py8decayer/src/SingleParticleGun.cc

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0027 /// \file eventgenerator/pythia/pythia8decayer/src/SingleParticleGun.cc
0028 /// \brief Implementation of the SingleParticleGun class
0029 ///
0030 /// \author J. Yarba; FNAL
0031
0032 #include "SingleParticleGun.hh"
0033
0034 #include "G4Event.hh"
0035 #include "G4ParticleDefinition.hh"
0036 #include "G4ParticleGun.hh"
0037 #include "G4ParticleTable.hh"
0038 #include "G4ThreeVector.hh"
0039 #include "Randomize.hh"
0040
0041 // SPECIAL CASE - needed for treatment of polarization for tau's
0042 #include "G4TauMinus.hh"
0043 #include "G4TauPlus.hh"
0044
0045 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
0046
0047 SingleParticleGun::SingleParticleGun(const G4String& pname, const double pmom)
0048   : G4VUserPrimaryGeneratorAction(), fGun(nullptr), fMomentum(pmom)
0049 {
0050   int nparts = 1;
0051   fGun = new G4ParticleGun(nparts);
0052
0053   G4ParticleTable* pdt = G4ParticleTable::GetParticleTable();
0054   G4ParticleDefinition* pd = pdt->FindParticle(pname);
0055   fGun->SetParticleDefinition(pd);
0056   fGun->SetParticlePosition(G4ThreeVector(0., 0., 0.));
0057   double mass = pd->GetPDGMass();
0058   double energy = std::sqrt(fMomentum * fMomentum + mass * mass);
0059   fGun->SetParticleEnergy(energy);
0060
0061   // NOTE: do not set momentum directions
0062   //       as it'll be generated event-by-event
0063 }
0064
0065 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
0066
0067 SingleParticleGun::~SingleParticleGun()
0068 {
0069   delete fGun;
0070 }
0071
0072 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
0073
0074 void SingleParticleGun::GeneratePrimaries(G4Event* evt)
0075 {
0076   if (!fGun) {
0077     G4cout << " ParticleGun is NOT set up; BAIL OUT from this event " << G4endl;
0078     G4cout << " Check if you are using ctor "
0079            << "SinglePartGun(const G4string& partname, const double pmom ) " << G4endl;
0080     return;
0081   }
0082
0083   double phi = -1. * CLHEP::pi + CLHEP::twopi * G4UniformRand();
0084   double theta = CLHEP::pi / 4. + CLHEP::halfpi * G4UniformRand();
0085
0086   double x = std::sin(theta) * std::cos(phi);
0087   double y = std::sin(theta) * std::sin(phi);
0088   double z = std::cos(theta);
0089
0090   fGun->SetParticleMomentumDirection(G4ThreeVector(x, y, z));
0091
0092   // SPECIAL CASE: for particle such as tau polarization should be -1 * p3
0093   //               while for tau_bar it should be +1
0094   //
0095   // NOTE: one could probably compare by name but that'd mean comparing strings...
0096   //
0097   if (fGun->GetParticleDefinition() == G4TauMinus::TauMinus()) {
0098     fGun->SetParticlePolarization(G4ThreeVector(-1.0 * x, -1.0 * y, -1.0 * z));
0099   }
0100   else if (fGun->GetParticleDefinition() == G4TauPlus::TauPlus()) {
0101     fGun->SetParticlePolarization(G4ThreeVector(x, y, z));
0102   }
0103
0104   fGun->GeneratePrimaryVertex(evt);
0105
0106   return;
0107 }
0108
0109 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......