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.^[why?]

HistoryEdit

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] (Here, * indicates a baryon decuplet.) 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 baryonsEdit

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^{[9][10][11][12]}

Particle	Symbol	Makeup	Rest mass (MeV/c2)	Isospin, I	Spin, parity, JP	Q	S	C	B	Mean lifetime (s)	Decays to
Xi[13]	Ξ0	u s s	1314.86±0.20	1/2	1/2(+)	0	−2	0	0	(2.90±0.09)×10−10	Λ0  +  π0
Xi[14]	Ξ−	d s s	1321.71±0.07	1/2	1/2(+)	−1	−2	0	0	(1.639±0.015)×10−10	Λ0  +  π−
Xi resonance[15]	Ξ0 (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]	Ξ0 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  Ξ0 c Decay Modes
charmed Xi resonance[16]	Ξ′+ c	u s c	2575.7±3.1	1/2	(1/2+)	+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+)	0	−1	+1	0	1.1×10−13	Ξ0 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[a][19]	Ξ+ cc	d c c	3518.9±0.9[a]	(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[20]	Ξ0 b	u s b	5792±3	(1/2)	(1/2 +)	0	−1	0	−1	1.42+0.28 −0.24×10−12[b]	See  Ξ b Decay Modes
bottom Xi or Cascade B[9][12][20]	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[21]	d s b	5935.02±0.02 ± 0.01 ± 0.50		1/2+	−1	−1	0	−1
	Ξ∗− b[21]	d s b	5955.33±0.12 ± 0.06 ± 0.50		3/2+	−1	−1	0	−1
	Ξ b(6227)−[22]	d s b	6226.9±2.0 ± 0.3 ± 0.2			−1	−1	0	−1		Λ b +  K− Ξ0 b +  π−
double bottom Xi[c]	Ξ0 bb	u b b		(1/2)	(1/2 +)	0	0	0	−2
double bottom Xi[c]	Ξ− bb	d b b		(1/2)	(1/2 +)	−1	0	0	−2
charmed bottom Xi[c]	Ξ+ cb	u c b		(1/2)	(1/2) +	+1	0	+1	−1
charmed bottom Xi[c]	Ξ0 cb	d c b		(1/2)	(1/2 +)	0	0	+1	−1

1. ^ ^a b c Some controversy exists about this data. See references
2. ^ 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.
3. ^ ^a b c d Particle (or quantity, i.e. spin) has neither been observed nor indicated

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