Commissioning and Calibration of the Upgraded Inner Tracking System of ALICE at the LHC

ALICE (A Large Ion Collider Experiment) is one of the four main experiments at the CERN Large Hadron Collider (LHC). Its primary goal is to study the Quark-Gluon Plasma (QGP), a state of matter that existed shortly after the Big Bang which can be recreated in Pb--Pb collisions at the LHC, at center-of-mass energies up to √s_{NN} = 5.36 TeV. In addition, ALICE investigates a wide range of Quantum Chromodynamics (QCD) phenomena, like for example strangeness production in proton--proton collisions at center-of-mass energies up to √s = 13.6 TeV. The ALICE experiment consists of multiple subdetectors that form a high-granularity system capable of tracking particles with transverse momentum down to about 0.15 GeV/c, identifying particles up to 20 GeV/c, and handling high particle densities. During the LHC Long Shutdown 2 (LS2, 2019–2022), ALICE underwent a significant upgrade to enhance its performance for LHC Run 3. The innermost detector, called Inner Tracking System (ITS), is essential for tracking particles and identifying primary and secondary vertices from collisions. As part of the LS2 upgrade, the ITS was completely replaced, resulting in significant improvements in impact parameter resolution, tracking efficiency, and readout capabilities. The upgraded ITS (ITS2) consists of seven layers of Monolithic Active Pixel Sensors (MAPS) called ALPIDE (ALice PIxel DEtector). These sensors feature a spatial resolution of about 5 μm and a low material budget, with the innermost layers achieving 0.36% X0 per layer. These features, along with the fact that the first detection layer was moved closer to the interaction point, result in a factor 2 improvement in impact parameter resolution at p_T = 1 GeV/c compared to Run 2. The readout capabilities of the ITS2 were also enhanced, allowing continuous data readout at rates of 67 kHz for Pb-Pb collisions and 202 kHz for pp collisions. These improvements allow ALICE to better study the QGP and perform new measurements with larger data samples. Covering a sensitive area of approximately 10 m^2, the ITS2 is the largest-scale application of MAPS in High-Energy Physics to date. To ensure stable operation during data collection, the ITS2 requires rigorous calibration and continuous monitoring of the calibration parameters. The main calibration procedures include the tuning of the pixel thresholds and masking noisy pixels. Additional scans are available for monitoring various aspects of the detector. The work for this thesis involves several studies conducted on the ITS2 during commissioning and operation, with the aim to assess the calibration results and performance of the upgraded detector. In the first part of this work, the calibration and monitoring strategies developed for the ITS2, along with the results obtained from them, are explored. These studies are important to verify the correct operability of the detector over time. The second part of this thesis is dedicated to the study of the possibility of MAPS detectors to perform Particle Identification (PID) by exploiting the Time Over Threshold (ToT) measurement under specific ITS2 configurations. This study is important mostly for the development of future detectors based on MAPS. The third part of this thesis is focused on the measurement of the ITS2 Inner Barrel detection efficiency. The detection efficiency is a key parameter of the ALPIDE sensors. In past test-beam measurements with single chips it was measured to be above 99% both for un-irradiated and irradiated chips. The goal of this part of the work is to estimate the detection efficiency of the current detector and its chips after several years of accumulated radiation from LHC collisions. This study was performed by exploiting the geometry of the ITS2, and in particular the overlaps of the staves along the azimuthal direction. Finally, this thesis presents a measurement of the non-prompt Ω- baryons, aimed at understanding the strangeness enhancement observed in pp collisions at the LHC. The Ω- baryons production in the data by means of the Distance of Closest Approach (DCA) of the baryon to the primary vertex, in order to define the percentage of strange baryons deriving from the heavier quarks feeddown. This measurement was only possible thanks to the enhanced impact parameter resolution and improved readout capabilities of the upgraded ITS2.