Investigating the Nucleon with Electromagnetic Probes

George Washington University, Washington DC

Investigators

Abstract

Protons are one of the basic building blocks of matter and even 100 years after their discovery, they are still not fully understood. In fact, the precise radius of the proton is a puzzle and multiple measurements of this quantity do not all agree. A precise measurement of a proton’s radius is important because it determines the interaction with, and cross-section for scattering, other particles. The proton’s radius even affects the atomic spectroscopy of the hydrogen atom, which is tightly linked to the value of the Rydberg constant that has been so critical to the interpretation of all atomic spectra, because the electron and proton wave functions overlap and the radius is an important parameter in determining the amount of overlap. In 2010, a measurement of the radius of the proton was made using muonic hydrogen (hydrogen where the electron has been replaced by a muon). This proved to be the most precise experimental determination of the proton radius ever made, but was massively inconsistent with the accepted value for the proton radius at that time. This became known as the Proton Radius Puzzle (PRP). Since its inception, many physicists have tried to resolve the PRP, to no avail. MUSE (the MUon proton Scattering Experiment) will make the world’s first measurement of the proton radius via elastic muon scattering at a precision which can address this 4% radius discrepancy. The focus of this project is the supervision and maintenance of the data acquisition system which reads out the MUSE detector system; and the analysis of the resulting data to compare scattering cross sections, form factors, and radii, and to extract charge asymmetries. The MUSE experiment has been fully constructed and this project will support engineering runs to further calibrate and test the equipment, followed by 12 months of measurement. Measurements will take place in the Paul Scherer Institute PiM1 beam line, which affords a 3.3 MHz secondary beam with a mixture of pions, muons and electrons. The beam momenta will be approximately 117MeV/c, 160 MeV/c, and 210 MeV/c. These measurements will cover a Q2 range of 0.002 – 0.07 GeV2. In the course of the above described activities, undergraduate students, PhD students, and postdoctoral researchers will be trained in the development of modern electronics systems and modern data analysis techniques. The PI will present this work as part of her involvement in the National Organizing Committee Chair Line of the Conferences for Undergraduate Women in Physics. 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.