Phenomenological spin-orbit three-body force

In order to improve the description of the N-d vector analyzing powers Ay and iT11 (the so-called Ay puzzle) a spin-orbit three-body force has been introduced. The L.S term in the NN potential has been modified including a two-parameter three-body function depending on the hyperradius \rho. The modification has been performed in channels where the pair nucleons (i,j) are coupled to spin Sij=1 and isospin Tij=1. Calculations have been done in the energy region from Elab=648 keV up to Elab=10 MeV. A noticeable improvement in the description of the polarization observables has been obtained