Observation of $ \Lambda_b^0 \to \psi(2S)pK^-$ and $ \Lambda_b^0 \to   J/\psi \pi^+ \pi^- pK^-$ decays and a measurement of the $\Lambda_b^0$ baryon   mass
  The decays $ \Lambda_b^0 \to \psi(2S)pK^-$ and $ \Lambda_b^0 \to J/\psi \pi^+ \pi^- pK^-$ are observed in a data sample corresponding to an integrated luminosity of 3fb$^{-1}$, collected in proton-proton collisions at 7 and 8TeV centre-of-mass energies by the LHCb detector. The $\psi(2S)$ mesons are reconstructed through the decay modes $\psi(2S)\to\mu^+\mu^-$ and $\psi(2S)\to J/\psi\pi^+\pi^-$. The branching fractions relative to that of $ \Lambda_b^0 \to J/\psi pK^-$ are measured to be \begin{eqnarray*}   \frac{\mathcal{B}(\Lambda_b^0 \to \psi(2S) pK^-)}   {\mathcal{B}(\Lambda_b^0 \to J/\psi pK^-)}   & = & (20.70\pm 0.76\pm 0.46\pm 0.37)\times10^{-2} \,,   \frac{\mathcal{B}(\Lambda_b^0 \to J/\psi \pi^+ \pi^- pK^-)}   {\mathcal{B}(\Lambda_b^0 \to J/\psi pK^-)}   & = & (20.86\pm 0.96\pm 1.34)\times10^{-2} \,, \end{eqnarray*} where the first uncertainties are statistical, the second are systematic and the third is related to the knowledge of $J/\psi$ and $\psi(2S)$ branching fractions.   The mass of the $\Lambda_b^0$ baryon is measured to be $$ M(\Lambda_b^0) = 5619.65 \pm 0.17 \pm 0.17\mathrm{MeV}/c^2,$$ where the uncertainties are statistical and systematic. 
LHCb collaboration: R. Aaij, C. Abell\'an Beteta, B. Adeva, M.   Adinolfi, A. Affolder, Z. Ajaltouni, S. Akar, J. Albrecht, ...
hep-ex