This thesis presents my work within the ATLAS experiment, covering both the calibration of the present Pixel Detector and the qualification of new pixel modules for the future Inner Tracker (ITk). The work can be addressed in two main aspects of tracking-detector performance: the stability of charge measurements in the current Pixel Detector during Run 2 and Run 3, and the preparation of radiation-hard pixel modules for operation at the High-Luminosity LHC. A major part of the thesis is devoted to the calibration and equalisation of the specific ionisation energy loss, dE/dx, measured with the ATLAS Pixel Detector. This observable is a key input to searches for heavy long-lived charged particles, which can release anomalously large amounts of charge in the silicon sensors. Since the collected charge depends on the detector region and evolves with radiation damage, I developed and validated, for the first time in ATLAS, a local cluster-level dE/dx equalisation procedure for the Run 2 and Run 3 datasets. The method corrects the response of different detector regions, defined by layer and module position, before computing the track-level dE/dx with a truncated mean. This is essential to prevent the truncation from preferentially rejecting clusters from less irradiated regions, such as outer layers or high-pseudorapidity modules, which would otherwise bias the track ionisation measurement. The resulting calibration improves the stability of the dE/dx response as a function of integrated luminosity, detector region, pseudorapidity, and track kinematics, enhancing the robustness of dE/dx-based mass reconstruction in long-lived particle searches. The second part of my work concerns the performance validation of 3D pixel sensors for the ITk Pixel Detector. I contributed to the characterisation of pre-production and early-production sensors before and after irradiation, using laboratory measurements, irradiation campaigns, and beam tests. These studies evaluate leakage current, noise, charge collection, hit efficiency, and radiation tolerance, showing that the tested devices can satisfy the ITk performance requirements under realistic operating conditions. I also investigated charge collection in 3D sensors read out with the ITkPix v2 front-end chip, studying the response of final ITk-compatible devices across the pixel cell. Finally, I contributed to the qualification of triplet modules for the ITk innermost layer at INFN Genova. This work included the development of the R05 triplet assembly procedure, component alignment, gluing, metrology, wire-bonding validation, electrical checks, shunt-LDO verification, and bump-bond integrity studies. Beam-test and irradiation results show that the triplet assembly and readout chain preserve the intrinsic sensor performance under conditions relevant for ITk operation. Overall, this thesis connects detector calibration, sensor characterisation, module assembly, and physics-analysis performance, contributing both to the exploitation of the present ATLAS Pixel Detector and to the construction of its replacement for the HL-LHC era.
The ATLAS tracking from the Pixel Detector dE/dx calibration to the construction of 3D pixel modules for the ITk Pixel Detector
RAVERA, SIMONE
2026-09-28
Abstract
This thesis presents my work within the ATLAS experiment, covering both the calibration of the present Pixel Detector and the qualification of new pixel modules for the future Inner Tracker (ITk). The work can be addressed in two main aspects of tracking-detector performance: the stability of charge measurements in the current Pixel Detector during Run 2 and Run 3, and the preparation of radiation-hard pixel modules for operation at the High-Luminosity LHC. A major part of the thesis is devoted to the calibration and equalisation of the specific ionisation energy loss, dE/dx, measured with the ATLAS Pixel Detector. This observable is a key input to searches for heavy long-lived charged particles, which can release anomalously large amounts of charge in the silicon sensors. Since the collected charge depends on the detector region and evolves with radiation damage, I developed and validated, for the first time in ATLAS, a local cluster-level dE/dx equalisation procedure for the Run 2 and Run 3 datasets. The method corrects the response of different detector regions, defined by layer and module position, before computing the track-level dE/dx with a truncated mean. This is essential to prevent the truncation from preferentially rejecting clusters from less irradiated regions, such as outer layers or high-pseudorapidity modules, which would otherwise bias the track ionisation measurement. The resulting calibration improves the stability of the dE/dx response as a function of integrated luminosity, detector region, pseudorapidity, and track kinematics, enhancing the robustness of dE/dx-based mass reconstruction in long-lived particle searches. The second part of my work concerns the performance validation of 3D pixel sensors for the ITk Pixel Detector. I contributed to the characterisation of pre-production and early-production sensors before and after irradiation, using laboratory measurements, irradiation campaigns, and beam tests. These studies evaluate leakage current, noise, charge collection, hit efficiency, and radiation tolerance, showing that the tested devices can satisfy the ITk performance requirements under realistic operating conditions. I also investigated charge collection in 3D sensors read out with the ITkPix v2 front-end chip, studying the response of final ITk-compatible devices across the pixel cell. Finally, I contributed to the qualification of triplet modules for the ITk innermost layer at INFN Genova. This work included the development of the R05 triplet assembly procedure, component alignment, gluing, metrology, wire-bonding validation, electrical checks, shunt-LDO verification, and bump-bond integrity studies. Beam-test and irradiation results show that the triplet assembly and readout chain preserve the intrinsic sensor performance under conditions relevant for ITk operation. Overall, this thesis connects detector calibration, sensor characterisation, module assembly, and physics-analysis performance, contributing both to the exploitation of the present ATLAS Pixel Detector and to the construction of its replacement for the HL-LHC era.
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