At the level of quark-gluon degrees of freedom, the Dyson-Schwinger equations
(DSEs) provide a Poincaré covariant, nonperturbative method for studying QCD
in the continuum. A hallmark of present-day DSE applications in hadron physics
is the existence of a symmetry preserving truncation that enables the
simultaneous exploration of phenomena such as: confinement, dynamical chiral
symmetry breaking, and bound state structure and interactions. In addition,
the DSEs provide a generating tool for perturbation theory and hence yield
model-independent results for the ultraviolet behavior of strong interaction
observables. This means that model studies facilitate the use of physical
processes to constrain the long-range behavior of the interaction between
light-quarks in QCD, which is poorly understood and whose elucidation is a
key goal of modern experimental programs. The last year saw numerous
noteworthy applications and successes. For example, we presented arguments
which support a view that chiral perturbation theory is inapplicable for
pion-like meson masses greater than m_{0-} ~ 0.45 GeV; that a
unique signal for the restoration of chiral symmetry via excitation of
mesons is equality between the pole residues for 0^{–+}(nS) and
0^{++}(nS) states when n, the radial quantum number, is large;
that chiral symmetry and its dynamical breakdown in QCD even place
constraints on properties of mesons composed of two heavy-quarks; and we
also provided a prediction for the ratio of neutron electric and magnetic
form factors.

At the level of meson and baryon degrees of freedom, we continue our effort to develop a dynamical coupled-channels model for use in analyzing the very extensive set of data for electromagnetic meson production reactions. A primary objective is the development of an interpretation for this data in terms of the quark-gluon substructure of nucleon resonances (N*). We aim to draw the connection between this data and the predictions made by QCD-based hadron models and numerical simulations of lattice-regularized QCD. In the past year we completed a major stage of this project by determining the hadronic interactions in the model by fitting pion-nucleon scattering data up to 2 GeV. We predicted the effect of a meson cloud on the form factors describing the transition to all known low-lying nucleon resonances. Methods for determining the resonance parameters from the partial-wave amplitudes were developed. We also made progress on projects focused on η and ω photoproduction, which aim at discovering highly excited resonances with masses close to 2 GeV. In addition, we continued to play a leading role in directing operations of the Excited Baryon Analysis Center (EBAC) at Jefferson Laboratory.

Relativistic quantum dynamics requires a unitary representation of space-time symmetries (Poincaré group) and localization of states, such that states localized in relatively space-like regions are causally independent. We have recently focused on the application and elucidation of complementary mathematical representations of hadron phenomena, and on a consistent treatment of medium energy electromagnetic few-body processes.

Nuclear astrophysics

Nuclear forces and nuclear systems

Theoretical Physics Research