DØ and CDF find same new baryon

After several years of independently gathering and analyzing data at Fermilab’s Tevatron, the DØ and CDF collaborations have reported the observation of the same new baryon within days of each other.

The DØ collaboration announced the first direct observation of the strange b baryon, Ξ_b^–, in a paper submitted to Physical Review Letters on 12 June. Then, at a packed Fermilab seminar on 15 June, no sooner had Eduard De La Cruz Burelo reported on DØ’s discovery than Dmitry Litvintsev from the CDF collaboration rose to present independent evidence for the very same particle. Consisting of three quarks – d, s and b – the Ξ_b^– is the first observed particle to be formed of quarks from all three generations.

The ALEPH and DELPHI experiments at LEP had previously found indirect evidence for the Ξ_b^– in the form of an excess of events with a Ξ^– and a lepton of the same sign. Now the two experiments at the Tevatron have been able to reconstruct fully the specific decay, Ξ_b^– → J/ΨΞ^–, where the products decay in their turn: J/Ψ → μ⁺μ^–, Ξ^– → Λ⁰π^– and Λ⁰ → pπ^–. While the Λ and Ξ^– have decay lengths of a few centimetres, the Ξ_b^– travels only a millimetre or so before it decays.

The analysis basically consists of searching for events with muon pairs that correspond to a J/Ψ together with a proton and two pions of the same sign. One pion and the proton must have a mass equivalent to the Λ and come from a vertex that is an appropriate distance from the origin of the second pion. The Λ and this pion should then have a mass equivalent to a Ξ^– and a common vertex corresponding to the Ξ^–’s decay point. The next step is to match the origins of the Ξ^– candidates with those of the J/Ψs to reconstruct the decay of the Ξ_b^–.

In an analysis of 1.3 fb^–1 of data collected during 2002–2006, the DØ collaboration found 19 candidate events for Ξ_b^– while the CDF collaboration found 17 candidates in 1.9 fb^–1. Both experiments measured the mass of the new particle, with consistent results. In their submitted paper, DØ gives the measured mass as 5.774 ± 0.011 (stat.) ± 0.015 (syst.) GeV/c², and quotes a significance of 5.5 σ for the observed signal. At the seminar, CDF presented a preliminary mass value of 5.7929 ± 0.0024 (stat.) ± 0.0017 (syst.) GeV/c², with a significance of 7.8 σ. DØ has also measured the ratio of the cross-section multiplied by the branching ratio of their observed Ξ_b^– events relative to that for the well-known Λ_b baryons. The measured ratio is 0.28 ± 0.13.

The discovery of the Ξ_b^– is the latest in a chain of discoveries made by CDF and DØ over the past few years. Last October, the CDF collaboration reported the observation of Σ_b particles, related to the Ξ_b. As the Tevatron delivers more and more data, the possibilities increase for the observation of even rarer processes.