Strong Evidence for<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Z</mml:mi><mml:mi>Z</mml:mi></mml:math>Production in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi><mml:mover accent="true"><mml:mi>p</mml:mi><mml:mo>¯</mml:mo></mml:mover></mml:math>Collisions at<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msqrt><mml:mi>s</mml:mi></mml:msqrt><mml:mo>=</mml:mo><mml:mn>1.96</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>TeV</mml:mi></mml:math>

We report the first evidence of $Z$ boson pair production at a hadron collider with a significance exceeding 4 standard deviations. This result is based on a data sample corresponding to $1.9\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of integrated luminosity from $p\overline{p}$ collisions at $\sqrt{s}=1.96\text{ }\text{ }\mathrm{TeV}$ collected with the Collider Detector at Fermilab II detector. In the $\ensuremath{\ell}\ensuremath{\ell}{\ensuremath{\ell}}^{\ensuremath{'}}{\ensuremath{\ell}}^{\ensuremath{'}}$ channel, we observe three $ZZ$ candidates with an expected background of ${0.096}_{\ensuremath{-}0.063}^{+0.092}$ events. In the $\ensuremath{\ell}\ensuremath{\ell}\ensuremath{\nu}\ensuremath{\nu}$ 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 $\ensuremath{\ell}\ensuremath{\ell}{\ensuremath{\ell}}^{\ensuremath{'}}{\ensuremath{\ell}}^{\ensuremath{'}}$ and $\ensuremath{\ell}\ensuremath{\ell}\ensuremath{\nu}\ensuremath{\nu}$ channels, we observe an excess of events with a probability of $5.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ to be due to the expected background. This corresponds to a significance of 4.4 standard deviations. The measured cross section is $\ensuremath{\sigma}(p\overline{p}\ensuremath{\rightarrow}ZZ)={1.4}_{\ensuremath{-}0.6}^{+0.7}(\mathrm{stat}+\mathrm{syst})\text{ }\text{ }\mathrm{pb}$, consistent with the standard model expectation.

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