ATLAS Trigger and Data Acquisition Upgrades for the High Luminosity LHC

The ATLAS experiment at CERN is planning a second phase of upgrades to prepare for the "High Luminosity LHC", with collisions due to start in 2026. In order to deliver an order of magnitude more data than previous runs, 14 TeV protons will collide with an instantaneous luminosity of 7.5 × 1034 cm－2s－1, resulting in much higher pileup and data rates than the current experiment was designed to handle. While this is essential to realise the physics programme, it is a huge challenge for the detector, trigger, data acquisition and computing. The detector upgrades themselves also present new requirements and opportunities for the trigger and data acquisition system. With the Technical Design Report written and construction due to start soon, the baseline design of the TDAQ upgrade will be described. The system comprises: a hardware-based low-latency real-time Trigger, Data Acquisition which combines custom readout with commodity hardware and networking, and an Event Filter which combines offline-like algorithms on a large commodity compute service augmented by fast hardware tracking. Throughout the system, use of precision algorithms running on FPGAs or commodity hardware are pushed to lower latencies and higher rates than before. Precision calorimeter reconstruction with offline-style clustering and jet-finding in FPGAs, and track reconstruction in Associative Memory and FPGAs are used to combat pileup in the Trigger and Event Filter respectively. The physics motivation and expected performance will be shown for key physics processes.