DIS scattering of polarized electrons and muons off polarized nucleon targets. This constrains the Lagrangian for processes involving nucleons and pseudoscalar mesons. This gives a most general lagrangian which contains many parameters one can constrain with data. The latter terms are responsible for charge-changing pion exchange etc. See Author and Abstract information and a proposal .
Most potentials give too little binding energy.
See the write-up by Rocco Schiavilla et al. Began discussing weak decays, especially beta-decay of Tritium. See the book by Povh et al., especially Chapter 15.5 and 17.6, and equivalent material in Wong. Also, for a reminder on weak interactions in general look at my Lecture 15 and Lecture 18 . This is the degree of forbiddenness.