Nucleon Structure Through Electron and Photon Scattering

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The goal of this research is to test the validity of accepted theories of the strong force, using experiments that push the limits of current knowledge. While the theories are believed to be the correct underlying theory of matter, precision experiments described in this proposal are designed to constrain the theoretical predictions. In particular, the so-called Standard Model provides the unification of electromagnetic and "weak" forces of nature, and one experiment is designed to measure the weak charge of the proton at a precision of a few percent. This experiment, called Q-weak, will be performed at Jefferson Lab and will clearly identify any necessary corrections to the Standard Model (or solidly confim it). In another experiment, also at Jefferson Lab, we are investigating a new kind of particle (sometimes called an "exotic" baryon) resonance that was predicted by group theory and global symmetries of the strong interaction. If this particle exists, then it is the first of its kind, called the pentaquark, and represents a marriage between the known 3-quark objects (like the proton) and quark-antiquark particles.Additional experiments are being carried out at the laser electron gamma source (LEGS) facility at Brookhaven National Lab. These experiments are also designed to test well-known principles of physics (gauge theories) that connects the "spin" of the proton to measurements of all scattered particles made from a photon beam hitting a proton or a neutron. This experiment requires the development of a novel solid HD (hydrogen deuteride) target that can be polarized to a high degree. The goal is to learn more about what causes the "spin" structure of the proton or neutron.