## main461

Back to index.

    // main461.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.

    // Authors:
    //            Torbjorn Sjostrand

    // Keywords:
    //            Rescattering
    //            Low energy
    //            Multiplicities

    // Compare charged multiplicity energy dependence in various treatments,
    // specifically the simplified one used for low-energy collisions in
    // rescattering with the full-fledged standard based on an MPI framework.

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

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

    int main() {

      // Number of events per energy point.
      int nEvent = 10000;

      // All subprocesses (0), nondiffractive (1) or single diffractive (2).
      int pick = 1;

      // Histograms.
      Hist multCh0("n_charged(e_CM), simple",      32, 1., 10000., true);
      Hist multCh1("n_charged(e_CM), interpolate", 32, 1., 10000., true);
      Hist multCh2("n_charged(e_CM), full MPI",    32, 1., 10000., true);

      // Loop over the three scenarios at fifteen energies.
      for (int ic = 0; ic < 3; ++ic) {

        // First scenario: low-energy handling as in rescattering.
        // Second scenario: variable-energy interpolating description.
        if (ic < 2) {

          // Create Pythia instance and set it up.
          Pythia pythia;
          Event& event = pythia.event;
          pythia.readString("Beams:allowVariableEnergy = on");
          pythia.readString("Beams:eCM = 10000.");
          // A raised eMinPert recovers the simple framework below new value.
          if (ic == 0) pythia.readString("Beams:eMinPert = 9900.");
          // Switch on all processes, nondiffractive or single diffractive.
          if (pick == 0) {
            pythia.readString("LowEnergyQCD:all = on");
            pythia.readString("SoftQCD:all = on");
          } else if (pick == 1) {
            pythia.readString("LowEnergyQCD:nonDiffractive = on");
            pythia.readString("SoftQCD:nonDiffractive = on");
          } else {
            pythia.readString("LowEnergyQCD:singleDiffractiveXB = on");
            pythia.readString("LowEnergyQCD:singleDiffractiveAX = on");
            pythia.readString("SoftQCD:singleDiffractive = on");
          }
          // If Pythia fails to initialize, exit with error.
          if (!pythia.init()) return 1;

          // Iterate over thirty energies.
          for (int ie = 2; ie < 32; ++ie) {
            double eCM = pow( 10., (ie + 0.5) / 8.);
            pythia.setKinematics(eCM);

            // Generate events. Update charged multiplicity.
            long nSuccess = 0;
            long nCharged = 0;
            for (int iEvent = 0; iEvent < nEvent; ++iEvent) {
              if (!pythia.next()) continue;
              ++nSuccess;
              nCharged += event.nFinal(true);
            }
            if (nSuccess == 0);
            else if (ic == 0) multCh0.fill(eCM, double(nCharged)/double(nSuccess));
            else              multCh1.fill(eCM, double(nCharged)/double(nSuccess));
          }

        // Third scenario: full generation energy by energy (only above 10 GeV).
        } else {
          for (int ie = 8; ie < 32; ++ie) {
            double eCM = pow( 10., (ie + 0.5) / 8.);

            // Create Pythia instance and set it up.
            Pythia pythia;
            Event& event = pythia.event;
            if (pick == 0) pythia.readString("SoftQCD:all = on");
            else if (pick == 1)  pythia.readString("SoftQCD:nonDiffractive = on");
            else pythia.readString("SoftQCD:singleDiffractive = on");
            pythia.settings.parm("Beams:eCM", eCM);

            // If Pythia fails to initialize, exit with error.
            if (!pythia.init()) {
              cout << "Pythia failed to initialize at eCM [GeV] = " << eCM << endl;
              return 1;
            }

            // Generate events. Update charged multiplicity.
            long nSuccess = 0;
            long nCharged = 0;
            for (int iEvent = 0; iEvent < nEvent; ++iEvent) {
              if (!pythia.next()) continue;
              ++nSuccess;
              nCharged += event.nFinal(true);
            }
            multCh2.fill( eCM, double(nCharged) / double(nSuccess) );
          }
        }
      }

      // Plot histograms.
      cout << multCh0 << multCh1 << multCh2;
      HistPlot hpl("plot461");
      hpl.frame("fig461", "Rise of charged multiplicity with energy",
        "eCM", "<n_charged>");
      hpl.add( multCh0, "-", "low-energy model");
      hpl.add( multCh1, "-", "interpolation");
      hpl.add( multCh2, "-", "high-energy model");
      hpl.plot();

      // Done.
      return 0;
    }