ATLAS Trigger

ATLAS Trigger
Name	ATLAS Trigger
Project	Large Hadron Collider
Location	CERN
Detector	ATLAS experiment
First operation	2009
Type	Trigger and Data Acquisition System

ATLAS Trigger The ATLAS Trigger is the real-time event selection system for the ATLAS experiment at the Large Hadron Collider (LHC). It reduces the raw collision rate produced by proton–proton collisions in the Collider ring to a manageable set of events for storage and analysis, coordinating with experiment subsystems such as the Inner Detector, Calorimeter, and Muon Spectrometer. The system operates within the CERNb computing and detector infrastructure alongside projects like Worldwide LHC Computing Grid and interfaces with collaborations including ATLAS Collaboration and institutions such as University of Oxford, Massachusetts Institute of Technology, and CERN Experimental Physics Department.

Overview

The Trigger serves as a hierarchical selection chain connecting front-end electronics in the ATLAS experiment to back-end storage systems used by experiments like CMS experiment and facilities linked to the European Organization for Nuclear Research. It mediates between the raw bunch-crossing frequency of the Large Hadron Collider and the data rates consumed by analyses from teams at Brookhaven National Laboratory, Lawrence Berkeley National Laboratory, Fermilab, and universities including University of Tokyo, University of Cambridge, Harvard University, Stanford University, and Imperial College London. The Trigger works in concert with accelerator operations managed by groups at CERN Accelerator Beam Department and coordinates timing with systems used by ALICE and LHCb.

Trigger Architecture and Components

The architecture is layered, comprising hardware-based low-latency components and software-based high-level processing similar to systems used in ATLAS Tile Calorimeter readout and contemporaries like the CMS High-Level Trigger. Hardware stages utilize custom electronics patterned after technologies in CERN Microelectronics Group developments and firmware approaches employed by European X-Ray Free-Electron Laser. Low-level triggers interface with subsystems such as the Liquid Argon Calorimeter, Muon Trigger Chambers, and the Pixel Detector, paralleling designs from institutions like Max Planck Institute for Physics and CEA Saclay. Back-end farms run high-throughput software on compute clusters provided by partners including Deutsches Elektronen-Synchrotron and National Institute for Nuclear and Particle Physics.

Trigger Algorithms and Selection Criteria

Algorithms implement physics-driven criteria to identify signatures of processes studied by collaborations such as those who discovered the Higgs boson and measured properties of the Top quark and W boson. Selection logic targets objects reconstructed from detector inputs: charged tracks in the Inner Detector, electromagnetic clusters in the Electromagnetic Calorimeter, and muon candidates in the Muon Spectrometer. Algorithms borrow pattern-recognition and machine-learning concepts tested in projects at MIT Lincoln Laboratory, Google DeepMind, and Oxford Machine Learning Research Group, and are influenced by analysis strategies used in measurements like CP violation studies and searches for supersymmetry and dark matter candidates. Trigger criteria are tuned for signatures from decays studied in experiments such as ATLAS Higgs Combination Group and searches inspired by results from Tevatron collaborations at Fermilab.

Performance, Calibration, and Monitoring

Performance metrics are evaluated against standards developed in coordination with detector teams at CERN Detector R&D Group and monitored by shift crews drawn from institutions such as University of Manchester, University of California, Berkeley, and University of Melbourne. Calibration uses control samples from processes like Z boson decays, J/psi resonances, and cosmic-ray commissioning runs previously used by experiments at SLAC National Accelerator Laboratory and DESY. Monitoring frameworks integrate dashboards and alarm systems similar to operational tools at European Grid Infrastructure and use statistical techniques validated in analyses of B meson decays and precision electroweak measurements published by groups including Particle Data Group contributors.

Operational History and Upgrades

Since first physics runs in 2009, the Trigger has evolved through major upgrades during the LHC long shutdowns, incorporating hardware improvements and software refactorings influenced by developments at CERN and partner laboratories like INFN and KEK. Milestones include commissioning for the Run 1 (LHC) Higgs analyses, adaptations for the higher-luminosity conditions of Run 2 (LHC), and preparatory redesigns for the High-Luminosity Large Hadron Collider (HL-LHC) era. Upgrades have paralleled technology transfers and cross-collaboration efforts with projects like the ATLAS Upgrade Project and infrastructures supported by funding agencies such as European Research Council and Science and Technology Facilities Council.

Use in Physics Analyses

Selected events from the Trigger feed physics groups performing measurements and searches that led to discoveries and precision results credited to collaborations including ATLAS Collaboration and comparative studies with results from CMS Collaboration, CDF (particle detector), and DØ experiment. Trigger choices directly impact analyses in sectors such as Higgs boson property measurements, top-quark physics, electroweak precision tests, and searches for new phenomena motivated by theories including Supersymmetry, Extra dimensions (physics), and Composite Higgs models. Data retained by the Trigger are archived and processed in workflows shared with computing centers like CERN Data Centre and the Worldwide LHC Computing Grid for final papers submitted to journals where collaborations from institutions like Princeton University, Yale University, University of Chicago, and École Polytechnique publish results.

Category:Particle physics