Studies of Nucleons at Fermilab and PSI

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

Although the proton has been studied for over 100 years, it still is not fully understood. Until about 15 years ago, there seemed to be agreement among various experiments for how large the proton is. All that came into question when an experiment using muons, the heavy cousins of the electrons, displayed a large, unexplained discrepancy in the size of the proton compared to using electrons. This discrepancy was entirely unexpected, as the proton's size should not depend on whether it is measured with electrons or muons. This became known as the proton radius puzzle. The main physics program supported by this award is an attempt at solving this 14-year old puzzle about the real size of the proton. This research program will train a postdoctoral fellow 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. The junior researchers will also have 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 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 by another research collaboration 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 the postdoc, and it will allow the U-M group to play a key role in the MUSE experiment at PSI. Education and training of postdoctoral fellows, as well as outreach aimed at the general public is an important aspect of this research program, particularly its international component, as it will expose them to physicists from many different countries, fostering cultural exchange and giving them the opportunity to develop their communication and leadership skills. 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.