## main09

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    // main09.cc is a part of the PYTHIA event generator.
    // Copyright (C) 2024 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:
    //            Biasing
    //            Second interaction

    // Generate a predetermined second hard interaction.

    #include "Pythia8/Pythia.h"

    using namespace Pythia8;

    int main() {

      // Generator.
      Pythia pythia;
      Event& event   = pythia.event;

      // Select first hard process (just a small sample of possibilities).
      //pythia.readString("HardQCD:all = on");
      pythia.readString("Top:all = on");
      //pythia.readString("WeakSingleBoson:ffbar2gmZ = on");
      //pythia.readString("WeakSingleBoson:ffbar2W = on");

      // Select second hard process (complete list of options).
      pythia.readString("SecondHard:generate = on");
      //pythia.readString("SecondHard:TwoJets = on");
      pythia.readString("SecondHard:PhotonAndJet = on");
      //pythia.readString("SecondHard:TwoPhotons = on");
      //pythia.readString("SecondHard:SingleGmZ = on");
      //pythia.readString("SecondHard:SingleW = on");
      //pythia.readString("SecondHard:TwoBJets = on");

      // Kinematics cuts, common for the two.
      pythia.readString("PhaseSpace:mHatMin = 40.");
      pythia.readString("PhaseSpace:pTHatMin = 20.");

      // Initialize for LHC at 8 TeV.
      pythia.readString("Beams:eCM = 8000.");
      pythia.init();

      // Histogram.
      Hist pTfirst("pT first collision",    100, 0., 400.);
      Hist pTsecond("pT second collision",  100, 0., 200.);
      Hist pTdiff("pT first-second collision", 100, -100., 300.);
      Hist nMult("number of multiparton interactions", 100, -0.5, 99.5);
      Hist bMore("b enhancement factor",    100, 0., 10.);
      Hist nChg("charged multiplicity", 100, -0.5, 999.5);

      // Generate events.
      for (int iev = 0; iev < 1000; ++iev) {
        pythia.next();

        // Histogram pT.
        double pT1 = pythia.info.pTMPI(0);
        double pT2 = pythia.info.pTMPI(1);
        pTfirst.fill( pT1 );
        pTsecond.fill( pT2 );
        pTdiff.fill( pT1 - pT2 );

        // Histogram multiparton interactions
        double nMPI = pythia.info.nMPI();
        nMult.fill( nMPI );
        bMore.fill( pythia.info.enhanceMPI() );

        // Histogram charged multiplicity.
        int nCharged = 0;
        for (int i = 0; i < event.size(); ++i)
          if (event[i].isFinal() && event[i].isCharged()) ++nCharged;
        nChg.fill( nCharged );

      }

      // Compare full statistics listing with what is set in info.
      pythia.stat();
      cout << scientific << setprecision(3) << "\n From pythia.info: sigma = "
           << pythia.info.sigmaGen() << " +- " << pythia.info.sigmaErr()
           << endl;

      // Print histograms.
      cout << pTfirst << pTsecond << pTdiff << nMult << bMore << nChg;

      // Done.
      return 0;
    }