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Modelling of top quark decays in $t\bar{t}γ$ production at the LHC

In this proceedings we briefly report on the state-of-the-art NLO QCD computation for the $pp\to t\bar{t}γ$ process in the di-lepton channel. We describe higher-order corrections to the $e^+ν_e \, μ^- \barν_μ\, b\bar{b}γ$ final state at the LHC with $\sqrt{s}=13$ TeV. Off-shell top quarks, double-, single- as well as non-resonant top-quark contributions along with all interference effects are consistently incorporated at the matrix element level. Results are presented in the form of fiducial integrated and differential cross sections. The impact of top quark modelling is scrutinised by an explicit comparison with the results in the narrow-width approximation. Both types of predictions are now available in the HELAC-NLO framework.