Towards the construction of a new tracker for the ATLAS detector at the HL-LHC

Particle tracking is one of the most fundamental aspects of high-energy physics research. By tracking the path of charged particles produced in high-energy collisions, physicists can study the properties of the particles and gain insight into the fundamental forces and interactions that govern the behavior of matter at the smallest scales. One of the most essential components of particle tracking is the interaction of charged particles with matter. When a charged particle passes through matter, it ionizes the atoms and molecules in its path, producing a trail of electrons and ions that can be detected and analyzed. Especially when dealing with high particle rates, semiconductor detectors are commonly used to detect the passage of ionizing particles, generating electrical signals that can be localized with high precision. By analyzing these signals, physicists can reconstruct the path of the particle and determine its properties. The need for high-precision tracking detectors arose in the 1970s with the development of the first colliders and the discovery of short-lived particles. These colliders produced particles with extremely high energies, making it difficult to detect them with traditional detectors. To detect these particles, compact detectors had to be developed and placed as close as possible to the point of interaction. These requirements led to the development of semiconductor detectors. Among semiconductor detectors, silicon detectors have become the most widely used tracking system in high-energy physics experiments. Silicons are ideal for tracking thanks to their low material cost and increasing industrial development in this material processing making them an attractive option for large-scale experiments. In particular, modern techniques have allowed the fabrication of pixel detectors which allow 3D spatial resolutions of the order of 10 μm. For these reasons, silicon pixel detectors are a standard choice in tracking systems in high-energy physics experiments. Pixel detectors have played a crucial role in the advancement of high-energy physics research, and the ATLAS and CMS experiments at the Large Hadron Collider (LHC) are excellent examples of this application. The LHC is the world’s largest and most powerful particle accelerator. Located at the European Organization for Nuclear Research (CERN) in Switzerland, it is designed to accelerate protons and heavy ions to nearly the speed of light and collide them at four experiment sites. The LHC is the culmination of decades of research and development, and its construction and operation represent an enormous technological and scientific achievement. The discovery of the Higgs boson in 2012, a missing piece in the Standard Model particle puzzle, was made possible also thanks to the high-precision tracking capabilities of pixel detectors. However, with an upgrade of the LHC planned, the High-Luminosity LHC, the ATLAS detector needs to be updated to cope with the increase in luminosity, improve the sensitivity of analyses already carried out, and widen the possibility of discoveries. As part of this upgrade, a new all-silicon detector, ITk, will replace the current ATLAS tracking system during the Long Shutdown 3 (2026-2028). The ITk detector will consist of a Pixel detector at a small radius and a large area Strip detector surrounding it. The main challenge facing the ITk detector will be the harder radiation environment, where both the level of radiation in the detector and the number of particles per square centimeter will increase by about a factor of seven. To address the radiation constraints, innovative high radiation-hard 3D pixel detectors will be used for the innermost layer of the ITk Pixel detector. The 3D sensor technology was used for the first time in HEP detectors in 2014 for the upgrade of the current ATLAS Pixel detector: a single layer (IBL) was partially equipped with 3D sensors for a surface of just 375 cm2. However, this first application paved the way for using 3D technology in future LHC detector upgrades. After several years of R&D, in 2020 ATLAS decided to use the 3D technology as baseline for the innermost pixel layer, with a cell size of 25x100 μm2 in the barrel and 50x50 μm2 in the forward region. Despite the decision to use 3D technology in the ITk detector based on first proto- type samples, there was the recommendation to continue with more prototypes and in pre-production the validation of the radiation hardness up to the ultimate fluence of 2 · 1016neq/cm2 and ionizing dose of 10 MGy. This Ph.D. thesis has been developed primarily inside the ITk Pixel project, focus- ing on two main specific aspects: the full qualification to the ultimate fluence of the 3D devices and the qualification of the local supports for the detector forward region. More specifically: Chapter 1 provides an overview of the LHC accelerator, including the machine performance in Run-2. Additionally, this chapter discusses the High Luminosity LHC pro gram, which aims to upgrade the LHC to increase by almost one order of magnitude its collision rate luminosity. The ATLAS experiment is described in detail in Chapter 2, including the detector configuration for Run-3, and the various subsystems that make up the detector. This chapter also discusses the physics goals of the ATLAS experiment, including the Higgs physics, W and Z physics, and SuperSymmetry searches. Chapter 3 provides an introduction to silicon detectors for high-energy physics, including a discussion of particles’ interactions with matter and the pn-junction. This chapter also explores the use of silicon detectors in particle physics experiments. Chapter 4 focuses on the ATLAS Inner Tracker for Phase II, which is part of the ATLAS upgrade program. This chapter also provides an overview of the Calorimeter, Muon Spectrometer, and Trigger and Data Acquisition systems upgrades. Additionally, this chapter discusses the ITk layout and its expected tracking performance. Chapter 5 explores the ITk 3D module qualification, both in Genova Laboratories and during the beam test campaigns carried out at Deutsches Elektronen-Synchrotron (DESY) and CERN. In the Genova laboratory standard measurements such as sensor IV and electronics tuning and characterization are performed, while the main results from the test beam campaigns are the detector efficiencies measurements. In particular, prototype modules, so-called RD53A, were tested in 2020 with an electron beam at DESY, obtaining an efficiency higher than 97% with both unirradiated and irradiated modules. In 2022 pre-production ITk 3D modules were tested with a protons beam at Proton Synchrotron (PS) at CERN, and nearly 99% efficiency was achieved with unirradiated modules, while at Super Proton Synchrotron (SPS) always at CERN, after irradiation the pre-production modules had shown the efficiency of 98,5% (99,9%) with modules placed perpendicularly (titled of 15 degrees) with respect to the beam. Therefore it is shown how the sensors meet the ITk requirement to have a mean efficiency higher than 96% (97%) in the perpendicular (tilted) configuration after irradiation. Chapter 6 focuses on various aspects of the ITk Outer Endcap Local Support structures. The section includes an overview of the Outer Endcap, details on the local support structures, and their manufacturing and assembly process. It also covers studies on foam density, metrology, and thermal properties, as well as information on the ITk Production Database, used to store all the results of the test done during the ITk detector construction. Finally, this section discusses the impact of detector active components misalignment on tracking performances, including the alignment procedure and misalignment studies with the ITk layout.