UC Berkeley

This dissertation presents the observation of same-sign WW electroweak production in proton-proton collisions with a center-of-mass energy of 13 TeV at the Large Hadron Collider using the ATLAS detector. The W bosons are required to decay leptonically, giving a signature of two leptons (electrons or muons) with the same electric charge, two jets with a large invariant mass and rapidity separation, and missing transverse energy. The same-sign WW electroweak fiducial cross section is measured using 36.1 fb^{-1} of data recorded in 2015 and 2016. A total of 122 candidate events are observed with a fitted background of 69 +/- 7 events, corresponding to an observed signal significance of 6.5 standard deviations. The measured fiducial cross section is 2.89 +0.51 -0.48 (stat) +0.29 -0.28 (syst) fb and is in agreement with Standard Model predictions.