Abstract
The measurements carried out at LEP and SLC projected us into the precision era of electroweak physics. This has also been relevant in the theoretical interpretation of LHCb and Belle measurements of rare B semileptonic decays, paving the road for new physics with the inference of lepton universality violation in {R}_{K^{left(ast right)}} ratios. The simplest explanation of these flavour anomalies — sizeable one-loop contributions respecting Minimal Flavour Violation — is currently disfavoured by electroweak precision data. In this work, we discuss how to completely relieve the present tension between electroweak constraints and one-loop minimal flavour violating solutions to {R}_{K^{left(ast right)}} . We determine the correlations in the Standard Model Effective Field Theory that highlight the existence of such a possibility. Then, we consider minimal extensions of the Standard Model where our effective-field-theory picture can be realized. We discuss how these solutions to b → sℓℓ anomalies, respecting electroweak precision and without any new source of flavour violation, may point to the existence of a Z′ boson at around the TeV scale, within the discovery potential of LHC, or to leptoquark scenarios.
Highlights
The interpretation of these experimental results as an imprint of heavy new dynamics has primarily been assessed in a model-independent fashion via the language of effective field theories (EFT) in [26,27,28,29,30] and more recently revisited in refs. [16, 17, 31,32,33,34,35]
The paper is organized as follows: in section 2 we review the ingredients of our EFT analysis; in section 3 we detail the strategy adopted for our combined EW+flavour fit in the Standard Model Effective Field Theory (SMEFT), the results from which are collected in section 4; in section 5 we discuss the most economic viable Z model in relation to our EFT results and mention possible alternative leptoquark scenarios
Under the reasonable assumption that the Z boson is mainly produced at tree level in association with the ttpair, in the blue region we show the 95% high-pT constraint stemming from the recasting of the pp → μ−μ+ttsearch at ATLAS [139], while in cyan we report the expected constraint on the model from the 4-tops analysis of CMS [140], 11In what follows, for η ∼ O(v) we will have ξT ∼ O(1); εt ∼ O(v/MT ). 12Note that even for masses as low as μS ∼ O(v), for η v and λS ∼ O(1), the interactions of S do not alter the phenomenology discussed here since the largest S-generated effects are still suppressed as O(ε2t /ξT2 )
Summary
The interpretation of these experimental results as an imprint of heavy new dynamics has primarily been assessed in a model-independent fashion via the language of effective field theories (EFT) in [26,27,28,29,30] and more recently revisited in refs. [16, 17, 31,32,33,34,35]. The NP picture depicted by these global analyses could accommodate a set of tensions related to the well-measured muonic channel of these B decays, in particular, to the angular analysis of B → K∗μ+μ− [36, 37] These measurements have very recently been updated by the LHCb collaboration [38]. It is important to acknowledge that the most up-to-date measurements of RD(∗) from the Belle collaboration — obtained by fully reconstructing the τ particle via the hadronic [83] and, more notably, leptonic [84] decay modes — turns out to be in good agreement with the SM [85,86,87,88] This fact may cast some doubt on the effective role one should really attribute to b → c transitions in the interpretation of the depicted B-physics crisis. The simplest resolution of these anomalies has been proposed in ref. [89], extending the SM with a single new Abelian gauge group, together with the presence of top- and muon-partners, resulting in a top-philic Z boson capable of evading present collider constraints [90] and responsible for the required LUV signatures
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