Nuclear and Particle Physics Theory
The Physics Department and the Institute of Nuclear and Particle Physics support some of the leading research groups in this basic area of physics. Faculty members are the spokesmen for experiments that test fundamental aspects of nucleon and nuclear structure. These include experiments at the Stanford Linear Accelerator Center (SLAC) on the origin of the nucleon's spin, the details of the charge distribution of the neutron at Thomas Jefferson National Accelerator Facility (TJNAF), and a precision measurement of pion beta decay at the Paul Scherrer Institute (PSI). At SLAC the inelastic scattering of polarized electrons from polarized nucleon targets allows a detailed investigation of the spin structures of the nucleon. These measurements provide the best determination of how the quarks and gluons contribute to the fundamental spin of the nucleon. There is active research and development of high-power polarized targets, using high-field superconducting magnets, low-temperature refrigerators, and high-frequency microwaves. Electron paramagnetic resonance characterization of these targets is proceeding together with theoretical and computational modeling of local hyperfine interactions that contribute to dynamic nuclear polarization. At TJNAF an extensive series of experiments has been approved, including a measurement of the electric charge form factor of the neutron. The experimental measurements are complemented by strong theoretical support in the Department. This theoretical effort involves work in relativistic chiral quark models; spontaneous chiral symmetry breaking; quantum theories based on light-front formalism; and perturbative quantum chromodynamics (QCD) phenomenology, including studies of power corrections to the nucleon/nuclear structure functions, quark-hadron duality and low Bjorken x physics.
The measurement of both the electron and hadron in electromagnetic nuclear reactions will provide new information about possible modifications of nucleon structure within the nuclear medium. The additional ability to measure the spin of the recoiling hadron with a polarimeter allows a determination of nuclear dynamics not accessible by standard methods.
Experiments at TJNAF use resonant pion production from the nucleon to study the structure and inelastic response of this fundamental three-quark system. Electro-production of excited states of the nucleon constitutes a major part of the research program for the large acceptance detector (CLAS). This detector consists of a large toroidal superconducting magnet, with a variety of detectors to track and identify particles over a large solid angle. Polarized nucleon targets have been developed for use in this detector.
The study of basic symmetries and conservation laws can provide some of the most precise information on the dynamics of the two short-range interactions, the electroweak and the strong. A precision measurement of the pion beta decay rate is taking place at PSI in Switzerland. This measurement provides the best theoretical means to study the weak coupling between the up and down quarks.
A program of experiments at the Laser Electron Gamma Source (LEGS) at Brookhaven National Laboratory examines the non-perturbative regime of QCD, including the possible resonant distortion of the nucleon. This work is complemented by research at the newly-commissioned low energy polarized photon source at the Triangle Universities Nuclear Laboratory (TUNL).