Precise determination of proton charge radius based on electron/proton elastic scattering in extremely low momentum region

It has become clear that the proton charge radius determined by high-energy electron scattering, hydrogen atom spectroscopy experiment, and spectroscopic experiments of μ hydrogen atom (hydrogen atom whose atom was substituted with μ-) had 4% (7σ) of deviation, which incurred a state called 'proton radius puzzle.' It is surprising that protons, whose internal structure has been studied in detail from the time of the construction of modern physics, still have such instability in the charge radius, which is one of the most fundamental physical quantities. Both the examination of experimental data and the reanalysis of past data were conducted, and a large number of additional tests were carried out and are planned. The authors carried out the measurement of the electron/proton elastic scattering cross section in the unprecedentedly extremely low momentum transition region at the low-energy electron linear accelerator of Tohoku University Photochemical Research Center, and aims to determine highly reliable proton charge radius without dependence on analyzed model as much as possible. Similar experiments are carried out or planned by other research institutions, but since the low-energy electron linear accelerator of Tohoku University can measure with the highest accuracy, the authors plan to start the measurement from FY2019 when a new measurement system is established. (A.O.)