Studies of the Structure of the Nucleon

This award to the Rutgers University Intermediate Energy Nuclear Physics group enables Profs. Gilman and Ransome and their graduate and undergraduate students to study the structure of the nucleon. The focus is on the shape of the proton - how the proton's charge is distributed. It includes the study of a high-profile puzzle: whether the proton size is the same when measured with two types of particles, electrons and muons. Muons are particles with the same characteristics as electrons but with more mass. In addition to the direct scientific goals of the project, the experiments provide undergraduate and graduate students and other early career scientists 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 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 continue taking production data and begin the analysis phase. The scattering cross sections are used to determine the proton form factors, from which the proton charge radius can be extracted. The group will compare scattering of positively and negatively charged particles to determine two-photon exchange corrections, and compare scattering of muons to electrons to determine if the proton radius is the same when determined by the two particles. In addition to the new measurements of MUSE, the group 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.