## main234

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    // main234.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:
    //            Hadronization

    // This is a simple test program.
    // It illustrates how to feed in a single particle (including a resonance)
    // or a toy parton-level configurations.

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

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

    // Single-particle gun. The particle must be a colour singlet.
    // Input: flavour, energy, direction (theta, phi).
    // If theta < 0 then random choice over solid angle.
    // Optional final argument to put particle at rest => E = m.

    void fillParticle(int id, double ee, double thetaIn, double phiIn,
      Event& event, ParticleData& pdt, Rndm& rndm, bool atRest = false,
      bool hasLifetime = false) {

      // Reset event record to allow for new event.
      event.reset();

      // Select particle mass; where relevant according to Breit-Wigner.
      double mm = pdt.mSel(id);
      double pp = sqrtpos(ee*ee - mm*mm);

      // Special case when particle is supposed to be at rest.
      if (atRest) {
        ee = mm;
        pp = 0.;
      }

      // Angles as input or uniform in solid angle.
      double cThe, sThe, phi;
      if (thetaIn >= 0.) {
        cThe = cos(thetaIn);
        sThe = sin(thetaIn);
        phi  = phiIn;
      } else {
        cThe = 2. * rndm.flat() - 1.;
        sThe = sqrtpos(1. - cThe * cThe);
        phi = 2. * M_PI * rndm.flat();
      }

      // Store the particle in the event record.
      int iNew = event.append( id, 1, 0, 0, pp * sThe * cos(phi),
        pp * sThe * sin(phi), pp * cThe, ee, mm);

      // Generate lifetime, to give decay away from primary vertex.
      if (hasLifetime) event[iNew].tau( event[iNew].tau0() * rndm.exp() );

    }

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

    // Simple method to do the filling of partons into the event record.

    void fillPartons(int type, double ee, Event& event, ParticleData& pdt,
      Rndm& rndm) {

      // Reset event record to allow for new event.
      event.reset();

      // Information on a q qbar system, to be hadronized.
      if (type == 1 || type == 12) {
        int    id = 2;
        double mm = pdt.m0(id);
        double pp = sqrtpos(ee*ee - mm*mm);
        event.append(  id, 23, 101,   0, 0., 0.,  pp, ee, mm);
        event.append( -id, 23,   0, 101, 0., 0., -pp, ee, mm);

      // Information on a g g system, to be hadronized.
      } else if (type == 2 || type == 13) {
        event.append( 21, 23, 101, 102, 0., 0.,  ee, ee);
        event.append( 21, 23, 102, 101, 0., 0., -ee, ee);

      // Information on a g g g system, to be hadronized.
      } else if (type == 3) {
        event.append( 21, 23, 101, 102,        0., 0.,        ee, ee);
        event.append( 21, 23, 102, 103,  0.8 * ee, 0., -0.6 * ee, ee);
        event.append( 21, 23, 103, 101, -0.8 * ee, 0., -0.6 * ee, ee);

      // Information on a q q q junction system, to be hadronized.
      } else if (type == 4 || type == 5) {

        // Need a colour singlet mother parton to define junction origin.
        event.append( 1000022, -21, 0, 0, 2, 4, 0, 0,
                      0., 0., 1.01 * ee, 1.01 * ee);

        // The three endpoint q q q; the minimal system.
        double rt75 = sqrt(0.75);
        event.append( 2, 23, 1, 0, 0, 0, 101, 0,
                              0., 0., 1.01 * ee, 1.01 * ee);
        event.append( 2, 23, 1, 0, 0, 0, 102, 0,
                       rt75 * ee, 0., -0.5 * ee,        ee );
        event.append( 1, 23, 1, 0, 0, 0, 103, 0,
                      -rt75 * ee, 0., -0.5 * ee,        ee );

        // Define the qqq configuration as starting point for adding gluons.
        if (type == 5) {
          int colNow[4] = {0, 101, 102, 103};
          Vec4 pQ[4];
          pQ[1] = Vec4(0., 0., 1., 0.);
          pQ[2] = Vec4( rt75, 0., -0.5, 0.);
          pQ[3] = Vec4(-rt75, 0., -0.5, 0.);

          // Minimal cos(q-g opening angle), allows more or less nasty events.
          double cosThetaMin =0.;

          // Add a few gluons (almost) at random.
          for (int nglu = 0; nglu < 5; ++nglu) {
            int iq = 1 + int( 2.99999 * rndm.flat() );
            double px, py, pz, e, prod;
            do {
              e =  ee * rndm.flat();
              double cThe = 2. * rndm.flat() - 1.;
              double phi = 2. * M_PI * rndm.flat();
              px = e * sqrt(1. - cThe*cThe) * cos(phi);
              py = e * sqrt(1. - cThe*cThe) * sin(phi);
              pz = e * cThe;
              prod = ( px * pQ[iq].px() + py * pQ[iq].py() + pz * pQ[iq].pz() )
                / e;
            } while (prod < cosThetaMin);
            int colNew = 104 + nglu;
            event.append( 21, 23, 1, 0, 0, 0, colNew, colNow[iq],
              px, py, pz, e, 0.);
            colNow[iq] = colNew;
          }
          // Update daughter range of mother.
          event[1].daughters(2, event.size() - 1);

        }

      // Information on a q q qbar qbar dijunction system, to be hadronized.
      } else if (type >= 6) {

        // The two fictitious beam remnant particles; needed for junctions.
        event.append( 2212, -12, 0, 0, 3, 5, 0, 0, 0., 0., ee, ee, 0.);
        event.append(-2212, -12, 0, 0, 6, 8, 0, 0, 0., 0., ee, ee, 0.);

        // Opening angle between "diquark" legs.
        double theta = 0.2;
        double cThe = cos(theta);
        double sThe = sin(theta);

        // Set one colour depending on whether more gluons or not.
        int acol = (type == 6) ? 103 : 106;

        // The four endpoint q q qbar qbar; the minimal system.
        // Two additional fictitious partons to make up original beams.
        event.append(  2,   23, 1, 0, 0, 0, 101, 0,
                      ee * sThe, 0.,  ee * cThe, ee, 0.);
        event.append(  1,   23, 1, 0, 0, 0, 102, 0,
                     -ee * sThe, 0.,  ee * cThe, ee, 0.);
        event.append(  2, -21, 1, 0, 0, 0, 103, 0,
                             0., 0.,  ee       , ee, 0.);
        event.append( -2,   23, 2, 0, 0, 0, 0, 104,
                      ee * sThe, 0., -ee * cThe, ee, 0.);
        event.append( -1,   23, 2, 0, 0, 0, 0, 105,
                     -ee * sThe, 0., -ee * cThe, ee, 0.);
        event.append( -2, -21, 2, 0, 0, 0, 0, acol,
                             0., 0., -ee       , ee, 0.);

        // Add extra gluons on string between junctions.
        if (type == 7) {
          event.append( 21, 23, 8, 5, 0, 0, 103, 106, 0., ee, 0., ee, 0.);
        } else if (type == 8) {
          event.append( 21, 23, 8, 5, 0, 0, 103, 108, 0., ee, 0., ee, 0.);
          event.append( 21, 23, 8, 5, 0, 0, 108, 106, 0.,-ee, 0., ee, 0.);
        } else if (type == 9) {
          event.append( 21, 23, 8, 5, 0, 0, 103, 107, 0., ee, 0., ee, 0.);
          event.append( 21, 23, 8, 5, 0, 0, 107, 108, ee, 0., 0., ee, 0.);
          event.append( 21, 23, 8, 5, 0, 0, 108, 106, 0.,-ee, 0., ee, 0.);
        } else if (type == 10) {
          event.append( 21, 23, 8, 5, 0, 0, 103, 107, 0., ee, 0., ee, 0.);
          event.append( 21, 23, 8, 5, 0, 0, 107, 108, ee, 0., 0., ee, 0.);
          event.append( 21, 23, 8, 5, 0, 0, 108, 109, 0.,-ee, 0., ee, 0.);
          event.append( 21, 23, 8, 5, 0, 0, 109, 106,-ee, 0., 0., ee, 0.);
        }

      // No more cases: done.
      }
    }

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

    int main() {

      // Loop over kind of events to generate:
      // 0 = single-particle gun.
      // 1 = q qbar.
      // 2 = g g.
      // 3 = g g g.
      // 4 = minimal q q q junction topology.
      // 5 = q q q junction topology with gluons on the strings.
      // 6 = q q qbar qbar dijunction topology, no gluons.
      // 7 - 10 = ditto, but with 1 - 4 gluons on string between junctions.
      // 11 = single-resonance gun.
      // 12 = q qbar plus parton shower.
      // 13 = g g plus parton shower.
      // It is easy to edit the line below to only study one of them.
      for (int type = 0; type < 14; ++type) {

        // Set particle species and energy for single-particle gun.
        int    idGun  = (type == 0) ? 15 : 25;
        double eeGun  = (type == 0) ? 20. : 125.;
        bool   atRest = (type == 0) ? false : true;

        // The single-particle gun produces a particle at the origin, and
        // by default decays it there. When hasLifetime = true instead a
        // finite lifetime is selected and used to generate a displaced
        // decay vertex.
        bool   hasLifetime = (type == 0) ? true : false;

        // Set typical energy per parton.
        double ee = 20.0;

        // Set number of events to generate and to list.
        int nEvent = 10000;
        int nList = 1;

        // Generator; shorthand for event and particleData.
        Pythia pythia;
        Event& event      = pythia.event;
        ParticleData& pdt = pythia.particleData;

        // Key requirement: switch off ProcessLevel, and thereby also PartonLevel.
        pythia.readString("ProcessLevel:all = off");

        // Avoid the standard scrutiny of mother/daughter relations.
        // But note that other event checks are done below.
        pythia.readString("Check:event = off");

        // Optionally switch off resonance decays.
        //pythia.readString("ProcessLevel:resonanceDecays = off");

        // For type = 12 and 13 showers are done by forceTimeShower,
        // so should not be done a second time.
        if (type == 12 || type == 13)
          pythia.readString("PartonLevel:FSRinResonances = off");

        // Optionally switch off ordinary decays.
        //pythia.readString("HadronLevel:Decay = off");

        // Switch off automatic event listing in favour of manual.
        pythia.readString("Next:numberShowInfo = 0");
        pythia.readString("Next:numberShowProcess = 0");
        pythia.readString("Next:numberShowEvent = 0");

        // Set true to also see space-time information in event listings.
        bool showScaleAndVertex = (type == 0) ? true : false;

        // Initialize.
        cout << "\n Now begin type = " << type << endl;
        // If Pythia fails to initialize, exit with error.
        if (!pythia.init()) return 1;

        // Book histograms.
        Hist epCons("deviation from energy-momentum conservation",100,0.,1e-4);
        Hist chgCons("deviation from charge conservation",57,-9.5,9.5);
        Hist nFinal("final particle multiplicity",100,-0.5,99.5);
        Hist dnparticledp("dn/dp for particles",100,0.,ee);
        Hist status85("multiplicity status code 85",50,-0.5,49.5);
        Hist status86("multiplicity status code 86",50,-0.5,49.5);
        Hist status83("multiplicity status code 83",50,-0.5,49.5);
        Hist status84("multiplicity status code 84",50,-0.5,49.5);
        Hist dndtheta("particle flow in event plane",100,-M_PI,M_PI);
        Hist dedtheta("energy flow in event plane",100,-M_PI,M_PI);
        Hist dpartondtheta("parton flow in event plane",100,-M_PI,M_PI);
        Hist dndyAnti("dn/dy primaries antijunction",100, -10., 10.);
        Hist dndyJun("dn/dy primaries junction",100, -10., 10.);
        Hist dndySum("dn/dy all primaries",100, -10., 10.);

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

          // Set up single particle, with random direction in solid angle.
          if (type == 0 || type == 11) fillParticle( idGun, eeGun, -1., 0.,
            event, pdt, pythia.rndm, atRest, hasLifetime);

          // Set up parton-level configuration.
          else fillPartons( type, ee, event, pdt, pythia.rndm);

          // To have partons shower they must be set maximum allowed scale.
          // (Can be set individually to restrict radiation differently.)
          if (type == 12 || type == 13) {
            double scale = ee;
            event[1].scale( scale);
            event[2].scale( scale);

            // Now actually do the shower, for range of partons, and max scale.
            // (Most restrictive of global and individual applied to each parton.)
            pythia.forceTimeShower( 1, 2, scale);
          }

          // Generate events. Quit if failure.
          if (!pythia.next()) {
            cout << " Event generation aborted prematurely, owing to error!\n";
            break;
          }

          // List first few events.
          if (iEvent < nList) {
            event.list(showScaleAndVertex);
            // Also list junctions.
            event.listJunctions();
          }

          // Initialize statistics.
          Vec4 pSum = - event[0].p();
          double chargeSum = 0.;
          if (type == 0) chargeSum = -event[1].charge();
          if (type == 4 || type == 5) chargeSum = -1;
          int nFin = 0;
          int n85 = 0;
          int n86 = 0;
          int n83 = 0;
          int n84 = 0;

          // Loop over all particles.
          for (int i = 0; i < event.size(); ++i) {
            int status = event[i].statusAbs();

            // Find any unrecognized particle codes.
            int id = event[i].id();
            if (id == 0 || !pdt.isParticle(id))
              cout << " Error! Unknown code id = " << id << "\n";

            // Find particles with E-p mismatch.
            double eCalc = event[i].eCalc();
            if (abs(eCalc/event[i].e() - 1.) > 1e-6) cout << " e mismatch, i = "
              << i << " e_nominal = " << event[i].e() << " e-from-p = "
              << eCalc << " m-from-e " << event[i].mCalc() << "\n";

            // Parton flow in event plane.
            if (status == 71 || status == 72) {
              double thetaXZ = event[i].thetaXZ();
              dpartondtheta.fill(thetaXZ);
            }

            // Origin of primary hadrons.
            if (status == 85) ++n85;
            if (status == 86) ++n86;
            if (status == 83) ++n83;
            if (status == 84) ++n84;

            // Flow of primary hadrons in the event plane.
            if (status > 80 && status < 90) {
              double eAbs = event[i].e();
              if (eAbs < 0.) {cout << " e < 0 line " << i; event.list();}
              double thetaXZ = event[i].thetaXZ();
              dndtheta.fill(thetaXZ);
              dedtheta.fill(thetaXZ, eAbs);

              // Rapidity distribution of primary hadrons.
              double y = event[i].y();
              dndySum.fill(y);
              if (type >= 6) {
                int motherId = event[event[i].mother1()].id();
                if (motherId > 0 ) dndyJun.fill(event[i].y());
                else dndyAnti.fill(event[i].y());
              }
            }

            // Study final-state particles.
            if (event[i].isFinal()) {
              pSum += event[i].p();
              chargeSum += event[i].charge();
              nFin++;
              double pAbs = event[i].pAbs();
              dnparticledp.fill(pAbs);
            }
          }

          // Fill histograms once for each event.
          double epDev = abs(pSum.e()) + abs(pSum.px()) + abs(pSum.py())
            + abs(pSum.pz());
          epCons.fill(epDev);
          chgCons.fill(chargeSum);
          nFinal.fill(nFin);
          status85.fill(n85);
          status86.fill(n86);
          status83.fill(n83);
          status84.fill(n84);
          if (epDev > 1e-3  || abs(chargeSum) > 0.1) event.list();

        // End of event loop.
        }

        // Print statistics and histograms.
        pythia.stat();
        cout << epCons << chgCons << nFinal << dnparticledp
             << dndtheta << dedtheta << dpartondtheta << dndySum;
        if (type >= 4) cout << status85 << status86 << status83
             << status84;
        if (type >= 6) cout << dndyJun << dndyAnti;

      // End of type loop and done.
      }
      return 0;
    }