Omega baryon

Subatomic hadron particle From Wikipedia, the free encyclopedia

Omega baryon

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.

Thumb
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.

Omega baryons

Thumb
Quark structure of omega baryon (Ω
)
More information Particle, Symbol ...
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
Close

† Particle (or quantity, i.e. spin) has neither been observed nor indicated.

Recent discoveries

Summarize
Perspective

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 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 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

References

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