main31.cc

// main31.cc is a part of the PYTHIA event generator.
// Copyright (C) 2014 Richard Corke, Torbjorn Sjostrand.
// PYTHIA is licenced under the GNU GPL version 2, see COPYING for details.
// Please respect the MCnet Guidelines, see GUIDELINES for details.

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

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

// Use userhooks to veto PYTHIA emissions above the POWHEG scale.

class PowhegHooks : public UserHooks {

public:

  // Constructor and destructor.
   PowhegHooks(int nFinalIn, int vetoModeIn, int vetoCountIn,
               int pThardModeIn, int pTemtModeIn, int emittedModeIn,
               int pTdefModeIn, int MPIvetoModeIn) : nFinal(nFinalIn),
               vetoMode(vetoModeIn), vetoCount(vetoCountIn),
               pThardMode(pThardModeIn), pTemtMode(pTemtModeIn),
               emittedMode(emittedModeIn), pTdefMode(pTdefModeIn),
               MPIvetoMode(MPIvetoModeIn) {};
  ~PowhegHooks() {}

//--------------------------------------------------------------------------

  // Routines to calculate the pT (according to pTdefMode) in a splitting:
  //   ISR: i (radiator after)  -> j (emitted after) k (radiator before)
  //   FSR: i (radiator before) -> j (emitted after) k (radiator after)
  // For the Pythia pT definition, a recoiler (after) must be specified.

  // Compute the Pythia pT separation. Based on pTLund function in History.cc
  double pTpythia(const Event &e, int RadAfterBranch, int EmtAfterBranch,
                  int RecAfterBranch, bool FSR) {

    // Convenient shorthands for later
    Vec4 radVec = e[RadAfterBranch].p();
    Vec4 emtVec = e[EmtAfterBranch].p();
    Vec4 recVec = e[RecAfterBranch].p();
    int  radID  = e[RadAfterBranch].id();

    // Calculate virtuality of splitting
    double sign = (FSR) ? 1. : -1.;
    Vec4 Q(radVec + sign * emtVec);
    double Qsq = sign * Q.m2Calc();

    // Mass term of radiator
    double m2Rad = (abs(radID) >= 4 && abs(radID) < 7) ?
                   pow2(particleDataPtr->m0(radID)) : 0.;

    // z values for FSR and ISR
    double z, pTnow;
    if (FSR) {
      // Construct 2 -> 3 variables
      Vec4 sum = radVec + recVec + emtVec;
      double m2Dip = sum.m2Calc();
      double x1 = 2. * (sum * radVec) / m2Dip;
      double x3 = 2. * (sum * emtVec) / m2Dip;
      z     = x1 / (x1 + x3);
      pTnow = z * (1. - z);

    } else {
      // Construct dipoles before/after splitting
      Vec4 qBR(radVec - emtVec + recVec);
      Vec4 qAR(radVec + recVec);
      z     = qBR.m2Calc() / qAR.m2Calc();
      pTnow = (1. - z);
    }

    // Virtuality with correct sign
    pTnow *= (Qsq - sign * m2Rad);

    // Can get negative pT for massive splittings
    if (pTnow < 0.) {
      cout << "Warning: pTpythia was negative" << endl;
      return -1.;
    }

#ifdef DBGOUTPUT
    cout << "pTpythia: rad = " << RadAfterBranch << ", emt = "
         << EmtAfterBranch << ", rec = " << RecAfterBranch
         << ", pTnow = " << sqrt(pTnow) << endl;
#endif

    // Return pT
    return sqrt(pTnow);
  }

  // Compute the POWHEG pT separation between i and j
  double pTpowheg(const Event &e, int i, int j, bool FSR) {

    // pT value for FSR and ISR
    double pTnow = 0.;
    if (FSR) {
      // POWHEG d_ij (in CM frame). Note that the incoming beams have not
      // been updated in the parton systems pointer yet (i.e. prior to any
      // potential recoil).
      int iInA = partonSystemsPtr->getInA(0);
      int iInB = partonSystemsPtr->getInB(0);
      double betaZ = - ( e[iInA].pz() + e[iInB].pz() ) /
                       ( e[iInA].e()  + e[iInB].e()  );
      Vec4 iVecBst(e[i].p()), jVecBst(e[j].p());
      iVecBst.bst(0., 0., betaZ);
      jVecBst.bst(0., 0., betaZ);
      pTnow = sqrt( (iVecBst + jVecBst).m2Calc() *
                    iVecBst.e() * jVecBst.e() /
                    pow2(iVecBst.e() + jVecBst.e()) );

    } else {
      // POWHEG pT_ISR is just kinematic pT
      pTnow = e[j].pT();
    }

    // Check result
    if (pTnow < 0.) {
      cout << "Warning: pTpowheg was negative" << endl;
      return -1.;
    }

#ifdef DBGOUTPUT
    cout << "pTpowheg: i = " << i << ", j = " << j
         << ", pTnow = " << pTnow << endl;
#endif

    return pTnow;
  }

  // Calculate pT for a splitting based on pTdefMode.
  // If j is -1, all final-state partons are tried.
  // If i, k, r and xSR are -1, then all incoming and outgoing
  // partons are tried.
  // xSR set to 0 means ISR, while xSR set to 1 means FSR
  double pTcalc(const Event &e, int i, int j, int k, int r, int xSRin) {

    // Loop over ISR and FSR if necessary
    double pTemt = -1., pTnow;
    int xSR1 = (xSRin == -1) ? 0 : xSRin;
    int xSR2 = (xSRin == -1) ? 2 : xSRin + 1;
    for (int xSR = xSR1; xSR < xSR2; xSR++) {
      // FSR flag
      bool FSR = (xSR == 0) ? false : true;

      // If all necessary arguments have been given, then directly calculate.
      // POWHEG ISR and FSR, need i and j.
      if ((pTdefMode == 0 || pTdefMode == 1) && i > 0 && j > 0) {
        pTemt = pTpowheg(e, i, j, (pTdefMode == 0) ? false : FSR);

      // Pythia ISR, need i, j and r.
      } else if (!FSR && pTdefMode == 2 && i > 0 && j > 0 && r > 0) {
        pTemt = pTpythia(e, i, j, r, FSR);

      // Pythia FSR, need k, j and r.
      } else if (FSR && pTdefMode == 2 && j > 0 && k > 0 && r > 0) {
        pTemt = pTpythia(e, k, j, r, FSR);

      // Otherwise need to try all possible combintations.
      } else {
        // Start by finding incoming legs to the hard system after
        // branching (radiator after branching, i for ISR).
        // Use partonSystemsPtr to find incoming just prior to the
        // branching and track mothers.
        int iInA = partonSystemsPtr->getInA(0);
        int iInB = partonSystemsPtr->getInB(0);
        while (e[iInA].mother1() != 1) { iInA = e[iInA].mother1(); }
        while (e[iInB].mother1() != 2) { iInB = e[iInB].mother1(); }

        // If we do not have j, then try all final-state partons
        int jNow = (j > 0) ? j : 0;
        int jMax = (j > 0) ? j + 1 : e.size();
        for (; jNow < jMax; jNow++) {

          // Final-state and coloured jNow only
          if (!e[jNow].isFinal() || e[jNow].colType() == 0) continue;

          // POWHEG
          if (pTdefMode == 0 || pTdefMode == 1) {

            // ISR - only done once as just kinematical pT
            if (!FSR) {
              pTnow = pTpowheg(e, iInA, jNow, (pTdefMode == 0) ? false : FSR);
              if (pTnow > 0.) pTemt = (pTemt < 0) ? pTnow : min(pTemt, pTnow);
  
              // FSR - try all outgoing partons from system before branching
              // as i. Note that for the hard system, there is no
              // "before branching" information.
              } else {
    
                int outSize = partonSystemsPtr->sizeOut(0);
                for (int iMem = 0; iMem < outSize; iMem++) {
                  int iNow = partonSystemsPtr->getOut(0, iMem);

                  // Coloured only, i != jNow and no carbon copies
                  if (iNow == jNow || e[iNow].colType() == 0) continue;
                  if (jNow == e[iNow].daughter1()
                    && jNow == e[iNow].daughter2()) continue;

                  pTnow = pTpowheg(e, iNow, jNow, (pTdefMode == 0)
                    ? false : FSR);
                  if (pTnow > 0.) pTemt = (pTemt < 0)
                    ? pTnow : min(pTemt, pTnow);
                } // for (iMem)
  
              } // if (!FSR)
  
          // Pythia
          } else if (pTdefMode == 2) {
  
            // ISR - other incoming as recoiler
            if (!FSR) {
              pTnow = pTpythia(e, iInA, jNow, iInB, FSR);
              if (pTnow > 0.) pTemt = (pTemt < 0) ? pTnow : min(pTemt, pTnow);
              pTnow = pTpythia(e, iInB, jNow, iInA, FSR);
              if (pTnow > 0.) pTemt = (pTemt < 0) ? pTnow : min(pTemt, pTnow);
  
            // FSR - try all final-state coloured partons as radiator
            //       after emission (k).
            } else {
              for (int kNow = 0; kNow < e.size(); kNow++) {
                if (kNow == jNow || !e[kNow].isFinal() ||
                    e[kNow].colType() == 0) continue;
  
                // For this kNow, need to have a recoiler.
                // Try two incoming.
                pTnow = pTpythia(e, kNow, jNow, iInA, FSR);
                if (pTnow > 0.) pTemt = (pTemt < 0)
                  ? pTnow : min(pTemt, pTnow);
                pTnow = pTpythia(e, kNow, jNow, iInB, FSR);
                if (pTnow > 0.) pTemt = (pTemt < 0)
                  ? pTnow : min(pTemt, pTnow);

                // Try all other outgoing.
                for (int rNow = 0; rNow < e.size(); rNow++) {
                  if (rNow == kNow || rNow == jNow ||
                      !e[rNow].isFinal() || e[rNow].colType() == 0) continue;
                  pTnow = pTpythia(e, kNow, jNow, rNow, FSR);
                  if (pTnow > 0.) pTemt = (pTemt < 0)
                    ? pTnow : min(pTemt, pTnow);
                } // for (rNow)
  
              } // for (kNow)
            } // if (!FSR)
          } // if (pTdefMode)
        } // for (j)
      }
    } // for (xSR)

#ifdef DBGOUTPUT
    cout << "pTcalc: i = " << i << ", j = " << j << ", k = " << k
         << ", r = " << r << ", xSR = " << xSRin
         << ", pTemt = " << pTemt << endl;
#endif

    return pTemt;
  }

//--------------------------------------------------------------------------

  // Extraction of pThard based on the incoming event.
  // Assume that all the final-state particles are in a continuous block
  // at the end of the event and the final entry is the POWHEG emission.
  // If there is no POWHEG emission, then pThard is set to SCALUP.

  bool canVetoMPIStep()    { return true; }
  int  numberVetoMPIStep() { return 1; }
  bool doVetoMPIStep(int nMPI, const Event &e) {
    // Extra check on nMPI
    if (nMPI > 1) return false;

    // Find if there is a POWHEG emission. Go backwards through the
    // event record until there is a non-final particle. Also sum pT and
    // find pT_1 for possible MPI vetoing
    int    count = 0;
    double pT1 = 0., pTsum = 0.;
    for (int i = e.size() - 1; i > 0; i--) {
      if (e[i].isFinal()) {
        count++;
        pT1    = e[i].pT();
        pTsum += e[i].pT();
      } else break;
    }
    // Extra check that we have the correct final state
    if (count != nFinal && count != nFinal + 1) {
      cout << "Error: wrong number of final state particles in event" << endl;
      exit(1);
    }
    // Flag if POWHEG radiation present and index
    bool isEmt = (count == nFinal) ? false : true;
    int  iEmt  = (isEmt) ? e.size() - 1 : -1;

    // If there is no radiation or if pThardMode is 0 then set pThard = SCALUP.
    if (!isEmt || pThardMode == 0) {
      pThard = infoPtr->scalup();
      
    // If pThardMode is 1 then the pT of the POWHEG emission is checked against
    // all other incoming and outgoing partons, with the minimal value taken
    } else if (pThardMode == 1) {
      pThard = pTcalc(e, -1, iEmt, -1, -1, -1);

    // If pThardMode is 2, then the pT of all final-state partons is checked
    // against all other incoming and outgoing partons, with the minimal value
    // taken
    } else if (pThardMode == 2) {
      pThard = pTcalc(e, -1, -1, -1, -1, -1);
    }

    // Find MPI veto pT if necessary
    if (MPIvetoMode == 1) {
      pTMPI = (isEmt) ? pTsum / 2. : pT1;
    }

#ifdef DBGOUTPUT
    cout << "doVetoMPIStep: Qfac = " << infoPtr->scalup()
         << ", pThard = " << pThard << endl << endl;
#endif

    // Initialise other variables
    accepted   = false;
    nAcceptSeq = nISRveto = nFSRveto = 0;

    // Do not veto the event
    return false;
  }

//--------------------------------------------------------------------------

  // ISR veto

  bool canVetoISREmission() { return (vetoMode == 0) ? false : true; }
  bool doVetoISREmission(int, const Event &e, int iSys) {
    // Must be radiation from the hard system
    if (iSys != 0) return false;

    // If we already have accepted 'vetoCount' emissions in a row, do nothing
    if (vetoMode == 1 && nAcceptSeq >= vetoCount) return false;

    // Pythia radiator after, emitted and recoiler after.
    int iRadAft = -1, iEmt = -1, iRecAft = -1;
    for (int i = e.size() - 1; i > 0; i--) {
      if      (iRadAft == -1 && e[i].status() == -41) iRadAft = i;
      else if (iEmt    == -1 && e[i].status() ==  43) iEmt    = i;
      else if (iRecAft == -1 && e[i].status() == -42) iRecAft = i;
      if (iRadAft != -1 && iEmt != -1 && iRecAft != -1) break;
    }
    if (iRadAft == -1 || iEmt == -1 || iRecAft == -1) {
      e.list();
      cout << "Error: couldn't find Pythia ISR emission" << endl;
      exit(1);
    }

    // pTemtMode == 0: pT of emitted w.r.t. radiator
    // pTemtMode == 1: min(pT of emitted w.r.t. all incoming/outgoing)
    // pTemtMode == 2: min(pT of all outgoing w.r.t. all incoming/outgoing)
    int xSR      = (pTemtMode == 0) ? 0       : -1;
    int i        = (pTemtMode == 0) ? iRadAft : -1;
    int j        = (pTemtMode != 2) ? iEmt    : -1;
    int k        = -1;
    int r        = (pTemtMode == 0) ? iRecAft : -1;
    double pTemt = pTcalc(e, i, j, k, r, xSR);

#ifdef DBGOUTPUT
    cout << "doVetoISREmission: pTemt = " << pTemt << endl << endl;
#endif

    // Veto if pTemt > pThard
    if (pTemt > pThard) {
      nAcceptSeq = 0;
      nISRveto++;
      return true;
    }

    // Else mark that an emission has been accepted and continue
    nAcceptSeq++;
    accepted = true;
    return false;
  }

//--------------------------------------------------------------------------

  // FSR veto

  bool canVetoFSREmission() { return (vetoMode == 0) ? false : true; }
  bool doVetoFSREmission(int, const Event &e, int iSys, bool) {
    // Must be radiation from the hard system
    if (iSys != 0) return false;

    // If we already have accepted 'vetoCount' emissions in a row, do nothing
    if (vetoMode == 1 && nAcceptSeq >= vetoCount) return false;

    // Pythia radiator (before and after), emitted and recoiler (after)
    int iRecAft = e.size() - 1;
    int iEmt    = e.size() - 2;
    int iRadAft = e.size() - 3;
    int iRadBef = e[iEmt].mother1();
    if ( (e[iRecAft].status() != 52 && e[iRecAft].status() != -53) ||
         e[iEmt].status() != 51 || e[iRadAft].status() != 51) {
      e.list();
      cout << "Error: couldn't find Pythia FSR emission" << endl;
      exit(1);
    }

    // Behaviour based on pTemtMode:
    //  0 - pT of emitted w.r.t. radiator before
    //  1 - min(pT of emitted w.r.t. all incoming/outgoing)
    //  2 - min(pT of all outgoing w.r.t. all incoming/outgoing)
    int xSR = (pTemtMode == 0) ? 1       : -1;
    int i   = (pTemtMode == 0) ? iRadBef : -1;
    int k   = (pTemtMode == 0) ? iRadAft : -1;
    int r   = (pTemtMode == 0) ? iRecAft : -1;

    // When pTemtMode is 0 or 1, iEmt has been selected
    double pTemt = 0.;
    if (pTemtMode == 0 || pTemtMode == 1) {
      // Which parton is emitted, based on emittedMode:
      //  0 - Pythia definition of emitted
      //  1 - Pythia definition of radiated after emission
      //  2 - Random selection of emitted or radiated after emission
      //  3 - Try both emitted and radiated after emission
      int j = iRadAft;
      if (emittedMode == 0 || (emittedMode == 2 && rndmPtr->flat() < 0.5)) j++;

      for (int jLoop = 0; jLoop < 2; jLoop++) {
        if      (jLoop == 0) pTemt = pTcalc(e, i, j, k, r, xSR);
        else if (jLoop == 1) pTemt = min(pTemt, pTcalc(e, i, j, k, r, xSR));
  
        // For emittedMode == 3, have tried iRadAft, now try iEmt
        if (emittedMode != 3) break;
        if (k != -1) swap(j, k); else j = iEmt;
      }

    // If pTemtMode is 2, then try all final-state partons as emitted
    } else if (pTemtMode == 2) {
      pTemt = pTcalc(e, i, -1, k, r, xSR);

    }

#ifdef DBGOUTPUT
    cout << "doVetoFSREmission: pTemt = " << pTemt << endl << endl;
#endif

    // Veto if pTemt > pThard
    if (pTemt > pThard) {
      nAcceptSeq = 0;
      nFSRveto++;
      return true;
    }

    // Else mark that an emission has been accepted and continue
    nAcceptSeq++;
    accepted = true;
    return false;
  }

//--------------------------------------------------------------------------

  // MPI veto

  bool canVetoMPIEmission() { return (MPIvetoMode == 0) ? false : true; }
  bool doVetoMPIEmission(int, const Event &e) {
    if (MPIvetoMode == 1) {
      if (e[e.size() - 1].pT() > pTMPI) {
#ifdef DBGOUTPUT
        cout << "doVetoMPIEmission: pTnow = " << e[e.size() - 1].pT()
             << ", pTMPI = " << pTMPI << endl << endl;
#endif
        return true;
      }
    }
    return false;
  }

//--------------------------------------------------------------------------

  // Functions to return information

  int    getNISRveto() { return nISRveto; }
  int    getNFSRveto() { return nFSRveto; }

private:
  int    nFinal, vetoMode, vetoCount, pThardMode, pTemtMode,
         emittedMode, pTdefMode, MPIvetoMode;
  double pThard, pTMPI;
  bool   accepted;
  // The number of accepted emissions (in a row)
  int nAcceptSeq;
  // Statistics on vetos
  unsigned long int nISRveto, nFSRveto;

};

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

int main(int, char **) {

  // Generator
  Pythia pythia;

  // Add further settings that can be set in the configuration file
  pythia.settings.addMode("POWHEG:nFinal",    2, true, false, 1, 0);
  pythia.settings.addMode("POWHEG:veto",      0, true, true,  0, 2);
  pythia.settings.addMode("POWHEG:vetoCount", 3, true, false, 0, 0);
  pythia.settings.addMode("POWHEG:pThard",    0, true, true,  0, 2);
  pythia.settings.addMode("POWHEG:pTemt",     0, true, true,  0, 2);
  pythia.settings.addMode("POWHEG:emitted",   0, true, true,  0, 3);
  pythia.settings.addMode("POWHEG:pTdef",     0, true, true,  0, 2);
  pythia.settings.addMode("POWHEG:MPIveto",   0, true, true,  0, 1);

  // 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 POWHEG settings
  int nFinal      = pythia.settings.mode("POWHEG:nFinal");
  int vetoMode    = pythia.settings.mode("POWHEG:veto");
  int vetoCount   = pythia.settings.mode("POWHEG:vetoCount");
  int pThardMode  = pythia.settings.mode("POWHEG:pThard");
  int pTemtMode   = pythia.settings.mode("POWHEG:pTemt");
  int emittedMode = pythia.settings.mode("POWHEG:emitted");
  int pTdefMode   = pythia.settings.mode("POWHEG:pTdef");
  int MPIvetoMode = pythia.settings.mode("POWHEG:MPIveto");
  bool loadHooks  = (vetoMode > 0 || MPIvetoMode > 0);

  // Add in user hooks for shower vetoing
  PowhegHooks *powhegHooks = NULL;
  if (loadHooks) {

    // Set ISR and FSR to start at the kinematical limit
    if (vetoMode > 0) {
      pythia.readString("SpaceShower:pTmaxMatch = 2");
      pythia.readString("TimeShower:pTmaxMatch = 2");
    }

    // Set MPI to start at the kinematical limit
    if (MPIvetoMode > 0) {
      pythia.readString("MultipartonInteractions:pTmaxMatch = 2");
    }

    powhegHooks = new PowhegHooks(nFinal, vetoMode, vetoCount,
        pThardMode, pTemtMode, emittedMode, pTdefMode, MPIvetoMode);
    pythia.setUserHooksPtr((UserHooks *) 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.
  if (powhegHooks) delete powhegHooks;
  return 0;
}