The design and performance of the ATLAS Inner Detector trigger in high pile-up collisions at 13 TeV at the Large Hadron Collider

The design and performance of the ATLAS Inner Detector (ID) trigger algorithms running online on the high level trigger (HLT) processor farm for 13 TeV LHC collision data with high pile-up are discussed. The HLT ID tracking is a vital component in all physics signatures in the ATLAS trigger for the precise selection of the rare or interesting events necessary for physics analysis without overwhelming the offline data storage in terms of both size and rate. To cope with the high expected interaction rates in the 13 TeV LHC collisions the ID trigger was redesigned during the 2013-15 long shutdown. The performance of the ID trigger in Run 2 from 13 TeV LHC collisions exceeded expectations as the pile-up increased throughout the run periods. The detailed efficiencies and resolutions of the trigger in a wide range of physics signatures spanning the entire Run 2 production luminosity data-taking are presented, to demonstrating that the trigger responded well under the extreme pile-up conditions. The performance of the ID trigger algorithms in ever higher pile-up collisions illustrates how the ID trigger continued to enable the ATLAS physics program and will continue to do so in the future.