Measurement of the $Z\gamma\to\nu\bar{\nu}\gamma$ Production Cross Section in $pp$ Collisions at $\sqrt{s}$ =13~TeV with the ATLAS Detector and Limits on Anomalous Triple Gauge Couplings

The production of $Z$ bosons in association with a high energy photon ($Z\gamma$ production) is studied in the neutrino decay channel of the $Z$ boson using $pp$ collisions at $\sqrt{s}$~=~13~TeV. The analysis uses a data sample with an integrated luminosity of 36.1~fb$^{-1}$ collected by the ATLAS detector in 2015 and 2016. $Z\gamma$ events with invisible decays of the $Z$ boson are selected by requiring significant missing transverse energy ($E_{T}^{miss}$) in conjunction with a single isolated photon with large transverse energy ($E_{T}$). The rate of $Z\gamma$ production is measured as a function of photon $E_{T}$, $E_{T}^{miss}$ and jet multiplicity. Evidence for anomalous triple gauge coupling is sought in $Z\gamma$ production with photon $E_{T}$ greater than 600 GeV. No excess is observed relative to the Standard Model expectation, and upper limits are set on the strength of $ZZ\gamma$ and $Z\gamma\gamma$ couplings.