Measurement of branching fractions, CP asymmetry, and isospin asymmetry for B→ργ decays using Belle and Belle II data

We present measurements of B⁺→ρ⁺γ and B⁰→ρ⁰γ decays using a combined data sample of 772 × 10⁶ BB̄ pairs collected by the Belle experiment and 387× 10⁶ BB̄ pairs collected by the Belle II experiment in e⁺e⁻ collisions at the Υ (4S) resonance. After an optimized selection, a simultaneous fit to the Belle and Belle II data sets yields 114± 12 B⁺→ρ⁺γ and 99± 12 B⁰→ρ⁰γ decays. The measured branching fractions are (13.1+2.0 +1.3-1.9 -1.2)× 10⁻⁷ and (7.5± 1.3+1.0-0.8)× 10⁻⁷ for B⁺→ρ⁺γ and B⁰→ρ⁰γ decays, respectively, where the first uncertainty is statistical and the second is systematic. We also measure the isospin asymmetry AI(B→ργ)=(10.9+11.2 +7.8-11.7 -7.3)% and the direct CP asymmetry ACP(B⁺→ρ⁺γ)=(-8.2± 15.2+1.6-1.2)%.