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

We present measurements of $B^{+}\to {\rho }^{+}\gamma$ and $B^0\to {\rho }^0\gamma$ decays using a combined data sample of $772\times {10}^6$ $B\bar{B}$ pairs collected by the Belle experiment and $387\times {10}^6$ $B\bar{B}$ pairs collected by the Belle II experiment in $e^{+}{e}^{-}$ collisions at the $\mathrm{{\rm Y}}(4S)$ resonance. After an optimized selection, a simultaneous fit to the Belle and Belle II data sets yields $114\pm 12$ $B^{+}\to {\rho }^{+}\gamma$ and $99\pm 12$ $B^0\to {\rho }^0\gamma$ decays. The measured branching fractions are $(13.1_{-1.9-1.2}^{+2.0+1.3})\times {10}^{-7}$ and $(7.5\pm 1.3_{-0.8}^{+1.0})\times {10}^{-7}$ for $B^{+}\to {\rho }^{+}\gamma$ and $B^0\to {\rho }^0\gamma$ decays, respectively, where the first uncertainty is statistical and the second is systematic. We also measure the isospin asymmetry $A_{\mathrm{I}}(B\to \rho \gamma )=(10.9_{-11.7-7.3}^{+11.2+7.8})\mathrm{\%}$ and the direct CP asymmetry $A_{CP}(B^{+}\to {\rho }^{+}\gamma )=(-8.2\pm 15.2_{-1.2}^{+1.6})\mathrm{\%}$.