Theory

The theory advances will focus on the foundation of applying QCD factorization theorems, including higher-order perturbative corrections and all-order resummations, to DVCS and other relevant hard exclusive processes with the goal of extracting GPDs. The theory pillar will also provide the necessary ingredients to extract the GPDs from a global analysis. In addition, we will explore non-perturbative methods to provide important insight to describe the GPDs, which can lead to a physics motivated parameterization for the global analysis.

In particular, our research will focus on the following subjects:

(1) Theory Advances in Deeply Virtual Compton Scattering

(2) Gluon Tomography with Exclusive Heavy Quarkonium Production

(3) New Processes and Observables for Probing GPDs

(4) Non-perturbative Methods for GPDs and Hadron Tomography

(5) Beyond GPDs: Wigner Distributions

The milestones from the theory working group are in three main categories: 1) application of perturbative QCD to hard exclusive processes; 2) non-perturbative methods, including and the Covariant Parton Model; and 3) processes at small-x.

Year 1: Analyze factorization for exclusive quarkonia production at leading power for all regions using SCET and NRQCD, including the large and small $Q^2$ regions and quarkonia production at threshold

Apply the light-front Hamiltonian method to compute the GPDs, explore the nucleon spin/mass sum rule, and help to unveil the parton correlation due to strong interaction non-perturbative physics

Year 2: Make quantitative connection of the GPD factorization formalism to the CGC/color-dipole formalism for various exclusive processes

Apply the Covariant Parton Model to the GPDs of quark and gluons, eventually the parton Wigner distributions

Year 3: Use SCET to investigate factorization at subleading power in DVCS, including hadron mass corrections and the factorization and resummation of potential endpoint singularities

Year 4: Perform large-$N_c$ analysis of hard exclusive pion production with $N \rightarrow \Delta$ transitions and a combined chiral $\times 1/N_c$ analysis of nucleon energy-momentum tensor form factors

Quantitative study of hard diffractive dijet and di-hadron production at future EIC and explore novel processes to probe the quark/gluon Wigner distribution in the valence and moderate $x$ region

Year 5: Study relativistic corrections and other subleading effects in heavy quarkonia production for cases where such corrections are likely to be important

Recent Highlights:

Colloquium: Gravitational Form Factors of the Proton

V. D. Burkert, L. Elouadrhiri, F. X. Girod, C. Lorce, P. Schweitzer, and P. E. Shanahan, e-Print: 2303.08347 [hep-ph], to appear in Review of Modern Physics, 2023

The matrix elements of the energy-momentum tensor are the key to understanding how the properties and structure of particles like the proton arise from their quark and gluon constituents. These matrix elements are described in terms of gravitational form factors, which encode such fundamental information as the proton’s mass, spin and the less well-known but equally fundamental D-term. Moreover, the form factor associated with the D-term is related to the stress tensor and internal forces in the proton, providing access to its `mechanical properties’, which has emerged as a vibrant research field in its own right. This article presents the immense progress in the physics of gravitational form factors including the mass and spin decompositions of the nucleon and its mechanical properties. The authors review at an accessible colloquium-style level the advances in theory, experiments, first-principle lattice QCD studies, model calculations and the interpretation of these fundamental quantities.