Bulletin of the American Physical Society
2005 2nd Joint Meeting of the Nuclear Physics Divisions of the APS and The Physical Society of Japan
Sunday–Thursday, September 18–22, 2005; Maui, Hawaii
Session 1WC: Workshop 3A: Spin Structure Studies at RHIC |
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Sponsoring Units: DNP JPS Chair: Naohito Saito, RIKEN Room: Ritz-Carlton Hotel Salon 2 |
Sunday, September 18, 2005 9:00AM - 9:30AM |
1WC.00001: Theoretical Overview of Longitudinal Spin Physics Invited Speaker: Opening the afternoon session of mainly longitudinal spin physics results, my overview will cover the theoretical underpinnings dating from the {\it Spin Crisis} to recent spin-related concepts, models, and expectations. The global analysis of experimental results -- such as to be presented after my talk -- provides a largely model-independent and coherent framework to extract the relevant theoretical information and the uncertainties thereof. I will outline the knowns and unknowns (incl.\ debatables) of the underlying building blocks: Perturbative expansion in terms of twist and coupling (higher orders) as well as statistical and computational tools. [Preview Abstract] |
Sunday, September 18, 2005 9:30AM - 10:00AM |
1WC.00002: Longitudinal Spin Asymmetry Measurements at PHENIX Invited Speaker: Measurements of the gluon helicity distibution ($\Delta g$), or the gluon spin contribution to the proton spin, is under way in the PHENIX experiment. $\Delta g$ is obtained from the double helicity asymmetry ($A_ {LL}$) for inclusive particle production with longitudinally polarized proton collisions at RHIC. First results for $A_{LL}$ at PHENIX have been obtained for inclusive neutral pion production at midrapidity and $\sqrt{s}$ = 200 GeV. The neutral pion is sensitive to the $\Delta g$ through a mixture of gluon-quark and gluon-gluon subprocesses. The rarer reaction, inclusive direct photon production, is a clean channel dominated by the gluon Compton process. By measuring $A_{LL}$ of direct photons, $\Delta g$ can be factored out in leading order. We have obtained the cross section of the direct photon, and will begin measurement of $A_{LL}$ this year. It is important to also obtain the cross section for these processes, as a basis to describe the measured asymmetries using next-to-leading order perturbative-QCD (NLO pQCD) calculations. The NLO pQCD calculations have shown good agreement with the cross section measurements for both neutral pion and direct photon production. In addition, development for future measurements of heavy-flavor production and weak-boson production is in progress. Heavy-flavor production is dominated by the gluon fusion process and the asymmetry measurement probes $\Delta g$. Parity-violating weak boson production at $\sqrt{s}$ = 500 GeV will be used to measure flavor-identified quark and anti-quark helicity distributions. By combining measurements of various channels at $\sqrt{s}$ = 200 GeV and 500 GeV with a wide $x$ coverage, we will be able to contribute to the understanding of the spin structure of the proton. [Preview Abstract] |
Sunday, September 18, 2005 10:00AM - 10:30AM |
1WC.00003: Longitudinal spin results from STAR Invited Speaker: One of the main goals of the spin physics program at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory is the precise determination of the spin-dependent gluon distribution function $\Delta g$ over a wide kinematic range in the momentum fraction $x_g$ by measurements of double spin asymmetries in collisions of longitudinally polarized protons at $\sqrt{s} = 200\,\mathrm{GeV}$ and $\sqrt{s} = 500\,\mathrm{GeV}$. We report on preliminary results for the double longitudinal spin asymmetry $A_{LL}$ in inclusive jet production at $\sqrt{s} = 200\,\mathrm{GeV}$ from an integrated luminosity of about 0.5 inverse pb and beam polarizations up to 45\%. The jet transverse energies are in the range of $5 < E_T < 20\,\mathrm{GeV}$. An overview of future longitudinal STAR measurements will be presented. [Preview Abstract] |
Sunday, September 18, 2005 10:30AM - 11:00AM |
1WC.00004: Study of longitudinal spin structure of nucleon in COMPASS Invited Speaker: The nucleon spin structure is studied in COMPASS at CERN by the measurement of the double spin asymmetry of the deep inelastic scattering of a polarized muon at 160 GeV off a polarized deuteron target. Thanks to the COMPASS spectrometer equipped with a RICH, semi-inclusive events are detected as well as the inclusive ones. This allows to access, in particular, the gluon polarization ($\Delta G/G)$ and flavor decomposition of the quark helicity distributions. The gluon polarization is studied via the photon-gluon fusion process(PGF), whereby a $q\bar {q}$ pair is produced from a gluon coupled with a virtual photon. Two different selections of the PGF are attempted: `open charm production' and `high $P_T $ hadron production' . In the former selection, the charmed meson decay, $D^0\to K^-+\pi ^+$, is selected. In the latter selection, events including two hadrons with high $P_T $ are selected. In the both selections, hard scale is satisfied with, respectively, the charm mass and the $P_T $ cut. Hence, one can rely on pQCD. From the double spin asymmetry of the PGF process, the gluon polarization is evaluated with a help of the polarized cross-section of the elementary process calculated by the pQCD. The `open charm production' has an advantage with less background although it gives poor statistics. On the other hand, the ``high $P_T $ hadron production' gives better statistics and larger background. The background comes from the leading process and the QCD Compton process. They are carefully evaluated with Monte Carlo simulation in order to extract the gluon polarization. The experiment has accumulated data both with a longitudinally and transversely polarized targets in its 3 years running from 2002 to 2004. The analysis of the data is in progress. The results of the analysis of the data obtained with a longitudinally polarized target are presented. [Preview Abstract] |
Sunday, September 18, 2005 11:00AM - 11:30AM |
1WC.00005: Helicity Distributions of Partons in the Nucleon from HERMES Experiment Invited Speaker: Deep inelastic electron scattering is one of the representative methods to reveal the inner structure of the nucleon. Electron itself is a point like particle. It is therefore suited to be used for the investigation of nucleon structure. When the scattered electron is detected and momentum analysed, 4 momenta of the exchanged virtual photon are determined. From these kinamatical variables, one can calculate Bjorken scaling variable X event by event. In addition to the scattered electron, produced hadrons are detected and identified. 'Nucleon spin problem' started by EMC in 1980's is being studied with novel experimental technique. HERMES is a deep inelastic scattering expeirment at DESY-HERA. Longitudinally polarized positoron/electron beam of 27.6 GeV is available at HERA. Physics results with longitudinally polarized hydrogen and deuteron targets are presented in this talk. These targets are internal gas targets. Produced hardons are detected and identified with Ring Imaging Cherenkov Counter (RICH). From the double spin asymmetry, the spin dependent (helicity) structure function was obtained. Quark flavor decomposition of helicity distributions was carried out making use of information on hadron identification. A positive distribution of u quark and negative distribution of d quark were extracted with a high precision. The polarizations of the sea quarks are small. [Preview Abstract] |
Sunday, September 18, 2005 11:30AM - 12:00PM |
1WC.00006: Spin Structure of the Nucleon in the Valence Quark Region Invited Speaker: With a minimal contamination from the sea quarks and gluons the valence quark region provides a clean region to study the spin structure of the nucleon. Precision deep inelastic scattering measurements of $g_{1}$ and $g_{2}$ spin-dependent structure functions and their higher moments (integrals over $x)$ offer an opportunity for testing our grasp of this structure. The valence quark distributions offer a good testing ground for constituent quark models of the nucleon. These distributions are a crucial input for calculating cross sections for hard processes in high-energy hadron-hadron colliders such as the LHC or the Fermilab Tevatron, in searches for the Higgs boson or for physics beyond the Standard Model. Last but not least, the higher moments of the quark distributions dominated by their valence contribution provide some of the cleanest tests of QCD. However, these tests require precision measurement in the large $x$ region were cross sections fall rapidly. I shall discuss some of the recent precision results of the nucleon spin-dependent structure functions obtained in this region and their impact on our overall understanding of the nucleon spin structure. I'll finish with remarks on prospects of extending these studies in the future using the 12 GeV upgrade of CEBAF at Jefferson Lab. [Preview Abstract] |
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