Physics:Omega baryon

Short description: Subatomic hadron particle

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.

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 5×10⁻²⁵ s, 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 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 United States physicist Murray Gell-Mann and, independently, by the Israeli physicist Yuval Ne'eman. Besides the Ω⁻, a charmed omega particle (Ω0c) 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. 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

Quark structure of omega baryon (Ω⁻)

Omega

Particle	Symbol	Quark content	Rest mass (MeV/c2)	JP	Q (e)	S	C	B'	Mean lifetime (s)	Decays to
Ω−	Strange quarkStrange quarkStrange quark	1672.45±0.29	3/2+	−1	−3	0	0	(8.21±0.11)×10−11	Lambda0 + Kaon- or Xi0 + Pion- or Xi- + Pion0
Ω0c	Strange quarkStrange quarkCharm quark	2697.5±2.6	1/2+	0	−2	+1	0	(268±24)×10−15	See Ω0c Decay Modes
Bottom omega	Ω−b	Strange quarkStrange quarkb	6054.4±6.8	1/2+	−1	−2	0	−1	(1.13±0.53)×10−12	Ω− + J/Psi (seen)
Double charmed omega†	Ω+cc	Strange quarkCharm quarkCharm quark		1/2+	+1	−1	+2	0
Charmed bottom omega†	Ω0cb	Strange quarkCharm quarkb		1/2+	0	−1	+1	−1
Double bottom omega†	Ω−bb	Strange quarkbb		1/2+	−1	−1	0	−2
Triple charmed omega†	Ω++ccc	Charm quarkCharm quarkCharm quark		3/2+	+2	0	+3	0
Double charmed bottom omega†	Ω+ccb	Charm quarkCharm quarkb		1/2+	+1	0	+2	−1
Charmed double bottom omega†	Ω0cbb	Charm quarkbb		1/2+	0	0	+1	−2
Triple bottom omega†	Ω−bbb	bbb		3/2+	−1	0	0	−3

† 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 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 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.

In March 2017, the LHCb collaboration announced the observation of five new narrow Ω0c states decaying to Ξ+cK⁻, where the Ξ+c was reconstructed in the decay mode pK⁻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.

See also

• Delta baryon
• Hyperon
• Lambda baryon
• List of mesons
• List of particles
• Nucleon
• Physics portal
• Sigma baryon
• Timeline of particle discoveries
• Xi baryon

External links

• Picture of the first event containing the Ω⁻, which happens to contain the complete decay chain of the Ω⁻.
• Science Daily – Discovery of the Ω−b
• Strangeness Minus Three - BBC Horizon 1964