## main469

Back to index.

    // main469.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:
    //            Space‑time picture
    //            Hadronic rescattering

    // Authors:
    //            Torbjörn Sjostrand

    // Space-time evolution of the hadronization process.
    // Output plotted by Python/Matplotlib/pyplot.

    #include "Pythia8/Pythia.h"

    using namespace Pythia8;

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

    int main() {

      // Allow hadronic rescattering or not.
      bool doRescatter = true;

      // Number of events.
      int nEvent  = 2000;
      int nAbort  = 5;
      int nStatus = 20;

      // Time/distance steps for histogramming and other counters.
      double tScale[90], sScale[100];
      int    ntPri[90], ntSec[90], ntRes[90], ntAll[90], nrPri[90], nrSec[90],
             nrRes[90], nrAll[90], nsPri[100], nsSec[100], nsRes[100], nsAll[100];
      for (int j = 0; j < 90; ++j) {
        tScale[j] = pow( 10., (j + 0.5) / 6.);
        ntPri[j] = ntSec[j] = ntRes[j]= ntAll[j] = nrPri[j] = nrSec[j]
          = nrRes[j] = nrAll[j] = 0;
      }
      for (int j = 0; j < 100; ++j) {
        sScale[j] = 0.2 * (j + 0.5);
        nsPri[j] = nsSec[j] = nsRes[j] = nsAll[j] = 0;
      }
      bool   isPri, isSec, isRes;
      int    iDau;
      double tProd, tDec, rProd, rDec, sProd, sDec, zProd, yProd;

      // Histogram particle time and space distributions.
      Hist tPri("primary hadrons",   90, 1., 1e15, true);
      Hist tSec("secondary hadrons", 90, 1., 1e15, true);
      Hist tRes("rescattered hadrons", 90, 1., 1e15, true);
      Hist tAll("all hadrons",       90, 1., 1e15, true);
      Hist rPri("primary hadrons",   90, 1., 1e15, true);
      Hist rSec("secondary hadrons", 90, 1., 1e15, true);
      Hist rRes("rescattered hadrons", 90, 1., 1e15, true);
      Hist rAll("all hadrons",       90, 1., 1e15, true);
      Hist sPri("primary hadrons",   100, 0., 20.);
      Hist sSec("secondary hadrons", 100, 0., 20.);
      Hist sRes("rescattered hadrons", 100, 0., 20.);
      Hist sAll("all hadrons",       100, 0., 20.);
      Hist qPri("primary hadrons",   100, 0., 10.);
      Hist qSec("secondary hadrons", 100, 0., 10.);
      Hist qRes("rescattered hadrons", 100, 0., 10.);
      Hist qAll("all hadrons",       100, 0., 10.);
      Hist yPri("primary hadrons",   100, -10., 10.);
      Hist ySec("secondary hadrons", 100, -10., 10.);
      Hist yRes("rescattered hadrons", 100, -10., 10.);
      Hist yAll("all hadrons",       100, -10., 10.);

      // Pythia generator. Event record shorthand.
      Pythia pythia;
      Event& event = pythia.event;
      pythia.readString("Beams:eCM = 13000.");

      // Process choice.
      pythia.readString("SoftQCD:nondiffractive = on");
      //pythia.readString("SoftQCD:inelastic = on");
      //pythia.readString("HardQCD:all = on");
      //pythia.readString("PhaseSpace:pTHatMin = 100.");

      // Vertex selection choices. Parton vertex can be removed.
      pythia.readString("Fragmentation:setVertices = on");
      pythia.readString("PartonVertex:setVertex = on");

      // Optionally switch on rescattering. Several parameters can be set.
      if (doRescatter) {
        pythia.readString("HadronLevel:Rescatter = on");
        //pythia.readString("Rescattering:nearestNeighbours = off");
      }

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

      // 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;
        }

        // Loop over all particles and analyze the hadrons.
        for (int i = 0; i < pythia.event.size(); ++i)
        if (event[i].isHadron() && event[i].statusAbs() > 20) {
          isPri = (event[i].statusAbs() / 10 == 8);
          isSec = (event[i].statusAbs() / 10 == 9);
          isRes = (event[i].statusAbs() / 10 == 15);
          iDau = event[i].daughter1();
          if (!isPri && !isSec && !isRes && ++nStatus < 20) cout << " status "
            << event[i].status() << " for hadron " << event[i].id()
            << " with mother id " << event[event[i].mother1()].id() << endl;

          // Particle production and decay time and radius, in units of fm.
          tProd = MM2FM * event[i].tProd();
          tDec  = (event[i].status() > 0) ? 1e15 : MM2FM * event[iDau].tProd();
          rProd = MM2FM * sqrt( pow2(event[i].xProd()) + pow2(event[i].yProd()) );
          rDec  = (event[i].status() > 0) ? 1e15
            : MM2FM * sqrt(pow2(event[iDau].xProd()) + pow2(event[iDau].yProd()));
          sProd = rProd;
          sDec  = rDec;
          zProd = MM2FM * event[i].zProd();
          yProd = 0.5 * log( max(1e-30, tProd + zProd)
                / max(1e-30, tProd - zProd) );

          // Check whether it exists at any of the time slots and count up.
          for (int j = 0; j < 90; ++j) if (tProd < tScale[j] && tDec > tScale[j]) {
            if (isPri) ++ntPri[j];
            if (isSec) ++ntSec[j];
            if (isRes) ++ntRes[j];
            ++ntAll[j];
          }

          // Check whether it exists at any of the radii slots and count up.
          for (int j = 0; j < 90; ++j) if (rProd < tScale[j] && rDec > tScale[j]) {
            if (isPri) ++nrPri[j];
            if (isSec) ++nrSec[j];
            if (isRes) ++nrRes[j];
            ++nrAll[j];
          }
          for (int j = 0; j < 100; ++j)
          if (sProd < sScale[j] && sDec > sScale[j]) {
            if (isPri) ++nsPri[j];
            if (isSec) ++nsSec[j];
            if (isRes) ++nsRes[j];
            ++nsAll[j];
          }

          // Histogram production r and y_tau.
          if (isPri) qPri.fill( sProd);
          if (isSec) qSec.fill( sProd);
          if (isRes) qRes.fill( sProd);
          qAll.fill( sProd);
          if (isPri) yPri.fill( yProd);
          if (isSec) ySec.fill( yProd);
          if (isRes) yRes.fill( yProd);
          yAll.fill( yProd);
        }

      // End of event loop. Final statistics.
      }
      pythia.stat();

      // Histogram particle time and space distributions.
      double wt = 1. / nEvent;
      for (int j = 0; j < 90; ++j) {
        tPri.fill( tScale[j], wt * ntPri[j]);
        tSec.fill( tScale[j], wt * ntSec[j]);
        tRes.fill( tScale[j], wt * ntRes[j]);
        tAll.fill( tScale[j], wt * ntAll[j]);
        rPri.fill( tScale[j], wt * nrPri[j]);
        rSec.fill( tScale[j], wt * nrSec[j]);
        rRes.fill( tScale[j], wt * nrRes[j]);
        rAll.fill( tScale[j], wt * nrAll[j]);
      }
      for (int j = 0; j < 100; ++j) {
        sPri.fill( sScale[j], wt * nsPri[j]);
        sSec.fill( sScale[j], wt * nsSec[j]);
        sRes.fill( sScale[j], wt * nsRes[j]);
        sAll.fill( sScale[j], wt * nsAll[j]);
      }
      qPri *= 10. * wt;
      qSec *= 10. * wt;
      qRes *= 10. * wt;
      qAll *= 10. * wt;
      yPri *= 5. * wt;
      ySec *= 5. * wt;
      yRes *= 5. * wt;
      yAll *= 5. * wt;
      cout << tPri << tSec << tRes << tAll << rPri << rSec << rRes << rAll
           << sPri << sSec << sRes  << sAll << qPri << qSec << qRes << qAll
           << yPri << ySec << yRes << yAll;

      // Write Python code that can generate a PDF file with the distributions.
      HistPlot hpl("plot469");
      hpl.frame( "fig469", "Hadron multiplicity at different times",
        "$ct$ (fm)", "$N_{\\mathrm{hadron}}(t)$");
      hpl.add( tPri, "-,red");
      hpl.add( tSec, "-,blue");
      hpl.add( tRes, "-,olive");
      hpl.add( tAll, "-,black");
      hpl.plot();
      hpl.frame( "", "Hadron multiplicity at different radii",
        "$r$ (fm)", "$N_{\\mathrm{hadron}}(r)$");
      hpl.add( rPri, "-,red");
      hpl.add( rSec, "-,blue");
      hpl.add( rRes, "-,olive");
      hpl.add( rAll, "-,black");
      hpl.plot();
      hpl.frame( "", "Hadron multiplicity at different radii",
        "$r$ (fm)", "$N_{\\mathrm{hadron}}(r)$");
      hpl.add( sPri, "-,red");
      hpl.add( sSec, "-,blue");
      hpl.add( sRes, "-,olive");
      hpl.add( sAll, "-,black");
      hpl.plot();
      hpl.frame( "", "Hadron production at different radii",
        "$r$ (fm)", "$\\mathrm{d}N_{\\mathrm{hadron}} / \\mathrm{d}r$");
      hpl.add( qPri, "-,red");
      hpl.add( qSec, "-,blue");
      hpl.add( qRes, "-,olive");
      hpl.add( qAll, "-,black");
      hpl.plot();
      hpl.frame( "", "Hadron production at different $y_{\\tau}$",
        "$y_{\\tau}$", "$\\mathrm{d}N_{\\mathrm{hadron}} / \\mathrm{d}y_{\\tau}$");
      hpl.add( yPri, "-,red");
      hpl.add( ySec, "-,blue");
      hpl.add( yRes, "-,olive");
      hpl.add( yAll, "-,black");
      hpl.plot();

      // Done.
      return 0;
    }