Strong Evidence for $ZZ$ Production in $p\overline{p}$ Collisions at $\sqrt{s}=1.96\mathrm{TeV}$

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 $1.9{\mathrm{fb}}^{-1}$ of integrated luminosity from $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ collected with the Collider Detector at Fermilab II detector. In the $\ell \ell {\ell }^{\prime }{\ell }^{\prime }$ channel, we observe three $ZZ$ candidates with an expected background of ${0.096}_{-0.063}^{+0.092}$ events. In the $\ell \ell \nu \nu$ 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 $\ell \ell {\ell }^{\prime }{\ell }^{\prime }$ and $\ell \ell \nu \nu$ 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 $\sigma (p\overline{p}\to ZZ)={1.4}_{-0.6}^{+0.7}(\mathrm{stat}+\mathrm{syst})\mathrm{pb}$, consistent with the standard model expectation.

Figure 1

Distribution of the discriminating variable ${log}_{10}(1-LR)$ for the $ZZ\to \ell \ell \nu \nu$ search.