Performance of the missing transverse momentum triggers for the ATLAS detector during Run-2 data taking

TL;DR

This paper evaluates the ATLAS detector's missing transverse momentum trigger performance during Run-2, demonstrating high efficiency despite increased luminosity and pile-up, through algorithm tuning and new approaches.

Contribution

It presents an analysis of trigger performance improvements and adaptations for high pile-up conditions during Run-2, ensuring high efficiency and manageable data rates.

Findings

Trigger efficiency >98% during Run-2

Maintained high performance with increased pile-up

Achieved acceptable data bandwidth usage

Abstract

The factor of four increase in the LHC luminosity, from $0.5\times 10^{34}\,\textrm{cm}^{-2}\textrm{s}^{-1}$ to $2.0\times 10^{34}\textrm{cm}^{-2}\textrm{s}^{-1}$, and the corresponding increase in pile-up collisions during the 2015-2018 data-taking period, presented a challenge for ATLAS to trigger on missing transverse momentum. The output data rate at fixed threshold typically increases exponentially with the number of pile-up collisions, so the legacy algorithms from previous LHC data-taking periods had to be tuned and new approaches developed to maintain the high trigger efficiency achieved in earlier operations. A study of the trigger performance and comparisons with simulations show that these changes resulted in event selection efficiencies of >98% for this period, meeting and in some cases exceeding the performance of similar triggers in earlier run periods, while at the same time keeping the necessary bandwidth within acceptable limits.