The Xi baryons or cascade particles are a family of subatomic hadron particles which have the symbol Ξ and have a +2, +1 or −1 elementary charge or are neutral. They are baryons containing three quarks: 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 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 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 collaboration reported the mass of the new state to be 5.774±0.019 GeV/c2; while the CDF collaboration measured the mass at 5.7929±0.0030 GeV/c2. The two results are consistent with each other. The Particle Data Group world average mass is 5.7924±0.0030 GeV/c2.
When unspecified, the non-up/down quark content of Xi baryons is strange. Thus a Ξ0
b contains one up, one strange, and one bottom quark, while a Ξ0
bb contains one up and two bottom quarks.
Experiments at the Large Hadron Collider have detected a Ξ∗0
b baryon (reported mass 5.945±2.8 GeV/c2) made up of three quarks (u s b).
| Particle | Symbol | Makeup | Rest mass MeV/c2 |
Isospin I |
Spin(Parity) JP |
Q | S | C | B | Mean lifetime s |
Decays to |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Xi | Ξ0 | uss | 1 314.86 ± 0.20 | 1⁄2 | 1⁄2+* | 0 | −2 | 0 | 0 | 2.90 ± 0.09×10−10 | Λ0 + π0 |
| Xi | Ξ− | dss | 1321.31 ± 0.13 | 1⁄2 | 1⁄2+* | −1 | −2 | 0 | 0 | 1.639±0.015×10−10 | Λ0 + π− |
| Xi resonance | Ξ0(1530) | uss | 1 531.80(32) | 1⁄2 | 3⁄2+ | 0 | −2 | 0 | 0 | Ξ + π | |
| Xi resonance | Ξ−(1530) | dss | 1 535.0(6) | 1⁄2 | 3⁄2+ | −1 | −2 | 0 | 0 | Ξ + π | |
| charmed Xi | 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 | Ξ0 c |
dsc | 2 471.0 ± 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 |
usc | 2 575.7±3.1 | 1⁄2 | 1⁄2+† | +1 | −1 | +1 | 0 | Ξ+ c + γ (Seen) |
|
| charmed Xi resonance | Ξ′0 c |
dsc | 2 578.0±2.9 | 1⁄2 | 1⁄2+† | 0 | −1 | +1 | 0 | 1.1×10−13 | Ξ0 c + γ (Seen) |
| double charmed Xi† | Ξ++ cc |
ucc | 1⁄2* | 1⁄2* +* | +2 | 0 | +2 | 0 | |||
| double charmed Xi | Ξ+ cc |
dcc | 3 518.9 ± 0.9 | 1⁄2* | 1⁄2* +* | +1 | 0 | +2 | 0 | <3.3×10−14 | Λ+ c + K− + π+ or p+ + D+ + K− |
| bottom Xi | Ξ0 b |
usb | 5 792 ± 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 |
dsb | 5 792.9 ± 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) |
| double bottom Xi† | Ξ0 bb |
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† | Ξ0 cb |
dcb | 1⁄2* | 1⁄2* +* | 0 | 0 | +1 | −1 |
Some controversy exists about this data. See references
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.
† Particle (or quantity, i.e. spin) has neither been observed nor indicated.