Search for the decay B$^0$ → K$^{ *0}$ τ$^+$ τ$^−$ at the Belle experiment

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