## main202

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    // main202.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:
    //            Parton distribution
    //            LHAPDF
    //            Minimum bias
    //            Tuning

    // Studies of hadron-level and parton-level minimum-bias quantities,
    // comparing the internal default PDF with an external one from LHAPDF.
    // Major differences indicate the need for major retuning, e.g. pT0Ref.

    #include "Pythia8/Pythia.h"
    #include "Pythia8/Plugins.h"

    using namespace Pythia8;

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

    int main() {

      // Machine: 1 = Tevatron, 2 = LHC. Statistics.
      int machine = 1;
      int nEvent  = 10000;

      // Results as inline histograms or pyplot ones.
      bool usepyplot = true;

      // Select new PDF set; obsolete LHAPDF5 file name conventions.
      //string pdfSet = "LHAPDF5:cteq5l.LHgrid";
      //string pdfSet = "LHAPDF5:cteq61.LHpdf";
      //string pdfSet = "LHAPDF5:cteq61.LHgrid";
      //string pdfSet = "LHAPDF5:MRST2004nlo.LHgrid";
      //string pdfSet = "LHAPDF5:MRST2001lo.LHgrid";

      // Select new PDF set; LHAPDF6 file name conventions.
      // (Bad/unoptimized choice, to illustrate that the PDF matters.)
      string pdfSet = "LHAPDF6:PDF4LHC15_nlo_asvar";

      // Histograms for hadron-level quantities.
      double nMax = (machine == 1) ? 199.5 : 599.5;
      Hist nChargedOld("n_charged old PDF", 100, -0.5, nMax);
      Hist nChargedNew("n_charged new PDF", 100, -0.5, nMax);
      Hist nChargedRat("n_charged new/old PDF", 100, -0.5, nMax);
      Hist ySpecOld("y charged distribution old PDF", 100, -10., 10.);
      Hist ySpecNew("y charged distribution new PDF", 100, -10., 10.);
      Hist ySpecRat("y charged distribution new/old PDF", 100, -10., 10.);
      Hist pTSpecOld("pT charged distribution old PDF", 100, 0., 20.);
      Hist pTSpecNew("pT charged distribution new PDF", 100, 0., 20.);
      Hist pTSpecRat("pT charged distribution new/old PDF", 100, 0., 20.);
      Hist avgPTnChOld("<pT>(n_charged) old PDF", 100, -0.5, nMax);
      Hist avgPTnChNew("<pT>(n_charged) new PDF", 100, -0.5, nMax);
      Hist avgPTnChRat("<pT>(n_charged) new/old PDF", 100, -0.5, nMax);

      // Histograms for parton-level quantities.
      Hist xDistOld("MPI log(x) distribution old PDF", 80, -8., 0.);
      Hist xDistNew("MPI log(x) distribution new PDF", 80, -8., 0.);
      Hist xDistRat("MPI log(x) distribution new/old PDF", 80, -8., 0.);
      Hist pTDistOld("MPI pT (=Q) distribution old PDF", 100, 0., 20.);
      Hist pTDistNew("MPI pT (=Q) distribution new PDF", 100, 0., 20.);
      Hist pTDistRat("MPI pT (=Q) distribution new/old PDF", 100, 0., 20.);

      // PDF path.
      string pdfPath;

      // Loop over one default run and one with new PDF.
      for (int iRun = 0; iRun < 2; ++iRun) {

        // Get starting time in seconds.
        clock_t tBegin = clock();

        // Generator.
        Pythia pythia;
        Event& event = pythia.event;
        pdfPath = pythia.settings.word("xmlPath") + "../pdfdata";

        // Generate minimum-bias events, with or without double diffraction.
        pythia.readString("SoftQCD:nonDiffractive = on");
        //pythia.readString("SoftQCD:doubleDiffractive = on");

        // Alternatively generate QCD jet events, above some threshold.
        //pythia.readString("HardQCD:all = on");
        //pythia.readString("PhaseSpace:pTHatMin = 50.");

        // Reduce output.
        pythia.readString("Next:numberShowEvent = 0");

        // In second run pick new PDF set.
        if (iRun == 1) {
          pythia.readString("PDF:pSet = " + pdfSet);

          // Need to change at least pT0Ref depending on choice of PDF.
          // One possibility: retune to same <n_charged>.
          //pythia.readString("MultipartonInteractions:pT0Ref = 2.17");
        }

        // Allow extrapolation of PDF's beyond x and Q2 boundaries, at own risk.
        // Default behaviour is to freeze PDF's at boundaries.
        pythia.readString("PDF:extrapolate = on");

        // Tevatron/LHC initialization.
        double eCM =  (machine == 1) ? 1960. : 13000.;
        pythia.settings.parm("Beams:eCM", eCM);
        if (machine == 1) pythia.readString("Beams:idB = -2212");
        // If Pythia fails to initialize, exit with error.
        if (!pythia.init()) return 1;

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

          // Generate events.  Skip if error.
          if (!pythia.next()) continue;

          // Statistics on multiplicity and pT.
          int    nCh   = 0;
          double pTsum = 0.;
          for (int i = 0; i < event.size(); ++i)
          if (event[i].isFinal() && event[i].isCharged()) {
            ++nCh;
            pTsum += event[i].pT();

            // Fill histograms for charged y and pT spectra.
            if (iRun == 0) {
              ySpecOld.fill( event[i].y() );
              pTSpecOld.fill( event[i].pT() );
            } else {
              ySpecNew.fill( event[i].y() );
              pTSpecNew.fill( event[i].pT()  );
            }
          }

          // Fill histograms for summed quantities.
          if (iRun == 0) {
            nChargedOld.fill( nCh );
            avgPTnChOld.fill( nCh, pTsum / max(1, nCh) );
          } else {
            nChargedNew.fill( nCh );
            avgPTnChNew.fill( nCh, pTsum / max(1, nCh) );
          }

          // Loop through event record and fill x of all incoming partons.
          for (int i = 1; i < event.size(); ++i)
          if (event[i].status() == -21 || event[i].status() == -31) {
            double x = 2. * event[i].e() / eCM;
            if (iRun == 0) xDistOld.fill( log10(x) );
            else           xDistNew.fill( log10(x) );
          }

          // Loop through multiparton interactions list and fill pT of all MPI's.
          for (int i = 0; i < pythia.info.nMPI(); ++i) {
            double pT = pythia.info.pTMPI(i);
            if (iRun == 0) pTDistOld.fill( pT );
            else           pTDistNew.fill( pT );
          }

        // End of event loop.
        }

        // Statistics.
        pythia.readString("Stat:showPartonLevel = on");
        pythia.stat();

        // Get finishing time in seconds. Print used time.
        clock_t tEnd = clock();
        double tUsed = double(tEnd - tBegin) / double(CLOCKS_PER_SEC);
        cout << "\n This subrun took " << tUsed << " seconds \n" << endl;

      // End of loop over two runs.
      }

      // Form <pT>(n_charged) ratios.
      avgPTnChOld /= nChargedOld;
      avgPTnChNew /= nChargedNew;

      // Inline histogram printout.
      if (!usepyplot) {

        // Take ratios of new to old distributions.
        nChargedRat  = nChargedNew / nChargedOld;
        ySpecRat     = ySpecNew    / ySpecOld;
        pTSpecRat    = pTSpecNew    / pTSpecOld;
        avgPTnChRat  = avgPTnChNew / avgPTnChOld;
        xDistRat     = xDistNew    / xDistOld;
        pTDistRat    = pTDistNew   / pTDistOld;

        // Print histograms.
        cout << nChargedOld << nChargedNew << nChargedRat
             << ySpecOld    << ySpecNew    << ySpecRat
             << pTSpecOld   << pTSpecNew   << pTSpecRat
             << avgPTnChOld << avgPTnChNew << avgPTnChRat
             << xDistOld    << xDistNew    << xDistRat
             << pTDistOld   << pTDistNew   << pTDistRat;

      // Write Python code that can generate a PDF file with the distributions.
      } else {
        HistPlot hpl("plot202");
        hpl.frame( "fig202", "Charged multiplicity", "$n_{\\mathrm{charged}}$",
          "Rate");
        hpl.add( nChargedOld, "-,blue", "default");
        hpl.add( nChargedNew, "--,red", "PDF4LHC15_nlo");
        hpl.plot();
        hpl.frame( "", "Charged rapidity", "$y$", "Rate");
        hpl.add( ySpecOld, "-,blue", "default");
        hpl.add( ySpecNew, "--,red", "PDF4LHC15_nlo");
        hpl.plot();
        hpl.frame( "", "Charged transverse momentum", "$p_{\\perp}$", "Rate");
        hpl.add( pTSpecOld, "-,blue", "default");
        hpl.add( pTSpecNew, "--,red", "PDF4LHC15_nlo");
        hpl.plot( 0., 20., 0.1, 1e6,  true, false);
        hpl.frame( "", "Average charged transverse momentum",
          "$n_{\\mathrm{charged}}$", "$\\langle p_{\\perp} \\rangle$");
        hpl.add( avgPTnChOld, "-,blue", "default");
        hpl.add( avgPTnChNew, "--,red", "PDF4LHC15_nlo");
        hpl.plot();
      }

      // Done.
      return 0;
    }