Light vector-like quarks with non-renormalizable couplings to the Higgs are a common feature of models trying to address the electroweak (EW) hierarchy problem by treating the Higgs as a pseudogoldstone boson of a global (approximate) symmetry. We systematically investigate the implications of the leading dimension five operators on Higgs phenomenology in presence of dynamical upand down-type weak singlet as well as weak doublet vector-like quarks. After taking into account constraints from precision EW and flavour observables we show that contrary to the renormalizable models, significant modifications of Higgs properties are still possible and could shed light on the role of vector-like quarks in solutions to the EW hierarchy problem. We also briefly discuss implications of higher dimensional operators for direct vector-like quark searches at the LHC. I. INTRODUCTIONThe recent LHC discovery of a Higgs boson [1] seems to experimentally complete the standard model (SM) picture of fundamental interactions. Although the SM works extremely well phenomenologically, the electro-weak (EW) hierarchy problem, as exemplified by the extreme UV sensitivity of the Higgs potential, provides a strong theoretical motivation for contemplating beyond SM physics at the TeV scale.On the other hand, direct searches for on-shell production of new degrees of freedom at the LHC together with ever more stringent constraints from measurements of flavor, EW and Higgs observables are starting to directly probe models addressing the naturalness problem of the SM. At present these experimental null-results are not yet conclusive, but viable new physics (NP) models' parameter space is becoming significantly reduced. In light of this, it has become crucial to focus on the minimal light particle content required to fulfil the naturalness conditions (see [2] for examples in supersymmetric theories).Vector-like quarks are expected to be the lightest new degrees of freedom in models addressing the EW hierarchy problem by treating the light Higgs as a pseudo-goldstone boson of a global symmetry, broken explicitly by the SM gauging and Yukawa couplings [3,4]. In such models, the dominant quadratic divergences in one-loop corrections to the Higgs boson mass coming from the top quark loop are canceled by contributions of dimension five operators of the form H † HQQ, where H is the Higgs doublet and Q a vector-like quark weak multiplet. In general, parametrizing single-and double-Higgs interactions of an arbitrary number of quark flavors f in the mass eigenbasis * Electronic address:svjetlana.fajfer@ijs.si † Electronic address:admir.greljo@ijs.si ‡ Electronic address:jernej.kamenik@ijs.si § Electronic address:ivana.mustac@ijs.si arXiv:1304.4219v2 [hep-ph] 23 Oct 20131 For a recent analysis of dimension six operator effects in composite Higgs scenarios without dynamical vector-like fermions see [6].2 For recent related studies in the context of explicit composite Higgs model realisations see [7].

The Standard Model fermion sector is enlarged by either one light singlet vector-like downtype quark or one light vector-like lepton doublet, which might be accommodated within a fivedimensional representation of SU (5). At low energies the inclusion of these states affects precisely measured observables in flavor physics, as well as electroweak precision measurements. These experimental results strongly constrain couplings of vector-like states to the Standard Model particles. Having these bounds, we investigate the impact of vector-like fermions on the mass matrices for down-type quarks and charged leptons in an SU (5) setting. We find that unitary transformations relating an arbitrary flavor basis to the mass eigenstate basis depend only on three free parameters. Then we discuss the parameter space constrained by low-energy data assuming vector-like quark and vector-like lepton masses to be 800 GeV and 400 GeV, respectively. We demonstrate that these two scenarios generate unique patterns for relevant proton decay widths. A further improvement of experimental bounds on proton decay modes would thus differentiate the allowed parameter space. We finally present two full-fledged SU (5) models that allow for gauge coupling unification with light vector-like fermions under consideration and discuss their viability. * Electronic address:ilja.dorsner@ijs.si † Electronic address:svjetlana.fajfer@ijs.si ‡ Electronic address:ivana.mustac@ijs.si