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 CB: Mini-Symposium: 3D View of the Nucleon and Its Spin II |
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Chair: Edward Kinney, University of Colorado at Boulder Room: Room 208 |
Friday, October 24, 2008 10:30AM - 11:06AM |
CB.00001: Longitudinal Spin Transfer of Hyperons in Polarized Proton-Proton Collisions Invited Speaker: We report on measurements of the longitudinal spin transfer $D_ {LL}$ in the inclusive production of $\Lambda$ and $\bar{ \Lambda}$ hyperons in 200\,GeV longitudinally polarized proton- proton collisions. At large transverse momenta $D_{LL}$ is sensitive to the helicity distributions of strange quarks and anti-quarks in the polarized proton, and to polarized fragmentation functions. The data were collected with the Solenoid Tracker At RHIC (STAR) in 2005 and correspond to a sampled integrated luminosity of $\sim\!3\mathrm{pb}^{-1}$ and $\sim\!50\%$ beam polarization. Preliminary results on $D_{LL}$, covering mid-rapidity and transverse hyperon momenta up to 4 GeV/c, will be presented and followed by an outlook on future hyperon spin transfer measurements. [Preview Abstract] |
Friday, October 24, 2008 11:06AM - 11:18AM |
CB.00002: Constraints on $\Delta$G from Recent Measurements of Inclusive Jet Production in Longitudinally Polarized pp Collisions at $\sqrt{s} = 200$ GeV W.W. Jacobs A major goal of the RHIC spin program is to determine the gluonic spin contribution to the proton. The large acceptance of the STAR detector is particularly suited for inclusive jet measurements at 200 GeV which arise in leading order from qq, and dominant qg and gg hard interactions. Robust measurements of the double longitudinal spin asymmetry A$_{LL}$ sensitive to the gluon polarization are aided by the large inclusive jet cross section and relative insensitivity to fragmentation functions. Mid-rapidity A$_{LL}$ data are presented from RHIC run 5 along with the more precise results from run 6 covering a transverse momentum range $5 <$ P$_{T} < 35$ GeV. Comparison of these data to predictions from parameterizations of the gluon polarization have been made and put significant new constraints on the gluon polarization contributions within the present kinematic range $0.03 < x < 0.3$ of acceptance. The STAR data and their systematics, current constraints from various prediction comparisons as well as results from inclusion of pp measurements in global analyses will be discussed. [Preview Abstract] |
Friday, October 24, 2008 11:18AM - 11:30AM |
CB.00003: Measurement of Double Longitudinal Spin Asymmetry in Heavy Flavor Production at $\sqrt(s)=200GeV$ at RHIC Xiaorong Wang One of the main goals of the RHIC-SPIN program is to determine the contribution of gluons to the proton spin. At RHIC energies, The heavy quark (charm and beauty) production is expected to be dominated by gluon-gluon interactions, so a measurement of the double longitudinal spin asymmetry A$_{\rm LL}$ in heavy quark production in polarized p+p collisions hence allow us to directly probe the polarized gluon distribution inside the proton. The PHENIX experiment collected 3.5 pb$^-1$ of data with beam polarization $\sim$50\% and 7.5 pb$^-1$ of data with beam polarization $\sim$60\% in the years 2005 and 2006, respectively. In this talk, we present the latest results on an A$_{\rm LL}$ measurement for J/$\psi$ and open charm production measured by the PHENIX detector. [Preview Abstract] |
Friday, October 24, 2008 11:30AM - 11:42AM |
CB.00004: Longitudinal Double-Spin Asymmetry and Cross Section for Inclusive $\pi^0$ Production in Polarized $p + p$ Collisions at RHIC Oleksandr Grebenyuk Important insight into the spin structure of the nucleon is provided by studying the gluon spin contribution to the spin of proton. Recent STAR measurements of inclusive jet production provide sensitive constraints on the integral of gluon polarization $\Delta g(x,Q^2)$ for fractional gluon momenta $0.03 < x < 0.3$ and hard scales. We present measurements of the longitudinal double-spin asymmetry and differential cross section for neutral pions produced in $p + p$ collisions at $\sqrt{s} = 200$ GeV. The measured cross section is in good agreement with NLO pQCD calculations for all measured $p_T$, $2 < p_T < 16\,\mathrm{GeV/c}$, corresponding to seven orders in magnitude, and can provide constraints on fragmentation functions. The longitudinal double-spin asymmetry data disfavor large and positive gluon polarization in the polarized nucleon. They provide sensitive constraints that are complementary to those obtained with inclusive jet probes. [Preview Abstract] |
Friday, October 24, 2008 11:42AM - 11:54AM |
CB.00005: On measuring the double longitudinal spin asymmetry $A_{LL}$ for non-photonic electrons in polarized $\vec{p}+\vec{p}$ collisions at STAR Priscilla Kurnadi The measurement of the gluon's spin structure function $\Delta g(x)$ in the proton is one of the main focuses of the STAR spin program. In polarized proton-proton collisions at RHIC, one of the processes sensitive to $\Delta g(x)$ is heavy flavor quark production through gluon fusion $gg \rightarrow q\bar{q}$. A way to access these heavy quarks is through the non-photonic electrons resulting from their semi-leptonic decays. This contribution will describe an analysis to obtain the first measurement of $A_{LL}$ for non-photonic electrons produced in $\sqrt{s} = 200 GeV$ $\vec{p}\vec{p}$ collisions at RHIC in 2005. [Preview Abstract] |
Friday, October 24, 2008 11:54AM - 12:06PM |
CB.00006: Future Measurements to Constrain $\Delta$g(x) at STAR James Sowinski Understanding how the spin of the proton is assembled from the spin and orbital motion of its partons has been a long term quest of the community. It is well known that the spin of the quarks contribute only about 1/3rd of the proton's spin. RHIC provides the opportunity to look directly at the distribution of gluon spins in the proton via hard partonic processes in high energy polarized pp collisions. Recent STAR measurements,\footnote{B. I. Abelev {\it et al.} (STAR), Phys. Rev. Lett. {\bf 100} (2008) 232003.} along with others, have demonstrated that the gluons also do not make large contributions above a partonic momentum fraction x of $\sim$0.02. However a better understanding of the x dependence of the gluon contribution, $\Delta$g(x), and its extension to lower x values, all at higher precision, are required to determine the integral contribution $\Delta$G. Additional measurements are planned to utilize the large acceptance of STAR along with detection of di-jet and photon-jet correlations to gain sensitivity to the x dependence. Recent additions in calorimetry allow extension of the measurements down to near 10$^{-3}$ in x. Improvements of beam luminosity and polarization as well as runs at $\sqrt{s}$=200 and 500 GeV/c$^2$ are expected. The planned measurements will be described and projections of their implications for our understanding of the gluon's contribution to the proton spin will be presented. [Preview Abstract] |
Friday, October 24, 2008 12:06PM - 12:18PM |
CB.00007: Photon and $\gamma$-Jet Reconstruction in the STAR Endcap EMC; Towards $\gamma$-Jet Constraints on $\Delta$G W.W. Jacobs The goal of determining the gluonic spin contribution to the proton is central to RHIC spin efforts. Recent results from inclusive jet measurements in polarized pp collisions at STAR and other experiments have indicated that gluons with momentum fraction x above $\sim 0.02$ do not contribute significantly to the integral $\Delta$G. $\gamma$-jet measurements provide a simpler although much more rare probe of the gluon spin contribution with sensitivity to the x dependence (event reconstruction of partonic kinematics) through measurements of the double longitudinal spin asymmetry A$_{LL}$. Several factors enhance the figure of merit of the primary gluon Compton process (g+q $\rightarrow$ q+$\gamma$) for these measurements in the Endcap and the detector itself was built to address various experimental difficulties, including suppressing the large $\pi^{\circ}$ backgrounds that grow at lower momentum transfer. The status of ongoing photon, direct $\gamma$ and $\gamma$-jet reconstruction and analyses efforts from the run 6 pp data sample and future prospects will be presented and discussed. [Preview Abstract] |
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