Tricking Landau-Yang: How to obtain the diphoton excess from a vector resonance

TL;DR

This paper proposes a novel explanation for the observed diphoton excess using a vector resonance decaying into a photon and a light scalar, which then decays into two photons, creating an apparent diphoton signal.

Contribution

It introduces a new mechanism involving a vector resonance and a light scalar to explain the diphoton excess without requiring new colored states.

Findings

The model can produce a large resonance width with perturbative couplings.

The decay chain mimics a diphoton final state despite involving three photons.

No new colored particles are necessary in this framework.

Abstract

We show that contrary to naive expectations the recently observed diphoton excess can be explained by a vector resonance, which decays to a photon and a light scalar $s$, followed by a decay of the scalar into two photons: $Z' \rightarrow \gamma s \rightarrow 3 \gamma$. As the two photons from the scalar decay are highly boosted, the experimental signature is an apparent diphoton final state. In fact all the necessary ingredients are naturally present in $Z'$ models: Additional fermions with electroweak quantum numbers are required in order to render the theory anomaly free and naturally induce the required effective couplings, while the hidden Higgs which gives mass to the $Z'$ can be very light. In particular no new coloured states are required in this framework. We also show that in such a setup the width of the resonance can be rather large, while all couplings remain perturbative.