Operation of the ATLAS trigger system in run 2

Abstract

The ATLAS experiment at the Large Hadron Collider employs a two-level triggersystem to record data at an average rate of 1 kHz from physics collisions,starting from an initial bunch crossing rate of 40 MHz. During the LHC Run 2(2015-2018), the ATLAS trigger system operated successfully with excellentperformance and flexibility by adapting to the various run conditionsencountered and has been vital for the ATLAS Run-2 physics programme. Forproton-proton running, approximately 1500 individual event selections wereincluded in a trigger menu which specified the physics signatures and selectionalgorithms used for the data-taking, and the allocated event rate andbandwidth. The trigger menu must reflect the physics goals for a given datacollection period, taking into account the instantaneous luminosity of the LHCand limitations from the ATLAS detector readout, online processing farm, andoffline storage. This document discusses the operation of the ATLAS triggersystem during the nominal proton-proton data collection in Run 2 with examplesof special data-taking runs. Aspects of software validation, evolution of thetrigger selection algorithms during Run 2, monitoring of the trigger system anddata quality as well as trigger configuration are presented.