Testing lepton flavour universality with (semi)-leptonic $$\varvec{D_{(s)}}$$D(s) decays
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Regular Article - Theoretical Physics
Testing lepton flavour universality with (semi)-leptonic D(s) decays Robert Fleischer1,2 , Ruben Jaarsma1, Gabriël Koole1,3,a 1
Nikhef, Science Park 105, 1098 XG Amsterdam, The Netherlands Faculty of Science, Vrije Universiteit Amsterdam, 1081 HV Amsterdam, The Netherlands 3 Max Planck Institute for Physics, Föhringer Ring 6, 80805 Munich, Germany
2
Received: 2 January 2020 / Accepted: 30 January 2020 © The Author(s) 2020
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 modelindependent analysis of NP contributions in (semi)-leptonic ¯ l decays of D(s) mesons which originate from c → d ν ¯ and c → s ν charged-current interactions. Starting from the most general low-energy effective Hamiltonian containing four-fermion operators and the corresponding shortdistance 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 |Vcd | and |Vcs | 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 anal+ → e+ νe ysis, we make also predictions for leptonic D(s) 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.
1 Introduction Forty-five years after the discovery of the charm quark, flavour physics has developed into a broad line of research, allowing us to probe the Standard Model (SM) with unprecedented sensitivity to new interactions and particles at energy scales far beyond the TeV regime which is directly accessible at the Large Hadron Collider (LHC) today [1]. In these explorations, decays of B mesons usually play the key role. Measurements by the BaBar, Belle and LHCb collaborations of processes originating from b → c− ν¯ and b → s+ − quark-level transitions indicate deviations from a e-mail:
[email protected] (corresponding author)
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the SM, where the following observables are in the focus ( = e, μ): R D (∗) ≡
B(B → D (∗) τ − ντ ) B(B → D (∗) − ν )
, R K (∗) ≡
B(B → K (∗) μ+ μ− ) B(B → K (∗) e+ e− )
.
(1) The experimental values of these ratios show tensions with respect to the corresponding SM predictions at the (2–3)σ level (for recent reviews, see, e.g., Refs. [2,3]). In particular, the data raise the exciting question of a possible violation of a central feature of the SM: lepton flavour universality (LFU). On the theory side, various models of New Physics (NP), i.e. physics lying beyond the SM, have been proposed that could explain the B-decay anomalies, allowing in particular also for violations of LFU. Imp
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