## main31

Back to index.

    // main31.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:
    //            Matching
    //            Merging
    //            Powheg

    // Example how to perform matching with POWHEG-BOX events,
    // based on the code found in include/Pythia8Plugins/PowhegHooks.h.

    #include "Pythia8/Pythia.h"
    #include "Pythia8Plugins/PowhegHooks.h"
    using namespace Pythia8;

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

    int main() {

      // Generator
      Pythia pythia;

      // Load configuration file
      pythia.readFile("main31.cmnd");

      // Read in main settings.
      int nEvent      = pythia.settings.mode("Main:numberOfEvents");
      int nError      = pythia.settings.mode("Main:timesAllowErrors");
      // Read in key POWHEG matching settings.
      int powhegVeto    = pythia.settings.mode("POWHEG:veto");
      int powhegMPIveto = pythia.settings.mode("POWHEG:MPIveto");
      bool loadHooks  = (powhegVeto > 0 || powhegMPIveto > 0);
      // Read in shower settings.
      int showerModel = pythia.settings.mode("PartonShowers:model");

      // Add in user hooks for shower vetoing.
      shared_ptr<PowhegHooks> powhegHooks;
      if (loadHooks) {

        // For POWHEG:veto >= 1, setup to do vetoed power showers.
        //   1) Setting pTmaxMatch = 2 forces PYTHIA's shower to sweep over the
        //      full phase space.
        //   2) Loading the POWHEG hooks will then veto any shower branchings that
        //      are judged (according to the POWHEG settings) to double-count the
        //      POWHEG one.
        if (powhegVeto > 0) {
          if (showerModel == 1 || showerModel == 3) {
            // For PYTHIA's simple shower (and also for Dire), the FSR and ISR
            // shower starting scales are set by the respective pTmaxMatch values.
            pythia.readString("TimeShower:pTmaxMatch = 2");
            pythia.readString("SpaceShower:pTmaxMatch = 2");
            // Use undamped power showers, except for cases for which there could
            // be an interplay with ISR branchings not simulated by the pure
            // shower, like ISR g->tt.
            pythia.readString("SpaceShower:pTdampMatch = 3");
            pythia.readString("TimeShower:pTdampMatch = 0");
          } else if (showerModel == 2) {
            // Vincia has common settings that apply to both ISR and FSR.
            pythia.readString("Vincia:pTmaxMatch = 2");
            // Use undamped power showers, except for cases for which there could
            // be an interplay with ISR branchings not simulated by the pure
            // shower, like g->tt.
            pythia.readString("Vincia:pTdampMatch = 3");
          }
        }

        // For POWHEG:MPIveto >= 1, also set MPI to start at the kinematical limit.
        if (powhegMPIveto > 0) {
          pythia.readString("MultipartonInteractions:pTmaxMatch = 2");
        }

        powhegHooks = make_shared<PowhegHooks>();
        pythia.setUserHooksPtr((UserHooksPtr)powhegHooks);
      }

      // Initialise and list settings
      pythia.init();

      // Counters for number of ISR/FSR emissions vetoed
      unsigned long int nISRveto = 0, nFSRveto = 0;

      // Begin event loop; generate until nEvent events are processed
      // or end of LHEF file
      int iEvent = 0, iError = 0;
      while (true) {

        // Generate the next event
        if (!pythia.next()) {

          // If failure because reached end of file then exit event loop
          if (pythia.info.atEndOfFile()) break;

          // Otherwise count event failure and continue/exit as necessary
          cout << "Warning: event " << iEvent << " failed" << endl;
          if (++iError == nError) {
            cout << "Error: too many event failures... exiting" << endl;
            break;
          }

          continue;
        }

        /*
         * Process dependent checks and analysis may be inserted here
         */

        // Update ISR/FSR veto counters
        if (loadHooks) {
          nISRveto += powhegHooks->getNISRveto();
          nFSRveto += powhegHooks->getNFSRveto();
        }

        // If nEvent is set, check and exit loop if necessary
        ++iEvent;
        if (nEvent != 0 && iEvent == nEvent) break;

      } // End of event loop.

      // Statistics, histograms and veto information
      pythia.stat();
      cout << "Number of ISR emissions vetoed: " << nISRveto << endl;
      cout << "Number of FSR emissions vetoed: " << nFSRveto << endl;
      cout << endl;

      // Done.
      return 0;
    }