Tensor correlations and P-wave pairing in neutron matter

Landau-Fermi liquid theory is a powerful effective theory for strongly interacting Fermi systems at low temperatures. It has been successfully applied to liquid He, nuclear matter and nuclei. However, in contrast to the interaction between He atoms, nuclear forces are complicated due to the large non-central spin-orbit and tensor components. Here, we report on the in-medium modification of the effective nucleon-nucleon interaction, paying particular attention to spin-dependent forces [1]. The form of the two-body interaction in vacuum is constrained by symmetries. In particular, in non-relativistic theories, Galilean invariance implies that the two-body interaction is independent of the particle-pair (cm) momentum P ≡ p1 + p2 = p3 + p4. In this case, the possible operators are scalar, spin-spin, spin-orbit, tensor and quadratic spin-orbit forces. In the medium, the Fermi sea defines a preferred frame, and the effective two-body interaction depends on the cm momentum. On the Fermi surface, this leads to novel non-central forces of the form