Composite Higgs models in disguise

We present a mechanism for disguising one composite Higgs model as another. Allowing the global symmetry of the strong sector to be broken by large mixings with elementary fields, we show that we can disguise one coset $\mathcal G/\mathcal H$ such that at low energies the phenomenology of the model is better described with a different coset $\mathcal G'/\mathcal H'$. Extra scalar fields acquire masses comparable to the rest of the strong sector resonances and therefore are no longer considered pNGBs. Following this procedure we demonstrate that two models with promising UV-completions can be disguised as the more minimal $SO(5)/SO(4)$ coset

On the impact of dimension-eight SMEFT operators on Higgs measurements

Using the production of a Higgs boson in association with a $W$ boson as a test case, we assess the impact of dimension-8 operators within the context of the Standard Model Effective Field Theory. Dimension-8--SM-interference and dimension-6-squared terms appear at the same order in an expansion in $1/\Lambda$, hence dimension-8 effects can be treated as a systematic uncertainty on the new physics inferred from analyses using dimension-6 operators alone. To study the phenomenological consequences of dimension-8 operators, one must first determine the complete set of operators that can contribute to a given process. We accomplish this through a combination of Hilbert series methods, which yield the number of invariants and their field content, and a step-by-step recipe to convert the Hilbert series output into a phenomenologically useful format. The recipe we provide is general and applies to any other process within the dimension $\le 8$ Standard Model Effective Theory. We quantify the effects of dimension-8 by turning on one dimension-6 operator at a time and setting all dimension-8 operator coefficients to the same magnitude. Under this procedure and given the current accuracy on $\sigma(pp \to h\,W^+)$, we find the effect of dimension-8 operators on the inferred new physics scale to be small, $\mathcal O(\text{few}\,\%)$, with some variation depending on the relative signs of the dimension-8 coefficients and on which dimension-6 operator is considered. The impact of the dimension-8 terms grows as $\sigma(pp \to h\,W^+)$ is measured more accurately or (more significantly) in high-mass kinematic regions. We provide a FeynRules implementation of our operator set to be used for further more detailed analyses.Comment: More operator coefficient choices explored, bugs in FeynRules implementation correcte

Composite Higgs models after Run 2

We assess the status of models in which the Higgs is a composite pseudo-Nambu Goldstone boson, in the light of the latest 13 TeV Run 2 Higgs data. Drawing from the extensive Composite Higgs literature, we collect together predictions for the modified couplings of the Higgs, in particular examining the different predictions for κ V and κ F . Despite the variety and increasing complexity of models on the market, we point out that many independent models make identical predictions for these couplings. We then look into further corrections induced by tree-level effects such as mass-mixing and singlet VEVs. We then investigate the compatibility of different models with the data, combining the Run 1 and recent Run 2 LHC data. We obtain a robust limit on the scale f of 600 GeV, with stronger limits for different choices of fermion embeddings. We also discuss how a deficit in a Higgs channel could pinpoint the type of Composite Higgs model responsible for it

Composite Higgses with seesaw EWSB

We introduce a new class of Composite Higgs models in which electroweak symmetry is broken by a seesaw-like mechanism. If a global symmetry is broken sequentially at different scales, two sets of pseudo-Goldstone bosons will arise, one set being typically heavier than the other. If two Composite Higgs doublets mix, then the mass-squared of the lighter state can be driven negative, and induce EWSB. We illustrate with the example SO(6) → SO(5) → SO(4), and derive an estimate of the light Higgs potential. We find that the introduction of an extra scale can ease many of the tensions present in conventional Composite Higgs models, especially those related to fine-tuning. In particular we find that we can significantly raise the upper bound on the mass of the elusive top partners

See-saw composite Higgs model at the LHC: Linking naturalness to the 750 GeV diphoton resonance

We explore the possibility of explaining the recent ∼750 GeV excesses observed by ATLAS and CMS in the γγ spectrum in the context of a compelling theory of naturalness. The potential spin-zero resonance responsible for the excesses also requires the existence of new heavy charged states. We show that both such features are naturally realized in a see-saw composite Higgs model for electroweak symmetry breaking, where the new pseudo-Goldstone bosons are expected to be comparatively heavier than the Standard Model Higgs, and the new fermions have masses in the TeV range. If confirmed, the existence of this new resonance could be the first stone in the construction of a new theory of naturalness

Strongly coupled physics beyond the standard model

This thesis is concerned with strongly coupled extensions to the Standard Model. The majority of the thesis is dedicated to the study of Composite Higgs models, which are a proposed solution to the hierarchy problem of the electroweak scale. In these models the Higgs is a composite pseudo-Nambu Goldstone boson which forms a part of a new strongly interacting sector. There are many different variations on the basic Composite Higgs theme { the current status of some of these variations is assessed in light of results from the Large Hadron Collider. A new kind of Composite Higgs model is presented and studied, which features an alternative mechanism for the breaking of electroweak symmetry. A mechanism for deforming one model into another is also discussed, which might find application to the UV completion of Composite Higgs models. The formalism used in the Composite Higgs literature is also applied to the study of inflation, where the inflaton is assumed to be a pseudo-Nambu Goldstone boson arising from strongly coupled dynamics. A study of the inflaton potential is performed and its cosmological implications discussed. A different extension to the Standard Model with interesting phenomenological consequences is also studied. Quirks are strongly interacting particles whose masses are significantly higher than their confining scale. If produced in colliders, they leave unusual tracks which current searches are mostly blind to. A new search strategy for these hypothetical particles is proposed