Measurement of the $ZZ$ Production Cross Section in $pp$ Collisions at $\sqrt{s}=13\mathrm{TeV}$ with the ATLAS Detector

The $ZZ$ production cross section in proton-proton collisions at 13 TeV center-of-mass energy is measured using $3.2{\mathrm{fb}}^{-1}$ of data recorded with the ATLAS detector at the Large Hadron Collider. The considered $Z$ boson candidates decay to an electron or muon pair of mass 66–116 GeV. The cross section is measured in a fiducial phase space reflecting the detector acceptance. It is also extrapolated to a total phase space for $Z$ bosons in the same mass range and of all decay modes, giving $16.7_{-2.0}^{+2.2}(\mathrm{stat})\genfrac{}{}{0ex}{}{+0.9}{-0.7}(\mathrm{syst})\genfrac{}{}{0ex}{}{+1.0}{-0.7}(\mathrm{lumi})\mathrm{pb}$. The results agree with standard model predictions.

Figure 1

(a) Invariant mass $m_{\ell \ell }$ of the leading-$p_{T,\ell \ell }$ vs the subleading-$p_{T,\ell \ell }$ lepton pair ($\ell \ell$), before the requirement $66\mathrm{GeV}<m_{\ell \ell }<116\mathrm{GeV}$ is applied. The dashed lines indicate this requirement. (b) Invariant mass, (c) transverse momentum, and (d) rapidity of the four-lepton system in selected events. The points represent experimental data. The filled histograms show the signal prediction from simulation, including the $q\overline{q}$ and loop-induced $gg$-initiated process. The contributions are stacked. In the simulation, the prediction from powheg+pythia8 combined with sherpa is scaled to the $\mathcal{O}({\alpha }_S^2)$ prediction. The uncertainties in the simulation are from the same sources as the $C_{ZZ}$ uncertainty. In addition, 6% $ZZ$ cross-section uncertainty and 5% integrated-luminosity uncertainty are included. The expected background of $0.62_{-0.11}^{+1.08}$ events is not shown as a histogram due to its small size.

Figure 2

(a) Comparison between measured fiducial cross sections and $\mathcal{O}({\alpha }_S^2)$ predictions. (b) Total cross section compared to measurements at lower center-of-mass energies by ATLAS, CMS, CDF, and D0 [11, 12, 13, 42, 43], and to a prediction from mcfm at $\mathcal{O}({\alpha }_S^1)$ accuracy for the $q\overline{q}$-initiated process and at $\mathcal{O}({\alpha }_S^2)$ accuracy for the loop-induced $gg$-initiated process. A full $\mathcal{O}({\alpha }_S^2)$ prediction (known to improve agreement at $\sqrt{s}=13\mathrm{TeV}$) was not yet available for all the different center-of-mass energies. Some data points are shifted horizontally to improve readability. The $ZZ$ cross section as function of $\sqrt{s}$ in the range 130–209 GeV was also measured at the LEP 2 $e^{+}{e}^{-}$ collider [44].