We propose a program of polarized electron scattering experiments, to use the weak neutral current and electromagnetic probes to study the flavor content and spin structure of the nucleon, and to conduct a search for new physics beyond the Standard Model. The experiments will be conducted primarily at the Thomas Jefferson National Accelerator Facility (Jefferson Lab). A series of experiments using parity violation in electron scattering at Jefferson Lab are focused on determining the elusive strange quark contribution to the nucleon ground state. These experiments include measurements in Hall A of the parity-violating asymmetry in forward-angle scattering from the proton, 4-He, and 208-Pb, at low momentum transfer. The G0 experiment in Hall C will measure parity-violating scattering from the proton at both forward and backward angles, and for a range of momentum transfers, and thus will definitively map out the strange quark vector form factors for the nucleon as a function of four-momentum transfer. A groundbreaking experiment has recently been approved at Jefferson Laboratory to measure the weak charge of the proton. The Qweak experiment will provide a precision test of the energy scale dependence of the weak mixing angle by precisely measuring the Weinberg angle at low momentum transfer and comparing to data from high-energy experiments. This will provide a uniquely sensitive probe of physics beyond the Standard Model. We are thus addressing possibly the single most important question in fundamental physics: how and where does the Standard Model break down? We have taken on an important component of the Qweak experiment, the Region III tracking chambers, the construction of which has been recently funded by an NSF Major Research Instrumentation grant. Another series of polarized electron scattering experiments at Jefferson Lab are focused on determining the spin structure of the nucleon using polarized 3-He targets. The GEN experiment will make the first measurement on the neutron electric form factor at large momentum transfers. This is particularly relevant given the recent controversial results for the electric form factor of the proton and new theoretical work on two-photon exchange mechanisms. In support of this program, research into promising new technology involving hybrid K-Rb cells is underway in the W&M polarized target lab. This physics program has a large educational component; eleven PhD graduate students are working on group-related activities, and support is requested from the NSF for five of these students. Each year we supervise an average of seven undergraduates in summer or senior-thesis research. Support for 1.5 post-doctoral positions is also requested from the NSF. The proximity of our group to Jefferson Lab, its strong ties to Jefferson Lab through affiliated faculty, and its ability to attract excellent William and Mary undergraduate students, ensure the success of the proposed research.
