Search

With the goal of increasing the precision of NLO QCD predictions for the pp → t t ¯ γ $$ pp\to t\overline{t}\gamma $$ process in the di-lepton top quark decay channel we present theoretical predictions for the ℛ = σ t t ¯ γ / σ t t ¯ $$ \mathrm{\mathcal{R}}={\sigma}_{t\overline{t}\gamma }/{\sigma}_{t\overline{t}} $$ cross section ratio. Results for the latter together with various differential cross section ratios are given for the LHC with the Run II energy of s = 13 $$ \sqrt{s}=13 $$ TeV. Fully realistic NLO computations for t t ¯ $$ t\overline{t} $$ and t t ¯ γ $$ t\overline{t}\gamma $$ production are employed. They are based on matrix elements for e + ν e μ − ν ¯ μ b b ¯ $$ {e}^{+}{\nu}_e{\mu}^{-}{\overline{\nu}}_{\mu }b\overline{b} $$ and e + ν e μ − ν ¯ μ b b ¯ γ $$ {e}^{+}{\nu}_e{\mu}^{-}{\overline{\nu}}_{\mu }b\overline{b}\gamma $$ processes and include all resonant and non-resonant diagrams, interferences, and off-shell effects of the top quarks and the W gauge bosons. Various renormalisation and factorisation scale choices and parton density functions are examined to assess their impact on the cross section ratio. Depending on the transverse momentum cut on the hard photon a judicious choice of a dynamical scale allows us to obtain 1%–3% percent precision on ℛ $$ \mathrm{\mathcal{R}} $$ . Moreover, for differential cross section ratios theoretical uncertainties in the range of 1%-6% have been estimated. Until now such high precision predictions have only been reserved for the top quark pair production at NNLO QCD. Thus, ℛ $$ \mathrm{\mathcal{R}} $$ at NLO in QCD represents a very precise observable to be measured at the LHC for example to study the top quark charge asymmetry or to probe the strength and the structure of the t- t ¯ $$ \overline{t} $$ -γ vertex. The latter can shed some light on possible new physics that can reveal itself only once sufficiently precise theoretical predictions are available.

A precise measurement of the top quark mass, a fundamental parameter of the Standard Model, is among the most important goals of top quark studies at the Large Hadron Collider. Apart from the standard methods, numerous new observables and reconstruction techniques are employed to improve the overall precision and to provide different sensitivities to various systematic uncertainties. Recently, the normalised inverse invariant mass distribution of the t t ¯ $$ t\overline{t} $$ system and the leading extra jet not coming from the top quark decays has been proposed for the pp → t t ¯ j $$ pp\to t\overline{t}j $$ production process, denoted as ℛ(m t pole , ρ s ). In this paper, a thorough study of different theoretical predictions for this observable, however, with top quark decays included, is carried out. We focus on fixed order NLO QCD calculations for the di-lepton top quark decay channel at the LHC with s = 13 $$ \sqrt{s}=13 $$ TeV. First, the impact on the extraction of m t is investigated and afterwards the associated uncertainties are quantified. In one approach we include all interferences, off-shell effects and non-resonant backgrounds. This is contrasted with a different approach with top quark decays in the narrow width approximation. In the latter case, two cases are employed: NLO QCD corrections to the pp → t t ¯ j $$ pp\to t\overline{t}j $$ production process with leading order decays and the more sophisticated case with QCD corrections and jet radiation present also in top quark decays. The top quark mass sensitivity of ℛ(m t pole , ρ s ) is investigated and compared to other observables: the invariant mass of the top anti-top pair, the minimal invariant mass of the b-jet and a charged lepton as well as the total transverse momentum of the t t ¯ j $$ t\overline{t}j $$ system. Once top quark decays are included the invariant mass of the t t ¯ $$ t\overline{t} $$ system shows better sensitivity to the top quark mass extraction and smaller dependence on the off-shell effects and non-resonant contributions of the top quark and the W gauge boson than the ℛ(m t pole , ρ s ) observable.