fastsim::Decayer Class Reference

Implementation of non-stable particle decays. More...

#include <Decayer.h>

Public Member Functions
void	decay (const Particle &particle, std::vector< std::unique_ptr< Particle > > &secondaries, CLHEP::HepRandomEngine &engine) const
	Decay particle using pythia. More...
	Decayer ()
	Default Constructor. More...
	~Decayer ()
	Default destructor. More...

Private Attributes
std::unique_ptr< Pythia8::Pythia >	pythia_
	Instance of pythia. More...
std::unique_ptr< gen::P8RndmEngine >	pythiaRandomEngine_
	Instance of pythia Random Engine. More...

Detailed Description

Implementation of non-stable particle decays.

Inspired by method Pythia8Hadronizer::residualDecay() in GeneratorInterface/Pythia8Interface/src/Py8GunBase.cc

Definition at line 34 of file Decayer.h.

Constructor & Destructor Documentation

Decayer()

fastsim::Decayer::Decayer

(

)

Default Constructor.

Definition at line 12 of file Decayer.cc.

                        : pythia_(new Pythia8::Pythia()), pythiaRandomEngine_(new gen::P8RndmEngine()) {
  pythia_->setRndmEnginePtr(pythiaRandomEngine_.get());
  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);
  }
}

References pythia_, and pythiaRandomEngine_.

~Decayer()

fastsim::Decayer::~Decayer

(

)

Default destructor.

Definition at line 10 of file Decayer.cc.

{ ; }

Member Function Documentation

decay()

void fastsim::Decayer::decay	(	const Particle &	particle,
		std::vector< std::unique_ptr< Particle > > &	secondaries,
		CLHEP::HepRandomEngine &	engine
	)		const

Decay particle using pythia.

Parameters

particle	The particle that should be decayed.
secondaries	The decay products.
engine	The Random Engine.

Definition at line 29 of file Decayer.cc.

                                                                 {
  // 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;
}

References funct::abs(), fastsim::Particle::charge(), fastsim::Particle::getMotherDeltaR(), fastsim::Particle::getMotherPdgId(), isExotic(), fastsim::Particle::momentum(), fastsim::Particle::pdgId(), fastsim::Particle::position(), and fastsim::Particle::simTrackIndex().

Referenced by FastSimProducer::createFSimTrack(), and FastSimProducer::produce().

Member Data Documentation

pythia_

std::unique_ptr<Pythia8::Pythia> fastsim::Decayer::pythia_

private

Instance of pythia.

Definition at line 53 of file Decayer.h.

Referenced by Decayer().

pythiaRandomEngine_

std::unique_ptr<gen::P8RndmEngine> fastsim::Decayer::pythiaRandomEngine_

private

Instance of pythia Random Engine.

Definition at line 54 of file Decayer.h.

Referenced by Decayer().