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ATLAS_2018_I1646686

All-hadronic boosted ttbar at 13 TeV
Experiment: ATLAS (LHC)
Inspire ID: 1646686
Status: VALIDATED
Authors:
  • Kyle Cormier
  • Christian Gutschow
References: Beams: p+ p+
Beam energies: (6500.0, 6500.0) GeV
Run details:
  • p + p -> ttbar (all-hadronic, boosted)

Measurements are made of differential cross-sections of highly boosted pair-produced top quarks as a function of top-quark and $t\bar{t}$ system kinematic observables using proton--proton collisions at a center-of-mass energy of $\sqrt{s}=13$ TeV. The data set corresponds to an integrated luminosity of 36.1 fb${}^{-1}$, recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Events with two large-radius jets in the final state, one with transverse momentum pT>500 GeV and a second with $p_\text{T} > 350$ GeV, are used for the measurement. The top-quark candidates are separated from the multijet background using jet substructure information and association with a $b$-tagged jet. The measured spectra are corrected for detector effects to a particle-level fiducial phase space and a parton-level limited phase space, and are compared to several Monte Carlo simulations by means of calculated $\chi^2$ values. The cross-section for $t\bar{t}$ production in the fiducial phase-space region is 292$\pm$7 (stat)$\pm$76(syst) fb, to be compared to the theoretical prediction of 384$\pm$36 fb.

Source code: ATLAS_2018_I1646686.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/PartonicTops.hh"
#include "Rivet/Math/LorentzTrans.hh"

namespace Rivet {


  /// @brief All-hadronic ttbar at 13 TeV
  class ATLAS_2018_I1646686 : public Analysis {
  public:

      /// Constructor
      DEFAULT_RIVET_ANALYSIS_CTOR(ATLAS_2018_I1646686);

      /// Book histograms and initialise projections before the run
      void init() {

        // Get options particle-level only.
        _mode = 0;
        if ( getOption("TMODE") == "PARTICLE" ) _mode = 0;
        if ( getOption("TMODE") == "BOTH" ) _mode = 1;

        //histogram booking
        _h["inclusive"] = bookHisto1D(1,1,1);
        bookHistograms("t_pt", 0, true);
        bookHistograms("t_y",  1, true);
        bookHistograms("t1_pt",         2);
        bookHistograms("t1_y",          3);
        bookHistograms("t2_pt",         4);
        bookHistograms("t2_y",          5);
        bookHistograms("tt_m",          6);
        bookHistograms("tt_pt",         7);
        bookHistograms("tt_y",          8);
        bookHistograms("tt_chi",        9);
        bookHistograms("tt_yboost",    10);
        bookHistograms("tt_pout",      11);
        bookHistograms("tt_dPhi",      12);
        bookHistograms("tt_Ht",        13);
        bookHistograms("tt_cosThStar", 14);

        // Projections
        Cut dressed_lep = (Cuts::abseta < 2.5) && (Cuts::pT >= 25*GeV);
        Cut eta_full = (Cuts::abseta < 5.0);

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

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

        // Projection to find the electrons
        PromptFinalState electrons(Cuts::abspid == PID::ELECTRON, true);
        DressedLeptons dressedelectrons(photons, electrons, 0.1, dressed_lep);
        declare(dressedelectrons, "elecs");
        DressedLeptons ewdressedelectrons(photons, electrons, 0.1, eta_full);

        // Projection to find the muons
        PromptFinalState muons(Cuts::abspid == PID::MUON, true);
        DressedLeptons dressedmuons(photons, muons, 0.1, dressed_lep);
        declare(dressedmuons, "muons");
        DressedLeptons ewdressedmuons(photons, muons, 0.1, eta_full);

        // Jet clustering.
        VetoedFinalState vfs;
        vfs.addVetoOnThisFinalState(ewdressedelectrons);
        vfs.addVetoOnThisFinalState(ewdressedmuons);

        FastJets jets(vfs, FastJets::ANTIKT, 0.4, JetAlg::DECAY_MUONS, JetAlg::DECAY_INVISIBLES);
        declare(jets, "jets");

        FastJets ljets(fs, FastJets::ANTIKT, 1.0, JetAlg::NO_MUONS, JetAlg::NO_INVISIBLES);
        declare(ljets, "ljets" );

        if (_mode != 0 ){
          PartonicTops partonTops;
          declare(partonTops, "partonicTops");
        }
      }


      void analyze(const Event& event) {

        const double weight = event.weight();

        if (_mode != 0){

          // Parton-level top quarks
          const Particles partonicTops = apply<PartonicTops>( event, "partonicTops").particlesByPt();
          FourMomentum top, tbar;
          bool foundT = false, foundTBar = false;
          for (const Particle& ptop : partonicTops) {
            const int pid = ptop.pid();
            if (pid == PID::TQUARK) {
              top = ptop.momentum();
              foundT = true;
            } else if (pid == -PID::TQUARK) {
              tbar = ptop.momentum();
              foundTBar = true;
            }
          }

          FourMomentum t1_parton, t2_parton, ttbar_parton;
          if ( foundT && foundTBar ) {
            t1_parton = top.pT2() > tbar.pT2() ? top : tbar;
            t2_parton = top.pT2() > tbar.pT2() ? tbar : top;
            ttbar_parton = t1_parton + t2_parton;
            
            if ( t1_parton.pT() > 500*GeV && t2_parton.pT() > 350*GeV) {
              
              const double chi_parton = calcChi(t1_parton, t2_parton);
              const double cosThetaStar_parton = abs(calcCosThetaStar(t1_parton, t2_parton));
              if (cosThetaStar_parton == -99) {
                MSG_DEBUG("ttbar going faster than light! Vetoing event. Try turning of partonic tops?");
                vetoEvent;
              }
              const double pout_parton = abs(calcPout(t1_parton, t2_parton));
              const double dPhi_parton = deltaPhi(t1_parton, t2_parton);
              
              const int randomChoice = rand() % 2;
              const FourMomentum& randomTopParton = (randomChoice == 0) ? t1_parton : t2_parton;
              
              fillParton("t_pt", randomTopParton.pT()/GeV, weight);
              fillParton("t_y",  randomTopParton.absrap(), weight);
              
              fillParton("t1_pt", t1_parton.pT()/GeV, weight);
              fillParton("t1_y",  t1_parton.absrap(), weight);
              fillParton("t2_pt", t2_parton.pT()/GeV, weight);
              fillParton("t2_y",  t2_parton.absrap(), weight);
              
              fillParton("tt_m",  ttbar_parton.mass()/TeV, weight);
              fillParton("tt_pt", ttbar_parton.pT()/GeV, weight);
              fillParton("tt_Ht", (t1_parton.pT() + t2_parton.pT())/GeV, weight);
              fillParton("tt_y",  ttbar_parton.absrap(), weight);

              fillParton("tt_yboost", 0.5 * abs(t1_parton.rapidity() + t2_parton.rapidity()), weight);
              fillParton("tt_chi", chi_parton, weight);
              fillParton("tt_cosThStar", cosThetaStar_parton, weight);
              fillParton("tt_pout", pout_parton/GeV, weight);
              fillParton("tt_dPhi", dPhi_parton, weight);
            }
          }
        }

        // Get and veto on dressed leptons
        const vector<DressedLepton> dressedElectrons = apply<DressedLeptons>(event, "elecs").dressedLeptons();
        const vector<DressedLepton> dressedMuons     = apply<DressedLeptons>(event, "muons").dressedLeptons();
        if (!dressedElectrons.empty()) vetoEvent;
        if (!dressedMuons.empty()) vetoEvent;

        // Get jets
        const Jets& all_jets  = apply<FastJets>( event, "jets").jetsByPt(Cuts::pT > 25*GeV && Cuts::abseta < 2.5);
        const FastJets& ljets_fj = apply<FastJets>( event, "ljets");
        const Jets all_ljets = ljets_fj.jetsByPt();

        // Trim the large-R jets
        Jets trimmedJets;
        fastjet::Filter trimmer(fastjet::JetDefinition(fastjet::kt_algorithm, 0.2), fastjet::SelectorPtFractionMin(0.05));
        for (const Jet& jet : all_ljets)
          trimmedJets += ljets_fj.trimJet(jet, trimmer);
        trimmedJets = sortByPt(trimmedJets);

        // Check large-R jets
        Jets ljets;
        vector<bool> b_tagged;
        for (const Jet& jet : trimmedJets) {

          if (jet.pT() < 250 * GeV)  continue;
          if (jet.pT() > 3000 * GeV) continue;
          if (jet.mass() > jet.pT()) continue;
          if (jet.abseta() > 2.0 )   continue;

          ljets += jet;
          b_tagged += jet.bTagged();
        }

        if (all_jets.size() < 2)  vetoEvent;
        if (ljets.size() < 2)     vetoEvent;

        // Identify top and anti top, compute some event variables
        const FourMomentum ttbar = ljets[0].momentum() + ljets[1].momentum();
        const FourMomentum t1 = ljets[0].momentum();
        const FourMomentum t2 = ljets[1].momentum();

        const double chi = calcChi(t1, t2);
        const double cosThetaStar = abs(calcCosThetaStar(t1, t2));
	if (cosThetaStar == -99) {
	  MSG_DEBUG("real ttbar going faster than light! This should not happen. Vetoing event.");
	  vetoEvent;
	}
        const double pout = abs(calcPout(t1, t2));
        const double dPhi = deltaPhi(t1, t2);

        if ( t2.pT() < 350*GeV)  vetoEvent;
        if ( t1.pT() < 500*GeV)  vetoEvent;

        // b-tagging for particle done on large-R jets
        if (!(b_tagged[0] && b_tagged[1]))  vetoEvent;

        // Continues with signal region cuts
        if ( abs(t1.mass() - 172.5 * GeV) > 50*GeV )  vetoEvent;
        if ( abs(t2.mass() - 172.5 * GeV) > 50*GeV )  vetoEvent;

        _h["inclusive"]->fill(0, weight);

        fillHistograms("t1_pt", t1.pT()/GeV, weight);
        fillHistograms("t1_y",  t1.absrap(), weight);
        fillHistograms("t2_pt", t2.pT()/GeV, weight);
        fillHistograms("t2_y",  t2.absrap(), weight);

        fillHistograms("tt_m",  ttbar.mass()/TeV, weight);
        fillHistograms("tt_pt", ttbar.pT()/GeV, weight);
        fillHistograms("tt_Ht", (t1.pT() + t2.pT())/GeV, weight);
        fillHistograms("tt_y",  ttbar.absrap(), weight);

        fillHistograms("tt_yboost", 0.5 * abs(t1.rapidity() + t2.rapidity()), weight);
        fillHistograms("tt_chi", chi, weight);
        fillHistograms("tt_cosThStar", cosThetaStar, weight);
        fillHistograms("tt_pout", pout/GeV, weight);
        fillHistograms("tt_dPhi", dPhi, weight);

      }


      void finalize() {
        // Normalize histograms
        const double sf = crossSection() / sumOfWeights();
        for (auto &hist : _h) {
          scale(hist.second, sf);
          if ((hist.first.find("_norm") != string::npos) && hist.second->integral(false)>0) hist.second->normalize(1.0, false);
        }
      }


      double calcChi(const FourMomentum& t1, const FourMomentum& t2) {
        double ystar = 0.5 * (t1.rapidity()-t2.rapidity());
        double chi = exp( 2 * abs(ystar));
        return chi;
      }

      double calcCosThetaStar(const FourMomentum& t1, const FourMomentum& t2) {
        FourMomentum ttbar = t1 + t2;
        LorentzTransform centreOfMassTrans;
        ttbar.setX(0);
        ttbar.setY(0);
        if (ttbar.betaVec().mod2() > 1){
          return -99;
        }
        centreOfMassTrans.setBetaVec( -ttbar.boostVector() );
        FourMomentum t1_star = centreOfMassTrans.transform(t1);
        double cosThetaStar;
        if (t1_star.p3().mod2() >= 0){
          cosThetaStar = t1_star.pz()/t1_star.p3().mod();
        } else {
          return -99;
        }
        return cosThetaStar;
      }

      double calcPout(const FourMomentum& t1, const FourMomentum& t2) {
        Vector3 t1V = t1.p3();
        Vector3 t2V = t2.p3();
        Vector3 zUnit = Vector3(0., 0., 1.);
        Vector3 vPerp = zUnit.cross(t1V);

        double pout = vPerp.dot(t2V)/vPerp.mod();
        return pout;
      }


    protected:

      size_t _mode;

    private:

      map<string, Histo1DPtr> _h;

      //some functions for booking, filling and scaling the histograms
      void fillHistograms(std::string name, double value, double weight) {
        _h[name]->fill(value, weight);
        _h[name + "_norm"]->fill(value, weight);
      }

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

      void bookHistograms(const std::string name, unsigned int index, bool onlyParton = false) {
        if (!onlyParton) {
          _h[name] = bookHisto1D(index, 1, 1 );
          _h[name + "_norm"] = bookHisto1D(index + 13, 1, 1 );
        }
        if (_mode != 0) {
          _h[name + "_parton"] = bookHisto1D(index + 82, 1, 1 );
          _h[name + "_parton_norm"] = bookHisto1D(index + 97, 1, 1 );
        }
      }

  };


  DECLARE_RIVET_PLUGIN(ATLAS_2018_I1646686);


}