UC Santa Cruz

This thesis presents a search for $WW$ and $WZ$ resonances using data from $pp$ collisions at $\sqrt{s}=13$ TeV using the ATLAS detector, corresponding to an integrated luminosity of 139 fb$^{-1}$. Diboson resonances are predicted in a number of Standard Model (SM) extensions, such as Extended Gauge Models, and extra dimensional models. This search looks for resonances where one $W$ boson decays leptonically and the other $W$ or $Z$ boson decays hadronically. This search is sensitive to diboson resonance production via vector-boson fusion (VBF), quark-antiquark annihilation (DY), and gluon-gluon fusion (ggF) mechanisms. No significant excess of events is observed with respect to the SM backgrounds, and constraints on the masses of new $W'$, $Z'$, and bulk-RS Gravitons are extended up to 3.7 TeV, depending on the model. As the dominant backgrounds in this search contain gluon-initiated jets, classifying jets as quark-initiated or gluon-initiated increases the sensitivity of this analysis to new physics. Towards this end, this thesis provides a calibrated quark-gluon tagger based on the multiplicity of charged particles within a jet.