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ATLAS_2012_I1199269

Inclusive diphoton $+ X$ events at $\sqrt{s} = 7$ TeV
Experiment: ATLAS (LHC)
Inspire ID: 1199269
Status: VALIDATED
Authors:
  • Giovanni Marchiori
References: Beams: p+ p+
Beam energies: (3500.0, 3500.0) GeV
Run details:
  • Inclusive diphoton $+ X$ events at $\sqrt{s} = 7$ TeV.

The ATLAS experiment at the LHC has measured the production cross section of events with two isolated photons in the final state, in proton-proton collisions at $\sqrt{s} = 7$ TeV. The full data set collected in 2011, corresponding to an integrated luminosity of 4.9 fb$^{-1}$, is used. The amount of background, from hadronic jets and isolated electrons, is estimated with data-driven techniques and subtracted. The total cross section, for two isolated photons with transverse energies above 25 GeV and 22 GeV respectively, in the acceptance of the electromagnetic calorimeter ($|\eta|<1.37$ and $1.52<|\eta|<2.37$) and with an angular separation $\Delta R>0.4$, is $44.0^{+3.2}_{-4.2}$ pb. The differential cross sections as a function of the di-photon invariant mass, transverse momentum, azimuthal separation, and cosine of the polar angle of the largest transverse energy photon in the Collins--Soper di-photon rest frame are also measured. The results are compared to the prediction of leading-order parton-shower and next-to-leading-order and next-to-next-to-leading-order parton-level generators.

Source code: ATLAS_2012_I1199269.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/IdentifiedFinalState.hh"
#include "Rivet/Projections/FastJets.hh"

namespace Rivet {


  /// @brief Measurement of isolated diphoton + X differential cross-sections
  ///
  /// Inclusive isolated gamma gamma cross-sections, differential in M(gg), pT(gg),
  /// dphi(gg), cos(theta*)_CS
  ///
  /// @author Giovanni Marchiori
  ///
  class ATLAS_2012_I1199269 : public Analysis {
  public:

    /// Constructor
    ATLAS_2012_I1199269()
      : Analysis("ATLAS_2012_I1199269")
    {    }


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

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

      FastJets fj(fs, FastJets::KT, 0.5);
      fj.useJetArea(new fastjet::AreaDefinition(fastjet::VoronoiAreaSpec()));
      declare(fj, "KtJetsD05");

      IdentifiedFinalState photonfs(Cuts::abseta < 2.37 && Cuts::pT > 22*GeV);
      photonfs.acceptId(PID::PHOTON);
      declare(photonfs, "Photon");

      _h_M            = bookHisto1D(1, 1, 1);
      _h_pT           = bookHisto1D(2, 1, 1);
      _h_dPhi         = bookHisto1D(3, 1, 1);
      _h_cosThetaStar = bookHisto1D(4, 1, 1);
    }


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

      // Require at least 2 photons in final state
      const Particles photons = apply<IdentifiedFinalState>(event, "Photon").particlesByPt();
      if (photons.size() < 2) vetoEvent;

      // Get jets, and corresponding jet areas
      vector<vector<double> > ptDensities(_eta_bins_areaoffset.size()-1);
      const auto clust_seq_area = apply<FastJets>(event, "KtJetsD05").clusterSeqArea();
      for (const Jet& jet : apply<FastJets>(event, "KtJetsD05").jets()) {
        const double area = clust_seq_area->area(jet); // implicit .pseudojet()
        if (area < 1e-3) continue;
        const int ieta = binIndex(jet.abseta(), _eta_bins_areaoffset);
        if (ieta != -1) ptDensities[ieta].push_back(jet.pT()/area);
      }

      // Compute median jet properties over the jets in the event
      vector<double> vptDensity; //, vsigma, vNjets;
      for (size_t b = 0; b < _eta_bins_areaoffset.size()-1; ++b) {
        vptDensity += ptDensities[b].empty() ? 0 : median(ptDensities[b]);
      }


      // Loop over photons and fill vector of isolated ones
      Particles isolated_photons;
      for (const Particle& photon : photons) {
        /// Remove photons in ECAL crack region
        if (inRange(photon.abseta(), 1.37, 1.52)) continue;
        // Compute isolation via particles within an R=0.4 cone of the photon
        const Particles& fs = apply<FinalState>(event, "FS").particles();
        FourMomentum mom_in_EtCone;
        for (const Particle& p : fs) {
          // Reject if not in cone
          if (deltaR(photon, p) > 0.4) continue;
          // Reject if in the 5x7 cell central core
          if (fabs(deltaEta(photon, p)) < 0.025 * 5 * 0.5 &&
              fabs(deltaPhi(photon, p)) < PI/128. * 7 * 0.5) continue;
          // Sum momentum
          mom_in_EtCone += p.momentum();
        }
        // Now figure out the correction (area*density)
        const double ETCONE_AREA = PI*sqr(0.4) - (7*.025)*(5*PI/128.); // cone area - central core rectangle
        const double correction = vptDensity[binIndex(photon.abseta(), _eta_bins_areaoffset)] * ETCONE_AREA;

        // Discard the photon if there is more than 4 GeV of cone activity
        // NOTE: Shouldn't need to subtract photon itself (it's in the central core)
        // NOTE: using expected cut at hadron/particle level, not at reco level
        if (mom_in_EtCone.Et() - correction > 4*GeV) continue;
        // Add isolated photon to list
        isolated_photons.push_back(photon);
      }

      // Require at least two isolated photons and select leading pT pair
      if (isolated_photons.size() < 2) vetoEvent;
      sortByPt(isolated_photons);
      const FourMomentum& y1 = isolated_photons[0].momentum();
      const FourMomentum& y2 = isolated_photons[1].momentum();

      // Leading photon should have pT > 25 GeV
      if (y1.pT() < 25*GeV) vetoEvent;

      // Require the two photons to be separated by dR > 0.4
      if (deltaR(y1, y2) < 0.4) vetoEvent;

      // Compute diphoton vector and fill histos
      const double weight = event.weight();
      FourMomentum yy = y1 + y2;
      const double costhetayy = 2 * y1.pT() * y2.pT() * sinh(y1.eta() - y2.eta()) / yy.mass() / add_quad(yy.mass(), yy.pT());
      _h_M->fill(yy.mass()/GeV, weight);
      _h_pT->fill(yy.pT()/GeV, weight);
      _h_dPhi->fill(mapAngle0ToPi(y1.phi() - y2.phi()), weight);
      _h_cosThetaStar->fill(costhetayy, weight);
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      scale(_h_M, crossSection()/sumOfWeights());
      scale(_h_pT, crossSection()/sumOfWeights());
      scale(_h_dPhi, crossSection()/sumOfWeights());
      scale(_h_cosThetaStar, crossSection()/sumOfWeights());
    }


  private:

    Histo1DPtr _h_M, _h_pT, _h_dPhi, _h_cosThetaStar;

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

  };


  DECLARE_RIVET_PLUGIN(ATLAS_2012_I1199269);

}