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ATLAS_2016_I1457605

Inclusive prompt photons at 8 TeV
Experiment: ATLAS (LHC)
Inspire ID: 1457605
Status: VALIDATED
Authors:
  • Mark Stockton
References: Beams: p+ p+
Beam energies: (4000.0, 4000.0) GeV
Run details:
  • Inclusive prompt photon production

A measurement of the cross section for the inclusive production of isolated prompt photons in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 8$ TeV is presented. The measurement covers the pseudorapidity ranges $|\eta^\gamma| < 1.37$ $1.56 < |\eta^\gamma| < 2.37$ in the transverse energy range $25 < E^\gamma_\text{T} < 1500$ GeV. The results are based on an integrated luminosity of 20.2 fb${}^{-1}$, recorded by the ATLAS detector at the LHC. Photon candidates are identified by combining information from the calorimeters and the inner tracker. The background is subtracted using a data-driven technique, based on the observed calorimeter shower-shape variables and the deposition of hadronic energy in a narrow cone around the photon candidate.

Source code: ATLAS_2016_I1457605.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/FastJets.hh"

namespace Rivet {


  /// Inclusive isolated prompt photon analysis with 2012 LHC data
  class ATLAS_2016_I1457605 : public Analysis {
  public:

    /// Constructor
    DEFAULT_RIVET_ANALYSIS_CTOR(ATLAS_2016_I1457605);

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

      FinalState fs;
      addProjection(fs, "FS");

      // Consider the final state jets for the energy density calculation
      FastJets fj(fs, FastJets::KT, 0.5);
      fj.useJetArea(new fastjet::AreaDefinition(fastjet::VoronoiAreaSpec()));
      addProjection(fj, "KtJetsD05");

      // Consider the leading pt photon with |eta| < 2.37 and pT > 25 GeV
      LeadingParticlesFinalState photonfs(PromptFinalState(FinalState(Cuts::abseta < 2.37 && Cuts::pT > 25*GeV)));
      photonfs.addParticleId(PID::PHOTON);
      addProjection(photonfs, "LeadingPhoton");

      // Book the dsigma/dEt (in eta bins) histograms
      for (size_t i = 0; i < _eta_bins.size() - 1; ++i) {
        if (fuzzyEquals(_eta_bins[i], 1.37)) continue; // skip this bin
        int offset = i > 2? 0 : 1;
        _h_Et_photon[i] = bookHisto1D(i + offset, 1, 1);
      }

    }


    /// Return eta bin for either dsigma/dET histogram (area_eta=false) or energy density correction (area_eta=true)
    size_t _getEtaBin(double eta_w, bool area_eta) const {
      const double eta = fabs(eta_w);
      if (!area_eta) {
        return binIndex(eta, _eta_bins);
      } else {
        return binIndex(eta, _eta_bins_areaoffset);
      }
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // Retrieve leading photon
      Particles photons = applyProjection<LeadingParticlesFinalState>(event, "LeadingPhoton").particles();
      if (photons.size() < 1)  vetoEvent;
      const Particle& leadingPhoton = photons[0];

      // Veto events with photon in ECAL crack
      if (inRange(leadingPhoton.abseta(), 1.37, 1.56)) vetoEvent;

      // Compute isolation energy in cone of radius .4 around photon (all particles)
      FourMomentum mom_in_EtCone;
      Particles fs = applyProjection<FinalState>(event, "FS").particles();
      for (const Particle& p : fs) {
        // Check if it's outside the cone of 0.4
        if (deltaR(leadingPhoton, p) >= 0.4) continue;
        // Except muons or neutrinos
        if (PID::isNeutrino(p.abspid()) || p.abspid() == PID::MUON) continue;
        // Increment isolation energy
        mom_in_EtCone += p.momentum();
      }
      // Remove the photon energy from the isolation
      mom_in_EtCone -= leadingPhoton.momentum();

      // Get the area-filtered jet inputs for computing median energy density, etc.
      vector<double> ptDensity;
      vector< vector<double> > ptDensities(_eta_bins_areaoffset.size()-1);
      const FastJets& fast_jets = applyProjection<FastJets>(event, "KtJetsD05");
      const auto clust_seq_area = fast_jets.clusterSeqArea();
      for (const Jet& jet : fast_jets.jets()) {
        const double area = clust_seq_area->area(jet);
        if (area > 1e-3 && jet.abseta() < _eta_bins_areaoffset.back())
          ptDensities.at( _getEtaBin(jet.abseta(), true) ) += 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]) : 0.0;
      }
      // Compute the isolation energy correction (cone area*energy density)
      const double etCone_area = PI * sqr(0.4);
      const double correction = ptDensity[_getEtaBin(leadingPhoton.abseta(), true)] * etCone_area;

      // Apply isolation cut on area-corrected value
      // cut is Etiso < 4.8GeV + 4.2E-03 * Et_gamma.
      if (mom_in_EtCone.Et() - correction > 4.8*GeV + 0.0042*leadingPhoton.Et()) vetoEvent;

      // Fill histograms
      const size_t eta_bin = _getEtaBin(leadingPhoton.abseta(), false);
      _h_Et_photon[eta_bin]->fill(leadingPhoton.Et(), event.weight());
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      double sf = crossSection() / (picobarn * sumOfWeights());
      for (size_t i = 0; i < _eta_bins.size()-1; ++i) {
        if (fuzzyEquals(_eta_bins[i], 1.37)) continue;
        scale(_h_Et_photon[i], sf);
      }
    }


  private:

    Histo1DPtr _h_Et_photon[5];

    const vector<double> _eta_bins = {0.00, 0.60, 1.37, 1.56, 1.81, 2.37 };
    const vector<double> _eta_bins_areaoffset = {0.0, 1.5, 3.0};

  };


  DECLARE_RIVET_PLUGIN(ATLAS_2016_I1457605);

}