Upgrade of the ATLAS tracking detector in preparation for the High-Luminosity phase of the LHC : Planar pixel module characterisation and calibration for the current and future ATLAS inner tracker

The LHC reached its original design in 2015 delivering center of mass energies of 13 TeV of proton collisions with instantaneous luminosity 1x10^34 cm^2s^-1. To maintain scientific progress and exploit the machine’s full capacity, the LHC plans to upgrade the luminosity about 7 times higher than its design value transforming the LHC into the High-Luminosity LHC (HL-LHC). In preparation for the HL-LHC phase in 2026 the experiments will have to be upgraded to maintain the same performance with the increase of the luminosity. The thesis focuses on the upgrade of the ATLAS pixel detector into an all-silicon Inner Tracker (ITk). The ITk will cover a high range of pseudorapidity 2.4 < |η| < 4. The ITk pixel sensors utilize the n-in-p technology assembled with the RD53A readout chip. The pixel modules will host 100 -150 mm thick sensors with a pixel geometry of 50x50 mm^2 and 25x100 mm^2. The performance of the ITk prototype pixel module is characterised with a particle beam to study the hit efficiency and position resolution of the sensor. To comply with ITk requirements the pixel module must achieve hit efficiency of at least 98% before irradiation and 97% after irradiation up to fluence of f = 5x10^15 n_eq/cm^2. Pixel modules that agree with these requirements are qualified for the production of the ITk detector. The investigated modules produced from FBK foundry show efficiency performance in well agreement with the ITk requirements for the pixel detector to operate in the HL-LHC phase of the LHC. The foundries participating in the production of the ITk pixel detector are qualified based on the performance of the sensors characterised in the Market Survey testbeam campaign. The methodology of characterising a pixel module with a particle beam and analyzing the results is discussed in detail to guide the reader to the concluded results of qualified modules for the ITk pixel detector. In parallel I present simulated performance of the ITk pixel sensors with the RD53A readout chip in the testbeam setup to study the impacts of multiple scattering in the setup. Results of residuals from simulated tracks are compared with testbeam data to show the compatibility between the simulation and the testbeam setup for the ITk pixel module. The results show the effects of multiple scattering on the performance of the pixel module. The simulation setup is prepared in allpix-squared framework and could be used by other users to simulate and predict the performance of the future ITk pixel modules. The work for this thesis also branched to investigate the calibration of the FE-I4B readout chip for the current IBL detector. It was found that a discrepancy of charge collection in the IBL module rises from a local threshold asymmetry when tuning the threshold of the FE-I4B chip using the charge injection circuit. The investigation led to the introduction of the novel threshold baseline tuning method to solve this local threshold asymmetry in the FE-I4B chip. The threshold baseline tuning method depends solely on counting hits and thus tunes the local threshold without using the charge injection circuit. The new tuning method indeed proves effective and therefore having a symmetrical charge collection throughout the IBL module. As the tuning algorithm is now implemented in the ATLAS pixel DAQ system, we are able to tune one module but the algorithm must be further improved to tune to complete detector. It is one step forward towards achieving higher quality of data for Run3.