Abstract
The growing experimental indication of Lepton Flavour Universality Violation (LFUV) both in charged- and neutral-current semileptonic B-decays, has triggered many theoretical interpretations of such non-standard phenomena. Focusing on popular scenarios where the explanation of these anomalies requires New Physics at the TeV scale, we emphasise the importance of including electroweak corrections to obtain trustable predictions for the models in question. We find that the most important quantum effects are the modifications of the leptonic couplings of the W and Z vector bosons and the generation of a purely leptonic effective Lagrangian. Although our results do not provide an inescapable no-go theorem for the explanation of the B anomalies, the tight experimental bounds on Z-pole observables and τ decays challenge an explanation of the current non-standard data. We illustrate how these effects arise, by providing a detailed discussion of the running and matching procedure which is necessary to derive the low-energy effective Lagrangian.
Highlights
The search for lepton flavour universality violation (LFUV) represents one of the most powerful tool to unveil New Physics (NP) phenomena, as the Standard Model (SM) predicts negligible Lepton Flavour Universality Violation (LFUV) effects
Many studies focused on the experimental signatures implied by the solution of these anomalies in specific scenarios, including kaon observables [23, 24], kinematic distributions in B decays [25,26,27,28,29,30,31,32,33,34,35], the lifetime of the Bc− meson [36], Υ and ψ leptonic decays [37], tau lepton searches [38, 39] and dark matter [40, 41]
After discussing our starting assumptions, in particular the operators dominating NP effects at the TeV scale, we present the minimal set of SU(2) × U(1) gauge-invariant operators involved in the renormalization group equations (RGE) flow from the TeV to the electroweak scale
Summary
The search for lepton flavour universality violation (LFUV) represents one of the most powerful tool to unveil New Physics (NP) phenomena, as the Standard Model (SM) predicts negligible LFUV effects. Of particular interest are those attempting to a simultaneous explanation of both charged- and neutral-current anomalies Such a task can be most naturally achieved assuming that NP intervenes through effective 4-fermion operators involving left-handed currents, (sLγμbL)(μLγμμL) and (cLγμbL)(τLγμνL), which are related by the SU(2)L gauge symmetry [42]. Another possibility consists in NP coupling to different fermion generations proportionally to the charged lepton mass squared [44] In this case LFUV does not necessarily imply LFV at an observable level, if NP preserves the lepton family numbers in the limit of massless neutrinos. We include the further running, dominated by pure electromagnetic effects, down to the tau lepton mass scale, after crossing the bottom threshold
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