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Abstract

The operation of the ATLAS trigger system during Run 2 of the LHC was highly successful for both $pp$ and heavy-ion data-taking as well as for a variety of special runs. The recorded data are of high quality with only minimal losses due to operational problems. The trigger's excellent performance and flexibility allowed collection of $139\,fb^{-1}$ of high-quality $pp$ collision data at $\sqrt{s}=13$ TeV over a wide range of running conditions, with peak instantaneous luminosities ranging from $0.5\times 10^{34} cm^{-2}s^{-1}$ to $2.1\times 10^{34}cm^{-2}s^{-1}$ and 28-60 pile-up collisions. A variety of tools and procedures, as detailed in this paper, have been used in both the offline and online environments to efficiently validate, monitor and assess the operation and the performance of the trigger system. They allowed maximal exploitation of the collisions delivered by the LHC in Run 2 for physics analyses, while respecting the limitations of the trigger system in terms of maximum read-out rates, available HLT computing resources and sustainable storage output bandwidth.