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.
Highlights
The Trigger and Data Acquisition (TDAQ) system [1] is an essential component of the ATLAS experiment [2] at the Large Hadron Collider (LHC) [3] as it is responsible for deciding in real time whether to record data from a given collision
The definition of the L1 configuration contains: the L1 topological (L1Topo) trigger menu, a list of algorithms and their parameters used in the L1Topo system; the muon and calorimeter pT thresholds and isolation requirements for each L1 trigger object; the L1 trigger menu, a list of L1 trigger thresholds and objects which are combined into L1 trigger items; the Central Trigger Processor (CTP) firmware logic, which is derived from the L1 trigger menu; and global settings, such as the L1 calorimeter (L1Calo) isolation settings
Once the release is validated, it is deployed to the online machines for data-taking and further tested in an ATLAS test run during an LHC inter-fill period to check that there are no problems configuring the High Level Trigger (HLT) with the new release in the online environment and that the release is compatible with software being used by other subsystems
Summary
The Trigger and Data Acquisition (TDAQ) system [1] is an essential component of the ATLAS experiment [2] at the Large Hadron Collider (LHC) [3] as it is responsible for deciding in real time whether to record data from a given collision. The TDAQ system operated efficiently during the LHC Run 1 (2009–2013) [4] at instantaneous luminosities up to 8 × 1033 cm−2 s−1 and primarily at centre-ofmass energies of 7 TeV and 8 TeV, collecting more than three billion proton-proton (pp) collision events. The operation of the trigger system under these challenging pp data-taking conditions is described in this document.
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