Light composite Higgs boson from higher representations versus electroweak precision measurements: Predictions for CERN LHC

Abstract

We investigate theories in which the technifermions in higher dimensional representations of the technicolor gauge group dynamically break the electroweak symmetry of the standard model. Somewhat surprisingly, for the two-index symmetric representation of the gauge group the lowest number of techniflavors needed to render the underlying gauge theory quasiconformal is two. This is exactly one doublet of technifermions with respect to the weak interactions promoting these theories to ideal candidates of walking type technicolor models. From the point of view of the weak interactions the two techniflavor theory has a Witten anomaly, which we cure by introducing a fourth family of leptons. An elegant feature of this model is that the techniquarks resemble an extra family of quarks arranged in the two-index symmetric representation of the SU(2) technicolor theory. We have also studied the theory with three technicolors and two techniflavors in the two-index symmetric representation of the gauge group, which does not require a fourth family of leptons. We confront the models with the recent electroweak precision measurements and demonstrate that the two technicolor theory is a valid candidate for electroweak symmetry breaking via new strong interactions. We investigate different hypercharge assignments for the two color theory and associated fourth family type of leptons. We find that the two technicolor theory is within the 90% confidence level contours defined by the oblique parameters. The electroweak precision measurements provide useful constraints on the relative mass splitting of the new leptons. In the case of a fourth family of leptons with ordinary lepton hypercharge the new heavy neutrino can be a natural candidate of cold dark matter. The mass of this heavy neutrino, constrained by the precision measurements, is slightly larger than the one of the neutral electroweak gauge boson while the charged lepton has a mass roughly twice as large as the associated neutrino. Extensions with a larger value of the hypercharge naturally featuring doubly charged leptons are also favored by precision measurements. We also propose theories in which the critical number of flavors needed to enter the conformal window is higher than the one with fermions in the two-index symmetric representation, but lower than in the walking technicolor theories with fermions only in the fundamental representation of the gauge group. The simplest theories are the ones in which we add a (techni)gluino to the theory, while the rest of the matter fermions remain in the fundamental representation of the gauge group. Although these theories share some features with split supersymmetric theories, they are introduced here to address the hierarchy problem. Because of the near-conformal/chiral phase transition, we show that the composite Higgs is very light compared to the intrinsic scale of the technicolor theory. For the two technicolor theory we predict the composite Higgs mass not to exceed 150 GeV. We also provide estimates for the Higgs mass in walking technicolor theories with fermions in the fundamental representation and show that in this case the mass is around 400 GeV.