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Time reversal

Cosmological development of the Universe requires existence of hitherto unknown mechanisms of CP or T violation. New T-violating phenomena may be generated e.g. by a $\Theta$-term in QCD interactions, or by the exchange of leptoquarks, Higgs or right handed weak bosons with complex couplings. T-violating correlation experiments determine spin polarisations and/or momenta of the particles involved in the decay process. The systems studied include the muon, neutron, 8Li and 19Ne nuclei, K+ and K0 mesons, $\Lambda^0$ and $\Sigma$-hyperons; and recently even polarised Z0. The achieved accuracies reach 10-3 in the T-violating amplitudes with respect to the regular weak interaction strength. Here, progress is achieved by a combination of new experimental techniques and development of the premium quality polarised beams and targets. T-violation in muon decay will be further investigated at PSI. New experiments with slow neutrons are in development at National Institute of Standards and Technology (NIST)-USA, and at the spallation neutron source under commissioning at PSI. These studies determine the complex amplitudes of the vector, axial vector, scalar or tensor weak interaction terms, which are related to the coupling constants and masses of the exchanged new gauge bosons. We quote the limits of   3 TeV/c2 from beta decay for the mass/amplitude ratios in lepto-quark exchange. T-violating electric dipole moments (EDM) arise at the Lagrangian level from the QCD's $\Theta$ term. In other theories, nonzero EDM's are induced e.g. via higher order processes (quantum loop corrections). Classical EDM tests are performed with polarised free neutrons. Present experiments restrict the neutron EDM to values less than 1.1 x 10-25 e.cm. Results on atomic EDM close to the above limit have also been obtained using complex systems : Hg and Tl atoms, or TIF molecules. New, challenging ideas are pursued to improve the sensitivity by three orders of magnitude. The complexities of the new experiments call for large international collaborations. Studies have already begun in the USA and Japan in the context of new spallation sources. The new EDM experiments will either establish a finite effect, or reject some popular theories such as Weinberg's multi-Higgs or left-right symmetric models. 
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Next: Other tests Up: Fundamental symmetries Previous: Parity violation 
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