The LHC Run 2 ATLAS trigger system: design, performance and plans

Abstract

In many high-energy physics experiments, online selection is vital to collect the most interesting collisions out of the large data volume. The ATLAS experiment at the Large Hadron Collider (LHC) utilizes a trigger system that consists of a hardware Level-1 (L1) and a software based high-level trigger (HLT), reducing the event rate from the LHC bunch-crossing rate of 40 MHz to an average recording rate of around 1000 Hz. In Run 2 of the LHC, started in spring 2015, the LHC is operating at a centre-of-mass energy of 13 TeV providing an instantaneous luminosity up to 1.4 ⋅ 1034 cm−2s−1 so far. The ATLAS trigger system has to cope with these challenges, while maintaining or improving upon the efficiency to select relevant physics processes. In this paper, the ATLAS trigger system for the 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 (SM) processes and to search for new phenomena. Finally, further developments planned for the rest of the LHC Run 2 are discussed. These include two new hardware subsystems for topological selections at L1 and full-scan tracking at the input to the HLT.