Semileptonic b → c transitions, and in particular the ratios R(, can be used to test the universality of the weak interactions. In light of the recent discrepancies between the experimental measurements of these observables by BaBar, Belle and LHCb and the Standard Model predicted values, we study the robustness of the latter. Our analysis reveals that R(D) might be enhanced by lepton mass effects associated to the mostly unknown scalar form factor. In constrast, the Standard Model prediction for R(D * ) is found to be more robust, since possible pollutions from B * contributions turn out to be negligibly small, which indicates that R(D * ) is a promising observable for searches of new physics.

We propose an alternative evaluation of the long-distance weak annihilation (WA, also called one-photon exchange in this paper) contribution to the rare semileptonic B ± → (π ± , K ± )ℓ + ℓ − (ℓ = e, µ) decays. This hadronic description at low energies is matched at intermediate energies to its short-distance counterpart in terms of quark and gluon degrees of freedom. Although the WA contribution does not contribute to solve the possible breaking of lepton-universality observed by LHCb in the B ± → K ± (µ + µ − /e + e − ) ratio, nor provides an important hadronic contamination to their decay rates, its contribution to the branching ratios (and direct CP asymmetry) of the B ± → π ± ℓ + ℓ − transitions turns out to be significant. This hadronic pollution should be taken into account when looking for new physics effects in decays into pions, which suggests to restrict these searches to squared lepton-pair invariant mass in the (1, 8) GeV 2 range. The interference of the one-photon exchange contribution with the dominant short-distance one-loop amplitude induces a sizable CP asymmetry in these rare decays, which calls for dedicated measurements.

We consider the exclusive P 4 decays, P → (P 1 P 2 ) V ν , where the subindex V means that the invariant mass of the pseudoscalar pair is taken within a small window around the mass of the vector meson V . Pole contributions beyond the dominant P → V (→ P 1 P 2 ) ν amplitude of P 4 decays are identified, which, in turn, affects the determination of the CKM matrix elements |V qq |. We evaluate the effects of those contributions in the extraction of bottom and charm quark mixings. An application to B → (ππ) ρ ν data from Belle collaboration, shows an increase in the extracted value of |V ub | in better agreement with determinations based on B → π ν decays. The effect of the ρ and D * pole contributions in the determination of |V cd | from the decay D → ππ −ν , has been also investigated.