Measurement of the $CP$-violating asymmetries in $B^0\to K_s^0{\pi }^0$ and of the branching fraction $B^0\to K^0{\pi }^0$

We present a measurement of the time-dependent $CP$-violating asymmetries in $B^0\to K_S^0{\pi }^0$ decays based on $383\times {10}^6$ $\mathit{{\rm Y}}(4S)\to B\overline{B}$ events collected by the BABAR experiment at the PEP-II asymmetric-energy $B$ Factory at the Stanford Linear Accelerator Center. We measure the direct $CP$-violating asymmetry $C_{K_S^0{\pi }^0}=0.24\pm 0.15\pm 0.03$ and the $CP$-violating asymmetry in the interference between mixing and decay $S_{K_S^0{\pi }^0}=0.40\pm 0.23\pm 0.03$, where the first errors are statistical and the second are systematic. On the same sample, we measure the decay branching fraction, obtaining $\mathcal{B}(B^0\to K^0{\pi }^0)=(10.3\pm 0.7\pm 0.6)\times {10}^{-6}$.

Figure 1

Distribution of (a) $m_{\mathrm{miss}}$, (b) $m_B$, (c) $L_2/L_0$, (d) $\cos {\theta }_B^{*}$, for background subtracted events on data (dots). The solid curve represents the shape of signal PDF, as obtained from the maximum-likelihood fit. The insets show the distribution of the data, and the PDF, for signal subtracted events. The binning of the $L_2/L_0$ PDF is coarser where the signal is well separated from the background to reduce the number of free parameters.

Figure 2

Background subtracted distributions of $\Delta t_{\mathrm{r}}$ for events where $B_{\mathrm{tag}}$ is tagged as (a) $B^0$ or (b) ${\overline{B}}^0$, and (c) the asymmetry $\mathcal{A}(\Delta t_{\mathrm{r}})$. The points are data and the curves are the PDF projections.