The ATLAS trigger in Run-2 at high luminosities

The ATLAS Trigger system has been operating very successfully during the LHC Run-2, between 2015 and 2018. This excellent performance has been vital for the ATLAS physics program, selecting interesting collision events for a wide variety of physics signatures with high efficiency. Compared to LHC Run-1, where inclusive triggers were sufficient to cover most of the ATLAS physics interests, the much higher luminosities of Run-2 made it necessary to increase in the number of customized selections. By the beginning of 2018 more than 1500 different ATLAS trigger selections had been carefully compiled into a "trigger menu", covering all aspects of the ATLAS physics program and following the rise in LHC luminosity. The ATLAS Run-2 trigger selection capabilities have been significantly improved compared to Run-1, in order to cope with the higher event rates and with the large number of simultaneous proton-proton interactions (pile-up). At the Level-1 trigger, the improvements undertaken resulted in selection efficiencies and event rates that are more pile-up-robust, as well as a reduction of fake candidate particles. A new hardware system, designed to analyse event-topologies, supports a more refined event selection at the Level-1. Exemplary are the application of angular and invariant mass cuts in low momentum di-lepton and di-jet triggers, which keeps the same efficiency for analyses whilst keeping the Level-1 rate at an affordable level. A hardware-based, high-rate track reconstruction, currently being commissioned, will enable the high-level software trigger to make use of tracking information at its full input rate. Such full-scan tracking has an important role in reducing the pile-up dependence of triggers based on isolated single leptons and those selecting on total transverse energy. Improvements to the high-level trigger software together with deploying more offline-like reconstruction techniques within the high-level trigger, dramatically improve the performance of the trigger selection to nearly that of the offline reconstruction. The presentation gives a comprehensive review of the ATLAS trigger system in Run-2, describing the design changes that were made after Run-1 in order to provide alternatives to the formerly inclusive trigger selections that could not be maintained for objects of low momenta. The main points of the 2018 trigger menu, representative for the Run-2 trigger strategy, will be shown. Some focus will be put on the new event-topology-based selections at the Level-1 trigger, symbolizing the paradigm change that the trigger selections must continue to undergo as the LHC continues to reach even higher luminosities. The trigger performance at high pile-up seen in 2018 will be presented. A strategy for Run-3 could be briefly discussed, which will see further improvement of the trigger selection capabilities.