Observation of $WZ$ Production

We report the first observation of the associated production of a $\W$ boson and a $\Z$ boson. This result is based on 1.1 ${\rm fb}^{-1}$ 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. We observe 16 $\WZ$ candidates passing our event selection with an expected background of $2.7 \pm 0.4$ events. A fit to the missing transverse energy distribution indicates an excess of events compared to the background expectation corresponding to a significance equivalent to six standard deviations. The measured cross section is $\sigma(p\overline{p}\rightarrow WZ) = 5.0^{+1.8}_{-1.6}$ pb, consistent with the standard model expectation.