New Physics Scenarios in $b \to c \ell {\bar \nu}_\ell$ decays

The latest BaBar measurements of the ratios ${\cal R}(D^{(*)})=\displaystyle{\frac{{\cal B}(B \to D^{(*)} \tau {\bar \nu}_\tau)}{{\cal B}(B \to D^{(*)} \mu {\bar \nu}_\mu)}}$ deviate from the Standard Model predictions at the global level of 3.4$\sigma$. A possibility to reproduce these experimental ratios without affecting other modes which do not show similar deviations is to consider new physics scenarios producing an additional tensor operator in the effective weak Hamiltonian. I describe the impact of such an operator in semileptonic $B \to D^{(*)}$ modes and in semileptonic $B$ and $B_s$ decays to excited positive parity charmed mesons. In particular, I discuss the most effective observables able to discriminate new physics from the Standard Model.