Upgrade of the Trigger System of the ATLAS Liquid Argon calorimeters

The ATLAS detector was designed and build 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. Liquid argon (LAr) sampling calorimeters are employed for all electromagnetic calorimetry in the pseudorapidity region |η| <3.2, and for hadronic calorimetry in the region from |η| = 1.5 to |η| = 4.9. 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 2019, 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 this increased trigger rate, an improved spatial granularity of the trigger primitives is proposed, in order to improve the identification performance for trigger signatures (e.g. electrons, photons, tau leptons, jets, total and missing energy) 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 refined trigger primitives are then transmitted to the first-level trigger system for extraction of improved trigger signatures. The general concept of the upgraded LAr calorimeter trigger together with the various electronics components to be developed will be presented. The R&D activities as well as architectural and performance studied undertaken by the ATLAS LAr Calorimeter group will be described.