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 BB¯ pairs collected by the Belle experiment and 387×106 BB¯ 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 datasets yields 114±12 B+→ρ+γ and 99±12 B0→ρ0γ decays. The measured branching fractions are (13.1-1.9-1.2+2.0+1.3)×10-7 and (7.6±1.3-0.8+1.0)×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)=(10.9-11.7-7.3+11.2+7.8)% and the direct CP asymmetry ACP(B+→ρ+γ)=(-8.2±15.2-1.3+2.0)%.