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D0_2010_S8570965

Direct photon pair production
Experiment: CDF (Tevatron Run 2)
Inspire ID: 846997
Status: VALIDATED
Authors:
  • Frank Siegert
References: Beams: p- p+
Beam energies: (980.0, 980.0) GeV
Run details:
  • All processes that can produce prompt photon pairs, e.g. $jj \to jj$, $jj \to j\gamma$ and $jj \to \gamma \gamma$. Non-prompt photons from hadron decays like $\pi$ and $\eta$ have been corrected for.

Direct photon pair production cross sections are measured using 4.2 fb$^{-1}$ of data. They are binned in diphoton mass, the transverse momentum of the diphoton system, the azimuthal angle between the photons, and the polar scattering angle of the photons. Also available are double differential cross sections considering the last three kinematic variables in three diphoton mass bins. Note, the numbers in version 1 of the arXiv preprint were missing the dM normalisation in the double differential cross sections. This has been reported to and fixed by the authors in v2 and the journal submission. HepData as well as the Rivet analysis have also been updated.

Source code: D0_2010_S8570965.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/IdentifiedFinalState.hh"
#include "Rivet/Tools/BinnedHistogram.hh"

namespace Rivet {


  /// @brief D0 direct photon pair production
  class D0_2010_S8570965 : public Analysis {
  public:


    D0_2010_S8570965()
      : Analysis("D0_2010_S8570965")
    {    }


    void init() {
      FinalState fs;
      declare(fs, "FS");

      IdentifiedFinalState ifs(Cuts::abseta < 0.9 && Cuts::pT > 20*GeV);
      ifs.acceptId(PID::PHOTON);
      declare(ifs, "IFS");

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

      std::pair<double, double> M_ranges[] = { std::make_pair(30.0, 50.0),
                                               std::make_pair(50.0, 80.0),
                                               std::make_pair(80.0, 350.0) };

      for (size_t i = 0; i < 3; ++i) {
        _h_pT_M.addHistogram(M_ranges[i].first, M_ranges[i].second, bookHisto1D(5+3*i, 1, 1));
        _h_dPhi_M.addHistogram(M_ranges[i].first, M_ranges[i].second, bookHisto1D(6+3*i, 1, 1));
        _h_costheta_M.addHistogram(M_ranges[i].first, M_ranges[i].second, bookHisto1D(7+3*i, 1, 1));
      }
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      const double weight = event.weight();

      Particles photons = apply<IdentifiedFinalState>(event, "IFS").particlesByPt();
      if (photons.size() < 2 ||
          (photons[0].pT() < 21.0*GeV)) {
        vetoEvent;
      }

      // Isolate photons with ET_sum in cone
      Particles isolated_photons;
      Particles fs = apply<FinalState>(event, "FS").particles();
      foreach (const Particle& photon, photons) {
        double eta_P = photon.eta();
        double phi_P = photon.phi();
        double Etsum=0.0;
        foreach (const Particle& p, fs) {
          if (p.genParticle()->barcode() != photon.genParticle()->barcode() &&
              deltaR(eta_P, phi_P, p.eta(), p.phi()) < 0.4) {
            Etsum += p.Et();
          }
        }
        if (Etsum < 2.5*GeV) {
          isolated_photons.push_back(photon);
        }
      }

      if (isolated_photons.size() != 2) {
        vetoEvent;
      }
      std::sort(isolated_photons.begin(), isolated_photons.end(), cmpMomByPt);

      FourMomentum y1=isolated_photons[0].momentum();
      FourMomentum y2=isolated_photons[1].momentum();
      if (deltaR(y1, y2)<0.4) {
        vetoEvent;
      }

      FourMomentum yy=y1+y2;
      double Myy = yy.mass()/GeV;
      if (Myy<30.0 || Myy>350.0) {
        vetoEvent;
      }

      double pTyy = yy.pT()/GeV;
      if (Myy<pTyy) {
        vetoEvent;
      }

      double dPhiyy = mapAngle0ToPi(y1.phi()-y2.phi());
      if (dPhiyy<0.5*M_PI) {
        vetoEvent;
      }

      double costhetayy = fabs(tanh((y1.eta()-y2.eta())/2.0));

      _h_M->fill(Myy, weight);
      _h_pT->fill(pTyy, weight);
      _h_dPhi->fill(dPhiyy, weight);
      _h_costheta->fill(costhetayy, weight);

      _h_pT_M.fill(Myy, pTyy, weight);
      _h_dPhi_M.fill(Myy, dPhiyy, weight);
      _h_costheta_M.fill(Myy, costhetayy, weight);
    }


    void finalize() {

      scale(_h_M, crossSection()/sumOfWeights());
      scale(_h_pT, crossSection()/sumOfWeights());
      scale(_h_dPhi, crossSection()/sumOfWeights());
      scale(_h_costheta, crossSection()/sumOfWeights());

      _h_pT_M.scale(crossSection()/sumOfWeights(), this);
      _h_dPhi_M.scale(crossSection()/sumOfWeights(), this);
      _h_costheta_M.scale(crossSection()/sumOfWeights(), this);

    }


  private:

    Histo1DPtr _h_M;
    Histo1DPtr _h_pT;
    Histo1DPtr _h_dPhi;
    Histo1DPtr _h_costheta;
    BinnedHistogram<double> _h_pT_M;
    BinnedHistogram<double> _h_dPhi_M;
    BinnedHistogram<double> _h_costheta_M;

  };



  // The hook for the plugin system
  DECLARE_RIVET_PLUGIN(D0_2010_S8570965);

}