Sea Quark Polarization Research Articles

Octet and decuplet baryon magnetic moments in the chiral quark model

Octet and decuplet baryon magnetic moments have been formulated within the \chiQM with configuration mixing incorporating the sea quark polarizations and their orbital angular momentum through the generalization of the Cheng-Li mechanism. When the parameters of \chiQM without configuration mixing are fixed by incorporating the latest data pertaining to $\bar u-\bar d$ asymmetry (E866) and the spin polarization functions, in the case of octet magnetic moments the results not only show improvement over the nonrelativistic quark model results but also give a non zero value for the right hand side of Coleman-Glashow sum rule, usually zero in most of the models. When effects of configuration mixing and ``mass adjustments'' due to confinement are included, in the case of p, \Sigma^+, \Xi^o, the \Sigma \Lambda transition magnetic moment and the violation of Coleman Glashow sum rule an almost perfect agreement with data is obtained. In the case of decuplet magnetic moments, we obtain a good overlap for \Delta^{++}, \Omega^- and the transition magnetic moment $\Delta N$ for which data are available. In case, we incorporate in our analysis the gluon polarization \Delta g, found phenomenologically through the relation \Delta\Sigma(Q^2)=\Delta\Sigma-\frac{3\alpha_s(Q^2)}{2\pi}\Delta g(Q^2), we not only obtain improvement in the quark spin distribution functions and magnetic moments but also the value of \Delta g comes out to be in good agreement with certain recent measurements as well as theoretical estimates.

MEASUREMENT OF POLARIZED QUARK DISTRIBUTIONS AT HERMES

The HERMES experiment has measured double spin asymmetries in the cross section for deep–inelastic scattering of longitudinally polarized positrons on longitudinally polarized 3 He (1995), 1 H (1996/97), and 2 D (1998) targets. A kinematic range of 0.023 < x < 0.6, and 1 GeV 2 < Q2 < 15 GeV 2 has been covered. From these asymmetries, based on inclusive and semi–inclusive measurements, polarized quark distributions are extracted as a function of x for up [Formula: see text] and down [Formula: see text] flavors as well as for valence and sea quarks. In the measured range, the up quark polarization is positive while the down quark polarization is negative. The sea quark polarization is compatible with zero. In this analysis all sea quark polarizations have been assumed to be flavor symmetric. Presently HERMES is analyzing a high statistics 2 D data sample taken in 1999-2000. For these data it is possible to identify pions and kaons with a RICH detector which was installed in 1998. This will enable an extraction of the individual sea quark flavors [Formula: see text], [Formula: see text], and [Formula: see text] without any symmetry assumptions.

Chiral symmetry and the nucleon spin structure functions

We carry out a systematic investigation of twist-two spin dependent structure functions of the nucleon within the framework of the chiral quark soliton model (CQSM) by paying special attention to the role of chiral symmetry of QCD. The importance of chiral symmetry is illustrated through the good reproduction of the recent SLAC data for the neutron spin structure function ${g}_{1}^{n}{(x,Q}^{2}).$ We also observe a substantial difference between the predictions of the longitudinally polarized distribution functions and those of the transversity distribution functions. That the chiral symmetry may be responsible for this difference is seen in the isospin dependence of the corresponding first moments, i.e., the axial and tensor charges. The CQSM predicts ${g}_{A}^{(0)}{/g}_{A}^{(3)}\ensuremath{\simeq}0.25$ for the ratio of the isoscalar to isovector axial charges, while ${g}_{T}^{(0)}{/g}_{T}^{(3)}\ensuremath{\simeq}0.46$ for the ratio of the isoscalar to isovector tensor charges, which should be compared with the prediction ${g}_{A}^{(0)}{/g}_{A}^{(3)}{=g}_{T}^{(0)}{/g}_{T}^{(3)}=3/5$ of the constituent quark model or of the naive MIT bag model without proper account of chiral symmetry. Another prominent prediction of the CQSM is the opposite polarization of the $\ifmmode \bar{u}\else \={u}\fi{}$ and $\overline{d}$ antiquarks, thereby indicating the SU(2) asymmetric sea quark (spin) polarization in the nucleon.

NLO QCD ANALYSIS OF POLARIZED DEEP INELASTIC SCATTERING

We have carried out a next-to-leading order QCD analysis of the experimental data on polarized DIS in the [Formula: see text] scheme. We have studied two models of the parametrizations of the input parton densities — the first due to Brodsky, Burkhardt and Schmidt (BBS), which gives a simultaneous parametrization for the polarized and unpolarized densities and in which the counting rules are strictly imposed; in the second, the input polarized densities are written in terms of the unpolarized ones in the generic form Δq(x)=f(x)q(x), with f(x) some simple smooth function. In both cases a good fit to the polarized data is achieved. As expected, the polarized data do not allow a precise determination of the polarized gluon density. Concerning the polarized sea quark densities, these are fairly well determined in the BBS model because of the interplay of polarized and unpolarized data, whereas in the second model, where only the polarized data are relevant, the polarized sea quark densities are largely undetermined.

Next-to-leading order QCD analysis of polarized deep inelastic scattering data

Abstract We present a next-to-leading order perturbative QCD analysis of world data on the spin dependent structure functions g1p, g1n, and g1d, including the new experimental information on the Q2 dependence of g1n. Careful attention is paid to the experimental and theoretical uncertainties. The data constrain the first moments of the polarized valence quark distributions, but only qualitatively constrain the polarized sea quark and gluon distributions. The NLO results are used to determine the Q2 dependence of the ratio g 1 F 1 and evolve the experimental data to a constant Q2 = 5 GeV2. We determine the first moments of the polarized structure functions of the proton and neutron and find agreement with the Bjorken sum rule.

STATUS OF THE PROTON SPIN PROBLEM

The proton spin problem triggered by the EMC experiment and its present status are closely examined. Recent experimental and theoretical progresses and their implications are reviewed. It is pointed out that the sign of the sea quark polarization generated perturbatively by hard gluons via the anomaly mechanism is predictable. It is negative if the gluon spin component is positive. We stress that the polarized nucleon structure function g1(x) is independent of the k⊥ factorization scheme chosen in defining the quark spin density and the hard photon–gluon scattering cross-section. Consequently, the anomalous gluon and sea quark interpretations for Γ1, the first moment of g1(x), are equivalent. It is the axial anomaly that accounts for the observed suppression of [Formula: see text].

A phenomenological model for the spin-dependent structure functions

A model for the polarized proton, neutron and deuteron structure functions embodying various features expected from the theory is presented. Each structure function is constructed from its asymptoticx behaviours, consistent with theoretical expectations at largex and smallx, while theQ2-dependence is obtained using the perturbative QCD evolution equations. By using QCD improvements on the first moments of various distributions, and including, from axial anomaly, a direct gluonic contribution tog1, good agreement with data on polarized deep inelastic scattering is obtained. Assuming that this gluonic component can be absorbed in a redefined polarized quark density, we reproduce the unexpected results on the Ellis-Jaffe sum rule and on spin content of the nucleon with no need of an initially strange sea quark polarization.

Parton distributions from W ± and Z production in polarized pp and pn collisions at RHIC

We study the production of W ± and Z gauge bosons in proton-proton and proton-neutron collisions up to a center-of-mass energy √ s = 0.5 TeV, in connection with the realistic possibility of having proton-deuteron collisions at RHIC with a high luminosity. We stress the importance of measuring unpolarized cross sections for a better determination of the flavor asymmetry of the quarks. Since polarized proton beams will be available at RHIC, we evaluate several spin-dependent observables, like double helicity asymmetries A LL as a test of the sea quark polarization, double transverse spin asymmetries A LL as a practical way to determine a new structure function, the nucleon transversity distribution h 1( x), and parity-violating asymmetries A L as a further test of our present knowledge of polarized parton distributions.

Soft gluon resummations for polarized Drell-Yan dimuon production

We calculate to O( α S) the transverse momentum differential cross section d Δσ/d Q 2d y d q T 2 for dimuon production in polarized hadron-hadron collisions. In the region q T 2 ⪡ Q 2 the resummation of large terms ∼ α s( q T 2) ln n ( q T 2)/ q T 2, n = 0, 1 is performed to all orders in α s in the double logarithmic approximation (DLA). The question of going beyond the DLA is discussed as well. From a numerical analysis we conclude that it should be possible to identify a sea-quark polarization in the proton in present and future experiments.

Polarized sea-quark and gluon distribution functions.

With the phenomenologically determined valence-quark spin densities in conjunction with the positivity of the unpolarized parton distributions, we extract the polarized sea-quark (gluon) distribution necessary to interpret the European Muon Collaboration measurement of the polarized proton structure function ${g}_{1}^{p}(x)$ in terms of sea quarks (gluons) alone. We then propose more realistic spin-dependent sea and gluon distribution functions which are parametrized in such a way that the first moment of ${g}_{1}^{p}(x)$ receives almost all contributions from the region $x>0.01$. A prediction of the polarized neutron structure function is given.

Z0 and test of sea-quark polarization.

The spin-spin asymmetry in the Drell-Yan process with polarized protons in both the beam and target is discussed including ${Z}^{0}$ effects. The role which this quantity could play to test sea-quark polarization is considered above the ${Z}^{0}$ peak. Numerical calculations are made for models with a polarized and an unpolarized sea.