Abstract
During Run 2 (2015–2018) the Large Hadron Collider has provided, at the World’s highest energy frontier, proton–proton collisions to the ATLAS experiment with high instantaneous luminosity (up to [Formula: see text]), placing stringent operational and physics requirements on the ATLAS trigger system in order to reduce the 40 MHz collision rate to a manageable event storage rate of 1 kHz, while not rejecting interesting collisions. The Level-1 trigger is the first rate-reducing step in the ATLAS trigger system with an output rate of up to 100 kHz and decision latency of less than 2.5 [Formula: see text]s. In Run 2, an important role was played by the Level-1 Topological Processor (L1Topo). This innovative system consists of two blades designed in AdvancedTCA form factor, mounting four individual state-of-the-art processors, and providing high input bandwidth and low latency data processing. Up to 128 topological trigger algorithms can be implemented to select interesting events by applying kinematic and angular requirements on electromagnetic clusters, hadronic jets, muons and total energy reconstructed in the ATLAS apparatus. This resulted in a significantly improved background rejection and enhanced acceptance of physics signal events, despite the increasing luminosity. The L1Topo system has become more and more important for physics analyses making use of low energy objects, commonly present in the Heavy Flavor or Higgs physics events, for example. An overview of the L1Topo architecture, simulation and performance results during Run 2 is presented alongside with upgrade plans for the L1Topo system to be installed for the future Run 3 data taking period.
Highlights
The ATLAS experiment [1] is a general-purpose detector at the Large Hadron Collider (LHC) [2], designed to explore physics at TeV energy scales with protonproton and heavy-ion collisions
Due to the high number of trigger objects sent to Level1 Topological Processor (L1Topo) from both Level-1 Calorimeter (L1Calo) and Level-1 Muon (L1Muon), 50 ns are spent in the reduction of the input lists for the decision algorithms in order to avoid a number of combinatorics that would result in a large resource usage
The Level-1 Topological processor allowed during Run 2 for selections including low pT objects due to the possibility of applying kinematic and angular requirements already at the first trigger level in ATLAS
Summary
The ATLAS experiment [1] is a general-purpose detector at the Large Hadron Collider (LHC) [2], designed to explore physics at TeV energy scales with protonproton and heavy-ion collisions. In order to minimize the amount of stored data while keeping the physically relevant collisions, the ATLAS experiment performs a real-time event selection in two levels. Having higher luminosity would increase the trigger rates, with the subsequent need for raising the energy thresholds in order to reduce the amount of data recorded This effect, was mitigated by the proper modifications done to the different trigger systems. The L1Calo system was upgraded during LS1 with the inclusion of new Multi-Chip Modules based on Field-Programmable Gate Array (FPGA) technology, and the L1Muon trigger was modified to suppress fake muon signals by introducing additional coincidence In this context, the addition of a new system in the real-time data path, the Level-1 Topological Trigger (L1Topo)[5], has proven of high relevance in the ATLAS trigger system during Run 2
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