Abstract
Motivated by additional experimental hints of Lepton Flavour Universality violation in B decays, both in charged- and in neutral-current processes, we analyse the ingredients necessary to provide a combined description of these phenomena. By means of an Effective Field Theory (EFT) approach, based on the hypothesis of New Physics coupled predominantly to the third generation of left-handed quarks and leptons, we show how this is possible. We demonstrate, in particular, how to solve the problems posed by electroweak precision tests and direct searches with a rather natural choice of model parameters, within the context of a U(2)q ×U(2)ℓ flavour symmetry. We further exemplify the general EFT findings by means of simplified models with explicit mediators in the TeV range: coloured scalar or vector leptoquarks and colour-less vectors. Among these, the case of an SU(2)L-singlet vector leptoquark emerges as a particularly simple and successful framework.
Highlights
In both cases the combination of the results leads to an evidence around the 4σ level for LFU violating contributions of non-SM origin, whose size is O(10%) compared to the corresponding charged- or neutral-current SM amplitudes
A class of motivated models includes those which are based on a U(2)q × U(2) flavour symmetry acting on the light generations of SM fermions [31, 32], and new massive bosonic mediators around the TeV scale: colour-less vector SU(2)L-triplets (W, B ) [13], vector SU(2)Lsinglet or -triplet leptoquarks (LQ) [17], or scalar SU(2)L-singlet and -triplet leptoquarks
Our analysis clearly demonstrates that a combined explanation of both charged- and neutral-current B-physics anomalies, consistent with the absence of deviations from the Standard Model so far observed in other low- and high-pT observables, is possible and does not require unnatural tunings in model space
Summary
We analyse the flavour structure and the constraints on the semi-leptonic four-fermion operators contributing at the tree-level to RKμe(∗) and RDτ (∗), taking into account the bounds from processes affected by the same effective operators both at the tree-level and beyond. We do not attempt a completely model-independent EFT-type analysis, but we keep the discussion sufficiently general under the main hypothesis of NP coupled predominantly to third-generation left-handed quarks and leptons. Only four-fermion operators built in terms of left-handed quarks and leptons have non-vanishing Wilson coefficients; 2. 3. operators containing flavour-blind contractions of the light fields have vanishing Wilson coefficients. We first discuss the consequences of these hypotheses on the structure of the relevant effective operators and proceed analysing the experimental constraints on their couplings
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