Precision Predictions for Flavour-Changing Neutral Current Decays

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Quark decays which are induced by flavour-changing neutral currents (FCNCs) represent important tools to explore the borders of the Standard Model (SM) of particle physics. In the SM, these decays are suppressed by small mixing angles in the Cabibbo-Kobayashi-Maskawa (CKM) matrix and/or small Yukawa couplings of light quarks (GIM mechanism). In extensions of the SM, similar effective suppression mechanism are not necessarily realized, and therefore “new physics” (NP) signals can show up in decay rates and asymmetries. On the theoretical side, FCNC-processes at low energies are described in terms of effective Hamiltonian which allows to take into account contributions from the SM as well as from its possible extensions, including, for instance, effects from right-handed currents or new sources of CP-violation. To date, we have already observed a multitude of processes based on b-quark transitions to s-quark; first data have also become available for b to d decay modes, which are further suppressed by smaller CKM angles. The situation for FCNCs involving up-type quarks (u, c, t) is less advanced, partly because of the still stronger suppression of the corresponding decay rates. Within this project, we plan to perform a comprehensive theoretical study of various exclusive FCNC decays of hadrons containing b-quarks. Such decays are ideally suited to test the fundamental interactions in and beyond the SM because, on the one hand, lots of new data can be expected from the LHC experiments in the coming years, and, at the same time the hadronic uncertainties in these modes are under reasonable theoretical control thanks to the systematic expansion in inverse powers of the heavy b-quark mass. By taking into account improved theoretical predictions on hadronic parameters, such as transition form factors, and by constructing optimized observables with reduced sensitivity on hadronic uncertainties, we are going to explore and classify various signatures of possible NP effects in angular distributions and decay asymmetries. In addition, we plan to investigate rare FCNC decays of c-quarks with similar theoretical methods.