On the Interpretation of a Possible $\sim 750$ GeV Particle Decaying into $\gamma \gamma$

TL;DR

This paper explores various theoretical models involving vector-like fermions to interpret a potential 750 GeV particle decaying into two photons, consistent with recent collider hints, and discusses their experimental signatures and constraints.

Contribution

It systematically analyzes multiple vector-like fermion models as explanations for the 750 GeV diphoton excess, including dark matter mediator scenarios, and assesses their compatibility with current experimental limits.

Findings

Models with vector-like fermions can explain the 750 GeV signal.

Predicted decay modes include $ZZ$, $Z\,\gamma$, and $W^+W^-$.

Couplings remain perturbative under current constraints.

Abstract

We consider interpretations of the recent $\sim 3 \sigma$ reports by the CMS and ATLAS collaborations of a possible $X(\sim 750~{\rm GeV})$ state decaying into $\gamma \gamma$ final states. We focus on the possibilities that this is a scalar or pseudoscalar electroweak isoscalar state produced by gluon-gluon fusion mediated by loops of heavy fermions. We consider several models for these fermions, including a single vector-like charge $2/3$ T quark, a doublet of vector-like quarks $(T, B)$, and a vector-like generation of quarks, with or without leptons that also contribute to the $X \to \gamma \gamma$ decay amplitude. We also consider the possibility that $X(750)$ is a dark matter mediator, with a neutral vector-like dark matter particle. These scenarios are compatible with the present and prospective direct limits on vector-like fermions from LHC Runs 1 and 2, as well as indirect constraints from electroweak precision measurements, and we show that the required Yukawa-like couplings between the $X$ particle and the heavy vector-like fermions are small enough to be perturbative so long as the $X$ particle has dominant decay modes into $gg$ and $\gamma \gamma$. The decays $X \to Z Z, Z \gamma$ and $W^+ W^-$ are interesting prospective signatures that may help distinguish between different vector-like fermion scenarios.