Measurement of the branching fraction of the decay $B^- → D^0 ρ(770)^-$ at Belle II

We measure the branching fraction of the decay B⁻ → D⁰ ρ(770)⁻ using data collected with the Belle II detector. The data contain 387 million BB̄ pairs produced in e⁺e⁻ collisions at the Υ(4S) resonance. We reconstruct 8360± 180 decays from an analysis of the distributions of the B⁻ energy and the ρ(770)⁻ helicity angle. We determine the branching fraction to be (0.939 ± 0.021(stat) ± 0.050(syst))%, in agreement with previous results. Our measurement improves the relative precision of the world average by more than a factor of two.