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The Cobalt 60 experiment

In 1956, following a theoretical prediction of Lee and Yang, a young women, Mrs Wu, impulsed with misters Ambler, Hayward, Hoppes and Hudson (Phys. Rev. 105, 1413 (1957)) show that electrons from beta decay of Cobalt 60 nuclei refrigerated by adiabatic demagnetization and whose spins were oriented in a magnetic field, are not symmetrically ejected over and under the plane perpendicular to the cobalt nuclei spins. There are more electrons going into the direction opposite to the cobalt nuclei spins.

The conclusion was the following: the space symmetry, an operation called P, is violated by the beta decays, that means by weak interaction. The mirror image of a beta decay is not a beta decay. It is the same for the neutrino. The left helicity neutrinos can interact with matter but right helicity neutrinos cannot.

This experiment showed also that the particle / anti-particle symmetry (the C symmetry) was also violated by weak interactions. Eight years later, J. Cronin and his team show that the product CP is also violated by weak interactions.

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Last update: 26/06/1999:
Didier Verkindt