### Session J13: Minisymposium on Hard Probes of the Quark-Gluon Plasma

 Sunday, April 13, 2008 10:45AM - 11:21AM J13.00001: Hard Probes of the Quark-Gluon Plasma: Introduction and Overview Invited Speaker: Abhijit Majumder The Quark-Gluon Plasma formed at the Relativistic Heavy-Ion collider (RHIC) is strongly interacting. Hard probes present the most rigorous tool in the study of the partonic substructure of the produced matter, where, the presence of a hard scale allows for the use of techniques based on perturbative QCD and factorization. These include a series of single jet observables, multi-particle jet correlations, jet-medium correlations as well as heavy quark and electromagnetic probes. The different approaches and approximation schemes used in the study of jet modification in dense matter are reviewed and major results reported. A variety of new jet correlation observables are outlined and their possible explanations and implications for the structure of the produced matter discussed. Sunday, April 13, 2008 11:21AM - 11:33AM J13.00002: Constraining energy-loss model parameters using multiple high-pt observables at RHIC Craig Ogilvie High-momentum partons lose energy as they travel through the dense QGP that is formed at RHIC. In a recent publication PHENIX has performed a quantitative comparison between various parton energy-loss models and experimental data on single-particle suppression (R$_{AA})$ versus pt. These data provided a constraint on the model parameters of medium opacity. It is important to also compare these model calculations with other complementary observables, such as R$_{AA}$ as a function of the reaction plane, or the suppression of two-particle correlations. Using other observables can more fully test energy-loss models, since a broader suite of observables not only tests for consistency but may also offer better sensitivity to the model parameters and sample path-length distributions differently than R$_{AA}$. In this talk I will present the status of using PHENIX results to constrain the theoretical models of energy-loss at RHIC. Sunday, April 13, 2008 11:33AM - 11:45AM J13.00003: Pseudorapidity asymmetry at high $p_{T}$ in $p(d)A$ collisions Adeola Adeluyi , Gergely Barnafoldi , George Fai , Peter Levai We calculate pseudorapity ($\eta$) asymmetry in $pBe$ and $dAu$ collisions in the framework of a next-to-leading order (NLO) pQCD-improved parton model. Our calculations are applicable in a wide range of kinematically accessible transverse momenta, $p_{T}$. The calculations [1,2] are tuned to describe existing spectra from pp collisions and asymmetric systems at midrapidity and large rapidities at FNAL and RHIC energies. We investigate the roles of nuclear shadowing and multiple scattering on the observed pseudorapity asymmetry [3]. Using this framework, we make predictions for pseudorapidity asymmetries at high $p_{T}$ and high $\eta$ at a wide range of energies up to LHC. [1] A. Adeluyi and G. Fai, Phys.\ Rev.\ C {\bf 76}, 054904 (2007) [2] G.G. Barnafoldi, P. Levai, G. Papp and G. Fai, Nucl.\ Phys.\ A {\bf 749}, 291 (2005) [3] B.I. Abelev {\it et al.} [STAR Collaboration], Phys.\ Rev. C {\bf 76}, 054903 (2007) Sunday, April 13, 2008 11:45AM - 11:57AM J13.00004: Prototype Performance of Novel Muon Telescope Detector at STAR David Tlusty , Lijuan Ruan A large area of muon telescope detector is proposed to measure muons of momentum at a few GeV/$c$ at mid-rapidity, allowing for the detection of di-muon pairs from QGP thermal radiation, quarkonia, light vector mesons, possible correlations of quarks and gluons as resonances in QGP, and Drell-Yan production as well as the measurement of heavy flavor hadrons through their semi-leptonic decays into single muons. The R$\&$D research has been carried out for this large area Muon Telescope Detector (MTD). The multi-gap resistive plate chamber technology with large module, long strips and two-end readout (Long-MRPC) was used for this research. The results from cosmic ray and beam test will be presented to address intrinsic timing and spatial resolution for Long-MRPC. Besides, a single prototype of MTD was installed in STAR during the 200 GeV Au+Au run in spring 2007. The detector consists of a long-MRPC layer between two layers of scintillator planes. They are placed outside of the magnet yoke that serves as hadron absorber. We will present results from this prototype run. Muon identification capability, timing and spatial resolution will be reported. We also discuss the implication of these tests on the physics performance and capabilities of full scale detector. Sunday, April 13, 2008 11:57AM - 12:09PM J13.00005: A Monte Carlo Event Generator for Ultra Peripheral Collisions Joseph Butterworth , Spencer Klein , Yury Gorbunov , Joakim Nystrand , Janet Seger Ultra peripheral collisions occur when the nuclei pass one another without overlapping. The intense electric fields present can be treated as a flux of photons; these photons can interact with the other nucleus, producing a range of particles, including vector mesons (Upsilon, J/Psi, rho, {\ldots}) and pairs of oppositely charged pions. In order to effectively study ultra peripheral collisions at the STAR experiment, a Monte Carlo event generator was created to optimize particle selection criteria and to assess on whether rarer physics processes are possible to study. The FORTRAN program was initially designed in 1995 to simulate ultra-relativistic Au-Au interactions. Since then, changes have been made to the program to fit the needs of newer experiments. I will present the kinds of physics that are modeled by the program such as photonuclear and photon-photon interactions, comparisons to data, and the current and future upgrades being made to the Monte Carlo. The upgrades include the consolidation of versions into C++, additional final states, asymmetric collisions of arbitrary nuclei, and improved expandability. Sunday, April 13, 2008 12:09PM - 12:21PM J13.00006: Analysis of Neutral Rho Decays from Ultra-Peripheral Collisions in Deuteron-Gold Interactions Stephen Hansen This work will investigate the production of the neutral rho meson in ultra-peripheral collisions (UPCs) of deuteron and gold (dAu) nuclei. A UPC occurs when the impact parameter for the collision of two charged particles is greater than the sum of their radii. The resulting interaction is electromagnetic in nature. These nuclei are collided in the Solenoidal Tracker at RHIC (STAR), a particle detector at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. I will present the kinematic variables of the produced rho mesons and compare them to Monte Carlo simulations. A ratio of rho production to direct pion par production will also be presented. I will also show how Monte Carlo simulations can be used to determine the geometric acceptance and reconstruction efficiency and to help differentiate between coherent and incoherent production. Sunday, April 13, 2008 12:21PM - 12:33PM J13.00007: Measurement of Electron-Muon Correlations from Semi-Leptonic D Decay in 200 GeV p+p Collisions at RHIC-PHENIX Tatia Engelmore Charm production is a valuable probe of the early stages of a heavy ion collision. Correlated electron-muon pairs are a signature of semi-leptonic D decays, and a measurement of D mesons provides information on charm quark energy loss in the hot medium. The energy loss of heavy quarks is still not fully understood, so it is vital to investigate different decay channels of charm mesons to better understand this process. Measurements of electron-muon pairs suffer less from background than $e^ {+}$$e^{-} or \mu^{+}$$\mu^{-}$ pairs since neither direct lepton production nor resonance decays produce this type of correlated signal. Another advantage is that because electrons are measured in the central arms and muons are measured in the forward region in PHENIX, open charm can be probed in a rapidity region different from previous dilepton measurements. Studying electron-muon pairs in p+p collisions provides an important baseline for the study of these processes in d+Au and Au+Au collisions. The data in this analysis was obtained during the 2006 RHIC run of p+p collisions at 200 GeV. The current status of this analysis will be presented.