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Xiangdong Ji

Publications and source records attributed to Xiangdong Ji.

At least 289 records · Page 16Linked to original sources
arXiv1997

Deeply Virtual Compton Scattering

We study in QCD the physics of deeply-virtual Compton scattering (DVCS)---the virtual Compton process in the large s and small t kinematic region. We show that DVCS can probe a new type of off-forward parton distributions. We derive an Altarelli-Parisi type of evolution equations for these distributions. We also derive their sum rules in terms of nucleon form-factors of the twist-two quark and gluon operators. In particular, we find that the second sum rule is related to fractions of the nucleon spin carried separately by quarks and gluons. We estimate the cross section for DVCS and compare it with the accompanying Bethe-Heitler process at CEBAF and HERMES kinematics.

hep-ph↗
arXiv1997

Spin-Dependent Twist-Four Matrix Elements from g_1 Data in the Resonance Region

Matrix elements of spin-dependent twist-four operators are extracted from recent data on the spin-dependent g_1 structure function of the proton and deuteron in the resonance region. We emphasize the need to include the elastic contributions to the first moments of the structure functions at Q^2 < 2 GeV^2. The coefficients of the 1/Q^2 corrections to the Ellis-Jaffe sum rules are found to be 0.04 \pm 0.02 and 0.03 \pm 0.04 GeV^2 for the proton and neutron, respectively.

hep-ph↗
arXiv1997

A Study of Off-Forward Parton Distributions

An extensive theoretical analysis of off-forward parton distributions (OFPDs) is presented. The OFPDs and the form factors of the quark energy-momentum tensor are estimated at a low-energy scale using a bag model. Relations among the second moments of OFPDs, the form factors, and the fraction of the nucleon spin carried by quarks are discussed.

hep-ph↗
arXiv1997

How Much of the Nucleon Spin is Carried by Glue?

We estimate in the QCD sum rule approach the amount of the nucleon spin carried by the gluon angular momentum: the sum of the gluon spin and orbital angular momenta. The result indicates that gluons contribute at least one half of the nucleon spin at scale of 1 GeV^2.

hep-ph↗
arXiv1996

Chiral Multiplets of Hadron Currents

Using group theoretical methods, we enumerate possible chiral representations in which hadron interpolating currents can be classified. We give simple examples of currents in each representation, some of which are well known. The classification enables one to find relations among current vacuum correlators in a chirally and/or axial U(1) symmetric phase of QCD in a straightforward way. Besides recovering many well-known relations among two-point correlation functions, a number of novel relations are found.

hep-ph↗
arXiv1996

Hunting for the Remaining Spin in the Nucleon

This talk consists of four parts. In part one, I give an elementary discussion on constructing a Lorentz-invariant spin sum rule for the nucleon. In part two, I discuss a gauge-dependent spin sum rule, explore its relation with the polarized gluon distribution, and introduce the complete evolution equation for the spin structure. In part three, I consider a gauge-invariant spin sum rule and the related evolution equation. The solution of the equation motivates the possibility that half of the nucleon spin may be carried by gluons at low energy scales. In the final part, I discuss deeply-virtual Compton scattering as a possible way to measure the canonical orbital angular momentum of quarks in the nucleon.

hep-ph↗
arXiv1996

Determination of flavor asymmetry for $Σ^{\pm}$ by the Drell-Yan process

Flavor asymmetries for the valence and sea quarks of the $Σ^{\pm}$ can be obtained from Drell-Yan experiments using charged hyperon beams on proton and deuteron targets. A large, measurable difference in sea quark asymmetries is predicted between SU(3) and pseudoscalar meson models. The latter predict that in $Σ^{+}$, $\bar{u}/\bar{d} \leq 1/2$, whereas the former predict $\bar{u}/\bar{d} \approx 4/3$. Estimates of valence quark asymmetries based on quark models also show large deviations from SU(3) predictions, which should be measurable.

hep-ph↗
arXiv1996

QCD Sum Rule Calculation for the Tensor Charge of the Nucleon

The nucleon's tensor charges (isovector $g^v_T=δu-δd$ and isoscalar $g^s_T=δu+δd$) are calculated using the QCD sum rules in the presence of an external tensor field. In addition to the standard quark and gluon condensates, new condensates described by vacuum susceptibilities are induced by the external field. The latter contributions to $g^v_T$ and $g^s_T$ are estimated to be small. After deriving some simplifying formulas, a detailed sum rule analysis yields $g^v_T=1.29 \pm 0.51$ and $g^s_T = 1.37 \pm 0.55$, or $δu=1.33 \pm 0.53$ and $δd = 0.04 \pm 0.02$ at the scale of 1 GeV$^2$.

hep-ph↗
arXiv1996

Gauge-Invariant Decomposition of Nucleon Spin and Its Spin-Off

I introduce a gauge invariant decomposition of the nucleon spin into quark helicity, quark orbital, and gluon contributions. The total quark (and hence the quark orbital) contribution is shown to be measurable through virtual Compton scattering in a special kinematic region where single quark scattering dominates. This deeply-virtual Compton scattering (DVCS) has much potential to unravel the quark and gluon structure of the nucleon.

hep-ph↗
arXiv1995

Spin Structure Functions of the Nucleon

I begin with a general discussion about importance of constructing a picture of the nucleon in terms of QCD degrees of freedom, emphasizing the role of spin structure functions. I then give a short overview on the theoretical and experimental status of the spin structure of the nucleon. Following that, I mention several upcoming experiments to measure the flavor and sea structure in polarized quark distributions and the polarized gluon distribution $Δg(x)$. Finally, I discuss other spin-related physics, such as the polarizabilities of gluon fields $χ_B $ and $χ_E$, the quark transversity distribution $h_1(x)$, and the spin structure functions $G_1$ and $G_2$ at low $Q^2$.

hep-ph↗
arXiv1995

Locality of the Strange Sea in the Nucleon

We introduce the concept of ``locality" for the strange sea in the nucleon, which measures proximity of the strange and anti-strange quarks in the momentum and coordinate spaces. The CCFR data for the strange and anti-strange distributions imply a ``local" strange sea in the momentum space, which is unexpected in QCD and is at variance with the simple meson-cloud model where the strangeness is generated from the virtual transition of the nucleon to a hyperon plus a kaon. We present a simple model to interpret the CCFR data and to correlate momentum and coordinate space locality, yielding an upper bound of 0.005 fm$^2$ on the strange radius. We also discuss significances of locality for other charge-conjugation-odd observables.

hep-ph↗
arXiv1995

Matching Perturbative and Non-Perturbative Physics With Power Accuracy in Heavy-Quark Effective Theory

I discuss a scheme to match perturbative and non-perturbative physics with power accuracy in the heavy-quark effective theory. I elaborate on two important aspects of the scheme: 1) a multi-loop subtraction of soft contributions from the perturbation series in the pole mass, 2) a perturbative regularization of the linearly-divergent heavy-quark self-energy in the lattice formulation of the heavy-quark effective theory.

hep-ph↗
arXiv1995

Exact Matching Condition for Matrix Elements in Lattice and $\overline{MS}$ Schemes

The exact matching condition is given for hadron matrix elements calculated in any two different schemes, in particular, in the lattice and dimensional regularization, (modified) minimal subtraction $\overline{\rm MS}$ schemes. The result provides insight into and permits to go beyond Lepage and Mackenzie's mean field theory of removing tadpole contributions in lattice operators.

hep-lat↗
arXiv1995

Gluon Condensate from Lattice QCD

After making some critical comments about the traditional method of extracting the gluon condensate from lattice QCD data, I present an alternative analysis. The result is more than a factor of five larger than the phenomenological value. Two closely related subjects, the effects of the infrared renormalons on the extraction and the Lepage and Mackenzie improvement on a lattice perturbation series, are also discussed.

hep-ph↗
arXiv1995

WILSON'S EXPANSION WITH POWER ACCURACY

Because of the infrared renormalons, it is difficult to get power accuracy in the traditional approach to the Wilson's operator product expansion. Based on a new perturbative renormalization scheme for power-divergent operators, I propose a practical version of the OPE that allows to calculate power corrections to desired accuracy. The method is applied to the expansion of the vector-current correlation function in QCD vacuum, in which field theoretical status of the gluon condensate is discussed.

hep-ph↗
arXiv1995

PHYSICS OF $Q^2$-DEPENDENCE IN THE NUCLEON'S $G_1(x,Q^2)$ STRUCTURE FUNCTION SUM RULE

I discuss in this talk the physics of the $Q^2$ dependence of the $G_1(x,Q^2)$ structure function sum rule. For $Q^2>3$ GeV$^2$, the $Q^2$ variation is controlled by pure QCD radiative corrections. For $0.5<Q^2<3$ GeV$^2$, the twist-four contribution becomes significant, but stays perturbative. For $Q^2$ below $\sim 0.05$, the sum rule is determined by low-energy theorems. The rapid change of the sum rule between 0.05 and 0.5 GeV$^2$ signals the transition between parton and hadron degrees of freedom.

hep-ph↗
arXiv1995

HADRON SUBSTRUCTURE PROBED WITH HADRON BEAMS

In this talk, I focus on the quark-gluon structure of hadrons probed using high-energy hadron beams. I start with a brief review on recent major achievements in measuring parton distributions of the nucleon, pion, and kaon, with hadron facilities at CERN and FNAL\@. Then I discuss a number of outstanding questions and interesting physics issues in the field, and point out their intellectual impact on nuclear physics as a whole. While advocating a continuing exploitation of hadron beams at CERN and FNAL, I strongly emphasize the role of a polarized RHIC, where a major nuclear physics program on the structure of hadrons can thrive.

hep-ph↗