Testing and characterization of the Inner Tracker pixel modules for the CMS upgrade at the HL‑LHC: from laboratory measurements to projected detector performance

From 2030, an upgraded Large Hadron Collider (HL-LHC) at CERN shall resume operations at a higher instantaneous luminosity, ranging between 5-7.5×10³⁴/cm²/s, enabling the CMS experiment to record, by 2040, an integrated luminosity of at least 3000/fb. To fully exploit this upgrade's potential for physics research, however, experimental challenges need to be addressed, such as the increased average pileup, ranging between 140–200 pp collisions per bunch crossing, and the unprecedented radiation damage. To this end, CMS is upgrading its detector. One of the main changes to CMS for the High Luminosity is a new Inner Tracker, replacing the Pixel system in use at the end of the LHC Run 3. Like its predecessor, the Inner Tracker is made out of hybrid silicon pixel modules, but features six times more granular segmentation with 25×100 µm² pixels, and a radiation tolerance allowing it to operate throughout the entire high luminosity program, corresponding to a maximum total ionizing dose of 1.2 Grad and a maximum 1 MeV neutron equivalent fluence of 2.3×10¹⁶/cm². Other key features of this detector are the capability of detecting signals collected in the silicon sensors down to a threshold of 1000 electrons, and the support for high hit-rate and trigger latency, up to 3.5 GHz/cm² and 12.5 µs respectively, while maintaining a high (> 98 %) single-hit efficiency. In this thesis, laboratory measurements on Inner Tracker hardware are presented, as well as a performance study for the same detector, based on Monte Carlo simulations of the CMS experiment. Concerning the laboratory measurements, two topics are discussed: the calibrations run as part of the production testing at wafer level of the readout chip of the Inner Tracker (the CROC), targeting its analog blocks (DACs, monitoring ADC and temperature sensors); and the use of X-rays on prototype modules for the absolute calibration of their threshold, and for the measurement of their hit detection efficiency at hit rates comparable with those expected at the HL-LHC. The performance study, instead, focuses on the track reconstruction inside jets with large transverse momentum, where the track density grows so large, that its detrimental effects on the tracking had to be addressed in the past via specialized workflows. The extension of these tools to the Inner Tracker is presented in this thesis, as well as some preliminary results from their application to simulated events with the upgraded detector.