## main261

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    // main261.cc is a part of the PYTHIA event generator.
    // Copyright (C) 2026 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:
    //            Userhooks
    //            Biasing

    // Example on how to enhance rare emission rates, in this case q -> q gamma.
    // To concentrate on the photons from the showers, MPI and hadronization
    // are switched off by default.

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

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

    int main() {

      // Histogram pT spectrum of photons and event weights.
      Hist gamNoEnh(   "gamma pT spectrum, no enhancement",   100, 0., 100.);
      Hist gamWithEnh( "gamma pT spectrum, with enhancement", 100, 0., 100.);
      Hist gamRatio("gamma pT spectrum, with/no enhancement", 100, 0., 100.);
      Hist gamBefWt(   "gamma pT spectrum, without weight",   100, 0., 100.);
      Hist eventWt(    "log10(event weight)",                 100, -7., 3.);

      // Compare generation without and with enhanced q -> q gamma emission.
      for (int iCase = 0; iCase < 2; ++iCase) {

        // Generator.
        Pythia pythia;
        pythia.readFile("main261.cmnd");
        int nEvent = pythia.mode("Main:numberOfEvents");

        // No enhancement.
        if (iCase == 0)
          pythia.readString("Enhancements:doEnhance = false");

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

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

          // Generate events. Find and histogram event weight.
          pythia.next();
          double weight = (iCase == 1) ? pythia.info.weight() : 1.;
          if (iCase == 1) eventWt.fill( log10(weight) );
          sumWt += weight;

          // Find all final-state photons and histogram them.
          for (int i = 0; i < pythia.event.size(); ++i)
          if (pythia.event[i].isFinal() && pythia.event[i].id() == 22) {
            double pT = pythia.event[i].pT();
            if (iCase == 0) gamNoEnh.fill(   pT, 1.);
            if (iCase == 1) gamBefWt.fill(   pT, 1.);
            if (iCase == 1) gamWithEnh.fill( pT, weight);
          }

        // End of event loop.
        }

        // Statistics.
        pythia.stat();
        cout << "\n Average event weight = " << scientific
             << sumWt / nEvent << endl;

        // End of case loop.
      }

      // Write histograms to output stream.
      gamRatio = gamWithEnh / gamNoEnh;
      cout << gamNoEnh << gamWithEnh << gamRatio << gamBefWt << eventWt;

      // Write histogram data to files.
      ofstream write;
      write.open("PTA_0.dat");
      gamNoEnh.table(write);
      write.close();
      write.open("PTA_1.dat");
      gamWithEnh.table(write);
      write.close();

      // Done.
      return 0;
    }