Measurement of the ZZ Production Cross Section in pp Collisions at √s = 13 TeV with the ATLAS Detector

CERN-LHC. 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 in 2015. 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, defined below.

Fiducial phase space definition:
- $e^+e^-e^+e^-$ or $e^+e^-\mu^+\mu^-$ or $\mu^+\mu^-\mu^+\mu^-$
- Leptons dressed by adding four momenta of final-state photons within $\Delta R(\ell,\gamma) = 0.1$ to lepton four momentum. Only photons not originating from hadron or $\tau$ decay are included. All further requirements refer to dressed leptons
- Leptons have transverse momentum $p_T > 20$ GeV
- Leptons have absolute pseudorapidity $|\eta| < 2.7$
- Leptons separated by $\Delta R(\ell,\ell) > 0.2$ from each other
- Form same-flavor oppositely-charged dileptons. In the case of four same-flavor leptons, form pairs in such a way that $|m_{\ell\ell,1} - m_Z| + |m_{\ell\ell,2} - m_Z|$ is minimised, where $m_{\ell\ell,1}$ ($m_{\ell\ell,2}$) is the mass of the first (second) dilepton system, and $m_Z$ is the Z boson pole mass
- Dileptons have invariant mass between 66 and 116 GeV

The measurement is inclusive with respect to hadronic activity, so any number of jets may be present in addition to the four-lepton system. Contributions from double parton scattering ($pp \rightarrow Z, Z$) are included in the measurement. For details please see the paper.

The cross section is also extrapolated to the total phase space and all Z boson decay modes. The result is model-dependent and includes phase space that was not experimentally accessible, so it should be used with caution. Whenever possible, the fiducial cross sections should be used instead, since they are only minimally model-dependent.