Revisiting Lepton Flavour Universality in B Decays

TL;DR

This paper revisits lepton flavour universality in B decays by including quantum effects through renormalization group evolution, revealing significant modifications to leptonic couplings and potential observable lepton flavour violation.

Contribution

It introduces a comprehensive analysis of quantum effects on LFU in B decays using RGEs from a high scale, highlighting their impact on leptonic couplings and LFV processes.

Findings

Quantum effects significantly modify Z boson leptonic couplings.

Large LFU breaking effects are predicted in Z and tau decays.

Observable LFV processes like tau -> mu ll are induced.

Abstract

Lepton flavour universality (LFU) in B-decays is revisited by considering a class of semileptonic operators defined at a scale Lambda above the electroweak scale v. The importance of quantum effects, so far neglected in the literature, is emphasised. We construct the low-energy effective Lagrangian taking into account the running effects from Lambda down to v through the one-loop renormalization group equations (RGE) 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 flavour violating (LFV) effects in the processes tau -> mu ll, tau -> mu rho, tau -> mu pi and tau -> mu eta^(') are induced.