Atlas Data Acquisition: from Run I to Run II

The experience gained during the first period of very successful data taking of the ATLAS experiment (Run I) has inspired a number of ideas for improvement of the Data AcQuisition (DAQ) system that are being put in place during the so-called Long Shutdown 1 of the Large Hadron Collider (LHC), in 2013/14. We have updated the data-flow architecture, rewritten an important fraction of the software and replaced hardware, profiting from state of the art technologies. This paper summarizes the main changes that have been applied to the ATLAS DAQ system and highlights the expected performance and functional improvements that will be available for the LHC Run II. Particular emphasis will be put on explaining the reasons for our architectural and technical choices, as well as on the simulation and testing approach used to validate this system.