## main505

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    // main505.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:
    //            BSM
    //            R‑hadron
    //            Displaced vertex

    // Example of of R-hadron production.
    // Several of the possibilities shown here, like displaced vertices,
    // are extras that need not be used for the basic setup.

    #include "Pythia8/Pythia.h"

    using namespace Pythia8;

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

    int main() {

      // Key settings to be used in the main program.
      // nGluino = 0, 1, 2 give stop pair, single gluino or gluino pair.
      int nGluino  = 2;
      int nEvent   = 200;
      int nAbort   = 3;
      int nList    = 0;
      double eCM   = 7000.;

      // Generator. Shorthand for the event.
      Pythia pythia;
      Event& event = pythia.event;

      // Set up beams: p p is default so only need set energy.
      pythia.settings.parm("Beams:eCM", eCM);

      // Squark pair: use stop-antistop as example.
      if (nGluino == 0) {
        pythia.readString("SUSY:gg2squarkantisquark = on");
        pythia.readString("SUSY:idA = 1000006");
        pythia.readString("SUSY:idB = 1000006");
      // Squark-gluino pair: also supersymmetric u has been made long-lived.
      // Stop does not work since then one would need inoming top PDF.
      // Nevertheless R-hadrons are numbered/named as if containing a stop.
      } else if (nGluino == 1) {
        pythia.readString("SUSY:qg2squarkgluino  = on");
        pythia.readString("SUSY:idA = 1000002");
        pythia.readString("RHadrons:idStop = 1000002");
        pythia.readString("SUSY:idB = 1000021");
      // Gluino pair.
      } else {
        pythia.readString("SUSY:gg2gluinogluino  = on");
      }

      // Use hacked sps1a file, with stop (+su) and gluino made long-lived.
      // This is based on the width being less than 0.2 GeV by default.
      pythia.readString("SLHA:file = sps1aNarrowStopGluino.spc");
      // Further hacked file, to test R-parity violating gluino decay.
      //pythia.readString("SLHA:file = sps1aNarrowStopGluinoRPV.spc");

      // Allow R-hadron formation.
      pythia.readString("Rhadrons:allow = on");

      // If you want to do the decay separately later,
      // you need to switch off automatic decays.
      pythia.readString("RHadrons:allowDecay = off");

      // Fraction of gluinoballs.
      pythia.readString("RHadrons:probGluinoball = 0.1");

      // Switch off key components.
      //pythia.readString("PartonLevel:MPI = off");
      //pythia.readString("PartonLevel:ISR = off");
      //pythia.readString("PartonLevel:FSR = off");
      //pythia.readString("HadronLevel:Hadronize = off");

      // Allow the R-hadrons to have secondary vertices: set c*tau in mm.
      // Note that width and lifetime can be set independently.
      // (Nonzero small widths are needed e.g. to select branching ratios.)
      pythia.readString("1000002:tau0 = 200.");
      pythia.readString("1000006:tau0 = 250.");
      pythia.readString("1000021:tau0 = 300.");

      // Checks. Optionally relax E-p-conservation.
      pythia.readString("Check:nErrList = 2");
      //pythia.readString("Check:epTolErr = 2e-3");

      // Possibility to switch off particle data and event listings.
      // Also to shop location of displaced vertices.
      pythia.readString("Init:showChangedSettings = on");
      pythia.readString("Init:showChangedParticleData = off");
      pythia.readString("Next:numberShowInfo = 1");
      pythia.readString("Next:numberShowProcess = 1");
      pythia.readString("Next:numberShowEvent = 0");
      pythia.readString("Next:showScaleAndVertex = on");

      // 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.);
      Hist decVtx("R-hadron decay vertex (mm from origin)", 100, 0., 1000.);

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

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

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

        // 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 );
            // Separation of R-hadron decay vertex from origin.
            // Don't be fooled by pAbs(); it gives the three-vector length
            // of any Vec4, also one representing spatial coordinates.
            double dist = event[i].vDec().pAbs();
            decVtx.fill( dist);

            // This is a place where you could allow a R-hadron shift of
            // identity, momentum and decay vertex to allow for detector effects.
            // Identity not illustrated here; requires a change of mass as well.
            // Toy model: assume an exponential energy loss, < > = 1 GeV,
            // but at most half of kinetic energy. Unchanged direction.
            // Note that event will no longer conserve energy and momentum.
            double eLossAvg = 1.;
            double eLoss = 0.;
            do { eLoss = eLossAvg * pythia.rndm.exp(); }
            while (eLoss > 0.5 * (event[i].e() - event[i].m()));
            double eNew = event[i].e() - eLoss;
            Vec4   pNew = event[i].p() * sqrt( pow2(eNew) - pow2(event[i].m()) )
                        / event[i].pAbs();
            pNew.e( eNew);
            event[i].p( pNew);
            // The decay vertex will be calculated based on the production vertex,
            // the proper lifetime tau and the NEW four-momentum, rather than
            // e.g. some average momentum, if you do not set it by hand.
            // This commented-out piece illustrates brute-force setting,
            // but you should provide real numbers from some tracking program.
            // With tau = 0 the decay is right at the chosen point.
            //event[i].tau( 0.);
            //event[i].vProd( 132., 155., 233., 177.);

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

        // If you have set R-hadrons stable above,
        // you can still force them to decay at this stage.
        pythia.forceRHadronDecays();
        if (iEvent < nList) pythia.event.list(true);

      // 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
           << decVtx;

      // Done.
      return 0;
    }