Performance of electron and photon triggers in ATLAS during LHC Run 2

TL;DR

This paper evaluates the performance of electron and photon triggers in the ATLAS detector during LHC Run 2, demonstrating high efficiencies across a broad energy range despite increased luminosity and interaction rates.

Contribution

It presents an optimized trigger strategy and performance assessment for electrons and photons in ATLAS during Run 2, accommodating higher luminosity and interaction rates.

Findings

Electron trigger efficiency is at least 75% for 31 GeV electrons.

Photon trigger efficiency exceeds 96% for 30 GeV photons.

Heavy-ion collision triggers maintain high efficiency above thresholds.

Abstract

Electron and photon triggers covering transverse energies from 5 GeV to several TeV are essential for the ATLAS experiment to record signals for a wide variety of physics: from Standard Model processes to searches for new phenomena in both proton-proton and heavy-ion collisions. To cope with a fourfold increase of peak LHC luminosity from 2015 to 2018 (Run 2), to 2.1$\times$10$^{34}$cm$^{-2}$s$^{-1}$, and a similar increase in the number of interactions per beam-crossing to about 60, trigger algorithms and selections were optimised to control the rates while retaining a high efficiency for physics analyses. For proton-proton collisions, the single-electron trigger efficiency relative to a single-electron offline selection is at least 75% for an offline electron of 31 GeV, and rises to 96% at 60 GeV; the trigger efficiency of a 25 GeV leg of the primary diphoton trigger relative to a tight offline photon selection is more than 96% for an offline photon of 30 GeV. For heavy-ion collisions, the primary electron and photon trigger efficiencies relative to the corresponding standard offline selections are at least 84% and 95%, respectively, at 5 GeV above the corresponding trigger threshold.