Bottom baryons decaying to a $J/\psi$ meson and a hyperon are reconstructed using $1.0{\mathrm{fb}}^{-1}$ of data collected in 2011 with the LHCb detector. Significant ${\Lambda }_b^0\to J/\psi \Lambda$, ${\Xi }_b^{-}\to J/\psi {\Xi }^{-}$ and ${\Omega }_b^{-}\to J/\psi {\Omega }^{-}$ signals are observed and the corresponding masses are measured to be $M({\Lambda }_b^0)=5619.53\pm 0.13(\mathrm{stat}.)\pm 0.45(\mathrm{syst}.)\mathrm{MeV}/c^2$, $M({\Xi }_b^{-})=5795.8\pm 0.9(\mathrm{stat}.)\pm 0.4(\mathrm{syst}.)\mathrm{MeV}/c^2$, $M({\Omega }_b^{-})=6046.0\pm 2.2(\mathrm{stat}.)\pm 0.5(\mathrm{syst}.)\mathrm{MeV}/c^2$, while the differences with respect to the ${\Lambda }_b^0$ mass are $M({\Xi }_b^{-})-M({\Lambda }_b^0)=176.2\pm 0.9(\mathrm{stat}.)\pm 0.1(\mathrm{syst}.)\mathrm{MeV}/c^2$, $M({\Omega }_b^{-})-M({\Lambda }_b^0)=426.4\pm 2.2(\mathrm{stat}.)\pm 0.4(\mathrm{syst}.)\mathrm{MeV}/c^2$. 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 $35{\mathrm{pb}}^{-1}$ of data collected in 2010 yields $M({\Lambda }_b^0)=5619.44\pm 0.13(\mathrm{stat}.)\pm 0.38(\mathrm{syst}.)\mathrm{MeV}/c^2$.

• Received 5 February 2013

DOI:https://doi.org/10.1103/PhysRevLett.110.182001

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© 2013 CERN, for the The LHCb Collaboration