Measurement of the $Z\gamma \to \nu \overline{\nu }\gamma$ Production Cross Section and Limits on Anomalous $ZZ\gamma$ and $Z\gamma \gamma$ Couplings in $p\overline{p}$ Collisions at $\sqrt{s}=1.96\mathrm{TeV}$

We present the first observation of the $Z\gamma \to \nu \overline{\nu }\gamma$ process at the Fermilab Tevatron at 5.1 standard deviations significance, based on $3.6{\mathrm{fb}}^{-1}$ of integrated luminosity collected with the D0 detector at the Fermilab Tevatron $p\overline{p}$ Collider at $\sqrt{s}=1.96\mathrm{TeV}$. The measured $Z\gamma$ production cross section multiplied by the branching fraction of $Z\to \nu \overline{\nu }$ is $32\pm 9(\mathrm{stat}+\mathrm{syst})\pm 2(\mathrm{lumi})\mathrm{fb}$ for the photon $E_T>90\mathrm{GeV}$. It is in agreement with the standard model prediction of $39\pm 4\mathrm{fb}$. We set limits on anomalous trilinear $Z\gamma \gamma$ and $ZZ\gamma$ gauge boson couplings, most of which are the most restrictive to date.

Figure 1

Photon $E_T$ spectrum in data (solid circles), sum of backgrounds (dashed-dotted line), and sum of the MC signal and background for the SM prediction (solid line) and for the ATGC prediction with $h_{30}^{\gamma }=0.09$ and $h_{40}^{\gamma }=0.005$ (dashed line). The shaded band corresponds to the $\pm 1\mathrm{s}.\mathrm{d}.$ total uncertainty on the predicted sum of the SM signal and background.

Figure 2

Two-dimensional bounds (ellipses) at 95% C.L. on $CP$-conserving (a) $Z\gamma \gamma$ and (b) $ZZ\gamma$ couplings. The crosses represent the one-dimensional bounds at the 95% C.L. setting all other couplings to zero. The dashed lines indicate the unitarity limits for $\Lambda =1.5\mathrm{TeV}$.