On the anomalous enhancement observed in $B \to D^{()} \, \tau \, {\bar \nu}_\tau $ decays

The BaBar measurements of the ratios ${\cal R}(D^{(*)})=\frac{{\cal B}(B \to D^{(*)} \tau {\bar \nu}_\tau)}{{\cal B}(B \to D^{(*)} \mu {\bar \nu}_\mu)}$ deviate from the standard model expectation, while new results on the purely leptonic $B \to \tau {\bar \nu}_\tau$ mode show a better consistency with the standard model, within the uncertainties. In a new physics scenario, one possibility to accomodate these two experimental facts consists in considering an additional tensor operator in the effective weak hamiltonian. We study the effects of such an operator in a set of observables, in semileptonic $B \to D^{(*)}$ modes as well as in semileptonic $B$ and $B_s$ decays to excited positive parity charmed mesons.