Abstract
The ATLAS experiment 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 effciency 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 the presence of energetic leptons, photons, jets or large missing energy. The trigger system exploits geometrical information on candidate objects, 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 which specifies the physics algorithms to be used during data taking and the bandwidth a given trigger is allocated to. Trigger menus reflect not only the physics goals of the collaboration for a given run, but also take into consideration the instantaneous luminosity of the LHC and limitations from the ATLAS detector readout and online processing farm. For the 2017 run, the ATLAS trigger has been enhanced to be able to handle higher instantaneous luminosities (up to 2:0x1034 cm-2s-1) and to ensure the selection robustness against higher average multiple interactions per bunch crossing. In these proceedings, we describe the design criteria for the trigger menus used for Run 2 at the LHC. We discuss several aspects of the process, from the validation of the algorithms, the fine-tuning of the prescales, 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 trigger algorithms.
Highlights
The trigger system is a fundamental component of the ATLAS [1] experiment at the LHC [2], since it is responsible for the online event selection for data storage
During the LHC shutdown (2013-2015), the trigger system underwent major upgrades to cope with the extremely challenging conditions expected for the upcoming years
Events for physics analyses using full event building are recorded at an average rate of approximately 1 kHz
Summary
The trigger system is a fundamental component of the ATLAS [1] experiment at the LHC [2], since it is responsible for the online event selection for data storage. In the LHC Run-1 (2009-2013), the ATLAS trigger system [3] operated efficiently at instantaneous luminosities of up to 8×1033 cm−2s−1 and at centre-of-mass energies of up to 8 TeV, collecting more than three billion events. During the LHC shutdown (2013-2015), the trigger system underwent major upgrades to cope with the extremely challenging conditions expected for the upcoming years. In the LHC Run-2 (2015-2018), the increased centre-of-mass energy to 13 TeV and higher instantaneous luminosities, surpassing the initial design luminosity of 1034 cm−2s−1, lead to a significant increase of rates, exceeding the capabilities of the original (Run-1) trigger system
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