The weak interaction

Weak interaction

Four interactions are known today: the gravitation, the electromagnetism, the strong interaction and the weak interaction. The last one was observed already 100 years ago, in 1896, by H. Becquerel, who was talking at that time only about uranium salts radiation or "uranic rays". He was discovering radioactivity. It is not before 1934 that weak interaction received a well established theoretical fundement with the work of Enrico Fermi.

Today, we talk about electro-weak interaction because, since 1974, thanks to the works A. Salam, S. Weinberg and S. Glashow, the electromagnetic interaction and the weak interaction have been unified in the same description. According to this theory, there is a boson, called the photon, which transports the electromagnetic interaction: this is the light. For the transport of the weak interaction, 3 other bosons are needed: the W+, the W- and the Z. When a radioactive nucleus emits an electron (beta decay), it is in fact a neutron within the nucleus that transforms into a proton by emitting a W- which, soon after, decays into an electron anti-neutrino pair.

The weak interaction has very astonishing properties. Until now, this is the only known interaction where has been observed a violation of parity (P), which is the spatial symetry, and a violation of the symetry between particles and anti-particles (C).

In 1957, Mrs Wu and Misters Ambler, Hayward, Hobbes and Hudson of the National Bureau of Standards verified an hypothesis made some month earlier by two theoricists, T.D.Lee and C.N.Yang: the mirror image of a physical process where weak interaction is involved, does not exist in nature (P violation). The experiment seemed to prove also that the C symetry was violated.

In 1964, J. Cronin and his team showed that, in the decays of the particles called neutral kaons, not only the spatial symetry P was violated but also, the combination CP was violated. This CP symetry breaking is very tiny but could have created an initial asymetry between matter and antimatter at the beginning of the universe. We would then all come from a very small overamount of matter (1 billionth) thanks to the CP symetry violation in weak interaction!...
Until now, the CP violation had been observed only in neutral kaons decays. Since 2004, the BaBar experiment, using the Standford accelerator, has shown that CP is violtated also in the B mesons decays, and much more strongly.

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Last update: 26/06/1999 : http://wwwlapp.in2p3.fr/neutrinos/aifaible.html

Didier Verkindt