## main212

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    // main212.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:
    //            Fastjet
    //            Jet finding

    // Simple example of fastjet analysis. Roughly follows analysis of:
    // T. Aaltonen et al. [CDF Collaboration],
    // Measurement of the cross section for W-boson production in association
    // with jets in ppbar collisions at sqrt(s)=1.96$ TeV
    // Phys. Rev. D 77 (2008) 011108
    // arXiv:0711.4044 [hep-ex]
    //
    // Cuts:
    //   ET(elec)     > 20GeV
    //   |eta(elec)|  < 1.1
    //   ET(missing)  > 30GeV
    //   ET(jet)      > 20GeV
    //   |eta(jet)|   < 2.0
    //   deltaR(elec, jet) > 0.52
    // Not used:
    //   mT(W)        > 20GeV
    //

    #include "Pythia8/Pythia.h"

    // This is the minimal interface needed to access FastJet directly.
    // A more sophisticated interface is demonstrated in main213.cc.
    #include "fastjet/PseudoJet.hh"
    #include "fastjet/ClusterSequence.hh"

    using namespace Pythia8;

    // Experimental cross section
    // sigma(W -> ev + >= n-jet; ET(n'th-jet) > 25GeV), n = 0, 1, 2, 3, 4
    const double expCrossSec[] = { 798.0, 53.5, 6.8, 0.84, 0.074 };

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

    int main() {

      // Settings.
      int  nEvent = 10000;
      bool doMPI  = true;

      // Generator.
      Pythia pythia;

      // Single W production.
      pythia.readString("WeakSingleBoson:ffbar2W = on");
      // Force decay W->ev.
      pythia.readString("24:onMode = off");
      pythia.readString("24:onIfAny = 11 12");
      // Multiparton Interactions.
      if (doMPI == false) pythia.readString("PartonLevel:MPI = off");

      // Initialisation, p pbar @ 1.96 TeV.
      pythia.readString("Beams:idB = -2212");
      pythia.readString("Beams:eCM = 1960.");

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

      // Histograms.
      Hist dSigma1("1-jet cross-section (E_jet1 > 20 GeV)", 70, 0.0, 350.0);
      Hist dSigma2("2-jet cross-section (E_jet2 > 20 GeV)", 38, 0.0, 190.0);
      Hist dSigma3("3-jet cross-section (E_jet3 > 20 GeV)", 16, 0.0, 80.0);
      Hist dSigma4("4-jet cross-section (E_jet4 > 20 GeV)",  7, 0.0, 35.0);
      Hist *dSigmaHist[5] = { NULL, &dSigma1, &dSigma2, &dSigma3, &dSigma4 };
      double dSigmaBin[5] = { 0.0, 350.0 / 70.0, 190.0 / 38.0,
                              80.0 / 16.0, 35.0 / 7.0 };

      // Fastjet analysis - select algorithm and parameters.
      double Rparam = 0.4;
      fastjet::Strategy               strategy = fastjet::Best;
      fastjet::RecombinationScheme    recombScheme = fastjet::E_scheme;
      fastjet::JetDefinition         *jetDef = NULL;
      jetDef = new fastjet::JetDefinition(fastjet::kt_algorithm, Rparam,
                                          recombScheme, strategy);

      // Fastjet input.
      std::vector <fastjet::PseudoJet> fjInputs;

      // Statistics for later.
      int nEventAccept25[5] = { 0, 0, 0, 0, 0 };
      int vetoCount[4] = { 0, 0, 0, 0 };
      const char *vetoStr[] = { "ET(elec)", "|eta(elec)|",
                                "ET(missing)", "deltaR(elec, jet)" };
      bool firstEvent = true;

      // Begin event loop. Generate event. Skip if error.
      for (int iEvent = 0; iEvent < nEvent; ++iEvent) {
        if (!pythia.next()) continue;

        // Need to find the electron from the W decay - cheat a bit here
        // and find it from the W in the event record.
        int idxW = -1;
        for (int i = pythia.event.size() - 1; i > 0; i--) {
          if (pythia.event[i].idAbs() == 24) {
            idxW = i;
            break;
          }
        }
        if (idxW == -1) {
          cout << "Error: Could not find W" << endl;
          continue;
        }

        // Find the electron from the W decay.
        int idxElec = idxW;
        while(true) {
          int daughter = pythia.event[idxElec].daughter1();
          if   (daughter == 0) break;
          else                 idxElec = daughter;
        }
        if (pythia.event[idxElec].idAbs() != 11 ||
           !pythia.event[idxElec].isFinal()) {
          cout << "Error: Found incorrect decay product of the W" << endl;
          continue;
        }

        // Electron cuts.
        if (pythia.event[idxElec].pT() < 20.0) {
          vetoCount[0]++;
          continue;
        }
        if (abs(pythia.event[idxElec].eta()) > 1.1) {
          vetoCount[1]++;
          continue;
        }

        // Reset Fastjet input.
        fjInputs.resize(0);

        // Keep track of missing ET.
        Vec4 missingETvec;

        // Loop over event record to decide what to pass to FastJet.
        for (int i = 0; i < pythia.event.size(); ++i) {
          // Final state only.
          if (!pythia.event[i].isFinal())        continue;

          // No neutrinos.
          if (pythia.event[i].idAbs() == 12 || pythia.event[i].idAbs() == 14 ||
              pythia.event[i].idAbs() == 16)     continue;

          // Only |eta| < 3.6.
          if (abs(pythia.event[i].eta()) > 3.6) continue;

          // Missing ET.
          missingETvec += pythia.event[i].p();

          // Do not include the electron from the W decay.
          if (i == idxElec)                      continue;

          // Store as input to Fastjet.
          fjInputs.push_back( fastjet::PseudoJet( pythia.event[i].px(),
            pythia.event[i].py(), pythia.event[i].pz(), pythia.event[i].e() ) );
        }
        if (fjInputs.size() == 0) {
          cout << "Error: event with no final state particles" << endl;
          continue;
        }

        // Run Fastjet algorithm.
        vector <fastjet::PseudoJet> inclusiveJets, sortedJets;
        fastjet::ClusterSequence clustSeq(fjInputs, *jetDef);

        // For the first event, print the FastJet details.
        if (firstEvent) {
          cout << "Ran " << jetDef->description() << endl;
          cout << "Strategy adopted by FastJet was "
               << clustSeq.strategy_string() << endl << endl;
          firstEvent = false;
        }

        // Extract inclusive jets sorted by pT (note minimum pT of 20.0 GeV).
        inclusiveJets = clustSeq.inclusive_jets(20.0);
        sortedJets    = sorted_by_pt(inclusiveJets);

        // Missing ET cut.
        double missingET = missingETvec.pT();
        if (missingET < 30.0) {
          vetoCount[2]++;
          continue;
        }

        // Keep track of jets with pT > 20/25 GeV.
        int  jetCount20 = 0, jetCount25 = 0;
        // For the deltaR calculation below.
        bool vetoEvent = false;
        fastjet::PseudoJet fjElec(pythia.event[idxElec].px(),
                                  pythia.event[idxElec].py(),
                                  pythia.event[idxElec].pz(),
                                  pythia.event[idxElec].e());

        for (unsigned int i = 0; i < sortedJets.size(); i++) {
          // Only count jets that have |eta| < 2.0.
          if (abs(sortedJets[i].rap()) > 2.0) continue;
          // Check distance between W decay electron and jets.
          if (fjElec.squared_distance(sortedJets[i]) < 0.52 * 0.52)
            { vetoEvent = true; break; }

          // Fill dSigma histograms and count jets with ET > 25.0.
          if (sortedJets[i].perp() > 25.0)
            jetCount25++;
          if (jetCount20 <= 3)
            dSigmaHist[++jetCount20]->fill(sortedJets[i].perp());
        }
        if (vetoEvent) { vetoCount[3]++; continue; }

        if (jetCount25 > 4) jetCount25 = 4;
        for (int i = jetCount25; i >= 0; i--)
          nEventAccept25[i]++;

      // End of event loop.
      }

      // Statistics.
      pythia.stat();

      // Output histograms.
      double sigmapb = pythia.info.sigmaGen() * 1.0E9;
      for (int i = 1; i <= 4; i++)
        (*dSigmaHist[i]) = ((*dSigmaHist[i]) * sigmapb) / nEvent / dSigmaBin[i];
      cout << dSigma1 << dSigma2 << dSigma3 << dSigma4 << endl;

      // Output cross-sections.
      cout << "Jet algorithm is kT" << endl;
      cout << "Multiparton interactions are switched "
           << ( (doMPI) ? "on" : "off" ) << endl;
      cout << endl << nEvent << " events generated. " << nEventAccept25[0]
           << " events passed cuts." << endl;
      cout << "Vetos:" << endl;
      for (int i = 0; i < 4; i++)
        cout << "  " << vetoStr[i] << " = " << vetoCount[i] << endl;
      cout << endl << "Inclusive cross-sections (pb):" << endl;
      for (int i = 0; i < 5; i++) {
        cout << scientific << setprecision(3)
             << "  " << i << "-jet - Pythia = "
             << ((double) nEventAccept25[i] / (double) nEvent) * sigmapb;
        cout << ", Experimental = " << expCrossSec[i];
        if (i != 0) {
          cout << scientific << setprecision(3)
               << ", Pythia ratio to " << i - 1 << "-jet = "
               << nEventAccept25[i - 1]
            ? ((double) nEventAccept25[i] / (double) nEventAccept25[i - 1])
            : numeric_limits<double>::quiet_NaN();
          cout << scientific << setprecision(3)
               << ", Experimental ratio to " << i - 1 << "-jet = "
               << expCrossSec[i - 1]
            ? expCrossSec[i] / expCrossSec[i - 1]
            : numeric_limits<double>::quiet_NaN();
        }
        cout << endl;
      }

      // Done.
      delete jetDef;
      return 0;
    }