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ATLAS_2013_I1244522

Photon + jets
Experiment: ATLAS (LHC)
Inspire ID: 1244522
Status: VALIDATED
Authors:
  • Josu Cantero
References: Beams: p+ p+
Beam energies: (3500.0, 3500.0) GeV
Run details:
  • p + p -> gamma + jet + X

Measurements of isolated photon plus jet production in $pp$ collisions at a centre-of-mass energy of 7 TeV with the ATLAS detector at the LHC using an integrated luminosity of $37 \text{pb}^{-1}$. Differential cross sections are presented as functions of photon transverse energy, jet transverse momentum and jet rapidity. In addition, the differential cross sections as functions of the difference between the azimuthal angles of the photon and the jet, the photon-jet invariant mass as well as the scattering angle in the photon-jet centre-of-mass frame have been measured.

Source code: ATLAS_2013_I1244522.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/PromptFinalState.hh"
#include "Rivet/Projections/LeadingParticlesFinalState.hh"
#include "Rivet/Projections/VetoedFinalState.hh"
#include "Rivet/Projections/FastJets.hh"

namespace Rivet {


  /// @brief Measurement of isolated gamma + jet + X differential cross-sections
  class ATLAS_2013_I1244522 : public Analysis {
  public:

    // Constructor
    ATLAS_2013_I1244522()
      : Analysis("ATLAS_2013_I1244522")
    {     }


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

      // Voronoi eta-phi tassellation with KT jets, for ambient energy density calculation
      FastJets fj(fs, FastJets::KT, 0.5);
      fj.useJetArea(new fastjet::AreaDefinition(fastjet::VoronoiAreaSpec()));
      declare(fj, "KtJetsD05");

      // Leading photon
      LeadingParticlesFinalState photonfs(PromptFinalState(FinalState(-2.37, 2.37, 45.0*GeV)));
      photonfs.addParticleId(PID::PHOTON);
      declare(photonfs, "LeadingPhoton");

      // FS excluding the leading photon
      VetoedFinalState vfs(fs);
      vfs.addVetoOnThisFinalState(photonfs);
      declare(vfs, "JetFS");

      // Jets
      FastJets jetpro(vfs, FastJets::ANTIKT, 0.6);
      jetpro.useInvisibles();
      declare(jetpro, "Jets");

      // Histograms
      _h_ph_pt      = bookHisto1D(1, 1, 1);
      _h_jet_pt     = bookHisto1D(2, 1, 1);
      _h_jet_rap    = bookHisto1D(3, 1, 1);
      _h_dphi_phjet = bookHisto1D(4, 1, 1);
      _h_costheta_biased_phjet = bookHisto1D(5, 1, 1);
      _h_mass_phjet            = bookHisto1D(6, 1, 1);
      _h_costheta_phjet        = bookHisto1D(7, 1, 1);

    }


    size_t getEtaBin(double eta) const {
      const double aeta = fabs(eta);
      return binIndex(aeta, _eta_bins_areaoffset);
    }


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

      // Get the photon
      Particles photons = apply<LeadingParticlesFinalState>(event, "LeadingPhoton").particles();
      if (photons.size() != 1 )  vetoEvent;
      const Particle& photon = photons[0];

      if (inRange(photon.abseta(), 1.37, 1.52))  vetoEvent;

      //Compute isolation energy in cone of radius .4 around photon (all particles)
      FourMomentum mom_in_EtCone;
      const Particles& fs = apply<VetoedFinalState>(event, "JetFS").particles();
      for (const Particle& p : fs) {
        // Check if it's outside the cone of 0.4
        if (deltaR(photon, p) >= 0.4) continue;
        // Increment isolation energy
        mom_in_EtCone += p.momentum();
      }

      // Get the jets
      Jets alljets = apply<FastJets>(event, "Jets").jetsByPt(40.0*GeV);
      Jets jets;
      for (const Jet& jet : alljets)
        if (deltaR(photon, jet) > 1.0) jets += jet;
      if (jets.empty())  vetoEvent;
      Jet leadingJet = jets[0];
      if (leadingJet.absrap() > 2.37) vetoEvent;

      // Get the area-filtered jet inputs for computing median energy density, etc.
      vector<double> ptDensity;
      vector< vector<double> > ptDensities(_eta_bins_areaoffset.size()-1);
      FastJets fast_jets = apply<FastJets>(event, "KtJetsD05");
      const auto clust_seq_area = fast_jets.clusterSeqArea();
      foreach (const Jet& jet, fast_jets.jets()) {
        const double area = clust_seq_area->area(jet);
        if (area > 1e-4 && jet.abseta() < _eta_bins_areaoffset.back())
          ptDensities.at( getEtaBin(jet.abseta()) ).push_back(jet.pT()/area);
      }

      // Compute the median energy density, etc.
      for (size_t b = 0; b < _eta_bins_areaoffset.size() - 1; ++b) {
        const int njets = ptDensities[b].size();
        ptDensity += (njets > 0) ? median(ptDensities[b]) : 0;
      }

      // Compute the isolation energy correction (cone area*energy density)
      const double etCone_area = PI*sqr(0.4) - (5.0*.025)*(7.0*PI/128.);
      const double correction = ptDensity[getEtaBin(photon.abseta())] * etCone_area;

      // Apply isolation cut on area-corrected value
      if (mom_in_EtCone.Et() - correction >= 4*GeV)  vetoEvent;

      // Fill histos
      const double weight = event.weight();
      const double dy = deltaRap(photon, leadingJet);
      const double costheta_yj = tanh(dy/2);
      _h_ph_pt->fill(photon.pT()/GeV, weight);
      _h_jet_pt->fill(leadingJet.pT()/GeV, weight);
      _h_jet_rap->fill(leadingJet.absrap(), weight);
      _h_dphi_phjet->fill(deltaPhi(photon, leadingJet), weight);
      _h_costheta_biased_phjet->fill(costheta_yj, weight);
      if (costheta_yj < 0.829022) {
        const FourMomentum yj = photon.momentum() + leadingJet.momentum();
        if (yj.mass() > 160.939*GeV) {
          if (fabs(photon.eta() + leadingJet.rap()) < 2.37) {
            _h_mass_phjet->fill(yj.mass()/GeV, weight);
            _h_costheta_phjet->fill(costheta_yj, weight);
          }
        }
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      const double sf = crossSection() / picobarn / sumOfWeights();
      scale(_h_ph_pt,                 sf);
      scale(_h_jet_pt,                sf);
      scale(_h_jet_rap,               sf);
      scale(_h_dphi_phjet,            sf);
      scale(_h_costheta_biased_phjet, sf);
      scale(_h_mass_phjet,            sf);
      scale(_h_costheta_phjet,        sf);
    }


  private:

    Histo1DPtr _h_ph_pt, _h_jet_pt, _h_jet_rap, _h_dphi_phjet, _h_costheta_biased_phjet, _h_mass_phjet, _h_costheta_phjet;

    const vector<double> _eta_bins_areaoffset = {0.0, 1.5, 3.0};

  };


  // The hook for the plugin system
  DECLARE_RIVET_PLUGIN(ATLAS_2013_I1244522);


}