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Jian-Jun Yang

Publications and source records attributed to Jian-Jun Yang.

At least 37 records · Page 2Linked to original sources
arXiv2001

Spin Transfers for Baryon Production in Polarized pp Collisions at RHIC-BNL

We consider the inclusive production of longitudinally polarized baryons in ${\vec p}p$ collisions at RHIC-BNL, with one longitudinally polarized proton. We study the spin transfer between the initial proton and the produced baryon as a function of its rapidity and we elucidate its sensitivity to the quark helicity distributions of the proton and to the polarized fragmentation functions of the quark into the baryon. We make predictions using an SU(6) quark spectator model and a perturbative QCD (pQCD) based model. We discuss these different predictions, and what can be learned from them, in view of the forthcoming experiments at RHIC-BNL.

hep-ph↗
arXiv2001

Up and Down Quark Contributions to Spin Content of Lambda from Fragmentation

We check the $u$ and $d$ quark contributions to the spin content of the $Λ$ by means of the $q\toΛ$ fragmentation and find that the $u$ and $d$ quarks of the $Λ$ are likely positively polarized. The parton distributions in the $Λ$ are given by a successful statistical model which can reproduce and correlate a vast body of polarized and unpolarized structure function and parton distribution data of the nucleon. With the Gribov-Lipatov relation between the quark distributions and fragmentation functions, the longitudinal spin transfer for the $Λ$ production in the polarized charged lepton deep inelastic scattering (DIS) process and the $Λ$-polarization in the neutrino (antineutrino) DIS process are predicted. The available experimental data suggests that the $u$ and $d$ quark contributions to the spin of the $Λ$ are positive. In addition, our results provide a collateral evidence for the SU(3) symmetry breaking in hyperon semileptonic decays of the octet baryons, which is very important for a deeper understanding of the proton 'spin crisis'.

hep-ph↗
arXiv2000

Nucleon Transversity Distribution from Azimuthal Spin Asymmetry in Pion Electroproduction

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 $A_{UL}^{\sin ϕ}$ and $A_{UL}^{\sin 2 ϕ}$ for $π^+$ and $π^-$ productions. We also show that the two models give similar predictions of $A_{UL}^{\sin ϕ}$ for $π^+$ production, whereas they give very different predictions of $A_{UL}^{\sin ϕ}$ for $π^-$ production at large $x$. Further precision measurement of $A_{UL}^{\sin ϕ}$ for $π^-$ production can provide a decisive test of different models.

hep-ph↗
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 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

Nuclear Effects on the Extraction of Neutron Structure Functions

Nuclear effects in light nuclei due to the presence of spin-one isosinglet 6-quark clusters are investigated. The quark distributions of 6-quark clusters are obtained by using a perturbative QCD (pQCD) based framework, which allows us to get a good description of the ratio of the deuteron structure function to the free nucleon structure function. Nuclear effects on the extraction of the neutron structure functions $F_2^n$ and $g_1^n$ are estimated. We find that the effect on the extracted spin-dependent neutron structure function is very different from that on the spin-independent neutron structure function. The effect enhances the Bjorken sum by about 10%, whereas its correction to the Gottfried sum is rather small. The formalism for calculating nuclear effects is further used to evaluate the spin-dependent structure function of the $^3$He nucleus and a good self-consistent check is obtained.

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↗
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

QCD condensate contributions to the effective quark potential in a covariant gauge

We discuss QCD condensate contributions to the gluon propagator both in the fixed-point gauge and in covariant gauges for the external QCD vacuum gluon fields with the conclusion that a covariant gauge is essential to obtain a gauge invariant QCD vacuum energy density difference and to retain the unitarity of the quark scattering amplitude. The gauge-invariant QCD condensate contributions to the effective one-gluon exchange potential are evaluated by using the effective gluon propagator which produces a gauge-independent quark scattering amplitude.

hep-ph↗
arXiv1998

Energy Loss in Nuclear Drell-Yan Process

By means of the nuclear parton distributions which can be used to provide a good explanation for the EMC effect in the whole x range, we investigate the energy loss effect in nuclear Drell-Yan process. When the cross section of lepton pair production is considered varying with the center-of-mass energy of the nucleon-nucleon collision, we find that the nuclear Drell-Yan(DY) ratio is suppressed due to the energy loss, which balances the overestimate of the DY ratio only in consideration of the effect of nuclear parton distributions.

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

Final-state Interaction Effects on Inclusive Two-particle Production in Electron-positron Annihilation

The final-state interaction effects on the inclusive two-particle production in electron-positron annihilation are investigated within the context of the one-photon annihilation approximation. Such effects are characterized by one structure function in the decomposition of the hadronic tensor. On the basis of the positivity, we derived an inequality to bound this structure function. The price to access it experimentally is to polarize longitudinally one of the initial-state beam, to say, the electron beam, and measure the corresponding single spin asymmetry. By combining the Callan-Gross relation with our positivity analysis, we obtain an upper bound for the single spin asymmetry considered.

hep-ph↗
arXiv1997

The Nuclear Effect Induced by Additional Parton Evolution and Recombination in Nuclear Environment

The relationship between the parton transverse momentum and longitudinal momentum, which is obtained by analysing the hadron tensor in the deep inelastic scattering (DIS) process, indicates that the transverse size of partons in a nucleon is sensitively affected by the effective nucleon mass. The change in this transverse size gives rise to an Additional Parton Evolution (APE) in the nuclear environment refer to the normal QCD parton evolution thus resulting in new parton distributions. The self-consistent integral equations for these new parton distributions are given. Having undergone the normal QCD evolution and the additional evolution in the nuclear environment, the partons with the small x participate in recombination and contribute to the nuclear shadowing effect. The reasonably good descriptions of the nuclear effect of structure functions, nuclear Drell-Yan ratios and nuclear gluon distributions are presented. A quantitative comparison between the APE model and the Q^2-rescaling model is made.

hep-ph↗
arXiv1996

Symmetry aspects of the pion leptoproduction and the upper limit of the Levelt-Mulders asymmetry

We examined the symmetry aspect of the semi-inclusive one-pion production in the deep inelastic scattering of a lepton beam off an unpolarized nucleon target, with an emphasis on the positivity restrictions on the corresponding structure functions. In combination with the Callan-Gross-type relation between two twist-two structure functions $W_1$ and $W_2$, we derived an upper bound on the Levelt-Mulders asymmetry, which occurs when the lepton beam is longitudinally polarized.

hep-ph↗