Precision Experiments at Jefferson Lab to Study Specific Electromagnetic Properties of Hadrons

North Carolina Agricultural & Technical State University, Greensboro NC

Investigators

Abstract

This award supports two high-profile projects: the high precision determination of the proton charge radius and the measurement of the probability for an eta-meson (one of the light elementary particles that interact through the strong force) to decay into two photons. The visible Universe consists mostly of protons; therefore understanding the proton's properties is one of the important tasks of contemporary nuclear physics. Fundamental symmetries and, most importantly, the ways in which these symmetries are broken, define the physical properties of the eta-meson. These properties, although very critical for the formation of the visible matter, are rather small and require high precision state-of-the-art experiments to be able to "look back" at the early stages of the development of our Universe. Under this award one undergraduate, one graduate student and one postdoctoral fellow will be trained in highly multidisciplinary experimental nuclear physics. This award will broaden the participation of under-represented groups in STEM, in that the majority of students at the PI's institution are African American. For the past many decades consistent results on the proton charge radius have been obtained between electron-proton scattering and traditional hydrogen spectroscopic measurements. However, recent ultra-high precision measurements of this quantity performed with a novel muonic-hydrogen spectroscopic method gave results up to eight-standard deviations smaller than the average value from all previous experiments. This clear discrepancy triggered what is now known as the "proton radius puzzle". The goal of the PRad experiment is to determine the proton charge radius to 0.5% precision. The projected accuracy of the eta --> 2 gamma decay width will significantly impact the eta-meson sector of the Particle Physics Group (PDG) by significantly improving the rest of eta-partial decay widths. As a result, it will critically improve the light quark mass ratio in a direct and mostly model independent way. 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.