First mass measurements at LHCb

The LHC opens new frontiers in heavy flavour physics through an unprecedented statistical reach for a variety of interesting states produced in pp collisions. The LHCb spectrometer provides a good mass resolution and is suitable for spectroscopy studies. We present first preliminary mass measurements of several $b$ hadrons and of the exotic $X(3872)$ meson, reconstructed in final states containing a $J/\psi$ using the data collected in 2010 by the LHCb experiment. An important aspect of the analysis is the calibration of the momentum scale using $J/\psi \to \mu^+ \mu^-$ decays, as well as the control of systematic uncertainties. While the already very competitive mass measurements for the $B^+$, $B^0$ and $B^0_s$ mesons receive similar contributions from systematic and statistical uncertainties, those of the $\Lambda_b$, $B^+_c$ and $X(3872)$ particles are dominated by statistical uncertainties, and will therefore substantially improve with more data in the future.