## main247

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    // main247.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.

    // Authors:
    //            Christian Bierlich

    // Keywords:
    //            Userhooks
    //            Performance
    //            Jets
    //            Hadronic rescattering

    // This main program illustrates the use of UserHooks to veto events
    // after hadronization, but before any subsequent processes such as
    // rescattering or Bose-Einstein.

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

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

    // Write own derived UserHooks class.

    class HadronUserHooks : public UserHooks {

    public:

      bool canVetoAfterHadronization() override {return true;}

      bool doVetoAfterHadronization(const Event& e) override {
        // This illustrates the use by requiring the presence of
        // a high pT track central in eta.
        for (const Particle& p : e) {
          // If a trigger particle is found, do not veto.
          if (p.isFinal() && p.isCharged() && p.pT() > 7 && abs(p.eta()) < 1.0)
            return false;
        }
        // This info message will be printed in the stat summary.
        loggerPtr->INFO_MSG("event vetoed by HadronUserHooks");
        // If no trigger particle is found, veto the event.
        return true;
      }

    };

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

    int main() {

      // Generator.
      Pythia pythia;

      // Make a histogram of the leading track.
      Hist leadingTrack("pTleading", 20, 0., 20.);

      // Select the parton-level pTHatMin below the cut on track-level.
      pythia.readString("HardQCD:all = on");
      pythia.readString("PhaseSpace:pTHatMin = 5.");

      // Add a process after hadronization to motivate the veto.
      pythia.readString("Fragmentation:setVertices = on");
      pythia.readString("PartonVertex:setVertex = on");
      pythia.readString("HadronLevel:Rescatter = on");
      pythia.readString("MultipartonInteractions:pT0Ref = 2.345");

      // Create and set user hook.
      auto hadronUserHooks = make_shared<HadronUserHooks>();
      pythia.setUserHooksPtr( hadronUserHooks);

      // Initialize.
      pythia.readString("Beams:eCM = 7000");

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

      // Collect sum of weights of accepted events.
      double sumW = 0.;

      // Begin event loop.
      for (int iEvent = 0; iEvent < 10000; ++iEvent) {

        // Generate events.
        if (!pythia.next()) continue;
        sumW+=pythia.info.weight();

        // Find highest-pT track.
        double pTmax = 0;
        for (int i = 0, N = pythia.event.size(); i < N; ++i) {
          Particle& p = pythia.event[i];
          if (p.isFinal() && p.isCharged() && abs(p.eta()) < 0.8)
            if (p.pT() > pTmax) pTmax = p.pT();
        }
        leadingTrack.fill(pTmax);
      // End of event loop.
      }

      // Statistics.
      pythia.stat();
      leadingTrack/=sumW;

      // Plot leading track pT spectrum.
      HistPlot hpl("plot247");
      hpl.frame("fig247", "Leading track pT", "$p_{\\perp}$", "Prob");
      hpl.add( leadingTrack, "h,black" , "leading track");
      hpl.plot();

      // Done.
      return 0;
    }