Studies of the Structure of the Nucleon

This award supports the study of a basic property of protons: how the charge of the proton is distributed within it. The motivation is that much of ordinary matter is made up of protons, but even simple properties like the charge distribution are hard to predict precisely, as they arise from complicated interactions of the quarks that make up the protons, rather than from the quarks themselves. The PIs and graduate students use electrons and muons (a heavier relative of electrons) to probe the proton, to measure its charge distribution and also to measure quantum mechanical corrections to determining the charge distribution. In addition to the direct scientific goals of the project, the experiments provide undergraduate and graduate students and other early career scientists with experience and training in working in international collaborations of modern scientific experiments, with state-of-the-art technology. This grant will support the continuation of the experimental nuclear physics research program of the intermediate energy experimental nuclear physics group at Rutgers University. The focus of the experimental activities is MUSE, a measurement of the scattering cross sections of both positively and negatively charged electrons and muons from a liquid hydrogen target, performed at the PiM1 beam line of the Paul Scherrer Institute in Switzerland. During this grant period the group will complete taking production data and work on analysis. The scattering cross sections are used to determine the proton form factors, from which the proton charge radius can be extracted. Comparing scattering of positively and negatively charged particles determines two-photon exchange corrections, which impact the radius determination. Comparing scattering of muons to electrons checks lepton universality, including the consistency of the proton radius when measured by the two species. In addition to the new measurements of MUSE, the group plans to take part in running-related Jefferson Lab experiments and will continue to be involved at a lower level in the analysis and publication of results of experiments run previously. The broader impact of the group has been primarily in the training of undergraduates, graduates, and postdocs. Students and post-docs who have worked with the group have gone on to careers in a variety of areas, including medical physics, national security, and financial systems, in addition to continued work in fundamental physics research. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.