Puzzling Out the Proton with Precision Scattering Experiments

Suny At Stony Brook, Stony Brook NY

Investigators

Abstract

Protons are one of the basic building blocks of all matter around us. Even 100 years after their discovery, they are still not fully understood. In fact, even the size of the proton has been a puzzle for over a decade now, after an extremely precise measurement in 2010 found a value much smaller than and in strong disagreement with previous determinations. The research supported by this grant aims to resolve this “Proton Radius puzzle”, which has found widespread interest beyond the physics community. The award supports the PI’s activity in the Muon Proton Scattering Experiment (MUSE) at the Paul Scherrer Institute in Villigen, Switzerland, which will provide crucial data to resolve the puzzle, and to determine the shape of the proton. The experiment will check if electrons and their heavier cousins, muons, interact in the same way, and will test our understanding of the scattering process by comparing electron and positron scattering. 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 precise nuclear physics experiments, modern detector and accelerator techniques, as well as in simulation and analysis algorithms. This international research project will expose them to researchers from all over the world, fostering cultural exchange and giving them the opportunity to develop their communication and leadership skills. The award further supports the group’s involvement in various outreach programs with a focus on high-schools and underrepresented groups. 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 and support the data taking efforts with shift work. 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.