Effective interpretations of a diphoton excess

TL;DR

This paper examines the theoretical consistency of interpreting a diphoton excess with scalar or pseudoscalar states, analyzing how such explanations impact Higgs phenomenology and effective couplings within an EFT framework.

Contribution

It provides a detailed EFT-based analysis of how diphoton excess explanations affect Higgs couplings and mixing, highlighting differences between scalar and pseudoscalar scenarios.

Findings

Scalar states can mix with the Higgs, affecting phenomenology.

Pseudoscalar models are more naturally protected from constraints.

Relations between Higgs data and diphoton signals are established.

Abstract

We discuss some consistency tests that must be passed for a successful explanation of a diphoton excess at larger mass scales, generated by a scalar or pseudoscalar state, possibly of a composite nature, decaying to two photons. Scalar states at mass scales above the electroweak scale decaying significantly into photon final states generically lead to modifications of Standard Model Higgs phenomenology. We characterise this effect using the formalism of Effective Field Theory (EFT) and study the modification of the effective couplings to photons and gluons of the Higgs. The modification of Higgs phenomenology comes about in a variety of ways. For scalar $0^+$states, the Higgs and the heavy boson can mix. Lower energy phenomenology gives a limit on the mixing angle, which gets generated at one loop in any such theory explaining the diphoton excess. Even if the mixing angle is set to zero, we demonstrate that a relation exists between lower energy Higgs data and a massive scaler decaying to diphoton final states. If the new boson is a pseudoscalar, we note that if it is composite, it is generic to have an excited scalar partner that can mix with the Higgs, which has a stronger coupling to photons. In the case of a pseudoscalar, we also characterize how lower energy Higgs phenomenology is directly modified using EFT, even without assuming a scalar partner of the pseudoscalar state. We find that naturalness concerns can be accommodated, and that pseudoscalar models are more protected from lower energy constraints.