Bulletin of the American Physical Society
2006 Division of Nuclear Physics Annual Meeting
Wednesday–Saturday, October 25–28, 2006; Nashville, Tennessee
Session DE: Ultrarelativistic Heavy Ions I |
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Sponsoring Units: DNP Chair: David Silvermyr, Oak Ridge National Laboratory Room: Gaylord Opryland Hermitage B |
Friday, October 27, 2006 2:00PM - 2:12PM |
DE.00001: Centrality Dependent Studies of Charged Particle Spectra at RHIC Selemon Bekele A major goal of the RHIC program is to create a deconfined state of nuclear matter at high temperatures and densities and to study the properties of this matter. A transition from a deconfined phase of quarks and gluons to hadronic matter requires significant rescattering of particles in the initial phase. The amount of rescattering is expected to increase with the size of the reaction region. It is therefore of interest to study reactions over a wide range of collision geometries as measured by centrality. Recent results from AuAu collisions at $\sqrt{s_{NN}} = 200$ GeV at RHIC show clear evidence of suppression of the hadron yields at mid-rapidity. Surprisingly, comparable suppression at forward rapidity has also been observed. While the suppression at mid-rapidity is believed to be due to final state effects, the cause of the suppression at forward rapidity is not very well understood. Comparing data from different collision systems may help us understand the underlying mechanism for the suppression at forward rapidity. We present preliminary results from the BRAHMS experiment on charged hadron spectra at pseudo-rapidity ~ 3.0 as a function of centrality in CuCu collisions at $\sqrt{s_{NN}} = 200$ GeV. [Preview Abstract] |
Friday, October 27, 2006 2:12PM - 2:24PM |
DE.00002: Centrality dependence of the N($\Omega$)/N($\phi$) ratios and $\phi$ anisotropic flow Sarah Blyth Due to its long lifetime and relatively small hadronic interaction cross-section, the $\phi$-meson is a clean probe for studying the properties of the hot and dense medium created in high-energy nuclear collisions. We present the first results of the centrality dependence of the N($\Omega$)/N($\phi$) ratios and $\phi$ anisotropic flow ($v_{2}$) from $\sqrt{s_{NN}}=$ 200 GeV Au+Au collisions measured by STAR at RHIC. In more central collisions, the eccentricity-scaled anisotropic flow ($v_{2}/\epsilon$) is large, indicating a stronger collective expansion at the early partonic stage. For $p_{T}>$2 GeV/$c$, the $v_{2}$ values are consistent with the $v_{2}$ values of other mesons, and expectations from parton recombination models. In addition, the N($\Omega$)/N($\phi$) ratio is found to increase linearly as a function of $p_{T}$, a characteristic of coalescence of thermal quarks for both $\phi$ and $\Omega$. In the most central collisions, the linear increase reaches up to $p_{T}\sim$4 GeV/$c$ implying that most of the multistrange hadrons are formed directly from thermalized $s$-quarks in Au+Au collisions at RHIC. [Preview Abstract] |
Friday, October 27, 2006 2:24PM - 2:36PM |
DE.00003: Why the $x_E$ distribution triggered by a pizero does not measure the fragmentation function Michael Tannenbaum Hard-scattering in pp collisions was discovered at the CERN-ISR in 1972 by measurements utilizing inclusive single or pairs of hadrons. Due to the steeply falling power-law $p_T$ spectrum of the scattered partons, the inclusive single particle (e.g. pizero) spectrum from jet fragmentation is dominated by trigger fragments with large $\langle{z_t}\rangle\sim 0.7-0.8$, where $z_t=p_{T_t}/p_{T{\rm jet}}$ is the fragmentation variable. It was generally assumed, following Feynman, Field and Fox, as shown by data from the CERN-ISR experiments, that the $p_{T_a}$ distribution of away side hadrons from a single particle trigger [with $p_{T_t}$], corrected for $\langle {z_t}\rangle$, would be the same as that from a jet-trigger and follow the same fragmentation function as observed in $e^+ e^-$ or DIS. PHENIX attempted to measure the fragmentation function from the away side $x_E\sim p_{T_a}/p_{T_t}$ distribution of charged particles triggered by a $\pi^0$ in p-p collisions and showed by explicit numerical calculation that the $x_E$ distribution was actually quite insensitive to the fragmentation function. The lack of sensitivity to the fragmentation function will be explained, and an analytic formula for the $x_E$ distribution given. The away-side distribution has the nice property that it both exhibits $x_E$ scaling and is directly sensitive to the ratio of the away jet $\hat{p}_{T_a}$ to that of the trigger jet, $\hat{p}_{T_t}$ and thus to the relative energy loss of the two jets escaping from the medium in RHI collisions. Applications to measurements from Au+Au collisions at RHIC will be presented, leading to some interesting conclusions. [Preview Abstract] |
Friday, October 27, 2006 2:36PM - 2:48PM |
DE.00004: A New Large-area Muon Telescope Detector at Mid-rapidity at RHIC Guoji Lin, Zhangbu Xu $\mu$ particle identification at middle and high $p_{T}$ range is crucial for many interesting physics in heavy ion collision experiment, like the dimuon continuum, the quarkonia production, and the Drell-Yan process. Due to the electromagnetic nature of interaction, $\mu$ carries information with direct sensitivity to the early stage of the high-energy nuclear collision before chemical freezeout. More importantly, $\mu$ is a background free probe compared to electron with no photon conversion background and much less Dalitz decay. A large-area Muon Telescope Detector (MTD) at mid-rapidity at RHIC is proposed and under investigation. In this talk the simulation of $\mu$ detection and hadron rejection will be shown. The preliminary result of the first test in run 6 p+p collisions will be discussed. A significant enhancement of high $p_{T}$ charged particles is achieved in this test run. Finally the plans for future upgrade will be presented. [Preview Abstract] |
Friday, October 27, 2006 2:48PM - 3:00PM |
DE.00005: Microscopic Transport Approaches to Analyzing Hadronic Matter Nasser Demir, Steffen Bass Ultra-relativistic heavy-ion collisions at RHIC are thought to have created a strongly interacting Quark-Gluon-Plasma (sQGP) with a very low shear viscosity in the deconfined phase. However, as the sQGP hadronizes it will evolve through a hadronic phase with rapidly increasing viscosity. In order to fully characterize the sQGP state, one has to separately determine the viscosity of the hadronic phase. Here, we present a calculation of the shear viscosity coefficient of hadronic matter in equilibrium for a range of initial conditions and energy and baryon number densities. The dynamics of the particles comprising this medium are simulated using the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) model in a box with periodic boundary conditions. Green-Kubo theory enables us to compute linear transport coefficients of a medium by examining fluctuations of the system's stress-energy tensor near equilibrium. We outline how to apply this approach to compute other transport coefficients, such as the baryon diffusion constant of hadronic matter. We also sketch an algorithm that combines the Green-Kubo formalism and our microscopic transport model to analyze the time evolution of the shear viscosity of a sytem gradually drifting out of equilibrium. [Preview Abstract] |
Friday, October 27, 2006 3:00PM - 3:12PM |
DE.00006: ABSTRACT WITHDRAWN |
Friday, October 27, 2006 3:12PM - 3:24PM |
DE.00007: Effect of the ``minimal'' viscosity on observables at RHIC Denes Molnar, Pasi Huovinen The goal of heavy ion experiments at the Relativistic Heavy Ion Collider (RHIC) is to create and study a novel hot and dense phase of quark-gluon matter, the so-called quark gluon plasma (QGP). Several features of the RHIC data can be reproduced using ideal hydrodynamics, which lead to the suggestion that the plasma could be a ``perfect fluid.'' However, ideal hydrodynamics assumes zero viscosity (and therefore no dissipation), contrary to general expectations based on quantum mechanics that imply finite rates and, therefore, a nonzero ``minimal viscosity.'' These expectations have been verified for strongly-coupled ${\cal N}=4$ supersymmetric Yang-Mills theories, for which the lower bound on the shear viscosity to entropy density ratio is $\eta/s = 1/(4\pi)$. Parton kinetic theory calculations based on microscopic $2\to 2$ rates do indicate that short, but non-zero, effective mean free paths generate sizable dissipative effects for conditions expected at RHIC. We show that these results imply that even a small $\eta/s$ ratio $\sim 0.1$ affects observables at RHIC and leads to significant deviations from ideal hydrodynamic behavior. [Preview Abstract] |
Friday, October 27, 2006 3:24PM - 3:36PM |
DE.00008: Anomalous Viscosity of an Expanding Quark-Gluon Plasma Steffen Bass, Masayuki Asakawa, Berndt Mueller We argue that an expanding quark-gluon plasma has an anomalous viscosity, which arises from interactions with dynamically generated color fields. We derive an expression for the anomalous viscosity in the turbulent plasma domain and apply it to the hydrodynamic expansion phase, when the quark-gluon plasma is near equilibrium. The anomalous viscosity dominates over the collisional viscosity for weak coupling and not too late times. This effect may provide an explanation for the apparent ``nearly perfect'' liquidity of the matter produced in nuclear collisions at the Relativistic Heavy Ion Collider without the assumption that it is a strongly coupled state. [Preview Abstract] |
Friday, October 27, 2006 3:36PM - 3:48PM |
DE.00009: Anti-particle to Particle Ratios in Cu+Cu Collisions Vasundhara Chetluru Anti-particle to particle ratio measurements in heavy-ion collisions are an interesting probe in the context of understanding the chemical freeze out parameters. In PHOBOS, particle identification is achieved by energy loss measurements in the two arm magnetic spectrometer. This detector, located at mid-rapidity, consists of 16 planes of highly segmented silicon pads, some of which are in a 2T magnetic field. This talk will present the analysis techniques and results of anti-particle to particle ratios for identified protons, kaons and pions from 200 and 62.4 GeV Cu+Cu collisions. We will discuss the centrality and transverse momentum dependence of the ratios and compare them to Au+Au, d+Au and p+p data. [Preview Abstract] |
Friday, October 27, 2006 3:48PM - 4:00PM |
DE.00010: Production of Phi Mesons in AA Collisions at $\sqrt{s_{NN}}$ = 62.4GeV measured by the PHENIX experiment Shengli Huang The phi meson mass centroid and width may provide information about partial chiral symmetry restoration in the hot and dense medium. The similar mass of the phi meson and the proton also makes the phi meson a good probe to study the baryon/meson anomaly in hadron production at intermediate transverse momentum (2 GeV/c $<$ p$_T$ $<$ 5 GeV/c). The PHENIX experiment has studied the production of phi mesons in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$ = 62.4 GeV using the $\phi \rightarrow$ K+ K- decay channel. We will present the latest results on transverse momentum spectra, invariant yields, nuclear modification factor(R$_{cp}$) and line-shape analysis (mass centroid and width) measured as a function of centrality. [Preview Abstract] |
Friday, October 27, 2006 4:00PM - 4:12PM |
DE.00011: Searching for the $D_{s}$ in STAR's d-Au Collisions Stephen Baumgart By studying the production of charm quarks at Relativistic Heavy Ion Collider we should be able to better understand the hard QCD processes which occur in high energy collisions. The STAR detector at RHIC has the potential to measure several different species of charmed hadrons. STAR has successfully measured $D^{0}$ spectra in d-Au and Au-Au collisions. Weak signals from the $D^{\pm}$ and $D^{*}$ were also observed. A measurement of the $D_{s}$ meson together with the measurements of the other observed charmed mesons would enable a more precise calculation of the total charm production cross-section. Simulations have shown that it may be possible to reconstruct the $D_{s}$ via its hadronic decay channels in the STAR data. We report on those simulations and the progress of this search in d-Au collisions at $\sqrt(s_{NN})$ = 200 GeV. [Preview Abstract] |
Friday, October 27, 2006 4:12PM - 4:24PM |
DE.00012: J/$\psi$ Measurements in $\sqrt{s_{NN}}$=200 GeV Au+Au Collisions Andrew Glenn Heavy quarkonia production is considered to be one of the most important probes of the hot and dense state created in relativistic heavy ion collisions. At RHIC energy, J/$\psi$ yields, especially the large feed-down contributions from $\chi_c$ and $\psi'$ states, are expected to be suppressed in a quark gluon plasma due to color screening. The PHENIX experiment at RHIC has measured J/$\psi$ production in $\sqrt{s_{NN}}$=200 GeV Au+Au collisions at both forward ($1.2<|y|<2.2$) and mid ($|y|<0.35$) rapidities. The most recent results for the centrality, rapidity and transverse momentum dependence of J/$\psi$ production will be discussed and compared with PHENIX baseline measurements and various theoretical calculations. [Preview Abstract] |
Friday, October 27, 2006 4:24PM - 4:36PM |
DE.00013: Non-photonic electron yields from Au+Au collisions at $\sqrt {s_{NN} } $ = 62 GeV and Cu+Cu collisions at $\sqrt {s_{NN} } $ = 200 GeV at STAR. Anders Knospe High-energy nucleus-nucleus collisions at RHIC allow physicists to study the behavior of nuclear matter at high temperatures and densities, where a new phase of matter, the quark-gluon plasma, is predicted to exist. All partons are thought to loose energy through gluon radiation as they pass through the hot and dense medium; it is expected that heavy quarks lose less energy in-medium than light quarks due to the suppression of small-angle gluon radiation (the dead cone effect), thus providing a sensitive probe of the medium. Heavy-quark production can be measured through the spectrum of non-photonic electrons, which is dominated by the decays of heavy-flavor mesons. The method used to identify non-photonic electrons in STAR data is described. Preliminary non-photonic electron spectra are being extracted for Au+Au collisions at $\sqrt {s_{NN} } $ = 62 GeV and for Cu+Cu collisions at $\sqrt {s_{NN} } $ = 200 GeV. They are compared to the non-photonic electron yields from $p+p$ and Au+Au collisions at $\sqrt {s_{NN} } $ = 200 GeV. This provides a test of the dependence of the non-photonic electron yield on collision energy and system size. [Preview Abstract] |
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