Measurement of the B⁰→ ρ(770)⁰γ branching fraction

The ratio between the branching fractions of the B⁰ → ρ(770)⁰γ and B⁰ → K^*(892)⁰γ decays is measured with proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb⁻¹. The measured value is (Formula presented.) where the first uncertainty is statistical and the second systematic. The branching fraction for B⁰→ ρ(770)⁰γ decays is hence obtained as (Formula presented.) where the last uncertainty is due to the branching fraction of the normalisation mode. This result assumes that both the ρ(770)⁰ and K^*(892)⁰ decays saturate the dihadron mass spectra considered in the analysis. It is consistent with the current world-average value and by far the most precise measurement to date.