Searches for $B^0$ Decays to Combinations of Two Charmless Isoscalar Mesons

We search for $B$ meson decays into two-body combinations of $\eta ,{\eta }^{\prime },\omega$, and $\varphi$ mesons from $89\times {10}^6$ $B\overline{B}$ pairs collected with the BABAR detector at the PEP-II asymmetric-energy $e^{+}{e}^{-}$ collider at SLAC. We find the branching fraction $\mathcal{B}(B^0\to \eta \omega )=({4.0}_{-1.2}^{+1.3}\pm 0.4)\times {10}^{-6}$ with a significance of $4.3\sigma$. For the other decay modes we set the following 90% confidence level upper limits on the branching fractions, in units of ${10}^{-6}$: $\mathcal{B}(B^0\to \eta \eta )<2.8$, $\mathcal{B}(B^0\to \eta {\eta }^{\prime })<4.6$, $\mathcal{B}(B^0\to {\eta }^{\prime }{\eta }^{\prime })<10$, $\mathcal{B}(B^0\to {\eta }^{\prime }\omega )<2.8$, $\mathcal{B}(B^0\to \eta \varphi )<1.0$, $\mathcal{B}(B^0\to {\eta }^{\prime }\varphi )<4.5$, and $\mathcal{B}(B^0\to \varphi \varphi )<1.5$.

Figure 1

Projections of the $B^0$ candidate $m_{\mathrm{E}\mathrm{S}}$ and $\Delta E$ for $B^0\to \eta \omega$. Points with errors represent data, shaded histograms the $B^0\to {\eta }_{3\pi }\omega$ subset, solid curves the full fit functions, and dashed curves the background functions. These plots are made with cuts on probability ratio and thus do not show all events in data samples.