Study of beauty hadron decays into pairs of charm hadrons

First observations of the decays ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{D}_{(s)}^{-}$ are reported using data corresponding to an integrated luminosity of $3,{\rm fb}^{-1}$ collected at 7 and 8 TeV center-of-mass energy in proton-proton collisions with the LHCb detector. In addition, the most precise measurement of the branching fraction $\mathcal{B}(B_s^0\to D^{+}{D}_s^{-})$ is made and a search is performed for the decays $B_{(s)}^0\to {\mathrm{\Lambda }}_c^{+}{\mathrm{\Lambda }}_c^{-}$. The results obtained are \begin{eqnarray*} $\mathcal{B}$(\Lambda_b^0 \to \Lambda_c^+ D^-)/$\mathcal{B}$(\Lambda_b^0 \to \Lambda_c^+ D_{s}^-) &=& 0.042 \pm 0.003({\rm stat}) \pm 0.003({\rm syst}), \left[\frac{$\mathcal{B}$(\Lambda_b^0 \to \Lambda_c^+ D_{s}^-)}{$\mathcal{B}$({\kern 0.2em}\overline{\kern -0.2em B}_d^0 \to D^+D_s^-)}\right]\big/\left[\frac{$\mathcal{B}$(\Lambda_b^0 \to \Lambda_c^+\pi^-)}{$\mathcal{B}$({\kern 0.2em}\overline{\kern -0.2em B}_d^0 \to D^+\pi^-)}\right] &=& 0.96 \pm 0.02({\rm stat}) \pm 0.06({\rm syst}), $\mathcal{B}$(B_s^0 \to D^+D_s^-)/$\mathcal{B}$({\kern 0.2em}\overline{\kern -0.2em B}_d^0 \to D^+D_s^-) &=& 0.038\pm0.004({\rm stat})\pm0.003({\rm syst}), $\mathcal{B}$({\kern 0.2em}\overline{\kern -0.2em B}^0 \to \Lambda_c^+ \Lambda_c^-)/$\mathcal{B}$({\kern 0.2em}\overline{\kern -0.2em B}d^0 \to D^+D_s^-) & < & 0.0022; [95% ; {\rm C.L.}], $\mathcal{B}$(B^0{s} \to \Lambda_c^+ \Lambda_c^-)/$\mathcal{B}$(B_s^0 \to D^+D_s^-) & < & 0.30; [95% ; {\rm C.L.}]. \end{eqnarray*} Measurement of the mass of the ${\mathrm{\Lambda }}_b^0$ baryon relative to the $B^0$ meson gives ${M(\Lambda_b^0) -M(B^0) = 339.72\pm 0.24({\rm stat}) \pm 0.18({\rm syst})}$MeV$/c^2$. This result provides the most precise measurement of the mass of the ${\mathrm{\Lambda }}_b^0$ baryon to date.