arXiv subjects

Jian-Jun Yang

Publications and source records attributed to Jian-Jun Yang.

At least 19 recordsLinked to original sources

arXiv2005

Collins Effect in Single Spin Asymmetries of the $p^{\uparrow}p \to πX$ Process

We investigate the Collins effect in single spin asymmetries (SSAs) of the $p^{\uparrow}p \to πX$ 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.

hep-ph↗

arXiv2005

Transverse single spin asymmetries in photon production

Transverse single-spin asymmetries (SSA) in inclusive reactions are now considered to be directly related to the transverse momentum ${\bf k}_{T}$ of the fundamental partons involved in the process. We find that the ideal probe to extract information on the gluon Sivers function is the transverse SSA of prompt photon production $p p^{\uparrow} \to γX$, at large $p_T$. The following related processes, $p p^{\uparrow} \to γ+ jet + X$, $p p^{\uparrow} \to γ^* + X \to μ^+ μ^- + X$ and $\bar{p} p^{\uparrow} \to γ+ X$ are also briefly discussed.

hep-ph↗

arXiv2004

Nuclear Antishadowing in Neutrino Deep Inelastic Scattering

The shadowing and antishadowing of nuclear structure functions in the Gribov-Glauber picture is due respectively to the destructive and constructive interference of amplitudes arising from the multiple-scattering of quarks in the nucleus. The effective quark-nucleon scattering amplitude includes Pomeron and Odderon contributions from multi-gluon exchange as well as Reggeon quark-exchange contributions. We show that the coherence of these multiscattering nuclear processes leads to shadowing and antishadowing of the electromagnetic nuclear structure functions in agreement with measurements. This picture leads to substantially different antishadowing for charged and neutral current reactions, thus affecting the extraction of the weak-mixing angle $θ_W$. We find that part of the anomalous NuTeV result for $θ_W$ could be due to the nonuniversality of nuclear antishadowing for charged and neutral currents. Detailed measurements of the nuclear dependence of individual quark structure functions are thus needed to establish the distinctive phenomenology of shadowing and antishadowing and to make the NuTeV results definitive.

hep-ph↗

arXiv2004

Particle-Antiparticle Asymmetries of $Λ$ Production in Hadron-Nucleon Collisions

The particle-antiparticle asymmetries of $Λ$ production in 250 GeV/c $π^{\pm}$, $K^{\pm}$, and $p$ --nucleon collisions are studied with two model parametrizations of quark to $Λ$ 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.

hep-ph↗

arXiv2004

Ratio of $\barΛ/Λ$ in Semi-inclusive Electroproduction

It is shown that the $\barΛ/Λ$ cross section ratio in semi-inclusive electroproduction of $Λ$ and $\barΛ$ hyperons in deep inelastic scattering of charged lepton on a nucleon target, can provide useful information on the quark to $Λ$ fragmentation functions. This ratio is calculated explicitly in a quark-diquark model, a pQCD based analysis, and an SU(3) symmetry model, with three different options for the contribution from the unfavored fragmentation functions. The $x$-dependence of this ratio is sensitive to the ratio of unfavored fragmentation functions over favored fragmentation functions, $D_{\bar{u}}^Λ(z)/D_{u}^Λ(z)$, whereas the $z$-dependence is sensitive to the flavor structure of the fragmentation functions, i.e., the ratio $D_u^Λ(z)/D_s^Λ(z)$. Future measurements by the HERMES Collaboration at DESY can discriminate between various cases.

hep-ph↗

arXiv2003

Sivers function in a spectator model with axial-vector diquarks

We perform a calculation of the Sivers function in a spectator model of the nucleon, with scalar and axial-vector diquarks. We make use of gluon rescattering to produce the nontrivial phases necessary to generate the Sivers function. The inclusion of axial-vector diquarks enables us to obtain a nonzero Sivers function for down quarks. Using the results of our model, we discuss the phenomenology of transvere single spin asymmetries in pi+, pi-, and pi0 production, which are currently analysed by the HERMES and COMPASS collaborations. We find that the inclusion of axial-vector diquarks substantially reduces the asymmetries.

hep-ph↗

arXiv2003

Collins fragmentation function from gluon rescattering

We estimate the Collins fragmentation function by introducing the effect of gluon rescattering in a model calculation of the fragmentation process. We include all necessary diagrams to the one-loop level and compute the nontrivial phases giving rise to the Collins function. We compare our results to the ones obtained from pion rescattering. We conclude that three out of four one-loop diagrams give sizeable contributions to the Collins function, and that the effect of gluon rescattering has a magnitude comparable to that of pion rescattering, but has opposite sign.

hep-ph↗

arXiv2003

Mass Suppression in Octet Baryon Production

There is a striking suppression of the cross section for production of octet baryons in $e^+ e ^-$ annihilation, as the mass of the produced hadron increases. We present a simple parametrization for the fragmentation functions into octet baryons guided by two input models: the SU(3) flavor symmetry part is given by a quark-diquark model, and the baryon mass suppression part is inspired by the string model. We need only eight free parameters to describe the fragmentation functions for all octet baryons. These free parameters are determined by a fit to the experimental data of octet baryon production in $e^+ e ^-$ annihilation. Then we apply the obtained fragmentation functions to predict the cross section of the octet baryon production in charged lepton DIS and find consistency with the available experimental data. Furthermore, baryon production in $pp$ collisions is suggested to be an ideal domain to check the predicted mass suppression.

hep-ph↗

arXiv2002

Understanding the proton spin "puzzle" with a new "minimal" 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. 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".

hep-ph↗

arXiv2002

Reanalysis of Azimuthal Spin Asymmetries of Meson Electroproduction

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.

hep-ph↗

arXiv2002

Nuclear Effects on the Extraction of $\sin^2θ_W$

We study the impact of nuclear effects on the extraction of the weak-mixing angle $\sin^2θ_W$ from deep inelastic (anti-)neutrino-nucleus scattering, with special emphasis on the recently announced NuTeV Collaboration 3$σ$ deviation of $\sin^2θ_W$ from its standard model value. We have found that nuclear effects, which are very important in electromagnetic deep inelastic scattering (DIS), are quite small in weak charged current DIS. In neutral current DIS processes, which contain the weak mixing angle, we predict that these effects play also an important role and may dramatically affect the value of $\sin^2θ_W$ extracted from the experimental data.

hep-ph↗

arXiv2002

Phenomenological Relation between Distribution and Fragmentation Functions

We study the relation between the quark distribution function q(x) and the fragmentation function D_q(z) based on a general form D_q(x) = C(z)z^{alpha}q(z) for valence and sea quarks. By adopting two known parametrizations of quark distributions for the proton, we find three simple options for the fragmentation functions that can provide a good description of the available experimental data on proton production in e^+ e^- inelastic annihilation. These three options support the revised Gribov-Lipatov relation D_q(z) = z q(z) at z --> 1, as an approximate relation for the connection between distribution and fragmentation functions. The three options differ in the sea contributions and lead to distinct predictions for antiproton production in the reaction p+p --> p-bar+X, thus they are distinguishable in future experiments at RHIC-BNL.

hep-ph↗

arXiv2001

$q \to Λ$ Fragmentation Function and Nucleon Transversity Distribution in a Diquark Model

Based on a simple quark-diquark model, we propose a set of unpolarized, longitudinally polarized and transversely polarized fragmentation functions for the $Λ$ by fitting the unpolarized $Λ$ production data in $e^+ e ^- $ annihilation. It is found that the helicity structure of the obtained $Λ$ fragmentation functions is supported by the all available experimental data on the longitudinal $Λ$ polarization. Within the same framework of the diquark model, the nucleon transversity distributions are presented and consistent descriptions of the available HERMES data on the azimuthal spin asymmetries in pion electroproduction are obtained. Furthermore, the spin transfers to the transversely polarized $Λ$ in the charged lepton DIS on a transversely polarized nucleon target are predicted for future experiments.

hep-ph↗

arXiv2001

Evidence for SU(3) symmetry breaking from hyperon production

We examine the SU(3) symmetry breaking in hyperon semileptonic decays (HSD) by considering two typical sets of quark contributions to the spin content of the octet baryons: Set-1 with SU(3) flavor symmetry and Set-2 with SU(3) flavor symmetry breaking in HSD. The quark distributions of the octet baryons are calculated with a successful statistical model. Using an approximate relation between the quark fragmentation functions and the quark distributions, we predict polarizations of the octet baryons produced in $e^+e^-$ annihilation and semi-inclusive deeply lepton-nucleon scattering in order to reveal the SU(3) symmetry breaking effect on the spin structure of the octet baryons. We find that the SU(3) symmetry breaking significantly affects the hyperon polarization. The available experimental data on the $Λ$ polarization seem to favor the theoretical predictions with SU(3) symmetry breaking. We conclude that there is a possibility to get a collateral evidence for SU(3) symmetry breaking from hyperon production. The theoretical errors for our predictions are discussed.

hep-ph↗

arXiv2001

Quark Fragmentation Functions in a Diquark Model for $Λ$ Production

Using a simple quark-diquark model, we extract a set of unpolarized and polarized fragmentation functions for the $Λ$ based on the available unpolarized $Λ$ production data in $e^+ e ^- $ annihilation. It is found that there is a strong SU(3) flavor symmetry breaking in the unpolarized $Λ$ fragmentation functions and that the polarized $u$ and $d$ quark contributions to the polarized $Λ$ fragmentation are positive in medium and large $z$ region. The predictions of $Λ$-polarization with the obtained quark fragmentation functions are compatible with the available data on the longitudinally polarized $Λ$ produced in $e^+e^-$-annihilation, polarized charged lepton deep inelastic scattering (DIS), and neutrino DIS. The spin asymmetry for the $Λ$ production in $p\vec{p}$ collisions is also predicted for a future test in experiments at RHIC-BNL.

hep-ph↗

arXiv2001

Azimuthal Spin Asymmetries of Pion Electroproduction

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 $π^-$ production from a proton target, although they can be neglected for $π^+$ and $π^0$ 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.

hep-ph↗

arXiv2001

Helicity and Transversity Distribution of Nucleon and $Λ$-Hyperon from $Λ$ Fragmentation

It is shown that $Λ$-hyperon fragmentation in charged lepton deep inelastic scattering (DIS) on a polarized nucleon target can provide sensitive information concerning the quark helicity and transversity distributions for both nucleon and $Λ$-hyperon at large $x$. Numerical predictions are given for the spin transfers of the produced $Λ$, when the target nucleon is polarized either longitudinally or transversely, and with the nucleon and $Λ$ quark distributions evaluated both in an SU(6) quark-spectator-diquark model and in a perturbative QCD (pQCD) based model. It is also shown that the predicted spin transfers have different behaviors for proton and neutron targets, and this can provide sensitive tests of different predictions for the quark helicity and transversity distributions of the $d$ valence quark of the proton at large $x$.

hep-ph↗

arXiv2001

Quark Distributions of Octet Baryons from SU(3) Symmetry

SU(3) symmetry relations between the octet baryons are introduced in order to connect both the unpolarized and polarized quark distributions of the octet baryons with those of the nucleon. Two different parametrizations of the nucleon quark distributions are used. A new scenario of quark flavor and spin structure of the $Λ$ is found and compared with two other models: a perturbative QCD based analysis and a quark diquark model. The $u$ and $d$ quarks inside the $Λ$ are predicted to be positively polarized at large Bjorken variable $x$ in the new scenario. By using an approximate relation connecting the quark fragmentation functions with the quark distributions, the hadron polarizations of the octet baryons in $e^+e^-$-annihilation, polarized charged lepton deep inelastic scattering (DIS) processes, and neutrino (antineutrino) DIS processes are predicted. The predictions for $Λ$ polarizations in several processes are compatible with the available data at large fragmentation momentum fraction $z$, and support the prediction of positively polarized $u$ and $d$ quarks inside the $Λ$ at large $x$. Predictions for Drell-Yan processes from $Σ^{\pm}$ and $Ξ^-$ beams on an isoscalar target are also given and discussed.

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