Measurements of b-baryon properties at LHCb

The LHCb experiment located at the Large Hadron Collider at CERN collected its first data from proton-proton collisions between December 2009 and December 2012. Based on data recorded in 2010, several software alignments of the LHCb tracking stations are performed. The procedure includes vertex and mass constraints from the $D^0 \rightarrow K^+ \pi^-$ decay. A precision of 15 $\mu$m is achieved and the alignment constants were used for high-precision measurements at LHCb with 2010 data. In addition the results of the alignments helped to improve the understanding of the procedure. Exploiting the data set recorded during the year 2011, the masses of the weakly-decaying $\Lambda_b^0, \Xi_b^-$ and $ \Omega_b^-$ baryons are measured: \begin{eqnarray*} M(\Lambda_b^0) & = & 5619.53 \pm 0.13 \rm \ (stat) \pm 0.45 \ (syst) \ MeV/c^2 , \\ M(\Xi_b^-) & = & 5795.8 \pm 0.9 \rm \ (stat) \pm 0.4 \ (syst) \ MeV/c^2 , \\ M(\Omega_b^-) & = & 6046.0 \pm 2.2 \rm \ (stat) \pm 0.5 \ (syst) \ MeV/c^2 . \\ \end{eqnarray*} The first (second) quoted uncertainty is statistical (systematic). These mass measurements are the most precise to date. Furthermore, the $\Omega_b^- $ mass measurement allowed to settle the discrepancy between the results of the CDF and D0 experiments, which disagree by more than six standard deviations. The $\Omega_b^-$ mass measurement presented here is in agreement with the CDF measurement and in disagreement with the D0 one. A preliminary search for the neutral $ \Xi_b^0$ baryon is also presented. A statistically significant signal is observed when both data sets from 2011 and 2012 are combined. Using this same data set, the lifetimes of the $\Xi_b^-$ and $\Omega_b^-$ baryons are measured: \begin{eqnarray*} \tau(\Xi_b^-) & = & 1.55 ^{+0.10}_{-0.09} \rm \ (stat) \pm 0.03 \ (syst) \ ps , \\ \tau(\Omega_b^-) & = & 1.54 ^{+0.26}_{-0.21} \rm \ (stat) \pm 0.05 \ (syst) \ ps . \\ \end{eqnarray*} These are the most precise lifetime measurements of these $b$ baryons to date. All $b$-baryon property measurements presented here provide a unique and valuable input for models in quantum chromodynamics at low energies.