Paper detail

Measurements of $Zγ$ and $Zγγ$ 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 $Zγ$ and $Zγγ$ production cross sections are measured with leptonic ($e^{+}e^{-}$, $μ^{+}μ^{-}$, $ν\barν$) 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 $E_T >$ 250 GeV from $\ell^{+}\ell^{-}γ$ events and with $E_T >$ 400 GeV from $ν\barνγ$ events are used to search for anomalous triple gauge-boson couplings $ZZγ$ and $Zγγ$. The yields of events with diphoton invariant mass $m_{γγ} >$ 200 GeV from $\ell^{+}\ell^{-}γγ$ events and with $m_{γγ} > $ 300 GeV from $ν\barνγγ$ events are used to search for anomalous quartic gauge-boson couplings $ZZγγ$ and $Zγγγ$. No deviations from Standard Model predictions are observed and limits are placed on parameters used to describe anomalous triple and quartic gauge-boson couplings.