The Idaho State University medium-energy nuclear physics group will conduct several key measurements at Jefferson Lab using high-energy electrons to precisely probe the underlying substructure of nucleons, where a nucleon is a proton or neutron, and they will make fundamental measurements on how force and matter interrelate. The group seeks to understand the nature of the strong and weak nuclear forces and how these forces govern the nucleon's constituents (quarks). Due to these forces, protons and neutrons are arranged slightly differently inside large nuclei as evidenced by neutrons tending to occupy the outer layers of heavy nuclei, with the protons being closer to the center. The group will lead an experiment to probe this outer neutron skin by making use of telltale signatures of the weak force in scattering electrons off complex nuclei. This issue further bears upon the structure of neutron stars. The group will also determine the precise and accurate diameter of the proton, where there are currently several conflicting measurements. This group has a strong tradition of bringing undergraduate and graduate students into their research program.
The CoPIs in this proposal are all co-spokespersons on experiments at Jefferson Lab to perform measurements on the underlying symmetry and structure of the quark and gluon components of the nucleon, the size of the proton, and the characteristics of the skin or surface of a nucleus. Dr. Tony Forest is continuing a measurement of the down quark fractional polarization in the nucleon to test predictions of pQCD via JLab experiment E12-06-109. Dr. Philip Cole is co-spokesperson on E12-09-003, an experiment that will measure the change of the dynamical quark mass as well as the nature of the nucleon structure and internal quark interactions as the probe's distance scale enters a regime that directly accesses quarks decoupled from the meson-baryon cloud. Dr. Cole is also a collaborator on the ELSA experiment BGO-OD, which will anchor N* measurements at the q2=0 point. Dr. Mahbub Khandaker will conduct a high-precision measurement of the proton charge radius in experiment E12-11-106 to resolve a current discrepancy between the proton radius extracted from muonic and electronic hydrogen experiments. Dr. Dustin McNulty is continuing his work using parity violation to precisely measure the neutron skin of lead and calcium to test nuclear model predictions of the difference between the radii of protons and neutrons in a heavy nucleus, E12-12-004.
