Bulletin of the American Physical Society
2007 Annual Meeting of the Division of Nuclear Physics
Volume 52, Number 10
Wednesday–Saturday, October 10–13, 2007; Newport News, Virginia
Session BD: Ultra Relativistic Heavy Ion Collisions I |
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Chair: Magdelena Djordjevic, The Ohio State University Room: Newport News Marriott at City Center Grand Salon IV |
Thursday, October 11, 2007 2:00PM - 2:12PM |
BD.00001: Initial Energy Density, Momentum and Flow in Heavy Ion Collisions Rainer Fries, Joseph Kapusta, Yang Li The very early phase of the collision of large nuclei at high energies is described by the color glass condensate. We compute the energy and momentum densities of the gluon field in a classical approximation at the earlierst stage of the collision. We also present constraints for the initial energy density, pressure and flow in the subsequent plasma phase. Phenomenological consequences for RHIC and LHC are discussed. [Preview Abstract] |
Thursday, October 11, 2007 2:12PM - 2:24PM |
BD.00002: Transport Coefficients of Hadronic Matter Nasser Demir, Steffen A. Bass Ultra-relativistic heavy-ion collisions at RHIC are thought to have created a Quark-Gluon-Plasma (QGP) with a very low shear viscosity in the deconfined phase. However, as the QGP hadronizes it will evolve through a hadronic phase with rapidly increasing viscosity. In order to fully characterize the QGP state, one has to separately determine the viscosity of the hadronic phase. We present a calculation of transport coefficients such as the shear viscosity, the shear viscosity to entropy density ratio $\frac{\eta}{s}$, and the diffusion coefficient as a function of $T$ and $\mu_B$ for nuclear densities in the range $(\rho_0 - 2 \rho_0)$. The hadronic medium is 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 such a medium by examining near-equilibrium fluctuations. We outline a scheme combining the Green-Kubo formalism and our microscopic transport model to extract the time-dependence of the shear viscosity of the matter created in an ultrarelativistic heavy ion collision. [Preview Abstract] |
Thursday, October 11, 2007 2:24PM - 2:36PM |
BD.00003: Produced Hadron Spectra in p + p Collisions at $\sqrt{s}$=200 GeV K. Hagel The rapidity dependence of particle production in high energy p + p collisions can provide important information of parton distribution functions and the transport of baryon number. In addition, p + p collisions provide an elementary reference for heavy ion collisions. Identified charged hadron spectra resulting from p + p collisions at RHIC have been measured over a wide range of rapidity with BRAHMS for $\sqrt{s}$=200 GeV. We will present the spectra of positive and negative $\pi$ and p. The spectra are analyzed to extract rapidity densities over the rapidity range from 0 to near 4 which we compare to rapidity distributions of the same species of produced hadrons in Au + Au collisions at the same energy. The proton rapidity densities are used to determine nuclear stopping while the pion yields constrain the total entropy production. [Preview Abstract] |
Thursday, October 11, 2007 2:36PM - 2:48PM |
BD.00004: High-pT pion and proton yields in p+p collisions at sqrt(s)=200GeV Yichun Xu, Lijuan Ruan Identified hadrons(pions, protons, and anti-protons) in mid-rapidity at high transverse momentum (pT$\sim $=10GeV) can be used to test the validity of pQCD in p+p collisions. They also provide a baseline for the study of color charge effect of parton energy loss in heavy ion collisions. However, with current event statistics from minimum-bias triggered p+p collisions the proton+anti-proton spectra are limited to pT$<\sim $7GeV/c. We report a study of charged hadron production in events triggered by high deposit energy in electromagnetic calorimeter positioned at mid-rapidity. Preliminary results on particle ratios and their pT dependence up to pT$\sim $12GeV/c will be presented. The effect of the above trigger can lead to relative enhancement/suppression of one of the sub-process (qq, qg, gg) of jet production, these aspects will also be discussed. [Preview Abstract] |
Thursday, October 11, 2007 2:48PM - 3:00PM |
BD.00005: Measurements of the dilepton continuum in p+p collisions at the PHENIX experiment at RHIC Torsten Dahms Among other penetrating probes, electromagnetic probes are ideal tools to investigate the full time evolution and dynamics of the hot and dense matter produced in high energy heavy ion collisions, as they undergo no interaction in the final state. In the dilepton continuum Dalitz decays of light hadrons and direct decays of vector mesons, which might be modified in the medium, add to hard processes, such as correlated charm production and to direct thermal photons, emitted as a blackbody radiation from the plasma. PHENIX has measured the dielectron continuum in heavy ion reactions at $\sqrt{s_{NN}} = 200$~GeV. Compared to peripheral collisions, which agree well to the cocktail of known hadronic sources, with more central collisions show indications of an enhanced yield in the low mass region. The intermediate mass region on the other hand hints at a suppression of the yield increasing with centrality which adds additional information to the charm medium modifications. A measurement of p+p reactions is crucial to provide a baseline reference necessary to interpret the signals in heavy ion collisions. The results of this analysis will be presented and compared to the results from heavy ion collisions. [Preview Abstract] |
Thursday, October 11, 2007 3:00PM - 3:12PM |
BD.00006: Dielectron Mass Spectra From $\sqrt {s_{NN} } $ = 200 GeV Heavy Ion Collisions at PHENIX Sarah Campbell The dielectron mass spectra is rich in physics signals including vector meson decays, hadron Dalitz decays, correlated semi-leptonic heavy flavor decays and direct virtual photon emission. In medium mass modifications of light vector mesons due to the QCD phase transition, the effects of chiral symmetry restoration as well as thermal radiation, may result in an additional signal in the low mass region. The PHENIX $\sqrt {s_{NN} } $ = 200 GeV Au+Au analysis shows the following centrality dependent trends: an excess in the the low mass region of the central Au+Au spectra over the expected reference cocktail of known hadronic sources, and a suppression pattern in the intermediate region, where correlated charm is expected to dominate. The status of the PHENIX $\sqrt {s_{NN} } $ = 200 GeV Cu+Cu analysis, in minimum bias and separated into centrality classes, will be presented. The Cu+Cu dataset will provide additional sensitivity in the study of centrality dependent trends. Comparisons between the Au+Au, Cu+Cu and p+p spectra with N$_{coll}$ and N$_{part }$scaling will clarify the nature of the many features seen in the Au+Au spectra. [Preview Abstract] |
Thursday, October 11, 2007 3:12PM - 3:24PM |
BD.00007: Chemical freeze-out in heavy-ion collisions and the quark-hadron phase transition Gregory Kestin, Ulrich Heinz Using the ideal hydrodynamic model we show that in heavy-ion collisions there exists a fundamental difference between ``chemical'' and ``kinetic'' freeze-out. This difference is exposed by showing that the ``chemical decoupling temperature,'' found experimentally to have no dependence on impact parameter, cannot be reproduced in simulations using a kinetic decoupling criterion. The ``kinetic freeze-out temperature,'' on the other hand, which is found to have impact parameter dependence, can be described quantitatively using such simulations. We show that kinetic decoupling necessarily leads to impact parameter dependence of the decoupling temperature. Chemical decoupling in heavy-ion collisions must therefore be controlled by a non-kinetic process, such as a phase transition. This supports the interpretation of the measured universal chemical decoupling temperature as the critical temperature for the quark-hadron phase transition. [Preview Abstract] |
Thursday, October 11, 2007 3:24PM - 3:36PM |
BD.00008: System Size Dependence of $\phi $-meson Production at RHIC Xinghua Shi The results on $\phi $-meson production in Au+Au collisions and comparison to model calculation indicate that the $\phi $ mesons are produced by coalescence of thermalized s-quarks in central collisions. The observation of $\phi $-meson elliptic flow in non-central collisions and its magnitude at intermediate p$_{T}$ being similar to other mesons has been considered as a clear signature of partonic collectivity at RHIC [1]. The Cu+Cu collisions provide a tool to probe the system size dependence of the $\phi $-meson production and collision dynamics at RHIC. In this talk we will present the STAR preliminary results on the $\phi $-meson production in Cu+Cu collisions at both 62.4 and 200 GeV from the STAR Collaboration. The transverse mass spectra, particle ratios, the strangeness enhancement and elliptic flow of $\phi $-mesons will be compared with the results from the Au+Au collisions at same beam energy in order to understand the system size dependence of $\phi $-meson production in high-energy collisions at RHIC. [Preview Abstract] |
Thursday, October 11, 2007 3:36PM - 3:48PM |
BD.00009: Production of the $\phi$ mesons at intermediate rapidity in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV Dipali Pal Study of the $\phi$ mesons produced in relativistic heavy-ion collisions can unfold properties of the hot and dense quark gluon matter produced in these reactions. Since the $\phi$ is a bound state of s and $\bar{s}$ quarks it is not subject to the canonical suppression of strangeness in p+p interactions and thus serves as a clean probe of strangeness enhancement in Au+Au collisions. The measurement of the $\phi$-meson spectra, yield, and temperature parameter gives information on strangeness enhancement and the bulk properties of the partonic matter. The BRAHMS experiment at RHIC has measured particles produced in high luminosity Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV over a broad range of rapidity and $p_{T}$. Using the efficient tracking system and high resolution time of flight wall of the mid-rapidity spectrometer (MRS), we have measured the $\phi$ mesons in the $K^{+}K^{-}$ decay channel at y $\sim$ 1. The first results of the $\phi ~\rightarrow ~K^{+}K^{-}$ measurements with focus on spectra and yield analysis will be presented. [Preview Abstract] |
Thursday, October 11, 2007 3:48PM - 4:00PM |
BD.00010: Thermodynamics of 2+1 flavor QCD at small quark masses Peter Petreczky I am going to present new results in 2+1 flavor lattice QCD simulations at finite temperature at small quark masses from RBC-Bielefeld Collaboration. The strange quark mass was fixed to its physical value, while light quark masses corresponding to pion mass of 200MeV was used. Calculations of the equation of state of strongly interacting matter, fluctuations of conserved charges and free energy of static quark anti-quark pair will be discussed. [Preview Abstract] |
Thursday, October 11, 2007 4:00PM - 4:12PM |
BD.00011: Gluon recombination in high parton density QCD: inclusive $\pi$-meson production Yang Li, Kirill Tuchin We argue that the collinear factorization of the fragmentation functions in high energy hadron and nuclei collisions breaks down at transverse momenta $k_T \alt Q_s/g$ due to high parton densities in the colliding hadrons and/or nuclei. We calculate, at next-to-leading order in projectile parton density and to all orders in target parton density, the double-inclusive cross section for production of a pair of gluons in the scalar $J=0^{++}$ channel. Using the low energy theorems of QCD we find the inclusive cross section for $\pi$-mesons production. [Preview Abstract] |
Thursday, October 11, 2007 4:12PM - 4:24PM |
BD.00012: Measurement of direct photons with a new $\pi^{0}$ tagging method in $\sqrt{s_{NN}}=200Gev$ AuAu Collision at PHENIX Haijiang Gong Direct photons provide a excellent probe to study the different stages of a heavy ion collision, especially the formation of a quark-gluon plasma(QGP), without being influenced by the strong interaction and hadronization processes. In the $p_{T}$ range of 1-3Gev/c thermal photons are expected to be the dominant source of direct photons and carry information about initial temperature of QGP. The yield of direct photons can be determined based on the inclusive photon yield and the background from hadronic decays. But due to the huge hadronic background at low $p_{T}$, the measurement is very challenging. A new method to determine direct photon excess yield above the background from decay photons is presented, which should lead to smaller systematic errors at low $p_{T}$. It uses strict photon identification in the EMCAL and a charged particle veto to extract a clean photon signal. The clean photons are then tagged with EMCAL photon candidates, which can be reconstructed with high efficiency, to determine the fraction of photons originating from $\pi^{0}$ decays. Many systematic uncertainties and detector effects cancel in this method. The result of this analysis and comparison with several other thermal photons measurements at PHENIX will be presented. [Preview Abstract] |
Thursday, October 11, 2007 4:24PM - 4:36PM |
BD.00013: Separation of different collision orientations in relativistic heavy ion reactions between deformed nuclei Chandra Nepali, George Fai, Declan Keane In relativistic uranium-uranium collisions, there is the potential to produce more extreme conditions of excited matter than is possible using spherical nuclei like gold or lead at the same incident energy. However, this potential is partly lost if it is not possible to distinguish experimentally between different collision orientations when the ions interact near zero impact parameter. Of special interest are the ``tip-tip'' orientation in which the long axes of both deformed nuclei are aligned with the beam axis, and the body-body orientation in which the long axes are both perpendicular to the beam axis and parallel to each other. In this talk, we report results of model simulations in which a promising level of discrimination can be achieved using a variety of experimental observables. [Preview Abstract] |
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