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. The ATLAS trigger has been successfully collecting collision data during the first run of the LHC (Run-1) between 2009-2013 at a centre-of-mass energy between 900 GeV and 8 TeV. In the second run of LHC (Run-2) starting from 2015, the LHC operates at centre-of-mass energy of 13 TeV and provides a higher luminosity of collisions. Also, the number of collisions occurring in a same bunch crossing increases. The ATLAS trigger system has to cope with these challenges, while maintaining or even improving the efficiency to select relevant physics processes. In this talk, first we will review the ATLAS trigger system upgrades that were implemented during the shutdown between Run-1 and Run-2. This includes changes to the L1 calorimeter and muon trigger systems, the introduction of a new L1 topological trigger module and the merging of the previously two-level HLT system into a single event filter farm. Then, we will show the impressive performance improvements in the HLT trigger algorithms used to identify leptons, hadrons and global event quantities like missing transverse energy. 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. Final states including leptons and photons had, for example, an important role in the discovery and measurement of the Higgs particle. The Run-2 condition demanded the optimisation of the trigger selections at each level, and in addition, new hardware selections are implemented at the L1. To improve the performance multivariate analysis techniques are introduced for the electron selections. Hadronic signatures are from the most promising in the high energy physics analysis program, broadly used for both Standard Model measurements and searches for new physics. These signatures include generic quark and gluon jets as well as jets originating from b-quarks or tau leptons decaying hadronically. We present an overview of how we trigger on hadronic signatures on the ATLAS experiment of the LHC, outlying the challenges of hadronic object trigger reconstruction and describing the improvements performed for the Run-2. We also discuss further critical developments envisaged for the rest for the Run-2. These include two new hardware components for topological selections at L1 and full-scan tracking in the input of the HLT.