The production of $Z$ bosons with one or two isolated high-energy photons is studied using $pp$ collisions at $\sqrt{s}=8\mathrm{TeV}$. The analyses use a data sample with an integrated luminosity of $20.3{\mathrm{fb}}^{-1}$ collected by the ATLAS detector during the 2012 LHC data taking. The $Z\gamma$ and $Z\gamma \gamma$ production cross sections are measured with leptonic ($e^{+}{e}^{-}$, ${\mu }^{+}{\mu }^{-}$, $\nu \overline{\nu }$) decays of the $Z$ boson, in extended fiducial regions defined in terms of the lepton and photon acceptance. They are then compared to cross-section predictions from the Standard Model, where the sources of the photons are radiation off initial-state quarks and radiative $Z$-boson decay to charged leptons, and from fragmentation of final-state quarks and gluons into photons. The yields of events with photon transverse energy $E_{\mathrm{T}}>250\mathrm{GeV}$ from ${\ell }^{+}{\ell }^{-}\gamma$ events and with $E_{\mathrm{T}}>400\mathrm{GeV}$ from $\nu \overline{\nu }\gamma$ 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 $m_{\gamma \gamma }>200\mathrm{GeV}$ from ${\ell }^{+}{\ell }^{-}\gamma \gamma$ events and with $m_{\gamma \gamma }>300\mathrm{GeV}$ from $\nu \overline{\nu }\gamma \gamma$ events are used to search for anomalous quartic gauge-boson couplings $ZZ\gamma \gamma$ and $Z\gamma \gamma \gamma$. No deviations from Standard Model predictions are observed and limits are placed on parameters used to describe anomalous triple and quartic gauge-boson couplings.

• Received 18 April 2016

DOI:https://doi.org/10.1103/PhysRevD.93.112002

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© 2016 CERN, for the ATLAS Collaboration