Direct determination of the neutron skin thickness of ⁴⁸Ca probed via proton elastic scattering and the ESPRI project

Equation of state (EOS) of the nuclear matter controls the properties of not only the finite quantum system, nuclei, but also the structure of neutron stars and the dynamics of supernovas. Presently, the isospin-asymmetric term of the EOS, the symmetry energy is much less certain. The density dependent term of the symmetry energy plays very important role to form the neutron-rich matter. Therefore, it is very essential for understanding the neutron star.

The neutron skin thickness (Δr_np) is the most important parameter to understand the density dependence of the symmetry energy. It is very difficult, however, to determine Δr_np accurately due to the theoretical model ambiguities.

Proton elastic scattering is one of the best probes to extract the direct information of proton and neutron distributions. We have successfully determined the model independent neutron distributions and the Δr_np of Sn and Pb isotopes. We will report a result of Δr_np in ⁴⁸Ca. Recently Δr_np in ⁴⁸Ca has received much attention because the ab initio calculation including a multi-nucleon force can be applied to Ca region. This helps us to directly understand the role of the multi-nucleon force which is essential for the EOS at high density.