Diphoton Excess through Dark Mediators

TL;DR

This paper explores whether the 750 GeV diphoton excess observed at the LHC could be explained by heavy resonances decaying into metastable light states that mimic photons, proposing models with specific decay chains and analyzing their experimental implications.

Contribution

It introduces models where heavy resonances decay into metastable states that produce collimated electron pairs, potentially explaining the diphoton excess and predicting testable signals in future experiments.

Findings

Models can reproduce the diphoton excess with metastable light states.

Predicted lifetimes and couplings are within reach of upcoming intensity frontier experiments.

The proposed decay chains can mimic genuine diphoton events at the LHC.

Abstract

Preliminary ATLAS and CMS results from the first 13 TeV LHC run have encountered an intriguing excess of events in the diphoton channel around the invariant mass of 750 GeV. We investigate a possibility that the current excess is due to a heavy resonance decaying to light metastable states, which in turn give displaced decays to very highly collimated $e^+e^-$ pairs. Such decays may pass the photon selection criteria, and successfully mimic the diphoton events, especially at low counts. We investigate two classes of such models, characterized by the following underlying production and decay chains: $gg \to S\to A'A'\to (e^+e^-)(e^+e^-)$ and $q\bar q \to Z' \to sa\to (e^+e^-)(e^+e^-)$, where at the first step a heavy scalar, $S$, or vector, $Z'$, resonances are produced that decay to light metastable vectors, $A'$, or (pseudo-)scalars, $s$ and $a$. Setting the parameters of the models to explain the existing excess, and taking the ATLAS detector geometry into account, we marginalize over the properties of heavy resonances in order to derive the expected lifetimes and couplings of metastable light resonances. We observe that in the case of $A'$, the suggested range of masses and mixing angles $\epsilon$ is within reach of several new-generation intensity frontier experiments.