Hyperon-nucleon three-body forces and strangeness in neutron stars

Three-body forces acting on a Lambda hyperon in a nuclear medium are investigated, with special focus on the socalled hyperon puzzle in neutron stars. The hyperon-nucleon two-body interaction deduced from SU(3) chiral effective field theory is employed at next-to-leading order. Hyperon-nucleon three-body forces are approximated using saturation by decuplet baryons and are transcribed to density-dependent effective two-body interactions. These together are taken as input in a Brueckner-Bethe-Goldstone equation with explicit treatment of the Lambda N <-> Sigma N and Lambda NN <-> Sigma NN coupled channels. Single-particle potentials of a A hyperon in symmetric nuclear matter and neutron matter are calculated. With parameters of the Lambda NN three-body force constrained by hypernuclear phenomenology, extrapolations to high baryon density are performed. By comparison of the Lambda and neutron chemical potentials at densities characteristic of the core of neutron stars it is found that the combined repulsive effects of two- and three-body correlations can make the appearance of Lambda hyperons in neutron stars energetically unfavourable, thus potentially offering a possible answer to a longstanding query.