Organizers:
Daniel Boer
KVI, University of Groningen
D.Boer@rug.nl
Markus Diehl
DESY
markus.diehl@desy.de
Richard Milner
MIT
milner@mit.edu
Raju Venugopalan
Brookhaven National Laboratory
raju@quark.phy.bnl.gov
Werner Vogelsang
University of Tübingen
werner.vogelsang@uni-tuebingen.de
Program Coordinator: Inge Dolan
inge@phys.washington.edu
(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"
Agenda
|
2 | 20–24 Sept |
open conceptual issues: factorization and universality,
spin and flavor structure, distributions and correlations
Agenda |
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
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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.
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