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Symmetry tests
Observables which are odd under the space inversion (parity) or time reversal transformations provide a simple means to search for deviations from maximal parity violation in the weak interaction or for new sources of T (or CP) violation in the strangeness-conserving sector probed by beta-decay experiments. Many scenarios beyond the SM predict the restoration of parity symmetry at some higher energy scale or the presence of new CP-violating phases beyond the standard electroweak CP phase of the CKM matrix. Precision experiments at low energies can often probe energy scales not otherwise accessible and are hence complementary to the searches for new physics performed at the highest possible energies. Significant improvements have been achieved in measurements of the longitudinal and transverse polarizations of beta particles from polarized nuclei and in other correlation parameters in nuclear and neutron decays. A precision level of a few 10-3 has been reached on several observables and provided new constraints on the parameter space of SM extensions. New exploratory experiments for parity and time-reversal symmetry tests have been performed using atom traps in which the radioactive atoms were polarized with lasers. Other such tests involve interference effects with the electromagnetic interaction in atoms (PNC) and the search for non-permanent electric dipole moments. In general, the sensitivity to symmetry violations in atoms is strongly enhanced for isotopes with high atomic numbers, which in addition may be radioactive. These experiments require beams or sources of very high intensity and purity as well as the development of polarization techniques.
