The hyperon puzzle: new hints from Quantum Monte Carlo calculations

The onset of strange baryons in neutron matter at densities of order 2-3 times saturation density has been questioned for a long time. The appearance of strange hadronic degrees of freedom in the core of neutron stars leads to a softening of the equation of state, hence reducing the value of the predicted maximum mass. Most of the present calculations of matter including hyperons predict low maximum masses (M 1.4-1.6 M), in agreement with earlier astrophysical observations. The situation has became more controversial after the recent discovery of very massive neutron stars (M 2 M), that seems to rule out the onset of strange hadronic degrees of freedom at high densities. We attempt to give our contribution to the discussion by revising the general problem of the hyperon-nucleon interaction by means of Auxiliary Field Diffusion Monte Carlo calculations. We employed a phenomenological approach showing that a refit of the three-body ΛNN force on the available hyperon-separation energies provides the strong repulsive contribution needed to correctly describe the systematics of medium-heavy mass Λ-hypernuclei. The same potential has then been used to determine the equation of state of neutron matter including lambda hyperons at high densities. Preliminary results on the study of the infinite medium suggest that the inclusion of the ΛNN force strongly affects the softening of the equation of state of hyper-matter. These results envisage the possibility of reconciling the presence of hyperons in the core of neutron stars with the recent astrophysical observations.