Low-energy electron scattering for nucleon and nuclei in an extremely low q region*

Electron scattering is a very powerful tool for studying the structure of nuclei. This is because the electron is a structureless particle, and the electromagnetic interaction is well understood and can be accurately calculated. Electron scattering in the extremely low momentum transfer region is particularly well suited for measuring the nuclear radius, since the radius is determined by the slope of the charge form factor at momentum transfer equal to 0. Such a measurement can reduce an extrapolation uncertainty. Although the radius is a very fundamental property of the nucleus, there are still important unsolved problems such as the proton and deuteron radius puzzle [1-2].

The high intensity electron linac in ELPH can provide an electron beam with energies of 10 – 60 MeV. The much lower energy electron beam compared to other electron scattering facilities facilitates access to the low momentum transfer region. A new beam line for the electron scattering and electromagnetic spectrometers have been built in ELPH to obtain the most reliable proton and deuteron charge radii for electron scattering. Commissioning has been successfully completed, confirming that these have sufficient performance for the electron scattering experiments. Therefore, we have started a physics run and aim to determine the radius in FY2024.

Furthermore, it has been shown that the 4^th moment of the charge distribution contains information about the neutron distribution in a nucleus [3]. To demonstrate this, the 4^th moment measurement of ²⁰⁸Pb is underway in ELPH.

[1] C. E. Carlson, Prog. Part. Nucl. Phys. 82, 59 (2015). [2] O. J. Hernandez et al., Phys. Lett. B, 778, 377 (2018). [3] H. Kurasawa, T. Suda, and T. Suzuki, Prog. Theor. Exp. Phys.013D02(2021).