Search for the decay B${}^0$ → K${}^{\ast 0}$ τ${}^{+}$ τ${}^{-}$ at the Belle experiment

This paper presents a search for the rare flavor-changing neutral current process $B^0\to K^{\ast 0}{\tau }^{+}{\tau }^{-}$ using data taken with the Belle detector at the KEKB asymmetric energy $e^{+}{e}^{-}$ collider. The analysis is based on the entire ${\rm Y}(4S)$ resonance data sample of $711\hspace{0.17em}\hspace{0.17em}f{b}^{-1}$, corresponding to $772\times {10}^{6}B\overline{B}$ pairs. In our search we fully reconstruct the companion B meson produced in the process $e^{+}{e}^{-}\to {\rm Y}(4S)\to B\overline{B}$ from its hadronic decay modes, and look for the decay $B^0\to K^{\ast 0}{\tau }^{+}{\tau }^{-}$ in the rest of the event. No evidence for a signal is found. We report an upper limit on the branching fraction $\mathcal{B}(B^0\to K^{\ast 0}{\tau }^{+}{\tau }^{-})<3.1\times {10}^{-3}$ at 90% confidence level. This is the first direct limit on $\mathcal{B}(B^0\to K^{\ast 0}{\tau }^{+}{\tau }^{-})$.