Measurement of $CP$ asymmetries in $B^0\to K^0{\pi }^0$ decays

We report measurements of $CP$ violation parameters in $B^0\to K^0{\pi }^0$ decays based on a data sample of $657\times {10}^{6}B\overline{B}$ pairs collected with the Belle detector at the KEKB $e^{+}{e}^{-}$ asymmetric-energy collider. We use $B^0\to K_S^0{\pi }^0$ decays for both mixing-induced and direct $CP$ violating asymmetry measurements and $B^0\to K_L^0{\pi }^0$ decays for the direct $CP$ violation measurement. The $CP$ violation parameters obtained are $\sin 2{\varphi }_1^{\mathrm{eff}}=+0.67\pm 0.31(\mathrm{stat})\pm 0.08(\mathrm{syst})$ and ${\mathcal{A}}_{K^0{\pi }^0}=+0.14\pm 0.13(\mathrm{stat})\pm 0.06(\mathrm{syst})$. The branching fraction of $B^0\to K^0{\pi }^0$ decay is measured to be $\mathcal{B}(B^0\to K^0{\pi }^0)=(8.7\pm 0.5(\mathrm{stat}.)\pm 0.6(\mathrm{syst}.))\times {10}^{-6}$. The observed ${\mathcal{A}}_{K^0{\pi }^0}$ value differs by 1.9 standard deviations from the value expected from an isospin sum rule.

Figure 1

$M_{\mathrm{bc}}\mathrm{\text{−}}\Delta E\mathrm{\text{−}}{\mathcal{R}}_{s/b}$ fit projections of $B^0\to K_S^0{\pi }^0$ candidates. The open histogram with the solid curve shows the fit result, the filled histogram is the $B\overline{B}$ background, and the dashed histogram is the sum of continuum and $B\overline{B}$ backgrounds. Each plot requires signal enhanced conditions for the other variables: $5.27\mathrm{GeV}/c^2<M_{\mathrm{bc}}<5.29\mathrm{GeV}/c^2$, $-0.15\mathrm{GeV}<\Delta E<0.1\mathrm{GeV}$ and ${\mathcal{R}}_{s/b}>0.7$.

Figure 2

$B^0\to K_L^0{\pi }^0$ fit projections for events with good tags: background subtracted $M_{\mathrm{bc}}$ (left) and ${\mathcal{R}}_{s/b}$ (right). The open solid histogram shows the fit result. The filled histogram is the $B\overline{B}$ background and the open dashed histogram is the sum of continuum and $B\overline{B}$ background.

Figure 3

The top plot shows the background subtracted $\Delta t$ distribution for $B^0$ and ${\overline{B}}^0$ tags where the solid (broken) curve represents the $\Delta t$ curve for $B^0$ (${\overline{B}}^0$) in the good tag region $0.5<r\le 1.0$. The bottom plot shows the background subtracted asymmetry defined as $(N_{{\overline{B}}^0}^{\mathrm{Sig}}-N_{B^0}^{\mathrm{Sig}})/(N_{{\overline{B}}^0}^{\mathrm{Sig}}+N_{B^0}^{\mathrm{Sig}})$ in each $\Delta t$ bin where $N_{B^0}^{\mathrm{Sig}}$ ($N_{\overline{B^0}}^{\mathrm{Sig}})$) is the $B^0$ (${\overline{B}}^0$) signal yield extracted in that $\Delta t$ bin. The solid curve shows the $CP$ asymmetry result expected from the fit.