Py8Decayer.cc

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#include "Py8Decayer.hh"

#include "Geant4/G4DynamicParticle.hh"
#include "Geant4/G4ParticleTable.hh"
#include "Geant4/G4Track.hh"
#include "Geant4/G4SystemOfUnits.hh"

#include <cstdlib>
#include <sstream>

using namespace Pythia8;

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Py8Decayer::Py8Decayer()
   : G4VExtDecayer("Py8Decayer"),
   fDecayer(nullptr)
{

   // use default path to the xml/configs but do NOT print banner
   //
   fDecayer = new Pythia( "../share/Pythia8/xmldoc", false );

   // this is the trick to make Pythia8 work as "decayer"
   //
   fDecayer->readString("ProcessLevel:all = off");

   fDecayer->readString("ProcessLevel:resonanceDecays=on");

   // shut off Pythia8 (default) verbosty
   //
   fDecayer->readString("Init:showAllSettings=false");
   fDecayer->readString("Init:showChangedSettings=false");
   fDecayer->readString("Init:showAllParticleData=false");
   fDecayer->readString("Init:showChangedParticleData=false");
   //
   // specify how many Py8 events to print out, at either level
   // in this particular case print out a maximum of 10 events
   //

   fDecayer->readString( "Next:numberShowProcess = 0" );

   //fDecayer->readString( "Next:numberShowEvent = 10" );
   ostringstream s;

   int nShowEvent   = 10;
   if (const char* env_nShowEvent = std::getenv("PY8_NSHOWEVENT")) {
     nShowEvent = std::atoi(env_nShowEvent);
     G4cout << "=====> set pythia8 Next:numberShowEvent to "
            << nShowEvent << " (" << env_nShowEvent << ")" << G4endl;
   }
   //s.str("");
   //s.clear();
   s << "Next:numberShowEvent = " << nShowEvent;
   //G4cout << "=====> set fDecayer->readString(" << s.str().c_str() << ")" << G4endl;
   fDecayer->readString( s.str().c_str() );


   fDecayer->init();

   // shut off decays of pi0's as we want Geant4 to handle them
   // if other immediate decay products should be handled by Geant4,
   // their respective decay modes should be shut off as well
   //
   fDecayer->readString("111:onMode = off");
}

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Py8Decayer::~Py8Decayer()
{

   delete fDecayer;

}

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G4DecayProducts* Py8Decayer::ImportDecayProducts(const G4Track& track)
{

   fDecayer->event.reset();

   G4DecayProducts* dproducts = nullptr;

   G4ParticleDefinition* pd = track.GetDefinition();
   int    pdgid   = pd->GetPDGEncoding();

   // check if pdgid is consistent with Pythia8 particle table
   //
   if ( !fDecayer->particleData.findParticle( pdgid ) )
   {
      G4cout << " can NOT find pdgid = " << pdgid
             << " in Pythia8::ParticleData" << G4endl;
      return dproducts;
   }

   if ( !fDecayer->particleData.canDecay(pdgid) )
   {
      G4cout << " Particle of pdgid = " << pdgid
             << " can NOT be decayed by Pythia8" << G4endl;
      return dproducts;
   }

   // NOTE: Energy should be in GeV

   fDecayer->event.append( pdgid, 1, 0, 0,
                           track.GetMomentum().x() / CLHEP::GeV,
                           track.GetMomentum().y() / CLHEP::GeV,
                           track.GetMomentum().z() / CLHEP::GeV,
                           track.GetDynamicParticle()->GetTotalEnergy() / CLHEP::GeV,
                           pd->GetPDGMass() / CLHEP::GeV );

   // specify polarization, if any

   // NOTE: while in Py8 polarization is a double variable ,
   //       in reality it's expected to be -1, 0., or 1 in case of "external" tau's,
   //       similar to LHA SPINUP; see Particle Decays, Hadron and Tau Decays in docs at
   //       https://pythia.org/manuals/pythia8305/Welcome.html
   //       so it's not able to handle anything like 0.99, thus we're rounding off
   fDecayer->event.back().pol(
      round( std::cos( track.GetPolarization().angle( track.GetMomentumDirection() ) )
      )
   );

   int npart_before_decay = fDecayer->event.size();

   fDecayer->next();

   int npart_after_decay = fDecayer->event.size();

   // create & fill up decay products
   //
   dproducts = new G4DecayProducts(*(track.GetDynamicParticle()));

   // create G4DynamicParticle out of each fDecayer->event entry (except the 1st one)
   // and push into dproducts

   for ( int ip=npart_before_decay; ip<npart_after_decay; ++ip )
   {

      // only select final state decay products (direct or via subsequent decays);
      // skip all others
      //
      // NOTE: in general, final state decays products will have
      //       positive status code between 91 and 99
      //       (in case such information could be of interest in the future)
      //
      if ( fDecayer->event[ip].status() < 0 ) continue;

      // should we also skip neutrinos ???
      // if so, skip products with abs(fDecayer->event[ip].id()) of 12, 14, or 16

      G4ParticleDefinition* pddec =
         G4ParticleTable::GetParticleTable()->FindParticle( fDecayer->event[ip].id() );
      if ( !pddec ) continue; // maybe we should print out a warning !
      G4ThreeVector momentum = G4ThreeVector( fDecayer->event[ip].px() * CLHEP::GeV,
                                              fDecayer->event[ip].py() * CLHEP::GeV,
                                              fDecayer->event[ip].pz() * CLHEP::GeV );
      dproducts->PushProducts( new G4DynamicParticle( pddec, momentum) );
   }

   return dproducts;

}

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