Search for New Resonances and Dark Matter Particles with BSM Higgs Boson Productions at the LHC

This dissertation presents searches for dark matter particles and new resonances at the LHC with the ATLAS experiment. The data used in these analyses are produced in proton-proton collisions at a center-of-mass energy of 13 TeV and are collected by the ATLAS experiment, with a total integrated luminosity of 36.1 fb-1. The experimental signature of new physics in this work involves large missing transverse energy (MET) plus a di-lepton pair from a Z boson decay. In the dark matter search program, it is hypothesized that the Higgs boson may couple to invisible particles who serve as dark matter candidates. The Higgs boson is produced in association with a Z boson, which is used as a `tag' particle through its charged lepton (ee or mm) decays. The dark matter signature is large MET since the dark matter particles, just like the neutrinos, escape from the detector (invisible). The challenge of searching for dark matter is to understand sources of MET produced in proton-proton collisions and design an efficient event selection criterion to separate the signal from the backgrounds. Selected data is compared with the expected background and the dark matter signal, assuming the Standard Model (SM) ZH production cross section. Data is found to be consistent with the background expectation. An upper limit on the branching fraction of the Higgs boson decaying to invisible particles of 67% is set at 95% confidence level (CL). The corresponding limits on the production cross-section () of beyond the SM Zh process with the invisible Higgs boson decays are also presented in a mass range of 110 to 400 GeV. The two-Higgs-doublet model and the electroweak singlet model are motivated theoretical models, which predict additional Higgs bosons near the TeV scale that may decay to pairs of Z bosons. The search for these non-SM Higgs bosons are also conducted with di-lepton plus MET final states, but by considering the decay mode ZZ -->llvv (v = e, or mu). The transverse mass spectrum of the ll+MET system is used as the discriminant to search for bumps created by new resonances. Comparing data with the expected signal and background, no evidence is found for new resonance production in the mass range between 300 GeV to 1500 GeV. Therefore, the limits on new resonance production cross-section times a branching ratio are set. This result is also interpreted as a search for a spin-2 Kaluza-Klein graviton excitation, GKK, in the context of the bulk Randall-Sundrum model via GKK --> ZZ --> llvv process. An upper limit on cross section times BR(GKK --> ZZ) is set as a function of the KK graviton mass between 600 GeV and 2 TeV.