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ATLAS_2012_I1203852

Measurement of the $ZZ(*)$ production cross-section in $pp$ collisions at 7 TeV with ATLAS
Experiment: ATLAS (LHC)
Inspire ID: 1203852
Status: VALIDATED
Authors:
  • Oldrich Kepka
  • Katerina Moudra
References: Beams: p+ p+
Beam energies: (3500.0, 3500.0) GeV
Run details:
  • Run with inclusive $Z$ events, with $Z$ decays to 4 leptons or 2 leptons + MET.

Measurement of the fiducial cross section for $ZZ(*)$ production in proton proton collisions at a centre-of mass energy of 7 TeV, is presented, using data corresponding to an integrated luminosity of 4.6/fb collected by the ATLAS experiment at the Large Hadron Collider. The cross-section is measured using processes with two $Z$ bosons decaying to electrons or muons or with one $Z$ boson decaying to electrons or muons and a second $Z$ boson decaying to neutrinos. The fiducial region contains dressed leptons in restricted $p_T$ and $\eta$ ranges. The selection has specific requirements for both production processes. A measurement of the normalized fiducial cross-section as a function of $ZZ$ invariant mass, leading $Z$ $p_T$ and angle of two leptons coming from the leading $Z$ is also presented for both signal processes.

Source code: ATLAS_2012_I1203852.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FastJets.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/IdentifiedFinalState.hh"
#include "Rivet/Projections/PromptFinalState.hh"
#include "Rivet/Projections/VetoedFinalState.hh"
#include "Rivet/Projections/DressedLeptons.hh"
#include "Rivet/Projections/MergedFinalState.hh"
#include "Rivet/Projections/MissingMomentum.hh"
#include "Rivet/Projections/InvMassFinalState.hh"

namespace Rivet {


  /// Generic Z candidate 
  struct Zstate : public ParticlePair {
    Zstate() { }
    Zstate(ParticlePair _particlepair) : ParticlePair(_particlepair) { }
    FourMomentum mom() const { return first.momentum() + second.momentum(); }
    operator FourMomentum() const { return mom(); }
    static bool cmppT(const Zstate& lx, const Zstate& rx) { return lx.mom().pT() < rx.mom().pT(); }
  };



  /// ZZ analysis
  class ATLAS_2012_I1203852 : public Analysis {
  public:

    /// Default constructor
    ATLAS_2012_I1203852()
      : Analysis("ATLAS_2012_I1203852")
    {    }


    void init() {

      // Get options 
      // Default does everything
      _mode = 0;
      if ( getOption("LMODE") == "LL" )  _mode = 1;
      if ( getOption("LMODE") == "LNU" ) _mode = 2;

      // NB Missing ET is not required to be neutrinos
      FinalState fs((Cuts::etaIn(-5.0, 5.0)));
      PromptFinalState pfs((Cuts::etaIn(-5.0, 5.0)));

      // Final states to form Z bosons
      vids.push_back(make_pair(PID::ELECTRON, PID::POSITRON));
      vids.push_back(make_pair(PID::MUON, PID::ANTIMUON));

      IdentifiedFinalState Photon(fs);
      Photon.acceptIdPair(PID::PHOTON);

      IdentifiedFinalState bare_EL(pfs);
      bare_EL.acceptIdPair(PID::ELECTRON);

      IdentifiedFinalState bare_MU(pfs);
      bare_MU.acceptIdPair(PID::MUON);


      if (_mode!=2) {

	// Selection 1: ZZ-> llll selection
	Cut etaranges_lep = Cuts::abseta < 3.16 && Cuts::pT > 7*GeV;

	DressedLeptons electron_sel4l(Photon, bare_EL, 0.1, etaranges_lep);
	declare(electron_sel4l, "ELECTRON_sel4l");
	DressedLeptons muon_sel4l(Photon, bare_MU, 0.1, etaranges_lep);
	declare(muon_sel4l, "MUON_sel4l");

	// Both ZZ on-shell histos
	book(_h_ZZ_xsect ,1, 1, 1);
	book(_h_ZZ_ZpT   ,3, 1, 1);
	book(_h_ZZ_phill ,5, 1, 1);
	book(_h_ZZ_mZZ   ,7, 1, 1);

	// One Z off-shell (ZZstar) histos
	book(_h_ZZs_xsect ,1, 1, 2);

      }

      if (_mode!=1) {

	// Selection 2: ZZ-> llnunu selection
	Cut etaranges_lep2 = Cuts::abseta < 2.5 && Cuts::pT > 10*GeV;

	DressedLeptons electron_sel2l2nu(Photon, bare_EL, 0.1, etaranges_lep2);
	declare(electron_sel2l2nu, "ELECTRON_sel2l2nu");
	DressedLeptons muon_sel2l2nu(Photon, bare_MU, 0.1, etaranges_lep2);
	declare(muon_sel2l2nu, "MUON_sel2l2nu");

	/// Get all neutrinos. These will not be used to form jets.
	IdentifiedFinalState neutrino_fs(Cuts::abseta < 4.5);
	neutrino_fs.acceptNeutrinos();
	declare(neutrino_fs, "NEUTRINO_FS");

	// Calculate missing ET from the visible final state, not by requiring neutrinos
	declare(MissingMomentum(Cuts::abseta < 4.5), "MISSING");

	VetoedFinalState jetinput;
	jetinput.addVetoOnThisFinalState(bare_MU);
	jetinput.addVetoOnThisFinalState(neutrino_fs);
	
	FastJets jetpro(fs, FastJets::ANTIKT, 0.4);
	declare(jetpro, "jet");

	// ZZ -> llnunu histos
	book(_h_ZZnunu_xsect ,1, 1, 3);
	book(_h_ZZnunu_ZpT   ,4, 1, 1);
	book(_h_ZZnunu_phill ,6, 1, 1);
	book(_h_ZZnunu_mZZ   ,8, 1, 1);

      }

    }


    /// Do the analysis
    void analyze(const Event& e) {


      if (_mode!=2) {

	////////////////////////////////////////////////////////////////////
	// preselection of leptons for ZZ-> llll final state
	////////////////////////////////////////////////////////////////////

	Particles leptons_sel4l;

	const vector<DressedLepton>& mu_sel4l = apply<DressedLeptons>(e, "MUON_sel4l").dressedLeptons();
	const vector<DressedLepton>& el_sel4l = apply<DressedLeptons>(e, "ELECTRON_sel4l").dressedLeptons();

	vector<DressedLepton> leptonsFS_sel4l;
	leptonsFS_sel4l.insert( leptonsFS_sel4l.end(), mu_sel4l.begin(), mu_sel4l.end() );
	leptonsFS_sel4l.insert( leptonsFS_sel4l.end(), el_sel4l.begin(), el_sel4l.end() );

	////////////////////////////////////////////////////////////////////
	// OVERLAP removal dR(l,l)>0.2
	////////////////////////////////////////////////////////////////////
	for (const DressedLepton& l1 : leptonsFS_sel4l) {
	  bool isolated = true;
	  for (DressedLepton& l2 : leptonsFS_sel4l) {
	    const double dR = deltaR(l1, l2);
	    if (dR < 0.2 && !isSame(l1, l2)) { isolated = false; break; }
	  }
	  if (isolated) leptons_sel4l.push_back(l1);
	}

	//////////////////////////////////////////////////////////////////
	// Exactly two opposite charged leptons
	//////////////////////////////////////////////////////////////////

	// calculate total 'flavour' charge
	double totalcharge = 0;
	for (const Particle& l : leptons_sel4l) totalcharge += l.pid();

	// Analyze 4 lepton events
	if (leptons_sel4l.size() == 4 && totalcharge == 0  ) {
	  Zstate Z1, Z2;

	  // Identifies Z states from 4 lepton pairs
	  identifyZstates(Z1, Z2,leptons_sel4l);

	  ////////////////////////////////////////////////////////////////////////////
	  // Z MASS WINDOW
	  //  -ZZ: for both Z: 66<mZ<116 GeV
	  //  -ZZ*: one Z on-shell: 66<mZ<116 GeV, one Z off-shell: mZ>20 GeV
	  ///////////////////////////////////////////////////////////////////////////

	  Zstate leadPtZ = std::max(Z1, Z2, Zstate::cmppT);

	  double mZ1   = Z1.mom().mass();
	  double mZ2   = Z2.mom().mass();
	  double ZpT   = leadPtZ.mom().pT();
	  double phill = fabs(deltaPhi(leadPtZ.first, leadPtZ.second));
	  if (phill > M_PI) phill = 2*M_PI-phill;
	  double mZZ   = (Z1.mom() + Z2.mom()).mass();

	  if (mZ1 > 20*GeV && mZ2 > 20*GeV) {
	    // ZZ* selection
	    if (inRange(mZ1, 66*GeV, 116*GeV) || inRange(mZ2, 66*GeV, 116*GeV)) {
	      _h_ZZs_xsect  -> fill(sqrtS()*GeV); ///< @todo xsec * GeV??
	    }

	    // ZZ selection
	    if (inRange(mZ1, 66*GeV, 116*GeV) && inRange(mZ2, 66*GeV, 116*GeV)) {
	      _h_ZZ_xsect  -> fill(sqrtS()*GeV); ///< @todo xsec * GeV??
	      _h_ZZ_ZpT    -> fill(ZpT);
	      _h_ZZ_phill  -> fill(phill);
	      _h_ZZ_mZZ    -> fill(mZZ);
	    }
	  }
	}
      }

      if (_mode!=1) {

	////////////////////////////////////////////////////////////////////
	/// preselection of leptons for ZZ-> llnunu final state
	////////////////////////////////////////////////////////////////////

	Particles leptons_sel2l2nu; // output
	const vector<DressedLepton>& mu_sel2l2nu = apply<DressedLeptons>(e, "MUON_sel2l2nu").dressedLeptons();
	const vector<DressedLepton>& el_sel2l2nu = apply<DressedLeptons>(e, "ELECTRON_sel2l2nu").dressedLeptons();

	vector<DressedLepton> leptonsFS_sel2l2nu;
	leptonsFS_sel2l2nu.insert( leptonsFS_sel2l2nu.end(), mu_sel2l2nu.begin(), mu_sel2l2nu.end() );
	leptonsFS_sel2l2nu.insert( leptonsFS_sel2l2nu.end(), el_sel2l2nu.begin(), el_sel2l2nu.end() );

	// Lepton preselection for ZZ-> llnunu
	if ((mu_sel2l2nu.empty() || el_sel2l2nu.empty()) // cannot have opposite flavour
	    && (leptonsFS_sel2l2nu.size() == 2) // exactly two leptons
	    && (leptonsFS_sel2l2nu[0].charge() * leptonsFS_sel2l2nu[1].charge() < 1 ) // opposite charge
	    && (deltaR(leptonsFS_sel2l2nu[0], leptonsFS_sel2l2nu[1]) > 0.3) // overlap removal
	    && (leptonsFS_sel2l2nu[0].pT() > 20*GeV && leptonsFS_sel2l2nu[1].pT() > 20*GeV)) { // trigger requirement
	  leptons_sel2l2nu.insert(leptons_sel2l2nu.end(), leptonsFS_sel2l2nu.begin(), leptonsFS_sel2l2nu.end());
	}
	if (leptons_sel2l2nu.empty()) vetoEvent; // no further analysis, fine to veto

	Particles leptons_sel2l2nu_jetveto;
	for (const DressedLepton& l : mu_sel2l2nu) leptons_sel2l2nu_jetveto.push_back(l.constituentLepton());
	for (const DressedLepton& l : el_sel2l2nu) leptons_sel2l2nu_jetveto.push_back(l.constituentLepton());
	double ptll = (leptons_sel2l2nu[0].momentum() + leptons_sel2l2nu[1].momentum()).pT();

	// Find Z1-> ll
	FinalState fs2((Cuts::etaIn(-3.2, 3.2)));
	InvMassFinalState imfs(fs2, vids, 20*GeV, sqrtS());
	imfs.calc(leptons_sel2l2nu);
	if (imfs.particlePairs().size() != 1) vetoEvent;
	const ParticlePair& Z1constituents = imfs.particlePairs()[0];
	FourMomentum Z1 = Z1constituents.first.momentum() + Z1constituents.second.momentum();

	// Z to neutrinos candidate from missing ET
	const MissingMomentum & missmom = applyProjection<MissingMomentum>(e, "MISSING");
	const FourMomentum Z2 = missmom.missingMomentum(ZMASS);
	double met_Znunu = missmom.missingEt(); //Z2.pT();

	// mTZZ
	const double mT2_1st_term = add_quad(ZMASS, ptll) + add_quad(ZMASS, met_Znunu);
	const double mT2_2nd_term = Z1.px() + Z2.px();
	const double mT2_3rd_term = Z1.py() + Z2.py();
	const double mTZZ = sqrt(sqr(mT2_1st_term) - sqr(mT2_2nd_term) - sqr(mT2_3rd_term));

	if (!inRange(Z2.mass(), 66*GeV, 116*GeV)) vetoEvent;
	if (!inRange(Z1.mass(), 76*GeV, 106*GeV)) vetoEvent;

	/////////////////////////////////////////////////////////////
	// AXIAL MET < 75 GeV
	////////////////////////////////////////////////////////////

	double dPhiZ1Z2 = fabs(deltaPhi(Z1, Z2));
	if (dPhiZ1Z2 > M_PI) dPhiZ1Z2 = 2*M_PI - dPhiZ1Z2;
	const double axialEtmiss = -Z2.pT()*cos(dPhiZ1Z2);
	if (axialEtmiss < 75*GeV) vetoEvent;

	const double ZpT   = Z1.pT();
	double phill = fabs(deltaPhi(Z1constituents.first, Z1constituents.second));
	if (phill > M_PI) phill = 2*M_PI - phill;


	////////////////////////////////////////////////////////////////////////////
	// JETS
	//    -"j": found by "jetpro" projection && pT() > 25 GeV && |eta| < 4.5
	//    -"goodjets": "j"  && dR(electron/muon,jet) > 0.3
	//
	// JETVETO: veto all events with at least one good jet
	///////////////////////////////////////////////////////////////////////////
	vector<Jet> good_jets;
	for (const Jet& j : apply<FastJets>(e, "jet").jetsByPt(25)) {
	  if (j.abseta() > 4.5) continue;
	  bool isLepton = 0;
	  for (const Particle& l : leptons_sel2l2nu_jetveto) {
	    const double dR = deltaR(l.momentum(), j.momentum());
	    if (dR < 0.3) { isLepton = true; break; }
	  }
	  if (!isLepton) good_jets.push_back(j);
	}
	size_t n_sel_jets = good_jets.size();
	if (n_sel_jets != 0) vetoEvent;
	

	/////////////////////////////////////////////////////////////
	// Fractional MET and lepton pair difference: "RatioMet"< 0.4
	////////////////////////////////////////////////////////////
	double ratioMet = fabs(Z2.pT() - Z1.pT()) / Z1.pT();
	if (ratioMet  > 0.4 ) vetoEvent;
	
	
	// End of ZZllnunu selection: now fill histograms
	_h_ZZnunu_xsect->fill(sqrtS()/GeV); ///< @todo xsec / GeV??
	_h_ZZnunu_ZpT  ->fill(ZpT);
	_h_ZZnunu_phill->fill(phill);
	_h_ZZnunu_mZZ  ->fill(mTZZ);

      }
    }

    /// Finalize
    void finalize() {
      const double norm = crossSection()/sumOfWeights()/femtobarn;

      if (_mode!=2) {

	scale(_h_ZZ_xsect, norm);
	normalize(_h_ZZ_ZpT);
	normalize(_h_ZZ_phill);
	normalize(_h_ZZ_mZZ);
	scale(_h_ZZs_xsect, norm);
      }

      if (_mode!=1) {

	scale(_h_ZZnunu_xsect, norm);
	normalize(_h_ZZnunu_ZpT);
	normalize(_h_ZZnunu_phill);
	normalize(_h_ZZnunu_mZZ);
      }
    }


  protected:
    // Data members like post-cuts event weight counters go here
    size_t _mode;


  private:

    void identifyZstates(Zstate& Z1, Zstate& Z2, const Particles& leptons_sel4l);
    Histo1DPtr _h_ZZ_xsect, _h_ZZ_ZpT, _h_ZZ_phill, _h_ZZ_mZZ;
    Histo1DPtr _h_ZZs_xsect;
    Histo1DPtr _h_ZZnunu_xsect, _h_ZZnunu_ZpT, _h_ZZnunu_phill, _h_ZZnunu_mZZ;
    vector< pair<PdgId,PdgId> > vids;
    const double ZMASS = 91.1876; // GeV


  };


  /// 4l to ZZ assignment -- algorithm
  void ATLAS_2012_I1203852::identifyZstates(Zstate& Z1, Zstate& Z2, const Particles& leptons_sel4l) {

    /////////////////////////////////////////////////////////////////////////////
    /// ZZ->4l pairing
    /// - Exactly two same flavour opposite charged leptons
    /// - Ambiguities in pairing are resolved by choosing the combination
    ///     that results in the smaller value of the sum |mll - mZ| for the two pairs
    /////////////////////////////////////////////////////////////////////////////

    Particles part_pos_el, part_neg_el, part_pos_mu, part_neg_mu;
    for (const Particle& l : leptons_sel4l) {
      if (l.abspid() == PID::ELECTRON) {
        if (l.pid() < 0) part_neg_el.push_back(l);
        if (l.pid() > 0) part_pos_el.push_back(l);
      }
      else if (l.abspid() == PID::MUON) {
        if (l.pid() < 0) part_neg_mu.push_back(l);
        if (l.pid() > 0) part_pos_mu.push_back(l);
      }
    }

    // ee/mm channel
    if ( part_neg_el.size() == 2 || part_neg_mu.size() == 2) {

      Zstate Zcand_1, Zcand_2, Zcand_3, Zcand_4;
      if (part_neg_el.size() == 2) { // ee
        Zcand_1 = Zstate( ParticlePair( part_neg_el[0],  part_pos_el[0] ) );
        Zcand_2 = Zstate( ParticlePair( part_neg_el[0],  part_pos_el[1] ) );
        Zcand_3 = Zstate( ParticlePair( part_neg_el[1],  part_pos_el[0] ) );
        Zcand_4 = Zstate( ParticlePair( part_neg_el[1],  part_pos_el[1] ) );
      } else { // mumu
        Zcand_1 = Zstate( ParticlePair( part_neg_mu[0],  part_pos_mu[0] ) );
        Zcand_2 = Zstate( ParticlePair( part_neg_mu[0],  part_pos_mu[1] ) );
        Zcand_3 = Zstate( ParticlePair( part_neg_mu[1],  part_pos_mu[0] ) );
        Zcand_4 = Zstate( ParticlePair( part_neg_mu[1],  part_pos_mu[1] ) );
      }

      // We can have the following pairs: (Z1 + Z4) || (Z2 + Z3)
      double minValue_1, minValue_2;
      minValue_1 = fabs( Zcand_1.mom().mass() - ZMASS ) + fabs( Zcand_4.mom().mass() - ZMASS);
      minValue_2 = fabs( Zcand_2.mom().mass() - ZMASS ) + fabs( Zcand_3.mom().mass() - ZMASS);
      if (minValue_1 < minValue_2 ) {
        Z1 = Zcand_1;
        Z2 = Zcand_4;
      } else {
        Z1 = Zcand_2;
        Z2 = Zcand_3;
      }

    // emu channel
    } else if (part_neg_mu.size() == 1 && part_neg_el.size() == 1) {
      Z1 = Zstate ( ParticlePair (part_neg_mu[0],  part_pos_mu[0] ) );
      Z2 = Zstate ( ParticlePair (part_neg_el[0],  part_pos_el[0] ) );
    }

  }


  // The hook for the plugin system
  DECLARE_RIVET_PLUGIN(ATLAS_2012_I1203852);

}