Revisiting Lepton Flavor Universality in B Decays

Lepton flavor universality (LFU) in B decays is revisited by considering a class of semileptonic operators defined at a scale. above the electroweak scale v. The importance of quantum effects, so far neglected in the literature, is emphasized. We construct the low-energy effective Lagrangian taking into account the running effects from. down to v through the one-loop renormalization group equations (RGEs) in the limit of exact electroweak symmetry and QED RGEs from v down to the 1 GeV scale. The most important quantum effects turn out to be the modification of the leptonic couplings of the vector boson Z and the generation of a purely leptonic effective Lagrangian. Large LFU breaking effects in Z and tau decays and visible lepton flavor violating effects in the processes tau -> mu ll, tau ->mu rho , tau -> mu pi, and tau -> mu eta((')) are induced.