Recently, the BESIII collaboration has reported numerous measurements of various D (s) meson semileptonic decays with significantly improved precision. Together with similar studies carried out at BABAR, Belle, and CLEO, new windows to a better understanding of weak and strong interactions in the charm sector have been opened. In light of new experimental data, we review the theoretical description and predictions for the semileptonic decays of D (s) to a pseudoscalar or a vector meson. This review is essentially an extended discussion of our recently published results obtained in the framework of the covariant confining quark model.

We study the semileptonic decays of the B c meson into final charmonium states within the standard model and beyond. The relevant hadronic transition form factors are calculated in the framework of the covariant confined quark model developed by us. We focus on the tau mode of these decays, which may provide some hints of new physics effects. We extend the standard model by assuming a general effective Hamiltonian describing the b → cτν transition, which consists of the full set of the four-fermion operators.We then obtain experimental constraints on the Wilson coefficients corresponding to each operator and provide predictions for the branching fractions and other polarization observables in different new physics scenarios.

We study the longitudinal, transverse, and normal polarization components of the tau lepton in the decays B0 → D ( * ) τ − ντ and discuss their role in searching for new physics (NP) beyond the standard model (SM). Starting with a model-independent effective Hamiltonian including non-SM four-Fermi operators, we obtain experimental constraints on different NP scenarios and investigate their effects on the polarization observables. In the SM the longitudinal and transverse polarizations of the tau lepton differ substantially from the corresponding zero lepton mass values of P L = −1 and P T = 0. In addition, P L and P T are very sensitive to NP effects. For the transverse polarization this holds true, in particular, for the effective tensor operator in the case of B0 → D * and for the scalar operator in the case of B0 → D. The T -odd normal polarization P N , which is predicted to be negligibly small in the SM, can be very sizable assuming NP complex Wilson coefficients.We also discuss in some detail how the three polarization components of the tau lepton can be measured with the help of its subsequent leptonic and semihadronic decays.