Low-energy Effects of Lepton Flavour Universality Violation

TL;DR

This paper investigates the low-energy implications of lepton flavour universality violation in B-decays, considering extended operators and electroweak corrections, revealing tensions between new physics explanations and experimental constraints.

Contribution

It generalizes previous models by including a broader set of operators and electroweak effects, analyzing their impact on low-energy observables and constraints.

Findings

Electroweak corrections are crucial for accurate predictions.

Constraints from Z-pole and tau decays challenge certain new physics scenarios.

Simultaneous explanation of charged and neutral-current anomalies remains difficult.

Abstract

The persisting anomalous data in semileptonic B-decays point towards New Physics models exhibiting large sources of Lepton Flavour Universality Violation. In this work we generalise previous studies by considering frameworks which include an enlarged set of semileptonic four-fermion operators invariant under the SM gauge group, with New Physics affecting mainly the third generation. We derive the low-energy effective Lagrangian including the leading electroweak corrections, mandatory to obtain reliable predictions. As a particularly interesting case, we analyse the scenario where the dominant New Physics effects are encoded in the Wilson coefficient C_9, as favoured by global fit analyses of b -> s data. We find that also in this case the stringent experimental bounds on Z-pole observables and tau decays challenge a simultaneous explanation of charged and neutral-current non-standard data.