The Run-2 ATLAS Trigger System: Design, Performance and Plan

In high-energy physics experiments, online selection is crucial to select interesting collisions from the large data volume. The ATLAS experiment at the Large Hadron Collider (LHC) utilizes the trigger system that consists of a hardware Level-1 (L1) and a software based high-level trigger (HLT), reducing the event rate from the design bunch-crossing rate of 40 MHz to an average recording rate of about 1000 Hz. In the LHC Run-2 starting from in 2015, the LHC operates at centre-of-mass energy of 13 TeV providing a luminosity up to $1.2 \cdot 10^{34} {\rm cm^{-2}s^{-1}}$. The ATLAS trigger system has to cope with these challenges, while maintaining or even improving the efficiency to select relevant physics processes. In this paper, the ATLAS trigger system for LHC Run-2 is reviewed. Secondly, the impressive performance improvements in the HLT trigger algorithms used to identify leptons, hadrons and global event quantities like missing transverse energy is shown. Electron, muon and photon triggers covering transverse energies from a few GeV to several TeV are essential for signal selection in a wide variety of ATLAS physics analyses to study Standard Model processes and to search for new phenomena. Finally, further critical developments envisaged for the rest for the Run-2 are discussed. These include two new hardware components for topological selections at L1 and full-scan tracking in the input of the HLT.