Holographic walking technicolor from D-branes

We investigate a model of dynamical electroweak symmetry breaking via a dual gravitational description. The gravity dual is obtained by embedding aD7-(D7) over bar pair of branes into a type IIB background that is dual to a walking gauge theory. We develop further a previous study of this model. In particular, we show that there is a nontrivial relation that needs to be satisfied in order for axial-vector modes to exist. Furthermore, we compute explicitly the electroweak S parameter. The result is positive-definite and, as was to be expected, much smaller than in earlier QCD-like D-brane constructions. We also find the masses and decay constants of the vector and axial-vector mesons in this model. This allows us to obtain another estimate for S by summing the contributions of the discrete states. It is noteworthy that, in contrast to previous holographic studies, the sum of the first several lowest-lying states does give a very good approximation to the full answer. (C) 2011 Elsevier B.V. All rights reserved.