Measurement of the branching fraction of the decay $B^{-}\to D^0\rho (770{)}^{-}$ at Belle II

We measure the branching fraction of the decay $B^{-}\to D^0\rho (770{)}^{-}$ using data collected with the Belle II detector. The data contain 387 million $B\overline{B}$ pairs produced in $e^{+}{e}^{-}$ collisions at the $\Upsilon \left(4S\right)$ resonance. We reconstruct $8360\pm 180$ decays from an analysis of the distributions of the $B^{-}$ energy and the $\rho (770{)}^{-}$ helicity angle. We determine the branching fraction to be $(0.939\pm 0.021\left(\mathrm{stat}\right)\pm 0.050\left(\mathrm{syst}\right))\%$, in agreement with previous results. Our measurement improves the relative precision of the world average by more than a factor of two.