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Search Results: 1 - 10 of 86701 matches for " MA Jian-Jun "
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The nucleon spin structure in a simple quark model
Bo-Qiang Ma,Ivan Schmidt,Jian-Jun Yang
Physics , 2000,
Abstract: We investigate the spin structure of the nucleon in an extended Jaffe-Lipkin quark model. In addition to the conventional $3q$ structure, different $(3q)(Q\bar{Q})$ 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.
Understanding the proton spin "puzzle" with a new "minimal" quark model
Bo-Qiang Ma,Ivan Schmidt,Jian-Jun Yang
Physics , 2002, DOI: 10.1007/s100500170014
Abstract: We investigate the spin structure of the nucleon in an extended Jaffe-Lipkin quark model. In addition to the conventional $3q$ structure, different $(3q)(Q\bar{Q})$ 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".
Reanalysis of Azimuthal Spin Asymmetries of Meson Electroproduction
Bo-Qiang Ma,Ivan Schmidt,Jian-Jun Yang
Physics , 2002, DOI: 10.1103/PhysRevD.66.094001
Abstract: The azimuthal spin asymmetries for pion production in semi-inclusive deep inelastic scattering of unpolarized charged lepton beams on longitudinally polarized nucleon targets, are reanalyzed by taking into account an important sign correction to previous formulas. It is found that different approaches of distribution functions and fragmentation functions may lead to distinct predictions on the azimuthal asymmetries measured in the HERMES experiments, thus the available data cannot be considered as a direct measurement of quark transversity distributions, although they still can serve to provide useful information on these distributions and on T-odd fragmentation functions. Predictions of the azimuthal spin asymmetries for kaon production are also presented, with different approaches of distribution and fragmentation functions. The unfavored fragmentation functions cannot be neglected for $K^-$ and $K^0_S$ production in semi-inclusive deep inelastic processes.
Flavor and Spin Structure of Octet Baryons at Large x
Bo-Qiang Ma,Ivan Schmidt,Jian-Jun Yang
Physics , 1999, DOI: 10.1016/S0550-3213(00)00021-3
Abstract: The quark flavor and spin distributions in octet baryons are calculated both in the SU(6) quark spectator diquark model and in a perturbative QCD (pQCD) based model. It is shown that the $\Lambda$ has the most significant difference in flavor structure at large $x$ between the two models, though the flavor and spin structure of other baryons can also provide tests of different models. The Drell-Yan process for $\Sigma^{\pm}$ beams on isoscalar targets can be used to test different predictions concerning the valence quark flavor structure of the $\Sigma^{\pm}$.
Quark Structure of $Λ$ from $Λ$-Polarization in Z Decays
Bo-Qiang Ma,Ivan Schmidt,Jian-Jun Yang
Physics , 1999, DOI: 10.1103/PhysRevD.61.034017
Abstract: The flavor and spin structure for the quark distributions of the $\Lambda$-baryon is studied in a perturbative QCD (pQCD) analysis and in the SU(6) quark-diquark model, and then applied to calculate the $\Lambda$-polarization of semi-inclusive $\Lambda$ production in $e^+e^-$-annihilation near the $Z$-pole. It is found that the quark-diquark model gives very good description of the available experimental data. The pQCD model can also give good description of the data by taking into account the suppression of quark helicities compared to the naive SU(6) quark model spin distributions. Further information is required for a clean distinction between different predictions concerning the flavor and spin structure of the $\Lambda$.
Nucleon Transversity Distribution from Azimuthal Spin Asymmetry in Pion Electroproduction
Bo-Qiang Ma,Ivan Schmidt,Jian-Jun Yang
Physics , 2000, DOI: 10.1103/PhysRevD.63.037501
Abstract: 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 $A_{UL}^{\sin \phi}$ and $A_{UL}^{\sin 2 \phi}$ for $\pi^+$ and $\pi^-$ productions. We also show that the two models give similar predictions of $A_{UL}^{\sin \phi}$ for $\pi^+$ production, whereas they give very different predictions of $A_{UL}^{\sin \phi}$ for $\pi^-$ production at large $x$. Further precision measurement of $A_{UL}^{\sin \phi}$ for $\pi^-$ production can provide a decisive test of different models.
Collins Effect in Single Spin Asymmetries of the $p^{\uparrow}p \to π X$ Process
Bo-Qiang Ma,Ivan Schmidt,Jian-Jun Yang
Physics , 2004, DOI: 10.1140/epjc/s2005-02136-x
Abstract: We investigate the Collins effect in single spin asymmetries (SSAs) of the $p^{\uparrow}p \to \pi X$ process, by taking into account the transverse momentum dependence of the microscopic sub-processes cross sections, with the transverse momentum in the Collins function integrated over. We find that the asymmetries due to the Collins effect can only explain the available data at best qualitatively, by using our choices of quark distributions in the quark-diquark model and a pQCD-based analysis, together with several options of the Collins function. Our results indicate the necessity to take into account contributions from other effects such as the Sivers effect or twist-3 contributions.
Azimuthal Spin Asymmetries of Pion Electroproduction
Bo-Qiang Ma,Ivan Schmidt,Jian-Jun Yang
Physics , 2001, DOI: 10.1103/PhysRevD.65.034010
Abstract: Azimuthal spin asymmetries, both for charged and neutral pion production in semi-inclusive deep inelastic scattering of unpolarized charged lepton beams on longitudinally and transversely polarized nucleon targets, are analyzed and calculated. Various assumptions and approximations in the quark distributions and fragmentation functions often used in these calculations are studied in detail. It is found that different approaches to the distribution and fragmentation functions may lead to quite different predictions on the azimuthal asymmetries measured in the HERMES experiments, thus their effects should be taken into account before using the available data as a measurement of quark transversity distributions. It is also found that the unfavored quark to pion fragmentation functions must be taken into account for $\pi^-$ production from a proton target, although they can be neglected for $\pi^+$ and $\pi^0$ production. Pion production from a proton target is suitable to study the $u$ quark transversity distribution, whereas a combination of pion production from both proton and neutron targets can measure the flavor structure of quark transversity distributions.
Flavor and Spin Structure of $Λ$-Baryon at Large x
Bo-Qiang Ma,Ivan Schmidt,Jian-Jun Yang
Physics , 1999, DOI: 10.1016/S0370-2693(00)00167-2
Abstract: It is shown that a perturbative QCD (pQCD) based analysis and the SU(6) quark-diquark model give significant different predictions concerning the flavor and spin structure for the quark distributions of the $\Lambda$-baryon near $x=1$. Detailed predictions for the ratios $u(x)/s(x)$ of unpolarized quark distributions, $\Delta s(x)/s(x)$ of valence strange quark, and $\Delta u(x)/u(x)$ of valence up and down quarks of the $\Lambda$ are given from the quark-diquark model and from a pQCD based model. It is found that the up and down quarks are positively polarized at large $x$, even though their net spin contributions to the $\Lambda$ might be zero or negative. The significant difference for $u(x)/s(x)$ between the two different approaches are predicted. The prediction of positively polarized up and down quarks inside the $\Lambda$ at large $x$ has been supported by the available data of $\Lambda$-polarization in $Z$ decays and also by the most recent HERMES result of spin transfer to the $\Lambda$ in deep elastic scattering of polarized lepton on the nucleon target.
Particle-Antiparticle Asymmetries of $Λ$ Production in Hadron-Nucleon Collisions
Bo-Qiang Ma,Ivan Schmidt,Jian-Jun Yang
Physics , 2004, DOI: 10.1016/j.physletb.2004.07.049
Abstract: The particle-antiparticle asymmetries of $\Lambda$ production in 250 GeV/c $\pi^{\pm}$, $K^{\pm}$, and $p$ --nucleon collisions are studied with two model parametrizations of quark to $\Lambda$ fragmentation functions. It is shown that the available data can be qualitatively explained by the calculated results in both the quark-diquark model and a pQCD based analysis of fragmentation functions. The differences in the two model predictions are significant for $K^{\pm}$ beams, and high precision measurements of the asymmetries with detailed $x_F$ and $P_T$ information can discriminate between different predictions.
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