https://doi.org/10.1140/epjc/s10052-020-7702-7
Regular Article - Theoretical Physics
Testing lepton flavour universality with (semi)-leptonic 
decays
1
Nikhef, Science Park 105, 1098 XG, Amsterdam, The Netherlands
2
Faculty of Science, Vrije Universiteit Amsterdam, 1081 HV, Amsterdam, The Netherlands
3
Max Planck Institute for Physics, Föhringer Ring 6, 80805, Munich, Germany
Received:
2
January
2020
Accepted:
30
January
2020
Published online:
20
February
2020
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 
mesons which originate from 
and 
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 
and 
of the Cabibbo–Kobayashi–Maskawa matrix and extract them from the 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 
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.
© The Author(s) 2020
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