TOP-12-028
Measurement of differential top-quark pair production cross sections in the dilepton final state in pp collisions at 8 TeV

Abstract

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.

Additional Material

Particle Level Definition of the Top Quark

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.

Object construction

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.

  • Lepton: A lepton to be considered in the reconstruction of the pseudotop is defined as a lepton from the prompt decay of a W-boson from the top quark. Only W decays to electron or muon are considered and all leptonic decays from tau are considered as background. The lepton 4-momentum is redefined by summing over all photons in a DR<0.1 cone around the lepton.

  • Jets: defined with anti-KT. Loop over all stable particles excluding neutrinos and photons used in the lepton definition. The R parameter is set to 0.5.

  • b-jets: A jet is a b-jet if a rescaled B-hadron is included in the jet. A rescaled B-hadron is treated as a stable B-hadron (that does not oscillate or decay to another B-hadron) the 4-momentum of which is scaled down by a factor of 10^(-20) and added to the list of particles for jet-clustering. This prescription provides an unambiguous way to identify b-jets.

  • ETmiss/neutrinos: The ETmiss is defined as the leading two neutrinos in the event. This choice is an appropriate particle-level definition, as a neutrino is a detectable object and can therefore be treated like any other stable particle.

  • Event selection (reconstruction-level):
    • Overlap removal: remove a jet if it is the closest one within DR<0.2 from an electron or muon.
    • Eta cuts:
      • |muon_eta| < 2.4
      • |electron_eta|< 2.4
      • |jets_eta |<2.4
    • PT cuts:
      • pt_jets > 30 GeV
      • pt_lepton > 20 GeV
    • dilepton invariant mass >20 GeV
    • MET>40 GeV (for same flavor only)
    • | m(l+l-) - M(Z) | > 15 GeV (for same flavor only)

Definition of pseudotops

  • Event selection (particle-level): All objects as defined above with appropriate isolation. Cuts on pT and eta corresponding to reconstruction level. The procedure is as follows:
    • Require >=2 jets
    • Require >= 2 b-jets
    • Require exactly 2 opposite sign leptons (el, mu)
    • If leptons have the same flavor, require m(l+l-) > 20 GeV and | m(l+l-) - M(Z) | > 15 GeV
    • Consider 2 leading pt neutrinos. Consider the 2 combinations ( nu_1,lep_+)( nu_2,lep_-) or ( nu_1,lep_-)( nu_2,lep_+) where _1 and _2 refer to pt ordering and _+ ,_- refer to the lepton sign. The (nu,lep) pair that minimizes the difference of the combined lepton-neutrino four momenta W1 and W2 with the mass of the particle data group, i.e. | m_W1 - M_W_true | + |m_W2-M_W_true | where M_W_true is the mass of the W boson from the PDG, is considered the proper combination to make the pseudoW+ and the pseudoW- from the di-lepton decay.
    • Analogously, consider the entire b-jet collection and the combined b-jet - W four momenta T1 and T2 . The pair that minimizes the quantity | m_T1 - M_top_true | + |m_T2-M_top_true | where M_top_true is the mass of the top quark from the PDG is considered the proper combination of bs and Ws to determine the pseudotops.

Unfolded distributions  
Figure Caption
DiffXS_HypToppT.png 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
DiffXS_HypTopRapidity.png 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
DiffXS_HypTTBarpT.png 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
DiffXS_HypTTBarRapidity.png 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
DiffXS_HypTTBarMass.png 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
DressGenLeptonpT.gif Particle level vs. generator-level Lepton transverse momentum after dilepton-mass requirement.pdf
GenRecoBJetpT.gif Generator level vs. reconstructed b-jet transverse momentum after full event selection. pdf
PseudoGenTopRapidity.gif Particle level vs. generated top rapidity after full event selection. pdf
PseudoGenToppT.gif Particle Level vs. Generated top transverse momentum after full event selection. pdf
PseudoGenTTBarMass.gif Particle level vs. generated top-pair mass after full event selection. pdf
PseudoGenTTBarpT.gif Particle level vs. generated top-pair transverse momentum after full event selection. pdf
PseudoGenTTBarRapidity.gif Particle level vs. generated top-pair rapidity after full event selection. pdf

Topic attachments
I Attachment History Action Size Date Who Comment
PDFpdf DiffXS_HypTTBarMass.pdf r1 manage 6.6 K 2013-04-26 - 08:47 AndreasMeyer  
PNGpng DiffXS_HypTTBarMass.png r1 manage 129.1 K 2013-04-26 - 09:05 AndreasMeyer  
PDFpdf DiffXS_HypTTBarRapidity.pdf r1 manage 6.5 K 2013-04-26 - 08:47 AndreasMeyer  
PNGpng DiffXS_HypTTBarRapidity.png r1 manage 137.4 K 2013-04-26 - 09:05 AndreasMeyer  
PDFpdf DiffXS_HypTTBarpT.pdf r1 manage 6.4 K 2013-04-26 - 08:47 AndreasMeyer  
PNGpng DiffXS_HypTTBarpT.png r1 manage 127.4 K 2013-04-26 - 09:05 AndreasMeyer  
PDFpdf DiffXS_HypTopRapidity.pdf r1 manage 6.4 K 2013-04-26 - 08:47 AndreasMeyer  
PNGpng DiffXS_HypTopRapidity.png r1 manage 132.5 K 2013-04-26 - 09:05 AndreasMeyer  
PDFpdf DiffXS_HypToppT.pdf r1 manage 6.6 K 2013-04-26 - 08:47 AndreasMeyer  
PNGpng DiffXS_HypToppT.png r1 manage 139.7 K 2013-04-26 - 09:05 AndreasMeyer  
GIFgif DressGenLeptonpT.gif r7 r6 r5 r4 r3 manage 18.3 K 2013-04-30 - 15:16 TylerDorland  
PDFpdf DressGenLeptonpT.pdf r1 manage 5.3 K 2013-04-30 - 15:30 TylerDorland  
GIFgif GenRecoBJetpT.gif r6 r5 r4 r3 r2 manage 28.8 K 2013-04-17 - 15:15 TylerDorland  
PDFpdf GenRecoBJetpT.pdf r1 manage 10.5 K 2013-04-30 - 15:30 TylerDorland  
GIFgif PseudoGenTTBarMass.gif r4 r3 r2 r1 manage 24.2 K 2013-04-30 - 15:54 TylerDorland  
PDFpdf PseudoGenTTBarMass.pdf r2 r1 manage 10.9 K 2013-04-30 - 15:57 TylerDorland  
GIFgif PseudoGenTTBarRapidity.gif r3 r2 r1 manage 14.5 K 2013-04-19 - 06:59 TylerDorland  
PDFpdf PseudoGenTTBarRapidity.pdf r1 manage 4.6 K 2013-04-30 - 15:30 TylerDorland  
GIFgif PseudoGenTTBarpT.gif r3 r2 r1 manage 23.7 K 2013-04-19 - 06:59 TylerDorland  
PDFpdf PseudoGenTTBarpT.pdf r1 manage 7.7 K 2013-04-30 - 15:30 TylerDorland  
GIFgif PseudoGenTopRapidity.gif r3 r2 r1 manage 29.8 K 2013-04-19 - 06:59 TylerDorland  
PDFpdf PseudoGenTopRapidity.pdf r1 manage 6.9 K 2013-04-30 - 15:30 TylerDorland  
GIFgif PseudoGenToppT.gif r3 r2 r1 manage 35.0 K 2013-04-19 - 06:59 TylerDorland  
PDFpdf PseudoGenToppT.pdf r1 manage 12.1 K 2013-04-30 - 15:30 TylerDorland  
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Topic revision: r21 - 2015-06-16 - AndreasMeyer
 
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