Overview

The last decade has taken hadron physics into a new era, with the emergence of a comprehensive approach to the description of hadron and nuclear structure. This framework encodes our knowledge of hadrons and nuclei in the Wigner distributions of the fundamental constituents. From the Wigner distributions, a natural interpretation of measured observables is provided via construction of quantities known as generalized parton distributions (GPDs) and transverse momentum-dependent partons distributions (TMDs): GPDs are key to a spatial tomography of hadrons; and TMDs allow for their momentum tomography. A new generation of experiments – for example at Jefferson Lab with the recently completed 12 GeV upgrade – will soon provide a tremendous amount of new empirical information on nucleon and nuclear GPDs and TMDs.

This INT program will bring together experts on GPDs and TMDs, as well as key people in related fields, for extensive discussion and collaboration aimed at insuring the success of this ambitious new direction in hadron physics. The GPD and TMD program at Jefferson Lab, which represents over 50% of allocated running time for planned 12 GeV experiments, will be the primary focus of this program. However, related efforts at COMPASS, Fermilab, RHIC and e⁺e^- colliders such as BES will also feature.

These new experimental resources have been complemented by significant recent advances in theory. Contemporary theory topics related to the spatial and momentum tomography of hadrons and nuclei will be explored in this program, such as the QCD evolution and factorization of TMDs, hadronization, the physical interpretation of the various distribution functions, the phenomenological extraction of GPDs, TMDs and fragmentation functions from data, together with their calculation within lattice and continuum QCD approaches. Emphasis will be placed on supporting the experimental program and guiding it to domains of maximum discovery potential.

This program aims to identify the key pathways that will lead to a deeper understanding of the strong interaction and articulate the critical theoretical and experimental steps that are necessary. This program will also serve as a focal point to discuss and consider aspects of the 2015 NSAC Long Range Plan, where the full operation of Jefferson Lab at 12 GeV is listed as a top priority and an Electron Ion Collider (EIC) is identified of the number one priority for new construction after FRIB.

Key Goals

This program will provide the opportunity to consider the current understanding of the multi-dimensional structure of hadrons and nuclei, to discuss the planned experimental programs at Jefferson Lab and elsewhere, identify avenues for improvement in these programs, and to pinpoint the key questions and measurements for the future. Key goals and outcomes include:

1. Refine the existing framework for the extraction of TMDs and fragmentation functions from data;

2. Formulate a clear program to measure the gluon TMDs from high to low x;

3. Develop a program in theory and experiment so that an accurate flavor decomposition of the nucleon GPDs and TMDs is realized;

4. Make progress in developing clear physical interpretations of the properties of TMDs, e.g., are TMDs related to orbital angular momentum;

5. Identify the GPDs and TMDs that can most readily highlight differences between the quark and gluon structure of the nucleon and nuclei;

6. Develop a comprehensive program to determine the nucleon and nuclear sea-quark GPDs and TMDs, utilizing all available facilities;

7. Identify the strengths, weaknesses, and avenues to improve the current theory predictions for GPDs, TMDs and fragmentation functions, and point the way to educated parametrizations for these functions that can be used to fit data.

Outline

The program will run for 5 weeks and consist of 2 workshops, one in the first week and the other in the final week of the program. The intervening weeks will have one or two seminars per-day, thereby providing time for discussion and collaboration. The outline of the program, with the general focus of each week is: