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ATLAS_2017_I1625109

Measurement of $ZZ -> 4\ell$ production at 13 TeV
Experiment: ATLAS (LHC)
Inspire ID: 1625109
Status: VALIDATED
Authors:
  • Stefan Richter
References: Beams: p+ p+
Beam energies: (6500.0, 6500.0) GeV
Run details:
  • pp -> Z(->ll)Z(->ll) + jets at 13 TeV

Measurements of $ZZ$ production in the $\ell^+\ell^-\ell^{\prime +}\ell^{\prime -}$-channel in proton--proton collisions at 13 TeV center-of-mass energy at the Large Hadron Collider are presented. The data correspond to 36.1 fb${}^{-1}$ of collisions collected by the ATLAS experiment in 2015 and 2016. Here $\ell$ and $\ell^\prime$ stand for electrons or muons. Integrated and differential $ZZ\to \ell^+\ell^-\ell^{\prime +}\ell^{\prime -}$-cross sections with $Z\to\ell^+\ell^-$-candidate masses in the range of 66 GeV to 116 GeV are measured in a fiducial phase space corresponding to the detector acceptance and corrected for detector effects. The differential cross sections are presented in bins of twenty observables, including several that describe the jet activity. The integrated cross section is also extrapolated to a total phase space and to all standard model decays of $Z$ bosons with mass between 66 GeV and 116 GeV, resulting in a value of 17.3$\pm$0.9[$\pm$0.6(stat)$\pm$0.5(syst)$\pm$0.6(lumi)] pb. The measurements are found to be in good agreement with the standard model. A search for neutral triple gauge couplings is performed using the transverse momentum distribution of the leading $Z$ boson candidate. No evidence for such couplings is found and exclusion limits are set on their parameters.

Source code: ATLAS_2017_I1625109.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/PromptFinalState.hh"
#include "Rivet/Projections/VetoedFinalState.hh"
#include "Rivet/Projections/DressedLeptons.hh"
#include "Rivet/Projections/FastJets.hh"

namespace Rivet {


  class ATLAS_2017_I1625109 : public Analysis {
  public:

    /// Constructor
    /// @brief measurement of on-shell ZZ at 13 TeV
    DEFAULT_RIVET_ANALYSIS_CTOR(ATLAS_2017_I1625109);

    /// @name Analysis methods
    //@{

    struct Dilepton {
      Dilepton() {};
      Dilepton(const ParticlePair & _leptons) : leptons(_leptons) {}

      FourMomentum momentum() const {
        return leptons.first.mom() + leptons.second.mom();
      }

      ParticlePair leptons;
    };


    struct Quadruplet {

      vector<DressedLepton> getLeptonsSortedByPt() const {
        vector<DressedLepton> out = { leadingDilepton.leptons.first, leadingDilepton.leptons.second,
                                      subleadingDilepton.leptons.first, subleadingDilepton.leptons.second };
        std::sort(out.begin(), out.end(), cmpMomByPt);
        return out;
      }

      Quadruplet(const Dilepton& dilepton1, const Dilepton& dilepton2) {
        if (dilepton1.momentum().pt() > dilepton2.momentum().pt()) {
          leadingDilepton = dilepton1;
          subleadingDilepton = dilepton2;
        }
        else {
          leadingDilepton = dilepton2;
          subleadingDilepton = dilepton1;
        }
        leptonsSortedByPt = getLeptonsSortedByPt();
      }

      FourMomentum momentum() const {
        return leadingDilepton.momentum() + subleadingDilepton.momentum();
      }

      double distanceFromZMass() const {
        return abs(leadingDilepton.momentum().mass() - Z_mass) + abs(subleadingDilepton.momentum().mass() - Z_mass);
      }

      Dilepton leadingDilepton;
      Dilepton subleadingDilepton;
      vector<DressedLepton> leptonsSortedByPt;
    };

    typedef vector<Quadruplet> Quadruplets;

    typedef std::pair<size_t, size_t> IndexPair;


    vector<IndexPair> getOppositeChargePairsIndices(const vector<DressedLepton>& leptons) {
      vector<IndexPair> indices = {};
      if (leptons.size() < 2) return indices;
      for (size_t i = 0; i < leptons.size(); ++i) {
        for (size_t k = i+1; k < leptons.size(); ++k) {
          const auto charge_i = leptons.at(i).charge();
          const auto charge_k = leptons.at(k).charge();
          if (charge_i == -charge_k) {
            indices.push_back(std::make_pair(i, k));
          }
        }
      }
      return indices;
    }

    bool indicesOverlap(IndexPair a, IndexPair b) {
      return (a.first == b.first || a.first == b.second || a.second == b.first || a.second == b.second);
    }


    bool passesHierarchicalPtRequirements(const Quadruplet& quadruplet) {
      const auto& sorted_leptons = quadruplet.leptonsSortedByPt;
      if (sorted_leptons.at(0).pt() < 20*GeV)  return false;
      if (sorted_leptons.at(1).pt() < 15*GeV)  return false;
      if (sorted_leptons.at(2).pt() < 10*GeV)  return false;
      return true;
    }

    bool passesDileptonMinimumMassRequirement(const Quadruplet& quadruplet) {
      const auto& leptons = quadruplet.leptonsSortedByPt;
      for (const Particle& l1 : leptons) {
        for (const Particle& l2 : leptons) {
          if (isSame(l1, l2)) continue;
          if ((l1.pid() + l2.pid() == 0) && ((l1.mom() + l2.mom()).mass() < 5.0*GeV))  return false;
        }
      }
      return true;
    }

    bool passesLeptonDeltaRRequirements(const Quadruplet& quadruplet) {
      const auto& leptons = quadruplet.leptonsSortedByPt;
      for (const Particle& l1 : leptons) {
        for (const Particle& l2 : leptons) {
          if (isSame(l1, l2))  continue;
          // Any lepton flavour:
          if (deltaR(l1.mom(), l2.mom()) < 0.1)  return false;
          // Different lepton flavour:
          if ((l1.abspid() - l2.abspid() != 0) && (deltaR(l1.mom(), l2.mom()) < 0.2))  return false;
        }
      }
      return true;
    }

    Quadruplets formQuadrupletsByChannel(const vector<DressedLepton>& same_flavour_leptons, vector<IndexPair> indices) {
      Quadruplets quadruplets = {};
      for (size_t i = 0; i <  indices.size(); ++i) {
        for (size_t k = i+1; k <  indices.size(); ++k) {
          const auto& pair_i = indices.at(i);
          const auto& pair_k = indices.at(k);
          if (indicesOverlap(pair_i, pair_k))  continue;
          const auto d1 = Dilepton({same_flavour_leptons.at(pair_i.first), same_flavour_leptons.at(pair_i.second)});
          const auto d2 = Dilepton({same_flavour_leptons.at(pair_k.first), same_flavour_leptons.at(pair_k.second)});
          const auto quadruplet = Quadruplet(d1, d2);
          if (passesHierarchicalPtRequirements(quadruplet)) quadruplets.push_back(quadruplet);
        }
      }
      return quadruplets;
    }

    Quadruplets formQuadrupletsByChannel(const vector<DressedLepton>& electrons, vector<IndexPair> e_indices,
                                         const vector<DressedLepton>& muons,     vector<IndexPair> m_indices) {
      Quadruplets quadruplets = {};
      for (const auto& pair_e : e_indices) {
        for (const auto& pair_m : m_indices) {
          const auto d1 = Dilepton({electrons.at(pair_e.first), electrons.at(pair_e.second)});
          const auto d2 = Dilepton({muons.at(pair_m.first), muons.at(pair_m.second)});
          const auto quadruplet = Quadruplet(d1, d2);
          if (passesHierarchicalPtRequirements(quadruplet)) quadruplets.push_back(quadruplet);
        }
      }
      return quadruplets;
    }


    Quadruplets getQuadruplets(const vector<DressedLepton>& electrons, const vector<DressedLepton>& muons) {
      const auto oc_electrons_indices = getOppositeChargePairsIndices(electrons);
      const auto oc_muons_indices = getOppositeChargePairsIndices(muons);

      const auto electron_quadruplets = formQuadrupletsByChannel(electrons, oc_electrons_indices);
      const auto muon_quadruplets = formQuadrupletsByChannel(muons, oc_muons_indices);
      const auto mixed_quadruplets = formQuadrupletsByChannel(electrons, oc_electrons_indices, muons, oc_muons_indices);

      auto quadruplets = electron_quadruplets;
      quadruplets.insert(quadruplets.end(), muon_quadruplets.begin(), muon_quadruplets.end());
      quadruplets.insert(quadruplets.end(), mixed_quadruplets.begin(), mixed_quadruplets.end());

      return quadruplets;
    }


    Quadruplet selectQuadruplet(const Quadruplets& quadruplets) {
      if (quadruplets.empty()) throw std::logic_error("Expect at least one quadruplet! The user should veto events without quadruplets.");
      Quadruplets sortedQuadruplets = quadruplets;
      std::sort(sortedQuadruplets.begin(), sortedQuadruplets.end(), [](const Quadruplet& a, const Quadruplet& b) {
        return a.distanceFromZMass() < b.distanceFromZMass();
      });
      return sortedQuadruplets.at(0);
    }

    /// Book histograms and initialise projections before the run
    void init() {
      const Cut presel = Cuts::abseta < 5 && Cuts::pT > 100*MeV;
      const FinalState fs(presel);

      // Prompt leptons, photons, neutrinos
      // Excluding those from tau decay
      const PromptFinalState photons(presel && Cuts::abspid == PID::PHOTON, false);
      const PromptFinalState bare_elecs(presel && Cuts::abspid == PID::ELECTRON, false);
      const PromptFinalState bare_muons(presel && Cuts::abspid == PID::MUON, false);

      // Baseline lepton and jet declaration
      const Cut lepton_baseline_cuts = Cuts::abseta < 2.7 && Cuts::pT > 5*GeV;
      const DressedLeptons elecs = DressedLeptons(photons, bare_elecs, 0.1, lepton_baseline_cuts);
      const DressedLeptons muons = DressedLeptons(photons, bare_muons, 0.1, lepton_baseline_cuts);
      declare(elecs, "electrons");
      declare(muons, "muons");

      VetoedFinalState jet_input(fs);
      jet_input.addVetoOnThisFinalState(elecs);
      jet_input.addVetoOnThisFinalState(muons);
      declare(FastJets(jet_input, FastJets::ANTIKT, 0.4), "jets");

      // // Book histograms
      _h["pT_4l"] = bookHisto1D(2, 1, 1);
      _h["pT_leading_dilepton"] = bookHisto1D(8, 1, 1);
      _h["pT_subleading_dilepton"] = bookHisto1D(14, 1, 1);
      _h["pT_lepton1"] = bookHisto1D(20, 1, 1);
      _h["pT_lepton2"] = bookHisto1D(26, 1, 1);
      _h["pT_lepton3"] = bookHisto1D(32, 1, 1);
      _h["pT_lepton4"] = bookHisto1D(38, 1, 1);
      _h["absy_4l"] = bookHisto1D(44, 1, 1);
      _h["deltay_dileptons"] = bookHisto1D(50, 1, 1);
      _h["deltaphi_dileptons"] = bookHisto1D(56, 1, 1);
      _h["N_jets"] = bookHisto1D(62, 1, 1);
      _h["N_central_jets"] = bookHisto1D(68, 1, 1);
      _h["N_jets60"] = bookHisto1D(74, 1, 1);
      _h["mass_dijet"] = bookHisto1D(80, 1, 1);
      _h["deltay_dijet"] = bookHisto1D(86, 1, 1);
      _h["scalarpTsum_jets"] = bookHisto1D(92, 1, 1);
      _h["abseta_jet1"] = bookHisto1D(98, 1, 1);
      _h["abseta_jet2"] = bookHisto1D(104, 1, 1);
      _h["pT_jet1"] = bookHisto1D(110, 1, 1);
      _h["pT_jet2"] = bookHisto1D(116, 1, 1);
    }


    /// Perform the per-event analysis
    void analyze(Event const & event) {
      const double weight = event.weight();

      const auto& baseline_electrons = apply<DressedLeptons>(event, "electrons").dressedLeptons();
      const auto& baseline_muons = apply<DressedLeptons>(event, "muons").dressedLeptons();

      // Form all possible quadruplets passing hierarchical lepton pT cuts
      const auto quadruplets = getQuadruplets(baseline_electrons, baseline_muons);

      if (quadruplets.empty())  vetoEvent;

      // Select the best quadruplet, the one minimising the total distance from the Z pole mass
      auto const quadruplet = selectQuadruplet(quadruplets);

      // Event selection on the best quadruplet
      if (!passesDileptonMinimumMassRequirement(quadruplet)) vetoEvent;
      if (!passesLeptonDeltaRRequirements(quadruplet)) vetoEvent;
      if (!inRange(quadruplet.leadingDilepton.momentum().mass(), 66*GeV, 116*GeV)) vetoEvent;
      if (!inRange(quadruplet.subleadingDilepton.momentum().mass(), 66*GeV, 116*GeV)) vetoEvent;

      // Select jets
      Jets alljets = apply<JetAlg>(event, "jets").jetsByPt(Cuts::pT > 30*GeV);
      for (const DressedLepton& lep : quadruplet.leptonsSortedByPt)
        ifilter_discard(alljets, deltaRLess(lep, 0.4));
      const Jets jets = alljets;
      const Jets centralJets = filterBy(jets, Cuts::abseta < 2.4);
      const Jets pt60Jets = filterBy(jets, Cuts::pT > 60*GeV);

      const auto& leadingDilepton = quadruplet.leadingDilepton.momentum();
      const auto& subleadingDilepton = quadruplet.subleadingDilepton.momentum();

      _h["pT_4l"]->fill((leadingDilepton + subleadingDilepton).pt()/GeV, weight);
      _h["pT_leading_dilepton"]->fill(leadingDilepton.pt()/GeV, weight);
      _h["pT_subleading_dilepton"]->fill(subleadingDilepton.pt()/GeV, weight);
      _h["pT_lepton1"]->fill(quadruplet.leptonsSortedByPt.at(0).pt()/GeV, weight);
      _h["pT_lepton2"]->fill(quadruplet.leptonsSortedByPt.at(1).pt()/GeV, weight);
      _h["pT_lepton3"]->fill(quadruplet.leptonsSortedByPt.at(2).pt()/GeV, weight);
      _h["pT_lepton4"]->fill(quadruplet.leptonsSortedByPt.at(3).pt()/GeV, weight);
      _h["absy_4l"]->fill((leadingDilepton + subleadingDilepton).absrapidity(), weight);
      _h["deltay_dileptons"]->fill(fabs(leadingDilepton.rapidity() - subleadingDilepton.rapidity()), weight);
      _h["deltaphi_dileptons"]->fill(deltaPhi(leadingDilepton, subleadingDilepton)/pi, weight);
      _h["N_jets"]->fill(jets.size(), weight);
      _h["N_central_jets"]->fill(centralJets.size(), weight);
      _h["N_jets60"]->fill(pt60Jets.size(), weight);

      // If at least one jet present
      if (jets.empty())  vetoEvent;
      _h["scalarpTsum_jets"]->fill(sum(jets, pT, 0.)/GeV, weight);
      _h["abseta_jet1"]->fill(jets.front().abseta(), weight);
      _h["pT_jet1"]->fill(jets.front().pt()/GeV, weight);

      // If at least two jets present
      if (jets.size() < 2)  vetoEvent;
      _h["mass_dijet"]->fill((jets.at(0).mom() + jets.at(1).mom()).mass()/GeV, weight);
      _h["deltay_dijet"]->fill(fabs(jets.at(0).rapidity() - jets.at(1).rapidity()), weight);
      _h["abseta_jet2"]->fill(jets.at(1).abseta(), weight);
      _h["pT_jet2"]->fill(jets.at(1).pt()/GeV, weight);
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      // Normalise histograms to cross section
      const double sf = crossSectionPerEvent() / femtobarn;
      for (map<string, Histo1DPtr>::iterator it = _h.begin(); it != _h.end(); ++it) {
        scale(it->second, sf);
      }
    }
    //@}

  private:
    /// @name Histograms
    //@{
    map<string, Histo1DPtr> _h;
    static constexpr double Z_mass = 91.1876;
    //@}
  };


  // The hook for the plugin system
  DECLARE_RIVET_PLUGIN(ATLAS_2017_I1625109);
}