## main244

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    // main244.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.

    // Keywords:
    //            Resonance decay
    //            External resonance

    // Simple illustration how to provide (a) your own resonance-width class,
    // and (b) your own cross-section class, with instances handed in to Pythia.
    // The hypothetical scenario is that top would have been so long-lived
    // that a toponium resonance Theta could form. Then production could
    // proceed via q qbar -> gamma*/Z* -> Theta, with decay either to
    // a fermion pair or (dominantly) to three gluons.
    // The implementation is not physically correct in any number of ways,
    // but should exemplify the strategy needed for realistic cases.

    #include "Pythia8/Pythia.h"

    using namespace Pythia8;

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

    // The ResonanceTheta class handles a toponium resonance.

    class ResonanceTheta : public ResonanceWidths {

    public:

      // Constructor.
      ResonanceTheta(int idResIn) {initBasic(idResIn);}

    private:

      // Locally stored properties and couplings.
      double normTheta2qqbar, normTheta2llbar, normTheta2ggg;

      // Initialize constants.
      virtual void initConstants();

      // Calculate various common prefactors for the current mass.
      // Superfluous here, so skipped.
      //virtual void calcPreFac(bool = false);

      // Calculate width for currently considered channel.
      virtual void calcWidth(bool = false);

    };

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

    // Initialize constants.

    void ResonanceTheta::initConstants() {

      // Dummy normalization of couplings to the allowed decay channels.
      normTheta2qqbar = 0.0001;
      normTheta2llbar = 0.0001;
      normTheta2ggg   = 0.001;
    }

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

    // Calculate width for currently considered channel.

    void ResonanceTheta::calcWidth(bool) {

      // Expression for Theta -> q qbar (q up to b). Colour factor.
      if (id1Abs < 6) widNow = 3. * normTheta2qqbar * mHat;

      // Expression for Theta -> l lbar (l = e, mu, tau).
      else if (id1Abs == 11  || id1Abs == 13 || id1Abs == 15)
        widNow = normTheta2llbar * mHat;

      // Expression for Theta -> g g g. Colour factor.
      else if (id1Abs == 21) widNow = 8. * normTheta2ggg * mHat;

    }

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

    // A derived class for q qbar -> Theta (toponium bound state).

    class Sigma1qqbar2Theta : public Sigma1Process {

    public:

      // Constructor.
      Sigma1qqbar2Theta() {}

      // Initialize process.
      virtual void initProc();

      // Calculate flavour-independent parts of cross section.
      virtual void sigmaKin();

      // Evaluate sigmaHat(sHat). Assumed flavour-independent so simple.
      virtual double sigmaHat() {return sigma;}

      // Select flavour, colour and anticolour.
      virtual void setIdColAcol();

      // Evaluate weight for decay angles.
      virtual double weightDecay( Event& process, int iResBeg, int iResEnd);

      // Info on the subprocess.
      virtual string name()       const {return "q qbar -> Theta";}
      virtual int    code()       const {return 621;}
      virtual string inFlux()     const {return "qqbarSame";}
      virtual int    resonanceA() const {return 663;}

    private:

      // Store flavour-specific process information and standard prefactor.
      int    idTheta;
      double mRes, GammaRes, m2Res, GamMRat, normTheta2qqbar, sigma;

      // Pointer to properties of Theta, to access decay width.
      ParticleDataEntryPtr particlePtr;

    };

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

    // Initialize process.

    void Sigma1qqbar2Theta::initProc() {

      // Store Theta mass and width for propagator.
      idTheta  = 663;
      mRes     = particleDataPtr->m0(idTheta);
      GammaRes = particleDataPtr->mWidth(idTheta);
      m2Res    = mRes*mRes;
      GamMRat  = GammaRes / mRes;

      // Same normlization as in ResonanceTheta for coupling strength.
      normTheta2qqbar = 0.0001;

      // Set pointer to particle properties and decay table.
      particlePtr = particleDataPtr->particleDataEntryPtr(idTheta);

    }

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

    // Evaluate sigmaHat(sHat); first step when inflavours unknown.

    void Sigma1qqbar2Theta::sigmaKin() {

      // Incoming width with colour factor.
      double widthIn  = normTheta2qqbar * mH / 3.;

      // Breit-Wigner, including some (guessed) spin factors.
      double sigBW    = 12. * M_PI / ( pow2(sH - m2Res) + pow2(sH * GamMRat) );

      // Outgoing width: only includes channels left open.
      double widthOut = particlePtr->resWidthOpen(663, mH);

      // Total answer.
      sigma = widthIn * sigBW * widthOut;

    }

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

    // Select identity, colour and anticolour.

    void Sigma1qqbar2Theta::setIdColAcol() {

      // Flavours trivial.
      setId( id1, id2, idTheta);

      // Colour flow topologies. Swap when antiquarks.
      setColAcol( 1, 0, 0, 1, 0, 0);
      if (id1 < 0) swapColAcol();

    }

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

    // Evaluate weight for Theta -> g g g.

    double Sigma1qqbar2Theta::weightDecay( Event& process, int iResBeg,
      int iResEnd) {

      // Should be Theta decay. (This is only option here, so overkill.)
      if (iResEnd != iResBeg || process[iResBeg].idAbs() != idTheta)
        return 1.;

      // Should be decay to three gluons.
      int i1 = process[iResBeg].daughter1();
      int i2 = i1 + 1;
      int i3 = i2 + 1;
      if (i3 != process[iResBeg].daughter2() || process[i1].id() != 21)
        return 1.;

      // Energy fractions x_i = 2 E_i/m_Theta of gluons in Theta rest frame.
      double x1 = 2. * process[i1].p() * process[iResBeg].p()
                / process[iResBeg].m2();
      double x2 = 2. * process[i2].p() * process[iResBeg].p()
                / process[iResBeg].m2();
      double x3 = 2. * process[i3].p() * process[iResBeg].p()
                / process[iResBeg].m2();

      // Matrix-element expression for Theta -> g g g.
      double wtME = pow2( (1. - x1) / (x2 * x3) )
        + pow2( (1. - x2) / (x1 * x3) ) + pow2( (1. - x3) / (x1 * x2) );
      double wtMEmax = 2.;
      return wtME / wtMEmax;

    }

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

    int main() {

      // Number of events to generate. Max number of errors.
      // Warning: generation of complete events is much slower than if you use
      // PartonLevel:all = off to only get cross sections, so adjust nEvent.
      int nEvent = 1000;
      int nAbort = 5;

      // Pythia generator.
      Pythia pythia;

      // Create the toponium resonance and a few production/decay channels.
      // Warning: many more exist, e.g. weak ones of one top quark.
      // Note: to obtain the correct width for the Breit-Wigner you must
      // include all channels, but you only need leave those on that you
      // want to study.
      pythia.readString("663:new = Theta void 3 0 0 342.0 0.2 300. 400. 0.");
      pythia.readString("663:addChannel = 1 0. 0 1 -1");
      pythia.readString("663:addChannel = 1 0. 0 2 -2");
      pythia.readString("663:addChannel = 1 0. 0 3 -3");
      pythia.readString("663:addChannel = 1 0. 0 4 -4");
      pythia.readString("663:addChannel = 1 0. 0 5 -5");
      pythia.readString("663:addChannel = 1 0. 0 11 -11");
      pythia.readString("663:addChannel = 1 0. 0 13 -13");
      pythia.readString("663:addChannel = 1 0. 0 15 -15");
      pythia.readString("663:addChannel = 1 0. 0 21 21 21");

      // Create instance of a class to calculate the width of Theta to the
      // above channels. Hand in pointer to Pythia.
      // Note: Pythia will automatically delete this pointer,
      // along with all other resonances.
      ResonanceWidthsPtr resonanceTheta = make_shared<ResonanceTheta>(663);
      pythia.addResonancePtr(resonanceTheta);

      // Create instance of a class to generate the q qbar -> Theta process
      // from an external matrix element. Hand in pointer to Pythia.
      SigmaProcessPtr sigma1Theta = make_shared<Sigma1qqbar2Theta>();
      pythia.addSigmaPtr(sigma1Theta);

      // Optionally only compare cross sections.
      //pythia.readString("PartonLevel:all = off");
      pythia.readString("Check:nErrList = 2");

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

      // Book histogram.
      Hist mTheta("Theta mass", 100, 300., 400.);

      // Begin event loop.
      int iAbort = 0;
      for (int iEvent = 0; iEvent < nEvent; ++iEvent) {

        // Generate events. Quit if many failures.
        if (!pythia.next()) {
          if (++iAbort < nAbort) continue;
          cout << " Event generation aborted prematurely, owing to error!\n";
          break;
        }

        // Fill Theta mass. End of event loop.
        mTheta.fill( pythia.process[5].m() );
      }

      // Final statistics. Print histogram.
      pythia.stat();
      cout << mTheta;

      // Done.
      return 0;
    }