Proton Charge Radius Measurement by Low-Energy Electron Scattering

The proton size, the root-mean-square charge radius, has been so far determined by electron scattering and hydrogen spectroscopy of the normal atoms as well as the muonic-hydrogen spectroscopy. The latest determination of the proton radius using muons, 0.84 fm disagrees by 4 % with one determined by electrons, 0.88 fm. Intensive discussions on possible reasons this discrepancy, including critical reviews of past analysis of experimental data, have been going on, but the situation is not yet clear. It is known that the radius determination from electron scattering data as well as hydrogen spectroscopy is somewhat model dependent. In order to diminish model dependence as much as possible in the radius determination, we are going to carry out electron scattering under the kinematics of the lowest-ever momentum transfer at Tohoku University. It is worth emphasizing here that this is the only place in the world, where such a low-energy electron scattering experiment is possible. In my talk, I will discuss in details on the on-going electron-scattering project at our laboratory.