First observations and branching fraction measurements of $\bar{B}_s^0$ to double-charm final states

First observations and measurements of the branching fractions of the $\bar{B}_s^0$ decay modes $\bar{B}_s^0\to D^+D^-$, $\bar{B}_s^0\to D_s^+D^-$, and $\bar{B}_s^0\to D^0\bar{D}^0$ are made using 1 fb$^{-1}$ of data collected by the LHCb detector. These branching fractions are normalized to $\bar{B}^0\to D^+D^-$, $B^0\to D_s^+D^-$ and $B^-\to D^0D_s^-$, respectively, from which we obtain ${\cal{B}}(\bar{B}_s^0\to D^+D^-)/{\cal{B}}(\bar{B}^0\to D^+D^-) = 1.00\pm 0.18\pm 0.09$, ${\cal{B}}(\bar{B}_s^0 \to D_s^+D^-)/{\cal{B}}(B^0\to D_s^+D^-)=0.048\pm0.008\pm0.004$, and ${\cal{B}}(\bar{B}_s^0\to D^0\bar{D}^0)/{\cal{B}}(B^-\to D^0D_s^-)=0.015\pm0.004\pm0.002$. The branching ratio of the decay $\bar{B}_s^0\to D_s^+D_s^-$ relative to $\bar{B}^0\to D^+D_s^-$ has also been measured and is found to be ${\cal{B}}(\bar{B}_s^0\to D_s^+D_s^-)/{\cal{B}}(\bar{B}^0\to D^+D_s^-)= 0.508\pm 0.026\pm 0.043$. This measurement is about five times more precise than the current world average.