Measurements of $CP$-Violating Asymmetries in $B^0\to K_s^0{\pi }^0$ Decays

We present a measurement of the time-dependent $CP$-violating (CPV) asymmetries in $B^0\to K_S^0{\pi }^0$ decays based on $124\times {10}^6$ ${\rm Y}(4S)\to B\overline{B}$ decays collected with the BABAR detector at the PEP-II asymmetric-energy $B$ factory at SLAC. In a sample containing $122\pm 16$ signal decays, we obtain the magnitudes of the direct CPV asymmetry $C_{K_S^0{\pi }^0}={0.40}_{-0.28}^{+0.27}\pm 0.09$ and of the CPV asymmetry in the interference between mixing and decay $S_{K_S^0{\pi }^0}={0.48}_{-0.47}^{+0.38}\pm 0.06$ where the first error is statistical and the second systematic.

Figure 1

Distribution of ${\mathrm{m}}_{\mathrm{E}\mathrm{S}}$ for events enhanced in signal decays. The dashed and solid curves represent the background and signal-plus-background contributions, respectively.

Figure 2

Distributions of $\Delta t$ for events enhanced in signal decays with $B_{\mathrm{t}\mathrm{a}\mathrm{g}}$ tagged as (a) $B^0$ or (b) ${\overline{B}}^0$, and (c) the asymmetry ${\mathcal{A}}_{K_S^0{\pi }^0}(\Delta t)$. The dashed and solid curves represent the fitted background and signal-plus-background contributions, respectively. The asymmetry projection corresponds to approximately 36 signal and 25 background events.