Speaker
Description
The Liquid Argon Calorimeters are employed by ATLAS for all electromagnetic calorimetry in the pseudo-rapidity region |η| < 3.2, and for hadronic and forward calorimetry in the region from |η| = 1.5 to |η| = 4.9. They also provide inputs to the first level of the ATLAS trigger. In 2022 the LHC started its Run 3 period with an increase in luminosity and and resulting in a pile-up of up to 65 average interactions per bunch crossing. To cope with these harsher conditions, a new trigger readout path was installed over Long Shutdown 2. This new path significantly improves the electromagnetic object trigger performance by maintaining or improving their efficiency without raising pT thresholds, and providing appreciable rate reductions. This was achieved by increasing the granularity of the objects available at trigger level by up to a factor of ten. The installation of this new trigger readout chain also required the update of the legacy system. More than 1500 boards of the precision readout were extracted from the ATLAS cavern, refurbished and re-installed. The legacy analog trigger readout that remain during the first years of Run 3 as a backup of the new digital trigger system was also updated. For the new system, 124 new on-detector boards have been added. Those boards that are operating in a radioactive environment are digitizing the calorimeter trigger signals at 40 MHz. The digital signal is sent to the off-detector system and processed online to provide the measured energy value for each unit of readout. In total up to 31 Tbps are analyzed by the processing system and more than 62 Tbps are generated for downstream reconstruction. To minimize the triggering latency the processing system had to be installed underground. The limited available space imposed a very compact hardware structure. To achieve a compact system, large FPGAs with high throughput have been mounted on ATCA mezzanine cards. In total no more than 3 ATCA shelves are used to process the signal from approximately 34000 channels. Given that modern technologies have been used compared to the previous system, all the monitoring and control infrastructure is being adapted and commissioned as well. This contribution will present the challenges of the commissioning and operation of the new digital trigger and its performance over the first three years of Run 3 operation, which allowed the complete retirement of the legacy analog trigger.
