Upgrade project and plans for the ATLAS detector and trigger

In the coming years different phases of upgrades for the LHC complex are foreseen, which will allow for the extension of the physics potential of its experiments. Through two different phases (namely Phase-I and Phase-II), the average luminosity will be increased by a factor 5-10 above the design luminosity of 1 × 10³⁴ cm^-2 s^-1. Consequently, the detectors and the infrastructure of the DAQ system of the experiments will need to be upgraded as well, to take into account the increased radiation level and particle rates foreseen at such high luminosity. In this paper we describe the proposed changes to the ATLAS detector and its trigger system, to face the increased number of interactions per bunch crossing. This will cause higher levels of pile-up and increased rates at each level of the trigger. The trigger system will improve its selectivity by benefiting from the increased granularity available at the trigger level, which will allow for a higher resolution. Different scenarios are compared, keeping in mind the requirements to achieve the expected physics potential of ATLAS in this high luminosity regime.