The Evolution of the Trigger and Data Acquisition System in the ATLAS Experiment

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The ATLAS experiment, which records the results of LHC proton-proton collisions, is upgrading its Trigger and Data Acquisition (TDAQ) system during the current LHC first long shutdown. The purpose of this upgrade is to add robustness and flexibility to the selection and the conveyance of the physics data, simplify the maintenance of the infrastructure, exploit new technologies and, overall, make ATLAS data-taking capable of dealing with increasing event rates.While the TDAQ system successfully operated well beyond the original design goals, the accumulated experience stimulated interest to explore possible evolutions. With higher luminosities, the required number and complexity of Level-1 triggers will increase in order to satisfy the physics goals of ATLAS, while keeping the total Level-1 rates at or below 100kHz. The Central Trigger Processor will be upgraded to increase the number ofmanageable inputs and accommodate additional hardware for improved performance, and a new Topological Processor will be included. A single homogeneous high level trigger system will be deployed. The current second and third trigger levels will be executed together on a unique hardware node. This design has many advantages: the radical simplification of the architecture, the flexible and automatically balanced distribution of the computing resources, the sharing of code and services on nodes.In this paper, we report on the design and the development status of the upgraded TDAQ system, with particular attention to the tests currently on-going to identify the required performance and to spot its possible limitations.