Electron scattering is the most powerful tool to study the internal structure of proton and atomic nuclei. Our research group utilizes this powerful tool for 1) precise determination of the proton size and 2) revealing the internal structure of short-lived exotic nuclei.

1) Precise determination of the proton size
Proton is the primary building block of the “visible” universe, and its internal structure has been intensively studied since its discovery.

It is a big surprise, however, to notice that its size, the root-mean-square charge radius, is seriously questioned even today: the radius determined by high-energy electron scattering disagrees seriously with one deduced from spectroscopic studies of the muonic hydrogen atom. This is referred to as “Proton Radius Puzzle”. Since one possible reason is suggested to be a non-identical nature of electron and muon except their masses, which is triggering much attention.

The radius determination is known to be somewhat model dependent. To determine the least model-dependent radius, we will perform elastic electron scattering off proton under the lowest-ever momentum transfer using low-energy electron beam available at our Research Center of Electron-Photon Science.

2) Electron scattering off short-lived exotic nucleus

Short-lived β-unstable exotic nuclei exhibit a variety of exotic structures which are totally beyond the “standard” nuclear-structure theory established by the studies of stable nuclei. It is well recognized today that the study of those peculiar structures is one of the most burning issues in modern nuclear physics.

Electron scattering, known to be the best way for the structural studies of atomic nuclei, has been strictly hampered for short-lived exotic nuclei due to obvious difficulties in target preparation of production-hard short-lived exotic nuclei.

We have recently completed the construction of the world’s first electron scattering facility at RIKEN RI Beam Factory, which is dedicated for short-lived exotic nuclei based on a novel target-forming technique, SCRIT (Self-Confining Radioactive-isotope Ion Target), that we have invented to overcome the difficulties. We are now ready to start long-awaited structure studies of exotic nuclei by electron scattering.