Abstract
We present a comparative study of various approaches for modelling of the {e}^{+}{v}_e{mu}^{-}{overline{v}}_{mu }boverline{b}gamma final state in toverline{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. We confirm that for the integrated cross sections the finite top quark width effects are small and of the order of mathcal{O}left({Gamma}_t/{m}_tright) . We show, however, that they are strongly enhanced for more exclusive observables. In addition, we investigate fractions of events where the photon is radiated either in the production or in the decay stage. We find that large fraction of isolated photons comes from radiative decays of top quarks. Based on our findings, selection criteria might be developed to reduce such contributions, that constitute a background for the measurement of the anomalous couplings in the toverline{t}upgamma vertex.
Highlights
Consistent with the SM, albeit has only been measured indirectly in ttproduction [8, 9]
First fully realistic theoretical predictions for ttγ have been presented in ref. [29] where top quark decays in the narrow width approximation (NWA) have been included, maintaining spin correlations of final state particles
In this paper we have presented a comparative study of various approaches for modelling of the e+νeμ−νμbbγ final state in ttγ production at the LHC
Summary
The NWA offers a conceptually easy and powerful method for computing cross sections for processes comprising the unstable resonances when the width (Γ) of the unstable particle is much smaller than its mass (M ), see e.g. [33, 34]. For the top quark the NWA allows to make predictions for realistic final states described in terms of light and b-jets, charged leptons as well as missing transverse momentum and gives the opportunity for direct comparisons with cross section measurements in the fiducial phase space regions. ) processes is still missing, even-though results for a variety of ttX processes exist in the literature.1 Such a MC program should include NLO QCD corrections to ttX production and top quark decays, retain all ttspin correlations and allow for arbitrary cuts on the final states. On the other hand, such automation will open a new path for performing higher order calculations for more complex processes such as ttbb, ttjj or tttt, for which only predictions with stable top quarks are available at NLO in QCD, see e.g. refs. PS accuracy, where top quarks are decayed by PS programs [53,54,55]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
