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ALEPH_1996_S3196992

Measurement of the quark to photon fragmentation function
Experiment: ALEPH (LEP Run 1)
Inspire ID: 398193
Status: VALIDATED
Authors:
  • Frank Siegert
References: Beams: e+ e-
Beam energies: (45.6, 45.6) GeV
Run details:
  • $e^+e^-\to$ jets with $\pi$ and $\eta$ decays turned off.

Earlier measurements at LEP of isolated hard photons in hadronic Z decays, attributed to radiation from primary quark pairs, have been extended in the ALEPH experiment to include hard photon production inside hadron jets. Events are selected where all particles combine democratically to form hadron jets, one of which contains a photon with a fractional energy $z > 0.7$. After statistical subtraction of non-prompt photons, the quark-to-photon fragmentation function, $D(z)$, is extracted directly from the measured 2-jet rate.

Source code: ALEPH_1996_S3196992.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/ChargedFinalState.hh"
#include "Rivet/Projections/IdentifiedFinalState.hh"
#include "Rivet/Projections/FastJets.hh"
#include "Rivet/Projections/Thrust.hh"

namespace Rivet {


  /// @brief ALEPH measurement of quark-to-photon fragmentation function
  class ALEPH_1996_S3196992 : public Analysis {
  public:

    /// Constructor
    ALEPH_1996_S3196992()
      : Analysis("ALEPH_1996_S3196992")
    {    }


    /// @name Analysis methods
    //@{

    void init() {
      // Set up projections
      FinalState fs;
      declare(FastJets(fs, FastJets::DURHAM, 0.7), "DurhamJets");
      IdentifiedFinalState ifs; //(Cuts::pT > 0);
      ifs.acceptId(PID::PHOTON);
      declare(ifs, "Photons");
      declare(Thrust(fs), "Thrust");
      declare(ChargedFinalState(), "CFS");

      // Book histograms
      _h_z_2jet_001 = bookHisto1D(1, 1, 1);
      _h_z_2jet_006 = bookHisto1D(2, 1, 1);
      _h_z_2jet_01  = bookHisto1D(3, 1, 1);
      _h_z_2jet_033 = bookHisto1D(4, 1, 1);
      _h_z_3jet_001 = bookHisto1D(5, 1, 1);
      _h_z_3jet_006 = bookHisto1D(6, 1, 1);
      _h_z_3jet_01  = bookHisto1D(7, 1, 1);
      _h_z_4jet_001 = bookHisto1D(8, 1, 1);
    }


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

      if (apply<FinalState>(event, "CFS").particles().size() < 2) vetoEvent;

      const Particles allphotons = apply<IdentifiedFinalState>(event, "Photons").particles();
      Particles photons;
      for (const Particle& photon : allphotons)
        if (fabs(cos(photon.theta())) < 0.95 && photon.E() > 5.0*GeV)
          photons.push_back(photon);
      if (photons.size() < 1) vetoEvent;

      const Thrust& thrust = apply<Thrust>(event, "Thrust");
      if (fabs(cos(thrust.thrustAxis().theta()))>0.95) vetoEvent;

      const FastJets& durjet = apply<FastJets>(event, "DurhamJets");


      const double weight = event.weight();
      for (const Particle& photon : photons) {

        PseudoJets jets_001 = durjet.clusterSeq()->exclusive_jets_ycut(0.01);
        for (const fastjet::PseudoJet& jet : jets_001) {
          if (particleInJet(photon, jet)) {
            double zgamma = photon.E()/jet.E();
            if (jets_001.size() == 2) _h_z_2jet_001->fill(zgamma, weight);
            else if (jets_001.size() == 3) _h_z_3jet_001->fill(zgamma, weight);
            else if (jets_001.size() > 3) _h_z_4jet_001->fill(zgamma, weight);
            break;
          }
        }

        PseudoJets jets_006 = durjet.clusterSeq()->exclusive_jets_ycut(0.06);
        for (const fastjet::PseudoJet& jet : jets_006) {
          if (particleInJet(photon, jet)) {
            double zgamma = photon.E()/jet.E();
            if (jets_006.size() == 2) _h_z_2jet_006->fill(zgamma, weight);
            else if (jets_006.size() == 3) _h_z_3jet_006->fill(zgamma, weight);
            break;
          }
        }

        PseudoJets jets_01 = durjet.clusterSeq()->exclusive_jets_ycut(0.1);
        for (const fastjet::PseudoJet& jet : jets_01) {
          if (particleInJet(photon, jet)) {
            double zgamma = photon.E()/jet.E();
            if (jets_01.size() == 2) _h_z_2jet_01->fill(zgamma, weight);
            else if (jets_01.size() == 3) _h_z_3jet_01->fill(zgamma, weight);
            break;
          }
        }

        PseudoJets jets_033 = durjet.clusterSeq()->exclusive_jets_ycut(0.33);
        for (const fastjet::PseudoJet& jet : jets_033) {
          if (particleInJet(photon, jet)) {
            double zgamma = photon.E()/jet.E();
            if (jets_033.size() == 2) _h_z_2jet_033->fill(zgamma, weight);
            break;
          }
        }

      }
    }


    bool particleInJet(const Particle& p, const fastjet::PseudoJet& jet) {
      for (const fastjet::PseudoJet& jetpart : jet.constituents()) {
        if (fuzzyEquals(jetpart.E(), p.E()) &&
            fuzzyEquals(jetpart.px(), p.px()) &&
            fuzzyEquals(jetpart.py(), p.py()) &&
            fuzzyEquals(jetpart.pz(), p.pz())) {
          return true;
        }
      }
      return false;
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      scale(_h_z_2jet_001, 1000.0/sumOfWeights());
      scale(_h_z_2jet_006, 1000.0/sumOfWeights());
      scale(_h_z_2jet_01, 1000.0/sumOfWeights());
      scale(_h_z_2jet_033, 1000.0/sumOfWeights());
      scale(_h_z_3jet_001, 1000.0/sumOfWeights());
      scale(_h_z_3jet_006, 1000.0/sumOfWeights());
      scale(_h_z_3jet_01, 1000.0/sumOfWeights());
      scale(_h_z_4jet_001, 1000.0/sumOfWeights());
    }

    //@}


  private:

    /// @name Histograms
    //@{
    Histo1DPtr _h_z_2jet_001, _h_z_2jet_006, _h_z_2jet_01, _h_z_2jet_033;
    Histo1DPtr _h_z_3jet_001, _h_z_3jet_006, _h_z_3jet_01;
    Histo1DPtr _h_z_4jet_001;
    //@}

  };


  // The hook for the plugin system
  DECLARE_RIVET_PLUGIN(ALEPH_1996_S3196992);


}