Singlets in composite Higgs models in light of the LHC 750 GeV diphoton excess

Abstract

Models of compositeness can successfully address the origin of the Higgs boson, as a pseudo Nambu Goldstone boson (pNGB) of a spontaneously broken global symmetry, and flavour physics via the partial compositeness mechanism. If the dynamics is generated by a confining gauge group with fermionic matter content, there exists only a finite set of models that have the correct properties to account for the Higgs and top partners at the same time. In this letter we explore the theory space of this class of models: remarkably, all of them contain - beyond the pNGB Higgs - a pNGB singlet, $a$, which couples to Standard Model gauge bosons via Wess-Zumino-Witten interactions, thus providing naturally a resonance in di-boson at the LHC. With the assumption that the recently reported di-photon excess at 750 GeV at the LHC arises from the a-resonance, we propose a generic approach on how to delineate the best candidate for composite Higgs models with top-partners. We find that constraints from other di-boson searches severely reduce the theory space of the models under consideration. For the models which can explain the di-photon excess, we make precise and testable predictions for the width and other di-boson resonance searches.