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Upgrade of the trigger system of the ATLAS liquid argon calorimeters

The ATLAS detector was designed and was built to study proton-proton collisions produced at the LHC at centre-of-mass energies up to 14 TeV and instantaneous luminosities up to 1034 cm-2s-1. The ATLAS Liquid Argon (LAr) calorimeters produce a total of 182,486 signals, which are digitized and processed by the front-end and back-end electronics for each triggered event. In addition, the front-end electronics sums analog signals to provide coarse-grained energy sums, called trigger towers, to the first-level trigger system, which is optimized for nominal LHC luminosities. In 2020, instantaneous luminosities of (2-3)×1034 cm-2s-1 are expected, far beyond that for which the detector was designed. In order to cope with the increased trigger rate, an improved spatial granularity of the trigger primitives is proposed, to improve the identification performance for trigger signatures at high background rejection rates. For these purposes, a new LAr Trigger Digitizer Board (LTDB) is being designed to receive higher granularity signals, digitize them on-detector and send them via fast optical links to a new digital processing system (DPS). The DPS applies digital filtering and identifies significant energy depositions in each trigger channel. The general concept of the upgraded LAr calorimeter trigger and expected performance as well as the various electronics components to be developed are described.