Chapter II. Superfluidity in Neutron Star Matter and Symmetric Nuclear Matter

Nucleon superfluids which are realized in neutron star interior and symmetric nuclear matter are studied with use of realistic nuclear forces, in the density domain from the subnuclear region to about 3<it>ρ</it><inf>0</inf> (<it>ρ</it><inf>0</inf> being the nuclear density). It is shown that characteristic aspects of nuclear forces manifest themselves in the appearance of several kinds of nucleon superfluids, which strongly depends on the density <it>ρ</it>. In this chapter emphasis is put on the pairing correlations where strong noncentral (tensor and spin-orbit) forces play important roles. A theoretical framework applicable to the nonzero angular-momentum pairing including the coupling due to tensor force is given by extending the usual BCS-Bogoliubov theory for the 1<it>S</it><inf>0</inf> pairing (the zero angular-momentum one). This formulation has been applied to the 3<it>P</it><inf>2</inf>+3<it>F</it><inf>2</inf> pairing in neutron matter (the dominant component of neutron stars) and the 3<it>S</it><inf>1</inf>+3<it>D</it><inf>1</inf> pairing in symmetric nuclear matter. In the former case, although spin-orbit force mainly contributes to the 3<it>P</it><inf>2</inf> attraction, the tensor coupling with the 3<it>F</it><inf>2</inf> component assists to realize the 3<it>P</it><inf>2</inf> superfluid. In the latter case, the tensor coupling to the 3<it>D</it><inf>1</inf> component plays a vital role to realize the 3<it>S</it><inf>1</inf> superfluid with a large energy gap. Results of the energy gaps calculated for such nonzero angular-momentum pairings, as well as those for the 1<it>S</it><inf>0</inf> pairing, are shown. We have found the realization of the following nucleon superfluids; the neutron 3<it>P</it><inf>2</inf> superfluid and the proton 1<it>S</it><inf>0</inf> one in the fluid core of neutron stars at <it>ρ</it>≃(0.7∼3)<it>ρ</it><inf>0</inf>, the neutron 1<it>S</it><inf>0</inf> superfluid in the inner crust of neutron stars at <it>ρ</it>≃(10−3∼0.5)<it>ρ</it><inf>0</inf>, and the 3<it>S</it><inf>1</inf> superfluid in symmetric nuclear matter at a wide range of <it>ρ</it> including <it>ρ</it><inf>0</inf>, contrary to the 1<it>S</it><inf>0</inf> one realized at <it>ρ</it>≲<it>ρ</it><inf>0</inf>/2. The properties of these superfluids and their implications are also discussed.