Heavy-Quark expansion for (B)over-bars → Ds(*) form factors and unitarity bounds beyond the SU(3)F limit

We carry out a comprehensive analysis of the full set of B over bar q -> Dq(*)form factors for spectator quarks q=u,d,s within the framework of the Heavy-Quark expansion (HQE) to order O alpha s,1. In addition to the available lattice QCD calculations we make use of two new sets of theoretical constraints: we produce for the first time numerical predictions for the full set of B over bar s -> Ds(*) form factors using Light-Cone sum rules with Bs-meson distribution amplitudes. Furthermore, we reassess the QCD three-point sum rule results for the Isgur-Wise functions entering all our form factors for both q=u,d and q=s spectator quarks. These additional constraints allow us to go beyond the commonly used assumption of SU(3)F symmetry for the B over bar s -> Ds(*) form factors, especially in the unitarity constraints which we impose throughout our analysis. We find the coefficients of the IW functions emerging at Oto be consistent with the naive Oexpectation, indicating a good convergence of the HQE. While we do not find significant SU(3) breaking, the explicit treatment of q=s as compared to a simple symmetry assumption renders the unitarity constraints more effective. We find that the (pseudo)scalar bounds are saturated to a large degree, which affects our theory predictions. We analyze the phenomenological consequences of our improved form factors by extracting |Vcb|from B over bar -> D(*)l nu decays and producing theoretical predictions for the lepton-flavour universality ratios R(D), R(D*) and R(Ds*), as well as the tau- and Dq* polarization fractions for the B over bar q -> Dq(*)tau nu modes.