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25 //
26 //
27 // Multibody "phase space" generator using Makoto Asai's NBody method.
28 //
29 // Author: Michael Kelsey (SLAC) <kelsey@slac.stanford.edu>
30
31 #include "G4HadPhaseSpaceNBodyAsai.hh"
32 #include "G4LorentzVector.hh"
33 #include "G4PhysicalConstants.hh"
34 #include "G4SystemOfUnits.hh"
35 #include "G4ThreeVector.hh"
36 #include "Randomize.hh"
37 #include <algorithm>
38 #include <functional>
39 #include <iterator>
40 #include <numeric>
41 #include <vector>
42
43
44 namespace {
45 // This wraps the existing #define in a true function
46 G4double uniformRand() { return G4UniformRand(); }
47 }
48
49
50 void G4HadPhaseSpaceNBodyAsai::
51 GenerateMultiBody(G4double initialMass,
52 const std::vector<G4double>& masses,
53 std::vector<G4LorentzVector>& finalState) {
54 if (GetVerboseLevel()) G4cout << GetName() << "::GenerateMultiBody" << G4endl;
55
56 finalState.clear();
57
58 //daughters' mass
59 G4int numberOfDaughters = (G4int)masses.size();
60 G4double sumofmasses =
61 std::accumulate(masses.begin(), masses.end(), 0.);
62
63 //Calculate daughter momentum
64 std::vector<G4double> daughtermomentum(numberOfDaughters);
65 std::vector<G4double> sm(numberOfDaughters);
66 G4double tmas;
67 G4double weight = 1.0;
68 G4int numberOfTry = 0;
69 G4int i;
70
71 std::vector<G4double> rd(numberOfDaughters);
72 do {
73 //Generate random number in descending order
74 rd[0] = 1.0;
75 std::generate(rd.begin()+1, rd.end(), uniformRand);
76 std::sort(rd.begin(), rd.end(), std::greater<G4double>());
77
78 if (GetVerboseLevel()>1) PrintVector(rd,"rd",G4cout);
79
80 //calcurate virtual mass
81 tmas = initialMass - sumofmasses;
82 G4double temp = sumofmasses;
83 for(i =0; i < numberOfDaughters; i++) {
84 sm[i] = rd[i]*tmas + temp;
85 temp -= masses[i];
86 if (GetVerboseLevel()>1) {
87 G4cout << i << " random number:" << rd[i]
88 << " virtual mass:" << sm[i]/GeV << " GeV/c2" <<G4endl;
89 }
90 }
91
92 //Calculate daughter momentum
93 weight = 1.0;
94 i = numberOfDaughters-1;
95 daughtermomentum[i] = TwoBodyMomentum(sm[i-1],masses[i-1],sm[i]);
96 if (GetVerboseLevel()>1) {
97 G4cout << " daughter " << i << ": momentum "
98 << daughtermomentum[i]/GeV << " GeV/c" <<G4endl;
99 }
100 for(i =numberOfDaughters-2; i>=0; i--) {
101 // calculate
102 daughtermomentum[i] = TwoBodyMomentum(sm[i],masses[i],sm[i+1]);
103 if(daughtermomentum[i] < 0.0) {
104 // !!! illegal momentum !!!
105 if (GetVerboseLevel()>0) {
106 G4cout << "G4HadPhaseSpaceNBodyAsai::Generate "
107 << " can not calculate daughter momentum "
108 << "\n initialMass " << initialMass/GeV << " GeV/c2"
109 << "\n daughter " << i << ": mass "
110 << masses[i]/GeV << " GeV/c2; momentum "
111 << daughtermomentum[i]/GeV << " GeV/c" << G4endl;
112 }
113 return; // Error detection
114 }
115
116 // calculate weight of this events
117 weight *= daughtermomentum[i]/sm[i];
118 if (GetVerboseLevel()>1) {
119 G4cout << " daughter " << i << ": momentum "
120 << daughtermomentum[i]/GeV << " GeV/c" <<G4endl;
121 }
122 }
123 if (GetVerboseLevel()>1) {
124 G4cout << " weight: " << weight <<G4endl;
125 }
126
127 // exit if number of Try exceeds 100
128 if (numberOfTry++ > 100) {
129 if (GetVerboseLevel()>0) {
130 G4cout << "G4HadPhaseSpaceNBodyAsai::Generate "
131 << " can not determine Decay Kinematics " << G4endl;
132 }
133 return; // Error detection
134 }
135 } while (weight > G4UniformRand()); /* Loop checking, 02.11.2015, A.Ribon */
136
137 if (GetVerboseLevel()>1) {
138 G4cout << "Start calculation of daughters momentum vector "<<G4endl;
139 }
140
141 G4double beta;
142
143 finalState.resize(numberOfDaughters);
144
145 i = numberOfDaughters-2;
146
147 G4ThreeVector direction = UniformVector(daughtermomentum[i]);
148
149 finalState[i].setVectM(direction, masses[i]);
150 finalState[i+1].setVectM(-direction, masses[i+1]);
151
152 for (i = numberOfDaughters-3; i >= 0; i--) {
153 direction = UniformVector();
154
155 //create daughter particle
156 finalState[i].setVectM(-daughtermomentum[i]*direction, masses[i]);
157
158 // boost already created particles
159 beta = daughtermomentum[i];
160 beta /= std::sqrt(beta*beta + sm[i+1]*sm[i+1]);
161 for (G4int j = i+1; j<numberOfDaughters; j++) {
162 finalState[j].boost(beta*direction);
163 }
164 }
165 }
166