Phenomenology of a light scalar: the dilaton

TL;DR

This paper develops a non-linear EFT framework to compare a light dilaton from near-conformal strong dynamics with a light composite Higgs (SILH). It identifies distinctive high-energy and flavor signatures, including energy-enhanced off-shell vector-boson scattering and potential tree-level FCNCs, that can help discriminate the two scenarios, especially when Higgs-dilaton mixing is significant. The analysis shows that warped higgsless models naturally realize the limiting case of maximal mixing and discusses the conditions under which the dilaton behaves like the Higgs. Overall, precision Higgs coupling measurements and high-energy collider probes are essential to determine the true nature of any observed light CP-even scalar.

Abstract

We make use of the language of non-linear realizations to analyze electro-weak 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 (as well as other scenarios admitting a light CP-even scalar not directly related to the breaking of the electro-weak symmetry) are off-shell events at high invariant mass involving two longitudinally polarized vector bosons and a dilaton, and tree-level flavor violating processes. Accommodating both electro-weak 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 electro-weak 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.