Limits on Anomalous Trilinear Gauge Couplings in $Z\gamma$ Events from $p\overline{p}$ Collisions at $\sqrt{s}=1.96\mathrm{TeV}$

Using $Z\gamma$ candidate events collected by the CDF detector at the Tevatron Collider, we search for potential anomalous (non-standard-model) couplings between the $Z$ boson and the photon. $Z\gamma$ couplings vanish at tree level and are heavily suppressed at higher orders; hence any evidence of couplings indicates new physics. Measurements are performed using data corresponding to an integrated luminosity of $4.9{\mathrm{fb}}^{-1}$ in the $Z\to \nu \overline{\nu }$ decay channel and $5.1{\mathrm{fb}}^{-1}$ in the $Z\to l^{+}{l}^{-}$ ($l=\mu$, $e$) decay channels. The combination of these measurements provides the most stringent limits to date on $Z\gamma$ trilinear gauge couplings. Using an energy scale of $\Lambda =1.5\mathrm{TeV}$ to allow for a direct comparison with previous measurements, we find limits on the $CP$-conserving parameters that describe $Z\gamma$ couplings to be $|h_3^{\gamma ,Z}|<0.022$ and $|h_4^{\gamma ,Z}|<0.0009$. These results are consistent with standard model predictions.

Figure 1

Comparison of the measured $E_T^{\gamma }$ distribution with the predicted distributions from both the SM and beyond-the-SM scenarios for $Z\to l^{+}{l}^{-}$ (top) and $Z\to \nu \overline{\nu }$ (bottom) candidate samples, at $\Lambda =1.5\mathrm{TeV}$. The beyond-the-SM scenarios chosen here can be excluded at 95% Bayesian credibility level in each sample. Note the greatest difference in the SM and beyond-the-SM cases is found off scale at $E_T^{\gamma }>200\mathrm{GeV}$; the lack of data events in this region indicates good agreement with the SM.