n. (context physics English) A subatomic hadron particle, denoted by Ξ, comprising three quarks: one up or down quark, and two heavier quarks.
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, +1, 0, or −1 e . Like all baryons, they contain three quarks: one up or down quark, and two more massive quarks. 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 observed at Brookhaven National Laboratory in 1964. The Xi spectrum is important to nonperturbative quantum chromodynamics (QCD).
The 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 .
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 contains one up, one strange, and one bottom quark, while a contains one up and two bottom quarks.
In 2012, the CMS experiment at the Large Hadron Collider detected a baryon (reported mass ). LHCb discovered two new Xi baryons in 2014: and