Three-nucleon forces and their importance in three-nucleon systems and heavier nuclei

In the past two decades, several laboratories have produced a large amount of data for cross sections, analyzing powers, and other spin observables from various reactions in the three-nucleon system. The experimental results are moderately described by only using the two-nucleon potentials in Faddeev-type calculations. The remaining discrepancies should, in principle, and aside from Coulomb and relativistic effects, be removed once the effects of three-nucleon forces are implemented. High precision data on elastic and break-up reactions show, however, that even after the inclusion of these effects, the picture is not complete yet and some ingredients are still missing in the calculations. With the advent of new frameworks within which two and three-nucleon forces can be properly implemented in the calculation of observables in heavy nuclei, it is essential that these forces are better understood.