Observation of $B^0\to K^{*0}{\overline{K}}^{*0}$ and Search for $B^0\to K^{*0}{K}^{*0}$

We report the observation of the $b\to d$ penguin-dominated decay $B^0\to K^{*0}{\overline{K}}^{*0}$ with a sample of $383.2\pm 4.2$ million $B\overline{B}$ pairs collected with the BABAR detector at the PEP-II asymmetric-energy $e^{+}{e}^{-}$ collider at the Stanford Linear Accelerator Center. The measured branching fraction is $\mathcal{B}(B^0\to K^{*0}{\overline{K}}^{*0})=[{1.28}_{-0.30}^{+0.35}\pm 0.11]\times {10}^{-6}$ and the fraction of longitudinal polarization is $f_L(B^0\to K^{*0}{\overline{K}}^{*0})={0.80}_{-0.12}^{+0.10}\pm 0.06$. The first error quoted is statistical and the second systematic. We also obtain an upper limit at the 90% confidence level on the branching fraction for $\mathcal{B}(B^0\to K^{*0}{K}^{*0})<0.41\times {10}^{-6}$.

Figure 1

Projections of the multidimensional fit onto (a) $m_{\mathrm{ES}}$; (b) $\Delta E$; (c) $K^{*0}$ mass; and (d) cosine of $K^{*0}$ helicity angle for $B^0\to K^{*0}{\overline{K}}^{*0}$ events selected with a requirement on the signal-to-total likelihood probability ratio, optimized for each variable, with the plotted variable excluded. The points with error bars show the data; the solid line shows signal-plus-background; the dashed line is the continuum background; the hatched region is the signal; and the shaded region is the $B\overline{B}$ background.

Figure 2

Distribution of $-2\ln \mathcal{L}(\mathcal{B},f_L)$ for $B^0\to K^{*0}{\overline{K}}^{*0}$ decays. The solid dot shows the central value and the curves give contours in steps of one sigma [$\Delta \sqrt{-2\ln \mathcal{L}(\mathcal{B},f_L)}=1$].