The ATLAS ITk Strip Detector System for the High Luminosity LHC Upgrade

The ATLAS experiment at the Large Hadron Collider is currently preparing major detector upgrades to meet the requirements of the High Luminosity LHC (HL-LHC), scheduled to begin in 2026. With the radiation damage and high density of tracks expected at the HL-LHC, the current Inner Detector would be inoperable, and will instead be replaced by an all-silicon Inner Tracker (ITk). The ITk consists of two sub-systems: a pixel detector close to the beam pipe, and a strip detector at larger radii. This paper presents results from an extensive design and prototyping effort for the ITk strip detector, including studies of the sensors, electronics, and support structures. The performance of the detector has been validated through testbeams, simulations, and thermal and electrical prototypes. Plans for the forthcoming production phase are also presented.