Observation of the Doubly Strange $b$ Baryon ${\Omega }_b^{-}$

We report the observation of the doubly strange $b$ baryon ${\Omega }_b^{-}$ in the decay channel ${\Omega }_b^{-}\to J/\psi {\Omega }^{-}$, with $J/\psi \to {\mu }^{+}{\mu }^{-}$ and ${\Omega }^{-}\to \Lambda K^{-}\to (p{\pi }^{-})K^{-}$, in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$. Using approximately $1.3{\mathrm{fb}}^{-1}$ of data collected with the D0 detector at the Fermilab Tevatron Collider, we observe $17.8\pm 4.9(\mathrm{stat})\pm 0.8(\mathrm{syst})$ ${\Omega }_b^{-}$ signal events at a mass of $6.165\pm 0.010(\mathrm{stat})\pm 0.013(\mathrm{syst})\mathrm{GeV}$. The significance of the observed signal is $5.4\sigma$, corresponding to a probability of $6.7\times {10}^{-8}$ of it arising from a background fluctuation.

Figure 1

The invariant mass distribution of the $\Lambda K$ pair before (a) and after (b) the BDT selection. Solid circles are from the right-sign $\Lambda K^{-}$ events while the histogram is from the wrong-sign $\Lambda K^{+}$ events without any additional normalization.

Figure 2

(a) The $M({\Omega }_b^{-})$ distribution of the ${\Omega }_b^{-}$ candidates after all selection criteria. The dotted curve is an unbinned likelihood fit to the model of a constant background plus a Gaussian signal. The mass distributions for the wrong-sign background (b), the ${\Omega }^{-}$ sideband events (c), and the $\Lambda$ sideband events (d).

Figure 3

The distribution of the proper decay length of the ${\Omega }_b^{-}$ candidates in the $\pm 3\sigma$ mass window around the observed peak along with the expected distribution from the MC ${\Omega }_b^{-}$ signal with a lifetime 1.54 ps plus the data background events.