Measurement of the masses of the $\Xi_b^{-}$ and $\Omega_b^{-}$

Preliminary measurements of the $\Xi_b^{-}$ and $\Omega_b^{-}$ masses are described. The decays of these bottom baryons with strangeness $\Xi_b^{-}\rightarrow J/\psi \Xi^{-}$ and $\Omega_b^{-} \rightarrow J/\psi \Omega^{-}$ are fully reconstructed. Using 0.62 fb$^{-1}$ of data collected with the LHCb detector at the LHC in 2011. \begin{align}M (\Xi_b^{-}) = 5796.5 \pm 1.2 (stat) 1.2 (syst) MeV/c^2; \\ M(\Omega_b^{-}) = 6050.3 \pm 4.5 (stat) 2.2 (syst) MeV/c^2. \\ \end{align}