Studies of Nucleons at Fermilab and PSI

The main physics program supported by this award is the study of the spin structure of nucleons and solving a 10-year old puzzle about the real size of the proton. Research will be carried out at two places: one at the Fermi National Accelerator Laboratory (Fermilab) near Chicago as part of a collaboration called SpinQuest, and the other at the Paul Scherrer Institute near Zurich, Switzerland, as part of a collaboration called MUSE.

The experiment at Fermilab is focused on a special process in proton-proton scattering, called the Drell-Yan process, which can identify light antiquarks in the nucleon and determine how those antiquarks contribute to the spin of the proton. The experiment in Switzerland aims to resolve the unexplained large discrepancy of the size of the proton measured with electron and muon scattering, where the muon is a heavy cousin of the electron.

This discrepancy was entirely unexpected, as the proton's size should not depend on whether it is measured with electrons or muons. This research program will train postdoctoral fellows and graduate and undergraduate students on cryogenic techniques and targets. The involvement of junior researchers in this effort is important to ensure that there will be sufficient expertise in the nuclear physics community and beyond to support the future need for cryogenic instrumentation.

Every group member has the opportunity to get involved in hardware, simulation and analysis projects and learn valuable skills in several programming languages and in the analysis of large data sets.

The SpinQuest experiment at Fermilab near Chicago will perform the first Sivers function measurement on sea quarks using the Drell-Yan process in proton-proton scattering to determine the sign and magnitude of the u-bar and d-bar Sivers distributions. These studies may shed light on the contribution of orbital angular momentum from sea quarks to the proton spin and enable measurements of a fundamental prediction of Quantum Chromodynamics that goes to the heart of the gauge formulation of QCD, known as the "sign change" of the Sivers function.

The studies of the Michigan group at PSI aim to reassess the proton charge radius and the discrepancies that remain by performing the first simultaneous measurement of elastic electron and muon scattering off the proton, which is afforded by a beam that contains positively and negatively charged electrons, muons and pions. This approach has never been attempted before and carries high promise to resolve the proton radius puzzle.

Due to their specific expertise in building and running cryogenic targets, the Michigan group will lead the unpolarized target effort at MUSE and support the polarized target effort at Fermilab. This program builds on the expertise the group has gained and is a natural extension of the group's work over the past years. It provides a balanced mix of hardware, simulation and analysis projects, which will benefit students and postdocs, and it will allow the U-M group to play a key role in the MUSE experiment at PSI and have a strong presence in the Drell-Yan program at Fermilab.

Education and training of undergraduate and graduate students and postdoctoral fellows, as well as outreach aimed at the general public, is an important aspect of this research program.

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.