Rivet analyses

title: ATLAS_2011_I921594

Inclusive isolated prompt photon analysis with full 2010 LHC data

Experiment: ATLAS (LHC 7TeV)

Inspire ID: 921594

Status: VALIDATED

Authors: - Giovanni Marchiori

References: - Expt page: ATLAS-STDM-2011-04 - arXiv: 1108.0253 - Phys.Lett. B706 (2011) 150-167

Beams: p+ p+

Beam energies: (3500.0, 3500.0)GeV

Run details: - Inclusive photon $+ X$ events (primary $\gamma$+jet events) at $\sqrt{s} = 7$~TeV.

A measurement of the differential cross-section for the inclusive production of isolated prompt photons in $pp$ collisions at a center-of-mass energy $\sqrt{s} = 7$ TeV is presented. The measurement covers the pseudorapidity ranges $|\eta|<1.37$ and $1.52<|\eta|<2.37$ in the transverse energy range $45<E_T<400$ GeV. The results are based on an integrated luminosity of 35 pb$^{-1}$, collected with the ATLAS detector at the LHC. The yields of the signal photons are measured using a data-driven technique, based on the observed distribution of the hadronic energy in a narrow cone around the photon candidate and the photon selection criteria. The results are compared with next-to-leading order perturbative QCD calculations and found to be in good agreement over four orders of magnitude in cross-section.

Source code:ATLAS_2011_I921594.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FastJets.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/LeadingParticlesFinalState.hh"

namespace Rivet {

/// Inclusive isolated prompt photon analysis with full 2010 LHC data class ATLAS_2011_I921594 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(ATLAS_2011_I921594);


/// 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>45 GeV
  LeadingParticlesFinalState photonfs(FinalState((Cuts::etaIn(-2.37, 2.37) && Cuts::pT >= 45 * GeV)));
  photonfs.addParticleId(PID::PHOTON);
  declare(photonfs, "LeadingPhoton");

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


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


/// Perform the per-event analysis
void analyze(const Event& event) {
  // Retrieve leading photon
  const 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, 1.52)) 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;
    // Don't count particles in the 5x7 central core
    if (deltaEta(leadingPhoton, p) < .025 * 5.0 * 0.5
        && deltaPhi(leadingPhoton, p) < (PI / 128.) * 7.0 * 0.5)
      continue;
    // Increment isolation energy
    mom_in_EtCone += p.momentum();
  }

  // Get the area-filtered jet inputs for computing median energy density, etc.
  vector<vector<double>> ptDensities(_eta_bins_areaoffset.size() - 1);
  FastJets fast_jets = apply<FastJets>(event, "KtJetsD05");
  const shared_ptr<fastjet::ClusterSequenceArea> clust_seq_area = fast_jets.clusterSeqArea();
  for (const Jet& jet : fast_jets.jets()) {
    const double area = clust_seq_area->area(jet); //< Implicit call to .pseudojet()
    if (area > 1e-4 && jet.abseta() < _eta_bins_areaoffset.back())
      ptDensities.at(_getEtaBin(jet.abseta(), true)).push_back(jet.pT() / area);
  }

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

  // Compute the isolation energy correction (cone area*energy density)
  const double ETCONE_AREA = M_PI * sqr(0.4) - (7.0 * .025) * (5.0 * PI / 128.);
  const double correction = ptDensity[_getEtaBin(leadingPhoton.abseta(), true)] * ETCONE_AREA;

  // Apply isolation cut on area-corrected value
  if (mom_in_EtCone.Et() - correction > 4 * GeV) 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() {
  for (size_t i = 0; i < _eta_bins.size() - 1; i++) {
    if (fuzzyEquals(_eta_bins[i], 1.37)) continue;
    scale(_h_Et_photon[i], crossSection() / picobarn / sumOfWeights());
  }
}

private:

Histo1DPtr _h_Et_photon[5];

const vector<double> _eta_bins = {0.00, 0.60, 1.37, 1.52, 1.81, 2.37};
const vector<double> _eta_bins_areaoffset = {0.0, 1.5, 3.0};

};

RIVET_DECLARE_PLUGIN(ATLAS_2011_I921594);

} ```