Electroweak Physics at the Large Hadron Collider with the ATLAS Detector: Standard Model Measurement, Supersymmetry Searches, Excesses, and Upgrade Electronics

This thesis presents searches for the production of charginos and neutralinos, supersymmetric partners of the gauge bosons, decaying via W and Z bosons. To understand the dominant background in these searches, WZ, the WZ cross section measurement is also performed. The measurement and searches were performed with both 36.1 fb$^{-1}$ and 139 fb$^{-1}$ of data collected at $\sqrt{s}$ = 13 TeV with the ATLAS detector. The WZ cross section is measured to be $50.6 \pm 3.6$ fb (statistical and systematic), which is consistent with the theoretical prediction calculated at next-to-next-to leading order in QCD. The supersymmetric searches are motivated by two models: Higgsino and wino production. The Higgsino search makes use of low-transverse-momentum leptons and leads to the first result of Higgsino production at a collider experiment since the Large Electron-Positron Collider. The wino production searches discussed in this thesis show a tension in the exclusion limit with one search finding an excess of data above the background prediction and another search targeting the same model and phase space which sees data consistent with the background prediction. The excess is followed up with 139 $^{-1}$ of data and the excess is found to be reduced. The signatures of electroweak SUSY will continue to be very important measurements throughout the life of the LHC but are difficult to perform because they require a lot of luminosity; as a result, these searches will remain interesting during the High-Luminosity LHC (HL-LHC) program, which will deliver 3000 fb $^{-1}$ of data. The increase in radiation during HL-LHC will require a new inner detector. To test the performance of this new detector, the thesis also describes the testing of one of the read-out chips, the HCC130, through calibration tests and irradiation.