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#include "FastSimulation/SimplifiedGeometryPropagator/interface/Decayer.h"
#include "FastSimulation/SimplifiedGeometryPropagator/interface/Particle.h"
#include "FastSimulation/SimplifiedGeometryPropagator/interface/ParticleManager.h"
#include "FWCore/ServiceRegistry/interface/RandomEngineSentry.h"
#include "GeneratorInterface/Pythia8Interface/interface/P8RndmEngine.h"
#include <Pythia8/Pythia.h>
#include "Pythia8Plugins/HepMC2.h"
fastsim::Decayer::~Decayer() { ; }
fastsim::Decayer::Decayer() : pythia_(new Pythia8::Pythia()), pythiaRandomEngine_(new gen::P8RndmEngine()) {
pythia_->setRndmEnginePtr(pythiaRandomEngine_);
pythia_->settings.flag("ProcessLevel:all", false);
pythia_->settings.flag("PartonLevel:FSRinResonances", false);
pythia_->settings.flag("ProcessLevel:resonanceDecays", false);
pythia_->init();
// forbid all decays
// (decays are allowed selectively in the decay function)
Pythia8::ParticleData& pdt = pythia_->particleData;
int pid = 0;
while (pdt.nextId(pid) > pid) {
pid = pdt.nextId(pid);
pdt.mayDecay(pid, false);
}
}
void fastsim::Decayer::decay(const Particle& particle,
std::vector<std::unique_ptr<fastsim::Particle> >& secondaries,
CLHEP::HepRandomEngine& engine) const {
// make sure pythia takes random numbers from the engine past through via the function arguments
edm::RandomEngineSentry<gen::P8RndmEngine> sentry(pythiaRandomEngine_.get(), &engine);
// inspired by method Pythia8Hadronizer::residualDecay() in GeneratorInterface/Pythia8Interface/src/Py8GunBase.cc
int pid = particle.pdgId();
// snip decay products of exotic particles or their children. These decay products are preserved from the event record.
// limitation: if exotic incurs heavy energy loss during propagation, the saved decay products could be too hard.
if (isExotic(pid) || isExotic(particle.getMotherPdgId())) {
return;
}
pythia_->event.reset();
// create a pythia particle which has the same properties as the FastSim particle
Pythia8::Particle pythiaParticle(pid,
93,
0,
0,
0,
0,
0,
0,
particle.momentum().X(),
particle.momentum().Y(),
particle.momentum().Z(),
particle.momentum().E(),
particle.momentum().M());
pythiaParticle.vProd(
particle.position().X(), particle.position().Y(), particle.position().Z(), particle.position().T());
pythia_->event.append(pythiaParticle);
int nentries_before = pythia_->event.size();
// switch on the decay of this and only this particle (avoid double decays)
pythia_->particleData.mayDecay(pid, true);
// do the decay
pythia_->next();
// switch it off again
pythia_->particleData.mayDecay(pid, false);
int nentries_after = pythia_->event.size();
if (nentries_after <= nentries_before)
return;
// add decay products back to the event
for (int ipart = nentries_before; ipart < nentries_after; ipart++) {
Pythia8::Particle& daughter = pythia_->event[ipart];
secondaries.emplace_back(new fastsim::Particle(
daughter.id(),
math::XYZTLorentzVector(daughter.xProd(), daughter.yProd(), daughter.zProd(), daughter.tProd()),
math::XYZTLorentzVector(daughter.px(), daughter.py(), daughter.pz(), daughter.e())));
// daughter can inherit the SimTrackIndex of mother (if both charged): necessary for FastSim (cheat) tracking
if (particle.charge() != 0 && std::abs(particle.charge() - daughter.charge()) < 1E-10) {
secondaries.back()->setMotherDeltaR(particle.momentum());
secondaries.back()->setMotherPdgId(particle.getMotherDeltaR() == -1 ? particle.pdgId()
: particle.getMotherPdgId());
secondaries.back()->setMotherSimTrackIndex(particle.simTrackIndex());
}
}
return;
}
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