=Date and Place=
Mar.28th(Tue.), 13:30~ at RIBF Hall(room 201)
(Research Center for Nuclear Physics, Osaka University)
np spin-correlation in the ground state studied by the sum-rules of the spin-M1 transition matrix elements and implications to the IS np pairing correlation
How are nucleons correlated in the ground state of atomic nuclei? It is one of fundamental questions to understand the strongly correlated nuclear system. Amongst many correlations, the correlation between a neutron and a proton, more specifically the isoscalar (IS) np pairing correlation, is thought to play a minor role in mean-field models while the isovector (IV) paring correlations are well descried by the quasi-particle description. However, the IS np paring is obviously stronger in the ground state of light nuclei, e.g. a deuteron or 6Li. How does the IS paring correlation manifest itself in heavier nuclei? We have obtained an evidence of np spin-correlation, that naturally emerges from the np pairing correlation, in sd-shell nuclei. The np spin-correlation function (npSCF) is proportional to the difference of the non energy-weighted sum-rule values of the IS and IV spin-M1 squared transition matrix elements. The npSCP takes a value of +1/4(-3/4) for the full spin-alignment (anti-alignment) that emerges from the IS (IV) np pairing in s-wave. The observed npSCFs showed a positive value of +0.1 for all the measured even-even self-conjugated nuclei in the sd-shell . The result shows stronger IS np-pairing than the IV np-pairing.
How can one understand the result with theoretical models? The shell-model with the USD interaction does not reproduce the positive npSCFs. Similar result was obtained for a large-scale shell model calculation . However the non-core shell-model predictions hint the positive npSCFs  that implies the importance of the core-polarization effect. The positive npSCFs are also predicted for 4He by ab initio calculations employing modern NN interactions [1,3]. Recent phenomenological study using shell-model wave functions could reproduce the positive npSCFs by introducing an enhanced IS spin-triplet pairing interaction and the Delta-hole coupling effect .
In this seminar, I will discuss in detail the experimental finding and its interpretations as well as possible future extensions to other excited states.
 H. Matsubara et al., Phys. Rev. Lett.  N. Shimizu, Y. Utsuno, and N. Tsunoda, private communication  W. Horiuchi et al., Phys. Rev. C 87, 034001 (2013). H. Sagawa et al., Phys. Rev. C 94, 041303(R).