First Measurement of $ZZ$ Production in $p\bar{p}$ Collisions at $\sqrt{s}$ = 1.96 TeV

We report the first measurement of the cross section for $\Z$ boson pair production at a hadron collider. This result is based on a data sample corresponding to 1.9 ${\rm fb}^{-1}$ of integrated luminosity from $p\overline{p}$ collisions at $\sqrt{s} = 1.96~{\rm TeV}$ collected with the CDF II detector at the Fermilab Tevatron. In the $\llll$ channel, we observe three $ZZ$ candidates with an expected background of $0.096^{+0.092}_{-0.063}$ events. In the $\llnunu$ channel, we use a leading-order calculation of the relative $\ZZ$ and $\WW$ event probabilities to discriminate between signal and background. In the combination of $\llll$ and $\llnunu$ channels, we observe an excess of events with a probability of $5.1\times 10^{-6}$ to be due to the expected background. This corresponds to a significance of 4.4 standard deviations. The measured cross section is $\sigma(p\overline{p}\rightarrow \ZZ) = 1.4^{+0.7}_{-0.6}$~(stat.+syst.)~pb, consistent with the standard model expectation.