NSF-BSF: Precision Scattering of Low-Energy Muons, Electrons, and Pions with MUSE

This award provides funding for the PI and his group at Hampton University, a Historically Black College and University (HBCU) in Virginia, USA to collaborate on a high precision experiment to measure the proton’s charge radius. The experiment is called the Muon Scattering Experiment (MUSE) and is in progress at Paul-Scherrer Institute in Switzerland.

The size of the proton is a fundamental quantity with considerable uncertainty due to conflicting measurements by various methods. The MUSE experiment will measure the proton charge radius via lepton-proton elastic scattering, and it will compare the interactions of muons and electrons of either charge with the proton at a fundamental level. The experiment will severely constrain possible explanations of the conflicting results and has the potential to reveal new physics beyond the Standard Model.

The project offers high-profile opportunities for students and postdocs at a minority serving institution at the forefront of international nuclear and particle physics research.

The MUSE experiment is underway to provide key insight in the quest to explain the so-called proton radius puzzle - the over five-standard deviation discrepancy between proton charge radius measurements with electronic and muonic probes, respectively. Despite recent progress reported by spectroscopy and scattering experiments, the puzzle is not nearly resolved but highlights the importance of systematic uncertainties.

Possible explanations of the puzzle also include new physics beyond the Standard Model. The MUSE experiment aims to address one of the most pressing questions in nuclear and particle science to date. MUSE uses mixed muon and electron beams of either charge (μ+/e+, μ−/e−) to make precise measurements of the lepton-proton elastic scattering cross sections and proton form factors over a broad kinematic range, with the goal to extract the proton charge radius for each probe.

It will determine with the highest precision, model independently and conclusively, if there is any proton charge radius difference observed in electron versus muon scattering. At the same time, it will determine the role of two-photon exchange and provide tight constraints on theory. The group supported by this award plays a major role in data collection and analysis.

It is responsible for the high-resolution beam particle tracking with GEM detectors, in a way not done before at low beam energies. The tracker is exposed to high rate density and beam flux, and needs to operate efficiently at a trigger rate of a few kHz.

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.