The decay constant of the holographic techni-dilaton and the 125 GeV boson

We critically discuss the possibility that the 125 GeV boson recently discovered at the LHC is the holographic techni-dilaton. a composite state emerging from a strongly-coupled model of electroweak symmetry breaking. This composite state differs from the SM for three main reasons. Its decay constant is in general larger than the electroweak scale, hence suppressing all the couplings to standard-model particles with respect to an elementary Higgs boson, with the exception of the coupling to photons and gluons, which is expected to be larger than the standard-model equivalent. We discuss three classes of questions. Is it possible to lower the decay constant, by changing the geometry of the holographic model? Is it possible to lower the overall scale of the strong dynamics, by modifying the way in which electroweak symmetry breaking is implemented in the holographic model? Is there a clear indication in the data that production mechanisms other than gluon-gluon fusion have been observed, disfavoring models in which the holographic techni-dilaton has a large decay constant? We show that all of these questions are still open, given the present status of theoretical as well as phenomenological studies, and that at present the techni-dilaton hypothesis yields a fit to the data which is either as good as the elementary Higgs hypothesis. or marginally better, depending on what sets of data are used in the fit. We identify clear strategies for future work aimed at addressing these three classes of open questions. In the process. we also compute the complete scalar spectrum of the two-scalar truncation describing the GPPZ model, as well as the decay constant of the holographic techni-dilaton in this model. (C) 2012 Elsevier B.V. All rights reserved.