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Les Houches

2023 Session

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ttbar + jets

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The idea is to compare the latest version of all generators to be able to see what the current status of the predictions is. So far, this has never been done for top pair production, including multi-jet merging.

The set-up is the following:

  • 13 TeV.
  • di-lepton top decays, with decays to e+mu, e+e and mu+mu.
  • Including hadronisation and underlying event/MPI.
  • Use MC truth for top reconstruction, i.e., the lepton, neutrino and b-jet momentum, using the “ghost” particle definition. The leptons have to have |eta(l)|<2.4 and pT(l)>20 GeV, b-jets must have |eta(b)|<2.4 and pT(b)>30GeV. Standard rivet dressing for the leptons with cone 0.1 (for both electrons and muons)
  • Jets (“j”) are defined with anti-kT algorithm, DeltaR=0.4, pT(jet)>30GeV and |eta|<4.7.
  • b-jets (“b”) have pT(jet)>30GeV and |eta|<2.4.
  • Light jets (“lj”) have pT(jet)>30GeV and |eta|<4.7, but do not contain a b, even if it's in the region 2.4<|eta|<4.7.


  • pT(top)
  • pT(ttbar)
  • pT(ttbar+jet)
  • m(t,tbar)
  • m(top)
  • DeltaPhi(l1,l2)
  • DeltaPhi(lj1,lj2)
  • m(b1,b2)
  • # of jets
  • # of b-jets
  • # of light jets
  • pT(j1), pT(j2), pT(j3), pT(j4)
  • pT(b1), pT(b2), pT(b3), pT(b4)
  • pT(lj1), pT(lj2), pT(lj3), pT(lj4)
  • asymmetries:
    • lepton
    • ttbar
  • Gap fraction: Q0, inclusive in Delta y

Samples generated by:


  • MadGraph5_aMC@NLO + Pythia8 with FxFx merging up to 2 extra jets at NLO
  • POWHEG+Pythia8 inclusive ttbar


  • POWHEG+pythia8 & 6, inclusive ttbar

Rikkert Frederix:

  • MadGraph5_aMC@NLO + Pythia8, inclusive ttbar

Simone Alioli: POWHEG+Pythia8 ttbar+0,1jet, with MinLo

Stefan Prestel:

  • Pythia8+UNLOPS merging up to 2 extra jets at NLO

Marek Schoenherr:

  • Sherpa+OpenLoops with merging up to 2 extra jets at NLO

Christian Reuschle:

  • Herwig++ (using e.g. MG5+OpenLoops) ttbar+0j NLO matched and ttbar+1j NLO matched

Note: the ttbar comparisons shown on the site may also be of interest and/or provide sanity checks:

Starting point: Rivet routines and data cards

Existing CMS rivet routines for ttbar

Available CMS Monte Carlo samples:
with NNPDF30NLO (5 flavour, alphas=0.118), top mass=172.5 GeV, TuneCUETP8M1 (Tune:ee 7, Tune:pp 18)

Powheg+Pythia8.205 ttbar card:

madgraph_aMC@NLO+Pythia8.205 0,1,2 jets NLO merged cards:
tt012j_5f_ckm_nlo_fxfx_customizecards.dat.txt tt012j_5f_ckm_nlo_fxfx_proc_card.dat.txt tt012j_5f_ckm_nlo_fxfx_run_card.dat.txt tt012j_5f_ckm_nlo_fxfx_madspin_card.dat.txt

Preliminary results

From CMS

A Rivet routine enables to get distributions for all the observables listed above,, which is making use of a helping class, and PseudoTop.hh.

This routine has been run for Powheg + Herwig++ on the one hand, and Powheg + Pythia 8 on the other hand. Distributions are gathered here, while .yoda files can be found here for Powheg + Herwig++ and there for Powheg + Pythia 8. Lepton and ttbar asymmetries and the gap fraction are represented with respect to the ttbar-system mass. In average, they are:

Powheg + Herwig++ Powheg + Pythia 8
Lepton asymmetry 0.0036 0.0006
ttbar asymmetry 0.007 0.005
Gap fraction 0.436 0.444
2015/groups/tools/ttjets.txt · Last modified: 2015/09/16 14:58 by rikkert.frederix