We study exclusive nonleptonic and semileptonic decays of the $B_c$-meson within a relativistic constituent quark model previously developed by us. For the nonleptonic decays we use the factorizing approximation. We update our model parameters by using new experimental data for the mass and the lifetime of the $B_c$ meson and the leptonic decay constants of the $D$-meson. We calculate the branching ratios for a large set of exclusive nonleptonic and semileptonic decays of the $B_c$ meson and compare our results with the results of other studies. As a guide for further experimental exploration we provide explicit formulas for the full angular decay distributions in the cascade decays $B_c^{-}\to J/\psi (\to l^{+}{l}^{-})+{\rho }^{-}(\to {\pi }^{-}{\pi }^0)$ and $B_c^{-}\to J/\psi (\to l^{+}{l}^{-})+W_{\mathrm{off}\mathrm{\text{−}}\mathrm{shell}}^{-}(\to l^{-}+{\overline{\nu }}_l)$.

We evaluate the decays $\overrightarrow{B}{K}^{*}\pi ,K(\rho ,\omega ,\phi )$ adding the long distance charming penguin contributions to the short distance: tree+penguin amplitudes. We estimate the imaginary part of the charming penguin contribution by an effective field theory inspired by heavy quark effective theory and parametrize its real part. The final results for branching ratios depend on only two real parameters and show a significant role of the charming penguin contributions. The overall agreement with the available experimental data is satisfactory.

A previous analysis of two-body Cabibbo-allowed nonleptonic decays of ${\mathit{D}}^0$ mesons and of Cabibbo-allowed and first-forbidden decays of ${\mathit{D}}^{+}$ and ${\mathit{D}}_{\mathit{s}}^{+}$ has been adjourned using more recent experimental data and extended to the Cabibbo-forbidden decays of ${\mathit{D}}^0$. Annihilation and W-exchange contributions as well as final state interaction effects (assumed to be dominated by nearby resonances) have been included and are in fact crucial to obtain a reasonable agreement with the experimental data, which show large flavor SU(3) violations. New fitting parameters are necessary to describe rescattering effects for Cabibbo-forbidden ${\mathit{D}}^0$ decays, given the lack of experimental information on isoscalar resonances. We keep their number to a minimum, three, using phenomenologically based considerations. We also discuss CP-violating asymmetries.

Recently there has been much interest in the tauonic semileptonic meson decays $B\to D+\tau +{\nu }_{\tau }$ and $B\to D^{*}+\tau +{\nu }_{\tau }$, where one has found larger rates than what is predicted by the Standard Model. We analyze the corresponding semileptonic baryon decays ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}+{\tau }^{-}+{\overline{\nu }}_{\tau }$ with particular emphasis on the lepton helicity flip and scalar contributions which vanish for zero lepton masses. We calculate the total rate, the differential decay distributions, the longitudinal and transverse polarization of the daughter baryon ${\mathrm{\Lambda }}_c^{+}$ and the $\tau$ lepton, and the lepton-side forward-backward asymmetries. The nonvanishing polarization of the daughter baryon ${\mathrm{\Lambda }}_c^{+}$ leads to hadron-side asymmetries in e.g. the decay ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Lambda }}^0+{\pi }^{+}$ and azimuthal correlations between the two final-state decay planes which we specify. We provide numerical results on these observables using results of the covariant confined quark model. We find large lepton mass effects in the $q^2$ spectra and in the polarization observables.

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 $b\to c\tau \nu$ 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.