Evolution of the trigger and data acquisition system in the ATLAS experiment

The ATLAS detector is designed to observe proton-proton collisions delivered by the LHC accelerator. The ATLAS Trigger and Data Acquisition (TDAQ) system is responsible for the selection and the conveyance of physics data, reducing the rate of stored events from the initial 40 MHz LHC frequency to several hundreds Hz. The TDAQ system is organized in a three-level selection scheme, including a hardware-based first-level trigger and second- and third-level triggers implemented as software systems distributed on commodity hardware nodes. While this architecture was successfully operated well beyond the original design goals, the accumulated experience stimulated interest to explore possible evolutions. In this paper, we report on the performance of the current system and design of the new system, with particular attention to the prototyping effort that allowed to spot possible limitations and to demonstrate benefits.