## main367

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    // main367.cc is a part of the PYTHIA event generator.
    // Copyright (C) 2025 Peter Skands, 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:
    //            Top
    //            R‑hadrons

    // Authors:
    //            Torbjörn Sjostrand

    // Fictitious scenario where the top is long-lived enough to hadronize.
    // This is studied using the R-hadron machinery, in e+e- or pp collisions.
    // Kindly disregard warning that wzDecayShowers find undecayed tops.

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

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

    int main() {

      // Key parameters: e+e- or pp, # events, t mass.
      bool doee    = true;
      int nEvent   = 1000;
      double mTop  = 171.;

      // Generator. Shorthand for the event and particle data.
      Pythia pythia;
      Event& event = pythia.event;
      ParticleData& pData = pythia.particleData;

      // Set up beams: e+e- or pp. Switch off ISR for former.
      if (doee) {
        pythia.readString("Beams:idA = 11");
        pythia.readString("Beams:idB = -11");
        pythia.readString("Beams:eCM = 500.");
        pythia.readString("PDF:lepton = off");
      } else {
        pythia.readString("Beams:eCM = 13000.");
      }

      // Set up top pair production.
      if (doee) {
        pythia.readString("Top:ffbar2ttbar(s:gmZ) = on");
      } else {
        pythia.readString("Top:gg2ttbar = on");
        pythia.readString("Top:qqbar2ttbar = on");
        pythia.readString("PhaseSpace:pTHatMin = 100.");
      }

      // Set top mass and width.
      pData.m0( 6, mTop);
      pData.mWidth( 6, 0.1);
      pData.doForceWidth( 6, true);

      // Allow R-hadron formation with top marked as stop.
      pythia.readString("Rhadrons:allow = on");
      pythia.readString("RHadrons:idStop = 6");

      // Set R-hadrons stable.
      //pythia.readString("RHadrons:allowDecay = off");

      // Switch off key components. Useful for first checks, but not full run.
      //pythia.readString("PartonLevel:MPI = off");
      //pythia.readString("PartonLevel:ISR = off");
      //pythia.readString("PartonLevel:FSR = off");
      //pythia.readString("HadronLevel:Hadronize = off");

      // Possibility to switch off particle data and event listings.
      //pythia.readString("Init:showChangedSettings = off");
      //pythia.readString("Init:showChangedParticleData = off");
      //pythia.readString("Next:numberShowInfo = 0");
      //pythia.readString("Next:numberShowProcess = 0");
      //pythia.readString("Next:numberShowEvent = 0");

      // Debug options.
      pythia.readString("Check:nErrList = 5");

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

      // Histograms.
      Hist nChargedH("charged multiplicity", 100, -0.5, 799.5);
      Hist dndyChargedH("dn/dy charged", 100, -10., 10.);
      Hist dndyRH("dn/dy R-hadrons", 100, -5., 5.);
      Hist pTRH("pT R-hadrons", 100, 0., 1000.);
      Hist xRH("p_RHadron / p_sparticle", 100, 0.9, 1.1);
      Hist mDiff("m(Rhadron) - m(sparticle)", 100, 0., 5.);

      // R-hadron flavour composition.
      map<int, int> flavours;

      // Begin event loop. Generate events. Skip. if failure.
      for (int iEvent = 0; iEvent < nEvent; ++iEvent) {
        if (!pythia.next()) continue;

        // Loop over final charged particles in the event.
        // The R-hadrons may not yet have decayed here.
        int nCharged = 0;
        Vec4 pSum;
        for (int i = 0; i < event.size(); ++i) {
          if (event[i].isFinal()) {
            pSum += event[i].p();
            if (event[i].isCharged()) {
              ++nCharged;
              dndyChargedH.fill( event[i].y() );
            }
          }
        }
        nChargedH.fill( nCharged );

        // Loop over final R-hadrons in the event: kinematic distribution
        for (int i = 0; i < event.size(); ++i) {
          int idAbs = event[i].idAbs();
          if (idAbs > 1000100 && idAbs < 2000000 && idAbs != 1009002) {
            ++flavours[ event[i].id() ];
            dndyRH.fill( event[i].y() );
            pTRH.fill( event[i].pT() );
            // Trace back to mother; compare momenta and masses.
            int iMother = i;
            while( event[iMother].statusAbs() > 100)
              iMother = event[iMother].mother1();
            double xFrac = event[i].pAbs() / event[iMother].pAbs();
            xRH.fill( xFrac);
            double mShift = event[i].m() - event[iMother].m();
            mDiff.fill( mShift );

          // End of loop over final R-hadrons.
          }
        }

      // End of event loop.
      }

      // Final statistics, flavour composition and histogram output.
      pythia.stat();
      cout << "\n Composition of produced R-hadrons \n    code            "
           << "name   times " << endl;
      for (map<int, int>::iterator flavNow = flavours.begin();
        flavNow != flavours.end(); ++flavNow)  cout << setw(8)
        << flavNow->first << setw(16) << pythia.particleData.name(flavNow->first)
        << setw(8) << flavNow->second << endl;
      cout << nChargedH << dndyChargedH << dndyRH << pTRH << xRH << mDiff;

      // Done.
      return 0;
    }