Spin and the Three-Nucleon Force

Theoretical descriptions of three-nucleon systems, which are based on a summation over pairwise NN interactions, fail to describe existing data. For instance, the binding energy of the triton is underpredicted as well as the unpolarized cross section in pd elastic scattering around θcm = 100 deg. It has been demonstrated that the inclusion of a three-nucleon-force, which has been adjusted to reproduce the measured binding energy of the triton, also reduces the discrepancy of the calculation with the measured cross section. In the case of many spin observables - the most notorious being the analyzing power in nd scattering below 10 MeV ('Ay puzzle') - inclusion of any one of the modelled three-nucleon-forces does not explain the data. Several precise data sets of spin observables exist between 100 MeV and the pion production threshold. A theoretical description of the three-nucleon system must be able to reproduce all spin observables simultaneously