Measurements of Zγ and Zγγ production in pp collisions at 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 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 − , νν ¯ ) 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 l + l − γ events and with E T >400  GeV from νν ¯ γ 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 l + l − γγ events and with m γγ >300  GeV from νν ¯ γγ 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.