Physics:Omega baryon

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Short description: Subatomic hadron particle
"Omega particle" redirects here. For usage of the term in Star Trek, see The Omega Directive.
Not to be confused with Omega meson.
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 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.

The earliest observed omega baryon was the Ω−, made of three strange quarks. It was first observed 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. 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 therefore has 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
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[6] 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.[7][8] 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 "Ωb 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.[6] 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.[9]

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+.[10][11] The states are named Ωc(3000)0, Ωc(3050)0, Ωc(3066)0, Ωc(3090)0 and Ωc(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

  1. ↑ Particle Data Group. "2010 Review of Particle Physics – Naming scheme for hadrons". http://pdg.lbl.gov/2011/reviews/rpp2011-rev-naming-scheme-hadrons.pdf. 
  2. ↑ A. Quadt (2006). "Top quark physics at hadron colliders". European Physical Journal C 48 (3): 835–1000. doi:10.1140/epjc/s2006-02631-6. Bibcode: 2006EPJC...48..835Q. https://cds.cern.ch/record/1339554. 
  3. ↑ V. E. Barnes (1964). "Observation of a Hyperon with Strangeness Minus Three". Physical Review Letters 12 (8): 204. doi:10.1103/PhysRevLett.12.204. Bibcode: 1964PhRvL..12..204B. http://teachers.web.cern.ch/teachers/archiv/HST2001/bubblechambers/omegaminus.pdf. 
  4. ↑ R. Nave. "The Omega baryon". http://hyperphysics.phy-astr.gsu.edu/hbase/particles/omega.html#c1. 
  5. ↑ Körner, J.G; Krämer, M; Pirjol, D (1994-01-01). "Heavy baryons". Progress in Particle and Nuclear Physics 33: 787–868. doi:10.1016/0146-6410(94)90053-1. Bibcode: 1994PrPNP..33..787K. 
  6. ↑ 6.0 6.1 T. Aaltonen et al. (CDF Collaboration) (2009). "Observation of the Ω−b and Measurement of the Properties of the Ξ−b and Ω−b". Physical Review D 80 (7). doi:10.1103/PhysRevD.80.072003. Bibcode: 2009PhRvD..80g2003A. 
  7. ↑ "Fermilab physicists discover "doubly strange" particle". Fermilab. 3 September 2008. http://www.fnal.gov/pub/presspass/press_releases/Dzero_Omega-sub-b.html. 
  8. ↑ V. Abazov et al. (DØ Collaboration) (2008). "Observation of the doubly strange b baryon Ω−b". Physical Review Letters 101 (23). doi:10.1103/PhysRevLett.101.232002. PMID 19113541. Bibcode: 2008PhRvL.101w2002A. 
  9. ↑ R. Aaij et al. (LHCb collaboration) (2013). "Measurement of the Λ0b, Ξ−b and Ω−b baryon masses". Physical Review Letters 110 (18). doi:10.1103/PhysRevLett.110.182001. PMID 23683191. Bibcode: 2013PhRvL.110r2001A. 
  10. ↑ "LHCb observes an exceptionally large group of particles". CERN. http://home.cern/about/updates/2017/03/lhcb-observes-exceptionally-large-group-particles. 
  11. ↑ R. Aaij et al. (LHCb collaboration) (2017). "Observation of five new narrow Ω0c states decaying to Ξ+cK−". Physical Review Letters 11801 (2017). doi:10.1103/PhysRevLett.118.182001. PMID 28524669. Bibcode: 2017PhRvL.118r2001A. 



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