New physics contributions in $Btopitaubar u$ and $Btotaubar u$

We study possible new physics contributions in $Btopitaubar u $ and $Btotaubar u$ employing the model-independent effective Lagrangian that describes the quark-level transition $b to u tau u$ at low energies. The decay rate of $Btopitaubar u$ and its theoretical uncertainty are evaluated using the $Btopi$ form factors given by recent lattice QCD studies. Comparing theoretical results with the current experimental data, $mathcal{B}(Btopitaubar u)<2.5times 10^{-4}$ and $mathcal{B}(Btotaubar u)=(1.14pm 0.22)times 10^{-4}$, we obtain constraints on the Wilson coefficients that quantify potential new physics. We also present the expected sensitivity of the SuperKEKB/Belle~II experiment.