Bulletin of the American Physical Society
APS April Meeting 2011
Volume 56, Number 4
Saturday–Tuesday, April 30–May 3 2011; Anaheim, California
Session G7: Relativistic Heavy Ion Collisions I |
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Sponsoring Units: DNP Chair: Miklos Gyulassy, Columbia University Room: Grand E |
Sunday, May 1, 2011 8:30AM - 8:42AM |
G7.00001: Direct Photons in Cu+Cu collisions at $\sqrt{s_{NN}} = 62.4GeV$ Tim Hester Direct-photons measured with the PHENIX detector at RHIC, in Au+Au collisions, have been found to scale with the number of binary (nucleon-nucleon) collisions at large transverse momenta. In contrast, earlier measurements from RHIC, using charged and neutral hadron spectra, showed a clear suppression with respect to pp collisions. That suppression was interpreted to be due to the energy loss of scattered partons in the produced dense medium in heavy-ion collisions. Thus, it is concluded that such an energy loss is not evident for direct-photons. In order to understand the turn-on of these effects, similar measurements have been subsequently made for Cu+Cu collisions at various collision energies. This work will present a new measurement of direct photons in Cu+Cu collisions at $\sqrt{s_{NN}} = 62.4GeV$. The smaller system size may provide new insight into the system size dependence of photon production. These data will be compared to pp collision data, as well as neutral pion spectra from the same collision data, to check for modification of particle production due to effects of the system created in the heavy ion collisions. [Preview Abstract] |
Sunday, May 1, 2011 8:42AM - 8:54AM |
G7.00002: Thermal Photon measurement using the Hadron Blind Detector: Status and Outlook Sky Rolnick Thermal photons are an important probe used in understanding the hot dense matter formed in heavy ion collisions. Previous measurements in the PHENIX experiment at RHIC have shed light on the rapid thermalization times observed in heavy ion collisions as well as put limits on the measured temperature range of the collision. Getting at the thermal photon contributions is challenging, particularly in the low and intermediate pT region, where direct photons can be experimentally accessed only by measuring dielectron pairs from internal/external conversions. One difficulty that arises in using dielectrons to measure thermal photons is the large combinatorial backgrounds present particularly in the low mass region $< 1 GeV/c^2$. The Hadron Blind Detector has been developed specifically to reduce backgrounds from conversion and Dalitz decays and will help improve the thermal photon measurement. In this talk I will present a review of latest results on thermal photons measured in the PHENIX experiment as well as current outlook on future thermal photon measurements. [Preview Abstract] |
Sunday, May 1, 2011 8:54AM - 9:06AM |
G7.00003: The high $p_T$ non-photonic electron measurements in $Au+Au$ collisions at $\sqrt{s}= 200$~GeV at RHIC/STAR Wenqin Xu Due to their large masses, heavy flavor quarks (charm and bottom) are believed to be unique probes to the strongly-coupled QCD matter created in high energy $Au+Au$ collisions at the Relativistic Heavy Ion Collider (RHIC). When heavy flavor quarks travel through the QCD medium, they are expected to lose less energy than light flavor quarks due to the dead-cone effect, if the dominant energy loss process is gluon radiation. However, the production of non-photonic electrons (NPE) from the semi-leptonic decay of charm and bottom hadrons are suppressed at high $p_T$ in Au+Au relative to $p$+$p$ and the NPE-hadron azimuthal correlations shows a broadening on the away side. These results suggest some significant contributions from other processes to heavy flavor quark energy loss. Which then calls for a high precision measurements of NPE production in both Au+Au and $p$+$p$ collisions. We will present the status of the NPE studies at midrapidity from a high statistics and low photon conversion background data set corresponding to $Au+Au$ collisions at $\sqrt{s}= 200$~GeV collected at RHIC in Run 2010. [Preview Abstract] |
Sunday, May 1, 2011 9:06AM - 9:18AM |
G7.00004: Ultraperipheral Heavy Ion Collisions at RHIC and LHC Anthony Baltz Higher order QED calculations of ultraperipheral heavy ion $e^+ + e^-$ pair production at RHIC have shown reductions from perturbation theory consistent with both STAR and PHENIX experimental measurements. The STAR and PHENIX data were triggered by Coulomb dissociation neutrons detected in Zero Degree Calorimeters. A phenomenological model based on photonuclear data from the giant dipole resonance region all the way to the kinematic cutoff in the vector dominance region is employed for the calculation of the Coulomb dissociation trigger of the $e^+ + e^-$ events. Ultraperipheral electromagnetic interactions can also produce quark anti-quark pairs in the form of meson events also triggered by Coulomb dissociation. Higher order QED calculations for $\mu^+ + \mu^-$ pair production predict reductions from perturbation theory comparable to the $e^+ + e^-$ case. For the the recent LHC heavy ion run of colliding 1.38 TeV/N Pb beams, the single Pb ion dissociation cross section is calculated to be 202.5 barns, an 11\% reduction from the 227.3 barn cross section previously calculated at LHC design energy of 2.76 TeV/N. [Preview Abstract] |
Sunday, May 1, 2011 9:18AM - 9:30AM |
G7.00005: Measurement of Upsilon production cross section in d+Au collisions at $\sqrt{s_{NN}}$=200 GeV Anthony Kesich Suppression of charmonia production in heavy-ion collisions, due to Debye screening of the quark-antiquark potential, was proposed as a clear signature of quark-gluon plasma (QGP) formation. However, further understanding of factors that influence charmonia production in heavy-ion collisions complicates this analysis. These factors include competing effects such as recombination enhancement and suppression due to comover absorption; these effects are negligible in $\Upsilon$ production. Furthermore, cold nuclear matter (CNM) effects can effect the rate of $\Upsilon$ production in heavy-ion collisions. These effects can be quantified by measuring production rates in d+Au collisions. We report on the measurement of the $\Upsilon$ production cross section via the $e^+e^-$ decay channel in d+Au collisions at $\sqrt{s_{NN}}$=200 GeV. We use the high-statistics data available from the 2008 run. [Preview Abstract] |
Sunday, May 1, 2011 9:30AM - 9:42AM |
G7.00006: Dynamical magnetic enhancement of light and heavy quark jet quenching at RHIC and LHC Alessandro Buzzatti, Miklos Gyulassy New Monte Carlo evaluations of Djordjevic's dynamical generalization of DGLV radiative energy loss are shown to enhance both light and heavy quark quenching similarly in nonuniform expanding geometries. Perturbative magnetic scattering effects in the HTL approximation might still be insufficient to explain present RHIC and future LHC data on heavy quark jet quenching. [Preview Abstract] |
Sunday, May 1, 2011 9:42AM - 9:54AM |
G7.00007: Testing Non-conformal Holographic Models of Jet Quenching in A+A at RHIC and LHC Andrej Ficnar, Jorge Noronha, Miklos Gyulassy We propose a new bottom-up, non-conformal holographic gravity dual model of quenching of both heavy and light quarks in ultra- relativistic nuclear collisions. We generalize the Gubser et al. string drag force model to interpolate between the trailing and falling string scenarios, depending on how the effective mass of the quark varies with temperature. We compare the differential nuclear modification of light and heavy quark jets in expanding sQGP with holographic thermodynamic properties constrained by lattice QCD calculations for both RHIC and new LHC conditions. [Preview Abstract] |
Sunday, May 1, 2011 9:54AM - 10:06AM |
G7.00008: Simulation of Asymptotically AdS Spacetimes with a Generalized Harmonic Evolution Scheme Hans Bantilan Motivated by the gauge/gravity duality, we introduce a numerical scheme based on generalized harmonic coordinates to solve the Einstein field equations on an asymptotically anti-de Sitter spacetime. We work in global AdS$_5$, which can be described by the $(t,r,\chi,\theta,\phi)$ coordinates adapted to the $R \times S^3$ boundary. We focus on solutions that preserve the SO (3) symmetry that acts to rotate the 2-spheres parametrized by $(\theta,\phi)$. This allows us to study axisymmetric physics in the bulk, corresponding to spherically symmetric physics in the boundary CFT. We present results from an ongoing study of black hole formation and subsequent quasinormal ring-down via scalar field collapse. This ring-down can be interpreted holographically as motions of an otherwise static plasma on $S^3 $, or as perturbations of a radial flow of thermal matter on Minkowski space, and is intended as a precursor to simulations in the gravity dual that have application to heavy-ion collisions. [Preview Abstract] |
Sunday, May 1, 2011 10:06AM - 10:18AM |
G7.00009: Energy loss from holography Mohammed Mia Using the dual classical gravity description for a non conformal quantum field theory, we analyze the energy loss mechanism for a heavy parton as it ploughs through a strongly interacting thermal medium. The medium is described by a gauge theory with matter in fundamental representation where the gauge coupling runs logarithmically with scale at low energies and at high energies, the theory reaches a conformal fixed point. By considering a string moving with constant velocity in the bulk geometry, we compute the drag experienced by a heavy parton as it traverses the medium. The drag coefficient depends non trivially on momentum as the dual geometry deviates from AdS space and in particular it decreases with increasing momentum. We also calculate the diffusion coefficients through computing average transverse and longitudinal momentum square of the heavy parton by considering the Nambu-Goto action of the string. Finally using our results for drag and diffusion coefficients, we compute nuclear modification factor $R_{AA}$ for charged particles and find agreement with the significant rise of $R_{AA}$ at large $p_T$ as observed in Pb-Pb collisions at the LHC. [Preview Abstract] |
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