Xi baryon

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In particle physics, Ξ (Xi) is a name given to a range of baryons with one up or down quark and two heavier quarks. They are sometimes called the cascade particles because of their unstable state, they decay rapidly into lighter particles through a chain of decays. The first discovery of the Xi particle was at Brookhaven National Laboratory in 1964.[1]

The Ξ−b particle is also known as the cascade B particle and contains quarks from all three families. It was discovered by D0 and CDF experiments at Fermilab. The discovery was announced on June 12, 2007. It was the first known particle made of quarks from all three quark families -- namely, a down quark, a strange quark, and a bottom quark. D0 experiment reported mass of the new state to be 5.774±0.19 GeV/c2; and CDF measured the mass at 5.7929±0.3 GeV/c2. The two results are consistent with each other. An unofficial weighted mean mass is 5.792±0.3 GeV/c2.

When unspecified, the non-up/down quark make up of Xi baryons is strange. Thus a Ξ0b is a down/strange/bottom Xi, and a Ξ0bb is a up/bottom/bottom Xi.

Xi Baryons[2][3][4][5]
Particle Symbol Makeup Rest mass
MeV/c²		 Isospin
I		 Spin(Parity)
JP		 Q S C B Mean lifetime
s		 Decays to
Xi [6] Ξ0 uss 1 314.86 ± 0.20 1⁄2 1⁄2+* 0 −2 0 0 2.90 ± 0.9×10−10 Λ0 + π0
Xi [7] Ξ− dss 1321.31 ± 0.13 1⁄2 1⁄2+* −1 −2 0 0 1.639±0.15×10−10 Λ0 + π−
Xi resonance [8] Ξ0(1530) uss 1 531.80(32) 1⁄2 3⁄2+ 0 −2 0 0 Ξ + Ξ
Xi resonance [8] Ξ−(1530) dss 1 535.0(6) 1⁄2 3⁄2+ −1 −2 0 0 Ξ + Ξ
charmed Xi[9] Ξ+c usc 2 467.9 ± 0.4 1⁄2 1⁄2* +* +1 −1 +1 0 4.42±0.26×10−13 See Ξ+c Decay Modes
charmed Xi[9] Ξ0c dsc 2 471.0 ± 0.4 1⁄2 1⁄2* +* 0 −1 +1 0 1.12+0.13−0.1×10−13 See Ξ0c Decay Modes
charmed Xi resonance [9] Ξ′+c usc 2 575.7±3.1 1⁄2 1⁄2+† +1 −1 +1 0 Ξ+c + γ (Seen)
charmed Xi resonance [9] Ξ′0c dsc 2 578.0±2.9 1⁄2 1⁄2+† 0 −1 +1 0 1.1×10−13 Ξ0c + γ (Seen)
double charmed Xi† Ξ++cc ucc 1⁄2* 1⁄2* +* +2 0 +2 0
double charmed Xi[a] [10] Ξ+cc dcc 3 518.9 ± 0.9[c] 1⁄2* 1⁄2* +* +1 0 +2 0 <3.3×10−14[a] Λ+c + K− + π+[e] or
p+ + D+ + K−[e]
bottom Xi [11] Ξ0b usb 5 792 ± 3 1⁄2* 1⁄2* +* 0 −1 0 −1 1.42+0.28−0.24×1012[b] See Ξb Decay Modes
bottom Xi or
Cascade B [11][12][13]		 Ξ−b dsb 5 792.9 ± 3.0 1⁄2* 1⁄2* +* −1 −1 0 −1 1.42×10−12 See Ξb Decay Modes
(l + J/ψ was also seen)
double bottom Xi† Ξ0bb ubb 1⁄2* 1⁄2* +* 0 0 0 −2
double bottom Xi† Ξ−bb dbb 1⁄2* 1⁄2* +* −1 0 0 −2
charmed bottom Xi† Ξ+cb ucb 1⁄2* 1⁄2* + +1 0 +1 −1
charmed bottom Xi† Ξ0cb dcb 1⁄2* 1⁄2* +* 0 0 +1 −1

[a] Some controversy exists about this data. See references
[b] This is actually a measurement of the average lifetime of b-baryons that decay to a jet containing a same sign Ξ±l± pair. Presumably the mix is mainly Ξb, with some Λb.
† Particle (or quantity, i.e. spin) has neither been observed nor indicated.

[edit] See also

[edit] References

  1. ^ The Xi Baryon
  2. ^ http://arxiv.org/PS_cache/arxiv/pdf/0706/0706.1690v2.pdf
  3. ^ Fermilab physicists discover "triple-scoop" baryon. Fermilab (2007-06-13). Retrieved on 2007-06-14.
  4. ^ Back-to-Back b Baryons in Batavia. Fermilab (2007-06-25). Retrieved on 2007-06-25.
  5. ^ http://www.slac.stanford.edu/spires/find/hep/www?eprint=arXiv:0707.0589 "Observation and mass measurement of the baryon Xi(b)-"
  6. ^ W.-M. Yao et al. (2006). "Particle listings - Neutral Xi". Journal of Physics G 33 (1). Particle Data Group. 
  7. ^ W.-M. Yao et al. (2006). "Particle listings - Negative Xi". Journal of Physics G 33 (1). Particle Data Group. 
  8. ^ a b Particle Data Groups: 2006 Review of Particle Physics - Xi(1530). Retrieved on 2008-04-20.
  9. ^ a b c d W.-M. Yao et al. (2006). "Particle listings - Charmed baryons". Journal of Physics G 33 (1). Particle Data Group. 
  10. ^ W.-M. Yao et al. (2006). "Particle listings - Double charmed positive Xi". Journal of Physics G 33 (1). Particle Data Group. 
  11. ^ a b W.-M. Yao et al. (2006). "Particle listings - Bottom Xis". Journal of Physics G 33 (1). Particle Data Group. 
  12. ^ V. M. Abazov et al. (2007). "Direct observation of the strange b baryon Xi(b)-.". Physical Review Letters 99: 052001. 
  13. ^ T. Aaltonen et al. (2007). "Observation and mass measurement of the baryon Xi(b)-.". Physical Review Letters 99: 052002. 

[edit] External links

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