Operation of the ATLAS trigger system in Run 2

TL;DR

The paper describes the successful operation and adaptation of the ATLAS trigger system during LHC Run 2, enabling efficient data collection at 1 kHz from 40 MHz collision rates for physics analysis.

Contribution

It details the implementation, operation, and evolution of the ATLAS trigger system during Run 2, including trigger menu configuration and system monitoring.

Findings

Trigger system operated successfully with excellent performance.

Approximately 1500 trigger selections used for data collection.

System adapted to varying run conditions and physics goals.

Abstract

The ATLAS experiment at the Large Hadron Collider employs a two-level trigger system to record data at an average rate of 1 kHz from physics collisions, starting from an initial bunch crossing rate of 40 MHz. During the LHC Run 2 (2015-2018), the ATLAS trigger system operated successfully with excellent performance and flexibility by adapting to the various run conditions encountered and has been vital for the ATLAS Run-2 physics programme. For proton-proton running, approximately 1500 individual event selections were included in a trigger menu which specified the physics signatures and selection algorithms used for the data-taking, and the allocated event rate and bandwidth. The trigger menu must reflect the physics goals for a given data collection period, taking into account the instantaneous luminosity of the LHC and limitations from the ATLAS detector readout, online processing farm, and offline storage. This document discusses the operation of the ATLAS trigger system during the nominal proton-proton data collection in Run 2 with examples of special data-taking runs. Aspects of software validation, evolution of the trigger selection algorithms during Run 2, monitoring of the trigger system and data quality as well as trigger configuration are presented.