Xi baryon

Xi baryon

Composition	Ξ0 : u s s Ξ− : d s s
Statistics	Fermionic
Family	Baryons
Interactions	Strong, weak, electromagnetic, and gravity
Types	2
Mass	Ξ0 : 1314.86±0.20 MeV/c2 Ξ− : 1321.71±0.07 MeV/c2
Spin	1⁄2
Strangeness	-2
Isospin	1⁄2

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). The first discovery of a charged Xi baryon was in cosmic ray experiments by the Manchester group in 1952. The first discovery of the neutral Xi particle was at Lawrence Berkeley Laboratory in 1959. It was also observed as a daughter product from the decay of the omega baryon (
Ω⁻
) observed at Brookhaven National Laboratory in 1964. 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 DØ 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 Ξ_c, Ξ_b, Ξ_cc, Ξ_cb, 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). (Here,"*" indicates a baryon decuplet.) The LHCb experiment at CERN discovered two new Xi baryons in 2014:
Ξ′⁻
_b and
Ξ^∗−
_b.

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.

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

Particle	Symbol	Makeup	Rest mass (MeV/c2)	Isospin, I	Spin, parity, JP	Q	S	C	B	Mean lifetime (s)	Decays to
Xi	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	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	Ξ0 (1530)	u s s	1531.80±0.32	1/2	3/2+	0	−2	0	0	(7.2±0.4)×10−23	Ξ + π
Xi resonance	Ξ− (1530)	d s s	1535.0±0.6	1/2	3/2+	−1	−2	0	0	6.6+1.6 −1.0×10−23	Ξ + π
charmed Xi	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	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	Ξ′+ c	u s c	2575.7±3.1	1/2	(1/2+)	+1	−1	+1	0		Ξ+ c + γ (Seen)
charmed Xi resonance	Ξ′0 c	d s c	2578.0±2.9	1/2	(1/2+)	0	−1	+1	0		Ξ0 c + γ (Seen)
double charmed Xi	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	Λ+ c + K− + π+ + π+
double charmed Xi	Ξ+ cc	d c c	3518.9±0.9	(1/2)	(1/2 +)	+1	0	+2	0	<3.3×10−14	Λ+ c + K− + π+ [e] or p+ + D+ + K− [e]
bottom Xi	Ξ0 b	u s b	5792±3	(1/2)	(1/2 +)	0	−1	0	−1	1.42+0.28 −0.24×10−12	See Ξ b decay modes
bottom Xi or Cascade B	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	d s b	5935.02±0.02 ± 0.01 ± 0.50		1/2+	−1	−1	0	−1	>8×10−21
	Ξ∗− b	d s b	5955.33±0.12 ± 0.06 ± 0.50		3/2+	−1	−1	0	−1	(3.99±0.75 ± 0.25)×10−22
	Ξ b(6227)−	d s b	6226.9±2.0 ± 0.3 ± 0.2			−1	−1	0	−1	3.64+1.55 −0.84 ±0.36×10−23	Λ b + K− Ξ0 b + π−
double bottom Xi	Ξ0 bb	u b b		(1/2)	(1/2 +)	0	0	0	−2
double bottom Xi	Ξ− bb	d b b		(1/2)	(1/2 +)	−1	0	0	−2
charmed bottom Xi	Ξ+ cb	u c b		(1/2)	(1/2) +	+1	0	+1	−1
charmed bottom Xi	Ξ0 cb	d c b		(1/2)	(1/2 +)	0	0	+1	−1

Table notes

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

External links

• Listings for Xi particles (Report). Particle Data Group. Archived from the original on 2012-12-12. Retrieved 2007-07-31.
• "Direct observation of the strange b baryon Xi(b)-". Stanford University.
• "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)-". Stanford University.