Measurement of ${\cal B}(B_s \rightarrow D^+_s D^-_s)$

The branching ratio ${\cal B}(B_s \rightarrow D^+_s D^-_s)$ provides an alternative way to estimate the width difference in the $B_s$ system. We report a preliminary result on the measurement of the branching ratio of the decay $B_s \to D^+_s D^-_s$ using 1 fb$^{-1}$ of data collected with the D\O\ detector of the Tevatron.