TheAypuzzle and the nuclear force

The nucleon-deuteron analyzing power $A_y$ in elastic nucleon-deuteron scattering poses a longstanding puzzle. At energies $E_{\mathrm{lab}}$ below approximately 30 MeV $A_y$ cannot be described by any realistic nucleon-nucleon (NN) force. The inclusion of existing three-nucleon forces does not improve the situation. Because of recent questions about the ${}^3{P}_J$ NN phases, we examine whether reasonable changes in the NN force can resolve the puzzle. In order to do this we investigate the effect on the ${}^3{P}_J$ waves produced by changes in different parts of the potential (viz., the central force, tensor force, etc.), as well as on the two-body observables and on $A_y.$ We find that it is not possible with reasonable changes in the NN potential to increase the three-body $A_y$ and at the same time to keep the two-body observables unchanged. We therefore conclude that the $A_y$ puzzle is likely to be solved by new three-nucleon forces, such as those of the spin-orbit type, which have not yet been taken into account.