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, 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.
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 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 Ξ0b contains one up, one strange,
and one bottom quark, while a Ξ0bb contains
one up and two bottom quarks.
In 2012, the CMS experiment at the Large Hadron
Collider detected a Ξ∗0b baryon (reported
mass 5945±2.8 MeV/c2).
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 ==
== 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
