ABSTRACT
Present measurements of bâcτν and bâuτν transitions differ from the standard model predictions of lepton flavor universality by almost 4σ. We examine new physics interpretations of this anomaly. An effective field theory analysis shows that minimal flavor violating models are not preferred as an explanation, but are also not yet excluded. Allowing for general flavor violation, right-right vector and right-left scalar quark currents are identified as viable candidates. We discuss explicit examples of two Higgs doublet models, leptoquarks as well as quark and lepton compositeness. Finally, implications for LHC searches and future measurements at the (super-)B factories are presented.
ABSTRACT
We compute the branching ratios for the rare top quark decays tâcγ and tâcZ mediated by effective flavor changing neutral couplings at next-to-leading order in QCD, including the effects due to operator mixing. After resumming contributions of the order of [α{s}log(Λ/m{t})]{n}, where Λ is the scale at which the effective operators are generated, using renormalization group methods, we compute finite matrix element corrections and study the effects of experimental kinematic cuts on the extracted branching ratios. We find that the tâcγ decay can also be used to probe the effective operators mediating tâcg processes, since the latter can naturally contribute 10% or more to the radiative decay. Conversely, any experimental signal of tâcg would indicate a natural lower bound on tâcZ, γ.