Technical Design Report for the ATLAS Inner Tracker Strip Detector

This is the first of two Technical Design Report documents that describe the upgrade of the central tracking system for the ATLAS experiment for the operation at the High Luminosity LHC (HL-LHC) starting in the middle of 2026. At this time the LHC will have been upgraded to reach a peak instantaneous luminosity of 7.5x10^34 cm^[-2]s^[-1], which corresponds to approximately 200 inelastic proton-proton collisions per beam crossing. The new Inner Tracker (ITk) will be operational for more than ten years, during which ATLAS aims to accumulate a total data set of 3,000 fb^[-1]. Meeting these requirements presents a unique challenge for the design of an all-silicon tracking system that consists of a pixel detector at small radius close to the beam line and a large-area strip tracking detector surrounding it. This document presents in detail the requirements of the new tracker, its layout and expected performance including the results of several benchmark physics studies at the highest numbers of collisions per beam crossing. The design, construction and expected performance of the large-area silicon-strip tracker are set out in detail. These include the results of measurements of prototype modules and associated support structures and the plans for their mass production. A short description is given of the state of the development of the pixel tracker, which will be presented in detail in the second Technical Design Report document one year from now. This document will also describe the management of the project, its schedule and major milestones. The costs of the new tracker and the collaboration that plans to build it are also presented.