Bulletin of the American Physical Society
2008 Annual Meeting of the Division of Nuclear Physics
Volume 53, Number 12
Thursday–Sunday, October 23–26, 2008; Oakland, California
Session MD: Hard Scattering in Relativistic Nuclear Collisions |
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Chair: Richard Seto, University of California at Riverside Room: Jewett Ballroom G-H |
Sunday, October 26, 2008 10:30AM - 10:42AM |
MD.00001: Systematic Comparison of Jet Energy-Loss Schemes in a 3D hydrodynamic medium Steffen Bass, Charles Gale, Abhijit Majumder, Chiho Nonaka, Guang-You Qin, Thorsten Renk, Joerg Ruppert We perform a systematic comparison of jet energy-loss calculations in the BDMPS/ASW, HT and AMY approaches. Since we use identical medium evolution in all three approaches we are in a unique position to isolate differences among the three calculations solely due to their energy-loss implementation. We find that the parameters of all three calculations can be adjusted to provide a good description of inclusive data on $R_{AA}$ versus transverse momentum. However, we do observe slight differences in their predictions for the centrality- and azimuthal angular dependence of $R_{AA}$ vs. $p_T$. We also note that the value of the transport coefficient $\hat{q}$ needed in the three approaches to describe the data differs significantly. We shall attempt to shed some light onto this {\it $\hat{q}$-puzzle}. [Preview Abstract] |
Sunday, October 26, 2008 10:42AM - 10:54AM |
MD.00002: Jet fragmentation functions for identified particles in p+p collisions at 200 GeV in the STAR experiment Elena Bruna According to theoretical predictions, jet quenching in heavy-ion collisions modifies the jet energy and multiplicity distributions, as well as the jet hadrochemical composition. The measurement of jet fragmentation functions in p+p collisions at 200 GeV provides a baseline to study jet modifications in Au+Au collisions at RHIC. A cone algorithm is used to reconstruct jets in the STAR Time Projection Chamber and Electromagnetic Calorimeter; a study of the jet energy resolution based on PYTHIA+GEANT simulations is reported. We present the results on distributions of jet fragments in p+p collisions at 200 GeV in STAR for charged hadrons and identified particles at different jet energies and cone radii. The results are compared to MLLA (modified leading logarithmic approximation) calculations which provide a good description of the data at higher jet energies. [Preview Abstract] |
Sunday, October 26, 2008 10:54AM - 11:06AM |
MD.00003: Full jet reconstruction in $p + p$ collisions at $\sqrt{s} = 200\,\mathrm{GeV}$ in PHENIX Yue Shi Lai Full jet reconstruction in acceptance-limited detectors is challenging and has been rarely attempted. We developed a Gaussian filter based jet reconstruction algorithm that not only reduces the sensitivity to the detector acceptance limit, but also reduces background effects from soft QCD in hadronic collisions and the heavy ion background. The algorithm in Monte- Carlo simulations has been found to be comparable and in some variables better than the conventional cone and $k_\perp$ algorithms~[1]. We present the first results in applying the Gaussian filter based jet reconstruction in $p + p$ collisions at $\sqrt{s} = 200\,\mathrm{GeV}$ using the PHENIX detector, thus demonstrating the applicability of jet reconstruction in an acceptance-limited detector. [1] Y.-S. Lai, B. A. Cole, ``Jet reconstruction in hadronic collisions by Gaussian filtering'', arXiv:0806.1499 (2008). [Preview Abstract] |
Sunday, October 26, 2008 11:06AM - 11:18AM |
MD.00004: Rapidity Densities of Produced Hadrons in p + p collision K. Hagel The characteristics of hadrons produced in p + p collisions at high energies provide important information on elementary processes. Such data can be, and have been, used to establish an elementary reference for heavy ion collisions at RHIC. The data can also be used to constrain calculations that model elementary processes. For p + p collisions at $\sqrt{s}$ = 62.4 and 200GeV, we present spectra and derived rapidity distributions of identified positive and negative pions, kaons and protons over the rapidity range from 0 to 3.5. The results of these measurements are compared to the results of various model calculations commonly used as a base for Heavy Ion comparisons. [Preview Abstract] |
Sunday, October 26, 2008 11:18AM - 11:30AM |
MD.00005: Hump-backed distribution without jet reconstruction in direct-$\gamma$-hadron correlations Michael Tannenbaum Borghini and Wiedemann proposed using the hump-backed or $\xi=\ln(1/z)$ distribution of jet fragments, which is a signature of QCD coherence for small values of particle momentum fraction, $z=p/E_{\rm jet}$, to explore the medium-modification of jets in heavy ion collisions. The use of the $\xi$ variable would emphasize the increase in the emission of fragments at small $z$ due to the medium induced depletion of the number of fragments at large $z$. It was presumed that full jet reconstruction would be required. However, one of the original measurements of the $\xi$ distribution in $e^+ e^-$ collisions on the $Z^0$ resonance at LEP was made using the inclusive distribution of $\pi^0$, which could be plotted in either the $z$ or the $\xi$ variable since the energy of the jets for di-jet events was known. A similar state of affairs exists for direct-$\gamma$-hadron correlations in p-p and A+A collisions since, modulo any $k_T$ effect, the jet recoiling from a direct-$\gamma$ has equal and opposite transverse momentum to the precisely measured $\gamma$. Thus, the $x_E$ or $z_T$ distribution of the away-side hadrons from a direct-$\gamma$ represents the away-jet fragmentation function, as suggested by Wang, Huang and Sarcevic, so that $dN/d\xi=z\,dN/dz$ can be derived. Examples from RHIC measurements will be given and compared to previous results. [Preview Abstract] |
Sunday, October 26, 2008 11:30AM - 11:42AM |
MD.00006: Direct photon-hadron correlations measured using the PHENIX detector Megan Connors Two-particle correlations have been used to study the medium created at RHIC. Since photons do not interact with the strongly coupled medium, they should escape with the same momentum that they had when originally produced by the hard scattering. Therefore, by triggering on a high momentum photon and making angular correlations with hadrons in the event, to good approximation, we know the momentum of the jet which produced these associated hadrons. This is important for understanding jet energy loss in the medium. To extract direct photon-hadron correlations from the large meson decay photon background, we use a statistical subtraction method and more recently an isolation-cut method in p+p which shows a large increase in precision. This talk will present the latest direct photon-hadron correlation results from the p+p and Au+Au collisions $\sqrt{S_{NN}}$ = 200 GeV measured at PHENIX. We use the data to study modification of the fragmentation function in Au+Au, as compared to p+p collisions. [Preview Abstract] |
Sunday, October 26, 2008 11:42AM - 11:54AM |
MD.00007: Study of High pT Muons with IceCube Lisa Gerhardt Study of High pT Muons with IceCube Muons with a large transverse momentum (pT) are produced in cosmic ray air showers via semileptonic decays of heavy quarks and the decay of high pT kaons and pions. These high pT muons will have a large lateral separation from the shower core. IceCube, a neutrino telescope consisting of a three-dimensional array of photodetectors buried in the ice of the South Pole and a surface air shower array, is well suited for the detection of high pT muons. The surface shower array can determine the energy, location and direction of the cosmic ray air shower while the in-ice array can do the same for the high pT muon. This makes it possible to measure the average separation as well as the shape of the pT spectrum of these muons which can be used to probe the composition of high energy cosmic rays. [Preview Abstract] |
Sunday, October 26, 2008 11:54AM - 12:06PM |
MD.00008: Longitudinal scaling of net-protons in AuAu and pp collisions at RHIC energies Flemming Videbaek BRAHMS has studied net-protons distributions in Au+Au and p+p collisions at $\sqrt{s_{NN}}$=62.4 and 200 GeV. Net-proton distributions reflect the net-baryon yields and can be used to extract the nuclear stopping in the collisions, thus providing information on baryon number transport and energy available for particle production. The talk will present final and preliminary results from the above mentioned systems. It will be shown that in p+p and in Au+Au central collisions that net-proton distributions exhibit longitudinal scaling once the target contribution to the projectile rapidity range is corrected for. The difference between p+p and Au+Au will be discussed. Aspects of future measurements at the LHC of net-baryons at mid-rapidity will be brought forth. [Preview Abstract] |
Sunday, October 26, 2008 12:06PM - 12:18PM |
MD.00009: Recent Updates of A Multi-Phase Transport (AMPT) Model Zi-Wei Lin We will present recent updates to the AMPT model, a Monte Carlo transport model for high energy heavy ion collisions, since its first public release in 2004 and the corresponding detailed descriptions in Phys. Rev. C 72, 064901 (2005). The updates often result from user requests. Some of these updates expand the physics processes or descriptions in the model, while some updates improve the usability of the model such as providing the initial parton distributions or help avoid crashes on some operating systems. We will also explain how the AMPT model is being maintained and updated. [Preview Abstract] |
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