Omega baryon

From Wikipedia, the free encyclopedia
Jump to: navigation, search
"Omega particle" redirects here. For the Star Trek episode, see The Omega Directive.
Bubble chamber trace of the first observed Ω baryon event at Brookhaven National Laboratory

The Omega baryons are a family of subatomic hadron 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.[1] 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,[2] 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.[3] 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 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 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[edit]

Omega
Particle Symbol Quark
content
Rest mass
(MeV/c2)
JP Q
(e)
S C B′ Mean lifetime
(s)
Decays to
Omega[6]
Ω

s

s

s
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

s

s

c
2697.5±2.6 1/2+ 0 −2 +1 0 (6.9±1.2)×10−14 See
Ω0
c
Decay Modes
Bottom Omega[8]
Ω
b

s

s

b
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

s

c

c
1/2+ +1 −1 +2 0
Charmed bottom Omega†
Ω0
cb

s

c

b
1/2+ 0 −1 −1 −1
Double bottom Omega†
Ω
bb

s

b

b
1/2+ −1 −1 0 −2
Triple charmed Omega†
Ω++
ccc

c

c

c
3/2+ +2 0 +3 0
Double charmed bottom Omega†
Ω+
ccb

c

c

b
1/2+ +1 0 +2 −1
Charmed double bottom Omega†
Ω0
cbb

c

b

b
1/2+ 0 0 +1 −2
Triple bottom Omega†
Ω
bbb

b

b

b
3/2+ −1 0 0 −3

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

Recent discoveries[edit]

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]

See also[edit]

References[edit]

  1. ^ Particle Data Group. "2010 Review of Particle Physics – Naming scheme for hadrons" (PDF). Retrieved 2011-12-26. 
  2. ^ A. Quadt (2006). "Top quark physics at hadron colliders". European Physical Journal C. 48 (3): 835–1000. Bibcode:2006EPJC...48..835Q. doi:10.1140/epjc/s2006-02631-6. 
  3. ^ V. E. Barnes; et al. (1964). "Observation of a Hyperon with Strangeness Minus Three" (PDF). Physical Review Letters. 12 (8): 204. Bibcode:1964PhRvL..12..204B. doi:10.1103/PhysRevLett.12.204. 
  4. ^ R. Nave. "The Omega baryon". HyperPhysics. Retrieved 2009-11-26. 
  5. ^ J. G. Körner; M. Krämer & D. Pirjol (1994). "Heavy Baryons". Progress in Particle and Nuclear Physics. 33: 787–868. arXiv:hep-ph/9406359Freely accessible. Bibcode:1994PrPNP..33..787K. doi:10.1016/0146-6410(94)90053-1. 
  6. ^ Particle Data Group. "2006 Review of Particle Physics –
    Ω
    "
    (PDF). Retrieved 2008-04-20.
     
  7. ^ Particle Data Group. "2006 Review of Particle Physics –
    Ω0
    c
    "
    (PDF). Retrieved 2008-04-20.
     
  8. ^ a b 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). arXiv:0905.3123Freely accessible. Bibcode:2009PhRvD..80g2003A. doi:10.1103/PhysRevD.80.072003.
     
  9. ^ "Fermilab physicists discover "doubly strange" particle". Fermilab. 3 September 2008. Retrieved 2008-09-04. 
  10. ^ V. Abazov et al. (DØ Collaboration) (2008). "Observation of the doubly strange b baryon
    Ω
    b
    ". Physical Review Letters. 101 (23): 232002. arXiv:0808.4142Freely accessible. Bibcode:2008PhRvL.101w2002A. doi:10.1103/PhysRevLett.101.232002.
     
  11. ^ R. Aaij et al. (LHCb collaboration) (2013). "Measurement of the
    Λ0
    b
    ,
    Ξ
    b
    and
    Ω
    b
    baryon masses". Physical Review Letters. 110 (18): 182001. arXiv:1302.1072Freely accessible. Bibcode:2013PhRvL.110r2001A. doi:10.1103/PhysRevLett.110.182001.
     

External links[edit]