The LHC-ATLAS experiment at CERN aims at recording about 1 kHz of physics collisions, starting with an LHC design bunch crossing rate of 40 MHz. To reduce the massive background rate while maintaining a high selection efficiency for rare physics events (such as beyond the Standard Model physics), a two-level trigger system is used. Events are selected based on physics signatures such as presence of energetic leptons, photons, jets or large missing energy. The trigger system exploits topological information, as well as multi-variate methods to carry out the necessary physics filtering. In total, the ATLAS online selection consists of thousands of different individual triggers.

A Trigger Menu (*) is a compilation of these triggers that specify the physics algorithms to be used during data-taking and the bandwidth a given trigger is allocated based on physics priority. Trigger Menus must reflect not only the physics goals of the collaboration for a given run period, but also take into consideration the instantaneous luminosity of the LHC and limitations from the ATLAS detector readout and offline processing farm. After the successful data-taking in Run1 and the first half of Run2 periods, the ATLAS trigger for the rest of Run2 has been enhanced to be able to handle higher instantaneous luminosities (up to 2.0×10^{34}cm^{-2}s^{-1}) and to ensure the selection robustness against higher average multiple interactions per bunch crossing.

In this presentation, we introduce the LHC trigger challenge and describe the design criteria for the ATLAS Trigger Menus used for Run2. We discuss several aspects of the process, from the fine-tuning of the prescales, the validation of the algorithms, and the monitoring tools that ensure the smooth operation of the trigger during data-taking. We also report on the physics performance of a few selected trigger algorithms.

(*) The speaker is a coordinator of the Trigger Menus & Signature-Performance group in the ATLAS experiment since 2016 January, after several years of expert experiences of this trigger area.