CDF discovers a heavy relative of the neutron

26 August 2011

The CDF collaboration at Fermilab has announced the observation of the Ξ0b, the latest entry in the periodic table of baryons. Although Fermilab’s Tevatron is not a dedicated bottom-quark factory, the sophisticated particle detectors employed there and large integrated luminosity of proton–antiproton collisions delivered to the experiments have made it a haven for discovering and studying almost all of the known bottom baryons. Experiments there discovered the Σb baryons in 2006, observed the Ξb baryon in 2007 and found the Ωb in 2009. The lightest bottom baryon, the Λb, was discovered at CERN.

The complex decay pattern of the neutral Ξ0b has made the observation of this particle significantly more challenging than that of its charged sibling. Combing through an integrated luminosity of 4.2 fb–1 of proton–antiproton collisions produced at a centre-of-mass energy of 1.96 TeV, the CDF collaboration isolated 25 examples in which the particles emerging from a collision revealed the distinctive signature of the Ξ0b, through its decay to Ξ+cπ and the subsequent decay chain. The analysis established the discovery at a level of 6.8 σ and measured the mass of the Ξ0b as 5787.8 ± 5.0(stat) ± 1.3(syst) MeV/c2.

CDF also observed a similar number of events for the charged Ξb, in the decay Ξb → Ξ0cπ, never previously observed; this served as an independent cross-check of the analysis.