Off-shell vs on-shell modelling of top quarks in photon associated production

Understand

We present a comparative study of various approaches for modelling of the $e^+ \nu_e \mu^- \bar{\nu}_\mu b\bar{b} \gamma$ final state in $t\bar{t}\gamma$ production at the LHC.

• Working at the NLO in QCD we compare the fully realistic description of the top quark decay chain with the one provided by the narrow-width-approximation.
• The former approach comprises all double, single and non-resonant diagrams, interferences, and off-shell effects of the top quarks.
• The latter incorporates only double resonant contributions and restricts the unstable top quarks to on-shell states.