Measurement of the ZZ production cross section and Z → Branching Fraction in pp Collisions at

Four-lepton production in proton–proton collisions, $\mathrm{pp}\to \left(Z/{\gamma }^{⁎}\right)\left(Z/{\gamma }^{⁎}\right)\to {\ell }^{+}{\ell }^{-}{\ell }^{\prime +}{\ell }^{\prime -}$, where $\ell ,{\ell }^{\prime }=e$ 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, $\sigma \left(\mathrm{pp}\to \mathrm{ZZ}\right)={14.6}_{-1.8}^{+1.9}\phantom{\rule{0.2em}{0ex}}\left(\text{stat}\right){\phantom{\rule{0.2em}{0ex}}}_{-0.3}^{+0.5}\phantom{\rule{0.2em}{0ex}}\left(\text{syst}\right)±0.2\phantom{\rule{0.2em}{0ex}}\text{(theo)}±0.4\phantom{\rule{0.2em}{0ex}}\left(\text{lumi}\right)\phantom{\rule{0.2em}{0ex}}\text{pb}$, is measured for events with two opposite-sign, same-flavor lepton pairs produced in the mass region $60<{m}_{{\ell }^{+}{\ell }^{-}},{m}_{{\ell }^{\prime +}{\ell }^{\prime -}}<120\text{}\text{GeV}$. The Z boson branching fraction to four leptons is measured to be $B\left(Z\to {\ell }^{+}{\ell }^{-}{\ell }^{\prime +}{\ell }^{\prime -}\right)={4.9}_{-0.7}^{+0.8}{\text{(stat)}}_{-0.2}^{+0.3}{\text{(syst)}}_{-0.1}^{+0.2}\phantom{\rule{0.2em}{0ex}}\text{(theo)}±0.1\phantom{\rule{0.2em}{0ex}}\left(\text{lumi}\right)×{10}^{-6}$ for the four-lepton invariant mass in the range $80<{m}_{{\ell }^{+}{\ell }^{-}{\ell }^{\prime +}{\ell }^{\prime -}}<100\text{}\text{GeV}$ and dilepton mass ${m}_{{\ell }^{+}{\ell }^{-}}>4\text{}\text{GeV}$ for all opposite-sign, same-flavor lepton pairs. The results are in agreement with standard model predictions.