March 6 - May 26, 2006
This program will bring together for an extended period theorists and experimentalists interested in the following five areas to deepen each others' appreciation of the issues involved, to coordinate effort, and to brainstorm new approaches:
Beyond the obvious challenge of quantitative comparison with experiment, high energy probes of hadron structure raise fundamental questions that can be clarified by lattice exploration, such as the origin of the nucleon spin, how quark and gluon substructure gives rise to scaling observed in form factors, and the transverse structure of the nucleon light cone wave function. Similarly, spectroscopy poses questions such as the role of diquark degrees of freedom in the baryon spectrum, including pentaquarks, and the role of flux tubes and their excitations.
In addition to the conventional spectroscopy of isolated hadrons, there is also a new spectroscopy in the quark-gluon plasma at high T > Tc. Recent lattice results indicate that the J/y survives until about T=(2-3)Tc, giving rise to a new picture of a strongly coupled Quark-Gluon plasma, including colored (qq,qg,gg) states, which are responsible for a large fraction of the pressure and entropy.
One can also explore hadron phenomenology based on topological objects in QCD. In the past, many aspects of the instanton liquid phenomenology have been observed in lattice calculations. One can similarly investigate the role of topological objects at high T > Tc, where, for example, instantons, monopoles or perhaps other objects are still needed to make pions massless as T g Tc.
Participants with expertise in these and related areas of phenomenology will play an essential role in exploring new ways to use lattice QCD to obtain new insight into these issues.
Chiral Perturbation Theory
Chiral perturbation theory is an essential tool for lattice QCD. Since the cost of lattice calculations in a box sufficiently large to contain a pion grows as mp-9, practical calculations rely on extrapolations based on partially quenched chiral perturbation theory to obtain physical results at the physical pion mass. In addition, chiral perturbation theory provides quantitative corrections for the effects of finite volume and for calculations that use a hybrid combination of staggered sea quarks for economic reasons and chiral valence quarks for essential physics reasons.
Since the technologies for large scale lattice calculations and for detailed chiral perturbation calculations are each so different and so demanding, the same theorists generally do not pursue both aspects. Hence, it will be valuable to get the practitioners of both aspects of lattice calculations together for an extended period.
Renormalization in Lattice Field Theory
Calculation of physical observables in lattice field theory requires extensive calculation of renormalization factors and matching of lattice regularization to some continuum regularization scheme used to analyze experimental data, such as . In addition, since some renormalization factors can be calculated nonperturbatively by numerical lattice calculations, it is valuable to plan a thorough program of cross-checks of perturbative and non-perturbative renormalization. Again, the technology of perturbative renormalization is so demanding that these calculations are done by different theorists than those who undertake large scale lattice calculations or chiral perturbation theory, so key members of this community will be an important component of the program.
Lattice Calculation of Hadron Structure
Theorists directly involved in large scale lattice calculations will benefit greatly from extended interaction with phenomenologists who are interested in how lattice QCD can contribute to understanding hadron structure and testing phenomenological models and with theorists interested in chiral perturbation theory and renormalization. In addition, there are many technical aspects of lattice calculations that will be beneficial to discuss. Finally, it will be valuable to brainstorm new approaches to some of the salient unsolved problems in lattice QCD, such as measuring gluon observables, calculating high moments of parton distributions, calculating form factors at high momentum transfer, and the fermion sign problem.
Experiments crucial to Hadron Structure and Spectroscopy
Given current and planned experiments at Bates, JLab and RHIC, such as measuring electromagnetic and strangeness form factors, generalized parton distributions, the nucleon spin structure, and nucleon-delta transition form factors; searching for pentaquarks and other exotics; and exploring bound states in the quark-gluon plasma, lattice QCD has potential for high impact on nuclear physics experiments. Hence, an essential component of this program will be visits and seminars by key experimentalists to explore how lattice calculations can most effectively complement and guide experiment.