One of the most challenging hurdles to the construction of realistic composite Higgs models is the generation of Yukawa couplings for the Standard Model fermions. This problem can be successfully addressed in approximate conformal theories that admit
a marginally relevant mixing between composite fermionic operators and the SM fermions. I argue that SU(3) gauge theories with light Dirac flavors in the fundamental representation feature all the ingredients under theoretical control, including a strongly-coupled IR fixed point, composite partners for all Standard Model fermions, absence of Landau poles at low energy, and a realistic phenomenology. These models acquire the status of compelling UV-completions of the SM if some spin-1/2 baryon operator has scaling dimension close to 2.5 within the conformal window, a possibility that can only be assessed via non-perturbative methods like lattice QCD. A distinctive collider signature is long-lived hadrons with fractional charges. Vacuum alignment is controlled by the Nambu-Goldstone bosons of the coset SU(4)xSU(4)/SU(4). With a technically natural choice of mixing for the top-quark, the exotic scalars with electro-weak charges acquire large positive masses and a compelling custodial-symmetric phenomenology is obtained. In the decoupling limit the symmetry breaking pattern effectively reduces to SU(4)->Sp(4) with a light Higgs.
We derive sufficient conditions that guarantee a robust solution of the strong CP problem in theories with spontaneous CP violation, and introduce a class of models satisfying these requirements. In the simplest scenarios the dominant contribution to
the topological angle arises at 3-loop order in the Yukawa couplings. A variety of realizations are possible on a warped extra dimension, which can simultaneously address the Planck-TeV hierarchy. Experimental signatures of this approach to the strong CP problem include flavor violation and vector-like partners of the top or bottom quarks.
The WIMP miracle suggests a new physics threshold ranging from the weak scale up to several tens of TeVs. Obtaining the correct dark matter density in many theories aiming to solve the hierarchy problem may thus require some amount of tuning of the w
eak scale, hinting at a possible connection between WIMP dark matter and unnaturalness. We point out that dark matter direct detection is a very efficient probe of these unnatural models, and that existing data already provide important clues to the nature of the associated WIMPs. We present a model-independent, relativistic analysis of the signatures of a gauge-singlet dark matter candidate of arbitrary spin, and discuss the current experimental bounds from LUX and XENON100. For complex WIMPs, dark matter direct detection is complementary to electroweak precision tests, and can even compete with flavor constraints if the dark matter has spin. Particularly relevant for future searches are couplings to the Higgs mass operator, which are expected to be large if the electroweak scale is finely tuned. Care is devoted to the RG evolution of the effective Lagrangian. We find that the CP-even scalar coupling to charm quarks is enhanced by about 20% compared to the one-loop estimate. When pushed in the unnatural regime, warped extra dimensions -- with or without custodial symmetry -- become attractive theories for flavor, the Higgs mass, and dark matter. The WIMP argument basically sets an upper bound on unnaturalness, whereas direct detection experiments select scalar or real particles as the most compelling dark matter candidates.
Generic extensions of the Standard Model that respect baryon and lepton numbers have accidentally stable particles. Typical examples are the lightest exotic neutral fermion, or neutralino, and fields with non-trivial lepton and baryon charges. In thi
s paper we show that an accidentally stable neutralino is a natural dark matter candidate in models with warped extra dimensions. We find that annihilation into other Kaluza-Klein resonances is often allowed and very efficient. The observed dark matter abundance may then be obtained with couplings of order unity and a compactification scale above the TeV. Light dark matter is also possible in the presence of unsuppressed couplings to the Higgs boson. In this latter case direct detection experiments will soon be able to probe a significant portion of the parameter space.
The discovery of the Higgs boson has put considerable pressure on theories that aim to solve the hierarchy problem. Scenarios in which the Higgs is a pseudo Nambu-Goldstone boson (NGB) of some new strong dynamics must possess a number of non-generic
features in order to pass the progressively stringent collider bounds and simultaneously meet our naturalness criteria. Among these features are the existence of light fermionic partners of the top quark and an efficient collective breaking of the Nambu-Goldstone symmetry. The top partners have to be not only parametrically lighter than the other composites, but also weakly coupled to them in order to suppress unwanted flavor-violating effects. A Natural pseudo-NGB Higgs model should also be able to fit the LHC Higgs data without fine-tuning. Among theories with comparable compositeness scales, those that predict smaller corrections in the Higgs couplings to the standard model particles are therefore preferred. A concrete implementation of these ingredients is discussed in a scenario based on the coset SU(5)/SO(5). The fit to the current LHC Higgs data is significantly improved compared to the minimal scenarios, and a fully natural explanation of both the weak scale and the Higgs boson mass can be attained. An important role is played by an independent quartic Higgs coupling generated by UV-sensitive loops involving electroweak doublets mixing with the top partners. The collider signature of this framework is shown to be rather model-dependent; in particular, the exotic scalars can alter the phenomenology of the top partners at a qualitative level.
We discuss flavor violation in large N Composite Higgs models. We focus on scenarios in which the masses of the standard model fermions are controlled by hierarchical mixing parameters, as in models of Partial Compositeness. We argue that a separatio
n of scales between flavor and Higgs dynamics can be employed to parametrically suppress dipole and penguin operators, and thus effectively remove the experimental constraints arising from the lepton sector and the neutron EDM. The dominant source of flavor violation beyond the standard model is therefore controlled by 4-fermion operators, whose Wilson coefficients can be made compatible with data provided the Higgs dynamics approaches a walking regime in the IR. Models consistent with all flavor and electroweak data can be obtained with a new physics scale within the reach of the LHC. Explicit scenarios may be realized in a 5D framework, the new key ingredient being the introduction of flavor branes where the wave functions of the bulk fermions end.
The new physics required to explain the anomalies recently reported by the D0 and CDF collaborations, namely the top forward-backward asymmetry (FBA), the like-sign dimuon charge asymmetry in semileptonic b decay, and the CDF dijet excess, has to fea
ture an amount of flavor symmetry in order to satisfy the severe constrains arising from flavor violation. In this paper we show that, once baryon number conservation is imposed, color & weak triplet scalars with hypercharge $Y=1/3$ can feature the required flavor structure as a consequence of standard model gauge invariance. The color & weak triplet model can simultaneously explain the top FBA and the dimuon charge asymmetry or the dimuon charge asymmetry and the CDF dijet excess. However, the CDF dijet excess appears to be incompatible with the top FBA in the minimal framework. Our model for the dimuon asymmetry predicts the observed pattern $h_dll h_s$ in the region of parameter space required to explain the top FBA, whereas our model for the CDF dijet anomaly is characterized by the absence of beyond the SM b-quark jets in the excess region. Compatibility of the color & weak triplet with the electroweak constraints is also discussed. We show that a Higgs boson mass exceeding the LEP bound is typically favored in this scenario, and that both Higgs production and decay can be significantly altered by the triplet. The most promising collider signature is found if the splitting among the components of the triplet is of weak scale magnitude.
