Letter of Intent for the Phase-II Upgrade of the ATLAS Experiment

This Letter of Intent presents a plan for preserving and improving the current detection capabilities of the ATLAS detector to meet the challenges and take advantage of operating at the High Luminosity LHC (HL-LHC). From 2024, the HL-LHC will provide unprecedented pp luminosities to ATLAS, resulting in an additional integrated luminosity of around 2500 fb-1 over ten years. This will present a unique opportunity to substantially extend the mass reach in searches for many signatures of new physics, in several cases well into the multi-TeV region, and to significantly extend the study of the properties of the Higgs boson. The increased luminosity and the accumulated radiation damage will render the current Inner Tracker no longer suitable for long term operations. It will need to be replaced with a new all silicon tracker to maintain tracking performance in the high occupancy environment and to cope with the increase of approximately a factor of ten in the total radiation fluence. New technologies are used to ensure that the system can survive this harsh radiation environment and to optimise the material distribution, while the new readout scheme allows the implementation of a track trigger contributing to the improvements in the ATLAS triggering capabilities. The very high luminosities also present significant challenges to the operation and performance of the rest of the detector systems as well as the trigger; the consequent high number of collisions per crossing will degrade the performance of ATLAS unless the LAr and Tile calorimeters and the Muon Spectrometer readout systems are upgraded. A new trigger architecture will be implemented exploiting the upgrades of the detector readout systems that will maintain and improve the event selection. The increased luminosity may also degrade the performance of the forward calorimetry. Options for upgrading the hadronic endcap calorimeter readout electronics and the forward calorimeter detector design are being investigated to address the performance degradation, if this proves to be required. Finally, the computing and software must be upgraded to meet the challenges of the increased luminosity and changes in computer architectures.