Higher order QCD predictions for top quark-physics at the LHC

Methods

The computation of the NLO QCD corrections to the pp → tt + X, X= H,γ,Z,W± process in lepton+jet top-quark decay channel poses new challenges on the technical side, as it involves one-loop diagrams with many external particles and virtual weak bosons in the loop. The calculation of such contributions can be done using HELAC-NLO, but it requires substantial modifications, e.g. improving the stability and efficiency of the Monte Carlo integration and including electroweak corrections. To this end, the one-loop evaluator of HELAC-NLO must be re-implemented with an extended set of Dyson-Schwinger recursive relations. In addition, the real radiation part in HELAC-Dipoles will have to include the possibility to treat photon radiation in unresolved configurations.

Results

Due to the cluster resources within one year we were able to provide NLO QCD corrections to complicated final states like for example 2 → 12 final states. Specifically, we calculated NLO QCD corrections to the tttt production in the 3 lepton and 4 lepton decay channels for the LHC Run III center-of-mass energy of sqrt{s} = 13.6 TeV. In addition, we computed the so-called complete NLO corrections to top-quark pair production with one and two isolated photons in the di-lepton top-quark decay channel. In this case higher-order QCD and electroweak effects as well as photon bremsstrahlung were consistently included at all stages, i.e. in production and top-quark decays. Finally, we were able to provide a complete description of top quark pair production in association with a hard photon in the di-lepton decay channel at NLO in QCD. This calculation included all resonant and non-resonant Feynman diagrams, interferences, and finite-width effects of the top quarks and W/Z gauge bosons. We provided the results for the process using the fixed-cone, smooth-cone and hybrid-photon isolation criteria. The fixed-cone isolation criterion allows contributions from collinear photon radiation off QCD partons, which requires the inclusion of parton-to-photon fragmentation processes. To this end, we included the latter contributions into our computational framework. We quantified the impact of different photon-isolation prescriptions on the integrated and differential cross-section predictions for the LHC at a centre-of-mass energy of 13.6 TeV.

Discussion

Our state-of-the-art NLO QCD results for ttγ will allow us to reproduce the photon-isolation prescription employed in ATLAS and CMS. This will help to improve future comparisons with the LHC data. The same applies to the results for the tttt process in the 4l and 3l decays channels. To this end the first comparison of our results for the ttj process in the di-lepton channel has been recently performed by the ATLAS collaboration at the LHC. The process 2 → 7, which includes top-quark decays to the dilepton final state and off-shell effects, was considered. Based on these calculations the top-quark mass was extracted using a Chi2 fit of the unfolded normalized differential cross-section distribution. The results obtained with the 2 → 3 and 2 →7 calculations were compatible within theoretical uncertainties, providing an important consistency check.

Additional Project Information

DFG classification: 309 Particles, Nuclei and Fields
Software: HELAC-NLO, HELAC-1LOOP, HELAC-DIPOLES, HEPlot, KALEU
Cluster: CLAIX

Publications