Short- vs. long-distance physics in : a data-driven analysis.

We analyze data on and decays in the whole dilepton invariant mass spectrum with the aim of disentangling short- vs. long-distance contributions. The sizable long-distance amplitudes from narrow resonances are taken into account by employing a dispersive approach. For each available bin and each helicity amplitude an independent determination of the Wilson coefficient , describing transitions at short distances, is obtained. The consistency of the values thus obtained provides an check of the absence of additional, sizable, long-distance contributions. The systematic difference of these values from the Standard Model expectation supports the hypothesis of a non-standard amplitude of short-distance origin.