Abstract
In scenarios of strongly coupled electroweak symmetry breaking, heavy composite particles of different spin and parity may arise and cause observable effects on signals that appear at loop levels. The recently observed process of Higgs to $\gamma \gamma$ at the LHC is one of such signals. We study the new constraints that are imposed on composite models from $H\to \gamma\gamma$, together with the existing constraints from the high precision electroweak tests. We use an effective chiral Lagrangian to describe the effective theory that contains the Standard Model spectrum and the extra composites below the electroweak scale. Considering the effective theory cutoff at $\Lambda = 4\pi v \sim 3 $ TeV, consistency with the $T$ and $S$ parameters and the newly observed $H\to \gamma\gamma$ can be found for a rather restricted range of masses of vector and axial-vector composites from $1.5$ TeV to $1.7$ TeV and $1.8$ TeV to $1.9$ TeV, respectively, and only provided a non-standard kinetic mixing between the $W^{3}$ and $B^{0}$ fields is included.
Highlights
One of the possible signals of composite Higgs boson models is the deviation of the h → γ γ channel from the Standard Model (SM) prediction, as it is a loop process sensitive to heavier virtual states
The recent h → γ γ signal reported by ATLAS and CMS collaborations [2–5], which is very close to the SM prediction, implies an additional constraint on composite models
It is important to explore the consequences of this new constraint on composite models, in conjunction with those previously known from electroweak precision measurements
Summary
One of the possible signals of composite Higgs boson models is the deviation of the h → γ γ channel from the Standard Model (SM) prediction, as it is a loop process sensitive to heavier virtual states. We will identify the lightest scalar, h, with the state of mass mh = 126 GeV discovered at the LHC All of these composite resonances are assumed to be lighter than the cutoff 4π v, so that they explicitly appear as fields in the effective chiral Lagrangian. Composite states of spin 2 and higher are assumed to be heavier than the cutoff, and so they are disregarded in this work These composite particles are important signatures of the strongly coupled scenarios of EWSB and they could manifest themselves either by direct production or as virtual states in loop corrections. The rate h → γ γ is important in our study as it is a one-loop process which is sensitive to the existence of extra vector and axial-vector particles In this sense, we are studying whether composite models are viable alternatives to electroweak symmetry breaking, given the current experimental success of the Standard Model [95].
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