A high-resolution dipole spectrometer for low-energy electron scattering to determine proton charge radius

The proton radius puzzle is the discrepancy between the proton radius determined by electron scattering and hydrogen spectroscopy, and that by muonic hydrogen spectroscopy. This discrepancy remains unresolved and is a topic of ongoing research in the world.

We are going to determine the proton radius by low-energy electron scattering at ELPH, Tohoku University. The proton radius is defined as a derivative of the charge form factor at the momentum transfer squared zero. Therefore, we are going to measure the scattering cross section under the lowest-ever momentum transfer squared using the low-energy (~60 MeV) electron beam.

Since the change in the charge form factor at momentum transfer measured in this experiment is only around 2 %, the cross section must be measured with an accuracy at 0.1%. We are constructing a high-resolution dipole spectrometer.

In this talk, I will discuss the design of the spectrometer and the evaluation of the background due to multiple scattering effect of low energy scattered electrons.