## main157

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    // main157.cc is a part of the PYTHIA event generator.
    // Copyright (C) 2024 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:
    //            Marius Utheim
    //            Philip Ilten

    // Keywords:
    //            Low energy
    //            Resonances

    // Illustration of how to create new resonance particles that can form
    // during rescattering. In this case, the new resonance is the chi_1c(3872)
    // tetraquark, which is produced from an initial D0 Dbar*0.

    #include "Pythia8/Pythia.h"
    using namespace Pythia8;

    //--------------------------------------------------------------------------

    int main() {

      // Initialize Pythia.
      Pythia pythia;

      // Define the chi_1c(3872)0 tetraquark.
      ParticleData& pd = pythia.particleData;
      pd.addParticle(9044111, "chi_1c(3872)0", "chi_1c(3872)bar0",
        2, 0, 0, 3.8720, 0.0012, 3.82, 3.92, 0., true);

      ParticleDataEntryPtr pde = pythia.particleData.findParticle(9044111);
      // Define decay channels. These can be used for resonance production.
      pde->addChannel(1, 0.3700, 3, 421,  -423); // D0 Dbar*0
      pde->addChannel(1, 0.0430, 3, 223,   443); // omega J/psi
      pde->addChannel(1, 0.0380, 3, 113,   443); // rho0 J/psi
      pde->addChannel(1, 0.0340, 3, 111, 20443); // pi0 chi_1c

      // Resonance production is only possible in hadron-hadron collisions.
      // These decay channels will not be used for resonance production, but
      // are relevant since they can change the particle composition.
      pde->addChannel(1, 0.0080, 3,  22,       443); // gamma J/psi
      pde->addChannel(1, 0.0450, 3,  22,    100443); // gamma psi(2S)
      pde->addChannel(1, 0.4900, 0, 421, -421, 111); // D0 Dbar0 pi0

      // Process specification. Resonance formation will be the dominant process.
      pythia.readString("LowEnergyQCD:all = on");
      pythia.readString("Beams:idA =  421");
      pythia.readString("Beams:idB = -423");
      pythia.readString("Beams:eCM = 3.9");

      // Number of events to generate/print.
      pythia.readString("Main:numberOfEvents  = 10");
      pythia.readString("Next:numberShowEvent = 10");

      // Initialize.
      if (!pythia.init()) {
        cout << " Pythia failed to initialize." << endl;
        return -1;
      }

      // Generate events. They will be printed to cout.
      int nEvent = pythia.mode("Main:numberOfEvents");
      for (int iEvent = 0; iEvent < nEvent; ++iEvent)
        pythia.next();

      // Print statistics.
      pythia.stat();

      // Done.
      return 0;
    }