Technical Design Report for the ATLAS Inner Tracker Pixel Detector

This is the second 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 that time the LHC will have been upgraded to reach a peak instantaneous luminosity of "7.5\times 10^{34} cm^{-2}s^{-1}", which corresponds to an average of about 200 inelastic proton-proton collisions per beam-crossing. The new Inner Tracker (ITk) will be operational for more than ten years, during which time ATLAS aims to accumulate a total data set of 4000 fb^{-1}. Many of the features of the tracker have already been presented in the first Technical Design Report that detailed the construction of the ITk Strip Tracker. That report was published in April 2017. This document focuses on the ITk Pixel Detector. A baseline design is described in detail, and the motivations for the chosen technologies are illustrated. In some cases, alternative solutions are also illustrated. In this case, we indicate the advantage in pursuing the other designs, and describe the time line for a decision. The design, construction and expected performance are set out in detail. When considering performance we pay particular attention to those parameters that are determined by the performance of the Pixel Detector. We describe in detail the design and construction of the Pixel Detector, including the results of measurements of prototype modules and associated support structures and we explain the status of the plans for their mass production. We present details of the decommissioning of the existing tracking detector and the replacement of the inner layers of the ITk Pixel Detector part way through the lifetime of the High Luminosity LHC. Finally, we describe the costing and schedule, including major milestones, to construct the detector.