Lepton flavor non-universality in B-meson decays from a U(2) flavor model

JHE from a 0 F-75005 , Paris , France 1 Wilberforce Road , Cambridge, CB3 0WA, U.K 2 CNRS , UMR 7589, LPTHE 3 91405 Orsay , France 4 DAMTP, University of Cambridge 5 Sorbonne Universites, UPMC Univ. Paris 06, UMR 7589, LPTHE 6 Laboratoire de Physique Theorique , Bat. 210 , Universite Paris-Sud We address the recent anomalies in semi-leptonic B-meson decays using a model of fermion masses based on the U(2) avor symmetry. The new contributions to b ! s`` transitions arise due to a tree-level exchange of a Z0 vector boson gauging a U(1) subgroup of the avor symmetry. They are controlled by a single parameter and are approximately aligned to the Standard Model prediction, with constructive interference in the e-channel and destructive interference in the -channel. The current experimental data on semi-leptonic B-meson decays can be very well reproduced without violating existing constraints from avor violation in the quark and lepton sectors. Our model will be tested by new measurements of b ! s`` transitions and also by future electroweak precision tests, direct Z0 searches, and -e conversion in nuclei. Rare Decays; Beyond Standard Model Lepton avor non-universality in B-meson decays U(2) avor model 1 Introduction 2 Phenomenology of b ! s`` transitions Constraints from avor violation and direct searches Constraints from F = 2 observables Semileptonic decays in b ! d and s ! d transitions b ! s`` transitions have produced signi cant deviations from the SM predictions. The as RK = 0:745+00::009872 [2] (deviating from the SM prediction by 2:6 ), as in this case the SM ! K + to B ! K e+e branching ratios measured 3 anomaly in B ! K + most notable is the ratio of B prediction RK fraction of Bs ! 1 can be calculated with a very good accuracy. The measured branching + [3] is also low compared to the SM prediction. These anomalies could well be the result of statistical uctuations, experimental problems, underestimated hadronic uncertainties, or a combination of all three. Nevertheless, it is intriguing that many of the discrepancies can be simultaneously explained by assuming new physics contributions to the Wilson coe cients of 4-fermion operators with a band s-quark and 2 leptons [4{15]. Such contributions can be easily generated in explicit models through the exchange of a new Z0 gauge boson [16{26] or leptoquarks [11, 27{34]. { 1 { In order to address the RK anomaly, lepton avor universality has to be broken, which generically also implies lepton avor violation (LFV) [30, 35{38]. It is tempting to connect these patterns of avor violation to the SM avor puzzle, i.e. the experimentally observed hierarchical structure of Yukawa couplings. Some papers in the literature have attempted to obtain this connection in the context of partial compositeness [19, 25, 28] or leptoquark models [30]. Other works [18, 22] considered gauged abelian symmetries that are able to reproduce some features of the CKM matrix (but not the SM quark mass hierarchies). However, up to now, no model has been proposed that directly connects the anomalies to the generation of fermion masses and mixings. The purpose of this work is to provide a predictive model of this kind. We address the anomalies in b ! s`` transitions in the context of a light Z0 vector boson, whose couplings to fermions are governed by an underlying U(2)F symmetry that explains fermion masses and mixings. The original U(2) models proposed in the context of supersymmetry [39, 40] have been disfavored by precision measurements in the B-factories [41], as they predicted the relation Vub=Vcb = pmu=mc which was not borne out experimentally. However, it is not di cult to modify this prediction with a more general U(1)F charge assignment, as demonstrated in ref. [42]. The model that we are presenting here is essentially a non-supersymmetric version of the one in ref. [42], in which the dominant source of deviations from the SM is due to the tree-level exchange of the Z0 gauge boson associated to the U(1)F avor group. Similarly to the supersymmetric model, the couplings of the Z0 are approximately U(2)F symmetric, and avor violating e ects in the quark sector are suppressed by the small CKM mixing angles involving the 3rd generation. In contrast to ref. [42], we do not demand that the U(1)F charges are compatible with SU(5) grand uni cation. This generalization gives us more freedom in the charged lepton sector to address the observed anomalies in b ! s`` transitions. Once this is achieved, the parametric freedom in the model is to a large extent xed by matching to the observed quark and charged lepton masses and quark mixing angles. In our scenario, the deviations in b ! s`` observables arise from a simple pattern of Z0 contributions to the 4 relevant Wilson coe cients C9e;e1;0 . Namely, the new contributions are aligned with the SM one (i.e. approximately left-handed) and controlled by a single parameter (the ratio of the U(1)F gauge coupling and the Z0 mass mu (...truncated)