Centre d'études et d'expertise sur les risques, l'environnement, la mobilité et l'aménagement

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A complete discussion of tree-level flavor-changing effects in the Randall-Sundrum (RS) model with brane-localized Higgs sector and bulk gauge and matter fields is presented. The bulk equations of motion for the gauge and fermion fields, supplemented by boundary conditions taking into account the couplings to the Higgs sector, are solved exactly. For gauge fields the Kaluza-Klein (KK) decomposition is performed in a covariant R ξ gauge. For fermions the mixing between different generations is included in a completely general way. The hierarchies observed in the fermion spectrum and the quark mixing matrix are explained naturally in terms of anarchic five-dimensional Yukawa matrices and wave-function overlap integrals. Detailed studies of the flavor-changing couplings of the Higgs boson and of gauge bosons and their KK excitations are performed, including in particular the couplings of the standard W ± and Z 0 bosons. A careful analysis of electroweak precision observables including the S and T parameters and the Z 0 bb couplings shows that the simplest RS model containing only Standard Model particles and their KK excitations is consistent with all experimental bounds for a KK scale as low as a few TeV, if one allows for a heavy Higgs boson (m h 1 TeV) and/or for an ultra-violet cutoff below the Planck scale. The study of flavor-changing effects includes analyses of the non-unitarity of the quark mixing matrix, anomalous righthanded couplings of the W ± bosons, tree-level flavor-changing neutral current couplings of the Z 0 and Higgs bosons, the rare decays t → c(u)Z 0 and t → c(u)h, and the flavor mixing among KK fermions. The results obtained in this work form the basis for general calculations of flavor-changing processes in the RS model and its extensions. arXiv:0807.4937v3 [hep-ph] 20 Oct 2008remaining issue of excessive contributions to the T parameter can be cured, e.g., by extending the bulk hypercharge group to SU (2) R × U (1) X and breaking it to U (1) Y on the UV brane [33]. An embedding of the SM fermions into the custodially symmetric SU (2) L × SU (2) R model, under which the left-handed bottom quark is symmetric under the exchange of SU (2) L and SU (2) R , allows one to protect the left-handed Z 0 bb coupling from vast corrections [34].Delocalized fermions have the further virtue of admitting a natural explanation of the flavor structure of the SM by harnessing the idea of split fermions [35]. In fact, it is perhaps not an overstatement to say that the RS scenario offers the best theory of flavor we have to date. Starting from anarchic 5D Yukawa couplings, the large mass hierarchies of the SM fermions can be generated without flavor symmetries by localizing the SM fermions at different points in the fifth dimension [21,26,36,37]. Given the large hierarchy of quark masses in the SM, small mixing angles in the Cabibbo-Kobayashi-Maskawa (CKM) matrix are a natural consequence of this scenario [37]. This way of generating fermion mass hierarchies also implies a certain amount of suppression of dangero...

We show that solving the flavor problem of the Standard Model with a simple Uð1Þ H flavor symmetry naturally leads to an axion that solves the strong CP problem and constitutes a viable Dark Matter candidate. In this framework, the ratio of the axion mass and its coupling to photons is related to the SM fermion masses and predicted within a small range, as a direct result of the observed hierarchies in quark and charged lepton masses. The same hierarchies determine the axion couplings to fermions, making the framework very predictive and experimentally testable by future axion and precision flavor experiments.