Rivet analyses

Measurement of associated Z + charm production in proton-proton collisions at sqrts = 8 TeV

Experiment: CMS (LHC)

Inspire ID: 1634835

Status: VALIDATED

Authors: - Hannes Jung - Juan Pablo Fernandez

References: - Eur.Phys.J.C 78 (2018) 287 - DOI: 10.1140/epjc/s10052-018-5752-x - arXiv: 1711.02143 - Expt page: CMS-SMP-15-009

Beams: p+ p+

Beam energies: (4000.0, 4000.0)GeV

Run details: - Z+c production

A study of the associated production of a Z boson and a charm quark jet (Z+c) and a comparison to production with a b quark jet (Z+b), in pp collisions at a centre-of-mass energy of 8 TeV are presented. The analysis uses a data sample cor- responding to an integrated luminosity of 19.7 fb−1, collected with the CMS detector at the CERN LHC. The Z boson candidates are identified through their decays into pairs of electrons or muons. Jets originating from heavy flavour quarks are iden- tified using semileptonic decays of c or b flavoured hadrons and hadronic decays of charm hadrons. The measurements are performed in the kinematic region with two leptons with pTl > 20 GeV, |ηl| < 2.1, 71 < mll < 111 GeV, and heavy flavour jets with pTjet > 25 GeV and |ηjet| < 2.5. The Z + c production cross section and the cross section ratio are also measured as a function of the transverse momentum of the Z boson and of the heavy flavour jet.

Source code:CMS_2017_I1634835.cc

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/DileptonFinder.hh"
#include "Rivet/Projections/FastJets.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/VetoedFinalState.hh"
#include "Rivet/Tools/Cuts.hh"

namespace Rivet {


  /// @brief Z+charm at 8 TeV
  class CMS_2017_I1634835 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(CMS_2017_I1634835);


    /// @name Analysis methods
    ///@{

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      DileptonFinder zeeFinder(91.2 * GeV, 0.1,
                               Cuts::abseta < 2.1 && Cuts::pT > 20 * GeV && Cuts::abspid == PID::ELECTRON,
                               Cuts::massIn(71 * GeV, 111 * GeV));
      declare(zeeFinder, "ZeeFinder");

      DileptonFinder zmumuFinder(91.2 * GeV, 0.1,
                                 Cuts::abseta < 2.1 && Cuts::pT > 20 * GeV && Cuts::abspid == PID::MUON,
                                 Cuts::massIn(71 * GeV, 111 * GeV));
      declare(zmumuFinder, "ZmumuFinder");

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

      FastJets akt04Jets(jetConstits, JetAlg::ANTIKT, 0.4);
      declare(akt04Jets, "AntiKt04Jets");

      book(_h_Z_pt_cjet, 4, 1, 1);
      book(_h_pt_cjet, 5, 1, 1);
      book(_h_Z_pt_bjet, "TMP/Z_pt_bjet", refData(4, 1, 2));
      book(_h_pt_bjet, "TMP/pt_bjet", refData(5, 1, 2));

      // book ratio histos
      book(_h_R_Z_pt_cjet, 4, 1, 2);
      book(_h_R_jet_pt_cjet, 5, 1, 2);

      book(counter_ee, "TMP/counter_ee");
      book(counter_mm, "TMP/counter_mm");
    }

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

      Particles leptons;
      Particle zcand;
      if (zees.size() == 1) {
        leptons = zeeFS.leptons();
        zcand = zees[0];
        counter_ee->fill();
      }
      if (zmumus.size() == 1) {
        leptons = zmumuFS.leptons();
        zcand = zmumus[0];
        counter_mm->fill();
      }

      // Cluster jets
      // NB. Veto has already been applied on leptons and photons used for dressing
      const FastJets& fj = apply<FastJets>(event, "AntiKt04Jets");
      Jets goodjets = fj.jetsByPt(Cuts::abseta < 2.5 && Cuts::pT > 25 * GeV);
      idiscardIfAnyDeltaRLess(goodjets, leptons, 0.5);

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

      Jets jc_final;
      Jets jb_final;

      // identification of bjets
      int n_btag = count(goodjets, hasBTag());

      for (const Jet& j : goodjets) {
        if (j.cTagged() && n_btag == 0) {
          jc_final.push_back(j);
        }
        if (j.bTagged()) {
          jb_final.push_back(j);
        }
      }
      // histogram filling

      if (goodjets.size() > 0) {
        if (jc_final.size() > 0) {
          _h_pt_cjet->fill(jc_final[0].pt() / GeV);
          _h_Z_pt_cjet->fill(zcand.pt() / GeV);
        }
        if (jb_final.size() > 0) {
          _h_pt_bjet->fill(jb_final[0].pt() / GeV);
          _h_Z_pt_bjet->fill(zcand.pt() / GeV);
        }
      }
    }

    /// Normalise histograms etc., after the run
    void finalize() {
      double norm = (sumOfWeights() != 0) ? crossSection() / picobarn / sumOfWeights() : 1.0;
      // account if we have electrons and muons in the sample
      if ((counter_ee->val() > 1.) && (counter_mm->val() > 1.)) {
        norm = norm / 2.;
      }

      divide(_h_pt_cjet, _h_pt_bjet, _h_R_jet_pt_cjet);
      divide(_h_Z_pt_cjet, _h_Z_pt_bjet, _h_R_Z_pt_cjet);

      scale(_h_Z_pt_cjet, norm);
      scale(_h_pt_cjet, norm);
    }

    ///@}

    /// @name Histograms

    Histo1DPtr _h_pt_cjet;
    Histo1DPtr _h_pt_bjet;
    Histo1DPtr _h_Z_pt_cjet, _h_Z_pt_bjet;

    Estimate1DPtr _h_R_jet_pt_cjet;
    Estimate1DPtr _h_R_Z_pt_cjet;

    CounterPtr counter_ee, counter_mm;
  };

  RIVET_DECLARE_PLUGIN(CMS_2017_I1634835);

} // namespace Rivet