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25 //
26 //
27 /// \file eventgenerator/pythia/pythia8decayer/src/SingleParticleGun.cc
28 /// \brief Implementation of the SingleParticleGun class
29 ///
30 /// \author J. Yarba; FNAL
31
32 #include "SingleParticleGun.hh"
33
34 #include "G4Event.hh"
35 #include "G4ParticleDefinition.hh"
36 #include "G4ParticleGun.hh"
37 #include "G4ParticleTable.hh"
38 #include "G4ThreeVector.hh"
39 #include "Randomize.hh"
40
41 // SPECIAL CASE - needed for treatment of polarization for tau's
42 #include "G4TauMinus.hh"
43 #include "G4TauPlus.hh"
44
45 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
46
47 SingleParticleGun::SingleParticleGun(const G4String& pname, const double pmom)
48 : G4VUserPrimaryGeneratorAction(), fGun(nullptr), fMomentum(pmom)
49 {
50 int nparts = 1;
51 fGun = new G4ParticleGun(nparts);
52
53 G4ParticleTable* pdt = G4ParticleTable::GetParticleTable();
54 G4ParticleDefinition* pd = pdt->FindParticle(pname);
55 fGun->SetParticleDefinition(pd);
56 fGun->SetParticlePosition(G4ThreeVector(0., 0., 0.));
57 double mass = pd->GetPDGMass();
58 double energy = std::sqrt(fMomentum * fMomentum + mass * mass);
59 fGun->SetParticleEnergy(energy);
60
61 // NOTE: do not set momentum directions
62 // as it'll be generated event-by-event
63 }
64
65 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
66
67 SingleParticleGun::~SingleParticleGun()
68 {
69 delete fGun;
70 }
71
72 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
73
74 void SingleParticleGun::GeneratePrimaries(G4Event* evt)
75 {
76 if (!fGun) {
77 G4cout << " ParticleGun is NOT set up; BAIL OUT from this event " << G4endl;
78 G4cout << " Check if you are using ctor "
79 << "SinglePartGun(const G4string& partname, const double pmom ) " << G4endl;
80 return;
81 }
82
83 double phi = -1. * CLHEP::pi + CLHEP::twopi * G4UniformRand();
84 double theta = CLHEP::pi / 4. + CLHEP::halfpi * G4UniformRand();
85
86 double x = std::sin(theta) * std::cos(phi);
87 double y = std::sin(theta) * std::sin(phi);
88 double z = std::cos(theta);
89
90 fGun->SetParticleMomentumDirection(G4ThreeVector(x, y, z));
91
92 // SPECIAL CASE: for particle such as tau polarization should be -1 * p3
93 // while for tau_bar it should be +1
94 //
95 // NOTE: one could probably compare by name but that'd mean comparing strings...
96 //
97 if (fGun->GetParticleDefinition() == G4TauMinus::TauMinus()) {
98 fGun->SetParticlePolarization(G4ThreeVector(-1.0 * x, -1.0 * y, -1.0 * z));
99 }
100 else if (fGun->GetParticleDefinition() == G4TauPlus::TauPlus()) {
101 fGun->SetParticlePolarization(G4ThreeVector(x, y, z));
102 }
103
104 fGun->GeneratePrimaryVertex(evt);
105
106 return;
107 }
108
109 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
110