## main421

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    // main421.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:
    //            Leif Lonnblad

    // Keywords:
    //            Heavy ions
    //            Rivet
    //            Angantyr

    // This is a simple test program, equivalent to main101.cc, but using the
    // Angantyr model for Heavy Ion collisions. It is still proton collisions,
    // but uses the Angantyr impact parameter description to select collisions.
    // It studies the charged multiplicity distribution at the LHC.

    // Optionally (by compiling with the flag -DRIVET and linking with rivet
    // - see output of the command "rivet-config --cppflags --libs" -
    // it will send the event to Rivet for an ATLAS jet-analysis.

    #include "Pythia8/Pythia.h"
    #include "Pythia8/HeavyIons.h"
    #include "Pythia8Plugins/ProgressLog.h"

    using namespace Pythia8;

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

    int main() {

      // Generator. Process selection. LHC initialization. Histogram.
      Pythia pythia;

      // This forces the HeavyIons model to be used even for pp collisons.
      pythia.readString("HeavyIon:mode = 2");

      pythia.readString("Beams:eCM = 7000.");
      pythia.readString("HardQCD:all = on");
      pythia.readString("PhaseSpace:pTHatMin = 20.");

      // Only do a couple generations in the fitting to cross sections.
      pythia.readString("HeavyIon:SigFitNGen = 4");

      // Try to load RIVET.
      int nEvents = 1000;
      pythia.readString("Init:plugins = {libpythia8rivet.so::RivetHooks}");
      if (pythia.settings.isWord("Rivet:fileName")) {
        nEvents = 10000;
        pythia.readString("Rivet:fileName = main421.yoda");
        pythia.readString("Rivet:analyses = {ATLAS_2010_I871366}");
      }
      else
        pythia.readString("Init:plugins = {}");

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

      // Book a histogram of the multiplicity distribution
      Hist mult("charged multiplicity", 100, -0.5, 799.5);

      // Initialise the printout of run progress information.
      ProgressLog logger(nEvents);

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

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

        // Intermittently report run progress.
        logger(iEvent);
      }

      // End of event loop. Statistics. Histogram. Done.
      pythia.stat();
      cout << mult;

      return 0;
    }