Higgs couplings in composite models

We study Higgs couplings in the composite Higgs model based on the coset SO(5)/SO(4). We show that the couplings to gluons and photons are insensitive to the elementary-composite mixings and thus not affected by light fermionic resonances. Moreover, at leading order in the mixings the Higgs couplings to tops and gluons, when normalized to the Standard Model (SM), are equal. These properties are shown to be direct consequences of the Goldstone symmetry, of the partial compositeness structure and of the assumption of CP invariance. In particular, they are independent of the details of the elementary-composite couplings, and they are also insensitive to derivative interactions of the Higgs with the composite resonances. We support our conclusions with an explicit construction where the SM fermions are embedded in the 14-dimensional representation of SO(5).

Effective Lagrangian Perspectives on Electroweak Symmetry Breaking

Andrea Thamm

We use an effective Lagrangian approach to address the question of the dynamics of electroweak symmetry breaking in the Standard Model (SM) and its relation to the hierarchy problem. Composite Higgs models provide a solution by describing the recently discovered Higgs-like scalar particle as a composite pseudo Nambu-Goldstone boson that dissolves into its constituents above a certain high energy scale. We discuss many features of the low energy description of composite Higgs models and present an explicit realisation in a flat extra dimension showing explicitly that top partners with masses below 1TeV are expected in a natural theory. Naturalness requires New Physics not much above the weak scale and hence motivates the search for direct and indirect evidence of physics beyond the SM at the LHC and future colliders. As an indirect probe at the LHC, we propose a dedicated analysis of single top production in association with a Higgs boson to lift the degeneracy in the sign of the top Yukawa coupling. We move on to an extensive study of WW scattering, double and triple Higgs production at future linear colliders to estimate their impact on the parameter space of a strongly interacting Higgs boson. Direct probes of New Physics at the LHC include the search for heavy vectors and fermions. We introduce a model-independent strategy to study narrow resonances which we apply to a heavy vector triplet of the SM for illustration. We conclude by summarising current constraints and the expected reach of future colliders on the parameter space of a minimal composite Higgs model. This thesis is based on the papers in Refs. [1–4].

EPFL2014

Composite Higgs as a Viable Scenario for Electro-Weak Symmetry Breaking

Jan Mrazek

This thesis presents a general discussion of the Composite Higgs scenario of Electro-Weak Symmetry Breaking (EWSB). We start by reviewing the Standard Model of Electro-Weak interaction, discussing its experimental tests and conceptual pitfalls. Emphasis is given to the effective field theory point of view. In particular, the inherent tension related to the stability of the Electro-Weak scale motivates us to explore the possibility of having the Higgs field emerging as a Nambu-Goldstone boson from a new strongly coupled sector. Our construction is to a large extent inspired by the picture of the long range dynamics of QCD. The main ingredients are the symmetry of the UV theory, the pattern of its spontaneous breakdown and the sources of explicit breaking. In QCD, the latter are provided by the light quark masses and by the electromagnetic interaction. In Composite Higgs models, the most relevant symmetry breaking couplings are those related to the generation of the third family quark Yukawas through partial compositeness. They generate a potential for the Higgs and thus trigger EWSB. The constraints on the scenario are exposed, with a particular emphasis on the composite Two Higgs Doublet Model (THDM). While a residual SO(4) symmetry is sufficient to ensure a realistic phenomenology in presence of a single composite Higgs doublet, an extended Higgs sector needs more symmetries. For two doublets we show how either CP or a ℤ2 symmetry can play this role and construct a model for each realisation relying on the SO(6)/SO(4) × SO(2) coset. Finally, we discuss the phenomenology of this scenario. In particular, we present de differences between an elementary and a composite THDM. We also conclude that composite fermions associated to the third family quarks seem to be the most promising experimental handles for these models. We discuss their discovery range at the LHC, and the possibility of measuring the structure of their couplings. This knowledge would allow important insight into the strong dynamics.

EPFL2011

A minimally tuned composite Higgs model from an extra dimension

Duccio Pappadopulo, Andrea Thamm, Riccardo Torre

We construct and study the 5D realization of a composite Higgs model with minimal tuning. The Higgs is a (pseudo-)Goldstone boson from the spontaneous breaking of a global SO(5) symmetry to an SO(4) subgroup. The peculiarity of our construction lies in the specific choice of the SO(5) representations of the 5D fermions from which the Standard Model fields arise as chiral zero modes. This choice reduces the tuning of these models to the minimal model-independent value allowed by electroweak precision tests. We analyse the main differences between our 5D construction and other descriptions in terms of purely 4D field theories. 5D models are generally more constrained and show a generic difficulty in accommodating a light Higgs without reintroducing large corrections to the (S) over cap parameter. We propose a specific construction in which this tension can be, even though accidentally, relaxed. We discuss the spectrum of the top partners in the viable regions of parameter space and predict the existence of light exotic quarks, gamma, of charge 8/3 whose striking decay channel gamma -> W(+)W(+)W(+)b can lead to either exclusion or confirmation of the model in the near future.

Springer2013

Composite Higgs as a Viable Scenario for Electro-Weak Symmetry Breaking

Jan Mrazek

EPFL2011