Xi baryon

The Xi baryons /ksaɪ bæriənz/ or cascade particles are a family of subatomic hadron particles which have the symbol Ξ and may have an electric charge (Q) of +2 e, +1 e, 0, or −1 e, where e is the elementary charge. Like all conventional baryons, they contain three quarks. Xi baryons, in particular, contain one up or down quark plus two more massive quarks: either strange, charm or bottom. They are historically called the cascade particles because of their unstable state; they decay rapidly into lighter particles through a chain of decays.[1] The first discovery of a charged Xi baryon was in cosmic ray experiments by the Manchester group in 1952.[2] The first discovery of the neutral Xi particle was at Lawrence Berkeley Laboratory in 1959.[3] It was also observed as a daughter product from the decay of the omega baryon (
Ω⁻
) observed at Brookhaven National Laboratory in 1964.[1] The Xi spectrum is important to nonperturbative quantum chromodynamics (QCD), such as Lattice QCD.

History

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 12 June 2007. It was the first known particle made of quarks from all three quark generations – namely, a down quark, a strange quark, and a bottom quark. The D0 and CDF collaborations reported the consistent masses of the new state. The Particle Data Group world average mass is 5.7924±0.0030 GeV/c².

Unless specified, the non-up/down quark content of Xi baryons is strange (i.e. there is one up or down quark and two strange quarks). However a
Ξ⁰
_b contains one up, one strange, and one bottom quark, while a
Ξ⁰
_bb contains one up and two bottom quarks.

In 2012, the CMS experiment at the Large Hadron Collider detected a
Ξ^∗0
_b baryon (reported mass 5945±2.8 MeV/c²).[4][5] The LHCb experiment at CERN discovered two new Xi baryons in 2014:
Ξ′⁻
_b and
Ξ^∗−
_b [6].

In 2017, the LHCb researchers reported yet another Xi baryon: the double charmed
Ξ⁺⁺
_cc baryon, consisting of two heavy charm quarks and one up quark. The mass of
Ξ⁺⁺
_cc is about 3.8 times that of a proton.[7][8]

List of Xi baryons

Xi baryons[9][10][11][12]

Particle	Symbol	Makeup	Rest mass MeV/c^2	Isospin I	Spin (parity) J^P	Q	S	C	B	Mean lifetime s	Decays to
Xi [13]	Ξ^0	u s s	1314.86±0.20	1/2	1/2^+*	0	−2	0	0	(2.90±0.09)×10^−10	Λ^0 + π^0
Xi [14]	Ξ^−	d s s	1321.71±0.07	1/2	1/2^+*	−1	−2	0	0	(1.639±0.015)×10^−10	Λ^0 + π^−
Xi resonance [15]	Ξ^0 (1530)	u s s	1531.80±0.32	1/2	3/2^+	0	−2	0	0		Ξ + π
Xi resonance [15]	Ξ^− (1530)	d s s	1535.0±0.6	1/2	3/2^+	−1	−2	0	0		Ξ + π
charmed Xi[16]	Ξ^+ c	u s c	2467.9±0.4	1/2	1/2* ^+*	+1	−1	+1	0	(4.42±0.26)×10^−13	See Ξ^+ c Decay Modes
charmed Xi[16]	Ξ^0 c	d s c	2471.0±0.4	1/2	1/2* ^+*	0	−1	+1	0	1.12+0.13 −0.10×10^−13	See Ξ^0 c Decay Modes
charmed Xi resonance [16]	Ξ′^+ c	u s c	2575.7±3.1	1/2	1/2^+[lower-alpha 1]	+1	−1	+1	0		Ξ^+ c + γ (Seen)
charmed Xi resonance [16]	Ξ′^0 c	d s c	2578.0±2.9	1/2	1/2^+[lower-alpha 1]	0	−1	+1	0	1.1×10^−13	Ξ^0 c + γ (Seen)
double charmed Xi[17]	Ξ^++ cc	u c c	3621.40±0.72 ± 0.27 ± 0.14	1/2*	1/2* ^+*	+2	0	+2	0	0.256+0.024 −0.022 ±0.014×10^−12 [18]	Λ^+ c + K^− + π^+ + π^+
double charmed Xi[lower-alpha 2] [19]	Ξ^+ cc	d c c	3518.9±0.9[lower-alpha 2]	1/2*	1/2* ^+*	+1	0	+2	0	<3.3×10^−14[lower-alpha 2]	Λ^+ c + K^− + π^+ ^[e] or p^+ + D^+ + K^− ^[e]
bottom Xi [20]	Ξ^0 b	u s b	5792±3	1/2*	1/2* ^+*	0	−1	0	−1	1.42+0.28 −0.24×10^−12[lower-alpha 3]	See Ξ b Decay Modes
bottom Xi or Cascade B[9][12][20][21]	Ξ^− b	d s b	5792.9±3.0	1/2*	1/2* ^+*	−1	−1	0	−1	1.42×10^−12	See Ξ b Decay Modes ( Ξ^− + J/ψ was also seen)
	Ξ′^− b[22]	d s b	5935.02±0.02 ± 0.01 ± 0.50		1/2+	−1	−1	0	−1
	Ξ^∗− b[22]	d s b	5955.33±0.12 ± 0.06 ± 0.50		3/2+	−1	−1	0	−1
	Ξ b(6227)^−[23]	d s b	6226.9±2.0 ± 0.3 ± 0.2			−1	−1	0	−1		Λ b + K^− Ξ^0 b + π^−
double bottom Xi[lower-alpha 1]	Ξ^0 bb	u b b		1/2*	1/2* ^+*	0	0	0	−2
double bottom Xi[lower-alpha 1]	Ξ^− bb	d b b		1/2*	1/2* ^+*	−1	0	0	−2
charmed bottom Xi[lower-alpha 1]	Ξ^+ cb	u c b		1/2*	1/2* ^+	+1	0	+1	−1
charmed bottom Xi[lower-alpha 1]	Ξ^0 cb	d c b		1/2*	1/2* ^+*	0	0	+1	−1

1. Particle (or quantity, i.e. spin) has neither been observed nor indicated
2. Some controversy exists about this data. See references
3. 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
   Ξ
   _b, with some
   Λ
   _b.

See also

• Delta baryon
• Hyperon
• Lambda baryon
• List of baryons
• List of mesons
• List of particles
• Nucleon
• Omega baryon
• Sigma baryon
• Timeline of particle discoveries

References

1. R. Nave. "The Xi Baryon". HyperPhysics. Retrieved 4 April 2013.
2. R. Armenteros; et al. (1952). "The properties of charged V-particles". Philosophical Magazine. 43 (341): 597. doi:10.1080/14786440608520216.
3. L. W. Alvarez; et al. (1959). "Neutral Cascade Hyperon Event". Physical Review Letters. 2 (5): 215. Bibcode:1959PhRvL...2..215A. doi:10.1103/PhysRevLett.2.215.
4. C. Simpson (28 April 2013). "New Particle Discovered with 'Higgs Boson' Machine". The Atlantic Wire. Retrieved 2013-04-04.
5. S. Chatrchyan; et al. (CMS Collaboration) (2012). "Observation of an excite excited Ξ_b baryon". Physical Review Letters. 108 (25): 252002. arXiv:1204.5955. Bibcode:2012PhRvL.108y2002C. doi:10.1103/PhysRevLett.108.252002. PMID 23004588.
6. "LHCb experiment observes two new baryon particles never seen before". 19 Nov 2014.
7. "The LHCb experiment is charmed to announce observation of a new particle with two heavy quarks" (Press release). CERN. 2017-07-06. Retrieved 2017-07-06.
8. Chang, Kenneth (2017-07-06). "CERN Physicists Find a Particle With a Double Dose of Charm". The New York Times. ISSN 0362-4331. Retrieved 2017-07-08.
9. V. Abazov et al. (D0 Collaboration) (2007). "Direct observation of the strange b baryon
   Ξ⁻
   _b". Physical Review Letters. 99 (5): 52001. arXiv:0706.1690. Bibcode:2007PhRvL..99e2001A. doi:10.1103/PhysRevLett.99.052001. hdl:10211.3/194385. PMID 17930744.
10. "Fermilab physicists discover "triple-scoop" baryon" (Press release). Fermilab. 13 June 2007. Retrieved 2007-06-14.
11. "Back-to-Back b Baryons in Batavia" (Press release). Fermilab. 25 June 2007. Retrieved 2007-06-25.
12. T. Aaltonen et al. (CDF Collaboration) (2007). "Observation and mass measurement of the baryon Xi(b)-". Physical Review Letters. 99 (5): 52002. arXiv:0707.0589. Bibcode:2007PhRvL..99e2002A. doi:10.1103/PhysRevLett.99.052002. hdl:10261/8530. PMID 17930745.
13. W.-M. Yao et al. (Particle Data Group) (2006). "Particle listings – Xi⁰" (PDF). Journal of Physics G. 33 (1): 1–1232. arXiv:astro-ph/0601168. Bibcode:2006JPhG...33....1Y. doi:10.1088/0954-3899/33/1/001.
14. W.-M. Yao et al. (Particle Data Group) (2006). "Particle listings – Xi⁻" (PDF). Journal of Physics G. 33 (1): 1–1232. arXiv:astro-ph/0601168. Bibcode:2006JPhG...33....1Y. doi:10.1088/0954-3899/33/1/001.
15. W.-M. Yao et al. (Particle Data Group) (2006). "Particle Listings – Xi(1530)" (PDF). Journal of Physics G. 33 (1): 1–1232. arXiv:astro-ph/0601168. Bibcode:2006JPhG...33....1Y. doi:10.1088/0954-3899/33/1/001.
16. W.-M. Yao et al. (Particle Data Group) (2006). "Particle listings – Charmed baryons" (PDF). Journal of Physics G. 33 (1): 1–1232. arXiv:astro-ph/0601168. Bibcode:2006JPhG...33....1Y. doi:10.1088/0954-3899/33/1/001.
17. "The LHCb experiment is charmed to announce observation of a new particle with two heavy quarks". 6 Jul 2017.
18. https://home.cern/about/updates/2018/05/long-live-doubly-charmed-particle
19. W.-M. Yao et al. (Particle Data Group) (2006). "Particle listings – Ξ⁺
    _cc" (PDF). Journal of Physics G. 33 (1): 1. arXiv:astro-ph/0601168. Bibcode:2006JPhG...33....1Y. doi:10.1088/0954-3899/33/1/001.
20. W.-M. Yao et al. (Particle Data Group) (2006). "Particle listings – Xi_b" (PDF). Journal of Physics G. 33 (1): 1–1232. arXiv:astro-ph/0601168. Bibcode:2006JPhG...33....1Y. doi:10.1088/0954-3899/33/1/001.
21. T. Aaltonen et al. (CDF Collaboration) (2007). "Observation and mass measurement of the baryon
    Ξ⁻
    _b". Physical Review Letters. 99 (5): 052002. arXiv:0707.0589. Bibcode:2007PhRvL..99e2002A. doi:10.1103/PhysRevLett.99.052002. hdl:10261/8530. PMID 17930745.
22. Aaij, R; et al. (LHCb Collaboration) (2015). "Observation of two new Ξ⁻
    _b baryon resonances". Physical Review Letters. 114 (6): 062004. arXiv:1411.4849. Bibcode:2015PhRvL.114f2004A. doi:10.1103/PhysRevLett.114.062004. PMID 25723210.
23. Aaij, R.; et al. (CMS Collaboration) (2018). "Observation of a new Ξ⁻
    _b resonance". Physical Review Letters. 121 (72002): 072002. arXiv:1805.09418. doi:10.1103/PhysRevLett.121.072002. PMID 30169090.

External links

• "Listings for Xi particles". Particle Data Group. Archived from the original on 2012-12-12. Retrieved 2007-07-31.
• "Direct observation of the strange b baryon Xi(b)-"
• "Fermilab physicists discover "triple-scoop" baryon" (Press release). Fermilab. 2007-06-18. Retrieved 2007-06-18.
• "Back-to-Back b Baryons in Batavia" (Press release). Fermilab. 2007-06-25. Retrieved 2007-06-25.
• "Observation and mass measurement of the baryon Xi(b)-"