Probing Nucleon Structure Through Muon and Photon Scattering

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 resulted in a proton size measurement much smaller than expected. This measurement disagreed with, but was much more precise than, all the other proton size measurements to date. The research supported by this award aims to resolve this issue, known as the "Proton Radius Puzzle" by making a simultaneous measurement of muon and electron scattering on the proton in the Muon Proton Scattering Experiment (MUSE). In addition to determining the proton size, the group will spend a small amount of its time investigating the internal structures and dynamics of the proton and neutron, by measuring quantities known as the polarizabilities. These measurements will be made by scattering light from protons and neutrons. Experiments will take place at laboratories in Switzerland, Germany and the US. Through these experiments, in addition to gaining knowledge about protons and neutrons, the postdoctoral researchers, 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. The polarizabilities of the proton and neutron will be also investigated via their influence on beam and target polarization asymmetries measured in Compton Scattering. Precise cross section measurements on liquid hydrogen and helium three and four within the A2 collaboration at the Mainzer Microtron in Mainz, Germany, and at lower energies in the High-Intensity Gamma Source (HIGS) at the Triangle Universities Nuclear Laboratory. This project involves participation in and leadership of construction, data taking and analysis of MUSE through the support of the PI, a graduate student, and postdoc. It will result in precise, simultaneous elastic muon and electron scattering data, and precise measurements of the scalar polarizabilities of the proton and neutron.