A minimally tuned composite Higgs model from an extra dimension

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.

Higgs couplings in composite models

Francesco Riva, Ennio Salvioni, Riccardo Torre

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).

Amer Physical Soc2013

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

Electroweak and Higgs Physics at Very High Energy

Lorenzo Ricci

Effective Field Theories (EFTs) allow a description of low energy effects of heavy new physics Beyond the Standard Model (BSM) in terms of higher dimensional operators among the SM fields. EFTs are not only an elegant and consistent way to describe heavy new physics but they represent, at the same time, a valuable experimental tool for collider searches. The Standard Model Effective Field Theory naturally parametrizes the space of models BSM and measuring its interactions is, nowadays, substantial part of the theoretical and the experimental program at the (HL-)LHC and at future colliders. In this thesis we address the theoretical challenges of this Beyond the Standard Model precision program, following three different paths.Firstly, we present some results towards the so-called high-

p_T

program at the (HL-)LHC, targeting to measure energy growing effects of higher dimensional operators in the tail of kinematic distributions. Concretely, we focus on dilepton production and we study the sensitivity to flavor universal dimension-six operators interfering with the SM and enhanced by the energy. We produce theoretical predictions for the SM and the dim-6 EFT operators at NLO-QCD, including 1-loop EW logs. Our predictions are based on event reweighting of SM Montecarlo simulations and allow an easy scan of the multi-dimensional new physics parameter space on data. Furthermore we asses the impact of the various sources of theoretical uncertainties and we study the projected sensitivity of (HL-)LHC to the EFT interactions under consideration and to concrete BSM scenario.We then turn to future colliders and in particular to very high energy lepton colliders. In this context we study the potential of such machines with about 10 TeV center of mass energy to probe Higgs, ElectroWeak and Top physics at 100 TeV via precise measurements of EFT interactions. A peculiar aspect of so energetic ElectroWeak processes is the prominent phenomenon of the EW radiation. On one hand we find that consistent and sufficiently accurate predictions require resummations, that we perform at double logarithmic order. On the other hand we show how the study of the radiation pattern can enhance the sensitivity to new physics. We assess our results in Composite Higgs and Top scenarios and minimal Z' models.Finally, we move to a top-down perspective and we perform a phenomenological study of composite Higgs models with partially composite Standard Models quarks. Starting from maximally symmetric scenarios that realize minimal flavor violation, we test various assumptions for the flavor structure of the strong sector. Among the different models we consider, we find that there is an optimal amount of symmetries that protects from (chromo-)electric dipoles and reduces, at the same time, constraints from other flavor observables.

EPFL2022

Electroweak and Higgs Physics at Very High Energy

Lorenzo Ricci

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EPFL2022