Measurements of $Z\gamma$ and $Z\gamma\gamma$ production in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

The production of Z bosons with one or two isolated high-energy photons is studied using $pp$ collisions at $\sqrt{s}$ = 8 TeV. The analyses use a data sample with an integrated luminosity of 20.3 $fb^{-1}$ collected by the ATLAS detector during the 2012 LHC data taking. The data are used to make tests of the electroweak sector of the Standard Model (SM) and search for deviations that could provide evidence for new physics beyond SM. The $Z\gamma$ and $Z\gamma\gamma$ production cross sections are measured with leptonic ($e^{+}e^{-}$, $\mu^{+}\mu^{-}$, $\nu\bar{\nu}$) decays of the Z boson and then compared to cross-section predictions from the SM where the sources of the photons are radiation off initial-state quarks or radiative Z boson decay to charged leptons. The yields of events with photon transverse energy $E_{T} > 250$ GeV from $l^{+}l^{-}\gamma$ events and with $E_{T} > 400$ GeV from $\nu\bar{\nu}\gamma$ events are used to search for anomalous triple gauge-boson couplings $ZZ\gamma$ and $Z\gamma\gamma$. The yields of events with diphoton invariant mass $m_{\gamma\gamma} > 200$ GeV from $l^{+}l^{-}\gamma\gamma$ events and with $m_{\gamma\gamma} > 300$ GeV from $\nu\bar{\nu}\gamma\gamma$ events are used to search for anomalous quartic gauge-boson couplings $ZZ\gamma\gamma$ and $Z\gamma\gamma\gamma$. No deviations from SM predictions are observed and limits are placed on parameters used to describe anomalous triple and quartic gauge-boson couplings.