Fermilab’s CDF experiment observes the Ω_b^–baryon

The CDF collaboration has announced the observation of a new particle, the Ω_b^– baryon, containing three quarks: two strange quarks and a bottom quark (ssb). The sighting of this “doubly strange” particle, predicted by the Standard Model, is significant because it strengthens physicists’ confidence in their understanding of how quarks form matter. However, it conflicts with a result announced in 2008 by CDF’s sister experiment, DØ.

The Ω_b^– is the latest entry in the “periodic table of baryons” illustrated in the figure. The Tevatron is unique in its ability to produce baryons containing the b quark, and the large data samples now available after many years of successful running have enabled experimenters to find and study these rare particles. The discovery of the Ω_b^– follows the first observations of two types of Σ_b baryons at the Tevatron in 2006 and the discovery there of the Ξ_b^– baryon in 2007.

Combing through almost 5 × 10¹¹ proton–antiproton collisions produced by the Tevatron, the CDF collaboration isolated 16 examples in which the particles emerging from collisions reveal the distinctive signature of the Ω_b^–, which travels only a fraction of a millimetre before it decays into lighter particles. CDF has performed the first ever measurement of the Ω_b^–‘s lifetime and obtained 1.13 + 0.53 – 0.40(stat.) ± 0.02(syst.) × 10^–12 s.

In August 2008, the DØ experiment announced its own observation of the Ω_b^– based on a smaller sample of data from the Tevatron. Interestingly, the new observation from CDF conflicts with this earlier result. The CDF collaboration measures the mass of the Ω_b^– to be 6054.4 ± 6.8(stat.) ± 0.9(syst.) MeV/c², compared with DØ’s findings of 6165 ± 10(stat.) ± 13(syst.) MeV/c². These two results are statistically inconsistent, leaving the teams from the two experiments wondering whether they are measuring the same particle. Furthermore, the experiments observed different rates of production for this particle. Perhaps most interesting is that neither experiment sees a hint of evidence for a particle at the mass value measured by the other.

Although the latest result announced by CDF agrees with theoretical expectations for the Ω_b^–, both in the measured production rate and in the mass value, further investigation is needed to solve the puzzle of these conflicting results.