Study of $Z\gamma$ Vector Boson Scattering and Upgrade of Liquid Argon Electronic Calibration Board with ATLAS detector

The 20th century witnessed an unprecedented surge in the exploration of the fundamental building blocks of the universe, driven significantly by the rise of quantum physics. This exploration culminated in the development of the Standard Model (SM), a comprehensive theoretical framework explaining fundamental particles and their interactions. Over the years, the predictions of the SM have undergone rigorous scrutiny, reaching a historic milestone with the discovery of the Higgs boson in 2012, serving as a robust confirmation of its validity. However, both theoretical arguments and experimental evidence strongly suggest its limitations as the ultimate theory governing particle physics.

Consequently, experiments at particle accelerators, such as the ATLAS detector at the Large Hadron Collider (LHC), gain paramount importance. They play a crucial role in testing the SM predictions and potentially uncovering indications of new particles or interactions beyond its established framework.

One particularly effective approach for assessing the SM involves examining the electroweak production of a vector boson pair associated with two jets. This method, notable for probing vector boson scattering (VBS) processes, provides direct insights into the electroweak (EW) sector of the SM. Accordingly, this thesis focuses on examining the electroweak production of a Z boson and photon accompanied by two jets ($EW- Z\gamma jj$). This study is important since it can access quartic gauge couplings $W W Z\gamma$, $ZZZ\gamma$, $ZZ\gamma\gamma$ and $Z\gamma\gamma\gamma$. The last three couplings are explicitly prohibited at the lowest order in the SM. Any deviation from the SM predictions for these couplings could signify hints of new physics beyond the SM.

However, observing the EW-$Z\gammajj$ process at the LHC poses significant challenges due to its low production rate and presence of substantial background events. Evidence of the process was found by both ATLAS and CMS detectors using proton-proton collision data collected in 2015 and 2016. Subsequently, ATLAS and CMS observed it using data collected from 2015 to 2018. This thesis details the analytical approach that led to the discovery of the EW-$Z\gammajj$ process, utilising data recorded by the ATLAS detector from 2015 to 2018. Additionally, the thesis includes differential cross-section measurements for the EW-$Z\gammajj$ process, which is pivotal for future Effective Field Theory (EFT) studies and refining the existing Monte Carlo description.

Furthermore, in anticipation of increased radiation doses at the upcoming High Luminosity Large Hadron Collider (HL-LHC), various components of the ATLAS detector require upgrades. Consequently, this thesis also focuses on upgrades of the electronic calibration boards for the ATLAS liquid argon electromagnetic calorimeter. This critical undertaking aims to ensure a
good detector resolution, vital for achieving precise measurements of the energies of electrons, photons, and hadrons with the ATLAS detector. The findings from these investigations will play a pivotal role in both the design and testing of the final calibration board for the HL-LHC.