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CMS_2016_PAS_TOP_15_006

Jet multiplicity in the top-quark pair lepton+jets final state at $\sqrt{s} = 8 \text{TeV}$
Experiment: CMS (LHC)
Inspire ID: 1476015
Status: VALIDATED
Authors:
  • Alexis Descroix
  • Frederik Schaaf
  • Markus Seidel
References:
  • CMS-PAS-TOP-15-006
Beams: p+ p+
Beam energies: (4000.0, 4000.0) GeV
Run details:
  • ttbar events at sqrt(s) = 8 TeV (inclusive decay modes)

The top-quark pair differential production cross section in $pp$ collisions at $\sqrt{s}=8 \text{TeV}$ as a function of the number of jets is measured in the lepton+jets (e/mu+jets) final state for an integrated luminosity of 19.7/fb. The cross section is presented in the visible phase space of the measurement.

Source code: CMS_2016_PAS_TOP_15_006.cc
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#include "Rivet/Analysis.hh"
#include "Rivet/Tools/Logging.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/FastJets.hh"
#include "Rivet/Projections/DressedLeptons.hh"
#include "Rivet/Projections/IdentifiedFinalState.hh"
#include "Rivet/Projections/VetoedFinalState.hh"
#include "Rivet/Tools/ParticleName.hh"
#include "Rivet/Tools/ParticleIdUtils.hh"

namespace Rivet {

  namespace { //< only visible in this compilation unit


    /// @brief Special dressed lepton finder
    ///
    /// Find dressed leptons by clustering all leptons and photons
    class SpecialDressedLeptons : public FinalState {
    public:

      /// Constructor
      SpecialDressedLeptons(const FinalState& fs, const Cut& cut)
        : FinalState(cut)
      {
        setName("SpecialDressedLeptons");
        IdentifiedFinalState ifs(fs);
        ifs.acceptIdPair(PID::PHOTON);
        ifs.acceptIdPair(PID::ELECTRON);
        ifs.acceptIdPair(PID::MUON);
        addProjection(ifs, "IFS");
        addProjection(FastJets(ifs, FastJets::ANTIKT, 0.1), "LeptonJets");
      }

      /// Clone on the heap
      virtual unique_ptr<Projection> clone() const {
        return unique_ptr<Projection>(new SpecialDressedLeptons(*this));
      }

      /// Retrieve the dressed leptons
      const vector<DressedLepton>& dressedLeptons() const { return _clusteredLeptons; }

      /// Perform the calculation
      void project(const Event& e) {
        _theParticles.clear();
        _clusteredLeptons.clear();

        vector<DressedLepton> allClusteredLeptons;
        const Jets jets = applyProjection<FastJets>(e, "LeptonJets").jetsByPt(5*GeV);
        for (const Jet& jet : jets) {
          Particle lepCand;
          for (const Particle& cand : jet.particles()) {
            const int absPdgId = cand.abspid();
            if (absPdgId == PID::ELECTRON || absPdgId == PID::MUON) {
              if (cand.pt() > lepCand.pt()) lepCand = cand;
            }
          }
          // Central lepton must be the major component
          if ((lepCand.pt() < jet.pt()/2.) || (lepCand.pdgId() == 0)) continue;

          DressedLepton lepton = DressedLepton(lepCand);
          for (const Particle& cand : jet.particles()) {
            if (cand == lepCand) continue;
            lepton.addPhoton(cand, true);
          }
          allClusteredLeptons.push_back(lepton);
        }

        for (const DressedLepton& lepton : allClusteredLeptons) {
          if (accept(lepton)) {
            _clusteredLeptons.push_back(lepton);
            _theParticles.push_back(lepton.constituentLepton());
            _theParticles += lepton.constituentPhotons();
          }
        }
      }

    private:

      /// Container which stores the clustered lepton objects
      vector<DressedLepton> _clusteredLeptons;

    };

  }



  /// Jet multiplicity in lepton+jets ttbar at 8 TeV
  class CMS_2016_PAS_TOP_15_006 : public Analysis {
  public:

    /// Constructor
    DEFAULT_RIVET_ANALYSIS_CTOR(CMS_2016_PAS_TOP_15_006);


    /// @name Analysis methods
    //@{

    /// Set up projections and book histograms
    void init() {

      // Complete final state
      FinalState fs;
      Cut superLooseLeptonCuts = Cuts::pt > 5*GeV;
      SpecialDressedLeptons dressedleptons(fs, superLooseLeptonCuts);
      addProjection(dressedleptons, "DressedLeptons");

      // Projection for jets
      VetoedFinalState fsForJets(fs);
      fsForJets.addVetoOnThisFinalState(dressedleptons);
      addProjection(FastJets(fsForJets, FastJets::ANTIKT, 0.5), "Jets");

      // Booking of histograms
      _normedElectronMuonHisto = bookHisto1D("normedElectronMuonHisto", 7, 3.5, 10.5,
                                             "Normalized differential cross section in lepton+jets channel", "Jet multiplicity", "Normed units");
      _absXSElectronMuonHisto = bookHisto1D("absXSElectronMuonHisto", 7, 3.5, 10.5,
                                            "Differential cross section in lepton+jets channel", "Jet multiplicity", "pb");
    }


    /// Per-event analysis
    void analyze(const Event& event) {
      const double weight = event.weight();

      // Select ttbar -> lepton+jets
      const SpecialDressedLeptons& dressedleptons = applyProjection<SpecialDressedLeptons>(event, "DressedLeptons");
      vector<FourMomentum> selleptons;
      for (const DressedLepton& dressedlepton : dressedleptons.dressedLeptons()) {
        // Select good leptons
        if (dressedlepton.pT() > 30*GeV && dressedlepton.abseta() < 2.4) selleptons += dressedlepton.mom();
        // Veto loose leptons
        else if (dressedlepton.pT() > 15*GeV && dressedlepton.abseta() < 2.5) vetoEvent;
      }
      if (selleptons.size() != 1) vetoEvent;
      // Identify hardest tight lepton
      const FourMomentum lepton = selleptons[0];

      // Jets
      const FastJets& jets   = applyProjection<FastJets>(event, "Jets");
      const Jets      jets30 = jets.jetsByPt(30*GeV);
      int nJets = 0, nBJets = 0;
      for (const Jet& jet : jets30) {
        if (jet.abseta() > 2.5) continue;
        if (deltaR(jet.momentum(), lepton) < 0.5) continue;
        nJets += 1;
        if (jet.bTagged(Cuts::pT > 5*GeV)) nBJets += 1;
      }
      // Require >= 4 resolved jets, of which two must be b-tagged
      if (nJets < 4 || nBJets < 2) vetoEvent;

      // Fill histograms
      _normedElectronMuonHisto->fill(min(nJets, 10), weight);
      _absXSElectronMuonHisto ->fill(min(nJets, 10), weight);
    }


    void finalize() {
      const double ttbarXS = !std::isnan(crossSectionPerEvent()) ? crossSection() : 252.89*picobarn;
      if (std::isnan(crossSectionPerEvent()))
        MSG_INFO("No valid cross-section given, using NNLO (arXiv:1303.6254; sqrt(s)=8 TeV, m_t=172.5 GeV): " <<
                 ttbarXS/picobarn << " pb");

      const double xsPerWeight = ttbarXS/picobarn / sumOfWeights();
      scale(_absXSElectronMuonHisto, xsPerWeight);

      normalize(_normedElectronMuonHisto);
    }

    //@}


  private:

    /// Histograms
    Histo1DPtr _normedElectronMuonHisto, _absXSElectronMuonHisto;

  };



  // The hook for the plugin system
  DECLARE_RIVET_PLUGIN(CMS_2016_PAS_TOP_15_006);

}