Rivet analyses

title: ATLAS_2016_I1457605

Inclusive prompt photons at 8 TeV

Experiment: ATLAS (LHC)

Inspire ID: 1457605

Status: VALIDATED

Authors: - Mark Stockton

References: - Expt page: ATLAS-STDM-2014-09 - arXiv: 1605.03495

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

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/FastJets.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/LeadingParticlesFinalState.hh"

include "Rivet/Projections/PromptFinalState.hh"

namespace Rivet {

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

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(ATLAS_2016_I1457605);

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

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

  // Consider the final state jets for the energy density calculation
  FastJets fj(fs, JetAlg::KT, 0.5);
  fj.useJetArea(new fastjet::AreaDefinition(fastjet::VoronoiAreaSpec()));
  declare(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);
  declare(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;
    book(_h_Et_photon[i], 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 = apply<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 = apply<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 = apply<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());
}


/// 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};

};

RIVET_DECLARE_PLUGIN(ATLAS_2016_I1457605);

} ```