7 TeV. We study the dependence of correlations on two-particle separation in pseudorapidity in order to find the separation which minimizes the correction without sacrificing the statistics too much. Measurements are performed for first to fifth harmonics. [Preview Abstract] |
Friday, October 28, 2011 11:42AM - 11:54AM |
HA.00007: Number and Transverse Momentum Two-Particle Correlation Studies in Pb+Pb Collisions at the LHC Claude Pruneau Observations of a ridge on the near-side, and a dip on the away-side of two-particle correlations measured in central Au + Au collisions have generated considerable interest at RHIC. Are the two phenomena connected? Do they result from jet interactions with the medium, or do they naturally arise from the rapid thermalization and hydrodynamic expansion of collision systems subject to large initial fluctuations? We present measurements, carried with the ALICE detector, of number ($R_2$) and transverse momentum ($\Delta p_t\Delta p_t$) correlation functions in Pb + Pb collisions. The two correlation functions are studied as a function of collision centrality for ++, -\ -, and +- charged particle pairs in various momentum ranges. The like-sign and unlike-sign correlations exhibit different evolution with collision centrality. We combine these correlations to study charge dependent (CD) and charge independent (CI) correlation functions. We characterize these distributions by studying Fourier decompositions of $\Delta\varphi$ projections of the $R_2$ and $\Delta p_t\Delta p_t$ correlation functions for different ranges of $\Delta \eta$. Of particular interest are the evolution of the ratios of 3rd, and 4th harmonics to the 2nd harmonics with number of participants. We will discuss these results in light of a MC Glauber model of the initial eccentricity of collision nucleon partic ipants. [Preview Abstract] |
Friday, October 28, 2011 11:54AM - 12:06PM |
HA.00008: Illuminating nuclei with Vector Mesons in the STAR Ultra Peripheral Collision Program Ramiro Debbe The Ultra Peripheral Collision (UPC) program of the STAR Collaboration at RHIC has collected a substantial set of events where $\rho$ and $J/\Psi$ vector mesons are detected with no other hadronic activity between the colliding nuclei. The vector meson production in heavy ion ultra peripheral interactions is well described as quark-antiquark dipoles fluctuating out of the strong electric fields of the ions. These dipoles then strongly interact with the other nucleus via color-singlet exchange. We will present the distribution of the momentum transfer t of the detected vector meson in UPC Au+Au events at $\sqrt{s_{NN}}=200$ GeV, around mid-rapidity. This distribution is connected to the partonic form factor of the nuclei as these interactions are seen as diffraction of beams of vector mesons on the hadronic component of nuclei. STAR's Zero Degree Calorimeters, designed to detect neutrons with beam momentum produced by weak excitations of the nuclei, can be used to separate events where the vector meson interacts coherently with the entire nucleus or incoherently with individual nucleons. The STAR UPC data sample is composed mainly of events with $\rho$ mesons, but last year's extensive run produced a good sample of events with $J/\Psi$ and we expect to be able to get a first glimpse of the ``imaging'' of nuclei with two wave lengths; the $J/\Psi$ is considered a small object as opposed to the more extended nature of the $\rho$ meson. [Preview Abstract] |
Friday, October 28, 2011 12:06PM - 12:18PM |
HA.00009: Ultraperipheral Pb+Pb reactions at LHC energies Edwin Norbeck, Yasar Onel The magnetic field midway between two Pb nuclei passing at 20 fm is 2 x 10$^{20}$ gauss at LHC energies (1144 TeV in PbPb center of mass). At these energies the Coulomb field of a passing Pb nucleus can be regarded as a cloud of real photons. The cross sections for $\gamma \gamma $ and $\gamma $A reactions are huge compared to 7 b for two Pb nuclei actually colliding. The reaction rate is limited by the 323 b cross section for breaking up the Pb nucleus or for the capture by a Pb ion of an e$^{-}$ from the many e$^{+}$ e$^{-}$ pairs that are formed. These products go down the beam pipe and eventually hit superconducting magnets. The $\gamma \gamma $ reactions can produce particles with mc$^{2}$ up to 100 GeV. The $\gamma $A reactions can produce particles with mc$^{2}$ more than 900 GeV. These ultraperipheral reactions are particularly clean. In proton-proton reactions, the reaction between two partons to produce something of interest is accompanied by a large background caused by many other parton-parton reactions. When $\gamma $A breaks up a Pb nucleus, the transverse energy is small so that the fragments continue in the original beam direction. A single neutron in the original beam direction provides a useful flag that shows that an ultraperipheral reaction has occurred. [Preview Abstract] |
Friday, October 28, 2011 12:18PM - 12:30PM |
HA.00010: Macroscopic Anomalous Effects in Hot QCD Fluid: A Status Report Jinfeng Liao Two fundamental features of QCD, the nontrivial topological nature of gauge fields together with the anomaly of light fermions, have been recently shown to lead to macroscopic anomalous effects in the hot QCD fluid created in relativistic heavy ion collisions. A number of recent exciting progresses will be reported here: (1) the Chiral Magnetic Effect (as a local P- and CP-odd phenomenon) predicts charge asymmetry fluctuations and particular patterns of charged hadron correlations, and progresses toward an accurate interpretation of the full data set, from most recent RHIC Beam Energy Scan all the way to LHC results, will be critically evaluated; (2) for fireballs with nonzero vector densities (e.g. in low-beam energy collisions), the Chiral Magnetic Wave induces an electric quadrupole of quark-gluon plasma and we predict the charge-dependent elliptic flow with v2(pi-)$>$v2(pi+); (3) for rotating hot fluid (i.e. QGP with angular momentum) in non- central collisions, a new category of anomalous hydrodynamic phenomena (Anomalous Magneto-Hydrodynamics) will also be presented, including the helical sound and the acoustic analog of the Faraday rotation in optics. Reference: PRC81:031901, PRC82:054902, PRC83:014905 (with Bzdak, Koch); In preparation (with Kharzeev); arXiv:1103.1307 to appear in PRL (with Burnier, Kharzeev, Yee). [Preview Abstract] |
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