Bulletin of the American Physical Society
2006 APS April Meeting
Saturday–Tuesday, April 22–25, 2006; Dallas, TX
Session L9: Ultrarelativistic Heavy Ions |
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Sponsoring Units: DNP Chair: Brant Johnson, Brookhaven National Laboratory Room: Hyatt Regency Dallas Cumberland B |
Sunday, April 23, 2006 3:15PM - 3:27PM |
L9.00001: Event-by-event K/pi Fluctuations at RHIC Gary Westfall We present the first results from measurements of event-by-event fluctuations in the K/$\pi$ ratio for Au+Au collisions at $\sqrt{s_{NN}}$ = 20, 62.4, 130 and 200 GeV at the Relativistic Heavy Ion Collider using the STAR detector. We observe that dynamical fluctuations in the K/$\pi$ ratio decrease as the collisions become more central. The excitation function for dynamical fluctuations in the K/$\pi$ ratio in central collisions will be presented. The results will be compared with similar fluctuations measured at SPS energies. [Preview Abstract] |
Sunday, April 23, 2006 3:27PM - 3:39PM |
L9.00002: Coherent Exclusive $\rho^{0}$ Production in Ultra Peripheral Relativistic Heavy Ion Collisions with STAR Yury Gorbunov We present a measurement of the coherent $\rho^{0}$ and direct $\pi^{+}\pi^{-}$ pair photo-production in ultra peripheral relativistic heavy ion collisions at $\sqrt{s_{NN}}$=200 GeV. At impact parameters larger then twice the nuclear radius, the nuclei do not physically collide, but interact via long-range electromagnetic fields. The process $AuAu \rightarrow Au^{*}Au^{*}\rho^{0}$ with accompanying mutual nuclear excitations is observed. The transverse momentum of the $\rho^{0}$ is peaked at low $p_{T}$ which is consistent with coherent coupling to both nuclei. We report $\rho^{0}$ production cross section accompanied by mutual nuclear break-up and the ratio of direct pion to $\rho^{0}$ production. [Preview Abstract] |
Sunday, April 23, 2006 3:39PM - 3:51PM |
L9.00003: A method for directly measuring bremsstrahlung photons from jets Janette Hanks Understanding the phenomenon of jet quenching is important to the study of heavy ion collisions and the medium they produce. Because of their lack of interaction with this medium, photons can provide information about the properties of the medium at the time of their production. In particular, Bremsstrahlung photons released by jets as they interact with the medium can be used for studying the evolution of the jets.$\backslash $pardof the many sources of photons, the useful Bremsstrahlung photons make up a very small part of the total photon cross section, and so far indirect measurements made by looking for differences in photon spectra have not produced statistically significant results. Photons associated with a jet can be selected for through photon-jet correlations; however, most of the photons correlated with jets come from pi0 decays and other background sources. These backgrounds can be measured by reconstructing the invariant mass of gamma pairs. Given the higher statistics of the most recent PHENIX datasets, it may be possible to use this method to directly measure Bremsstrahlung photon spectra. [Preview Abstract] |
Sunday, April 23, 2006 3:51PM - 4:03PM |
L9.00004: Near field properties in relativistic heavy ion collisions Yang Li, Rainer Fries, Joseph Kapusta We study the properties of the soft gluon field produced in relativistic heavy ion collisions. In the spirit of McLerran-Venugopalan model, we write the field potential in a power series of the proper time $\tau$ and solve the Yang-Mills equation along with color current conservation equations simultaneously. We find that the classical gluon field at small $\tau$, i.e., the near field, is mainly longitudinal. We also calculate the energy-momentum tensor of the field. This gluon field will decay and thermalize into a quark gluon plasma. Our results can be used as the initial conditions for the consequent relativistic hydrodynamic description of the dense parton matter. [Preview Abstract] |
Sunday, April 23, 2006 4:03PM - 4:15PM |
L9.00005: Two-particle number and transverse momentum correlations in Au-Au collisions at RHIC Michael Daugherity The related studies of two-particle correlations and event-by-event fluctuations have played important roles in the search for new physics through the experimental study of relativistic heavy ion collisions. We present a general method of determining two-particle correlations and show the relationship between these correlations and event-by-event fluctuations [1]. Data from the STAR experiment at RHIC for Au-Au collisions at $\sqrt{s_{NN}}$ = 62 and 200 GeV will be presented that show the energy and centrality dependences of several correlation measures. Since this analysis method is minimally biased and requires no high-$p_t$ triggers, these results provide unique access for studying the hot, dense medium produced at RHIC, as well as shed new light on the sources of non-statistical fluctuations. [1] J. Adams et al. (STAR Collaboration), nucl-ex/0509030. [Preview Abstract] |
Sunday, April 23, 2006 4:15PM - 4:27PM |
L9.00006: Interpreting two-particle correlation and autocorrelation observables for proton-proton and Au-Au collision data from STAR Lanny Ray Central collisions between Au ions at RHIC produce several thousand charged particles, about a thousand of which are detected with the large acceptance Time Projection Chamber of STAR. Two-particle correlations and autocorrelations are presented for the 130 GeV (per nucleon pair) data for charged particles for large ranges of relative transverse momentum (2 GeV/c), pseudorapidity (2 units), and azimuthal angle (180 deg) [1]. We obtain similar correlation observables for high energy proton-proton data (CM energy 200 GeV) taken with STAR. We argue that combined analyses of inclusive single-particle distributions and two-particle correlation data will provide deeper insight into the nature of the hot, dense medium produced at RHIC than can be achieved with analyses of singles data alone. We present the evolution of the correlation distributions for Au-Au as the collision centrality increases from very peripheral to the most central. The results of the analyses suggest the occurrence of semi-hard parton scattering, strong medium induced modification of the fragmentation pattern for the scattered partons, dissipation of the parton transverse momentum to the bulk medium, development of opacity, and an alteration of the way in which the longitudinally expanding medium hadronizes. [1] J. Adams et al., nucl-ex/0408012; nucl-ex/0411003; nucl-ex/0406035. Supported in part by The U.S. Dept. of Energy. [Preview Abstract] |
Sunday, April 23, 2006 4:27PM - 4:39PM |
L9.00007: Advantage of U+U over Au+Au collisions at the Relativistic Heavy Ion Collider Chandra Nepali, George Fai, Declan Keane In a graph of $v_{2}/\epsilon$ vs. $(1/S)(dN_{ch}/dy)$, STAR data [1] reach the reported perfect fluid value of $v_{2}/\epsilon$ for the most central Au+Au collisions at 200 GeV/nucleon. It is interesting to see whether a limit is reached asymptotically by the data when $(1/S)(dN_{ch}/dy)$ is increased further. Collisions of deformed uranium nuclei have been suggested to increase this quantity at fixed beam energy [2-4]. We have studied U+U collisions in a Monte-Carlo Glauber model and attempted to simulate realistic experimental conditions. We have examined $(1/S)(dN_{ch}/dy)$ under different centrality cuts and compared with Au+Au. The most desired U+U configuration (tip-to-tip) increases the $(1/S)(dN_{ch}/dy)$ by about 35\% compared to Au+Au at $b = 0$ fm in the ideal case. In the context of real experiments and with available centrality cuts at the Relativistic Heavy Ion Collider, the predicted increase in $(1/S)(dN_{ch}/dy)$ is about 16\% which we deem still a worthwhile gain. \\ $[ 1 ]$ J.~Adams {\it et al.} [STAR], Nucl.\ Phys.\ A {\bf 757}, 102 (2005) \\ $[ 2 ]$ E.~V.~Shuryak, Phys.\ Rev.\ C {\bf 61}, 034905 (2000) \\ $[ 3 ]$ B.~A.~Li, Phys.\ Rev.\ C {\bf 61}, 021903 (2000) \\ $[ 4 ]$ U.~W.~Heinz and A.~Kuhlman, Phys.\ Rev.\ Lett.\ {\bf 94}, 132301 (2005). [Preview Abstract] |
Sunday, April 23, 2006 4:39PM - 4:51PM |
L9.00008: Studying Cosmic-Ray-Type Events Made in the Laboratory with CMS and CASTOR Edwin Norbeck, Y. Onel, A. Panagiotou, M. Murray The LHC at CERN will produce collisions between heavy ions at energies heretofore seen only in cosmic rays, 1144 TeV in the center of mass for Pb+Pb. Even with a colliding beam, the core of the shower appears at very small, forward angles. The small-angle, 0.1$^{o}$ to 0.8$^{o}$, reaction products will be studied with a novel quartz-tungsten calorimeter, CASTOR, and a thin tracking detector, TOTEM-2. Products going into larger angles will be measured by the rest of the huge CMS experiment. These studies will provide, for the first time, a much needed calibration for cosmic-ray air-shower detectors. Cosmic-ray literature describes a wide variety of exotic events from collisions between ions up to Fe+air. Even more exotic events could be expected from Pb+Pb. CASTOR is designed to look for a variety of exotic phenomena that have been predicted. ``Strangelets'' are metastable, almost neutral, nuclei with nearly equal numbers of s, u, and d quarks. Matter, comprised of u, d, and s quarks is believed to form the core of neutron stars. ``Centauros'' or ``disoriented chiral condensates,'' whose nature has not been understood so far, would appear in CASTOR as an unusual distribution of the charges of pions. Also, at these angles and energy the gluons are expected to overlap to form a classical field known as a ``color glass condensate.'' [Preview Abstract] |
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