Role of Perturbed Angular Correlations in Symmetry Tests

Abstract

The most familiar "rules" for testing the invariance of nuclear electromagnetic transitions under space and time inversions are based on the theory of unperturbed angular correlations. We show that the perturbation of an angular correlation can increase the efficiency of such experiments by removing the need for using polarized particles. An external field can reorient the spin of the intermediate state, changing an alignment into a polarization. The theory of such "off-diagonal" perturbations is discussed, with attention to two specific models: electric quadrupole interaction with an axially symmetric electric field gradient, and magnetic dipole interaction with external and internal magnetic fields. The quadrupole interactions are found to be very efficient in producing nuclear orientation of odd rank. A survey of possible applications of these results is given, with emphasis on the decay of ${\mathrm{Ta}}^{181}$.