Multi-Probe Investigation of the Nucleon

George Washington University, Washington DC

Investigators

Abstract

Protons, neutrons, and electrons are the building blocks of all matter with which we are most familiar. Despite the scientific efforts to comprehend their nature, there are many things about them that remain to be understood. In 2010, a measurement of the size of the proton gave a much smaller than expected result. This measurement disagreed with, but was much more precise than, all the other proton size measurements to date. Since that time, many other people have tried to re-measure the proton size using different approaches, but the discrepancy persists. The research supported by this award aims to resolve this issue, known as the "Proton Radius Puzzle" by making the first simultaneous measurement of muon and electron scattering on the proton in the Muon Proton Scattering Experiment (MUSE). Through this experiment, in addition to gaining knowledge about protons, the postdoctoral researcher, undergraduate, and graduate students will gain knowledge and experience of modern detection techniques, and simulation and reconstruction software, which are employed in many applications such as industrial or medical imaging. They will also participate in cultural exchanges with researchers from many countries worldwide, and develop their communication, organization, and leadership skills. MUSE will make the world's first measurement of the proton radius via elastic muon scattering at a precision which can address the 4% proton radius puzzle. The experiment will be performed at the Paul Scherer Institute in Switzerland. The simultaneous measurement of elastic e-plus / e-minus and mu-plus/mu-minus scattering on the proton in the MUSE experiment will allow resolution of the proton radius puzzle: the 7-sigma discrepancy between the charge radius of the proton measured in the excitation spectrum of muonic hydrogen and that measured in the excitation spectrum of the atomic hydrogen. Measuring with both electrons and muons simultaneously improves the systematic uncertainty, and provides a test of lepton universality. Measuring both charge states allows for the direct measurement and determination, and then cancellation, of any two-photon effects in both muon- and electron-scattering. This project involves participation in and leadership of data taking and analysis of MUSE through the support of the PI, a graduate student, undergraduate student, and a postdoc. It will result in precise, simultaneous elastic muon and electron scattering data. 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.