Measurement of $b$-hadron masses with exclusive $J/\psi X$ decays in 2010 data

We report on $b$-hadron mass measurements with the exclusively reconstructed decays $B^+\to J/\psi K^{+}, B^0\to J/\psi K^{0}_{rm S}, B_s^0\to J/\psi\phi,\Lambda_b\to J/\psi\Lambda$ and $B_c^+\to J/\psi \pi^{+}$ using 35 pb$^{-1}$ of data collected with the LHCb detector during the 2010 physics run of the LHC. The momentum scale is calibrated with $J/\psi \to \mu^+\mu^-$ decays. The preliminary results are \begin{align}M(B^+\to J/\psi K^{+}) = 5279.27 \pm 0.11 (stat) 0.20 (syst) MeV/c^2,\\ M(B^0\to J/\psi K^{*0}) = 5279.54 \pm 0.15 (stat) 0.16 (syst) MeV/c^2, \\ M(B^0\to J/\psi K^{0}_{\rm S}) = 5279.61 \pm 0.29 (stat) 0.20 (syst) MeV/c^2, \\ M(B_s^0 \to J/\psi\phi) = 5366.60 \pm 0.28 (stat) 0.21 (syst) MeV/c^2, \\ M(\Lambda_b \to J/\psi\Lambda) = 5619.48 \pm 0.70 (stat) 0.19 (syst) MeV/c^2, \\ M(B_c^+ \to J/\psi\pi^{+}) = 6268.0 \pm 4.0 (stat) 0.6 (syst) MeV/c^{2}. \end{align}