Puzzling out the Proton with Precision Scattering Experiments

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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. The research supported by this grant aims to resolve this “Proton Radius puzzle”. The Muon Proton Scattering Experiment (MUSE) at the Paul Scherrer Institute, Switzerland, will scatter a beam of electrons and muons off protons to measure the shape of the proton. The work of undergraduate and graduate students as well as postdoctoral researchers is central to these efforts, and they will gain knowledge and experience in precision nuclear physics experiments, modern detector and accelerator techniques, as well as in simulation and analysis algorithms. This international research effort will expose them to researchers from all over the world, fostering cultural exchange and give them opportunity to develop their communication and leadership skills.MUSE will make use of the worldwide unique beam available at the Paul Scherrer Institute to measure the lepton-proton cross section. It is the first measurement of muon-proton scattering that is precise enough to address the 4% effect at the heart of the proton radius puzzle. Through the measurement of muons of both charges, as well as electrons and positrons over a wide kinematic range, MUSE will provide crucial data to verify the existing e-p scattering results, test radiative corrections including two-photon-exchange by comparison of the two charges, and search for a violation of lepton universality by comparison of the two lepton families. Dr. Bernauer’s group will play a crucial role in the development of the software and the analysis of the data. Additional activities with the MAMI-A1 and MESA-MAGIX collaborations in Mainz, Germany, aim to reduce the leading systematic uncertainties on the proton electric form factor and will prepare a future experiment at MESA which will reduce the uncertainty of the magnetic form factor by two orders of magnitude. This will lead to an unprecedented precision on both the magnetic radius and the Zemach radius of the proton. The latter quantity is also accessible in spectroscopy and is, like the charge radius, a rare point of overlap between atomic and nuclear 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.