Omega baryon

"Omega particle" redirects here. For usage of the term in Star Trek, see The Omega Directive.

Not to be confused with Omega meson.

Omega baryons (often called simply omega particles) are a family of subatomic hadrons 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 down quarks.^[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.

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 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 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 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.

The naming convention of baryons has become such that those with no light (i.e. up or down) valence quarks are called omega baryons. By default, the quarks are strange quarks, but those with one or more the strange quarks replaced by charm or bottom quarks have a subscript c or b, respectively.

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
Omega[6]	Ω−	sss	1672.45±0.29	⁠3/2⁠+	−1	−3	0	0	(8.21±0.11)×10−11	Λ0 + K− or Ξ0 + π− or Ξ− + π0
Charmed omega[7]	Ω0 c	ssc	2697.5±2.6	⁠1/2⁠+	0	−2	+1	0	(268±24)×10−15	See Ω0 c Decay Modes
Bottom omega[8]	Ω− b	ssb	6054.4±6.8	⁠1/2⁠+	−1	−2	0	−1	(1.13±0.53)×10−12	Ω− + J/ψ (seen)
Double charmed omega†	Ω+ cc	scc		⁠1/2⁠+	+1	−1	+2	0
Charmed bottom omega†	Ω0 cb	scb		⁠1/2⁠+	0	−1	+1	−1
Double bottom omega†	Ω− bb	sbb		⁠1/2⁠+	−1	−1	0	−2
Triple charmed omega†	Ω++ ccc	ccc		⁠3/2⁠+	+2	0	+3	0
Double charmed bottom omega†	Ω+ ccb	ccb		⁠1/2⁠+	+1	0	+2	−1
Charmed double bottom omega†	Ω0 cbb	cbb		⁠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.^[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⁺
_cK⁻
, where the Ξ⁺
_c was reconstructed in the decay mode pK⁻
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.

See also

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