Measurement of the ZZ production cross section and $\mathrm{Z} \to \ell\ell\ell'\ell'$ branching fraction in pp collisions at $\sqrt{s}=13~\mathrm{TeV}$

Four-lepton production in proton-proton collisions, ${\mathrm{pp}} \rightarrow {\mathrm{Z}} / \gamma^* {\mathrm{Z}}/ \gamma^* \rightarrow \ell\ell\ell'\ell'$, where ($\ell, \ell' = $ e or $\mu$), is studied at a center-of-mass energy of $13~\mathrm{TeV}$ with the CMS detector at the LHC. The data sample corresponds to an integrated luminosity of $2.6~\mathrm{fb}^{-1}$. The ZZ production cross section, $\sigma(\mathrm{pp} \to \mathrm{ZZ}) = 14.6 ^{+1.9}_{-1.8} \mathrm{(stat)} ^{+0.5}_{-0.3} \mathrm{(syst)} \pm 0.2 \mathrm{(theo)} \pm 0.4 \mathrm{(lum)}~\mathrm{pb}$, is measured for both Z bosons produced in the mass region $60 < m_{\mathrm{Z}} < 120~\mathrm{GeV}$. The Z branching fraction to four leptons is measured to be $\mathcal{B}(\mathrm{Z} \to \ell\ell\ell'\ell') = 4.9 _{-0.7}^{+0.8} \mathrm{(stat)} _{-0.2}^{+0.3} \mathrm{(syst)} _{-0.1}^{+0.2} \mathrm{(theo)} \pm 0.1 \mathrm{(lum)} \times 10^{-6}$ for the four-lepton mass in the range $80 < m_{\ell\ell\ell'\ell'} < 100~\mathrm{GeV}$ and dilepton mass $m_{\ell\ell'} > 4~\mathrm{GeV}$ for all opposite-charge lepton pairs. The results are compared to standard model predictions.