arXiv subjects

Bo-Qiang Ma

Publications and source records attributed to Bo-Qiang Ma.

At least 289 records · Page 16Linked to original sources

arXiv2000

The Flavor and Spin Structure of Hyperons from Quark Fragmentation

We systematically study the hadron longitudinal polarizations of the octet baryons at large $z$ from quark fragmentations in $e^+e^-$-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 $e^+e^-$-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.

hep-ph↗

arXiv2000

The Quark Distributions of Baryons

The recent progress in the investigations on the quark structure of the $Λ$ and $Σ$ baryons are reviewed. It is shown that the quark structure of $Λ$ and $Σ$ hyperons can provide a new domain to test various theories concerning the spin and flavor structure of the nucleon. The $Λ$ and $Σ$ Physics might be new directions to explore the quark distributions of baryons both theoretically and experimentally.

hep-ph↗

arXiv2000

The nucleon spin structure in a simple quark model

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.

hep-ph↗

arXiv2000

The Quark-Antiquark Asymmetry of the Nucleon Sea from $Λ$ and $\barΛ$ Fragmentation

We present a general analysis of the spin transfer for $Λ$ and $\barΛ$ production in deep-inelastic scattering of polarized charged leptons on the nucleon, and find that the pattern of different behaviors of $Λ$ and $\barΛ$ production observed by the E665 Collaboration suggests the possibility of quark-antiquark asymmetries either in the quark to $Λ$ fragmentation functions and/or in the quark and antiquark distributions of the target proton. We also point out that the strange-antistrange asymmetry of the nucleon sea may produce an observable contribution to the different behaviors of $Λ$ and $\barΛ$ production. We find that a softer $\bar{s}(x)$ than $s(x)$ as predicted by the light-cone baryon-meson fluctuation model of intrinsic quark-antiquark pairs of the nucleon sea might lead to a reasonable picture. However, the magnitude is still too small to explain the E665 data and the conclusion has also strong model-dependence. This may suggest the importance of quark-antiquark asymmetry in the quark to $Λ$ fragmentation functions, provided that the E665 data are confirmed.

hep-ph↗

arXiv2000

Flavor and Spin Structure of Octet Baryons at Large x

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 $Λ$ 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 $Σ^{\pm}$ beams on isoscalar targets can be used to test different predictions concerning the valence quark flavor structure of the $Σ^{\pm}$.

hep-ph↗

arXiv2000

$Λ$, $\barΛ$ Polarization and Spin Transfer in Lepton Deep-Inelastic Scattering

The flavor and helicity distributions of the $Λ$ and $\barΛ$ 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 $Λ$ and $\barΛ$, and calculate the $z$-dependence of the longitudinal spin transfer to the $Λ$ and $\barΛ$ in lepton deep-inelastic scattering (DIS). It is shown that the spin transfer to the $Λ$ is compatible with the first HERMES results at DESY and further tests are suggested. We also make predictions for the $z$-dependence of the $Λ$ and $\barΛ$ 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 $Λ$ (or sea antiquarks in $\barΛ$) are negatively polarized, whereas sea antiquarks in the $Λ$ (or sea quarks in $\barΛ$) are positively polarized. The asymmetry of the polarized fragmentation functions of sea quarks and antiquarks to $Λ$ and $\barΛ$ provides a way to understand the different behaviour between the $Λ$ and $\barΛ$ spin transfers observed in the recent E665 experiment at FNAL.

hep-ph↗

arXiv2000

Flavor and Spin Structure of $Λ$-Baryon at Large x

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 $Λ$-baryon near $x=1$. Detailed predictions for the ratios $u(x)/s(x)$ of unpolarized quark distributions, $Δs(x)/s(x)$ of valence strange quark, and $Δu(x)/u(x)$ of valence up and down quarks of the $Λ$ 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 $Λ$ 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 $Λ$ at large $x$ has been supported by the available data of $Λ$-polarization in $Z$ decays and also by the most recent HERMES result of spin transfer to the $Λ$ in deep elastic scattering of polarized lepton on the nucleon target.

hep-ph↗

arXiv2000

The Spin Structure of the Proton

It is shown that the proton "spin crisis'' or "spin puzzle" can be understood by the relativistic effect of quark transversal motions due to the Melosh-Wigner rotation. The quark helicity $Δq$ measured in polarized deep inelastic scattering is actually the quark spin in the infinite momentum frame or in the light-cone formalism, and it is different from the quark spin in the nucleon rest frame or in the quark model. The flavor asymmetry of the Melosh-Wigner effect for the valence $u$ and $d$ quarks and the intrinsic sea $q \bar{q}$ pairs are also the important ingredients in a SU(6) quark-spectator-diquark model framework to understand the "spin puzzle". Such a picture of the spin structure can be tested by use of several simple relations to measure the quark spin distributions in the quark model.

hep-ph↗

arXiv1999

Quark Structure of $Λ$ from $Λ$-Polarization in Z Decays

The flavor and spin structure for the quark distributions of the $Λ$-baryon is studied in a perturbative QCD (pQCD) analysis and in the SU(6) quark-diquark model, and then applied to calculate the $Λ$-polarization of semi-inclusive $Λ$ 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 $Λ$.

hep-ph↗

arXiv1999

High Precision Test of QCD at Beijing Electron Positron Collider

The generalized Crewther relation relates the cross section ratio $R=σ(e^+e^- \to {\rm hadrons})/σ(e^+e^- \to μ^+ μ^-)$ in $e^+e^-$ annihilation with the Bjorken sum rule or the Gross-Llewellyn Smith sum rule in deep inelastic scattering and provides a fundamental connection for observables in Quantum Chromodynamics (QCD) without scale or scheme ambiguities. The ratio $R$ can be measured at the upgrated Bejing Electron Positron Collider or the $τ$-Charm factory with higher precision and thus can be served for a high precision test of QCD in the Standard Model.

hep-ph↗

arXiv1999

Quark Flavor Separation in $Λ$-Baryon Fragmentation

It is shown that neutrino and antineutrino deep inelastic scattering of unpolarized and polarized $Λ$ and $\barΛ$ productions, can provide a clean separation of unpolarized and polarized fragmentation functions of a quark into a $Λ$, for both light-flavor quarks and antiquarks and also for strange quarks. Combining with $Λ$ and $\barΛ$ productions in polarized electron deep inelastic scattering, one can systematically measure or check the various flavor and spin dependent fragmentation functions. Such measurements can provide crucial tests of different predictions concerning the spin structure of hadrons and the quark-antiquark asymmetry of the nucleon sea.

hep-ph↗

arXiv1998

The Quark Spin Distributions of the Nucleon

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 $Δq_{RF}(x)$ are connected with the quark helicity distributions $Δq(x)$ and the quark transversity distributions $δq(x)$ by an approximate relation: $Δq_{RF}(x) + Δq(x)=2 δq(x)$. 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 $x$-dependent quark transversity distributions $δq(x)$ and quark spin distributions $Δq_{RF}(x)$ in a light-cone SU(6) quark-spectator model, and present discussions on possible effect from the sea quark-antiquark pairs.

hep-ph↗

arXiv1998

The Quark Orbital Angular Momentum in a Light-Cone Representation

We perform an analysis of the quark angular momentum in a light-cone representation by taking into account the effect due to the Melosh-Wigner rotation and find that there is a relativistic correction factor connecting the quark orbital angular momentum with the quark model spin distribution: $L_q(x)={ }Δq_{QM}(x)$. The quark orbital angular momentum $L_q(x)$ and the quark helicity distribution $Δq(x)$ are connected to the quark model spin distribution $Δq_{QM}(x)$ by a relation: $\frac{1}{2}Δq(x)+ L_q(x)=\frac{1}{2}Δq_{QM}(x)$, which means that one can decompose the quark model spin contribution $Δq_{QM}(x)$ by a quark helicity term $Δq(x)$ {\it plus} an orbital angular momentum term $L_q(x)$. There is also a new relation connecting the quark orbital angular momentum with the measurable quark helicity distribution and transversity distribution ($δq(x)$): $Δq(x)+L_q(x)=δq(x)$, from which we may have new sum rules connecting the quark orbital angular momentum with the nucleon axial and tensor charges.

hep-ph↗

arXiv1998

Electromagnetic Transition Form Factor of Pseudoscalar Meson and $η-η'$ Mixing

The electromagnetic transition form factors of $η$ and $η'$ are calculated in the light-cone perturbation theory. We show that it is unreliable to determine the η-\etap'$ mixing angle without any additional normalization conditions other than their decay widthes to two photons. The possible intrinsic $c\bar{c}$ component in the flavor singlet is investigated. The heavy quark pair has distinct properties from the light ones in electromagnetic transition processes of pseudoscalar mesons. It is possible to explore the size of $c\bar{c}$ component and our numerical results disfavor a large portion of $c\bar{c}$ component.

hep-ph↗

arXiv1998

The Non-perturbative Effect on $R=σ_L /σ_T$ from QCD Vacuum

We investigate the non-perturbative effects on the ratio $R=σ_L /σ_T$ in lepton-nucleon deep inelastic scattering by taking into account the lowest dimensional condensate contributions from the QCD vacuum. By combining conventional perturbative QCD corrections and the Georgi- Politzer target-mass effects with the non-perturbative effects from the QCD vacuum, we give a good description of the $Q^2$ and $x$ dependences of $R$ in comparison with the recent experimental data.

hep-ph↗

arXiv1997

New Sum Rules for Nucleon Tensor Charges

Two new sum rules for the quark tensor charges of the nucleon are proposed, based on a relation connecting the quark transversity distributions to the quark helicity distributions and the quark model spin distributions, and on the sum rules for the quark helicity distributions. The two sum rules are useful for an estimate of the values of the quark tensor charges $δU$ and $δD$ from the measured quantities of $Γ^p$, $Γ^n$, $g_A/g_V$ and $ΔS$, and two model correction factors with limited uncertainties. We predict a small value for the sum of the quark tensor charges compared to most other predictions, in analogy to the unexpectedly small quark helicity sum which gave rise to the proton ``spin puzzle''.

hep-ph↗

arXiv1997

Strange Magnetic Moment and Isospin Symmetry Breaking

The small mass difference $m_n-m_p=1.3 MeV$ between the proton and neutron leads to an excess of $n=π^- p$ over $p=π^+ n$ fluctuations which can be calculated by using a light-cone meson-baryon fluctuation model of intrinsic quark-antiquark pairs of the nucleon sea. The Gottfried sum rule violation may partially be explained by isospin symmetry breaking between the proton and neutron and the same effect introduces correction terms to the anomalous magnetic moments and the anomalous weak magnetic moments of the proton and neutron. We also evaluated the strange magnetic moment of the nucleon from the lowest strangeness $K Λ$ fluctuation and found a non-trivial influence due to isospin symmetry breaking in the experimental measurements of the strange magnetic moment of the nucleon.

hep-ph↗

arXiv1997

The Proton Spin Structure in a Light-Cone Qaurk-Spectator-Diquark Model

It is shown that the proton spin problem raised by the Ellis-Jaffe sum rule violation does not in conflict with the SU(6) quark model, provided that the relativistic effect from the quark transversal motions, the flavor asymmetry between the $u$ and $d$ valence quarks, and the intrinsic quark-antiquark pairs generated by the non-perturbative meson-baryon fluctuations of the nucleon sea are taken into account. The meson-baryon fluctuations of the nucleon sea provide a consistent picture for connecting the proton spin structure with a number of other anomalies observed in the proton's structure, such as the flavor asymmetry of the nucleon sea implied by the violation of the Gottfried sum rule and the discrepancy between two different measurements of the strange quark distributions in the nucleon sea.

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