## main363

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    // main363.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:
    //            Philip Ilten

    // Keywords:
    //            Onia
    //            Helaconia

    // An example where the quarkonia hard process (p p -> J/psi g) is
    // automatically produced externally with HelacOnia, read in, and the
    // remainder of the event is then produced by Pythia (MPI, showers,
    // and hadronization). A comparison is made between events produced
    // with Pythia at LO and HelacOnia at LO.

    // For this example to run, HelacOnia must be installed and the
    // command "exe" (set by default as "ho_cluster") must be available
    // via the command line. Note that this example has only been tested
    // with HelacOnia version 2.2.4; due to rapid HelacOnia development,
    // this example may not work with other versions. For more details on
    // the LHAHelacOnia class see the comments of
    // Pythia8Plugins/LHAHelacOnia.h.

    #include "Pythia8/Pythia.h"
    #include "Pythia8Plugins/LHAHelaconia.h"

    using namespace Pythia8;

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

    // A simple method to run Pythia, analyze the events, and fill a histogram.

    bool run(Pythia& pythia, Hist& hist, int nEvent) {
      pythia.readString("Random:setSeed = on");
      pythia.readString("Random:seed = 1");
      if (!pythia.init()) return false;
      for (int iEvent = 0; iEvent < nEvent; ++iEvent) {
        if (!pythia.next()) continue;
        for (int i = 0; i < pythia.event.size(); ++i) {
          if (pythia.event[i].id() != 443) continue;
          hist.fill(pythia.event[pythia.event[i].iBotCopyId()].pT());
          break;
        }
      }
      pythia.stat();
      return true;
    }

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

    int main() {

      // The name of the HelacOnia executable.
      // You must prepend this string with the path to the executable
      // on your local installation, or otherwise make it available.
      string exe("ho_cluster");

      // Create the histograms.
      Hist pyPtPsi("Pythia dN/dpT J/psi", 100, 0., 20.);
      Hist hoPtPsi("HelacOnia dN/dpT J/psi", 100, 0., 20.);

      // Produce leading-order events with Pythia.
      {
        Pythia pythia;
        pythia.readString("Beams:eCM = 13000.");
        pythia.readString("Charmonium:gg2ccbar(3S1)[3S1(1)]g = on,off");
        pythia.readString("PhaseSpace:pTHatMin = 2");
        // If we fail to initialize, abort.
        if (!run(pythia, pyPtPsi, 1000)) return 1;
      }

      // Produce leading-order events with HelacOnia.
      {
        Pythia pythia;
        shared_ptr<LHAupHelaconia> helaconiaPtr;
        helaconiaPtr = make_shared<LHAupHelaconia>(&pythia, "helaconiarun", exe);
        helaconiaPtr->readString("generate g g > cc~(3S11) g");
        helaconiaPtr->readString("set energy_beam1 = 6500");
        helaconiaPtr->readString("set energy_beam2 = 6500");
        helaconiaPtr->readString("set minptconia = 2");
        pythia.setLHAupPtr((LHAupPtr)helaconiaPtr);
        // If we fail to initialize, abort.
        if(!run(pythia, hoPtPsi, 1000)) return 1;
      }

      // Print the histograms.
      cout << pyPtPsi;
      cout << hoPtPsi;
      return 0;
    }