Search for $B^0 \to \pi^- \tau^+ \nu_\tau$ with hadronic tagging at Belle

Understand

We search for the process $B^0 \to \pi^- \tau^+ \nu_\tau$ using the full Belle data set of $711\,{\rm fb}^{-1}$, corresponding to $772 \times 10^6 B\bar{B}$ pairs, collected at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider.

• We reconstruct one $B$ meson in a hadronic decay and search for the $B^0 \to \pi^- \tau^+ \nu_\tau$ process in the remainder of the event.
• No significant signal is observed and an upper limit of $\mathcal{B}(B^0 \to \pi^- \tau^+ \nu_\tau) < 2.5 \times 10^{-4}$ is obtained at the $90\%$ confidence level.