We report the first direct observation of the strange $b$ baryon ${\Xi }_b^{-}({\overline{\Xi }}_b^{+})$. We reconstruct the decay ${\Xi }_b^{-}\to J/\psi {\Xi }^{-}$, with $J/\psi \to {\mu }^{+}{\mu }^{-}$, and ${\Xi }^{-}\to \Lambda {\pi }^{-}\to p{\pi }^{-}{\pi }^{-}$ in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$. Using $1.3{\mathrm{fb}}^{-1}$ of data collected by the D0 detector, we observe $15.2\pm 4.4(\mathrm{stat}{)}_{-0.4}^{+1.9}(\mathrm{syst})$ ${\Xi }_b^{-}$ candidates at a mass of $5.774\pm 0.011(\mathrm{stat})\pm 0.015(\mathrm{syst})\mathrm{GeV}$. The significance of the observed signal is $5.5\sigma$, equivalent to a probability of $3.3\times {10}^{-8}$ of it arising from a background fluctuation. Normalizing to the decay ${\Lambda }_b\to J/\psi \Lambda$, we measure the relative rate $\frac{\sigma ({\Xi }_b^{-})\times \mathcal{B}({\Xi }_b^{-}\to J/\psi {\Xi }^{-})}{\sigma ({\Lambda }_b)\times \mathcal{B}({\Lambda }_b\to J/\psi \Lambda )}=0.28\pm 0.09(\mathrm{stat}{)}_{-0.08}^{+0.09}(\mathrm{syst})$.

• Received 12 June 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.052001