Measurement of the total ZZ production cross section in the four-lepton channel using 5.8 fb$^{−1}$ of ATLAS data at $\sqrt s$ = 8 TeV

The ZZ production cross section has been measured by the ATLAS experiment using the four-lepton decay channel, where the term lepton implies electrons and muons. A data sample of 5.8 fb$^{−1}$ collected in $pp$ collisions at $\sqrt s$ = 8 TeV at the LHC in 2012 is used. Events are selected by requiring four leptons with $p_\mathrm{T}$ > 15 GeV, forming two opposite-sign same-flavour lepton pairs with mass between 66 GeV and 116 GeV, with the leading lepton required to have $p_\mathrm{T} > 25$ GeV. We observe 85 candidate events in the acceptance region $|\eta| < 2.5$ for muons ($|\eta| < 2.47$ for electrons) with a background expectation of 1.3$\pm$1.2(stat.)$\pm$0.5(syst.). The $ZZ\to\ell^+\ell^-\ell^{\prime+}\ell^{\prime-}$ production cross section, measured in a fiducial phase-space corresponding closely to the acceptance defined above, is 21.0$^{+2.4}_{−2.2}$(stat.)$^{+0.6}_{−0.5}$(syst.)$\pm$0.8(lumi.) fb. After correcting for the $ZZ\to\ell^+\ell^-\ell^{\prime+}\ell^{\prime-}$ branching fraction and the fiducial acceptance, this extrapolates to a total $ZZ$ production cross section of 9.3$^{+1.1}_{−1.0}$(stat.)$^{+0.4}_{−0.3}$(syst.)$\pm$0.3(lumi.) pb, where the total cross section is defined with both $Z$ bosons with mass between 66 GeV and 116 GeV. This is consistent with the Standard Model expectation of 7.4$\pm$0.4 pb.