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One of the main challenges in nuclear and particle physics in the last 20 years has been to understand how the proton''s spin is built up from its quark and gluon constituents. Quark models generally predict that about 60% of the proton''s spin should be carried by the spin of the quarks inside, whereas high energy scattering experiments have shown that the quark spin contribution is small OCo only about 30%. This result has been the underlying motivation for about 1000 theoretical papers and a global program of dedicated spin experiments at BNL, CERN, DESY and Jefferson Laboratory to map the individual quark and gluon angular momentum contributions to the proton''s spin, which are now yielding exciting results. This book gives an overview of the present status of the field: what is new in the data and what can be expected in the next few years. The emphasis is on the main physical ideas and the interpretation of spin data. The interface between QCD spin physics and the famous axial U(1) problem of QCD (eta and etaprime meson physics) is also highlighted. Sample Chapter(s). Chapter 1: Introduction (159 KB). Contents: Spin Experiments and Data; Dispersion Relations and Spin Sum Rules; g 1 Spin Sum Rules; Fixed Poles; The Axial Anomaly, Gluon topology and g (0) A; Chiral Symmetry and Axial U(1) Dynamics; QCD Inspired Models of the Proton Spin Problem; The Spin-Flavour Structure of the Proton; QCD Fits to g 1 Data; Polarized Quark Distributions; Polarized Glue o g(x, Q 2 ); Transversity; Deeply Virtual Compton Scattering and Exclusive Processes; Polarized Photon Structure Functions; Conclusions and Open Questions: How Does the Proton Spin?. Readership: Academics, as well as physicists working on particle and nuclear physics at the interface of theory and experiment.
We report a new STAR measurement of the longitudinal double-spin asymmetry A{sub LL} for inclusive jet production at mid-rapidity in polarized p + p collisions at a center-of-mass energy of (square root)s = 200 GeV. The data, which cover jet transverse momenta 5
We report precision measurements of the Feynman-x (x{sub F}) dependence, and first measurements of the transverse momentum (p{sub T}) dependence, of transverse single spin asymmetries for the production of [pi]° mesons from polarized proton collisions at √s = 200 GeV. The x{sub F} dependence of the results are in fair agreement with perturbative QCD (pQCD) model calculations that identify orbital motion of quarks and gluons within the proton as the origin of the spin effects. Results for the p{sub T} dependence at fixed x{sub F} are not consistent with these same pQCD-based calculations.
We report a new measurement of the midrapidity inclusive jet longitudinal double-spin asymmetry, ALL, in polarized pp collisions at center-of-mass energy √s = 200 GeV. The STAR data place stringent constraints on polarized parton distribution functions extracted at next-to-leading order from global analyses of inclusive deep-inelastic scattering (DIS), semi-inclusive DIS, and RHIC pp data. Lastly, the measured asymmetries provide evidence at the 3? level for positive gluon polarization in the Bjorken-x region x > 0.05 .
The SPIN 2002 Proceedings describe the recent advances in the field of spin physics. The topics cover research in high energy and nuclear physics and the study of the nuclear spin structure. The symposium also covers advances in polarized proton and electron acceleration and storage as well as polarized ion sources and targets. The first measurement of spin physics observables in pp collisions at center of mass energy of 200 Gev was announced at this symposium.
One of the primary goals of the spin physics program at the STAR experiment is to constrain the polarized gluon distribution function, [delta]g(X,Q2), by measuring the double longitudinal spin asymmetry, ALL, of various final-state channels. Neutral pions provide a potentially powerful final state because they are copiously produced in p+p collisions and have few backgrounds. In 2009, STAR took 14 pb-1 of integrated luminosity of 200 GeV p+p collisions, with average beam longitudinal polarization of 59%. Neutral pions produced in these collisions can be identified using STAR's large-acceptance electromagnetic calorimeter, with help from tracking from the STAR Time Projection Chamber. This work presents a measurement of the inclusive neutral pion ALL from this data, based on a new [pi]0 reconstruction algorithm. A comparison to theoretical predictions and other experimental results suggests that the current best-fit value of AG, the gluon contribution to the proton spin, is too small and that [delta]G is actually comparable in magnitude to the quark contribution to the proton spin [delta][sigma].
We report the first measurement of the transverse momentum dependence of double spin asymmetries in semi-inclusive production of pions in deep inelastic scattering off the longitudinally polarized proton. Data have been obtained using a polarized electron beam of 5.7 GeV with the CLAS detector at the Thomas Jefferson National Accelerator Facility (JLab). A significant non-zero $\sin2\phi$ single spin asymmetry was also observed for the first time indicating strong spin-orbit correlations for transversely polarized quarks in the longitudinally polarized proton. The azimuthal modulations of single spin asymmetries have been measured over a wide kinematic range.