Observation Of EW Vector-Boson Scattering And Search For New Resonance In ZZ Events With The ATLAS Experiment

In 2012,the Higgs boson was discovered by ATLAS and CMS experiments at the Large Hadron Collider(LHC),which revealed the mystery of the origin of elementary particle mass and laid the most critical cornerstone for the Standard Model(SM)of particle physics.It is an important milestone in particle physics research.However,the spontaneous symmetry breaking and the Higgs mechanism are not the whole story,since mass bosons have self-interactions.For the vector boson scattering(VBS)process,the scattering amplitude is proportional to the square of the energy of the center of mass.In the case of high energy,the strong interaction of W or Z will causes the scattering amplitude to diverge.Therefore,the SM Higgs boson is introduced in the electroweak(EW)interaction theory to ensure the unity of the vector boson scattering process.Studies of VBS processes are important to study the mechanism of electroweak symmetry breaking.These processes are rare in LHC experiments and are very challenging research topics.At the LHC,the large amount of data collected during the Run2 period provides a supreme opportunity to study the VBS processes.Both ATLAS and CMS experiments have claimed observations of electroweak WW,WZ and ZZ productions since 2018,with the observed significance of more than 5 σ for each process.The first study shown in this dissertation focuses on the observation of the EW ZZ vector boson scattering process associated with two jets production at the LHC.The study is conducted by searching for purely EW production of two Z bosons associated with two forward/backward jets(denote as jj).Two decay channels,one with four charged leptons(l+l-l+l-jj)and the other with two charged leptons and two neutrinos(l+l-vvjj),are analyzed.The l+l-l+l-jj channel is clean with minor fake lepton background contaminations,and provides the most of sensitivity.The l+l-vvj j channel has a larger branching ratio and could complement the l+l-l+l-jj channel,but the background is six times larger compared to the four charged lepton channel.In addition to the irreducible SM ZZ background,the major contaminations in the signal region are from WZ,non-resonant l+lprocess and Z+jets,which are estimated using data-driven methods.The challenge of the analyses is to extract the EW VBS events from the large strong production of ZZjj events.Multivariate variables are trained to enhance the separation between the EW ZZjj signal and the QCD ZZjj component.The analysis has resulted in the first observation of EW ZZjj production with an observed significance of 5.7 σ over the background-only hypothesis.The inclusive ZZjj production cross sections are measured to be 1.27±0.12(stat)±0.08(syst)fb and 1.13±0.28(stat)±0.18(syst)fb,forl+l-l+l-jj and l+l-vvjj channels in the corresponding fiducial phase space,respectively.The measured cross sections are found to be consistent with the SM predictions of 1.16±0.04(stat)±0.20(theo)fb and 1.07±0.01(stat)±0.12(theo)fb for two channels,respectively.The observation of EW ZZjj production is an important milestone in studies of the EW VVjj(V=W,Z)productions at the LHC.The second analysis presented is the search for new heavy resonances decaying into a pair of Z bosons,with one Z decays to charged lepton pair and another Z decays to a neutrino pair(a l+l-vv final state).Such new resonances are predicted by theoretical models beyond the Standard Model,including the two-Higgs-doublet model and the bulk Randall-Sundrum graviton model.In this analysis different heavy particle masses ranging from 300 GeV to 2000 GeV for the hypothetical resonances are considered.The background contaminations are similar to the backgrounds in the l+l-vvj j channel of the EW ZZjj measurement.The results are interpreted as upper limits at 95%confidence level on the production cross section of a spin-0 or spin-2 resonance using the CLS method.The observed upper limits range from 305(400)fb for a mass of mH300 GeV to 4-5(3-4)fb for signal masses above 1.4 TeV in the gluon-gluon fusion(Vector Boson Fusion)production with the narrow width approximation.For a heavy Higgs boson with a large width only the gluon-gluon fusion(ggF)production mode is considered.Here,the observed limits range from 260 fb for a heavy Higgs boson with a mass of 400 GeV and a mass width of 15%to 4 fb for Higgs bosons with masses above 1.4 TeV and a mass width of 1%.For the graviton model,the results show that the graviton is excluded up to a mass of 1750 GeV.