Rivet analyses

Measurement of the quark to photon fragmentation function

Experiment: ALEPH (LEP Run 1)

Inspire ID: 398193

Status: VALIDATED

Authors: - Frank Siegert

References: - Z.Phys.C69:365-378,1996 - DOI: 10.1007/s002880050037

Beams: e+ e-

Beam energies: (45.6, 45.6)GeV

Run details: - e⁺e⁻→ jets with π and η 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_I398193.cc`

// -*- 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 {


  /// ALEPH measurement of quark-to-photon fragmentation function
  class ALEPH_1996_I398193 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(ALEPH_1996_I398193);


    /// @name Analysis methods
    /// @{

    void init() {
      // Set up projections
      FinalState fs;
      declare(FastJets(fs, JetAlg::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
      book(_h_z_2jet_001 ,1, 1, 1);
      book(_h_z_2jet_006 ,2, 1, 1);
      book(_h_z_2jet_01  ,3, 1, 1);
      book(_h_z_2jet_033 ,4, 1, 1);
      book(_h_z_3jet_001 ,5, 1, 1);
      book(_h_z_3jet_006 ,6, 1, 1);
      book(_h_z_3jet_01  ,7, 1, 1);
      book(_h_z_4jet_001 ,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");

      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);
            else if (jets_001.size() == 3) _h_z_3jet_001->fill(zgamma);
            else if (jets_001.size() > 3) _h_z_4jet_001->fill(zgamma);
            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);
            else if (jets_006.size() == 3) _h_z_3jet_006->fill(zgamma);
            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);
            else if (jets_01.size() == 3) _h_z_3jet_01->fill(zgamma);
            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);
            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;
    /// @}

  };



  RIVET_DECLARE_ALIASED_PLUGIN(ALEPH_1996_I398193, ALEPH_1996_S3196992);

}

Aliases: - ALEPH_1996_S3196992

Rivet analyses

Measurement of the quark to photon fragmentation function

Source code:ALEPH_1996_I398193.cc

Source code:`ALEPH_1996_I398193.cc`