Hyperon-mixed neutron star matter and neutron stars

Effective Sigma(-)n and Sigma(-)Sigma(-) interactions are derived from the G-matrix calculations for {n+Sigma(-)} matter and employed in the investigation of hyperon mixing in neutron star matter. The threshold densities p(t)(Y) at which hyperons start to appear are between 2p(0) and 3p(0) (where p(0) is the normal nuclear density) for both Lambda and Sigma(-), and their fractions increase rapidly with baryon density, reaching 10% already for p similar or equal to p(t) + p(0). The mechanism of hyperon mixing and single-particle properties, such as the effective mass and the potential depth, are analyzed taking into account the roles of YN and NN interactions. The resulting equation of state is found to be too soft to sustain the observed neutron star mass M-obs = 1.44M(circle dot). We discuss the reason for this and stress the necessity of the extra repulsion for YN and YY interactions to resolve this crucial problem. It is remarked that p(t)(Y) would be as large as 4p(0) for neutron stars compatible with M-obs. A comment is given regarding the effects on the Y-mixing problem from a less attractive LambdaLambda interaction, newly suggested by the NAGARA event.