https://doi.org/10.1140/epja/s10050-021-00489-6
Regular Article - Theoretical Physics
Neutron–proton spin–spin correlations in the ground states of 
nuclei
1
Grand Accélérateur National d’Ions Lourds, CEA/DRF–CNRS/IN2P3, Bvd Henri Becquerel, 14076, Caen, France
2
Nuclear Science Division, Lawrence Berkeley National Laboratory, 94720, Berkeley, CA, USA
Received:
8
March
2021
Accepted:
16
May
2021
Published online:
2
June
2021
We present expressions for the matrix elements of the spin–spin operator 
in a variety of coupling schemes. These results are then applied to calculate the expectation value 
in eigenstates of a schematic Hamiltonian describing neutrons and protons interacting in a single-l shell through a Surface Delta Interaction. The model allows us to trace as a function of the competition between the isovector and isoscalar interaction strengths and the spin–orbit splitting of the 
shells. We find negative values in the ground state of all even–even nuclei, contrary to what has been observed in hadronic inelastic scattering at medium energies. We discuss the possible origin of this discrepancy and indicate directions for future theoretical and experimental studies related to neutron–proton spin–spin correlations.
© The Author(s), under exclusive licence to Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature 2021
