31,290 research outputs found

    The Quark Spin Distributions of the Nucleon

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    The quark helicity measured in polarized deep inelastic scattering is different from the quark spin in the rest frame of the nucleon. We point out that the quark spin distributions ΔqRF(x)\Delta q_{RF}(x) are connected with the quark helicity distributions Δq(x)\Delta q(x) and the quark transversity distributions δq(x)\delta q(x) by an approximate relation: ΔqRF(x)+Δq(x)=2δq(x)\Delta q_{RF}(x) + \Delta q(x)=2 \delta q(x). This relation will be useful in order to measure the rest frame (or quark model) spin distributions of the nucleon once the quark helicity distributions and quark transversity distributions are themselves measured. We also calculate the xx-dependent quark transversity distributions δq(x)\delta q(x) and quark spin distributions ΔqRF(x)\Delta q_{RF}(x) in a light-cone SU(6) quark-spectator model, and present discussions on possible effect from the sea quark-antiquark pairs.Comment: 16 latex pages including a ps figure, many changes in abstract, text and references, Final version to appear in Phys.Lett.

    Electromagnetic Form Factors of Nucleons in a Light-cone Diquark Model

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    We investigate the electromagnetic form factors of nucleons within a simple relativistic quark spectator-diquark model using the light-cone formalism. Melosh rotations are applied to both quark and vector diquark. It is shown that the difference between vector and scalar spectator diquarks reproduces the right electric form factor of neutrons, and both the form factors GE(Q2)G_E(Q^2) and GM(Q2)G_M(Q^2) of the proton and neutron agree with experimental data well up to Q2=2 GeV2Q^2=2 ~\rm{GeV}^2 in this simple model.Comment: 16 pages, Revtex4, minor changes, to appear in Phys. Rev.

    Understanding the proton spin "puzzle" with a new "minimal" quark model

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    We investigate the spin structure of the nucleon in an extended Jaffe-Lipkin quark model. In addition to the conventional 3q3q structure, different (3q)(QQˉ)(3q)(Q\bar{Q}) admixtures in the nucleon wavefunction are also taken into account. The contributions to the nucleon spin from various components of the nucleon wavefunction are discussed. The effect due to the Melosh-Wigner rotation is also studied. It is shown that the Jaffe-Lipkin term is only important when antiquarks are negatively polarized. We arrive at a new "minimal" quark model, which is close to the naive quark model, in order to understand the proton spin "puzzle".Comment: 19 latex pages, Published in Eur.Phys.J.

    Nucleon Transversity Distribution from Azimuthal Spin Asymmetry in Pion Electroproduction

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    The azimuthal asymmetry observed by the HERMES collaboration in semi-inclusive pion production in deep inelastic scattering of unpolarized positron on the longitudinally polarized proton target, can provide information of the quark transversity distributions of the nucleon. We show that the quark transversity distributions predicted both by the light-cone quark-spectator-diquark model and by a pQCD inspired model can give consistent descriptions of the available HERMES data for the analyzing powers AULsinϕA_{UL}^{\sin \phi} and AULsin2ϕA_{UL}^{\sin 2 \phi} for π+\pi^+ and π\pi^- productions. We also show that the two models give similar predictions of AULsinϕA_{UL}^{\sin \phi} for π+\pi^+ production, whereas they give very different predictions of AULsinϕA_{UL}^{\sin \phi} for π\pi^- production at large xx. Further precision measurement of AULsinϕA_{UL}^{\sin \phi} for π\pi^- production can provide a decisive test of different models.Comment: 11 latex pages, 1 figure, version to appear in PR

    The Flavor and Spin Structure of Hyperons from Quark Fragmentation

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    We systematically study the hadron longitudinal polarizations of the octet baryons at large zz from quark fragmentations in e+ee^+e^--annihilation, polarized charged lepton deep inelastic scattering (DIS) process, and neutrino (antineutrino) DIS process, based on predictions of quark distributions for the octet baryons in the SU(6) quark-spectator-diquark model and a perturbative QCD based counting rule analysis. We show that the e+ee^+e^--annihilation and polarized charged lepton DIS process are able to distinguish between the two different predictions of the hyperon polarizations. We also find that the neutrino/antineutrino DIS process is ideal in order to study both the valence content of the hyperons and the antiquark to hyperon (quark to anti-hyperon) fragmentations, which might be related to the sea content of hyperons.Comment: 32 latex pages, 11 figures, a misprint in Table 1 corrected, reference updated, to appear in PR

    Λ\Lambda, Λˉ\bar{\Lambda} Polarization and Spin Transfer in Lepton Deep-Inelastic Scattering

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    The flavor and helicity distributions of the Λ\Lambda and Λˉ\bar{\Lambda} hyperons for both valence and sea quarks are calculated in a perturbative QCD (pQCD) based model. We relate these quark distributions to the fragmentation functions of the Λ\Lambda and Λˉ\bar{\Lambda}, and calculate the zz-dependence of the longitudinal spin transfer to the Λ\Lambda and Λˉ\bar{\Lambda} in lepton deep-inelastic scattering (DIS). It is shown that the spin transfer to the Λ\Lambda is compatible with the first HERMES results at DESY and further tests are suggested. We also make predictions for the zz-dependence of the Λ\Lambda and Λˉ\bar{\Lambda} longitudinal polarizations in neutrino (antineutrino) DIS processes. We investigate the sea contribution to the fragmentation functions, and we test a possible scenario where sea quarks in Λ\Lambda (or sea antiquarks in Λˉ\bar{\Lambda}) are negatively polarized, whereas sea antiquarks in the Λ\Lambda (or sea quarks in Λˉ\bar{\Lambda}) are positively polarized. The asymmetry of the polarized fragmentation functions of sea quarks and antiquarks to Λ\Lambda and Λˉ\bar{\Lambda} provides a way to understand the different behaviour between the Λ\Lambda and Λˉ\bar{\Lambda} spin transfers observed in the recent E665 experiment at FNAL.Comment: 23 latex pages, 5 figures, references added to the last version, to appear in Eur. Phys. J.
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