Geant4_10
G4HadPhaseSpaceNBodyAsai.cc
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26 // $Id$
27 //
28 // Multibody "phase space" generator using Makoto Asai's NBody method.
29 //
30 // Author: Michael Kelsey (SLAC) <kelsey@slac.stanford.edu>
31 
33 #include "G4LorentzVector.hh"
34 #include "G4PhysicalConstants.hh"
35 #include "G4SystemOfUnits.hh"
36 #include "G4ThreeVector.hh"
37 #include "Randomize.hh"
38 #include <algorithm>
39 #include <functional>
40 #include <iterator>
41 #include <numeric>
42 #include <vector>
43 
44 
45 namespace {
46  // This wraps the existing #define in a true function
47  G4double uniformRand() { return G4UniformRand(); }
48 }
49 
50 
53  const std::vector<G4double>& masses,
54  std::vector<G4LorentzVector>& finalState) {
55  if (GetVerboseLevel()) G4cout << GetName() << "::GenerateMultiBody" << G4endl;
56 
57  finalState.clear();
58 
59  //daughters' mass
60  G4int numberOfDaughters = masses.size();
61  G4double sumofmasses =
62  std::accumulate(masses.begin(), masses.end(), 0.);
63 
64  //Calculate daughter momentum
65  std::vector<G4double> daughtermomentum(numberOfDaughters);
66  std::vector<G4double> sm(numberOfDaughters);
67  G4double tmas;
68  G4double weight = 1.0;
69  G4int numberOfTry = 0;
70  G4int i;
71 
72  std::vector<G4double> rd(numberOfDaughters);
73  do {
74  //Generate random number in descending order
75  rd[0] = 1.0;
76  std::generate(rd.begin()+1, rd.end(), uniformRand);
77  std::sort(rd.begin(), rd.end(), std::greater<G4double>());
78 
79  if (GetVerboseLevel()>1) PrintVector(rd,"rd",G4cout);
80 
81  //calcurate virtual mass
82  tmas = initialMass - sumofmasses;
83  G4double temp = sumofmasses;
84  for(i =0; i < numberOfDaughters; i++) {
85  sm[i] = rd[i]*tmas + temp;
86  temp -= masses[i];
87  if (GetVerboseLevel()>1) {
88  G4cout << i << " random number:" << rd[i]
89  << " virtual mass:" << sm[i]/GeV << " GeV/c2" <<G4endl;
90  }
91  }
92 
93  //Calculate daughter momentum
94  weight = 1.0;
95  i = numberOfDaughters-1;
96  daughtermomentum[i] = TwoBodyMomentum(sm[i-1],masses[i-1],sm[i]);
97  if (GetVerboseLevel()>1) {
98  G4cout << " daughter " << i << ": momentum "
99  << daughtermomentum[i]/GeV << " GeV/c" <<G4endl;
100  }
101  for(i =numberOfDaughters-2; i>=0; i--) {
102  // calculate
103  daughtermomentum[i] = TwoBodyMomentum(sm[i],masses[i],sm[i+1]);
104  if(daughtermomentum[i] < 0.0) {
105  // !!! illegal momentum !!!
106  if (GetVerboseLevel()>0) {
107  G4cout << "G4HadPhaseSpaceNBodyAsai::Generate "
108  << " can not calculate daughter momentum "
109  << "\n initialMass " << initialMass/GeV << " GeV/c2"
110  << "\n daughter " << i << ": mass "
111  << masses[i]/GeV << " GeV/c2; momentum "
112  << daughtermomentum[i]/GeV << " GeV/c" << G4endl;
113  }
114  return; // Error detection
115  }
116 
117  // calculate weight of this events
118  weight *= daughtermomentum[i]/sm[i];
119  if (GetVerboseLevel()>1) {
120  G4cout << " daughter " << i << ": momentum "
121  << daughtermomentum[i]/GeV << " GeV/c" <<G4endl;
122  }
123  }
124  if (GetVerboseLevel()>1) {
125  G4cout << " weight: " << weight <<G4endl;
126  }
127 
128  // exit if number of Try exceeds 100
129  if (numberOfTry++ > 100) {
130  if (GetVerboseLevel()>0) {
131  G4cout << "G4HadPhaseSpaceNBodyAsai::Generate "
132  << " can not determine Decay Kinematics " << G4endl;
133  }
134  return; // Error detection
135  }
136  } while (weight > G4UniformRand());
137 
138  if (GetVerboseLevel()>1) {
139  G4cout << "Start calulation of daughters momentum vector "<<G4endl;
140  }
141 
142  G4double beta;
143 
144  finalState.resize(numberOfDaughters);
145 
146  i = numberOfDaughters-2;
147 
148  G4ThreeVector direction = UniformVector(daughtermomentum[i]);
149 
150  finalState[i].setVectM(direction, masses[i]);
151  finalState[i+1].setVectM(-direction, masses[i+1]);
152 
153  for (i = numberOfDaughters-3; i >= 0; i--) {
154  direction = UniformVector();
155 
156  //create daughter particle
157  finalState[i].setVectM(-daughtermomentum[i]*direction, masses[i]);
158 
159  // boost already created particles
160  beta = daughtermomentum[i];
161  beta /= std::sqrt(beta*beta + sm[i+1]*sm[i+1]);
162  for (G4int j = i+1; j<numberOfDaughters; j++) {
163  finalState[j].boost(beta*direction);
164  }
165  }
166 }
G4int GetVerboseLevel() const
subroutine sort(A, N)
Definition: dpm25nuc7.f:4670
double weight
Definition: plottest35.C:25
int G4int
Definition: G4Types.hh:78
#define G4UniformRand()
Definition: Randomize.hh:87
G4GLOB_DLL std::ostream G4cout
void PrintVector(const std::vector< G4double > &v, const G4String &name, std::ostream &os) const
const G4String & GetName() const
G4double TwoBodyMomentum(G4double M0, G4double M1, G4double M2) const
virtual void GenerateMultiBody(G4double initialMass, const std::vector< G4double > &masses, std::vector< G4LorentzVector > &finalState)
#define G4endl
Definition: G4ios.hh:61
double G4double
Definition: G4Types.hh:76
G4ThreeVector UniformVector(G4double mag=1.) const