Measurement of the ZZ production cross section and limits on anomalous neutral triple gauge couplings with the ATLAS detector

This thesis presents measurements of the ZZ production cross section in proton-proton collisions at √s = 7 TeV and at √s = 8 TeV, using data recorded by the ATLAS experiment at the LHC in 2011 and 2012. Events are selected which are consistent with two Z bosons decaying to electrons or muons. The cross section is first measured in a fiducial phase-space corresponding closely to the detector acceptance. For the 7 TeV measurement, this phase-space is defined by requiring four electrons or muons with pT > 7 GeV and |eta|<3.16, with a minimum separation between any pair of leptons (electrons or muons) of delta(R)=0.2. The leptons must form two opposite-sign same-flavour pairs, each with invariant mass 66 < m(ll) < 116 GeV. The fiducial cross section times branching ratio to four electrons or muons measured in a dataset corresponding to an integrated luminosity of 4.6 fb−1 is 25.2 +3.3 −3.0 (stat) +1.2 −1.0 (syst) +1.1 −0.9 (lumi) fb. For the 8 TeV measurement, the fiducial phase-space is defined in the same way, except with the lepton pseudo-rapidity requirement tightened to |eta| < 2.7. The fiducial cross section measured in a dataset corresponding to an integrated luminosity of 20 fb−1 is 20.8 +1.3 -1.2 (stat) +1.0 -0.9 (syst) +0.6 -0.6(lumi) fb. Additionally, a fiducial cross section allowing one of the Z bosons to be off shell is measured at √s = 7 TeV by relaxing the mass requirement on one of the lepton pairs to ml−l+ > 20 GeV. This is found to be 27.8 +3.6 -3.4 (stat) +1.8 -1.6 (syst) +1.1 -1.0 (lumi) fb. These results are then used to derive the total cross section for ZZ production with Z bosons in the mass range 66 GeV to 116 GeV, by correcting for the acceptance of the fiducial phase-space and the Z -> ll branching fractions. The total cross section is measured to be 7.0 +0.9 -0.8 (stat) +0.4 -0.3 (syst) +0.3 -0.2 (lumi) pb at √s = 7 TeV and − 7.1 +0.4 -0.4 (stat) +0.4 -0.3 (syst) +0.2 -0.2 (lumi) pb at √s = 8 TeV, which is consistent with the Standard Model expectation of 5.9 +/- 0.2 pb at √s = 7 TeV and 7.2 +/- 0.2 pb at √s = 8 TeV, calculated to next-to-leading order in QCD. The differential cross section in bins of three kinematic variables is also presented. The differential event yield as a function of the transverse momentum of the highest transverse momentum Z boson is used to set limits on the strength of anomalous ZZZ and ZZgamma neutral triple gauge boson couplings, which are forbidden in the Standard Model. The limits obtained with the √s = 8 TeV data are the most constraining to date.

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