main35

Back to index.

// main35.cc is a part of the PYTHIA event generator.
// Copyright (C) 2020 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.

void run(Pythia* pythia, Hist& hist, int nEvent) {
  pythia->readString("Random:setSeed = on");
  pythia->readString("Random:seed = 1");
  pythia->init();
  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();
}

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

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 = new Pythia();
  pythia->readString("Beams:eCM = 13000.");
  pythia->readString("Charmonium:gg2ccbar(3S1)[3S1(1)]g = on,off");
  pythia->readString("PhaseSpace:pTHatMin = 2");
  run(pythia, pyPtPsi, 1000);
  delete pythia;

  // Produce leading-order events with HelacOnia.
  pythia = new Pythia();
  shared_ptr helaconiaPtr;
  helaconiaPtr = make_shared(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);
  run(pythia, hoPtPsi, 1000);
  delete pythia;

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