New physics contributions in $B\to\pi\tau\bar\nu$ and $B\to\tau\bar\nu$

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