We are facing a serious confusion about the proton size, referred to as “Proton Radius Puzzle.” The proton charge radius determined by electron scattering and hydrogen spectroscopy, and that determined by muonic hydrogen spectroscopy disagree seriously, and the reason is not yet clear. It is a big surprise to realize that we do not yet fully pin down the most basic feature of the proton. In this article, we discuss on an on-going project at the Research Center for Electron-Photon Science (ELPH), Tohoku University to determine the proton charge radius by low-energy electron scattering under the lowest-ever momentum transfer, which is crucial to extract the radius reliably from electron scattering data. It is worth emphasizing that ELPH is the only facility in the world, where such a low-energy electron scattering experiment is possible.
