測量Belle II中$\Bz \to \KS \piz$之衰變率及宇稱不對稱性

Abstract

本論文紀錄了Belle II實驗中$\Bz\to \KS\piz$之衰變率及宇稱不對稱性的測量，透過非訊號端之$B$介子($B_{tag}$)衰變的風味，使用時間積分方法來測量直接宇稱不對稱性參數。本測量使用Belle II於2019-2022年測量的數據，由$\epem$對撞器SuperKEKB在$\FourS$能量碰撞產生，總光度為$\LUMI$，得到 \begin{equation*} \mathcal{B}(\Bz \to \Kz \piz) = (10.16 \pm 0.65 \pm 0.65) \times 10^{-6}, \end{equation*} \begin{equation*} \mathcal{A}(\Bz \to \Kz \piz) = -0.06 \pm 0.15 \pm 0.05, \end{equation*} \begin{equation*} I_{K\pi} = -0.03 \pm 0.13 \pm 0.05. \end{equation*} 其中誤差項第一項為統計誤差，第二項為系統誤差。 加法原理之計算合併其他Belle II實驗中$B\to K\pi$衰變率及宇稱不對稱性的測量，計算結果符合標準模型之預測($I_{K\pi}=0$)。

We report updated measurements of the branching ratio and the direct $\CP$-violating asymmetry in $\Bz \to \KS \piz$ decay at Belle II. The flavour information of the decay is obtained from the pair-produced neutral B partner $B_{tag}$, and the direct $\CP$ asymmetry parameter is determined with the time-integrated method. We analyse the 2019-2022 Belle II data from $\epem$ collisions produced by the asymmetric-energy SuperKEKB collider at the $\FourS$ resonance. The data analysed contain 387 million $\BB$ pairs and correspond to an integrated luminosity of $\LUMI$. We obtain the branching fraction and the direct $\CP$ asymmetry

\begin{equation*} \mathcal{B}(\Bz \to \Kz \piz) = (10.16 \pm 0.65 \pm 0.65) \times 10^{-6}, \end{equation*} \begin{equation*} \mathcal{A}(\Bz \to \Kz \piz) = -0.06 \pm 0.15 \pm 0.05, \end{equation*} where the first uncertainties are statistical and the second systematic. With other Belle II $B\to K\pi$ measurements of branching fractions and direct $\CP$ asymmetries, we compute the $K\pi$ isospin sum rule to be \begin{equation*} I_{K\pi} = -0.03 \pm 0.13 \pm 0.05, \end{equation*} which is in agreement with the standard model prediction of zero.