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OPAL_2004_I648738

Quark and gluon jet fragmentation functions
Experiment: OPAL (LEP)
Inspire ID: 648738
Status: VALIDATED
Authors:
  • Daniel Reichelt [d.reichelt@cern.ch]
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 $e^+e^-\to q\bar{q}$ events are required, while for gluon fragmentation the fictonal $e^+e^-\to gg$ 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^-\to 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 $Q_\mathrm{jet}=E_\mathrm{jet}\sin(\theta/2)$ 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
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// -*- 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(3,0.)
    {    }


    /// @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
      _histo_xE.push_back(bookHisto1D(ih+5,1,1));
      _histo_xE.push_back(bookHisto1D(ih+5,1,2));
      if(ih<5) _histo_xE.push_back(bookHisto1D(ih+5,1,3));
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      const double weight = event.weight();
      // find the initial quarks/gluons
      ParticleVector initial;
      for (const GenParticle* p : Rivet::particles(event.genEvent())) {
	const GenVertex* pv = p->production_vertex();
	const PdgId pid = abs(p->pdg_id());
	if(!( (pid>=1&&pid<=5) || pid ==21) ) continue;
	bool passed = false;
	for (const GenParticle* pp : particles_in(pv)) {
	  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].pdgId()==-initial[1].pdgId()) {
	PdgId pid = abs(initial[0].pdgId());
	if(pid>=1&&pid<=4) 
	  itype=0;
	else
	  itype=1;
      }
      assert(itype<_histo_xE.size());

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

    }


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


  private:

    vector<double> _sumW;

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


  };

  // The hook for the plugin system
  DECLARE_RIVET_PLUGIN(OPAL_2004_I648738);

}