Abstract
The ATLAS Experiment uses a two-level triggering system to identify and record proton-proton collision events containing a wide variety of physics signatures. It reduces the event rate from the bunch crossing rate of 40 MHz to an average recording rate of 1 kHz, whilst maintaining high efficiency for interesting collision events. It is composed of an initial hardware-based level-1 trigger followed by a software-based high-level trigger. A central component of the high-level trigger is the calorimeter trigger. This is responsible for processing data from the electromagnetic and hadronic calorimeters in order to identify electrons, photons, taus, jets and missing transverse energy. This paper presents the performance of the high-level calorimeter trigger in Run-2, noting the improvements that have been made in response to the challenges of operating at high luminosity.
Highlights
The ATLAS Trigger is responsible for the online selection of events to be written to disk
Centre-of-mass energy and luminosity increased in Run-2 → both leading to increased pile-up
Plan for Run-2 was to run full-scan topo-clustering at earlier stage in High-Level Trigger (HLT) – to improve harmony with offline But... topo-clustering was already one of the most CPU intensive algorithms in Run-1
Summary
The ATLAS Trigger is responsible for the online selection of events to be written to disk. Software based (~40k CPU farm) Average latency ~350 ms Full detector / Full granularity is possible but not at 100 kHz rate (Jets / Missing E (MET)). Can run special, HLT-specific and offline-like algorithms within RoIs (Egamma / Taus). Centre-of-mass energy and luminosity increased in Run-2 → both leading to increased pile-up
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