Searches for $B^0$ decays to $\eta K^0$, $\eta \eta$, ${\eta }^{\prime }{\eta }^{\prime }$, $\eta \varphi$, and ${\eta }^{\prime }\varphi$

We search for $B^0$ meson decays into two-body combinations of $K^0$, $\eta$, ${\eta }^{\prime }$, and $\varphi$ mesons in $324\times {10}^6$ $B\overline{B}$ pairs collected with the BABAR detector at the PEP-II asymmetric-energy $e^{+}{e}^{-}$ collider at SLAC. We measure the following branching fractions (upper limits at 90% confidence level) in units of ${10}^{-6}$: $\mathcal{B}(B^0\to \eta K^0)={1.8}_{-0.6}^{+0.7}\pm 0.1(<2.9)$, $\mathcal{B}(B^0\to \eta \eta )={1.1}_{-0.4}^{+0.5}\pm 0.1(<1.8)$, $\mathcal{B}(B^0\to \eta \varphi )=0.1\pm 0.2\pm 0.1(<0.6)$, $\mathcal{B}(B^0\to {\eta }^{\prime }\varphi )={0.2}_{-0.3}^{+0.4}\pm 0.1(<1.0)$, and $\mathcal{B}(B^0\to {\eta }^{\prime }{\eta }^{\prime })={1.0}_{-0.6}^{+0.8}\pm 0.1(<2.4)$, where the first error is statistical and the second systematic.

Figure 1

Signal enhanced projections on $m_{\mathrm{ES}}$ (left) and $\Delta E$ (right) in the decays: (a, b) $\eta K_S^0$, (c, d) $\eta \eta$, (e, f) $\eta \varphi$, (g, h) ${\eta }^{\prime }\varphi$, (i, j) ${\eta }^{\prime }{\eta }^{\prime }$. Points with error bars (statistical only) represent the data (combined measurements), the solid line the full fit function, and the dashed line its background component.