Search for Vector Boson Scattering in semi-leptonic final states with the ATLAS detector. Contribution to the Inner Tracker Upgrade in view of the High-Luminosity LHC

In this thesis the Vector Boson Scat- tering (VBS) process in semileptonic final states is studied. In semileptonic VBS final states, one of the two bosons decays leptonically while the se- cond hadronically into a pair of quarks. Due to the very low production cross section, VBS is a very challenging channel to measure. What makes this process distinguishable with respect to other processes in LHC, is the two forward jets that ac- company the boson scattering. During my thesis I worked in many aspects of the analysis such as ; the background modelling, the MVA discriminant, the systematic uncertainties considered in the measu- rement, as well as, the statistical analysis of the results. Forward jet performance is of paramount im- portance in this analysis. Therefore, the thesis work starts with a detailed study of optimizing the jet re- construction, in particular developing an algorithm to mitigate the pileup jets in the forward region ; in ATLAS, in order to make use of valid physics data , in any analysis, the physical process (na- mely hard-scatter process) has to be distinguished from secondary collision processes (namely pile-up interactions). This is mainly achieved by the use of tracking information. The forward part of the ATLAS detector, lacking this information is thus highly challenging. An alternative way of tagging in the forward region, based on momentum conser- vation between forward activity and activity hap- pening within the tracking coverage, is being em- ployed in this algorithm. The algorithm has been optimized and various methods have been tested. The actual ATLAS Inner Tracker is compatible with the LHC design luminosity of 10$^{34}$ cm$^{-2}$ s${-1}$. The foreseen increase of luminosity towards the High Luminosity (HL) LHC phase requires a fun- damental re-design of the complete inner detector due to both, increased radiation damage, and sub- stantial occupancy of the sub- detectors. For the HL-LHC, the current inner detector of ATLAS will be replaced by an all-silicon Inner Tracker (ITk). The ITk pixel detector design features a much hi- gher active area and granularity with respect to the current pixel detector. Two different silicon ba- sed detector technologies are considered ; planar- pixel sensors and 3D-pixel sensors. The sensors are equipped with a new readout chip, able to meet all the requirements needed for the high lumino- sity LHC. A prototype version of the readout chip, called RD53A, was designed by the RD53A colla- boration. In the third part of this thesis a characte- rization of the RD53A readout chip is performed. The much larger number of modules and the much finer granularity used by ITk, results in a si- gnificant increase in the power density in the detec- tor. For this reason a serial powering (SP) scheme has been chosen. In this scheme, the readout chips of the pixel modules are powered in series by a constant current ; while the sensors of several mo- dules will be connected to a common supply line for the depletion voltage. This architecture results in an effective forward bias on some sensors under certain operating conditions. Although the forward bias will be small, it can still lead to non negligible currents between the sensor backside and the rea- dout chip, in particular for irradiated sensors with large saturation currents. Therefore, the behavior of such a serial powering chain is also studied as part of this thesis.