## main381

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    // main381.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:
    //            Higgs
    //            Electron‑positron

    // Authors:
    //            Torbjörn Sjostrand

    // Simple example of Higgs pruduction at future e+e- colliders.

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

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

    int main() {

      // Number of events.
      int nEvent = 25000;

      // Generator. Incoming beams. (Switch off iniial-state photon radiation.)
      Pythia pythia;
      pythia.readString("Beams:idA = -11");
      pythia.readString("Beams:idB = 11");
      pythia.readString("Beams:eCM = 500.");
      //pythia.readString("PDF:lepton = off");

      // All Higgs production channels.
      pythia.readString("HiggsSM:all = on");
      //pythia.readString("23:onMode = off");
      //pythia.readString("23:onIfAny = 1 2 3 4 5");

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

      Hist mult("charged multiplicity all", 50, -1., 99.);
      Hist multS("charged multiplicity Higgsstrahlung", 50, -1., 99.);
      Hist multG("charged multiplicity gammagamma fusion", 50, -1., 99.);
      Hist multZ("charged multiplicity ZZ fusion", 50, -1., 99.);
      Hist multW("charged multiplicity WW fusion", 50, -1., 99.);

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

        // Find number of all final charged particles and fill histogram.
        int nCharged = 0;
        for (int i = 0; i < pythia.event.size(); ++i)
          if (pythia.event[i].isFinal() && pythia.event[i].isCharged())
            ++nCharged;
        mult.fill( nCharged );

        // Split by process type.
        int code = pythia.info.code();
        if      (code == 904) multS.fill( nCharged );
        else if (code == 903) multG.fill( nCharged );
        else if (code == 906) multZ.fill( nCharged );
        else if (code == 907) multW.fill( nCharged );
        else cout << " Warning: unexpected process code " << code << endl;

      // End of event loop. Statistics. Normalize and print histograms.
      }
      pythia.stat();
      mult  /= double(nEvent);
      multS /= double(nEvent);
      multG /= double(nEvent);
      multZ /= double(nEvent);
      multW /= double(nEvent);
      cout << mult << multS << multG << multZ << multW;

      //Plot histograms.
      HistPlot hpl("plot381");
      hpl.frame( "fig381", "Charged multiplicity for Higgs production "
        "in a 500 GeV e$^+$e$^-$ collider", "$n_{\\mathrm{charged}}$",
        "Probability");
      hpl.add( mult, "h,black", "all events");
      hpl.add( multS, "h,red", "Higgsstrahlung");
      hpl.add( multW, "h,blue", "WW fusion");
      hpl.add( multZ, "h,olive", "ZZ fusion");
      hpl.add( multG, "h,magenta", "$\\gamma\\gamma$ fusion");
      hpl.plot();

      // Done
      return 0;
    }