Abstract
In view of recent experimental indications of violations of Lepton Flavor Universality (LFU) in B decays, we analyze constraints and implications of LFU interactions, both using an effective theory approach, and an explicit dynamical model. We show that a simple dynamical model based on a SU(2) L triplet of massive vector bosons, coupled predominantly to third generation fermions (both quarks and leptons), can significantly improve the description of present data. In particular, the model decreases the tension between data and SM predictions concerning: i) the breaking of τ-μ universality in B→D(*)lν decays; ii) the breaking of μ-e universality in B → Kl+l− decays. Indirectly, the model might also decrease the discrepancy between exclusive and inclusive determinations of |V cb | and |V ub |. The minimal version of the model is in tension with ATLAS and CMS direct searches for the new massive vectors (decaying into τ+τ− pairs), but this tension can be decreased with additional non-standard degrees of freedom. Further pre-dictions of the model both at low- and high-energies, in view of future high-statistics data, are discussed.
Highlights
In addition to these LFU ratios, whose deviation from unity would clearly signal physics beyond the Standard Model (SM), B-physics data exhibit other tensions with SM expectations in semi-leptonic observables
Lepton Flavor Universality is not a fundamental symmetry: within the Standard Model it is an approximate accidental symmetry broken only by the Yukawa interactions. This specific symmetry and symmetry-breaking pattern results in tiny deviations from LFU in helicity-conserving amplitudes, within the SM, and it implies that LFU tests are clean probes of physics beyond the SM
Motivated by a series of recent experimental results in B physics pointing to possible violations of LFU, both in charged and in neutral currents, in this paper we have consider a simplified dynamical model able to describe these effects in a unified way
Summary
We assume an approximate U(2)q × U(2) flavor symmetry, under which the light generations of QiL and LiL transform as 2q × 1 and 1q × 2 , respectively, and all other fermions are singlets. We further assume that the underlying dynamics responsible for the effective interaction in eq (2.1) involves, in first approximation, only third generation SM fermions (the left-handed 1q ×1 fermions) In this limit, the flavor couplings in eq (2.2) are λqij, = δi3δ3j. The flavor couplings in eq (2.2) are λqij, = δi3δ3j The corrections to this limit are expected to be generated by appropriate U(2)q × U(2) breaking spurions, connected to the generation of subleading terms in the Yukawa couplings for the SM light fermions.
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