Prospects of Finding New Fundamental Physics and Upgrade Studies with the ATLAS detector at the LHC

This thesis presents the work done to develop and qualify the Pixel Data Transmission Chain of the Inner Tracker (ITk) of the ATLAS experiment. To meet High Luminosity - Large Hadron Collider (HL-LHC) conditions, the ATLAS Inner Detector will adopt an all-silicon tracker with high-speed, radiation-resistant data transmission, of which a key stage is the electrical-to-optical component called optoboard. The optoboard is the fun- damental component of the optosystem, which is responsible for the recovery, aggregation and electrical-optical conversion of all ITk Pixel data links. This thesis describes the de- sign of the optosystem and the key components of the ITk Pixel data transmission chain, namely, the GBCR which restores the data that is attenuated after electrical transmis- sion from the modules, the lpGBT which multiplexes and de-multiplexes the outgoing and incoming signal and the VTRx+ which is responsible for the electrical-to-optical conversion using a 850nm laser diode. Functionality tests were performed to validate the design of the optoboard and to analyse and qualify the signal over the electrical part of the ITk Pixel data transmission chain to ensure that the the transmitted data is not distorted or lost and the required specifications fulfilled. Overall, the ITk Pixel electrical data transmission chain from the pixel modules to the optoboard is able to satisfy the requirements of BER < 10^-12 for a signal loss < 20 dB with lowest observed total jitter of ~ 110 ps. Furthermore, the Optosystem and the ITk Pixel Data Transmission Chain has successfully passed the final design review and is scheduled to be installed in ATLAS in the next long shutdown in 2026-2028. This thesis also documents the results of the analysis which searches for 3rd generation squarks with final states consisting of top and bottom quarks and missing transverse energy (EmissT) using the ATLAS detector at the T Large Hadron Collider (LHC). The Standard Model (SM) of Particle Physics accurately predicts observed experimental outcomes related to the fundamental constituents of mat- ter. However, there are still many unanswered questions. Supersymmetry (SUSY) is an extension of the SM that aims to fill these gaps by predicting a super partner particle for each particle in the SM. This analysis has been performed earlier using Run 1 and 36fb^-1of Run 2 data. This time a dedicated search is performed using the full LHC Run 2 data of 139fb^-1at sqrt(s) = 13TeV to look for the pair production of 3rd generation squarks decaying asymmetrically into a tb+EmissT signature resulting in 1 lepton, 2 b-jets miss T and ET final state. The analysis was divided into bulk and compressed signal regions based on the mass difference between the top squark and neutralino. The author’s work had been focused on the developing and validating the entire signal grid and defining and analysing the bulk region. In this signal region, this search has placed expected exclusion limits on stop mass upto 980 GeV for N1 mass = 110 GeV assuming m( N1, C1 ) =1 GeV. This search stop/sbottom increases the limit by ~ 150 GeV and 60 GeV when compared to the one lepton region and the combined bb+EmissT and tb+EmissT observed exclusion contours of the previous version of the analysis.