Rivet analyses

Quark and gluon jet fragmentation functions

Experiment: OPAL (LEP)

Inspire ID: 648738

Status: VALIDATED

Authors: - Daniel Reichelt

References: - Eur.Phys.J. C37 (2004) no.1, 25-47 - hep-ex/0404026

Beams: e+ e-

Beam energies: (6.4, 6.4); (13.4, 13.4); (24.0, 24.0); (45.6, 45.6); (46.5, 46.5); (48.5, 48.5); (98.5, 98.5)GeV

Run details: - For quark fragmentation e⁺e⁻ → qq̄ events are required, while for gluon fragmentation the fictional e⁺e⁻ → gg process is required.

Measurement of the fragmentation functions for quarks and gluons at LEP. Useful for studying the properties of gluon jets and the differences between quark and gluon jets. For the study of gluon jets the fictional $e^+e-g g $ process must be used. The data in histograms labelled “hemisphere fragmentation” are measured from jets defined by hemispheres, with an energy scale defined by $E=\sqrt{s}/2$, while data in histograms named “durham fragmentation” are measured from jets defined by the durham algorithm with Q_jet = E_jetsin (θ/2) as the energy scale. The rivet analysis defines the jet flavour from the initial quarks, as the data are corrected for impurities, and defines all jets as hemispheres with energy scale $E=\sqrt{s}/2$.

Source code:`OPAL_2004_I648738.cc`

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/ChargedFinalState.hh"

namespace Rivet {


  class OPAL_2004_I648738 : public Analysis {
  public:

    /// Constructor
    OPAL_2004_I648738()
      : Analysis("OPAL_2004_I648738"), _sumW(3), _histo_xE(3)
    {    }


    /// @name Analysis methods
    /// @{
    void init() {
      declare(FinalState(), "FS");
      declare(ChargedFinalState(), "CFS");
      unsigned int ih=0;
      if (inRange(0.5*sqrtS()/GeV, 4.0, 9.0)) {
    ih = 1;
      }
      else if (inRange(0.5*sqrtS()/GeV, 9.0, 19.0)) {
    ih = 2;
      }
      else if (inRange(0.5*sqrtS()/GeV, 19.0, 30.0)) {
    ih = 3;
      }
      else if (inRange(0.5*sqrtS()/GeV, 45.5, 45.7)) {
    ih = 5;
      }
      else if (inRange(0.5*sqrtS()/GeV, 30.0, 70.0)) {
    ih = 4;
      }
      else if (inRange(0.5*sqrtS()/GeV, 91.5, 104.5)) {
    ih = 6;
      }
      assert(ih>0);
      // book the histograms
      book(_histo_xE[0], ih+5,1,1);
      book(_histo_xE[1], ih+5,1,2);
      if(ih<5) book(_histo_xE[2] ,ih+5,1,3);
      book(_sumW[0], "_sumW_0");
      book(_sumW[1], "_sumW_1");
      book(_sumW[2], "_sumW_2");
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // find the initial quarks/gluons
      Particles initial;
      for (ConstGenParticlePtr p : HepMCUtils::particles(event.genEvent())) {
        ConstGenVertexPtr pv = p->production_vertex();
        const PdgId pid = abs(p->pdg_id());
        if(!( (pid>=1&&pid<=5) || pid ==21) ) continue;
        bool passed = false;
        for (ConstGenParticlePtr pp : HepMCUtils::particles(pv, Relatives::PARENTS)) {
          const PdgId ppid = abs(pp->pdg_id());
          passed = (ppid == PID::ELECTRON || ppid == PID::HIGGS ||
                    ppid == PID::ZBOSON   || ppid == PID::GAMMA);
          if(passed) break;
        }
        if(passed) initial.push_back(Particle(*p));
        }
        if(initial.size()!=2) {
          vetoEvent;
        }
        // type of event
        unsigned int itype=2;
        if(initial[0].pid()==-initial[1].pid()) {
          PdgId pid = abs(initial[0].pid());
          if(pid>=1&&pid<=4)
            itype=0;
          else
            itype=1;
        }
        assert(itype<_histo_xE.size());

      // fill histograms
      _sumW[itype]->fill(2.);
      const Particles& chps = apply<FinalState>(event, "CFS").particles();
      for(const Particle& p : chps) {
        double xE = 2.*p.E()/sqrtS();
    _histo_xE[itype]->fill(xE);
      }

    }


    /// Normalise histograms etc., after the run
    void finalize() {
      for(unsigned int ix=0;ix<_histo_xE.size();++ix) {
    if(_sumW[ix]->val()>0.) scale(_histo_xE[ix],1./ *_sumW[ix]);
      }
    }
    /// @}


  private:

    vector<CounterPtr> _sumW;

    /// @name Histograms
    /// @{
    vector<Histo1DPtr> _histo_xE;
    /// @}


  };

  RIVET_DECLARE_PLUGIN(OPAL_2004_I648738);

}

Rivet analyses

Quark and gluon jet fragmentation functions

Source code:OPAL_2004_I648738.cc

Source code:`OPAL_2004_I648738.cc`