## main204

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    // main204.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:
    //            Photon beam
    //            Parton distribution
    //            LHAPDF

    // This is a simple test program.
    // It illustrates how to interface an external process with an incoming photon
    // in a hadron beam, using a set containing QED evolution.
    // All input is specified in the main204.cmnd file.

    #include "Pythia8/Pythia.h"

    using namespace Pythia8;

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

    int main() {

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

      // Read in commands from external file.
      pythia.readFile("main204.cmnd");

      // Extract settings to be used in the main program.
      int nEvent = pythia.mode("Main:numberOfEvents");

      // Initialize. Either of two options, to be picked in main204.cmnd.
      // 1) Read in external event with incoming photon in the ME,
      // from a pre-generated .lhe file (thanks to SANC and R. Sadykov).
      // 2) Use internal fermion gamma -> W+- fermion' process.

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

      // Histograms for pT distribution in gluon production vertex.
      Hist pTprim( "pT of photon production, no ISR", 100, 0., 100.);
      Hist pTwith( "pT of photon production, with ISR", 100, 0., 100.);

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

        // Generate events. Quit if failure.
        if (!pythia.next()) {
          break;
        }

        // Analyze event to find branching where photon is produced.
        int iGam = (event[3].id() == 22) ? 3 : 4;
        int iGamMother = iGam;
        for ( ; ; ) {
          iGamMother = event[iGam].mother1();
          if (iGamMother < iGam || event[iGamMother].id() != 22) break;
          iGam = iGamMother;
        }

        // Find and histogram pT in this branching.
        if (iGamMother < iGam) pTprim.fill( event[iGam].pT() );
        else {
          int iQ = iGamMother;
          int size = event.size();
          do ++iQ;
          while (event[iQ].status() != -43 && iQ < size - 1);
          if (event[iQ].status() == -43) pTwith.fill( event[iQ].pT() );
        }

      // End of event loop.
      }

      // Final statistics and histogram output.
      pythia.stat();
      cout << pTprim << pTwith;

      return 0;
    }