## main403

Back to index.

    // main403.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.

    // Authors:
    //            Peter Skands

    // Keywords:
    //            Vincia
    //            Top

    // This test program is a basic check of Vincia showers for pp > tt at LHC.
    // Also illustrates how various components of Vincia can be switched on/off
    // in a command file, and measures the run time (eg to compare options
    // and/or compare with Pythia).

    #include <time.h>
    #include "Pythia8/Pythia.h"
    using namespace Pythia8;

    int main() {

      //************************************************************************
      // Generator.
      Pythia pythia;

      // Read user settings from file
      pythia.readFile("main403.cmnd");

      //************************************************************************

      // Extract settings to be used in the main program.
      // Number of events, generated and listed ones.
      int nEvent         = pythia.settings.mode("Main:numberOfEvents");
      int showerModel    = pythia.settings.mode("PartonShowers:Model");
      bool hadronisation = pythia.settings.flag("HadronLevel:all");
      Event& event       = pythia.event;

      //************************************************************************

      // Initialize
      if(!pythia.init()) { return EXIT_FAILURE; }

      //************************************************************************

      // Histograms
      string modelName = "Pythia";
      if (showerModel == 2) modelName = "Vincia";
      double scale = 1;
      if (hadronisation) scale = 4;
      // Include stat uncertainties on histograms (last argument = true).
      Hist hNFinal(modelName + " nFinal", 100, -0.5, double(scale*200.-0.5),
        false, true);
      Hist hNGam(modelName + " nPhotons", 100, -0.5, double(scale*100.-0.5),
        false, true);
      Hist hNEle(modelName + " nElectrons", 100, -0.5, 99.5, false , true);
      Hist pTt(modelName + " pT(t)", 100, 0.1, 500., true, true);
      Hist yt(modelName + " y(t)", 20, -5.0, 5.0, false, true);
      Hist mt(modelName + " m(t)", 100, 150.0, 200.0, false, true);
      Hist pTtt(modelName + " pT(tt)", 100, 0.1, 500., true, true);
      Hist ytt(modelName + " y(tt)", 20, -5.0, 5.0, false, true);
      Hist mtt(modelName + " m(tt)", 100, 0.0, 1000.0, false, true);

      // Measure the cpu runtime.
      clock_t start, stop;
      double t = 0.0;
      start = clock();

      // Begin event loop. Generate event. Abort if error.
      for (int iEvent = 0; iEvent < nEvent; ++iEvent) {

        if (!pythia.next()) {
          cout << " Event generation aborted prematurely, owing to error!\n";
          cout<< "Event number was : "<<iEvent<<endl;
          break;
        }

        // Check for weights
        double weight = pythia.info.weight();

        // Find final copies of t and tbar.
        int i1 = event[5].iBotCopyId();
        int i2 = event[6].iBotCopyId();
        pTt.fill(event[i1].pT(), 0.5);
        yt.fill(event[i1].y(), 0.5);
        mt.fill(event[i1].m(), 0.5);
        pTt.fill(event[i2].pT(), 0.5);
        yt.fill(event[i2].y(), 0.5);
        mt.fill(event[i2].m(), 0.5);
        // Form 4-vector of final ttbar system.
        Vec4 pSum = event[i1].p() + event[i2].p();
        pTtt.fill(pSum.pT());
        ytt.fill(pSum.rap());
        mtt.fill(pSum.mCalc());

        // Count number of final-state particles.
        // Also count photons and electrons, to test QED.
        double nFinal = 0;
        double nGam   = 0;
        double nEle   = 0;
        for (int i = 1; i<event.size(); ++i) {
          if (!event[i].isFinal()) continue;
          nFinal++;
          if (event[i].idAbs() == 22) ++nGam;
          else if (event[i].id() == 11) ++nEle;
        }
        hNFinal.fill(nFinal, weight);
        hNGam.fill(nGam, weight);
        hNEle.fill(nEle, weight);

      }

      // End of event loop. Determine run time.
      stop = clock(); // Stop timer
      t = (double) (stop-start)/CLOCKS_PER_SEC;

      // Statistics. Histograms.
      pythia.stat();
      cout << hNFinal << hNGam << hNEle;
      cout << yt << mt << pTt << endl;
      cout << ytt << mtt << pTtt << endl;

      // Print runtime
      cout << "\n" << "|----------------------------------------|" << endl;
      cout << "| CPU Runtime = " << t << " sec" << endl;
      cout << "|----------------------------------------|" << "\n" << endl;

      // Done.
      return 0;
    }