Measurement of the ZZ production cross section and Z → ℓ⁺ℓ⁻ℓ^′+ℓ^′− branching fraction in pp collisions at s=13 TeV

Four-lepton production in proton–proton collisions, pp→(Z/γ^⁎)(Z/γ^⁎)→ℓ⁺ℓ⁻ℓ^′+ℓ^′−, where ℓ,ℓ^′=e or μ, is studied at a center-of-mass energy of 13 TeV with the CMS detector at the LHC. The data sample corresponds to an integrated luminosity of 2.6 fb⁻¹. The ZZ production cross section, σ(pp→ZZ)=14.6_−1.8 ^+1.9(stat)_−0.3 ^+0.5(syst)±0.2(theo)±0.4(lumi)pb, is measured for events with two opposite-sign, same-flavor lepton pairs produced in the mass region 60<m_ℓ⁺ℓ⁻,m_{ℓ^′+ℓ^′−}<120 GeV. The Z boson branching fraction to four leptons is measured to be B(Z→ℓ⁺ℓ⁻ℓ^′+ℓ^′−)=4.9_−0.7 ^+0.8 (stat)_−0.2 ^+0.3 (syst)_−0.1 ^+0.2(theo)±0.1(lumi)×10⁻⁶ for the four-lepton invariant mass in the range 80<m_{ℓ⁺ℓ⁻ℓ^′+ℓ^′−}<100 GeV and dilepton mass m_ℓ⁺ℓ⁻>4 GeV for all opposite-sign, same-flavor lepton pairs. The results are in agreement with standard model predictions.