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Concept
Baryon Oméga
Résumé
The omega baryons are a family of subatomic hadron (a baryon) particles that are represented by the symbol _Omega and are either neutral or have a +2, +1 or −1 elementary charge. They are baryons containing no up or down quarks. Omega baryons containing top quarks are not expected to be observed. This is because the Standard Model predicts the mean lifetime of top quarks to be roughly 5e-25s, which is about a twentieth of the timescale for strong interactions, and therefore that they do not form hadrons.
The first omega baryon discovered was the _Omega-, made of three strange quarks, in 1964. The discovery was a great triumph in the study of quark processes, since it was found only after its existence, mass, and decay products had been predicted in 1961 by the American physicist Murray Gell-Mann and, independently, by the Israeli physicist Yuval Ne'eman. Besides the _Omega-, a charmed omega particle (_Charmed Omega0) was discovered in 1985, in which a strange quark is replaced by a charm quark. The _Omega- decays only via the weak interaction and has therefore a relatively long lifetime. Spin (J) and parity (P) values for unobserved baryons are predicted by the quark model.
Since omega baryons do not have any up or down quarks, they all have isospin 0.
† Particle (or quantity, i.e. spin) has neither been observed nor indicated.
The _Bottom Omega- particle is a "doubly strange" baryon containing two strange quarks and a bottom quark. A discovery of this particle was first claimed in September 2008 by physicists working on the DØ experiment at the Tevatron facility of the Fermi National Accelerator Laboratory. However, the reported mass of 6165MeV/c2 was significantly higher than expected in the quark model. The apparent discrepancy from the Standard Model has since been dubbed the "_Bottom Omega puzzle". In May 2009, the CDF collaboration made public their results on the search for the _Bottom Omega- based on analysis of a data sample roughly four times the size of the one used by the DØ experiment.
Source officielle
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