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 ρ. The modification has been performed in channels where the pair of 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.