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The Xi baryons 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, Ξ particles contain three quarks. Ξ baryons, in particular, contain either one up or one down quark and two other, more massive quarks. The two more massive quarks are any two of strange, charm, or bottom (doubles allowed). For notation, the assumption is that the two heavy quarks in the Ξ are both strange; subscripts "c" and "b" are added for each even heavier charm or bottom quark that replaces one of the two presumed strange quarks.

They are historically called the cascade particles because of their unstable state; they are typically observed to decay rapidly into lighter particles, through a chain of decays (cascading decays).[2] The first discovery of a charged Xi baryon was in cosmic ray experiments by the Manchester group in 1952.[3] The first discovery of the neutral Xi particle was at Lawrence Berkeley Laboratory in 1959.[4] It was also observed as a daughter product from the decay of the omega baryon (
Ω
) observed at Brookhaven National Laboratory in 1964.[2] The Xi spectrum is important to nonperturbative quantum chromodynamics (QCD), such as lattice QCD.[why?]

## History

The
Ξ
b
particle is also known as the cascade B particle and contains quarks from all three families. It was discovered by 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 DØ 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.

For notation, the assumption is that the two heavy quarks are both strange, denoted by a simple Ξ ; a subscript "c" is added for each constituent charm quark, and a "b" for each bottom quark. Hence Ξc, Ξb, Ξcc, Ξcb, etc. 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).[5][6] (Here,"*" indicates a baryon decuplet.) The LHCb experiment at CERN discovered two new Xi baryons in 2014:
Ξ′
b
and
Ξ∗−
b
.[7]

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.[8][9]

## List of Xi baryons

Isospin and spin values in parentheses have not been firmly established by experiments, but are predicted by the quark model and are consistent with the measurements.

Xi baryons[10][11][12][13]
Particle Symbol Makeup Rest mass
(MeV/c2)
Isospin,
I
Spin, parity,
JP
Q S C B Mean lifetime
(s)
Decays to
Xi[14]
Ξ0
1314.86±0.20 1/2 1/2(+) 0 −2 0 0 (2.90±0.09)×10−10
Λ0
+
π0
Xi[15]
Ξ
1321.71±0.07 1/2 1/2(+) −1 −2 0 0 (1.639±0.015)×10−10
Λ0
+
π
Xi resonance[16]
Ξ0
(1530)
1531.80±0.32 1/2 3/2+ 0 −2 0 0 (7.2±0.4)×10−23[a]
Ξ
+
π
Xi resonance[16]
Ξ
(1530)
1535.0±0.6 1/2 3/2+ −1 −2 0 0 6.6+1.6
−1.0
×10−23
[a]

Ξ
+
π
charmed Xi[17]
Ξ+
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[17]
Ξ0
c
2471.0±0.4 1/2 (1/2+) 0 −1 +1 0 1.12+0.13
−0.10
×10−13
See
Ξ0
c
decay modes
charmed Xi resonance[17]
Ξ′+
c
2575.7±3.1 1/2 (1/2+) +1 −1 +1 0
Ξ+
c
+
γ
(Seen)
charmed Xi resonance[17]
Ξ′0
c
2578.0±2.9 1/2 (1/2+) 0 −1 +1 0
Ξ0
c
+
γ
(Seen)
double charmed Xi[8]
Ξ++
cc
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[b][19]
Ξ+
cc
3518.9±0.9[b] (1/2) (1/2+) +1 0 +2 0 <3.3×10−14[b]
Λ+
c
+
K
+
π+
[e] or

p+
+
D+
+
K
[e]
bottom Xi[20]
Ξ0
b
5792±3 (1/2) (1/2+) 0 −1 0 −1 1.42+0.28
−0.24
×10−12
[c]
See
Ξ
b
decay modes
bottom Xi or

Ξ
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
[21]
5935.02±0.02 ± 0.01 ± 0.50 1/2+ −1 −1 0 −1 >8×10−21[b][a]

Ξ∗−
b
[21]
5955.33±0.12 ± 0.06 ± 0.50 3/2+ −1 −1 0 −1 (3.99±0.75 ± 0.25)×10−22[a]

Ξ
b
(6227)[22]
6226.9±2.0 ± 0.3 ± 0.2 −1 −1 0 −1 3.64+1.55
−0.84

±0.36×10−23[a]

Λ
b
+
K

Ξ0
b
+
π
double bottom Xi[d]
Ξ0
bb
(1/2) (1/2+) 0 0 0 −2
double bottom Xi[d]
Ξ
bb
(1/2) (1/2+) −1 0 0 −2
charmed bottom Xi[d]
Ξ+
cb
(1/2) (1/2)+ +1 0 +1 −1
charmed bottom Xi[d]
Ξ0
cb
(1/2) (1/2+) 0 0 +1 −1
Table notes
1. The resonance width (Γ) was reported for this species. Here the conversion τ = ħ/Γ is given instead.
3. ^ This is actually a measurement of the average lifetime of b-baryons that decay to a jet containing a same-sign
Ξ
±

±
pair. Presumably the mix is mainly
Ξ
b
,
with some
Λ
b
.
4. ^ a b c d Particle (or quantity, i.e. spin) has neither been observed nor indicated

## References

1. ^ Zyla, P. A.; et al. (Particle Data Group) (2020). "Review of Particle Physics". Progress of Theoretical and Experimental Physics. 2020 (8): 083C01. doi:10.1093/ptep/ptaa104. hdl:11585/772320.
2. ^ a b Nave, R. "The Xi Baryon". HyperPhysics. Retrieved 4 April 2013.
3. ^ Armenteros, R.; et al. (Manchester group) (1952). "The properties of charged V-particles". Philosophical Magazine. 43 (341): 597. doi:10.1080/14786440608520216.
4. ^ Alvarez, L.W.; et al. (1959). "Neutral Cascade Hyperon Event". Physical Review Letters. 2 (5): 215. Bibcode:1959PhRvL...2..215A. doi:10.1103/PhysRevLett.2.215.
5. ^ Simpson, C. (28 April 2013). "New particle discovered with 'Higgs boson' machine". The Atlantic Wire. Retrieved 2013-04-04.
6. ^ Chatrchyan, S.; et al. (CMS Collaboration) (2012). "Observation of a new Ξb baryon". Physical Review Letters. 108 (25): 252002. arXiv:1204.5955. Bibcode:2012PhRvL.108y2002C. doi:10.1103/PhysRevLett.108.252002. PMID 23004588. S2CID 119204847.
7. ^ "LHCb experiment observes two new baryon particles never seen before" (Press release). Bern, CH: CERN. 19 November 2014.
8. ^ a b "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.
9. ^ 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.
10. ^ a b Abazov, V.; 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. S2CID 11568965.
11. ^ "Fermilab physicists discover "triple-scoop" baryon" (Press release). Fermilab. 13 June 2007. Retrieved 2007-06-14.
12. ^ "Back-to-back b baryons in Batavia" (Press release). Fermilab. 25 June 2007. Retrieved 2007-06-25.
13. ^ a b Aaltonen, T.; et al. (CDF Collaboration) (2007). "Observation and mass measurement of the baryon Ξ
b
". Physical Review Letters. 99 (5): 52002. arXiv:0707.0589. Bibcode:2007PhRvL..99e2002A. doi:10.1103/PhysRevLett.99.052002. hdl:10261/8530. PMID 17930745. S2CID 45025038.
14. ^ Yao, W.-M.; 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.
15. ^ Yao, W.-M.; 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.
16. ^ a b Yao, W.-M.; 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.
17. ^ a b c d Yao, W.-M.; 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.
18. ^ "Long-lived doubly-charmed particle". Updates. CERN. May 2018.
19. ^ Yao, W.-M.; 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. ^ a b Yao, W.-M.; et al. (Particle Data Group) (2006). "Particle listings – Xib" (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. ^ a b 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. S2CID 6055559.
22. ^ Aaij, R.; et al. (CMS Collaboration) (2018). "Observation of a new Ξ
b
resonance". Physical Review Letters. 121 (72002): 072002. arXiv:1805.09418. Bibcode:2018PhRvL.121g2002A. doi:10.1103/PhysRevLett.121.072002. PMID 30169090. S2CID 52139397.