Phenomenology of a light scalar: the dilaton
We make use of the language of nonlinear realizations to analyze electroweak symmetry breaking scenarios in which a light dilaton emerges from the breaking of a nearly conformal strong dynamics and compare the phenomenology of the dilaton to that of the well motivated light composite Higgs scenario. We argue that in addition to departures in the decay/production rates into massless gauge bosons mediated by the conformal anomaly characterizing features of the light dilaton scenario are enhancements in off shell events at high invariant mass involving two longitudinally polarized vector bosons and a dilaton, and tree level flavor violating processes. Accommodating both electroweak precision measurements and flavor constraints appears especially challenging in the ambiguous scenario in which the Higgs and the dilaton fields strongly mix. We show that warped higgsless models of electroweak symmetry breaking are explicit and tractable realizations of this limiting case. The relation between the naive radion profile often adopted in the study of holographic realizations of the light dilaton scenario and the actual dynamical dilaton field is clarified in the Appendix.