Measurement of the ZZ production cross section and Z $\to \ell^+\ell^-\ell'^+\ell'^-$ branching fraction in pp collisions at $\sqrt s$=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<mℓ+ℓ−,mℓ′+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<mℓ+ℓ−ℓ′+ℓ′−<100 GeV and dilepton mass mℓ+ℓ−>4 GeV for all opposite-sign, same-flavor lepton pairs. The results are in agreement with standard model predictions.