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CMS_2017_I1499471

Measurements of the associated production of a Z boson and b jets in pp collisions at $\sqrt{s} = 8$ TeV
Experiment: CMS (LHC)
Inspire ID: 1499471
Status: VALIDATED
Authors:
  • Fabio Cossutti
References:
  • DOI:10.1140/epjc/s10052-017-5140-y
  • arXiv: 1611.06507
  • CMS-SMP-14-010
Beams: p+ p+
Beam energies: (4000.0, 4000.0) GeV
Run details:
  • Run MC generators with Z decaying leptonically into both electrons and muons at 8 TeV CoM energy. For jet-flavour inclusive samples, order of 20 million unweighted events can give a reasonable global comparison, but precision in the high jet multiplicity region/high jet pt may require substantially larger samples or statistical enhancement of high jet multiplicities.

Measurements of the associated production of a Z boson with at least one jet originating from a b quark in proton-proton collisions at $\sqrt{s} = 8$ TeV are presented. Differential cross sections are measured with data collected by the CMS experiment corresponding to an integrated luminosity of 19.8 $\text{fb}^{-1}$. Z bosons are reconstructed through their decays to electrons and muons. Cross sections are measured as a function of observables characterizing the kinematics of the b jet and the Z boson. Ratios of differential cross sections for the associated production with at least one b jet to the associated production with any jet are also presented. The production of a Z boson with at least two b jets is investigated, and differential cross sections are measured for the dijet system. Results are compared to theoretical predictions, testing two different flavour schemes for the choice of initial-state partons. Note of the authors: the Z boson selection and b-tagging algorithms used in the code are based on standard Rivet code and do not exactly coincide with those used in the paper. They anyway reproduce the paper results at the percent level or better for most of the bins.

Source code: CMS_2017_I1499471.cc
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#include "Rivet/Analysis.hh"
#include "Rivet/Tools/Cuts.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/VetoedFinalState.hh"
#include "Rivet/Projections/DressedLeptons.hh"
#include "Rivet/Projections/FastJets.hh"
#include "Rivet/Projections/ZFinder.hh"

//#define DebugLog 

namespace Rivet {

  class CMS_2017_I1499471 : public Analysis {
  public:
    
    /// Constructor
    DEFAULT_RIVET_ANALYSIS_CTOR(CMS_2017_I1499471);
    
    /// Book histograms and initialise projections before the run
    void init() {

#ifdef DebugLog
      // set optionally the verbosity for the internal Rivet message system
      getLog().setLevel(0);
#endif      

      FinalState fs; ///< @todo No cuts?
      VisibleFinalState visfs(fs);

      ZFinder zeeFinder(fs, Cuts::abseta < 2.4 && Cuts::pT > 20*GeV, PID::ELECTRON, 71.0*GeV, 111.0*GeV, 0.1 );
      declare(zeeFinder, "ZeeFinder");

      ZFinder zmumuFinder(fs, Cuts::abseta < 2.4 && Cuts::pT > 20*GeV, PID::MUON, 71.0*GeV, 111.0*GeV, 0.1 );
      declare(zmumuFinder, "ZmumuFinder");

      VetoedFinalState jetConstits(visfs);
      jetConstits.addVetoOnThisFinalState(zeeFinder);
      jetConstits.addVetoOnThisFinalState(zmumuFinder);

      FastJets akt05Jets(jetConstits, FastJets::ANTIKT, 0.5);
      declare(akt05Jets, "AntiKt05Jets");
      
      //Histograms booking
      
      book(_h_first_bjet_pt_b ,1,1,1);
      book(_h_first_bjet_abseta_b ,3,1,1);
      book(_h_Z_pt_b ,5,1,1);
      book(_h_HT_b ,7,1,1);
      book(_h_Dphi_Zb_b ,9,1,1);
      
      book(_h_first_jet_pt_ratio ,2,1,1);
      book(_h_first_jet_abseta_ratio ,4,1,1);
      book(_h_Z_pt_ratio ,6,1,1);
      book(_h_HT_ratio ,8,1,1);
      book(_h_Dphi_Zj_ratio ,10,1,1);
      
      book(_h_first_jet_pt, "first_jet_pt", refData(1,1,1) ); // (*_h_first_bjet_pt_b);
      book(_h_first_jet_abseta, "first_jet_abseta", refData(3,1,1) ); // (*_h_first_bjet_abseta_b);
      book(_h_Z_pt, "Z_pt", refData(5,1,1) ); // (*_h_Z_pt_b);
      book(_h_HT, "HT", refData(7,1,1) ); // (*_h_HT_b);
      book(_h_Dphi_Zj, "Dphi_Zj", refData(9,1,1) ); // (*_h_Dphi_Zb_b);

      book(_h_first_bjet_pt_bb ,11,1,1);
      book(_h_second_bjet_pt_bb ,12,1,1);
      book(_h_Z_pt_bb ,13,1,1);
      book(_h_bb_mass_bb ,14,1,1);
      book(_h_Zbb_mass_bb ,15,1,1);
      book(_h_Dphi_bb ,16,1,1);
      book(_h_DR_bb ,17,1,1);
      book(_h_DR_Zbmin_bb ,18,1,1);
      book(_h_A_DR_Zb_bb ,19,1,1);

      book(_h_bjet_multiplicity ,20,1,1);

    }
                
        
    /// Perform the per-event analysis
    void analyze(const Event& event) {
            
      const ZFinder& zeeFS = applyProjection<ZFinder>(event, "ZeeFinder");
      const ZFinder& zmumuFS = applyProjection<ZFinder>(event, "ZmumuFinder");

      const Particles& zees = zeeFS.bosons();
      const Particles& zmumus = zmumuFS.bosons();

      // We did not find exactly one Z. No good.
      if (zees.size() + zmumus.size() != 1) {
        MSG_DEBUG("Did not find exactly one good Z candidate");
        vetoEvent;
      }

      //event identification depending on mass window
      bool ee_event=false;
      bool mm_event=false;
            
      if (zees.size() == 1) { ee_event = true; }
      if (zmumus.size() == 1) { mm_event = true; }
      const Particles& theLeptons = zees.size() ? zeeFS.constituents() : zmumuFS.constituents();

      // Cluster jets
      // NB. Veto has already been applied on leptons and photons used for dressing
      const FastJets& fj = applyProjection<FastJets>(event, "AntiKt05Jets");
      const Jets& jets = fj.jetsByPt(Cuts::abseta < 2.4 && Cuts::pT > 30*GeV);

      // Perform lepton-jet overlap and HT calculation
      double Ht = 0;
      Jets goodjets;
      for (const Jet& j : jets) {
        // Decide if this jet is "good", i.e. isolated from the leptons
        /// @todo Nice use-case for any() and a C++11 lambda
        bool overlap = false;
        for (const Particle& l : theLeptons) {
          if (Rivet::deltaR(j, l) < 0.5) {
            overlap = true;
            break;
          }
        }

        // Fill HT and good-jets collection
        if (overlap) continue;
        goodjets.push_back(j);
        Ht += j.pT();
      }

      // We don't care about events with no isolated jets
      if (goodjets.empty()) {
        MSG_DEBUG("No jets in event");
        vetoEvent;
      }

      Jets jb_final;
            
      //identification of bjets
            
      for (const Jet& j : goodjets) {
        if ( j.bTagged() ) { jb_final.push_back(j); }
      }
            
      //Event weight
      const double w = 0.5;
            
      //histogram filling

      if ((ee_event || mm_event) && goodjets.size() > 0) {
        
        FourMomentum j1(goodjets[0].momentum());

        _h_first_jet_pt->fill(j1.pt(),w);
        _h_first_jet_abseta->fill(fabs(j1.eta()),w);
        if ( ee_event ) _h_Z_pt->fill(zees[0].pt(),w);
        if ( mm_event ) _h_Z_pt->fill(zmumus[0].pt(),w);
        _h_HT->fill(Ht,w);
        if ( ee_event ) _h_Dphi_Zj->fill(deltaPhi(zees[0], j1),w);
        if ( mm_event ) _h_Dphi_Zj->fill(deltaPhi(zmumus[0], j1),w);
        
        if ( jb_final.size() > 0 ) { 

          FourMomentum b1(jb_final[0].momentum());

          _h_bjet_multiplicity->fill(1.,w);

          _h_first_bjet_pt_b->fill(b1.pt(),w);
          _h_first_bjet_abseta_b->fill(fabs(b1.eta()),w);
          if ( ee_event ) _h_Z_pt_b->fill(zees[0].pt(),w);
          if ( mm_event ) _h_Z_pt_b->fill(zmumus[0].pt(),w);
          _h_HT_b->fill(Ht,w);
          if ( ee_event ) _h_Dphi_Zb_b->fill(deltaPhi(zees[0], b1.phi()),w);
          if ( mm_event ) _h_Dphi_Zb_b->fill(deltaPhi(zmumus[0], b1.phi()),w);

          if ( jb_final.size() > 1 ) {

            FourMomentum b2(jb_final[1].momentum());

            _h_bjet_multiplicity->fill(2.,w);

            _h_first_bjet_pt_bb->fill(b1.pt(),w);
            _h_second_bjet_pt_bb->fill(b2.pt(),w);
            if ( ee_event ) _h_Z_pt_bb->fill(zees[0].pt(),w);
            if ( mm_event ) _h_Z_pt_bb->fill(zmumus[0].pt(),w);

            FourMomentum bb = add(b1,b2);
            FourMomentum Zbb;
            if (ee_event) Zbb = add(zees[0],bb);
            if (mm_event) Zbb = add(zmumus[0],bb);

            _h_bb_mass_bb->fill(bb.mass(),w);
            _h_Zbb_mass_bb->fill(Zbb.mass(),w);

            _h_Dphi_bb->fill(deltaPhi(b1,b2),w);
            _h_DR_bb->fill(deltaR(b1,b2),w);

            double DR_Z_b1(0.), DR_Z_b2(0.);
            if ( ee_event ) {
              DR_Z_b1 = deltaR(zees[0],b1);
              DR_Z_b2 = deltaR(zees[0],b2);
            }
            if ( mm_event ) {
              DR_Z_b1 = deltaR(zmumus[0],b1);
              DR_Z_b2 = deltaR(zmumus[0],b2);
            }

            double DR_Zb_min = DR_Z_b1;
            double DR_Zb_max = DR_Z_b2;
            if ( DR_Zb_min > DR_Zb_max ) {
              DR_Zb_min = DR_Z_b2;
              DR_Zb_max = DR_Z_b1;
            }
            double A_Zbb = (DR_Zb_max - DR_Zb_min)/(DR_Zb_max + DR_Zb_min);

            _h_DR_Zbmin_bb->fill(DR_Zb_min,w);
            _h_A_DR_Zb_bb->fill(A_Zbb,w);

          }
          
        }
                                           
      }

    }
   
        
    /// Normalise histograms etc., after the run
    void finalize() {

      const double norm = (sumOfWeights() != 0) ? crossSection()/picobarn/sumOfWeights() : 1.0;

      MSG_INFO("Cross section = " << std::setfill(' ') << std::setw(14) << std::fixed << std::setprecision(3) << crossSection() << " pb");
      MSG_INFO("# Events      = " << std::setfill(' ') << std::setw(14) << std::fixed << std::setprecision(3) << numEvents() );
      MSG_INFO("SumW          = " << std::setfill(' ') << std::setw(14) << std::fixed << std::setprecision(3) << sumOfWeights());
      MSG_INFO("Norm factor   = " << std::setfill(' ') << std::setw(14) << std::fixed << std::setprecision(6) << norm);

      scale( _h_first_bjet_pt_b, 100. );
      scale( _h_first_bjet_abseta_b, 100. );
      scale( _h_Z_pt_b, 100. );
      scale( _h_HT_b, 100. );
      scale( _h_Dphi_Zb_b, 100. );

      divide( _h_first_bjet_pt_b , _h_first_jet_pt , _h_first_jet_pt_ratio );
      divide( _h_first_bjet_abseta_b , _h_first_jet_abseta , _h_first_jet_abseta_ratio );
      divide( _h_Z_pt_b , _h_Z_pt , _h_Z_pt_ratio );
      divide( _h_HT_b , _h_HT , _h_HT_ratio );
      divide( _h_Dphi_Zb_b , _h_Dphi_Zj , _h_Dphi_Zj_ratio );

      scale( _h_first_bjet_pt_b, norm/100. );
      scale( _h_first_bjet_abseta_b, norm/100. );
      scale( _h_Z_pt_b, norm/100. );
      scale( _h_HT_b, norm/100. );
      scale( _h_Dphi_Zb_b, norm/100. );

      scale( _h_first_bjet_pt_bb, norm);
      scale( _h_second_bjet_pt_bb, norm);
      scale( _h_Z_pt_bb, norm);
      scale( _h_bb_mass_bb, norm);
      scale( _h_Zbb_mass_bb, norm);
      scale( _h_Dphi_bb, norm);
      scale( _h_DR_bb, norm);
      scale( _h_DR_Zbmin_bb, norm);
      scale( _h_A_DR_Zb_bb, norm);

      scale( _h_bjet_multiplicity, norm );

    }


  private:

    /// @name Histograms
    
    Histo1DPtr     _h_first_jet_pt, _h_first_bjet_pt_b;
    Histo1DPtr     _h_first_jet_abseta, _h_first_bjet_abseta_b;
    Histo1DPtr     _h_Z_pt, _h_Z_pt_b;
    Histo1DPtr     _h_HT, _h_HT_b;
    Histo1DPtr     _h_Dphi_Zj, _h_Dphi_Zb_b;

    Scatter2DPtr     _h_first_jet_pt_ratio;
    Scatter2DPtr     _h_first_jet_abseta_ratio;
    Scatter2DPtr     _h_Z_pt_ratio;
    Scatter2DPtr     _h_HT_ratio;
    Scatter2DPtr     _h_Dphi_Zj_ratio;
    
    Histo1DPtr     _h_first_bjet_pt_bb, _h_second_bjet_pt_bb;
    Histo1DPtr     _h_Z_pt_bb;
    Histo1DPtr     _h_bb_mass_bb, _h_Zbb_mass_bb;
    Histo1DPtr     _h_Dphi_bb, _h_DR_bb, _h_DR_Zbmin_bb, _h_A_DR_Zb_bb;
    
    Histo1DPtr     _h_bjet_multiplicity;

  };
  
  
  // The hook for the plugin system
  DECLARE_RIVET_PLUGIN(CMS_2017_I1499471);
  
}