First mass measurements at LHCb

We present preliminary mass measurements of the $X(3872)$ particle and five different $b$ hadrons, based on a sample of $pp$ collisions at $\sqrt{s} = 7$ TeV collected in 2010 with the LHCb detector and corresponding to an integrated luminosity of 35 pb$^{-1}$. Signal candidates are fully reconstructed in final states containing a $J/\psi$ meson. The momentum scale is calibrated at the $10^{-4}$ level using a large sample of $J/\psi \to \mu^+\mu^-$ decay. While the $X(3872)$ mass result is still dominated by its statistical uncertainty, the $b$-hadron results already represent the most precise measurements to date.