Fermionic extensions of the Standard Model in light of the Higgs couplings

As the Higgs boson properties settle, the constraints on the Standard Model extensions tighten. We consider all possible new fermions that can couple to the Higgs, inspecting sets of up to four chiral multiplets. We confront them with direct collider searches, electroweak precision tests, and current knowledge of the Higgs couplings. The focus is on scenarios that may depart from the decoupling limit of very large masses and vanishing mixing, as they offer the best prospects for detection. We identify exotic chiral families that may receive a mass from the Higgs only, still in agreement with the $h\gamma\gamma$ signal strength. A mixing $\theta$ between the Standard Model and non-chiral fermions induces order $\theta^2$ deviations in the Higgs couplings. The mixing can be as large as $\theta\sim 0.5$ in case of custodial protection of the $Z$ couplings or accidental cancellation in the oblique parameters. We also notice some intriguing effects for much smaller values of $\theta$, especially in the lepton sector. Our survey includes a number of unconventional pairs of vector-like and Majorana fermions coupled through the Higgs, that may induce order one corrections to the Higgs radiative couplings. We single out the regions of parameters where $h\gamma\gamma$ and $hgg$ are unaffected, while the $h\gamma Z$ signal strength is significantly modified, turning a few times larger than in the Standard Model in two cases. The second run of the LHC will effectively test most of these scenarios.Comment: v2: a few clarifications and references added, improved treatment of the constraint from Z-b-bbar, Higgs couplings updated by using the combined ATLAS+CMS fi

Common origin of \theta_{13} and \Delta m^2_{12} in a model of neutrino mass with quaternion symmetry

The smallness of the 1-3 lepton mixing angle $\theta_{13}$ and of the neutrino mass-squared-difference ratio $\Delta m^2_{12}/\Delta m^2_{23}$ can be understood as the departure from a common limit where they both vanish. We discuss in general the conditions for realizing the mass degeneracy of a pair of neutrinos and show that the vanishing of a CP violating phase is needed. We find that the discrete quaternion group Q of eight elements is the simplest family symmetry which correlates the smallness of $\Delta m^2_{12}$ to the value of $\theta_{13}$. In such a model we predict $0.12\lesssim \sin\theta_{13} \lesssim 0.2$ if the ordering of the neutrino mass spectrum is normal, and $\sin\theta_{13}\lesssim 0.12$ if it is inverted.Comment: revtex, 7 pages, 6 pdf figures; Appendix added discussing in detail the alignment of VEV

Dynamical Clockwork Axions

The clockwork mechanism is a novel method for generating a large separation between the dynamical scale and interaction scale of a theory. We demonstrate how the mechanism can arise from a sequence of strongly-coupled sectors. This framework avoids elementary scalar fields as well as ad hoc continuous global symmetries, both of which are subject to serious stability issues. The clockwork factor, $q$, is determined by the consistency of the strong dynamics. The preserved global $U(1)$ of the clockwork appears as an accidental symmetry, resulting from discrete or $U(1)$ gauge symmetries, and it is spontaneously broken by the chiral condensates. We apply such a dynamical clockwork to construct models with an effectively invisible QCD axion from TeV-scale strong dynamics. The axion couplings are determined by the localisation of the Standard Model interactions along the clockwork sequence. The TeV spectrum includes either coloured hadrons or vector-like quarks. Dark matter can be accounted for by the axion or the lightest neutral baryons, which are accidentally stable.Comment: 41 page

Fermion masses and mixing in models with SO(10) x A_4 symmetry

We study the flavour sector in models where the three families of matter are unified in a $(16,3)$ representation of the $SO(10)\times A_4$ group. The necessary ingredients to realize tri-bi-maximal mixing in the lepton sector are identified systematically. The non-renormalizable operators contributing to the fermion mass matrices play an important role. We also present a mechanism to explain the inter-family mass hierarchy of quarks and charged leptons, which relies on a `universal seesaw' mechanism and is compatible with tri-bi-maximal mixing.Comment: 24 pages, revte

Flavour violation in supersymmetric SO(10) unification with a type II seesaw mechanism

We study flavour violation in a supersymmetric SO(10) implementation of the type II seesaw mechanism, which provides a predictive realization of triplet leptogenesis. The experimental upper bounds on lepton flavour violating processes have a significant impact on the leptogenesis dynamics, in particular they exclude the strong washout regime. Requiring successful leptogenesis then constrains the otherwise largely unknown overall size of flavour-violating observables, thus yielding testable predictions. In particular, the branching ratio for mu -> e gamma lies within the reach of the MEG experiment if the superpartner spectrum is accessible at the LHC, and the supersymmetric contribution to epsilon_K can account for a significant part of the experimental value. We show that this scenario can be realized in a consistent SO(10) model achieving gauge symmetry breaking and doublet-triplet splitting in agreement with the proton decay bounds, improving on the MSSM prediction for alpha_3(m_Z), and reproducing the measured quark and lepton masses.Comment: 40 pages, 10 figures. Accepted for publication in JHE