Daniel Boer
KVI, University of Groningen

Markus Diehl

Richard Milner

Raju Venugopalan
Brookhaven National Laboratory

Werner Vogelsang
University of Tübingen

Program Coordinator:
Inge Dolan
(206) 685-4286

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Gluons and the quark sea at high energies: distributions, polarization, tomography

September 13 to November 19, 2010

Report from the INT program "Gluons and the quark sea at high energies:
distributions, polarization, tomography"


This INT program will address open questions about the dynamics of gluons and sea quarks in the nucleon and in nuclei. Answers to these questions are crucial for a deeper understanding of hadron and nuclear structure in QCD at high energies. Many of them are relevant for understanding QCD final states at the LHC, which often provide a background for physics beyond the standard model. The topics addressed in this program have important ramifications for understanding the matter produced in heavy-ion collisions at RHIC and the LHC.

These issues motivate arguments for a Electron Ion Collider (EIC) that will provide a precise imaging of gluons and sea quarks in hadrons and explore the physics of strong color fields in nuclei. An EIC was endorsed at the last US Nuclear Science Advisory Committee Long Range Plan as the next major project in high-energy nuclear physics. The community must now work out the physics case for such a facility, showing on one side that its projected parameters and performance will be adequate for its physics goals, and on the other side that we have the theoretical tools to analyze the envisaged measurements. It is also important to situate the proposal with respect to other planned or proposed facilities.

We plan to organize the program activities along the following timeline:

week dates topics
1 13–17 Sept Workshop on "Perturbative and Non-Perturbative Aspects of QCD at Collider Energies"
2 20–24 Sept open conceptual issues: factorization and universality, spin and flavor structure, distributions and correlations
3–5 27 Sept –15 Oct small x, saturation, diffraction, nuclear effects; connections to p+A and A+A physics; fragmentation/hadronization in vacuum and in medium
Agenda for week 3
Agenda for week 4
Agenda for week 5
6–7 18–29 Oct parton densities (unpolarized and polarized), fragmentation functions, electroweak physics
Agenda for week 6
Agenda for week 7
8–9 1–12 Nov longitudinal and transverse nucleon structure; spin and orbital effects (GPDs, TMDs, and all that)
Agenda for week 8
Agenda for week 9
10 16–19 Nov Workshop on "The Science Case for an EIC"
Agenda for week 10

To facilitate the organization of the program there are convenors for the following subtopics:

Key issues on which we hope to achieve progress are:

  • Factorization is the cornerstone for the application of QCD at high energies. There are various incarnations of this concept, such as standard collinear factorization, dipole or kt-factorization at small x, kt-factorization in spin physics, and the more recent development of fully unintegrated parton distributions. While important progress has been made in each of these areas, their scope and interrelations are often not fully understood.

    The program aims to bring together experts from the different communities working on these issues and thus to help elucidate the open questions of the field, from foundations to practical application.

  • An outstanding open question in high-energy QCD is where a non-linear regime of strong color fields sets in and how it can be described quantitatively. In the infinite momentum frame, this corresponds to saturating gluon densities. Partons in the saturation regime can be described as a Color Glass Condensate. Results from HERA and RHIC have provided hints of saturation, and further insight will be provided by ongoing measurements at RHIC and in pp and AA collisions at the LHC. However, given the complexity of hadronic collisions, measurements at a high-energy and high-luminosity ep and eA collider will prove crucial in understanding the properties of this novel regime of QCD.

    The program should help consolidate and refine our understanding of the HERA, RHIC, and LHC experiments, as well as identify key channels and observables at the EIC, from inclusive measurements to diffractive or exclusive final states, which will formulate the corresponding accelerator and detector requirements.

  • While the last decade has brought major progress in the determination of parton distributions inside the proton, outstanding questions remain to be answered. These include the detailed flavor structure or the unpolarized sea (relevant for flavor sensitive new-physics processes at the LHC), the helicity carried by sea quarks and the gluon (central to the spin decomposition of the proton), and the distribution of gluons and sea quarks inside nuclei (indispensable for a quantitative analysis of heavy-ion collisions).

    In the program we aim to identify the needs for future ep and eA measurements in this area, to quantify their experimental feasibility, and to sharpen the theoretical tools required for extracting parton distributions from data.

  • A counterpart to the initial conditions of high-energy collisions, described by parton distributions or similar quantities, is the formation of final states through hadronization. Quantitative knowledge of fragmentation functions is often prerequisite for determining parton densities. The energy loss of quarks and gluons in "cold matter" is both of intrinsic interest and crucial for quantifying our understanding of parton energy loss in the quark-gluon plasma. Investigating the energy loss of charm and bottom quarks at EIC may resolve puzzles revealed by RHIC results on heavy-quark energy loss in hot matter.

    The potential of future ep and eA measurements in this area remains to be studied in detail.

  • Several recent theoretical developments emphasize the importance of transverse degrees of freedom, both in momentum and in position space. Keywords are spatial tomography, and the detailed study of spin-orbit correlation and orbital angular momentum. Existing measurements are encouraging, but it is widely felt that a new collider facility is needed to go beyond the exploratory stage of this field.

    Key questions the community has to answer at this stage are "Which kind of measurements are feasible for a given accelerator and detector?" and "Exactly how can such measurements be turned into information about the physics governing hadrons and nuclei?"

  • Finally, measurements in the electroweak sector could significantly add to the physics case for a new facility. A clear view of what such measurements will require in terms of machine parameters will be of great value for the further planning process.