Measurement of the branching fractions and $CP$ asymmetries of $B^{+}\to {\pi }^{+}{\pi }^0$ and $B^{+}\to K^{+}{\pi }^0$ decays in 2019-2021 Belle II data

We determine the branching fractions $\mathcal{B}$ and $CP$ asymmetries ${\mathcal{A}}_{CP}$ of the decays $B^{+}\to {\pi }^{+}{\pi }^0$ and $B^{+}\to K^{+}{\pi }^0$. The results are based on a data set containing 198 million bottom-antibottom meson pairs corresponding to an integrated luminosity of $190{\text{fb}}^{-1}$ recorded by the Belle II detector in energy-asymmetric electron-positron collisions at the $\mathrm{{\rm Y}}(4S)$ resonance. We measure $\mathcal{B}(B^{+}\to {\pi }^{+}{\pi }^0)=(6.12\pm 0.53\pm 0.53)\times {10}^{-6}$, $\mathcal{B}(B^{+}\to K^{+}{\pi }^0)=(14.30\pm 0.69\pm 0.79)\times {10}^{-6}$, ${\mathcal{A}}_{CP}(B^{+}\to {\pi }^{+}{\pi }^0)=-0.085\pm 0.085\pm 0.019$, and ${\mathcal{A}}_{CP}(B^{+}\to K^{+}{\pi }^0)=0.014\pm 0.047\pm 0.010$, where the first uncertainties are statistical and the second are systematic. These results improve a previous Belle II measurement and agree with the world averages.