Parity-violating interaction effects in thenpsystem

We investigate parity-violating observables in the $np$ system, including the longitudinal asymmetry and neutron-spin rotation in $\vec{n}p$ elastic scattering, the photon asymmetry in $\vec{n}p$ radiative capture, and the asymmetries in deuteron photodisintegration $d(\vec{\gamma },n)p$ in the threshold region and electrodisintegration $d(\vec{e},e^{\prime })np$ in quasielastic kinematics. To have an estimate of the model dependence for the various predictions, a number of different, latest-generation strong-interaction potentials—Argonne $v_{18}$, Bonn 2000, and Nijmegen I—are used in combination with a weak-interaction potential consisting of $\pi$-, $\rho$-, and $\omega$-meson exchanges—the Desplanques-Donoghue-Holstein (DDH) model. The complete bound and scattering problems in the presence of parity-conserving, including electromagnetic, and parity-violating potentials are solved in both configuration and momentum space. The issue of electromagnetic current conservation is examined carefully. We find large cancellations between the asymmetries induced by the parity-violating interactions and those arising from the associated pion-exchange currents. In the $\vec{n}p$ capture, the model dependence is nevertheless quite small, because of constraints arising through the Siegert evaluation of the relevant $E_1$ matrix elements. In quasielastic electron scattering these processes are found to be insignificant compared to the asymmetry produced by $\gamma \text{−}Z$ interference on individual nucleons. These two experiments, then, provide clean probes of different aspects of weak-interaction physics associated with parity violation in the $np$ system. Finally, we find that the neutron-spin rotation in $\vec{n}p$ elastic scattering and asymmetry in deuteron disintegration by circularly polarized photons exhibit significant sensitivity both to the values used for the weak vector-meson couplings in the DDH model and to the input strong-interaction potential adopted in the calculation. This reinforces the conclusion that these short-ranged meson couplings are not in themselves physical observables; rather, the parity-violating mixings are the physically relevant parameters.