Observation of the decay $B^0\to {\eta }^{\prime }{K}^{*}(892{)}^0$

We report a search for charmless hadronic decays of neutral $B$ mesons to ${\eta }^{\prime }{K}^{*}(892{)}^0$. The results are based on a $711{\mathrm{fb}}^{-1}$ data sample that contains $772\times {10}^{6}B\overline{B}$ pairs, collected at the $\mathrm{\Upsilon }(4S)$ resonance with the Belle detector at the KEKB asymmetric energy $e^{+}{e}^{-}$ collider. We observe the decay for the first time with a significance of 5.0 standard deviations and obtain its branching fraction $\mathcal{B}[B^0\to {\eta }^{\prime }{K}^{*}(892{)}^0]=[2.6\pm 0.7(\text{stat})\pm 0.2(\text{syst})]\times {10}^{-6}$. We also measure the $CP$-violating asymmetry as ${\mathcal{A}}_{CP}[B^0\to {\eta }^{\prime }{K}^{*}(892{)}^0]=-0.22\pm 0.29(\text{stat})\pm 0.07(\text{syst})$.

Figure 1

Projections of the fit results onto (a) $M_{\text{bc}}$, (b) $\mathrm{\Delta }E$, (c) $C_{NB}^{\prime }$, and (d) $\cos {\theta }_H$. Each distribution is shown in the signal-enhanced regions of the other three observables: $M_{\text{bc}}>5.27\mathrm{GeV}/c^2$, $-0.10\mathrm{GeV}<\mathrm{\Delta }E<0.06\mathrm{GeV}$, and $2.0<C_{NB}^{\prime }<8.0$. Data are points with error bars; the fit results are shown by solid curves. Contributions from signal, continuum $q\overline{q}$, generic $B\overline{B}$, and rare $B\overline{B}$ including nonresonant background are shown by dashed, dotted, dash-dotted, and dash-double-dotted curves, respectively.

Figure 2

Distributions of (a) fit likelihood and (b) $-2\ln ({\mathcal{L}}_0/{\mathcal{L}}_{\max })$ as a function of the branching fraction. Solid curves are after taking the systematic uncertainty into account, while dashed ones are only with the statistical uncertainty.