Measurement of the $\Lambda_b^0$, $\Xi_b^-$ and $\Omega_b^-$ baryon masses

Bottom baryons decaying to a $J/\psi$ meson and a hyperon are reconstructed using $\rm 1.0~fb^{-1}$ of data collected in 2011 with the LHCb detector. Significant $\Lambda_b^0 \rightarrow J/\psi \Lambda$, $\Xi_b^-\rightarrow J/\psi \Xi^-$ and $\Omega_b^- \rightarrow J/\psi \Omega^-$ signals are observed and the corresponding masses are measured to be \begin{eqnarray} M(\Lambda_b^0) = 5619.53 \pm 0.13 (stat) \pm 0.45 (syst) MeV/c^2, \cr M(\Xi_b^-) = 5795.8 \pm 0.9 (stat) \pm 0.4 (syst) MeV/c^2, \cr M(\Omega_b^-) = 6046.0 \pm 2.2 (stat) \pm 0.5 (syst) MeV/c^2,\end{eqnarray} while the diffierences with respect to the $\Lambda_b^0$ mass are \begin{eqnarray} M(\Xi_b^-) - M(\Lambda_b^0) = 176.2 \pm 0.9 (stat) \pm 0.1 (syst) MeV/c^2, \cr M(\Omega_b^-) - M(\Lambda_b^0) = 426.4 \pm 2.2 (stat) \pm 0.4 (syst) MeV/c^2.\end{eqnarray} These are the most precise mass measurements of the $\Lambda_b^0$, $\Xi_b^-$ and $\Omega_b^-$ baryons to date. Averaging the above $\Lambda_b^0$ mass measurement with that published by LHCb using $\rm 35~pb^{-1}$ of data collected in 2010 yields $M(\Lambda_b^0) = \rm 5619.44 \pm 0.13\,(stat) \pm 0.38\,(syst)$ MeV/$c^2$.