## main143

Back to index.

    // main143.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:
    //            Rene Brun
    //            Axel Naumann and Bernhard Meirose

    // Keywords:
    //            Analysis
    //            Root

    // This is a simple test program, based on main101.cc,
    // but modified to use ROOT for histogramming.
    // It studies the charged multiplicity distribution at the LHC.

    // WARNING: for currently unknown reasons it may hang
    // with an empty canvas on a Mac.

    // Header file to access Pythia 8 program elements.
    #include "Pythia8/Pythia.h"

    // ROOT, for histogramming.
    #include "TH1.h"

    // ROOT, for interactive graphics.
    #include "TVirtualPad.h"
    #include "TApplication.h"

    // ROOT, for saving file.
    #include "TFile.h"

    using namespace Pythia8;

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

    int main(int argc, char* argv[]) {

      // Create the ROOT application environment.
      TApplication theApp("hist", &argc, argv);

      // Create Pythia instance and set it up to generate hard QCD processes
      // above pTHat = 20 GeV for pp collisions at 14 TeV.
      Pythia pythia;
      pythia.readString("HardQCD:all = on");
      pythia.readString("PhaseSpace:pTHatMin = 20.");
      pythia.readString("Beams:eCM = 14000.");

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

      // Create file on which histogram(s) can be saved.
      TFile* outFile = new TFile("hist.root", "RECREATE");

      // Book histogram.
      TH1F *mult = new TH1F("mult","charged multiplicity", 100, -0.5, 799.5);

      // Begin event loop. Generate event; skip if generation aborted.
      for (int iEvent = 0; iEvent < 1000; ++iEvent) {
        if (!pythia.next()) continue;

        // Find number of all final charged particles.
        int nCharged = 0;
        for (int i = 0; i < pythia.event.size(); ++i)
          if (pythia.event[i].isFinal() && pythia.event[i].isCharged())
            ++nCharged;

        // Fill charged multiplicity in histogram. End event loop.
        mult->Fill( nCharged );
      }

      // Statistics on event generation.
      pythia.stat();

      // Show histogram. Possibility to close it.
      mult->Draw();
      std::cout << "\nDouble click on the histogram window to quit.\n";
      gPad->WaitPrimitive();

      // Save histogram on file and close file.
      mult->Write();
      delete outFile;

      // Done.
      return 0;
    }