Nuclear Physics Studies at Fermilab and PSI

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The main physics program supported by this grant is directed at the study of those properties of nucleons and nuclei that are influenced by the size of the proton. As the proton is a nucleon, and one of the principal components of nuclei, knowing its size precisely is critical to understanding fundamental nuclear structure. Yet measurement of the proton's size through two independent means has yielded different results. The source of this difference has been a 10-year old puzzle that this project hopes to solve. Research will be carried out at two places: one at the Fermi National Accelerator Laboratory (Fermilab) near Chicago as part of a collaboration called SeaQuest, and the other at the Paul Scherrer Institute near Zurich, Switzerland, as part of a collaboration called MUSE. Education and training of both undergraduate and graduate students, as well as outreach aimed at the general public, is an important aspect of this research program. The experiment at Fermilab is focused on Drell-Yan production of dimuon pairs to determine the light anti-quark distributions in the nucleon. It will provide a better understanding how light anti-quarks are distributed in unpolarized and polarized nucleons, and how orbital angular momentum might contribute to the nucleon spin. This information will provide direct input to theoretical models of the proton, and shed light on the origin of the proton's spin. The MUSE experiment will simultaneously measure elastic scattering with electrons and muons off protons which is afforded by a beam that contains positively and negatively charged electrons, muons and pions. The experiment aims to resolve the unexplained large discrepancy of the size of the proton measured with electron and muon scattering. This discrepancy was entirely unexpected, as the proton's size should not depend on whether it is measured with electrons or muons. This approach has never been attempted before. In addition, the Michigan group will lead the operation of a liquid hydrogen target at the MUSE experiment, and lead the analysis effort at the SeaQuest experiment.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.