Search For Heavy Diboson Resonances And Vhh Production With The ATLAS Detector At LHC

After the discovery of Higgs boson on LHC experiment,the measurements on the Higgs boson properties have become one of the most important experimental physics topics.It not only can provide a better understanding of Spontaneous Symmetry Breaking(SSB)mechanism,but also can be an important gateway leading into the indirect search for new physics.In the meantime,the direct search for new physical signals is also a popular topic in the LHC community.Based on full ATLAS Run II data,this thesis performed a search targeting the Vhh production for the first time at ATLAS experiment,in order to search the Di-Higgs production predicted by the Standard Model and potential new physics signals in this process.This thesis also directly searches for heavy diboson resonance in the semileptonic decay process for new physics evidence.Using 139 fb-1 of the protonproton collision data at(?)=13 TeV recorded by the ATLAS detector,a search for heavy diboson resonance with 1-lepton final state was performed,which shows no significant excess in the signal mass range of 300 GeV to 5 TeV.Upper limits at 95%confidence level on the corresponding production cross-section are given for the new physics predicted by the RS graviton,Radion and HVT theoretical models,respectively.The search for the Vhh production for the first time on the ATLAS experiment was performed using the same dataset.Preliminary results do not show any significant excesses.For the non-resonant Vhh production,the results give constraints on the hhh coupling coefficient C3 of the range-34.4<C3<33.3 and the hhVV coupling coefficient C2V of the range-8.0<C2V<9.8 at the 95%confidence level.For the resonance VH(hh)and A→ZH production,upper limits were set and significances were calculated at the 95%confidence level.Significances of larger than 2 times of the standard deviation were observed.The most significant excesses of VH(hh)events are at ZH(mH=550 GeV)and WH(mH=315 GeV)while most significant excesses of A→ZH events correspond to the NWA signal at(mA,mH)=(800,300)GeV and the LWA signal at(mA,mH)=(420,320)GeV.