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The Omega baryons are a family of subatomic hadron (a baryon) particles that are represented by the symbol
Ω
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 6975500000000000000♠5×10−25 s, which is about a twentieth of the timescale for strong interactions, and therefore that they do not form hadrons.
Contents
The first Omega baryon discovered was the
Ω−
, 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
Ω−
, a charmed Omega particle (
Ω0
c) was discovered, in which a strange quark is replaced by a charm quark. The
Ω−
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.
Omega baryons
† Particle (or quantity, i.e. spin) has neither been observed nor indicated.
Recent discoveries
The
Ω−
b 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 7003616500000000000♠6165±16 MeV/c2 was significantly higher than expected in the quark model. The apparent discrepancy from the Standard Model has since been dubbed the "
Ω
b puzzle". In May 2009, the CDF collaboration made public their results on the search for the
Ω−
b based on analysis of a data sample roughly four times the size of the one used by the DØ experiment. CDF measured the mass to be 7003605440000000000♠6054.4±6.8 MeV/c2, which was in excellent agreement with the Standard Model prediction. No signal has been observed at the DØ reported value. The two results differ by 7002111000000000000♠111±18 MeV/c2, which is equivalent to 6.2 standard deviations and are therefore inconsistent. Excellent agreement between the CDF measured mass and theoretical expectations is a strong indication that the particle discovered by CDF is indeed the
Ω−
b. In February 2013 the LHCb collaboration published a measurement of the
Ω−
b mass that is consistent with, but more precise than, the CDF result.
In March 2017, the LHCb collaboration announced the observation of five new narrow
Ω0
c states decaying to
Ξ+
c
K−
, where the
Ξ+
c was reconstructed in the decay mode
p
K−
π+
. The states are named
Ω
c(3000)0,
Ω
c(3050)0,
Ω
c(3066)0,
Ω
c(3090)0 and
Ω
c(3119)0. Their masses and widths were reported, but their quantum numbers could not be determined due to the large background present in the sample.