B¯ → D¯ D decays and the extraction of f _d/f _u at hadron colliders

We perform a detailed model-independent phenomenological analysis of B¯ → D¯ D decays. Employing an SU(3) _F analysis including symmetry-breaking contributions together with a conservative power counting for various suppression effects, we obtain updated Standard Model predictions for all branching fractions and CP asymmetries from a fit to the available experimental data, testable at Belle II and the LHC experiments. These results include all relevant suppressed contributions, thereby providing upper limits on subleading Standard Model (SM) effects like “penguin pollution”, enabling searches for physics beyond the SM. Importantly, allowing in the same fit for the production fractions of charged and neutral B mesons to be different, we find f _d/f _u = 0.86 ± 0.05, which is 2.5σ away from unity, which, if confirmed, would have important phenomenological consequences.