Testing lepton flavour universality with (semi)-leptonic varvec{D_{(s)}} decays

Abstract

Data in B-meson decays indicate violations of lepton flavour universality, thereby raising the question about such phenomena in the charm sector. We perform a model-independent analysis of NP contributions in (semi)-leptonic decays of D_{(s)} mesons which originate from c rightarrow d bar{{ell }} nu _l and c rightarrow s bar{{ell }} nu _{ell } charged-current interactions. Starting from the most general low-energy effective Hamiltonian containing four-fermion operators and the corresponding short-distance coefficients, we explore the impact of new (pseudo)-scalar, vector and tensor operators and constrain their effects through the interplay with current data. We pay special attention to the elements |V_{cd}| and |V_{cs}| of the Cabibbo–Kobayashi–Maskawa matrix and extract them from the D_{(s)} decays in the presence of possible NP decay contributions, comparing them with determinations utilizing unitarity. We find a picture in agreement with the Standard Model within the current uncertainties. Using the results from our analysis, we make also predictions for leptonic D_{(s)}^+ rightarrow e^+ nu _e modes which could be hugely enhanced with respect to their tiny Standard Model branching ratios. It will be interesting to apply our strategy at the future high-precision frontier.