Measurement of the $Z(\rightarrow\ell^+\ell^-)\gamma$ production cross-section in $pp$ collisions at $\sqrt{s} =13$ TeV with the ATLAS detector
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The ATLAS collaboration

Abstract (data abstract)
CERN-LHC. Measurement of the $Z\gamma\rightarrow \ell^+ \ell^- \gamma$ production cross section in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector. The production of a prompt photon in association with a Z boson is studied in proton--proton collisions at a centre-of-mass energy $\sqrt{s} =$~13 TeV. The analysis uses a data sample with an integrated luminosity of 139 fb$^{-1}$ collected by the ATLAS detector at the LHC from 2015 to 2018. The production cross-section for the process $pp\to\ell^{+}\ell^{-}\gamma+X$ ($\ell = e, \mu$) is measured within a fiducial phase space region defined by kinematic requirements on the photon and the leptons, and by isolation requirements on the photon. An experimental precision of 2.9% is obtained on the fiducial cross-section. Differential cross-sections are measured as a function of each of six kinematic variables characterising the $\ell^{+}\ell^{-}\gamma$ system. The data are compared to theoretical predictions based on next-to-leading-order and next-to-next-to-leading-order perturbative QCD calculations. The impact of next-to-leading-order electroweak corrections is also considered. The fiducial region is defined as: - opposite sign same flavour lepton pair (electron or muon) - lepton $p_{\mathrm{T}}>30, 25$ GeV, $|\eta|<2.47$, dressed - photon $E_{\mathrm{T}}>30$ GeV, $|\eta|<2.37$. - $m_{\ell\ell}>40$ GeV, $m_{\ell\ell}+m_{\ell\ell\gamma}>182$ GeV - $\Delta R(\ell, \gamma)>0.4$ - isolation energy (the scalar sum of $E_{\mathrm{T}}$ for all stable particles except neutrinos and muons within a cone of $\Delta R=0.2$) around photon must be less than 7% of the photon $E_{\mathrm{T}}$.

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