Testing the standard model with precision calculations of semileptonic B-decays

Measurements in the flavour sector are very important to test the Standard Model, since most of the free parameters are related to flavour physics. We are discussing semileptonic B-meson decays, from which an important parameter, |V cb |, is extracted. First we discuss higher-order non-perturbative corrections in inclusive semileptonic decays of B mesons. We identify the relevant hadronic matrix elements up to 1/m b 5 and estimate them using an approximation scheme. Within this approach the effects on the integrated rate and on kinematic moments are estimated. Similar estimates are presented for B → X s +γ decays. Furthermore we investigate the role of so-called "intrinsic-charm" operators in this decay, which appear first at order 1/m b 3 in the heavy-quark expansion. We show by explicit calculation that-at scales μ ≤ m c -the contributions from "intrinsic-charm" effects can be absorbed into short-distance coefficient functions multiplying, for instance, the Darwin term. Then, the only remnant of "intrinsic charm" are logarithms of the form ln(m c 2 / m b 2 ), which can be resummed by using renormalization-group techniques. As long as the dynamics at the charm quark scale is perturbative, α s (m c ) << 1, this implies that no additional non-perturbative matrix elements aside from the Darwin and the spin-orbit term have to be introduced at order 1/m b 3 . However, "intrinsic charm" leads at the next order to terms with inverse powers of the charm mass: 1/m b 3 Ã 1/m c 2 . Parametrically they complement the estimate of the potential impact of 1/m b 4 contributions, which we will explore. In this context, we draw semiquantitative conclusions for the expected scale of weak annihilation in semileptonic B decays, both for its valence and non-valence components. The last part is dedicated to a complementary measurement of |V cb | from exclusive B→ D (∗) l ― ν l . Since this determination shows a slight tension with respect to the inclusive one, we investigate wether a non standard model contribution may distort the extraction.