Search for the decay $B^0\to K^{*0}{\tau }^{+}{\tau }^{-}$ at the Belle experiment

This paper presents a search for the rare flavor-changing neutral current process $B^0\to K^{*0}{\tau }^{+}{\tau }^{-}$ using data taken with the Belle detector at the KEKB asymmetric energy $e^{+}{e}^{-}$ collider. The analysis is based on the entire $\mathrm{\Upsilon }(4S)$ resonance data sample of $711{\mathrm{fb}}^{-1}$, corresponding to $772\times {10}^{6}B\overline{B}$ pairs. In our search we fully reconstruct the companion $B$ meson produced in the process $e^{+}{e}^{-}\to \mathrm{\Upsilon }(4S)\to B\overline{B}$ from its hadronic decay modes, and look for the decay $B^0\to K^{*0}{\tau }^{+}{\tau }^{-}$ in the rest of the event. No evidence for a signal is found. We report an upper limit on the branching fraction $\mathcal{B}(B^0\to K^{*0}{\tau }^{+}{\tau }^{-})<3.1\times {10}^{-3}$ at 90% confidence level. This is the first direct limit on $\mathcal{B}(B^0\to K^{*0}{\tau }^{+}{\tau }^{-})$.

Figure 1

Fit results for $M_{\mathrm{miss}}^2$ (upper) and $E_{\mathrm{ECL}}^{\text{extra}}$ (lower) for the decays $B^0\to D^{(*)}\ell {\nu }_{\ell }$. The dots with error bars represent the data, and the blue line indicates the fitted results. The dashed lines indicate different fit components. $E_{\mathrm{ECL}}^{\text{extra}}$ is plotted with the selection $M_{\mathrm{miss}}^2<0.5{\mathrm{GeV}}^2/c^4$.

Figure 2

Distribution of $E_{\mathrm{ECL}}^{\text{extra}}$ combined for all signal modes. The dots with error bars show the data, the blue line shows the fitted results with the background-only model, and the dashed lines show fit results for the different components. A signal (red region) with a branching fraction of $3.1\times {10}^{-3}$, corresponding to the upper limit at 90% CL, is superimposed on the top of the fit.