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ATLAS_2014_I1310835

H(125) -> 4l at 8 TeV
Experiment: ATLAS (LHC)
Inspire ID: 1310835
Status: VALIDATED
Authors:
  • Jonathan Stahlman
  • Christian Gutschow
References: Beams: p+ p+
Beam energies: (4000.0, 4000.0) GeV
Run details:
  • p + p -> H (-> 4 l) + X at 8 TeV

Measurements of fiducial and differential cross sections of Higgs boson production in the $H\to ZZ^\ast\to 4\ell$ decay channel are presented. The cross sections are determined within a fiducial phase space and corrected for detection efficiency and resolution effects. They are based on 20.3 fb$^{-1}$ of $pp$ collision data, produced at $\sqrt{s}=8$ TeV centre-of-mass energy at the LHC and recorded by the ATLAS detector. The differential measurements are performed in bins of transverse momentum and rapidity of the four-lepton system, the invariant mass of the subleading lepton pair and the decay angle of the leading lepton pair with respect to the beam line in the four-lepton rest frame, as well as the number of jets and the transverse momentum of the leading jet. The measured cross sections are compared to selected theoretical calculations of the Standard Model expectations. No significant deviation from any of the tested predictions is found.

Source code: ATLAS_2014_I1310835.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FastJets.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/PromptFinalState.hh"
#include "Rivet/Projections/DressedLeptons.hh"

namespace Rivet {

  /// @brief H(125)->ZZ->4l at 8 TeV
  class ATLAS_2014_I1310835 : public Analysis {
  public:

    /// Default constructor
    DEFAULT_RIVET_ANALYSIS_CTOR(ATLAS_2014_I1310835);

    void init() {
      const FinalState fs(Cuts::abseta < 5.0);

      PromptFinalState photons(Cuts::abspid == PID::PHOTON);

      PromptFinalState bare_el(Cuts::abspid == PID::ELECTRON);

      PromptFinalState bare_mu(Cuts::abspid == PID::MUON);

      // Selection: lepton selection
      Cut etaranges_el = Cuts::abseta < 2.47 && Cuts::pT > 7*GeV; 
      DressedLeptons electron_sel4l(photons, bare_el, 0.1, etaranges_el, false);
      addProjection(electron_sel4l, "electrons");
 
      Cut etaranges_mu = Cuts::abseta < 2.7 && Cuts::pT > 6*GeV;
      DressedLeptons muon_sel4l(photons, bare_mu, 0.1, etaranges_mu, false);
      addProjection(muon_sel4l, "muons");

      FastJets jetpro(fs, FastJets::ANTIKT, 0.4, JetAlg::NO_MUONS, JetAlg::NO_INVISIBLES);
      addProjection(jetpro, "jet");

      // Book histos
      _h_pt           = bookHisto1D(1, 1, 1);
      _h_rapidity     = bookHisto1D(2, 1, 1);
      _h_m34          = bookHisto1D(3, 1, 1);
      _h_costheta     = bookHisto1D(4, 1, 1);
      _h_njets        = bookHisto1D(5, 1, 1);
      _h_leadingjetpt = bookHisto1D(6, 1, 1);

    }



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

      const double weight = e.weight();

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

      const vector<DressedLepton>& mu_sel4l = applyProjection<DressedLeptons>(e, "muons").dressedLeptons();
      const vector<DressedLepton>& el_sel4l = applyProjection<DressedLeptons>(e, "electrons").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() );

      /////////////////////////////////////////////////////////////////////////////
      /// H->ZZ->4l pairing
      /////////////////////////////////////////////////////////////////////////////
 
      size_t el_p = 0;
      size_t el_n = 0;
      size_t mu_p = 0; 
      size_t mu_n = 0;
      
      for (const Particle& l : leptonsFS_sel4l) {
        if (l.abspid() == PID::ELECTRON) {
          if (l.pid() < 0)  ++el_n;
          if (l.pid() > 0)  ++el_p;
        }
        else if (l.abspid() == PID::MUON) {
          if (l.pid() < 0)  ++mu_n;
          if (l.pid() > 0)  ++mu_p;
        }
      }
            
      bool pass_sfos = ( (el_p >=2 && el_n >=2) || (mu_p >=2 && mu_n >=2) || (el_p >=1 && el_n >=1 && mu_p >=1 && mu_n >=1) );
      
      if (!pass_sfos)  vetoEvent;

      Zstate Z1, Z2, Zcand;
      size_t n_parts = leptonsFS_sel4l.size();
      size_t l1_index = 0;
      size_t l2_index = 0;

      // determine Z1 first
      double min_mass_diff = -1;
      for (size_t i = 0; i < n_parts; ++i) {
        for (size_t j = 0; j < n_parts; ++j) {
          if (i >= j)  continue;

          if (leptonsFS_sel4l[i].pid() != -1*leptonsFS_sel4l[j].pid())  continue; //only pair SFOS leptons

          Zcand = Zstate( ParticlePair(leptonsFS_sel4l[i], leptonsFS_sel4l[j]) );
          double mass_diff = fabs( Zcand.mom().mass() - 91.1876 );
         
          if (min_mass_diff == -1 || mass_diff < min_mass_diff) {
            min_mass_diff = mass_diff;
            Z1 = Zcand;
            l1_index = i;
            l2_index = j;
          }
        }
      }

      //determine Z2 second
      min_mass_diff = -1;
      for (size_t i = 0; i < n_parts; ++i) {
        if (i == l1_index || i == l2_index)  continue;
        for (size_t j = 0; j < n_parts; ++j) {
          if (j == l1_index || j == l2_index || i >= j)  continue;

          if (leptonsFS_sel4l[i].pid() != -1*leptonsFS_sel4l[j].pid())  continue; // only pair SFOS leptons

          Zcand = Zstate( ParticlePair(leptonsFS_sel4l[i], leptonsFS_sel4l[j]) );
          double mass_diff = fabs( Zcand.mom().mass() - 91.1876 );

          if (min_mass_diff == -1 || mass_diff < min_mass_diff) {
            min_mass_diff = mass_diff;
            Z2 = Zcand;
          }
        }
      }

      Particles leptons_sel4l;
      leptons_sel4l.push_back(Z1.first);
      leptons_sel4l.push_back(Z1.second);
      leptons_sel4l.push_back(Z2.first);
      leptons_sel4l.push_back(Z2.second);

      ////////////////////////////////////////////////////////////////////////////
      // Kinematic Requirements
      ///////////////////////////////////////////////////////////////////////////
      
      //leading lepton pT requirement
      std::vector<double> lepton_pt;
      for (const Particle& i : leptons_sel4l) lepton_pt.push_back(i.pT() / GeV);
      std::sort(lepton_pt.begin(), lepton_pt.end(), [](const double pT1, const double pT2) { return pT1 > pT2; });
      
      if (!(lepton_pt[0] > 20*GeV && lepton_pt[1] > 15*GeV && lepton_pt[2] > 10*GeV))  vetoEvent;
      
      //invariant mass requirements
      if (!(inRange(Z1.mom().mass(), 50*GeV, 106*GeV) && inRange(Z2.mom().mass(), 12*GeV, 115*GeV)))  vetoEvent;
      
      //lepton separation requirements
      for (unsigned int i = 0; i < 4; ++i) {
        for (unsigned int j = 0; j < 4; ++j) {
          if (i >= j) continue;
          double dR = deltaR(leptons_sel4l[i], leptons_sel4l[j]);
          bool sameflavor = leptons_sel4l[i].abspid() == leptons_sel4l[j].abspid();

          if ( sameflavor && dR < 0.1)  vetoEvent;
          if (!sameflavor && dR < 0.2)  vetoEvent;
        }
      }

      // J/Psi veto requirement
      for (unsigned int i = 0; i < 4; ++i) {
        for (unsigned int j = 0; j < 4; ++j) {
          if (i >= j) continue;
          if ( leptons_sel4l[i].pid() != -1*leptons_sel4l[j].pid() )  continue;
          if ((leptons_sel4l[i].momentum() + leptons_sel4l[j].momentum()).mass() <= 5*GeV)  vetoEvent;
        }
      }
 
      // 4-lepton invariant mass requirement
      double m4l = (Z1.mom() + Z2.mom()).mass();
      if (!(inRange(m4l, 118*GeV, 129*GeV)))  vetoEvent;
  
  
      ////////////////////////////////////////////////////////////////////////////
      // Higgs observables
      ///////////////////////////////////////////////////////////////////////////
      FourMomentum Higgs = Z1.mom() + Z2.mom();

      double H4l_pt       = Higgs.pt()/GeV; 
      double H4l_rapidity = Higgs.absrap(); 
      LorentzTransform HRF_boost;
      //HRF_boost.mkFrameTransformFromBeta(Higgs.boostVector());
      HRF_boost.setBetaVec(- Higgs.boostVector());
      FourMomentum Z1_in_HRF = HRF_boost.transform( Z1.mom() );
      double H4l_costheta = fabs(cos( Z1_in_HRF.theta())); 
      double H4l_m34      = Z2.mom().mass()/GeV;
      
      ////////////////////////////////////////////////////////////////////////////
      // Jet observables
      ///////////////////////////////////////////////////////////////////////////

      Jets jets;
      for (const Jet& jet : applyProjection<FastJets>(e, "jet").jetsByPt(Cuts::pT > 30*GeV && Cuts::absrap < 4.4)) {
        bool overlaps = false;
        for (const Particle& lep : leptonsFS_sel4l) {
          if (lep.abspid() != PID::ELECTRON)  continue;
          const double dR = deltaR(lep, jet);
          if (dR < 0.2) { overlaps = true; break; }
        }
        if (!overlaps) jets += jet;
      }
      size_t n_jets = jets.size();
      if (n_jets > 3)  n_jets = 3;

      std::vector<double> jet_pt;
      for (const Jet& i : jets) jet_pt.push_back(i.pT()/GeV);

      double leading_jet_pt = n_jets? jet_pt[0] : 0.;

      ////////////////////////////////////////////////////////////////////////////
      // End of H->ZZ->llll selection: now fill histograms
      ////////////////////////////////////////////////////////////////////////////


      _h_pt->fill(H4l_pt, weight);
      _h_rapidity->fill(H4l_rapidity, weight);
      _h_costheta->fill(H4l_costheta, weight);
      _h_m34->fill(H4l_m34, weight);
      _h_njets->fill(n_jets + 1, weight);
      _h_leadingjetpt->fill(leading_jet_pt, weight);


    }


    /// 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(); }
    };

    /// Finalize
    void finalize() {

      const double norm = crossSection()/sumOfWeights()/femtobarn;
      std::cout << "xsec: " << crossSection() << std::endl;
      std::cout << "sumw: " << sumOfWeights() << std::endl;
      std::cout << "femb: " << femtobarn << std::endl;
      std::cout << "norm: " << norm << std::endl;

      scale(_h_pt, norm);
      scale(_h_rapidity, norm);
      scale(_h_costheta, norm);
      scale(_h_m34, norm);
      scale(_h_njets, norm);
      scale(_h_leadingjetpt, norm);
    }


  private:

    Histo1DPtr _h_pt, _h_rapidity, _h_costheta;
    Histo1DPtr _h_m34, _h_njets, _h_leadingjetpt;

  };


  // The hook for the plugin system
  DECLARE_RIVET_PLUGIN(ATLAS_2014_I1310835);

}