Normalized differential top-quark pair production cross sections are measured in pp-collisions at a centre-of-mass energy of 8 TeV at the LHC with the CMS detector using a dataset recorded in 2012 corresponding to an integrated luminosity of 12.1 fb-1. The measurement is performed in the dilepton decay channels (m+m-, e+e- and m+/- e-/+). The ttbar production cross section is measured as a function of kinematic kinematic properties of the final-state charged leptons and jets associated to b quarks, as well as those of the top quarks and the ttbar system. The data are compared with several predictions from perturbative QCD calculations up to approximate next-to-next-to-leading-order precision. No significant deviations from the standard model are observed.
It is a distinctive property of the top quark that it decays before hadronisation. Top-quark measurements generally imply an event selection that is based on the top quark final states as measurable in the experiment (i.e. after hadronisation and decay of the top quark decay products). The extrapolation from the measured (particle-level) observables back to the top-quark (parton-level) is then inherently model and scheme dependent.
The goal of the analysis presented here, is to define and measure top-quark distributions for which model dependencies are minimized. A new particle-level definition of top-antitop quark pair events is introduced, in the following referred to as "pseudotop". Normalized differential pseudotop production cross-sections are measured as functions of the top quarks and the ttbar system as reconstructed from their final state particles, using a simple kinematic reconstruction algorithm. Detector effects are determined and corrected for, using Monte Carlo generator information at particle-level only, i.e. after full matrix element, parton shower and hadronisation steps. As per standard top quark cross section measurements, backgrounds from non-ttbar events are subtracted.
This analysis is performed in the context of presenting a unified experimental definition of the particle-level top quark.
The following information is also documented at the Particle Level Top Twiki.
All objects below are defined for generator level objects, i.e. objects that have gone through no detector simulation. For details on the reconstruction of Monte Carlo and data, please refer to TOP-12-028. For this analysis, there is no change in the kinematic reconstruction algorithm for data and Monte Carlo after detector simulation, only for generator/particle-level objects.
Unfolded distributions | |
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Figure | Caption |
Normalised differential particle-level tt production cross section as a function of the pT of the particle-level top quark. The superscript t refers to both top and antitop. The inner (outer) error bars indicate the statistical (combined statistical and systematic) uncertainty. The measurement is compared to predictions from MadGraph, POWHEG, and MC@NLO Monte Carlo generators. The MadGraph prediction is shown both as a curve and as a binned histogram. pdf | |
Normalised differential particle-level ttbar production cross section as a function of the rapidity of the particle-level top quark. The superscript t refers to both top and antitop. The inner (outer) error bars indicate the statistical (combined statistical and systematic) uncertainty. The measurement is compared to predictions from MadGraph, POWHEG, and MC@NLO Monte Carlo generators. The MadGraph prediction is shown both as a curve and as a binned histogram. pdf | |
Normalised differential particle-level ttbar production cross section as a function of the pT of the particle-level ttbar system. The inner (outer) error bars indicate the statistical (combined statistical and systematic) uncertainty. The measurement is compared to predictions from MadGraph, POWHEG, and MC@NLO Monte Carlo generators. The MadGraph prediction is shown both as a curve and as a binned histogram. pdf | |
Normalised differential particle-level ttbar production cross section as a function of the rapidity of the particle-level ttbar system. The inner (outer) error bars indicate the statistical (combined statistical and systematic) uncertainty. The measurement is compared to predictions from MadGraph, POWHEG, and MC@NLO Monte Carlo generators. The MadGraph prediction is shown both as a curve and as a binned histogram. pdf | |
Normalised differential particle-level ttbar production cross section as a function of the mass of the ttbar system. The inner (outer) error bars indicate the statistical (combined statistical and systematic) uncertainty. The measurement is compared to predictions from MadGraph, POWHEG, and MC@NLO Monte Carlo generators. The MadGraph prediction is shown both as a curve and as a binned histogram. pdf | |
Particle level vs. generator-level Lepton transverse momentum after dilepton-mass requirement.pdf | |
Generator level vs. reconstructed b-jet transverse momentum after full event selection. pdf | |
Particle level vs. generated top rapidity after full event selection. pdf | |
Particle Level vs. Generated top transverse momentum after full event selection. pdf | |
Particle level vs. generated top-pair mass after full event selection. pdf | |
Particle level vs. generated top-pair transverse momentum after full event selection. pdf | |
Particle level vs. generated top-pair rapidity after full event selection. pdf |