Deuteron-Like Proton-Neutron Correlations in Nuclei

Spin-triplet proton-neutron correlation is the most direct manifestation of the tensor-force effects in nuclei. Since the tensor force is a rank-2 tensor in the spin space, it doesn’t affect spin-single pairs but provides (strong) attraction to spin-triplet pairs.

As is well known, the strong proton-neutron attraction due to the tensor force can modify the magic core, which is often considered inert. In the talk, I plan to discuss two cases where the effects of the strong proton-neutron attraction are demonstrated: One is the proton density polarization in the Z=20 proton magic core in 48Ca, driven by eight neutrons added in the f7/2 shell[1]. The other is a significant change of the neutron structure in 25F induced by a single proton in the d5/2 orbit.

One of the most direct ways to approach the spin-triplet proton-neutron correlation is to knock it out. The proton-induced knockout reactions offer the most versatile and productive approaches to nuclear structure at intermediate-energy facilities such as GSI/FAIR, RIBF, and RCNP. Under the ONOKORO project, with which we aim to comprehensively investigate clustering in nuclei using the cluster knockout reactions, we conducted the deuteron knockout (p,pd) reaction experiments. The results from the first experiment for stable carbon and oxygen isotopes show that the number of deuteron-like proton-neutron pairs in those nuclei is 1.6 for 12C and 1.9 for 16O, which amounts to 40% and 32% of the maximum number of pairs in the p-shell.
The deuteron knockout reaction studies are being extended to radioactive nuclei using the RI beam at RIBF and the newly constructed detector telescope TOGAXSI, specialized for inverse-kinematics cluster knockout reaction.

In the colloquium, after the general overview about the deuteron-like proton-neutron correlation in nuclei and its relevance to the short-range correlation, I discuss experimental approaches to the physics case, with some emphasis on the knockout reaction studies. 

[1] J. Zenihiro, T. Uesaka et al., Prog. Theo. Exp. Phys. 2021, 023D05 (2021).
[2] T.L. Tang, S. Kawase et al., Phys. Rev. Lett. 124, 212502 (2020).
[3] C.S. Lee et al., Prog. Theo. Exp. Phys. 2026, 053D01 (2026).