Development of calibration techniques and performance analysis of the CMS Inner Tracker for the High Luminosity phase of LHC

The Large Hadron Collider (LHC) is the world's largest and highest-energy particle ac- celerator. In 2029 LHC will enter a new era, named High Luminosity LHC, or HL-LHC. Dur- ing this new phase, the machine will be able to provide an enormous amount of data that will give access to many physics opportunities. However, this comes with much harsher exper- imental conditions: a significant increase in the number of simultaneous collisions during a sin- gle bunch crossing and in the radiation to which the detector parts will be exposed. In order to fully exploit the physics potential during the HL-LHC era, the Compact Muon Solenoid (CMS) will undergo major upgrades, known as CMS Phase–II Upgrades. In particular, a complete new tracking system is being developed and will be built. The main purpose of my thesis work is to conceive calibration techniques and to analyze the performance of silicon pixel sensor modules for the CMS Phase–II Inner Tracker (IT), the cru- cial tracker subsystem sitting in proximity of the interaction point. One of the most important pa- rameter for the IT operation is the threshold of each pixel cell. Only particle signals above this value will be actually recorded. The threshold needs to be carefully optimized to maximize ef- ficiency and performance in general while keep- ing spurious effects (e.g. noise) under control. The threshold can be calibrated by acting on the relevant registers in the readout electronics and this is done by the DAQ system within a dedicated procedure. In the context of my work, I demonstrated that one of those registers, called "Fine Delay", is of paramount importance for the tuning procedure. In particular, if its value is set to the so-called optimal value, the noise and overall device performance improves. The Optimal Fine Delay can be found by running a ded- icated procedure that I designed, implemented and tested. Moreover, within the scope of this work, a spurious instrumental effect, i.e. a time– dependent, periodic variation of the threshold, has been observed. A detailed study of this effect is essential, given the importance of the threshold for a tracking system. In particular, I thoroughly studied the threshold oscillation phenomena in both sampling modes, the Synchronous Mode and the Asynchronous Mode. It turns out that, in the former mode, a threshold oscillation is to be expected, being intrinsically related on the working principle of the front-end stage of the readout electronics. On the other hand, the oscillation observed in Asynchronous Mode is unexpected, and it can be explained only taking into account an os- cillation of the comparator voltage in the read- out chip front end. CMS will operate in Synchronous mode. Within my thesis work, I have been able to demonstrate that threshold oscillations in Syn- chronous mode will not affect the detector per- formance. This is a crucial finding with respect to the accurate knowledge of the device that sev- eral hundreds of physicists will rely on for data analyses. To conclude, the core of my thesis work has been the deep comprehension of the tool (i.e. the detector module of the CMS Inner Tracker) and then, thanks to this, the improve- ment and optimization of its performance. This is a mandatory step to exploit the CMS Inner Tracker at HL-LHC, an incredibly powerful measurement instrument. This process has al- lowed me to learn as a student and to grow as a physicist.