Bulletin of the American Physical Society
2012 Fall Meeting of the APS Division of Nuclear Physics
Volume 57, Number 9
Wednesday–Saturday, October 24–27, 2012; Newport Beach, California
Session DE: Heavy Ions: Jets, Dielelectrons, and the Initial State |
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Chair: Dave Morrison, Brookhaven National Laboratory Room: Garden I |
Thursday, October 25, 2012 10:30AM - 10:42AM |
DE.00001: Probing jet quenching via conditionally triggered two-particle correlations in Au+Au at $\sqrt{s_{NN}} =200$ GeV Eric Vazquez A hot dense medium exceeding the critical energy density for the formation of a partonic state of matter has been created in heavy ion collisions at RHIC and LHC. This dense matter is extremely opaque to high energy partons and therefore leads to strong modifications of di-jets traversing the medium. These modifications are typically studied by comparing the di-hadron azimuthal correlation functions in Au+Au collisions to those in p+p collisions. However, di-hadron correlations measured in events triggered on a single hadron are subject to a surface bias due to the larger probability of a high-$p_T$ parton emerging with little or no energy loss from production points near the surface of the medium. This effect can be reduced by studying events in which the production of two high-$p_T$ hadrons at large azimuthal separation is required. We present the status of such an analysis performed at PHENIX using Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV. [Preview Abstract] |
Thursday, October 25, 2012 10:42AM - 10:54AM |
DE.00002: Event by Event Methods for Identification of Direct Photon-Hadron Correlations at RHIC Justin Frantz Direct photon - hadron pair correlations serve as an excellent probe of the hot and dense medium created in the heavy ion collision at RHIC. Application of event-by-event based methods such as isolation cuts that would reject background photons are challenging in high multiplicity events for the relatively low photon energies statistically achievable at RHIC. We present some analysis techniques to deal with these challenges, and the status of possible application of these techniques within the PHENIX experiment. [Preview Abstract] |
Thursday, October 25, 2012 10:54AM - 11:06AM |
DE.00003: Two particle correlations with Particle Identification in $\sqrt{s_{NN}}$=62.4 GeV Au+Au collisions Matthew Mendoza It is well known that the novel state of matter known as the Quark Gluon Plasma created at the Relativistic Heavy Ion Collider suppresses high transverse momentum jets. A study of the baryon-to-meson production ratio in Au+Au collisions has shown a clear increase in the intermediate $p_T$ region for more central (head-on) collisions and in comparison to elementary systems. PHENIX's Time of Flight detector, used in conjunction with the Aerogel Cherenkov Counter, can provide particle identification with good proton/meson separation for $p_T$ $<$ $10$ GeV/c. I will present studies of particle identified di-jet production using these detectors in a two particle correlation analysis which could help to elucidate the underlying physics of the baryon excess anomaly. [Preview Abstract] |
Thursday, October 25, 2012 11:06AM - 11:18AM |
DE.00004: Measurement of Di-electron Continuum in p+p at $\sqrt{s}=200$ GeV collisions by PHENIX using the Hadron Blind Detector Sky Rolnick Dielectrons provide a very important probe for studying the hot dense nuclear matter created in heavy ion collisions at RHIC. Since dielectrons are color neutral and produced during all stages of the collision, they provide access to an abundance of information including thermal sources, Dalitz decays, vector meson resonances, correlated open charm and bottom decay, and Drell-Yan processes. Previous measurements from PHENIX have indicated an unexpectedly large enhancement of dielectrons in Au+Au collisions in the low mass region $\ (0.3-0.8 GeV/c^2)$, a possible signal of chiral symmetry restoration.\footnote{S.~Afanasiev {\it et al.} [PHENIX Collaboration], arXiv:0706.3034 [nucl-ex].} These measurements were limited by large systematic uncertainties, primarily from a poor S/B ratio. In 2009 the PHENIX experiment was upgraded with the addition of the Hadron Blind Detector which will improve the background rejection by allowing removal of pairs from both partially reconstructed Dalitz decays and photon conversions. In this talk, we will report on the results obtained from 2009 data in p+p using the HBD which will serve as a baseline for the Au+Au results obtained in 2010. [Preview Abstract] |
Thursday, October 25, 2012 11:18AM - 11:30AM |
DE.00005: Centrality and $p_{T}$ dependence study of Dielectron Production in $\sqrt{s_{NN}}$=200 GeV Au+Au collisions at STAR Yi Guo Dilepton production has been proposed as a penetrating probe for the hot and dense nuclear systems created in high-energy nuclear collisions. Due to their relatively weak final-state interaction, dileptons escape the interaction region undistorted. Since dileptons originate from all stages of a heavy ion reaction, their sources vary with the kinematic phase space under consideration: the low mass region (LMR: mass$<$1.1GeV/$c^{2}$),the intermediate mass region (IMR: 1.1$<$mass$<$3GeV/$c^{2}$) and the high mass region (HMR: mass$>$3GeV/$c^{2}$). According to the time-energy correlation, the dilepton distributions, especially in the IMR and HMR, provide information on early collision dynamics in heavy ion collisions. We will present a systematic study of dielectron production in $\sqrt{s_{NN}}$=200GeV Au+Au collisions taken by STAR experiment in the year 2011. The dataset involves nearly 750 million Au+Au minimum bias events which is about a factor of two compared to that collected in year 2010. The dielectron pair transverse momentum and centrality dependence of the delectron invariant mass distribution will be discussed. The results will be compared to hadron decay cocktails as well as theoretical calculations on vector meson in-medium modifications and the QGP thermal radiation. [Preview Abstract] |
Thursday, October 25, 2012 11:30AM - 11:42AM |
DE.00006: Prompt Photon Measurements with the PHENIX MPC-EX Detector John Lajoie The MPC-EX detector is a Si-W preshower extension to the existing PHENIX Muon Piston Calorimeter (MPC). The MPC-EX consists of eight layers of alternating W absorber and Si mini-pad sensors. Covering a large pseudorapidity range, $3.1 < |\eta| < 3.8$, the MPC-EX and MPC access low-x partons in the Au nucleus in d+Au collisions through prompt photon measurements. With the addition of the MPC-EX, the neutral pion reconstruction range extends to energies $>$ 80 GeV, a factor of four improvement over current capabilities. Not only will the MPC-EX strengthen PHENIX's existing forward $\pi^0$ and jet measurements, it also provides the necessary $\pi^0$ rejection to make a prompt photon measurement feasible. With this $\pi^0$ rejection, prompt photon yields at high $p_{T}$, $p_{T} > 3$ GeV, can be statistically extracted using a double ratio method. The prompt photon $R_{dAu}$ measured with the MPC-EX will quantify the level of gluon shadowing or saturation in the Au nucleus at low-x, $x\sim$ 10$^{-3}$, with a projected systematic error band a factor of four smaller than current global fits to current measurements. [Preview Abstract] |
Thursday, October 25, 2012 11:42AM - 11:54AM |
DE.00007: Measuring the gluon distribution of nuclei: diffractive e+A collisions at eRHIC Matthew Lamont Despite the vast array of exciting results coming from the RHIC and LHC heavy-ion programmes over the last decade, a quantitative understanding of many of the physics processes is still lacking. One of the prime reasons for this is the complete lack of understanding of the gluon distribution in nuclei. We know from HERA that the gluon dominates the structure of the nucleon at small parton momentum fractions $x_{Bj}$. Although it is not yet measured, we expect this to be the case for nuclei too. In fact, due to geometrical considerations, it is expected that gluons will be even more dominant in nuclei than in nucleons at the same value of $x_{Bj}$. In order to probe the gluon distribution, one needs to look at cold-nuclear matter via e+A collisions. Whilst p+A collisions can offer some expects of this, one cannot control the kinematical variables. A measurement of diffractive processes, where the nucleus stays intact after the collision, will help to provide insight into the gluon distribution and it was one of the important measurements identified in the Fall 2010 INT programme and the EIC White Paper. I will present results for simulations of diffraction at a high luminosity eRHIC facility, where an electron beam is added to the existing RHIC collider complex. [Preview Abstract] |
Thursday, October 25, 2012 11:54AM - 12:06PM |
DE.00008: Photons from Spectators in Relativistic Heavy-Ion Reactions Edwin Norbeck, Yasar Onel In a high energy Pb-Pb collision the outer parts of the Pb nuclei that do not become part of the quark gluon plasma are more than passive spectators. In reasonably central collisions the ``spectator'' matter is completely disintegrated into protons and neutrons. It experiences an electric field of 2 x 10$^{28}$ V/m at the LHC design energy of 5.5 TeV/nucleon pair. In such a huge electric field the acceleration of a d or u quark or an electron is so large that almost all of the transferred energy goes into radiation. The large electric field can produce e$^{+ }$ e$^{-}$ pairs as it interacts with spectator matter. The electric field lasts for such a short time that an electron would acquire only 18 MeV if there were no radiation. If the radiation were isotropic in the spectator frame, the Lorentz transformation would put half of the radiation into a narrow cone that is about 10 cm wide at a distance of 140 m, where the CMS experiment has detectors. GeV photons are seen at 100,000 ft, which may be from spectators formed by collisions of cosmic Fe nuclei with air at 140,000 ft. Some of these photons arrive as clusters of a dozen or so, all within an area less than 0.01 mm across. This suggests that some of spectator radiation may occur as solitons. [Preview Abstract] |
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