Holographic theories of electroweak symmetry breaking without a Higgs boson

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Abstract
Recently, realistic theories of electroweak symmetry breaking have been constructed in which the electroweak symmetry is broken by boundary conditions imposed at a boundary of higher dimensional spacetime. These theories have equivalent 4D dual descriptions, in which the electroweak symmetry is dynamically broken by nontrivial infrared dynamics of some gauge interaction, whose gauge coupling g̃ and size N satisfy g2N≳16π2. Such theories allow one to calculate electroweak radiative corrections, including the oblique parameters S, T and U, as long as g2N/16π2 and N are sufficiently larger than unity. We study how the duality between the 4D and 5D theories manifests itself in the computation of various physical quantities. In particular, we calculate the electroweak oblique parameters in a warped 5D theory where the electroweak symmetry is broken by boundary conditions at the infrared brane. We show that the value of S obtained in the minimal theory exceeds the experimental bound if the theory is in a weakly coupled regime. This requires either an extension of the minimal model or departure from weak coupling. A particularly interesting scenario is obtained if the gauge couplings in the 5D theory take the largest possible values—the value suggested by naive dimensional analysis. We argue that such a theory can provide a potentially consistent picture for dynamical electroweak symmetry breaking: corrections to the electroweak observables are sufficiently small while realistic fermion masses are obtained without conflicting with bounds from flavor violation. The theory contains only the standard model quarks, leptons and gauge bosons below ≃2TeV, except for a possible light scalar associated with the radius of the extra dimension. At ≃2TeV increasingly broad string resonances appear. An analysis of top-quark phenomenology and flavor violation is also presented, which is applicable to both the weakly coupled and strongly coupled cases.

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