Measurements of isospin asymmetry and difference of direct $CP$ asymmetries in inclusive $B \to X_s \gamma$ decays

Abstract

We report measurements of isospin asymmetry $\Delta_{0-}$ and difference of direct $CP$ asymmetries $\Delta A_{CP}$ between charged and neutral $B \to X_s \gamma$ decays. This analysis is based on the data sample containing $772 \times 10^6 B\bar{B}$ pairs that was collected with the Belle detector at the KEKB energy-asymmetric $e^+ e^-$ collider. Using a sum-of-exclusive technique with invariant $X_s$ mass up to 2.8~GeV/$c^2$, we obtain $\Delta_{0-} = \bigl[-0.48 \pm 1.49 {\rm (stat.)} \pm 0.97 {\rm (syst.)} \pm 1.15 {(f_{+-}/f_{00})}\bigr]$\% and $\Delta A_{CP} = \bigl[+3.69 \pm 2.65 {\rm (stat.)} \pm 0.76{\rm (syst.)}\bigr]$\%, where the last uncertainty for $\Delta_{0-}$ is due to the uncertainty on the production ratio of $B^+B^-$ to $B^0\bar{B}^0$ in $\Upsilon(4S)$ decays. The measured value of $\Delta_{0-}$ is consistent with zero, allowing us to constrain the resolved photon contribution in the $B \to X_s \gamma$, and improve the branching fraction prediction. The result for $\Delta A_{CP}$ is consistent with the prediction of the SM. We also measure the direct $CP$ asymmetries for charged and neutral $B \to X_s \gamma$ decays. All the measurements are the most precise to date.