Measurement of $B\to K{}^{\ast }(892)\gamma$ decays at Belle II

We present measurements of $B\to K{}^{\ast }(892)\gamma$ decays using $365{\mathrm{f}\mathrm{b}}^{-1}$ of data collected from 2019 to 2022 by the Belle~II experiment at the SuperKEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The data sample contains $(387\pm 6)\times {10}^6$ $\mathrm{{\rm Y}}(4S)$ events. We measure branching fractions ($\mathcal{B}$) and $C\text{\hspace{0.17em}⁣}P$ asymmetries (${\mathcal{A}}_{C\text{\hspace{0.17em}⁣}P}$) for both $B^0\to K{}^{\ast 0}\gamma$ and $B^{+}\to K{}^{\ast +}\gamma$ decays. The difference in $C\text{\hspace{0.17em}⁣}P$ asymmetries ($\mathrm{\Delta }{\mathcal{A}}_{C\text{\hspace{0.17em}⁣}P}$) and the isospin asymmetry (${\mathrm{\Delta }}_{0+}$) between these neutral and charged channels are also measured. We obtain the following branching fractions and $C\text{\hspace{0.17em}⁣}P$ asymmetries: $\mathcal{B}(B^0\to K{}^{\ast 0}\gamma )=(4.14\pm 0.10\pm 0.11)\times {10}^{-5}$, $\mathcal{B}(B^{+}\to K{}^{\ast +}\gamma )=(4.04\pm 0.13{}_{-0.15}^{+0.13})\times {10}^{-5}$, ${\mathcal{A}}_{C\text{\hspace{0.17em}⁣}P}(B^0\to K{}^{\ast 0}\gamma )=(-3.3\pm 2.3\pm 0.4)\mathrm{\%}$, and ${\mathcal{A}}_{C\text{\hspace{0.17em}⁣}P}(B^{+}\to K{}^{\ast +}\gamma )=(-0.7\pm 2.9\pm 0.5)\mathrm{\%}$. The measured difference in $C\text{\hspace{0.17em}⁣}P$ asymmetries is $\mathrm{\Delta }{\mathcal{A}}_{C\text{\hspace{0.17em}⁣}P}=(+2.6\pm 3.8\pm 0.6)\mathrm{\%}$, and the measured isospin asymmetry is ${\mathrm{\Delta }}_{0+}=(+4.8\pm 2.0\pm 1.8)\mathrm{\%}$. The first uncertainties listed are statistical and the second are systematic. These results are consistent with world-average values and theory predictions.