Study Of Physics With ZZ Production With The ATLAS Detector

The thesis presents my research in High Energy Physics at the LHC with diboson ZZ productions in proton-proton collisions at (?)s=8TeV. The ZZ productions at the LHC provide a most sensitive tool to test the Standard Model (SM) at the energy frontier and to study the Higgs physics. This thesis reports three measurements using the ZZ decay mode of ZZâ†'l+l-vv (l=e,μ) with data, corresponding to an integrated luminosity of20.3fb-1, collected by the ATLAS detector in2012. Major results of the measurements on the ZZ production cross section, the Higgs boson off-shell signal strength, and high mass Higgs boson searches are briefly given below.The ZZ production cross section measurement provides a precision test of the SM and a foundation for Higgs physics studies at the LHC. The total cross section is measured to be σZZtot=8.84-0.88+1.04(stat)-0.85+0.87(syst)-0.28+0.33(lumi) pb, which is consistent with the exper-imental uncertainty compared to the SM prediction with next-to-next-to-leading order in QCD calculation of7.74-0.31+0.35pb. The selected ZZ events are analyzed to search for anomalous triple gauge couplings for the ZZ production. In absence of deviations from the SM predictions, the most stringent limits on the anomalous coupling parameters are set at95%confident level.Due to the off-shell effects of the recently discovered Higgs boson at the LHC, the Standard Model ZZ production can be enhanced in the mass range above2mz thresh-old. Measurements of the ZZ production in the mass range above the2mz threshold provide a unique opportunity to measure the off-shell signal strength of the Higgs bo-son. Using the ZZâ†'2l2v final state, the95%confidence level (CL) upper limit on the off-shell signal strength is determined to be in the range of9.6-12.4, with8.8-13.3expected. From a combination of ZZâ†'4l, ZZâ†'2l2v and WWâ†'evμv channels, the observed (expected) upper limit of5.1-8.6(6.7-11.0) is obtained. In each case the upper limit range is determined by varying the unknown ggâ†'ZZ and ggâ†'WW continuum background k-factor from higher-order QCD corrections between half and twice the value of the known signal k-factor. Assuming the Higgs boson couplings are independent from the energy scale of the Higgs production, The off-shell Higgs production analysis with ZZ and WW channels, combined with the on-shell Higgs measurements, yields an observed (expected)95%CL upper limit on the scaled Higgs width, ΓH/ΓHSM, in a range of4.5-7.5(6.5-11.2) using variations of the background k-factors for higher order QCD corrections. Assuming that the unknown ggâ†'ZZ background k-factor is equal to the Higgs pro-duction k-factor, this translates into an observed (expected)95%CL upper limit on the Higgs boson total width of22.7(33.0) MeV.A search for additional Higgs bosons with a mass range from240to1000GeV using a channel of Hâ†'ZZâ†'llvv is conducted. Data are found to agree with a background-only hypothesis, and a model-independent limit on the cross section times branching ratio for a narrow-width resonance is derived. In the gluon-gluon fusion (ggF) channel, a95%CL limit on the σgg F x BR(Hâ†'ZZ) is set to be227fb (22fb) for mH=400GeV (mHh=1TeV); the corresponding expected limits are209fb and33fb, respectively. In the VBF channel a95%CL limit on σVBF x BR(Hâ†'ZZ) of248fb (29fb) is set for mH=400GeV (mH=1TeV); the corresponding expected limits are136fb and20fb, respectively. Limits are also set on the production rate of additional Higgs bosons in the two-Higgs doublet models (2HDM). A large fraction of low tan β is excluded depending on cos(β-α); for cos(β-α)=±0.1, the region with tan β<1is excluded in the range of250<mH<350GeV at95%CL.