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) 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.
Centre d'études et d'expertise sur les risques, l'environnement, la mobilité et l'aménagement
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We construct a general class of pseudo-Goldstone composite Higgs models, within the minimal SO(5)/SO(4) coset structure, that are not necessarily of moose-type. We characterize the main properties these models should have in order to give rise to a Higgs mass around 125 GeV. We assume the existence of relatively light and weakly coupled spin 1 and 1/2 resonances. In absence of a symmetry principle, we introduce the Minimal Higgs Potential (MHP) hypothesis: the Higgs potential is assumed to be one-loop dominated by the SM fields and the above resonances, with a contribution that is made calculable by imposing suitable generalizations of the first and second Weinberg sum rules. We show that a 125 GeV Higgs requires light, often sub-TeV, fermion resonances. Their presence can also be important for the models to successfully pass the electroweak precision tests. Interestingly enough, the latter can also be passed by models with a heavy Higgs around 320 GeV. The composite Higgs models of the moose-type considered in the literature can be seen as particular limits of our class of models.1 From now on a pNGB Higgs will always be assumed. 2 This is not the case for little Higgs models where a hierarchy between v and f can naturally be realized, but the explicit working implementations of this idea are a bit cumbersome. We will not consider little Higgs models in this paper.3 The idea of partially composite SM fermions dates back to [9], but only extra dimensions have allowed us to appreciate its full power [10].3 directly related by deconstruction to five-dimensional models, where the Higgs mass can at least be assumed to be calculable, and characterize the main properties these models should have in order to give rise to a Higgs mass at around 125 GeV. More specifically, we focus on the minimal SO(5)/SO(4) coset structure and consider models with an arbitrary number of spin 1 ("vector" and "axial") and spin 1/2 resonances. These resonances are assumed to be the only ones below the cut-off of the model at Λ = 4πf . Partial compositeness is assumed. The divergencies of the Higgs potential are cancelled by imposing that certain form factors, both in the gauge and in the fermion sectors, vanish sufficiently fast for large euclidean values of the momentum. These conditions are straightforward generalizations of the first and second Weinberg sum rules [15] and guarantee that the calculable part of the one-loop Higgs potential is finite. Being the Higgs potential a UV-sensitive quantity, and in absence of a symmetry mechanism protecting it, we will simply assume that the one-loop form factors above represent the main contributions to the potential, with higher-loop and higher-order operators giving only a sub-leading correction. We will denote the above assumption as the Minimal Higgs Potential (MHP) hypothesis. This is by far the strongest assumption underlying our construction. A similar approach is known to describe quite well the pion mass difference in QCD (see [16] for a very nice review), in which case the knowledge of ...
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