Parity Violation in Pp and Np Experiments

Parity violation experiments involving only two nucleons provide a way to study the non-leptonic, strangeness-conserving part of the weak interaction in a clean measurement free of nuclear structure uncertainties. Such measurements are particularly appropriate for discussion at this conference as their success depends critically on the ability to accurately control and measure spin. Although simple in principle, the experiments are technically very demanding and great pains must be taken both in the preparation of the incident polarized beams and the measurement of the resultant parity violating asymmetries , which may be masked by a multitude of systematic effects. At low and intermediate energies, pp experiments are sensitive to the medium range part of the parity violating nucleon-nucleon force, usually parameterized in terms of rho and omega meson exchange. The pion does not contribute to parity violation in the pp experiments, as the π 0 is its own antiparticle and parity violation would also imply CP violation. I review existing pp measurements with particular emphasis on the recent 221 MeV pp measurement at TRIUMF which permitted the weak meson-nucleon coupling constants h pp ρ and h pp ω to be determined separately for the first time. The np experiments, on the other hand, are used to extract the weak pion nucleon coupling, f π , describing the longest range part of the parity violating nucleon-nucleon force. The np system is the only two nucleon system that is sensitive to f π. I also review these experiments, with specific details of the np → dγ experiment now under preparation at Los Alamos National Laboratory. 1 target target p p detector detector detector detector SCATTERING (low energies) TRANSMISSION (intermediate and high energies) Figure 1: Types of pp experiments. The low-energy experiments use scattering geometry, while the intermediate and high-energy experiments use transmission geometry.