Photons, Photon Jets and Dark Photons at 750 GeV and Beyond

TL;DR

This paper analyzes how to distinguish photon jets from isolated photons at the LHC, especially in the context of the 750 GeV diphoton excess, by studying conversion probabilities and decay properties of new particles.

Contribution

It provides a quantitative method to differentiate photon jets from isolated photons and explores the impact of particle lifetime and dark photon decays on detection sensitivity.

Findings

Photon conversion rates can effectively distinguish photon jets from isolated photons.

Sensitivity to photon jets varies with the lifetime of decaying particles.

Results are applicable to future searches for new physics involving photons.

Abstract

In new physics searches involving photons at the LHC, one challenge is to distinguish scenarios with isolated photons from models leading to "photon jets". For instance, in the context of the 750 GeV diphoton excess, it was pointed out that a true diphoton resonance $S \to \gamma\gamma$ can be mimicked by a process of the form $p p \to S \to a a \to 4\gamma$, where $S$ is a new scalar with a mass of 750 GeV and $a$ is a light pseudoscalar decaying to two collinear photons. Photon jets can be distinguished from isolated photons by exploiting the fact that a large fraction of photons convert to an $e^+e^-$ pair inside the inner detector. In this note, we quantify this discrimination power, and we study how the sensitivity of future searches differs for photon jets compared to isolated photons. We also investigate how our results depend on the lifetime of the particle(s) decaying to the photon jet. Finally, we discuss the extension to $S\to A^\prime A^\prime\to e^+e^-e^+e^-$, where there are no photons at all but the dark photon $A^\prime$ decays to $e^+e^-$ pairs. Our results will be useful in future studies of the putative 750 GeV signal, but also more generally in any new physics search involving hard photons.