Session B14: Nucleon Spin Structure at High Energy

Latest Results on the Flavor-Separated Parton Distribution Functions of the Nucleon at HERMES

Through semi-inclusive deep-inelastic scattering (DIS), the HERMES collaboration has accessed many features of the structure of the nucleon. By taking into account the identity of final-state hadrons and their correlation with particular struck-quark flavors in the DIS process, the individual quark contributions to the nucleon structure functions have been unraveled. New results on the quark helicity distributions, $\Delta q(x)$, extracted from double-spin inclusive and hadron asymmetries, will be presented. Previously overlooked data have been included and improved analysis methods, which better leverage the full statistical power of the data, are utilized. Additionally, an improved method for estimating the fragmentation model contribution to the systematic uncertainties of the result has been included. This new analysis significantly reduces previously published systematic and statistical uncertainties and explores previously uninvestigated features of the quark polarizations. In addition, other new HERMES results related to the extraction of parton distribution functions will be presented.   

Measurement of the Double Longitudinal Spin Asymmetry for Inclusive Hadron Production in 200 GeV Polarized p+p Collisions at RHIC

A primary goal of the STAR-spin program is the measurement of the gluon polarization, delta g, in the proton. The STAR detector, with its large-acceptance tracking and calorimetery, provides a uniquely suited environment for asymmetry measurements in a number of different final-state channels in polarized p+p collisions. These asymmetries will provide significant contributions to a global analysis of delta g. We present here the most recent measurements of the double longitudinal spin asymmetries ($A_{LL}$) for the inclusive production of both neutral and charged pions at mid-rapidity. These asymmetries are compared to NLO pQCD calculations for different polarization scenarios and are used to provide constraints on delta g. Charged pions are of particular interest as they are sensitive to the sign of delta g. Results and continuing analyses are presented from RHIC runs 5 and 6.   

Inclusive Jet and Dijet Cross Section and Longitudinal Double Spin

The polarized gluon distribution in the proton, delta G, is of particular interest to the STAR Spin program. Double spin asymmetry, A{\_}LL, of the jets production rate is sensitive to delta G. With it's large acceptance electromagnetic calorimetry and tracking system, STAR is well suited for these measurements. We report the status of the measurements of inclusive jet and dijet cross section and A{\_}LL for the polarized proton-proton collision data taken during RHIC 2005 and 2006 run.   

Inclusive $\pi^0$ Production in Longitudinally Polarized pp Collisions at $\sqrt{s}$=200 GeV Using the STAR Endcap Electromagnetic Calorimeter

Measurement of the double-spin asymmetry $A_{LL}$ for inclusive $\pi^0$ production in polarized pp collisions can provide important constraints on gluonic contributions to the proton's spin.$^1$ The STAR Endcap Electromagnetic Calorimeter (EEMC) is well suited for these studies, providing full azimuthal coverage for $1.086 \leq \eta \leq 2$, and with separate readout of two pre- and one post-shower layers, and a fine grain scintillator-based shower maximum detector (SMD) that can distinguish between single photons or electrons, charged hadrons, and neutral mesons ($\pi^0$'s and $\eta$'s) via the observed transverse shower profile. The EEMC also provides fast triggering on significant energy deposition in individual towers, trigger patches, or jet patches ($\Delta\eta \times \Delta\phi \approx 0.007, 0.06, 1$, respectively). Details of the $\pi^0$ reconstruction algorithm employed, and the current status of analysis of the 2006 longitudinally polarized pp data set (sampled luminosity $\approx$~3.5~pb$^{-1} $) will be presented. \break $^1$ B.~Jager, M.~Stratmann and W.~Vogelsang, Phys.\ Rev.\ D {\bf 70}, 034010 (2004) [arXiv:hep-ph/0404057].   

The STAR Forward GEM Tracker

The STAR collaboration is preparing a tracking detector upgrade, the Heavy Flavor Tracker (HFT) and the Forward GEM Tracker (FGT) to further investigate fundamental properties of the new state of strongly interacting matter produced in relativistic-heavy ion collisions at RHIC and to provide fundamental studies of the proton spin structure and dynamics in high-energy polarized proton-proton collisions at RHIC. The FGT will focus on novel spin physics measurements in high-energy polarized proton-proton collisions, determining the flavor dependence ($\Delta \bar{u}$ versus $\Delta \bar{d}$) of the polarized sea. These polarized parton distribution functions (PDFs) are only weakly constrained by polarized fixed target experiments, giving limited insight into the underlying mechanisms which produce the polarized sea. STAR plans to probe these PDFs using parity violating W production and decay. The production of $W^{-(+)}$ bosons provides an ideal tool to study the spin-flavor structure of the proton. $W^{-(+)}$ bosons are produced in $\bar{u}+d\,(\bar{d}+u)$ collisions and can be detected through their leptonic decays, $e^{-}+\bar{\nu}_{e}\,(e^{+}+\nu_{e})$, where only the respective charged lepton is measured. The sensitivity of those measurements is enhanced in the forward direction. The discrimination of $\bar{u}+d (\bar{d}+u)$ quark combinations requires distinguishing between high $p_{T}$ $e^{-(+)}$ through their opposite charge sign, which in turn requires precise tracking information. An upgrade of the STAR forward tracking system is needed to provide the required tracking precision for charge sign discrimination. This upgrade will consist of six triple-GEM detectors with two dimensional readout arranged in disks along the beam axis. The FGT design, the performance of triple-GEM prototype detectors based on industrially produced GEM foils along with the status of the FGT construction and the anticipated installation schedule will be presented.