Measurement of $CP$ asymmetry in $B^0\to K_s^0{\pi }^0{\pi }^0$ decays

We present a measurement of the time-dependent $CP$ asymmetry for the neutral $B$-meson decay into the $CP=+1$ final state $K_S^0{\pi }^0{\pi }^0$, with $K_S^0\to {\pi }^{+}{\pi }^{-}$. We use a sample of approximately 227 million $B$-meson pairs recorded at the $\mathit{{\rm Y}}(4S)$ resonance with the BABAR detector at the PEP-II $B$-Factory at SLAC. From an unbinned maximum likelihood fit, we extract the mixing-induced $CP$-violation parameter $S=0.72\pm 0.71\pm 0.08$ and the direct $CP$-violation parameter $C=0.23\pm 0.52\pm 0.13$, where the first uncertainty is statistical and the second systematic.

Figure 1

Distribution of the event variables (a) $m_{\mathrm{ES}}$, (b) $\Delta E/\sigma (\Delta E)$, and (c) NN output in 10 bins after reconstruction and a requirement on the ratio of signal likelihood to the signal-plus-background likelihood, calculated without the plotted variable. The efficiencies of the likelihood ratio requirements for (a), (b), and (c) are 10%, 26%, and 55%, respectively. The solid line represents the fit result for the total event yield and the dotted line for the total background. Plot (d) shows the ratio of the signal likelihood to signal-plus-background likelihood with all variables included, data (dots) with the fit result superimposed. Plot (e) shows the same quantity as (d) close to 1 and with a linear scale.

Figure 2

Plots (a) and (b) show the $\Delta t$ distributions of $B^0$- and ${\overline{B}}^0$-tagged $B^0\to K_S^0{\pi }^0{\pi }^0$ candidates. The solid lines refer to the fit for all events; the dashed lines correspond to the total background. Plot (c) shows the raw asymmetry (see text). A requirement is applied on the event likelihood to suppress background.