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CMS_2018_I1667854

Differential cross section of Z boson production in association with jets in proton-proton collisions at $\sqrt{s} = 13\,$TeV
Experiment: CMS (LHC)
Inspire ID: 1667854
Status: VALIDATED
Authors:
  • cms-pag-conveners-smp@cern.ch
  • Philippe Gras
References:
  • CMS-SMP-16-015
  • https://inspirehep.net/record/1667854
Beams: p+ p+
Beam energies: (6500.0, 6500.0) GeV
Run details:
  • Run MC generators with Z decaying leptonically into both electrons and muons at 13 TeV CoM energy. If only one of the two decay channels is included, set LMODE accordingly.

The production of a Z boson, decaying to two charged leptons, in association with jets in proton-proton collisions at a centre-of-mass energy of 13 TeV is measured. Data recorded with the CMS detector at the LHC are used that correspond to an integrated luminosity of 2.19$\,$fb$^{-1}$. The cross section is measured as a function of the jet multiplicity and its dependence on the transverse momentum of the Z boson, the jet kinematic variables (transverse momentum and rapidity), the scalar sum of the jet momenta, which quantifies the hadronic activity, and the balance in transverse momentum between the reconstructed jet recoil and the Z boson. The measurements are compared with predictions from four different calculations. The first two merge matrix elements with different parton multiplicities in the final state and parton showering, one of which includes one-loop corrections. The third is a fixed-order calculation with next-to-next-to-leading order accuracy for the process with a Z boson and one parton in the final state. The fourth combines the fully differential next-to-next-to-leading order calculation with next-to-next-to-leading logarithm resummation and parton showering.

Source code: CMS_2018_I1667854.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FastJets.hh"
#include "Rivet/Projections/VisibleFinalState.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/VetoedFinalState.hh"
#include "Rivet/Projections/DressedLeptons.hh"

namespace Rivet {


  /// @brief Differential cross section of Z boson production in association with jets at 13 TeV
  ///
  /// @note Code copied from CMS_2015_I1410737 and adapted by P. Gras to CMS-SMP-16-015,
  class CMS_2018_I1667854 : public Analysis {
  public:

    /// Constructor
    DEFAULT_RIVET_ANALYSIS_CTOR(CMS_2018_I1667854);


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

      // Get options from the new option system; defaults to combined e+mu
      _mode = 2;
      if ( getOption("LMODE") == "EL" ) _mode = 0;
      if ( getOption("LMODE") == "MU" ) _mode = 1;
      if ( getOption("LMODE") == "EMU" ) _mode = 2;

      // Projections
      FinalState fs;
      VisibleFinalState visfs(fs);
      VetoedFinalState fs_notaudecay(fs);
      fs_notaudecay.addDecayProductsVeto(PID::TAU);
      fs_notaudecay.addDecayProductsVeto(-PID::TAU);

      IdentifiedFinalState bareMuons(fs_notaudecay);
      bareMuons.acceptIdPair(PID::MUON);
      declare(DressedLeptons(fs, bareMuons, /*dRmax = */0.1,
                             Cuts::abseta < 2.4 && Cuts::pT > 20*GeV), "muons");

      IdentifiedFinalState bareElectrons(fs_notaudecay);
      bareElectrons.acceptIdPair(PID::ELECTRON);
      declare(DressedLeptons(fs, bareElectrons, /*dRmax =*/ 0.1,
                             Cuts::abseta < 2.4 && Cuts::pT > 20*GeV), "electrons");

      FastJets jets(visfs, FastJets::ANTIKT, 0.4);
      declare(jets, "jets");

      // Histograms
      book(_h_excmult_jets_tot, 1, 1, 1);
      book(_h_incmult_jets_tot, 2, 1, 1);
      book(_h_zpt1, 3, 1, 1);
      book(_h_leading_jet_pt_tot, 4, 1, 1);
      book(_h_second_jet_pt_tot, 5, 1, 1);
      book(_h_third_jet_pt_tot, 6, 1, 1);
      book(_h_leading_jet_y_tot, 7, 1, 1);
      book(_h_second_jet_y_tot, 8, 1, 1);
      book(_h_third_jet_y_tot, 9, 1, 1);
      book(_h_ht1_tot, 10, 1, 1);
      book(_h_ht2_tot, 11, 1, 1);
      book(_h_ht3_tot, 12, 1, 1);
      book(_h_ptbal1, 13, 1, 1);
      book(_h_ptbal2, 14, 1, 1);
      book(_h_ptbal3, 15, 1, 1);
      book(_h_jzb, 16, 1, 1);
      book(_h_jzb_ptHigh, 17, 1, 1);
      book(_h_jzb_ptLow, 18, 1, 1);
    }


    /// @brief Z boson finder
    ///
    /// @note We don't use the standard ZFinder class in order to stick to
    /// the definition of the publication that is simpler than the ZFinder algorithm.
    ///
    /// @param leptons pt-ordered list of electrons or muons from which to build the Z boson
    std::unique_ptr<Particle> zfinder(const Particles& leptons) {
      if (leptons.size() < 2) return 0;
      if (leptons[0].charge()*leptons[1].charge() > 0) return 0;
      std::unique_ptr<Particle> cand(new Particle(PID::ZBOSON, leptons[0].mom() + leptons[1].mom()));
      if (cand->mass() < 71.*GeV || cand->mass() > 111.*GeV) return 0;
      return cand;
    }


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

      // Get leptons
      const Particles& muons = apply<DressedLeptons>(event, "muons").particlesByPt();
      const Particles& electrons = apply<DressedLeptons>(event, "electrons").particlesByPt();

      // Look for Z->ee
      std::unique_ptr<Particle> z = zfinder(electrons);
      const Particles* dressedLeptons = 0;
      if (z.get() != nullptr) {
        dressedLeptons = &electrons;
        if (_mode == 1)
          vetoEvent;
      } else { // look for Z->mumu
        z = zfinder(muons);
        if (z.get() != nullptr) {
          dressedLeptons = &muons;
          if (_mode == 0)
            vetoEvent;
        } else { // no Z boson found
          vetoEvent;
        }
      }

      // Cluster jets
      const FastJets& fj = apply<FastJets>(event, "jets");
      const Jets& jets = fj.jetsByPt(Cuts::absrap < 2.4 && Cuts::pT > 30*GeV);

      // Remove jets overlapping with any of the two selected leptons
      Jets goodjets = filter_discard(jets, [dressedLeptons](const ParticleBase& j) {
          return deltaR(j, (*dressedLeptons)[0]) < 0.4 ||  deltaR(j, (*dressedLeptons)[1]) < 0.4;
        });

      // Compute jet pt scalar sum, H_T:
      double ht = sum(goodjets, [](const ParticleBase& j){return j.pT();}, 0.);

      // Fill jet number integral histograms
      _h_excmult_jets_tot->fill(goodjets.size());
      /// @todo Could be better computed by toIntegral transform on exclusive histo
      for (size_t iJet = 0; iJet <= goodjets.size(); iJet++ )
        _h_incmult_jets_tot->fill(iJet);

      if (goodjets.size() < 1) return;

      // Hadronic recoil:
      FourMomentum recoil;
      for (const auto& j: goodjets) {
        recoil += j.momentum();
      }

      // Jet-Z balance = |recoil_T| - |pt(Z)|
      double jzb  = recoil.pT() - z->pT();

      // pT balance:
      double ptbal = (recoil + z->momentum()).pT();

      // Fill leading-jet histograms
      _h_zpt1->fill(z->pT());
      const Jet& j1 = goodjets[0];
      _h_leading_jet_pt_tot->fill(j1.pT()/GeV);
      _h_leading_jet_y_tot->fill(j1.absrapidity());
      _h_ht1_tot->fill(ht/GeV);
      _h_jzb->fill(jzb/GeV);
      if (z->pT() > 50*GeV) {
        _h_jzb_ptHigh->fill(jzb/GeV);
      } else {
        _h_jzb_ptLow->fill(jzb/GeV);
      }
      _h_ptbal1->fill(ptbal/GeV);

      // Fill 2nd jet histograms
      if (goodjets.size() < 2) return;
      const Jet& j2 = goodjets[1];
      _h_second_jet_pt_tot->fill(j2.pT()/GeV);
      _h_second_jet_y_tot->fill(j2.absrapidity());
      _h_ht2_tot->fill(ht/GeV);
      _h_ptbal2->fill(ptbal/GeV);

      // Fill 3rd jet histograms
      if (goodjets.size() < 3) return;
      const Jet& j3 = goodjets[2];
      _h_third_jet_pt_tot->fill(j3.pT()/GeV);
      _h_third_jet_y_tot->fill(j3.absrapidity());
      _h_ht3_tot->fill(ht/GeV);
      _h_ptbal3->fill(ptbal/GeV);
    }


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

      // Normalisation factor
      double norm = (sumOfWeights() != 0) ? crossSection()/sumOfWeights() : 1.0;
      // When running in combined mode, need to average to get lepton xsec
      if (_mode == 2) norm /= 2.;

      // 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_excmult_jets_tot, norm);
      scale(_h_incmult_jets_tot, norm);
      scale(_h_zpt1, norm);
      scale(_h_leading_jet_pt_tot, norm);
      scale(_h_second_jet_pt_tot, norm);
      scale(_h_third_jet_pt_tot, norm);
      scale(_h_leading_jet_y_tot, norm);
      scale(_h_second_jet_y_tot, norm);
      scale(_h_third_jet_y_tot, norm);
      scale(_h_ht1_tot, norm);
      scale(_h_ht2_tot, norm);
      scale(_h_ht3_tot, norm);
      scale(_h_ptbal1, norm);
      scale(_h_ptbal2, norm);
      scale(_h_ptbal3, norm);
      scale(_h_jzb, norm);
      scale(_h_jzb_ptHigh, norm);
      scale(_h_jzb_ptLow, norm);
    }


  protected:

    size_t _mode;


  private:

    /// @name Histograms
    /// @{
    Histo1DPtr _h_excmult_jets_tot,  _h_incmult_jets_tot;
    Histo1DPtr _h_leading_jet_pt_tot, _h_second_jet_pt_tot;
    Histo1DPtr _h_third_jet_pt_tot, _h_fourth_jet_pt_tot;
    Histo1DPtr _h_leading_jet_y_tot, _h_second_jet_y_tot;
    Histo1DPtr _h_third_jet_y_tot, _h_fourth_jet_y_tot;
    Histo1DPtr _h_ht1_tot, _h_ht2_tot, _h_ht3_tot, _h_ht4_tot;
    Histo1DPtr _h_ptbal1, _h_ptbal2, _h_ptbal3;
    Histo1DPtr _h_jzb, _h_jzb_ptHigh, _h_jzb_ptLow;
    Histo1DPtr _h_zpt1;
    /// @}

  };



  DECLARE_RIVET_PLUGIN(CMS_2018_I1667854);

}