## main211

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    // main211.cc is a part of the PYTHIA event generator.
    // Copyright (C) 2025 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

    // This is a simple test program.
    // It studies jet production at the LHC, using SlowJet,
    // here as a Pythia-centric wrapper for FastJet.

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

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

    int main() {

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

      // Generator. LHC process and output selection. Initialization.
      Pythia pythia;
      pythia.readString("Beams:eCM = 14000.");
      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;

      // Standard parameters for a jet finder.
      double radius   = 0.7;
      double pTjetMin = 10.;
      double etaMax   = 4.;
      // Exclude neutrinos (and other invisible) from study.
      int    nSel     = 2;

      // Set up SlowJet jet finder, with anti-kT clustering (-1)
      // and pion mass assumed for non-photons.
      SlowJet slowJet( -1, radius, pTjetMin, etaMax, nSel, 1);

      // Histograms.
      Hist nJets("number of jets", 50, -0.5, 49.5);
      Hist pTjets("pT for jets", 100, 0., 500.);
      Hist yJets("y for jets", 100, -5., 5.);
      Hist phiJets("phi for jets", 100, -M_PI, M_PI);
      Hist distJets("R distance between jets", 100, 0., 10.);
      Hist pTdiff("pT difference between jets", 100, -100., 400.);

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

        // Analyze Slowet jet properties. List first few.
        slowJet.analyze( pythia.event );
        if (iEvent < nListJets) slowJet.list();

        // Fill SlowJet inclusive jet distributions.
        nJets.fill( slowJet.sizeJet() );
        for (int i = 0; i < slowJet.sizeJet(); ++i) {
          pTjets.fill( slowJet.pT(i) );
          yJets.fill( slowJet.y(i) );
          phiJets.fill( slowJet.phi(i) );
        }

        // Fill SlowJet distance between jets.
        for (int i = 0; i < slowJet.sizeJet() - 1; ++i)
        for (int j = i + 1; j < slowJet.sizeJet(); ++j) {
          double dY = slowJet.y(i) - slowJet.y(j);
          double dPhi = abs( slowJet.phi(i) - slowJet.phi(j) );
          if (dPhi > M_PI) dPhi = 2. * M_PI - dPhi;
          double dR = sqrt( pow2(dY) + pow2(dPhi) );
          distJets.fill( dR );
        }

        // Fill SlowJet pT-difference between jets (to check ordering of list).
        for (int i = 1; i < slowJet.sizeJet(); ++i)
          pTdiff.fill( slowJet.pT(i-1) - slowJet.pT(i) );

      // End of event loop. Statistics. Histograms.
      }
      pythia.stat();
      cout << nJets << pTjets << yJets << phiJets << distJets << pTdiff;

      // Done.
      return 0;
    }