Triggering in the ATLAS Experiment

The ATLAS experiment at the LHC can record about 1 kHz of physics collisions, out of an LHC design bunch crossing rate of 40MHz. To achieve a high selection efficiency for rare physics events (such as beyond the StandardModel physics) while reducing the large background rate, a two-level trigger system is used. The event selection is based on physics signatures, such as the presence of energetic leptons, photons, jets or missing energy. In addition, the trigger system exploits algorithms using topological information and multivariate methods to carry out the filtering for the many physics analyses pursued by the ATLAS collaboration. In Run 2, around 1500 individual selections, the trigger chains, are included in the trigger menu specifying the selection algorithms to be used for data taking. Trigger menus must reflect the physics goals for a given data-taking period, taking the instantaneous luminosity of the LHC and limitations from the ATLAS detector readout and offline processing farm into account. An overview of the 2015–2018 trigger system and its performance is presented.