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Semileptonic and purely leptonic decays of B meson to $\tau$, such as $B\toD^{(\ast)}\tau\nu_{\tau}$ and $B\to\tau\nu_\tau$ are studied. Recognizing thatthere already were some weak hints of possible deviations from the SM in themeasurements of $\mathcal{B}(B\to\tau\nu_\tau)$ by \Babar~and Belle and thefact that detection of the $\tau$ also occurs in the measurements of $B\toD^{(\ast)}\tau\nu_{\tau}$, we stress the importance of joint studies of theseprocesses, whenever possible. For this purpose, as an illustration, weintroduce the observable, $\mathcal{R}(D^{(*)})/\mathcal{B}(B\to\tau\nu_\tau)$where, for one thing, the unknown systematics due to $\tau$ identification areexpected to largely cancel. We show that all measurements of this observableare consistent with the existing data, within somewhat largish experimentalerrors, with the predictions of the SM. We stress that precise experimentalmeasurement and comparison with theory of the branching ratio for $B \to \tau\nu_\tau$ is extremely important for a reliable search of new physics.Furthermore, in view of the anticipated improved precision in experiments inthe next few years, in addition to $\mathcal{R}(D^{(*)})$, host of other ratiosanalogous to $\mathcal{R}(D^{(*)})/\mathcal{B}(B \to \tau \nu_{\tau})$ in theSM are suggested for lattice calculations as well, so that for more stringenttests of the SM, correlations in lattice calculations can be properly takeninto account to enhance precision.

Correlating new physics signals in $B \to D^{(*)} \tau \nu_{\tau}$ with $B \to \tau \nu_{\tau}$