Working Group Convenors: Brenna Flaugher (Fermilab), Ed Kinney (Colorado), Paul Mackenzie (Fermilab), George Sterman (Stony Brook)
This group should consider the full range of topics associated with the strong interactions, including critical tests of Quantum Chromodynamics, the developing area of hadronic physics including our understanding of hadron (particularly nucleon) structure, the fundamental parameters of QCD including the strong coupling constant and the quark masses, the ramifications of the richness of QCD under unusual conditions, and QCD as a tool for calculations and measurements of cross sections and decay rates.
An important responsibility of this group is to interact with the other working groups on common problems. QCD has a strong influence on almost all measurements in particle physics, via the scattering cross sections and backgrounds at hadron colliders, fragmentation in electron-positron colliders and weak or strong matrix elements in hadron decays. A solid understanding of the QCD issues underlies many measurements (and discoveries) in HEP.
Charge:
Status and prospects. Provide a compact summary of the current status of QCD, catalogue the new information on QCD that may become available in the next decade, either through experimental measurement or improved theoretical techniques, and--after consultation with the other physics working groups--report on the interrelation of QCD with other topics in particle physics. Assess the current state of our knowledge of the quark masses and the strong coupling constant. What issues surround the precise definition and meaning of quark masses? What are the limitations to current knowledge, and how might they be overcome? How do uncertainties in the QCD parameters propagate into predictions for observables? Survey the range of experimental studies of QCD and ask which important experiments are not yet being undertaken, and what kinds of instruments will be needed to make them happen.
The technology of perturbative QCD. Survey the current state of the art in making reliable perturbative calculations in QCD--not just at very high energies, but in all the domains in which QCD is applied. What are the points at which current methods encounter unresolved issues? What are the prospects for major advances over the coming decade? What calculations will be required by the coming generation of experiments? How can we ensure that the needed theoretical work is done?
Nonperturbative methods. Survey the current state of the art in making reliable nonperturbative calculations in QCD--by lattice gauge theory, sum rules, and other approaches. What are the points at which current methods encounter unresolved issues? What are the prospects for major advances over the coming decade? What calculations will be required by the coming generation of experiments? How can we ensure that the needed theoretical work is done?
Confinement and the hadron spectrum. How close have we come to a quantitative understanding of the hadron spectrum through lattice QCD? What are the prospects for a complete solution (including dynamical fermions) over the next decade, and what developments are required to make that happen? What insights into the mechanism of confinement come from developments in string theory and supersymmetric gauge theories, and what do they suggest for investigations (on the lattice, or by other methods) of theories other than four-dimensional QCD that might yield important lessons?
Hadron structure.
Static properties. Briefly summarize what is known about the static properties of the nucleon and other hadrons, and discuss the areas in which improvements are needed. In consultation with the experimental working groups, consider the kinds of measurements (by improvements in traditional methods, using intense neutrino beams, etc.) that could yield the desired information.
Parton distribution functions. Make a critical assessment
of the current crop of parton distribution functions, with attention
to how well they reproduce the data from which they are extracted,
how precisely they respect important theoretical constraints,
and how well they serve the needs of their users. Evaluate the
newly available parton distributions with uncertainties, and characterize
what would be an ideal set of parton distribution functions. What
are the current theoretical and experimental limitations on the
reliability of parton distribution functions?
On a related topic, consider what is currently known about fragmentation
functions, and what needs to be known for applications that
will be important over the coming decade.
Partons and the structure of hadrons. What progress can we expect in relating the parton degrees of freedom in the infinite momentum frame to the structure of hadrons in the rest frame? What are the prospects for developing quantitative tools and physical pictures to make this link?
Hadronic physics. The study of strongly interacting matter is an area of fruitful interaction between nuclear and particle physics, and many important questions involve experimental results and theoretical tools from both disciplines. The QCD working group should report on the state of hadronic physics, considering a few key issues (such as chiral symmetry breaking and the development of sound models and approximations, particularly those based on effective field theory, to QCD) to give form to the discussion. What role can high-energy experiments play in advancing our understanding of hadronic physics? The group should coordinate with the working group on Flavor Physics.
Spin. What is the value of spin observables, and of polarized beams and targets, in probing the implications of QCD and in looking for new phenomena?
Diffraction. What are the important issues in diffractive physics that must be addressed by theory and experiment? Are there special situations in which diffractive phenomena can be an effective tool in the search for new physics?
Compositeness. The idealization that quarks and leptons are elementary is one of the foundations of the standard model. What are the prospects for finding, or setting limits on, a compositeness scale over the next decade and beyond, in all the instruments we might contemplate? Examine theoretical scenarios for composite quarks and leptons (in consultation with the Scales beyond 1 TeV working group). What special considerations might present themselves for the top quark, or for the third generation?
The richness of QCD. Explore the novel phase structure of QCD under unusual conditions, including the prospects for observing and understanding the quark-gluon plasma and the consequences of phenomena such as color superconductivity. What are the implications of heavy-ion experiments for our understanding of QCD? What other experimental approaches might yield similar, or complementary, information? What lessons can we expect for the quark-hadron phase transition and other phenomena in the early universe?
Background information should be developed, to the extent possible, before the beginning of Snowmass 2001. This activity should be coordinated with the convenors of the instrument-oriented physics groups. During Snowmass, many of the specific experimental questions can be addressed in the instrument-oriented physics sessions, reserving the QCD and Strong Interactions sessions for other issues, comparisons, synthesis, and discussion.
Organizing Committee Contacts: Heidi Schellman