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

Abstract

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.

Comments: 17 pages, 4 figures, 1 table; revised version accepted by journal