Theoretical Nuclear and Particle Physics Group
The research interests of the Nuclear and Particle theory group cover a wide range of problems in the physics of the strong and electroweak interactions and in the many- body theory of strongly interacting systems. Major emphasis is put on studies of structure and dynamics of the baryons and few-nucleon systems and their response to electroweak probes; heavy quark physics; perturbative and non-perturbative aspects of quantum-chromodynamics.
Research on baryon structure revolves around measurements that have been made elsewhere, and which will eventually be reproduced at Jefferson Lab with higher precision. Decay rates of baryon resonances to two-body final states have been calculated in a relatively simple model, which reproduces reasonably well the masses of baryons and mesons. Attempting to reproduce the couplings of the baryons in this model provides us with the opportunity to understand what its shortcomings and strengths are, thus leading to a better understanding of the strong interactions.
Studies of few-body systems are aimed at producing a quantitative understanding of the structure and dynamics of light nuclei A£8 based on realistic interaction models, and electroweak current operators constructed consistently with these interactions. Exact Monte Carlo methods suitable for the nuclear many-body problem are being developed and applied to investigate a wide variety of properties depending on the bound and continuum spectra of the nuclei, such as, for example, very low-energy electroweak capture reactions, or the response to lepton an hadronic probes in the quasi-elastic regime.
Research is also being conducted on the relativistic description of strongly interacting many-body systems using two approaches: the first is based on a covariant dynamical equation and a one-boson-exchange interaction constrained by fitting deuteron properties and nucleon-nucleon elastic scattering data at low and moderate energies. Presently this model is being applied to a covariant, gauge invariant calculation of elastic and inelastic electron deuteron scattering.
The second approach is based on the Bakamijan-Thomas and Foldy et al. formulation of a relativistic many-body theory of particles interacting via potentials. In this method, the nuclear Hamiltonian is written as the sum of square-root kinetic energy operators, two- and three-nucleon rest-frame interactions, and their boost corrections. The latter are determined from the rest-frame interactions by the requirements of relativistic covariance dictated by the commutation relations of the Poincaré group. Consistent expressions for nuclear electromagnetic current operator have also been derived. Quantum Monte Carlo methods are being developed to calculate, with the relativistic Hamiltonian and current, properties of light nuclei.
It has been recently realized that hadrons containing at least one heavy quark exhibit symmetries beyond those usually associated with quantum chromodynamics. Current research is focused on using these additional symmetries to glean information on the strong interaction, in a model-independent way. In particular, weak decays of such hadrons are being studied to understand the effects of the interplay between the weak and strong interactions.
Studies of hadronic form factors and other elastic processes provide important information about the internal structure of hadrons and the dynamics of quarks and gluons. Theoretical studies based on QCD sum rules aim at a unified description of perturbative and nonperturbative aspects of quantum chromodynamics. They provide relations between experimentally measurable properties, such as form factors, and condensates, the functions describing the structure of the QCD vacuum. A new trend within the QCD sum rule approach is to incorporate nonlocal condensates, universal functions describing the momentum distribution of quarks and gluons in vacuum.
Another direction of study includes a QCD analysis of virtual Compton scattering and closely related polarization effects in electron-nucleon scattering revealing the spin structure of the nucleons. QCD sum rules and quark-hadron duality are applied to calculate nucleon matrix element of the gluon spin tensor which describes both the gluon spin and gluon orbital momentum in the nucleon.