Observation of $B^{-}\to \overline{p}\Lambda D^0$ at Belle

We study $B^{-}$ meson decays to $\overline{p}\Lambda D^{(*)0}$ final states using a sample of $657\times {10}^{6}B\overline{B}$ events collected at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The observed branching fraction for $B^{-}\to \overline{p}\Lambda D^0$ is $({1.43}_{-0.25}^{+0.28}\pm 0.18)\times {10}^{-5}$ with a significance of 8.1 standard deviations, where the uncertainties are statistical and systematic, respectively. Most of the signal events have the $\overline{p}\Lambda$ mass peaking near threshold. No significant signal is observed for $B^{-}\to \overline{p}\Lambda D^{*0}$ and the corresponding upper limit on the branching fraction is $4.8\times {10}^{-5}$ at the 90% confidence level.

Figure 1

The $\Delta M$ distribution of the $B^{-}\to \overline{p}\Lambda D^{*0}$ MC sample with fit curves overlaid, where $\Delta M$ denotes the mass difference between $D^{*0}$ and $D^0$. The solid curve is the overall fit result, the dashed curve shows the CF background and the black filled squares are the MC events.

Figure 2

Distributions of $\Delta E$ (a, c) for $M_{\mathrm{bc}}>5.27\mathrm{GeV}/c^2$ and of $M_{\mathrm{bc}}$ (b, d) for $|\Delta E|<0.05\mathrm{GeV}$; the top row is the fit result for $B^{-}\to \overline{p}\Lambda D^0$, $D^0\to K^{-}{\pi }^{+}$ (a, b) and the bottom row for $B^{-}\to \overline{p}\Lambda D^0$, $D^0\to K^{-}{\pi }^{+}{\pi }^0$ (c, d). The points with error bars are data; the solid curve shows the fit; the dashed curve represents the signal, and the dotted curve indicates continuum background.

Figure 3

Differential branching fraction ($d\mathcal{B}/d{M}_{\overline{p}\Lambda }$) as a function of the $\overline{p}\Lambda$ mass for $B^{-}\to \overline{p}\Lambda D^0$. Note that the last bin with the central value of $3\mathrm{GeV}/c^2$ has a bin width of $0.8\mathrm{GeV}/c^2$. The solid curve is a fit with a threshold function.

Figure 4

Distributions of $\Delta E$ (a, c) for $M_{\mathrm{bc}}>5.27\mathrm{GeV}/c^2$ and of $M_{\mathrm{bc}}$ (b, d) for $|\Delta E|<0.05\mathrm{GeV}$; the top row is the fit result for $B^{-}\to \overline{p}\Lambda D^{*0}$ in the $\Delta M$ sideband region (a, b) and the bottom row for $B^{-}\to \overline{p}\Lambda D^{*0}$ in the $\Delta M$ signal region (c, d). The points with error bars are data; the solid curve shows the result of the fit; the dot-dashed and dotted curve indicates the CF and continuum background; the dashed curve represents the signal.