Study of tracking and software performance of the ATLAS Inner Tracker in staged-installation scenarios at the High-luminosity LHC

The current baseline construction schedule of the Pixel detector of the ATLAS Phase-2 Inner Tracker (ITk) is in tension with its foreseen installation date during Long Shutdown (LS) 3. Different staged-installation scenarios are being discussed that could help advancing the construction schedule. This would provide enough time contingency to ensure completion of the detector before the start of the run period. In the considered scenarios, only part of the nominal ITk Pixel detector would be installed during LS3 and be present for the Run-4 data-taking, while the rest of the detector would be installed during LS4. This note presents a study of the implications of the proposed staging scenarios on track reconstruction, physics and technical performance. Tracking and flavour-tagging performance are more severely impacted when taking into account detector defects, while a significant increase in computing power is needed to guarantee reasonable tracking performance even in the absence of defects, with major implications for the tracking reconstruction in trigger.