Collaborative Research: Equipment for and Running of the PSI MUSE Experiment

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A fundamental goal of subatomic physics is the precise determination of the physical properties of subatomic particles such as the proton and neutron. The radius of the proton is one of these properties. The radius can be measured by scattering electrons, or heavier particles called muons, off protons. Another technique is to measure the energy levels of hydrogen, an atom consisting of a proton plus an electron, or muonic hydrogen, an atom consisting of a proton with the electron replaced by a muon. To date, the radius has been measured by scattering of electrons off protons, and by the energy levels of hydrogen and muonic hydrogen. The proton radius derived from the measurement of the energy levels in muonic hydrogen disagrees with the radius derived from the measurements of the energy levels in hydrogen, and with the measurements of the proton radius by scattering of electrons, an issue known as "the proton radius puzzle." The discrepancy has attracted wide attention, because it may indicate the existence of previously unobserved and unexpected differences between the interactions of muons and electrons. A dedicated MUon proton Scattering Experiment (MUSE), which will run at the Paul Scherrer Institute in Switzerland and directly compare the scattering of muons and electrons from protons, is highly anticipated by experts in the field for its potential to resolve the puzzle. This project will cover the prototyping and development of several crucial components of the MUSE experiment detectors. The project will involve the training of students and young scientists, at the undergraduate, graduate, post-doctoral, and junior faculty levels. The institutions involved in the project have trained large numbers of students of each type, including from minority populations. The training they have received in the process of doing basic research has led to careers in a variety of areas, from medical physics to national security, in addition to continued work in fundamental physics research. The MUSE experiment will broaden the perspective of American students by having them work in an international collaboration at an international laboratory, which will prepare them effectively to become prominent global scientists of the next generation. With the broad interest in the proton radius puzzle, MUSE has the potential to be broadly inspirational beyond the current scientific community.