Measurement of branching fraction and direct $CP$ asymmetry in $B^0\to {\rho }^0{\pi }^0$ decays

We report a measurement of the branching fraction of the decay $B^0\to {\rho }^0{\pi }^0$, using $386\times {10}^6$ $B\overline{B}$ pairs collected at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We detect ${51}_{-13}^{+14}$ signal events with a significance of 4.2 standard deviations, including systematic uncertainties, and measure the branching fraction to be $\mathcal{B}(B^0\to {\rho }^0{\pi }^0)=({3.12}_{-0.82}^{+0.88}(\mathrm{stat})\pm 0.33(\mathrm{syst}{)}_{-0.68}^{+0.50}(\mathrm{\text{model}}))\times {10}^{-6}$. We also perform the first measurement of direct $CP$ violating asymmetry in this mode.

Figure 1

(a) Distribution of signal (continuum) events in $r\mathrm{\text{−}}\mathcal{R}$ space shown with open (shaded) proportional boxes; the marked region (top-right) indicates the area selected. (b) (c) Distributions of $\Delta E(M_{\mathrm{bc}})$ in the signal region of $M_{\mathrm{bc}}(\Delta E)$. Projection of the fit result is shown as the thick solid curve; the thin solid line represents the signal component; the dashed, dotted and dash-dotted curves represent, respectively, the cumulative background components from continuum processes, $b\to c$ decays, and charmless $B$ backgrounds. (d), (e) Distributions of fit yields in $m_{{\pi }^{+}{\pi }^{-}}$ and $\cos {\theta }_{\mathrm{hel}}^{\rho }$ variables for ${\rho }^0{\pi }^0$ candidate events. Points with error bars represent data fit results, and the histograms show signal MC expectation; the selection requirements described in the text are shown as dashed lines.

Figure 2

$\Delta E$ and $M_{\mathrm{bc}}$ distributions (with projections of the fit results) shown separately for events tagged as $q=+1$ (left) and $q=-1$ (right).