The sizable difference of the proton charge radius derived from electron scattering and the Lamb-shift measurement of muonic hydrogen caused a considerable confusion in our community, and triggered intensive discussions about its possible reasons. The problem is not yet settled even today, and it is called as "Proton Radius Puzzle".
In the seminar, I will discuss our project of electron scattering experiments off proton using a 60 MeV electron linac of Tohoku University. Elastic electron-scattering cross section will be measured in the lowest-ever momentum transfer region, Q2 = 0.0003 - 0.005 (GeV/c)2, and the charge form factor, GE(Q2), is extracted by using the Rosenbluth separation technique. Since the charge radius is defined as a derivative of GE(Q2) at Q2 = 0, this project will provide the least model-dependent proton charge radius ever determined by electron scattering, which will be crucial to understand the puzzle. Such study including the Rosenbluth separation needs an energy-variable low-energy electron accelerator, and our "old" low-energy electron linac of Tohoku University just fits the purpose.
In my talk, I will firstly review "Proton Radius Puzzle" and discuss the experimental methods so far employed to determine the proton charge radius. Then I will discuss the details of our project which is now underway at Tohoku University.