## main214

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    // main214.cc is a part of the PYTHIA event generator.
    // Copyright (C) 2026 Torbjorn Sjostrand.
    // PYTHIA is licenced under the GNU GPL v2 or later, see COPYING for details.
    // Please respect the MCnet Guidelines, see GUIDELINES for details.

    // Keywords:
    //            Basic usage
    //            Jet finding
    //            Slowjet
    //            Hadronization

    // Example how to compare "parton-level" and "hadron-level" properties.

    #include "Pythia8/Pythia.h"
    using namespace Pythia8;

    //==========================================================================

    // Generic routine to extract the particles that existed right before
    // the hadronization machinery was invoked.

    void getPartonLevelEvent( Event& event, Event& partonLevelEvent) {

      // Copy over all particles that existed right before hadronization.
      partonLevelEvent.reset();
      for (int i = 0; i < event.size(); ++i)
      if (event[i].isFinalPartonLevel()) {
        int iNew = partonLevelEvent.append( event[i] );

        // Set copied properties more appropriately: positive status,
        // original location as "mother", and with no daughters.
        partonLevelEvent[iNew].statusPos();
        partonLevelEvent[iNew].mothers( i, i);
        partonLevelEvent[iNew].daughters( 0, 0);
      }

    }

    //==========================================================================

    // Generic routine to extract the particles that exist after the
    // hadronization machinery. Normally not needed, since SlowJet
    // contains the standard possibilities preprogrammed, but this
    // method illustrates further discrimination.

    void getHadronLevelEvent( Event& event, Event& hadronLevelEvent) {

      // Iterate over all final particles.
      hadronLevelEvent.reset();
      for (int i = 0; i < event.size(); ++i) {
        bool accept = false;
        if (event[i].isFinal()) accept = true;

        // Example 1: reject neutrinos (standard option).
        int idAbs = event[i].idAbs();
        if (idAbs == 12 || idAbs == 14 || idAbs == 16) accept = false;

        // Example 2: reject particles with pT < 0.1 GeV (new possibility).
        if (event[i].pT() < 0.1) accept = false;

        // Copy over accepted particles, with original location as "mother".
        if (accept) {
          int iNew = hadronLevelEvent.append( event[i] );
          hadronLevelEvent[iNew].mothers( i, i);
        }
      }

    }

    //==========================================================================

    int main() {

      // Number of events, generated and listed ones.
      int nEvent    = 1000;
      int nListEvts = 1;
      int nListJets = 5;

      // Generator. LHC process and output selection. Initialization.
      Pythia pythia;
      pythia.readString("Beams:eCM = 13000.");
      pythia.readString("HardQCD:all = on");
      pythia.readString("PhaseSpace:pTHatMin = 200.");
      pythia.readString("Next:numberShowInfo = 0");
      pythia.readString("Next:numberShowProcess = 0");
      pythia.readString("Next:numberShowEvent = 0");

      // If Pythia fails to initialize, exit with error.
      if (!pythia.init()) return 1;

      // Parton and Hadron Level event records. Remeber to initalize.
      Event partonLevelEvent;
      partonLevelEvent.init("Parton Level event record", &pythia.particleData);
      Event hadronLevelEvent;
      hadronLevelEvent.init("Hadron Level event record", &pythia.particleData);

      //  Parameters for the jet finders. Need select = 1 to catch partons.
      double radius   = 0.5;
      double pTjetMin = 10.;
      double etaMax   = 4.;
      int select      = 1;

      // Set up anti-kT clustering, comparing parton and hadron levels.
      SlowJet antiKTpartons( -1, radius, pTjetMin, etaMax, select);
      SlowJet antiKThadrons( -1, radius, pTjetMin, etaMax, select);

      // Histograms.
      Hist nJetsP("number of jets, parton level", 20, -0.5, 19.5);
      Hist nJetsH("number of jets, hadron level", 20, -0.5, 19.5);
      Hist pTallP("pT for jets, parton level", 100, 0., 500.);
      Hist pTallH("pT for jets, hadron level", 100, 0., 500.);
      Hist pThardP("pT for hardest jet, parton level", 100, 0., 500.);
      Hist pThardH("pT for hardest jet, hadron level", 100, 0., 500.);
      Hist pTdiff("pT for hardest jet, hadron - parton", 100, -100., 100.);

      // Begin event loop. Generate event. Skip if error.
      for (int iEvent = 0; iEvent < nEvent; ++iEvent) {
        if (!pythia.next()) continue;

        // Construct parton and hadron level event.
        getPartonLevelEvent( pythia.event, partonLevelEvent);
        getHadronLevelEvent( pythia.event, hadronLevelEvent);

        // List first few events.
        if (iEvent < nListEvts) {
          pythia.event.list();
          partonLevelEvent.list();
          hadronLevelEvent.list();
        }

        // Analyze jet properties and list first few analyses.
        antiKTpartons.analyze( partonLevelEvent );
        antiKThadrons.analyze( hadronLevelEvent );
        if (iEvent < nListJets) {
          antiKTpartons.list();
          antiKThadrons.list();
        }

        // Fill jet properties distributions.
        nJetsP.fill( antiKTpartons.sizeJet() );
        nJetsH.fill( antiKThadrons.sizeJet() );
        for (int i = 0; i < antiKTpartons.sizeJet(); ++i)
          pTallP.fill( antiKTpartons.pT(i) );
        for (int i = 0; i < antiKThadrons.sizeJet(); ++i)
          pTallH.fill( antiKThadrons.pT(i) );
        if ( antiKTpartons.sizeJet() > 0 && antiKThadrons.sizeJet() > 0) {
          pThardP.fill( antiKTpartons.pT(0) );
          pThardH.fill( antiKThadrons.pT(0) );
          pTdiff.fill( antiKThadrons.pT(0) - antiKTpartons.pT(0) );
        }

      // End of event loop. Statistics. Histograms.
      }
      pythia.stat();
      cout << nJetsP << nJetsH << pTallP << pTallH
           <<  pThardP << pThardH << pTdiff;

      // Write Python code to generate comparison plots.
      HistPlot hpl("plot214");
      hpl.frame("fig214", "Number of jets in event",
        "$n_{\\mathrm{jet}}$", "Rate");
      hpl.add( nJetsP, "h,black", "partons");
      hpl.add( nJetsH, "h,red", "hadrons");
      hpl.plot();
      hpl.frame("", "Transverse momentum of all jets",
        "$_{\\perp}$", "Rate");
      hpl.add( pTallP, "-,black", "partons");
      hpl.add( pTallH, "-,red", "hadrons");
      hpl.plot();
      hpl.frame("", "Transverse momentum of hardest jet",
        "$_{\\perp}$", "Rate");
      hpl.add( pThardP, "-,black", "partons");
      hpl.add( pThardH, "-,red", "hadrons");
      hpl.plot();

      // Done.
      return 0;
    }