Status of the ATLAS ITk Inner System Upgrade for the HL-LHC

The Large Hadron Collider (LHC) at CERN is the world’s largest particle accelerator, producing rare particles, such as the Higgs boson, by colliding bunches of protons near the speed of light. While the Higgs was discovered in 2012, little was learned about the particle’s properties. This has motivated an upgrade of the LHC to the High Luminosity LHC (HL-LHC), increasing the integrated dataset by a factor of 10. The ATLAS Detector surrounding the interaction point must be upgraded to withstand the more intense collision environment. SLAC has been working in conjunction with other laboratories around the world to construct the two innermost layers of the silicon pixel Inner Tracker (ITk). The Inner Tracker is essential for particle’s trajectory reconstruction, revealing properties such as charge and momentum.

I will present an overview of SLAC’s contribution to the HL-LHC, including the three main phases of assembly: reception, loading, and integration. I will detail what has been accomplished so far in the pre-production phase, as well as the hurdles in quality assurance and quality control yet to be overcome. SLAC has already achieved major milestones, including conducting delamination tests on pre-production modules, revealing the potential lifetime of a module in the detector. Loaded Local Support (LLS) structures act as the building blocks of the Inner Tracker, and successful assembly and testing were demonstrated in March 2025.