Lambda-nuclear interactions and hyperon puzzle in neutron stars

Brueckner theory is used to investigate the in-medium properties of a.-hyperon in nuclear and neutron matter, based on hyperon-nucleon interactions derived within SU(3) chiral effective field theory (EFT). It is shown that the resulting Lambda single-particle potential U-Lambda(p(Lambda) = 0, rho) becomes strongly repulsive for densities rho of two-to-three times that of normal nuclear matter. Adding a density-dependent effective Lambda N-interaction constructed from chiral Lambda NN three-body forces increases the repulsion further. Consequences of these findings for neutron stars are discussed. It is argued that for hyperon-nuclear interactions with properties such as those deduced from the SU(3) EFT potentials, the onset for hyperon formation in the core of neutron stars could be shifted to much higher density which, in turn, could pave the way for resolving the so-called hyperon puzzle.