Search for the ${\mathbf{B}}^0\to {\mathbf{K}}_{\mathrm{S}}^0{\mathbf{\tau }}^{+}{\mathbf{\tau }}^{-}$ decay

Abstract

We present the first search for $B^0\to K_{\mathrm{S}}^0{\tau }^{+}{\tau }^{-}$ decays. We look for signal decays in $B^0{\bar{B}}^0$ events produced in asymmetric-energy electron-positron collisions. This work uses samples from the Belle and Belle~II detectors, comprising 1.16 billion ${\rm Y}(4S)$ events. In ${\rm Y}(4S)\to B^0{\bar{B}}^0$ decays, the non-signal ${\bar{B}}^0$ meson is fully reconstructed in a hadronic channel. For the signal $B^0$ meson, $\tau$-lepton decays into final states with a single charged particle are selected. A multivariate classifier is used to combine several discriminating inputs into a single fit observable. We observe no evidence for the signal and set an upper limit on the branching fraction $\mathcal{B}(B^0\to K_{\mathrm{S}}^0{\tau }^{+}{\tau }^{-})<8.3\times 10^{-4}$ at the 90\% confidence level. Combining this with the recent measurement of the isospin-partner decay $B^{+}\to K^{+}{\tau }^{+}{\tau }^{-}$, we determine an upper limit $\mathcal{B}(B\to K{\tau }^{+}{\tau }^{-})<5.4\times 10^{-4}$ at the 90\% confidence level.