Hard photons in hadroproduction of top quarks with realistic final states

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We present a comparative study of various approaches for modelling of the e + ν e µ −ν µ bbγ final state in ttγ 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 O(Γ t /m t ). 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 ttγ vertex.

Section: Phenomenological Results For Integrated Cross Sectionsmentioning

confidence: 99%

Section: Implementing the Nwa In Helac-nlomentioning

confidence: 99%

Section: Lhc Setupmentioning

confidence: 99%

“…The latter is used to describe the dynamics of the whole process, while the former only the top quark decays. For more details we refer the reader to our previous publications [15,30].…”

Section: Lhc Setupmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

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

Bevilacqua

Hartanto

Kraus

et al. 2020

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

Bevilacqua

Hartanto

Kraus

et al. 2019

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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.

NNLO QCD corrections to three-photon production at the LHC

Chawdhry

Czakon

Mitov

et al. 2020

126

146

We compute the NNLO QCD corrections to three-photon production at the LHC. This is the first NNLO QCD calculation for a 2 → 3 process. Our calculation is exact, except for the scale-independent part of the two-loop finite remainder which is included in the leading color approximation. We estimate the size of the missing two-loop corrections and find them to be phenomenologically negligible. We compare our predictions with available 8 TeV measurement from the ATLAS collaboration. We find that the inclusion of the NNLO corrections eliminates the existing significant discrepancy with respect to NLO QCD predictions, paving the way for precision phenomenology in this process.