Precise predictions for $$ t\overline{t}\gamma /t\overline{t} $$ cross section ratios at the LHC

Abstract

With the goal of increasing the precision of NLO QCD predictions for the pp → ttγ process in the di-lepton top quark decay channel we present theoretical predictions for the R = σ ttγ /σ tt 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 TeV. Fully realistic NLO computations for tt and ttγ production are employed. They are based on matrix elements for e + ν e µ −ν µ bb and e + ν e µ −ν µ bbγ 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. 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 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-γ vertex. The latter can shed some light on possible new physics that can reveal itself only once sufficiently precise theoretical predictions are available.

Top quark physics

“…3, right. In all these works [19][20][21] the contribution of off-shell effects have been estimated to below a few percent for the total cross section which is consistent with the parametric approximation O(Γ t m t ) of the NWA. This has to be compared with typical NLO scale uncertainties of about 10%.…”

“…[19,20] and Ref. [21] have appeared that show the remarkable capabilities of modern computational tools. In Ref.…”

“…Ref. [21] deals with off-shell effects in associated top quark pair production with a photon. This process is of particular interest for measurements of the top quark electroweak couplings and searches for New Physics.…”

Towards constraining dark matter at the LHC: higher order QCD predictions for t$$ \overline{t} $$ + Z (Z → νℓ$$ \overline{v} $$ℓ)

“…Moreover, the differences coming from NLO results for various PDF sets are comparable (usually even higher) to the individual estimates of PDF systematics. We have checked that this is the case for similar processes, namely for pp → e + ν e µ −ν µ bbj+X [43,44] and pp → e + ν e µ −ν µ bbγ + X production [45,60]. Results with the recomputed NLO JHEP11(2019)001 QCD corrections to the pp → e + ν e µ −ν τ bbν τντ + X production process for MMHT14 and NNPDF3.0 PDF sets are summarised in table 4.…”

On the ratio of $t\bar{t}\gamma$ and $t\bar{t}$ cross sections at the LHC

“…Given its large cross section and similar behaviour with respect to radiative corrections [18][19][20], tt production is a particularly suitable candidate for normalizing the absolute ttγ predictions. In this contribution we present selected results from our recent work [17] where we focus on the observable R = σ ttγ /σ tt and explore its performance in accuracy in the context of the LHC Run II.…”