Parity-non-conservation in nuclear forces at low energy : phenomenology and questions

The information available from the analysis of parity-non-conserving (pnc) effects in nuclear systems at low energy is considered. While five pieces of information corresponding to the five transitions between S and P nucleon–nucleon states should be at least determined, only three are presently more or less known. They are the strength of the pnc proton–nucleus and neutron–nucleus forces and that of the proton–proton force. Although it is not complete, this knowledge already provides a valuable discussion when it is compared to theoretical expectations. The discussion concerns the strength of the pnc π NN coupling constant, which determines the long-range part of pnc isovector forces, and the contribution of the pnc proton–proton force to the pnc proton–nucleus force. Both are too low in comparison with current expectations. Several issues to overcome these difficulties, that appeared with the advent of more accurate measurements of pnc effects in 18 F and proton–proton scattering, are examined. They include an update of earlier estimates of the pnc π NN coupling constant, the role of collective effects in nuclei and new contributions to the pnc NN force, yet undetermined. Experiments, which may provide the missing information, are discussed. (Elsevier