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CMS_2019_I1764472

Measurement of the differential ttbar production cross section as a function of the jet mass and top quark mass in boosted hadronic top quark decays.
Experiment: CMS (LHC)
Inspire ID: 1764472
Status: VALIDATED
Authors:
  • Dennis Schwarz
  • Roman Kogler
  • Johannes Haller
References:
  • TOP-19-005
  • Phys.Rev.Lett. 124 (2020) 20, 202001
  • arXiv: 1911.03800
Beams: p+ p+
Beam energies: (6500.0, 6500.0) GeV
Run details:
  • ttbar events at sqrt(s) = 13 TeV, lepton+jets selection at particle level

A measurement of the ttbar production cross section as a function of the jet mass of hadronic decays of boosted top quarks is presented. The measurement is carried out in the lepton+jets channel. As leptons, muons and electrons are defined with originate from the W boson decay. Jets are clustered from all stable particles excluding neutrinos in a two-step procedure using the XCone algorithm. At first, two large jets (R=1.2) are found aiming at a reconstruction of the two top quarks. Using the constituents of those jets, XCone is run again finding three subjets with R=0.4. A jet representing the hadronic and leptonic top quark decay is found by the angular distance to the leading lepton in the event. The final jets are constructed as the sum of the subjet four-momenta. The jet identified as the one containing the hadronic top quark decay is required to have \pt > 400 GeV. Furthermore, the jet mass of this jet has to be larger than the invariant mass of the combined system of second jet and lepton. Both, the differential and normalized differential ttbar production cross sections are measured as a function of the jet mass.

Source code: CMS_2019_I1764472.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/PartonicTops.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/VetoedFinalState.hh"
#include "Rivet/Projections/PromptFinalState.hh"
#include "Rivet/Projections/FastJets.hh"
#include "Rivet/Projections/DressedLeptons.hh"
#include "Rivet/Projections/ChargedLeptons.hh"

#include "fastjet/contrib/Nsubjettiness.hh"
#include "fastjet/contrib/XConePlugin.hh"

namespace Rivet {


  /// @brief Measurement of the jet mass for boosted top quarks at 13 TeV
  class CMS_2019_I1764472 : public Analysis {
  public:

    /// Constructor
    DEFAULT_RIVET_ANALYSIS_CTOR(CMS_2019_I1764472);


    /// @name Analysis methods
    //@{

    void init() {

      // Prompt leptons
      ChargedLeptons charged_leptons;
      PromptFinalState prompt_leptons(charged_leptons);
      declare(prompt_leptons, "PromptLeptons");

      // Final state particles for jet clustering
      VetoedFinalState fs_jets;
      fs_jets.vetoNeutrinos();

      // First XCone jet clustering step
      fastjet::contrib::PseudoXConePlugin* plugin_xcone = new fastjet::contrib::PseudoXConePlugin(2, 1.2, 2.0);
      declare(FastJets(fs_jets, plugin_xcone), "FatJets");

      // Partonic tops for decay channel definition
      declare(PartonicTops(PartonicTops::DecayMode::E_MU, false), "LeptonicTops");
      declare(PartonicTops(PartonicTops::DecayMode::HADRONIC), "HadronicTops");

      // Book histograms
      book(_hist_mass, "d01-x01-y01");
      book(_hist_mass_norm, "d02-x01-y01");
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {

      // Decay mode check
      const Particles& leptonicTops = apply<PartonicTops>(event, "LeptonicTops").particlesByPt();
      const Particles& hadronicTops = apply<PartonicTops>(event, "HadronicTops").particlesByPt();
      if (leptonicTops.size() != 1 || hadronicTops.size() != 1) vetoEvent;


      // Get prompt leptons
      const PromptFinalState& prompt_leptons = apply<PromptFinalState>(event, "PromptLeptons");
      const Particles & leptons = prompt_leptons.particles();
      if(leptons.empty()) vetoEvent;

      // Select leading lepton
      Particle lepton;
      for(const Particle& l : leptons){
        if(l.pT() > lepton.pT()) lepton = l;
      }
      if(lepton.pT() < 60*GeV) vetoEvent;

      // Get the fat jets
      const Jets& fatjets = applyProjection<FastJets>(event, "FatJets").jets();

      // Get index of hadronic jet by distance to lepton
      int ihad = 0;
      int ilep = 1;

      double dR0 = deltaR(lepton, fatjets.at(0));
      double dR1 = deltaR(lepton, fatjets.at(1));

      if(dR0 < dR1){
        ihad = 1;
        ilep = 0;
      }

      // Get jet constituents
      const Particles & phad = fatjets.at(ihad).particles();
      const Particles & plep = fatjets.at(ilep).particles();

      // Cluster subjets
      FinalState fs_dummy;
      fastjet::JetDefinition::Plugin* plugin_subhad = new fastjet::contrib::PseudoXConePlugin(3, 0.4, 2.0);
      fastjet::contrib::PseudoXConePlugin* plugin_sublep = new fastjet::contrib::PseudoXConePlugin(3, 0.4, 2.0);
      FastJets hadsubcluster(fs_dummy, plugin_subhad);
      FastJets lepsubcluster(fs_dummy, plugin_sublep);
      hadsubcluster.calc(phad);
      lepsubcluster.calc(plep);

      Jets subjets_had = hadsubcluster.jets();
      Jets subjets_lep = lepsubcluster.jets();

      // Subtract the lepton four vector from closest subjet if dR<0.4
      Jets subjets_had_clean;
      double dRmin_had = 0.4;
      unsigned int i_dRmin_had = 0;
      bool found_match_had = false;
      for(unsigned int i=0; i<subjets_had.size(); i++){
        double dR = deltaR(subjets_had[i], lepton);
        if(dR < dRmin_had){
          dRmin_had = dR;
          i_dRmin_had = i;
          found_match_had = true;
        }
      }
      for(unsigned int i=0; i<subjets_had.size(); i++){
        Jet subjet = subjets_had[i];
        if(found_match_had && i == i_dRmin_had) subjet = Jet(subjets_had[i].momentum()-lepton.momentum(), subjets_had[i].particles(), subjets_had[i].tags());
        subjets_had_clean.push_back(subjet);
      }
      std::sort(subjets_had_clean.begin(), subjets_had_clean.end(), cmpMomByPt);

      // do the same for lep jets
      Jets subjets_lep_clean;
      double dRmin_lep = 0.4;
      unsigned int i_dRmin_lep = 0;
      bool found_match_lep = false;
      for(unsigned int i=0; i<subjets_lep.size(); i++){
        double dR = deltaR(subjets_lep[i], lepton);
        if(dR < dRmin_lep){
          dRmin_lep = dR;
          i_dRmin_lep = i;
          found_match_lep = true;
        }
      }
      for(unsigned int i=0; i<subjets_lep.size(); i++){
        Jet subjet = subjets_lep[i];
        if(found_match_lep && i == i_dRmin_lep) subjet = Jet(subjets_lep[i].momentum()-lepton.momentum(), subjets_lep[i].particles(), subjets_lep[i].tags());
        subjets_lep_clean.push_back(subjet);
      }
      std::sort(subjets_lep_clean.begin(), subjets_lep_clean.end(), cmpMomByPt);

      // Subjet cuts
      if(subjets_had_clean.size() != 3) vetoEvent;
      if(subjets_lep_clean.size() != 3) vetoEvent;
      for (Jet jet : subjets_had_clean){
          if(jet.pT() < 30*GeV) vetoEvent;
          if(jet.abseta() > 2.5) vetoEvent;
      }

      // Combine subjets to final jets
      FourMomentum hadjet;
      for(Jet subjet : subjets_had_clean){
        if(subjet.abseta() < 2.5) hadjet += subjet.momentum();
      }
      FourMomentum lepjet;
      for(Jet subjet : subjets_lep_clean){
        if(subjet.abseta() < 2.5) lepjet += subjet.momentum();
      }

      // Jet pT cuts
      if(hadjet.pT() < 400*GeV) vetoEvent;
      if(lepjet.pT() < 10*GeV) vetoEvent;

      // m(hadjet) > m(lepjet+lepton)
      FourMomentum secondJetLepton = lepjet + lepton.momentum();
      if(hadjet.mass() < secondJetLepton.mass()) vetoEvent;

      // Fill histograms
      _hist_mass->fill(hadjet.mass()/GeV);
      _hist_mass_norm->fill(hadjet.mass()/GeV);

    }

    /// Normalise and scale histograms
    void finalize() {
      const double sf = crossSection() / femtobarn / sumOfWeights();
      scale(_hist_mass, sf);
      normalize(_hist_mass_norm, 1.0, false);
    }

    //@}


  private:

    // Histograms
    Histo1DPtr _hist_mass, _hist_mass_norm;

  };



  DECLARE_RIVET_PLUGIN(CMS_2019_I1764472);

}