Proton Tomography Through Deeply Virtual Compton Scattering

In this prize talk, I recall some of the history surrounding the discovery of deeply virtual Compton scattering, and explain why it is an exciting experimental tool to obtain novel tomographic pictures of the nucleons at Jefferson Lab 12 GeV facility and the planned Electron-Ion Collider in the United States.Comment: 17 pages, 13 figures; Feshbach Prize in theoretical nuclear physics talk at APS April Meeting, Salt Lake City, April 18, 201

Comment on "Spin and Orbital Angular Momentum in Gauge Theories" by X. S. Chen et. al. (PRL100, 232002 (2008))

The individual parts of the total angular momentum operator in interacting theories cannot satisfy the canonical angular momentum commutation rule, including those proposed in the above paper. Furthermore, the operators in the new proposal a) are non-local in general gauge, b) do not have proper Lorentz transformation properties, and c) do not have any known physical measurements.Comment: 2 pages, no figur

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 $\Delta g(x)$. Finally, I discuss other spin-related physics, such as the polarizabilities of gluon fields $\chi_B$ and $\chi_E$, the quark transversity distribution $h_1(x)$, and the spin structure functions $G_1$ and $G_2$ at low $Q^2$.Comment: 11 pages in REVTeX. Plenary talk presented at Baryon '9

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

Purely-Nonperturbative Composite Operators and Parton Distributions

A class of purely-nonperturbative (PNP) composite operators is defined in Quantum Chromodynamics, which is perturbative scheme and scale independent and are useful to describe the internal structure of a strong interacting system. The operator product expansion in terms of the new operators cleanly separates the perturbative and nonperturbative physics without introducing any factorization scale. A number of PNP observables of the nucleon is briefly discussed including the PNP parton distributions. In particular, the fraction of the nucleon momentum carried by the purely-nonperturbative gluons is found to be around 16%.Comment: 8 pages, latex, no figure

Parton Physics on Euclidean Lattice

I show that the parton physics related to correlations of quarks and gluons on the light-cone can be studied through the matrix elements of frame-dependent, equal-time correlators in the large momentum limit. This observation allows practical calculations of parton properties on an Euclidean lattice. As an example, I demonstrate how to recover the leading-twist quark distribution by boosting an equal-time correlator to a large momentum.Comment: 8 pages, 1 figur

Parton Physics from Large-Momentum Effective Field Theory

Parton physics, when formulated as light-front correlations, are difficult to study non-perturbatively, despite the promise of light-front quantization. Recently an alternative approach to partons have been proposed by re-visiting original Feynman picture of a hadron moving at asymptotically large momentum. Here I formulate the approach in the language of an effective field theory for a large hadron momentum $P$ in lattice QCD, LaMET for short. I show that using this new effective theory, parton properties, including light-front parton wave functions, can be extracted from lattice observables in a systematic expansion of $1/P$, much like that the parton distributions can be extracted from the hard scattering data at momentum scales of a few GeV.Comment: 10 pages, 1 fig. Science China, 201

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.Comment: 8 pages in REVTE

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.Comment: 13 pages, ReVTeX, 8 figures (available by hardcopy only) Talk presented at the NSAC/DNP town meeting on ``Nuclear Physics with Intermediate and High-Energy Hadron Probes,'' Argonne, January 29--30, 199

Threshold Resummation for Drell-Yan Process in Soft-Collinear Effective Theory

We consider Drell-Yan process in the threshold region $z\to 1$ where large logarithms appear due to soft-gluon radiations. We present a soft-collinear effective theory approach to re-sum these Sudakov-type logarithms following an earlier treatment of deep inelastic scattering, and the result is consistent with that obtained earlier through perturbative QCD factorizationComment: A version to appear in Phyr. Rev.