Search for resonant $WZ$ production in the fully leptonic final state with the ATLAS detector

Diboson resonance searches are an essential test of electroweak symmetry breaking theories beyond the Standard Model (SM) of particle physics. Vector or scalar resonances decaying to dibosons are predicted by various models going beyond the SM, such as Grand Unified theories, Little Higgs models, Composite Higgs models or models with extended Higgs sector (such as SUSYor two-Higgs-doublet models). This thesis presents a search for resonant $WZ$ production in the fully leptonic decay channel $\ell\nu~\ell\ell$ ($\ell =e$ or $\mu$) with two production modes: quark-antiquark fusion or vector-boson fusion. Using 36.1 fb$^{-1}$ of recent data collected by the ATLAS detector in $pp$ collisions delivered by the Large Hadron Collider (LHC) at 13 TeV, constraints are obtained on models going beyond theSM. Since this analysis considers the fully leptonic decays of the vector bosons, events with exactly three leptons in the final state and a substantial missing transverse energy are selected. One signal region is established for each production mode of the resonance, either by quark-antiquark or vector boson fusion. For heavy vectorial resonances produced by quark fusion the $W$ and $Z$ bosons are required to carry a substantial fraction of the resonance energy ($p_{\mathrm{T}}^W/m_{WZ}>0.35$ and $p_{\mathrm{T}}^Z/m_{WZ}>0.35)$). Events with resonances produced by vector boson fusion are characterised by two jets with a large invariant mass and a large separation in pseudorapidity. Therefore, in the search for this production mode, events are required to have at least two jets with an invariant mass ($m_{jj}$) greater than 500~GeV and a separation in pseudorapidity ($\Delta\eta_{jj}$) of at least $|\Delta\eta_{jj}|>3.5$. In each signal region the distribution of the invariant mass of the $WZ$ system will then be examined to determine the presence or absence of new resonances that manifest themselves as localised excesses in the invariant mass of the diboson system ($m_{WZ}$). No significant excess was observed in the signal regions. Limits have then been set on the cross section times branching ratio for a heavy vector resonance produced by either quark-antiquark or vector boson fusion. Additionally, limits on the coupling parameters and masses are obtained for a charged Higgs boson in the Georgi-Machacek produced by vector boson fusion.