Bubble chamber trace of the first observed Ω baryon event at Brookhaven National Laboratory, adapted from original tracing. The tracks of neutral particles (dashed lines) are not visible in the bubble chamber. The collision of a K− meson with a proton creates an Ω−, a K0 and a K+. The Ω− decays into a π− and a Ξ0, which in turn decays into a Λ0 and a π0. The Λ0 decays into a proton and a π−. The π0, invisible due to its short lifetime, decays into two photons (γ), which in turn each create an electron-positron pair.
Omega baryons (often called simply Omega particles) are a family of subatomichadrons which are represented by the symbol Ω and are either charge neutral or have a +2, +1 or −1 elementary charge. Additionally, they contain no up or downquarks.[1] Omega baryons containing top quarks are also not expected to be observed. This is because the Standard Model predicts the mean lifetime of top quarks to be roughly 5×10−25 s,[2] which is about a twentieth of the timescale necessary for the strong interactions required for Hadronization, the process by which hadrons form from quarks and gluons.
The first omega baryon was the Ω− , it was made of three strange quarks, and was discovered in 1964.[3] The discovery was a great triumph in the study of quarks, since it was found only after its existence, mass, and decay products had been predicted in 1961 by the AmericanphysicistMurray Gell-Mann and, independently, by the IsraeliphysicistYuval Ne'eman. Besides the Ω− , a charmed omega particle ( Ω0 c) was discovered in 1985, 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.[4]Spin (J) and parity (P) values for unobserved baryons are predicted by the quark model.[5]
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.
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.[9][10] However, the reported mass of 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.[8]CDF measured the mass to be 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 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.[11]
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+ .[12][13] 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.
