Time reversal  Fundamental symmetries  Fundamental symmetries

Parity violation

At present, the intriguing question is whether the weak interaction violates parity in a maximal way. The left-right symmetric extensions of the Standard Model restore parity symmetry at energies which exceed significantly the mass-equivalent of the hypothetical right handed $W^{\pm}_{R}$ bosons. Measurements of the relative polarisation of leptons, emitted anti- and parallel to the spin direction of the parent, bear enhanced sensitivity for effects induced by $W^{\pm}_{R}$'s. This method has already been applied successfully in nuclear beta decay. ¹²N, ¹⁷F, ²¹Na, ¹⁰⁷In nuclei are a subject of refined or new investigations at CERN (ISOLDE), PSI, and in university laboratories such as Louvain-la-Neuve. A new study of this kind is in progress at PSI for muon decay. Recent discrepancies in the absolute beta decay asymmetries in free neutron decay should be soon resolved in a series of experiments with cold neutrons decaying in the beam. In the future these experiments will be supplemented by studies with ultra-cold neutrons polarised to almost 100% and stored in bottles and traps. A particularly elegant alternative, insensitive to the Standard Model uncertainties and systematic effects, is provided by a simultaneous measurement of the emission asymmetries of the electron and neutrino with respect to the neutron spin. The next series of parity violation experiments at low energies with polarised muons, neutrons and nuclei will provide results in excess of   500 GeV/c² for the mass of the right handed bosons. For certain theoretical scenarios such limits are difficult to reach in measurements at high energy colliders. Precision - experiments on atomic parity-violation (Cs, Fr, ...) probe the Z^o-coupling and so provide results complementary to the ones obtained at high energy.