On lepton flavour universality in semileptonic Bcηc, J/ψ decays


We discuss Bcηc and BcJ/ψ semileptonic decays within the Standard Model (SM) and beyond. The relevant transition form factors, being the main source of theoretical uncertainties, are calculated in the sum rule approach and are provided in a full q2 range. We calculate the semileptonic branching fractions and find for the ratios, \( \left.{R}_{\eta_c}\right|\mathrm{S}\mathrm{M}=0.32\pm 0.02 \), RJ/ψ |SM = 0.23 ± 0.01. Both predictions are in agreement with other existing calculations and support the current tension in RJ/ψ at 2σ level with the experiment. To extend the potential of testing the SM in the semileptonic Bc decays, we consider the forward-backward asymmetry and polarization observables. We also study the 4-fold differential distributions of \( {B}_c\to J/\psi \left(J/\psi \to {\tilde{\ell}}^{-}{\tilde{\ell}}^{+}\right){\ell}^{-}{\overline{\nu}}_{\ell } \), where \( \tilde{\ell} = e,\mu \), in the presence of different new physics scenarios and find that the new physics effects can significantly modify the angular observables and can also produce effects which do not exist in the SM. Using the constraints on the new physics couplings from the recent combined analysis of BaBar, Belle and LHCb data on semileptonic BD(∗) decays, where the effects of new physics could be visible, we find that these different new physics scenarios are not able to simultaneously explain the current experimental value of RJ/ψ.

A preprint version of the article is available at ArXiv.


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Leljak, D., Melić, B. & Patra, M. On lepton flavour universality in semileptonic Bcηc, J/ψ decays. J. High Energ. Phys. 2019, 94 (2019). https://doi.org/10.1007/JHEP05(2019)094

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  • Beyond Standard Model
  • Heavy Quark Physics