Studies Of Electroweak Symmetry Breaking In ZZ Production In Purely Leptonic Decay With ATLAS Detector

This dissertation presents my research in the field of Particle Physics with the ATLAS experiment at the Large Hadron Collider(LHC).The LHC is the largest and most powerful collider in the world,and it is the proton-proton collider with the centre-of-mass energy currently at 13 TeV.With its unprecedented energy,the LHC was built as a bridge between the theories and the experiment.The ATLAS experiment is a general-purpose particle detector experiment with the largest volume at the LHC.The experiment was designed to take advantage of the extramely high energy and luminosity available at the LHC and observe phenomena that involve highly massive particles which were not observable using earlier lower-energy accelerators.And it was one of the two LHC experiments involved in the discovery of the Higgs boson in July 2012.This dissertation focus on the studies with two Z bosons production decaying into lll'l' final state,where l stands for electron or muon,using 139 fb-1 of 13 TeV proton-proton(pp)collision data collected by ATLAS experiment at the LHC.With small amount of backgrounds and very high signal-to-background ratio,the ZZ production in lll'l'channel provides a most clean and sensitive tool to test the Standard Model(SM)at the energy frontier and to study the Higgs physics.First of all,studies including the measure-ment on SM ZZjj production cross section and the observation of Vector Boson Scat-tering(VBS)process.The fiducial cross section for SM ZZjj production is measured to be ?ZZjj tot=1·27±0.14[f b],where 9%out of 11%uncertainty is from data statistic,and found to be compatible with the SM prediction of 1.14 ± 0.20[f b].The electroweak production of two jets in association with a Z-boson pair(EW-ZZjj)is observed with a significant deviation from the background-only hypothesis corresponding to a statistical significance of 5.5 ?.Following with the observation,the prospect study for the EW-ZZjj production at the High luminosity LHC(HL-LHC)using 3000 fb-1 simulated pp collision data at a centre-of-mass energy of 14 TeV is presented,with a expected signifi-cance of around 7 ?.Meanwhile,a search for heavy resonances decaying into a pair of Z bosons to lll'l'final state is also conducted in this dissertation.Different mass ranges for the hypothet-ical resonances are considered,depending on the signal models and spanning between 200 GeV and 2000 GeV.Data is found to agree with a background-only hypothesis,thus,the results are interpreted as upper limits on production cross section for sevaral differ-ent models,including heavy Higgs like(spin-0)narrow-width approximation(NWA)and large-width approximation(LWA),as well as the Randall-Sundrum model with a gravi-ton excitation spin-2 resonance(RSG).The signal is assumed to generate dominatly via gluon-gluon Fusion(ggF)production mode and Vector Boson Fusion(VBF)production mode.Both ggF and VBF channels are studied in NWA,while for LWA,only ggF chan-nel is studied due to worse resolution in higher mass region and the lack of statistic for VBF process.In addition,mass of RS Graviton is constrained,m(GKK)<1500 GeV is excluded at 95%CL by ZZ?lll'l' analysis.The ZZ to lll'l' production presented in this dissertation are consistent with the SM prediction.This result completes the observation of weak boson scattering that is a new milestone in the study of electroweak symmetry breaking.In the meantime,no indication of new physics is observed.We are looking forward to the HL-LHC,with greatly increased luminosity and higher centre-of-mass energy,which should enhance the sensitivity for new physics search and precise measurement for rare process.