Importance of tensor forces in nuclear structure

Recent ab-initio calculations of light nuclei demonstrate the importance of the pion for binding nuclei. It was found that about 80% of attraction is due to pions. The pion interaction is written as shown below, where S12 is the tensor operator and provides the contribution of the tensor forces in the first term of the right hand side of the first equation. The second term is the spin-spin term of the central forces.
As seen in the equation, the tensor force is as important as the central forces in the pion exchange interactions. However, the tensor force has not been explicitly handled in nuclear models except for the lightest nuclei such as the deuteron and 4He. Moreover, recent studies of nuclei far from the stability line show the importance of the tensor forces through changes of magic numbers and a peculiar mixing of s- and p-waves in the neutron-halo nucleus 11Li.
An important property of the tensor force is that it produces a strong correlation between a proton and neutron pair (pn pairing) in a nucleus and introduces high-momentum nucleons in nuclei. In fact, it is these tensor-correlated nucleons that provide the main part of the binding energy of the deuteron and 4He. Most of nuclear models, however, are based on the mean-field picture and do not include the correlations induced by the tensor forces.