Observation Of Electroweak Production Of ZZjj With The ATLAS Detector At The LHC

The electroweak symmetry breaking is one of the most fundamental questions in par-ticle physics.In the Standard Model,the electroweak symmetry breaking is described by the Higgs mechanism.In 2012,the Higgs boson was discovered by both the ATLAS experiment[1]and CMS-experiment[2]at the Large Hadron Collider(LHC).With the discovery of the Higgs boson,the Standard Model theory has been completed.But this is not the end of particle physics,many properties of the Higgs boson still remain as mystery.The nature of the electroweak symmetry breaking still need to be understood.Besides direct measurement of the properties of the Higgs boson,the scattering of two vector bosons provides another way to understand the nature of the electroweak symme-try breaking.The presence of the Higgs boson prevents the amplitudes of the scattering from violating unitarity at TeV scale.Therefore,vector boson scattering processes are important for testing the Standard Model and probing the nature of physics beyond the Standard Model.From 2015 to 2018,the second phase of running(Run 2)of the LHC,more and more data are collected by the ATLAS detector.The high collision ener-gies and luminosities of the LHC provide an unprecedented opportunity to study vector boson scattering.At the LHC,the scattering of massive electroweak gauge bosons is accessible through the purely electroweak production of two jets and two gauge bosons(VVjj).Among all the electroweak VVjj processes,the ZZjj process is a clean and competitive process to study scattering of two vector bosons.This thesis presents the first observation of electroweak ZZjj process using proton-proton collision data corresponding to an integrated luminosity of 139 fb-1 recorded at a center-of-mass energy of 13 TeV by the ATLAS detector.To enhance the statis-tics,two final states of the ZZjj process,lllljj and llvvjj,are considered.The lllljj channel has low background,and its fully reconstructed final state provides max-imal information.While the llvvjj channel has relatively large ZZ decay branch ratio.The measured inclusive cross-sections of the lllljj and llvvjj channels are 1.27±0.12(stat)±0.08(syst)fb and 1.22±0.30(stat)±0.18(syst)fb.The predicted in-clusive cross-sections of the lllljj and llvvjj channels are 1.14±0.04(stat)±0.20(syst)fb and 1.07±0.01(stat)±0.12(syst)fb.The measured and predicted inclusive cross-sections are consistent with each other within uncertainties.To extract the electroweak production of the ZZjj process,statistical tests are performed in both the individual lllljj and llvvjj channels and in the combined channel.Combining these two final states,the background only hypothesis is rejected with an observed significance of 5.5? and an expected significance of 4.3 ?.The observation of electroweak ZZjj pro-duction is a new milestone reached in the study of electroweak VVjj production.The observation of this rare process and the cross-section measurements can help us test the Standard Model and understand the nature of electroweak symmetry breaking.