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ATLAS_2017_I1614149

Resolved and boosted ttbar l+jets cross sections at 13 TeV
Experiment: ATLAS (LHC)
Inspire ID: 1614149
Status: VALIDATED
Authors:
  • Francesco La Ruffa
  • Steffen Henkelmann
  • Federica Fabbri
  • Christian Gutschow
References: Beams: p+ p+
Beam energies: (6500.0, 6500.0) GeV
Run details:
  • non-all-hadronic ttbar production at 13 TeV

Measurements of differential cross-sections of top-quark pair production in fiducial phase-spaces are presented as a function of top-quark and $t\bar{t}$ system kinematic observables in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 13$ TeV. The data set corresponds to an integrated luminosity of 3.2 fb${}^{-1}$, recorded in 2015 with the ATLAS detector at the CERN Large Hadron Collider. Events with exactly one electron or muon and at least two jets in the final state are used for the measurement. Two separate selections are applied that each focus on different top-quark momentum regions, referred to as resolved and boosted topologies of the $t\bar{t}$ final state. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations by means of calculated $\chi^2$ and $p$-values.

Source code: ATLAS_2017_I1614149.cc
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// -*- C++ -*
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/VetoedFinalState.hh"
#include "Rivet/Projections/IdentifiedFinalState.hh"
#include "Rivet/Projections/PromptFinalState.hh"
#include "Rivet/Projections/DressedLeptons.hh"
#include "Rivet/Projections/FastJets.hh"
#include "Rivet/Projections/MissingMomentum.hh"
#include "Rivet/Tools/fjcontrib/Njettiness.hh"
#include "Rivet/Tools/fjcontrib/Nsubjettiness.hh"
#include "Rivet/Tools/fjcontrib/NjettinessPlugin.hh"

namespace Rivet {


  class ATLAS_2017_I1614149 : public Analysis {
  public:

    /// Constructor
    ///@brief: Resolved and boosted ttbar l+jets cross sections at 13 TeV
    DEFAULT_RIVET_ANALYSIS_CTOR(ATLAS_2017_I1614149);

    void init() {
      // Eta ranges
      Cut eta_full = (Cuts::abseta < 5.0);
      Cut lep_cuts = (Cuts::abseta < 2.5) && (Cuts::pT > 25*GeV);

      // All final state particles
      FinalState fs(eta_full);

      IdentifiedFinalState all_photons(fs);
      all_photons.acceptIdPair(PID::PHOTON);

      // Get photons to dress leptons
      IdentifiedFinalState ph_id(fs);
      ph_id.acceptIdPair(PID::PHOTON);

      // Projection to find the electrons
      IdentifiedFinalState el_id(fs);
      el_id.acceptIdPair(PID::ELECTRON);

      PromptFinalState photons(ph_id);
      photons.acceptTauDecays(true);
      addProjection(photons, "photons");

      PromptFinalState electrons(el_id);
      electrons.acceptTauDecays(true);
      DressedLeptons dressedelectrons(photons, electrons, 0.1, lep_cuts);
      addProjection(dressedelectrons, "elecs");
      DressedLeptons ewdressedelectrons(all_photons, electrons, 0.1, eta_full);

      // Projection to find the muons
      IdentifiedFinalState mu_id(fs);
      mu_id.acceptIdPair(PID::MUON);

      PromptFinalState muons(mu_id);
      muons.acceptTauDecays(true);
      DressedLeptons dressedmuons(photons, muons, 0.1, lep_cuts);
      addProjection(dressedmuons, "muons");
      DressedLeptons ewdressedmuons(all_photons, muons, 0.1, eta_full);

      // Projection to find MET
      declare(MissingMomentum(fs), "MET");

      // remove prompt neutrinos from jet clustering
      IdentifiedFinalState nu_id(fs);
      nu_id.acceptNeutrinos();
      PromptFinalState neutrinos(nu_id);
      neutrinos.acceptTauDecays(true);

      // Jet clustering.
      VetoedFinalState vfs(fs);
      vfs.addVetoOnThisFinalState(ewdressedelectrons);
      vfs.addVetoOnThisFinalState(ewdressedmuons);
      vfs.addVetoOnThisFinalState(neutrinos);
      FastJets jets(vfs, FastJets::ANTIKT, 0.4);
      jets.useInvisibles(true);
      addProjection(jets, "jets");

      // Addition of the large-R jets
      VetoedFinalState vfs1(fs);
      vfs1.addVetoOnThisFinalState(neutrinos);
      FastJets fjets(vfs1, FastJets::ANTIKT, 1.);
      fjets.useInvisibles(JetAlg::NO_INVISIBLES);
      fjets.useMuons(JetAlg::NO_MUONS);
      addProjection(fjets, "fjets");

      bookHists("top_pt_res", 15);
      bookHists("top_absrap_res", 17);
      bookHists("ttbar_pt_res", 19);
      bookHists("ttbar_absrap_res", 21);
      bookHists("ttbar_m_res", 23);
      bookHists("top_pt_boost", 25);
      bookHists("top_absrap_boost", 27);

    }


    void analyze(const Event& event) {

      const double weight = event.weight();

      // Get the selected objects, using the projections.
      vector<DressedLepton> electrons = apply<DressedLeptons>(event, "elecs").dressedLeptons();
      vector<DressedLepton> muons     = apply<DressedLeptons>(event, "muons").dressedLeptons();
      const Jets& jets  = apply<FastJets>(event, "jets").jetsByPt(Cuts::pT > 25*GeV && Cuts::abseta < 2.5);
      const PseudoJets& all_fjets  = apply<FastJets>(event, "fjets").pseudoJetsByPt();


      // get MET
      const Vector3 met = apply<MissingMomentum>(event, "MET").vectorMPT();

      Jets bjets, lightjets;
      for (const Jet& jet : jets) {
        bool b_tagged = jet.bTags(Cuts::pT > 5*GeV).size();
        if ( b_tagged && bjets.size() < 2)  bjets +=jet;
        else lightjets += jet;
      }

      // Implementing large-R jets definition
      // trim the jets
      PseudoJets trimmed_fatJets;
      float Rfilt = 0.2;
      float pt_fraction_min = 0.05;
      fastjet::Filter trimmer(fastjet::JetDefinition(fastjet::kt_algorithm, Rfilt), fastjet::SelectorPtFractionMin(pt_fraction_min));
      for (PseudoJet pjet : all_fjets)  trimmed_fatJets += trimmer(pjet);
      trimmed_fatJets = fastjet::sorted_by_pt(trimmed_fatJets);
      PseudoJets trimmed_jets;
      for (unsigned int i = 0; i < trimmed_fatJets.size(); ++i) {
        FourMomentum tj_mom = momentum(trimmed_fatJets[i]);
        if (tj_mom.pt() <= 300*GeV)  continue;
        if (tj_mom.abseta() >= 2.0)  continue;
        trimmed_jets.push_back(trimmed_fatJets[i]);
      }

      bool single_electron = (electrons.size() == 1) && (muons.empty());
      bool single_muon = (muons.size() == 1) && (electrons.empty());

      DressedLepton *lepton = NULL;
      if (single_electron)   lepton = &electrons[0];
      else if (single_muon)  lepton = &muons[0];

      if (!single_electron && !single_muon) vetoEvent;

      bool pass_resolved = true;
      bool num_b_tagged_jets = (bjets.size() == 2);
      if (!num_b_tagged_jets)  pass_resolved = false;

      if (jets.size() < 4) pass_resolved = false;

      bool pass_boosted = true;
      int fatJetIndex = -1;
      bool passTopTag = false;
      bool passDphi = false;
      bool passAddJet = false;
      bool goodLepJet = false;
      bool lepbtag = false;
      bool hadbtag=false;
      vector<int> lepJetIndex;
      vector<int> jet_farFromHadTopJetCandidate;
      if (met.mod() < 20*GeV)  pass_boosted = false;
      if (pass_boosted) {
        double transmass = _mT(lepton->momentum(), met);
        if (transmass + met.mod() < 60*GeV)  pass_boosted = false;
      }
      if (pass_boosted) {
        if (trimmed_jets.size() >= 1) {
          for (unsigned int j = 0; j<trimmed_jets.size(); ++j) {
            if (tau32( trimmed_jets.at(j), 1. ) < 0.75 &&
                momentum(trimmed_jets.at(j)).mass() > 100*GeV &&
                momentum(trimmed_jets.at(j)).pt() > 300*GeV &&
                momentum(trimmed_jets.at(j)).pt() < 1500*GeV &&
                fabs(momentum(trimmed_jets.at(j)).eta()) < 2.) {
              passTopTag = true;
              fatJetIndex = j;
              break;
            }
          }
        }
      }
      if(!passTopTag && fatJetIndex == -1)  pass_boosted = false;
      if (pass_boosted) {
        double dPhi_fatjet = deltaPhi(lepton->phi(), momentum(trimmed_jets.at(fatJetIndex)).phi());
        double dPhi_fatjet_lep_cut  = 1.0; //2.3
        if (dPhi_fatjet > dPhi_fatjet_lep_cut ) {
          passDphi = true;
        }
      }
      if (!passDphi)   pass_boosted = false;
      if (bjets.empty())   pass_boosted = false;
      if (pass_boosted) {
        for (unsigned int sj = 0; sj < jets.size(); ++sj) {
          double dR = deltaR(jets.at(sj).momentum(), momentum(trimmed_jets.at(fatJetIndex)));
          if(dR > 1.5) {
            passAddJet = true;
            jet_farFromHadTopJetCandidate.push_back(sj);
          }
        }
      }
      if (!passAddJet)  pass_boosted = false;
      if (pass_boosted) {
        for (int ltj : jet_farFromHadTopJetCandidate) {
          double dR_jet_lep = deltaR(jets.at(ltj).momentum(), lepton->momentum());
          double dR_jet_lep_cut = 2.0;//1.5
          if (dR_jet_lep < dR_jet_lep_cut) {
            lepJetIndex.push_back(ltj);
            goodLepJet = true;
          }
        }
      }
      if(!goodLepJet)  pass_boosted = false;
      if (pass_boosted) {
        for (int lepj : lepJetIndex) {
          lepbtag = jets.at(lepj).bTags(Cuts::pT > 5*GeV).size();
          if (lepbtag) break;
        }
      }
      double dR_fatBjet_cut = 1.0;
      if (pass_boosted) {
        for (const Jet& bjet : bjets) {
          hadbtag |= deltaR(momentum(trimmed_jets.at(fatJetIndex)), bjet) < dR_fatBjet_cut;
        }
      }

      if (!(lepbtag || hadbtag))  pass_boosted = false;

      FourMomentum pbjet1; //Momentum of bjet1
      FourMomentum pbjet2; //Momentum of bjet
      int Wj1index = -1, Wj2index = -1;

      if (pass_resolved) {

        if ( deltaR(bjets[0], *lepton) <= deltaR(bjets[1], *lepton) ) {
          pbjet1 = bjets[0].momentum();
          pbjet2 = bjets[1].momentum();
        } else {
          pbjet1 = bjets[1].momentum();
          pbjet2 = bjets[0].momentum();
        }

        double bestWmass = 1000.0*TeV;
        double mWPDG = 80.399*GeV;
        for (unsigned int i = 0; i < (lightjets.size() - 1); ++i) {
          for (unsigned int j = i + 1; j < lightjets.size(); ++j) {
            double wmass = (lightjets[i].momentum() + lightjets[j].momentum()).mass();
            if (fabs(wmass - mWPDG) < fabs(bestWmass - mWPDG)) {
              bestWmass = wmass;
              Wj1index = i;
              Wj2index = j;
            }
          }
        }

        FourMomentum pjet1 = lightjets[Wj1index].momentum();
        FourMomentum pjet2 = lightjets[Wj2index].momentum();

        // compute hadronic W boson
        FourMomentum pWhadron = pjet1 + pjet2;
        double pz = computeneutrinoz(lepton->momentum(), met);
        FourMomentum ppseudoneutrino( sqrt(sqr(met.x()) + sqr(met.y()) + sqr(pz)), met.x(), met.y(), pz);

        //compute leptonic, hadronic, combined pseudo-top
        FourMomentum ppseudotoplepton = lepton->momentum() + ppseudoneutrino + pbjet1;
        FourMomentum ppseudotophadron = pbjet2 + pWhadron;
        FourMomentum pttbar = ppseudotoplepton + ppseudotophadron;

        fillHists("top_pt_res", ppseudotophadron.pt()/GeV, weight);
        fillHists("top_absrap_res", ppseudotophadron.absrap(), weight);
        fillHists("ttbar_pt_res", pttbar.pt()/GeV, weight);
        fillHists("ttbar_absrap_res", pttbar.absrap(), weight);
        fillHists("ttbar_m_res", pttbar.mass()/GeV, weight);
      }

      if (pass_boosted) {// Boosted selection
        double hadtop_pt= momentum(trimmed_jets.at(fatJetIndex)).pt() / GeV;
        double hadtop_absrap= momentum(trimmed_jets.at(fatJetIndex)).absrap();
        fillHists("top_pt_boost", hadtop_pt, weight);
        fillHists("top_absrap_boost", hadtop_absrap, weight);
      }
    }


    void finalize() {
      // Normalize to cross-section
      const double sf = (crossSection() / sumOfWeights());
      for (HistoMap::value_type& hist : _h) {
        scale(hist.second, sf);
        if (hist.first.find("_norm") != string::npos)  normalize(hist.second);
      }
    }


    void bookHists(std::string name, unsigned int index) {
      _h[name] = bookHisto1D(index, 1 ,1);
      _h[name + "_norm"] = bookHisto1D(index + 1, 1 ,1);
    }


    void fillHists(std::string name, double value, double weight) {
      _h[name]->fill(value, weight);
      _h[name + "_norm"]->fill(value, weight);
    }


    double _mT(const FourMomentum &l, const Vector3 &met) const {
      return  sqrt(2.0 * l.pT() * met.mod() * (1 - cos(deltaPhi(l, met))) );
    }


    double tau32(const fastjet::PseudoJet &jet, double jet_rad) const {
      double alpha = 1.0;
      fjcontrib::Nsubjettiness::NormalizedCutoffMeasure normalized_measure(alpha, jet_rad, 1000000);
      // WTA definition
      // Nsubjettiness::OnePass_WTA_KT_Axes wta_kt_axes;
      // as in JetSubStructure recommendations
      fjcontrib::Nsubjettiness::KT_Axes kt_axes;

      /// NsubjettinessRatio uses the results from Nsubjettiness to calculate the ratio
      /// tau_N/tau_M, where N and M are specified by the user. The ratio of different tau values
      /// is often used in analyses, so this class is helpful to streamline code.
      fjcontrib::Nsubjettiness::NsubjettinessRatio tau32_kt(3, 2, kt_axes, normalized_measure);

      double tau32 = tau32_kt.result(jet);
      return tau32;
    }


    double computeneutrinoz(const FourMomentum& lepton, const Vector3 &met) const {
      //computing z component of neutrino momentum given lepton and met
      double pzneutrino;
      double m_W = 80.399; // in GeV, given in the paper
      double k = (( sqr( m_W ) - sqr( lepton.mass() ) ) / 2 ) + (lepton.px() * met.x() + lepton.py() * met.y());
      double a = sqr ( lepton.E() )- sqr ( lepton.pz() );
      double b = -2*k*lepton.pz();
      double c = sqr( lepton.E() ) * sqr( met.mod() ) - sqr( k );
      double discriminant = sqr(b) - 4 * a * c;
      double quad[2] = { (- b - sqrt(discriminant)) / (2 * a), (- b + sqrt(discriminant)) / (2 * a) }; //two possible quadratic solns
      if (discriminant < 0)  pzneutrino = - b / (2 * a); //if the discriminant is negative
      else { //if the discriminant is greater than or equal to zero, take the soln with smallest absolute value
        double absquad[2];
        for (int n=0; n<2; ++n)  absquad[n] = fabs(quad[n]);
        if (absquad[0] < absquad[1])  pzneutrino = quad[0];
        else                          pzneutrino = quad[1];
      }

      return pzneutrino;
    }


  private:

    /// @name Objects that are used by the event selection decisions
    typedef map<string, Histo1DPtr> HistoMap;
    HistoMap _h;

  };


  DECLARE_RIVET_PLUGIN(ATLAS_2017_I1614149);


}