Progress on Proton Charge Radius Measurements
NC A&T State University
for the PRad Collaboration
The proton charge radius is one of the fundamental quantities in physics. For the past seventy years it has been measured through elastic electron-proton scattering and ordinary hydrogen spectroscopy methods. Over the years, results from both methods generally agreed with each other within their experimental uncertainties. Unexpectedly, in 2010 (and 2013) two experiments from newly developed muonic hydrogen atomic spectroscopy method reported results up to six standard deviations smaller values than the accepted average from all previous experiments performed on ordinary hydrogen. This discrepancy triggered the well-known proton radius puzzle in hadronic physics. This talk will discuss the post-2010 progress in proton radius measurement methods together with new experimental results and plans for new experiments. In particular, a novel magnetic-spectrometer-free electron-proton scattering experiment (PRad), performed at Jefferson Lab in 2016, will be presented emphasizing its methods and results. Finally, the current status of the proton radius puzzle will be presented and discussed.