  Bottom baryons decaying to a J/\psi\ meson and a hyperon are reconstructed
using 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 M(\Lambda_b^0) = 5619.53 \pm 0.13
(stat) \pm 0.45 (syst) MeV/c^2, M(\Xi_b^-) = 5795.8 \pm 0.9 (stat) \pm 0.4
(syst) MeV/c^2, M(\Omega_b^-) = 6046.0 \pm 2.2 (stat) \pm 0.5 (syst) MeV/c^2,
while the differences with respect to the \Lambda_b^0 mass are
M(\Xi_b^-)-M(\Lambda_b^0) = 176.2 \pm 0.9 (stat) \pm 0.1 (syst) MeV/c^2,
M(\Omega_b^-)-M(\Lambda_b^0) = 426.4 \pm 2.2 (stat) \pm 0.4 (syst) MeV/c^2.
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 35 pb^{-1} of data collected in 2010 yields
M(\Lambda_b^0) = 5619.44 \pm 0.13 (stat) \pm 0.38 (syst) MeV/c^2.
