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25 //
26 //
27 // Multibody "phase space" generator using GEN
28 //
29 // Author: Michael Kelsey (SLAC) <kelsey@slac
30
31 #include "G4HadPhaseSpaceGenbod.hh"
32 #include "G4LorentzVector.hh"
33 #include "G4PhysicalConstants.hh"
34 #include "G4ThreeVector.hh"
35 #include "Randomize.hh"
36 #include <algorithm>
37 #include <functional>
38 #include <iterator>
39 #include <numeric>
40 #include <vector>
41
42
43 namespace {
44 // Wrap #define in a true function, for pass
45 G4double uniformRand() { return G4UniformRan
46 }
47
48
49 // Constructor initializes everything to zero
50
51 G4HadPhaseSpaceGenbod::G4HadPhaseSpaceGenbod(G
52 : G4VHadPhaseSpaceAlgorithm("G4HadPhaseSpace
53 nFinal(0), totalMass(0.), massExcess(0.),
54
55
56 // C++ re-implementation of GENBOD.F (Raubold-
57
58 void G4HadPhaseSpaceGenbod::
59 GenerateMultiBody(G4double initialMass,
60 const std::vector<G4double>& masses,
61 std::vector<G4LorentzVector>& finalState
62 if (GetVerboseLevel()) G4cout << GetName() <
63
64 finalState.clear();
65
66 Initialize(initialMass, masses);
67
68 const G4int maxNumberOfLoops = 10000;
69 nTrials = 0;
70 do { // Apply accept/reject to get di
71 ++nTrials;
72 FillRandomBuffer();
73 FillEnergySteps(initialMass, masses);
74 } while ( (!AcceptEvent()) && nTrials < maxN
75 if ( nTrials >= maxNumberOfLoops ) {
76 G4ExceptionDescription ed;
77 ed << " Failed sampling after maxNumberOfL
78 G4Exception( " G4HadPhaseSpaceGenbod::Gene
79 }
80 GenerateMomenta(masses, finalState);
81 }
82
83 void G4HadPhaseSpaceGenbod::
84 Initialize(G4double initialMass, const std::ve
85 if (GetVerboseLevel()>1) G4cout << GetName()
86
87 nFinal = masses.size();
88 msum.resize(nFinal, 0.); // Initialize bu
89 msq.resize(nFinal, 0.);
90
91 std::partial_sum(masses.begin(), masses.end(
92 std::transform(masses.begin(), masses.end(),
93 std::multiplies<G4double>());
94 totalMass = msum.back();
95 massExcess = initialMass - totalMass;
96
97 if (GetVerboseLevel()>2) {
98 PrintVector(msum, "msum", G4cout);
99 PrintVector(msq, "msq", G4cout);
100 G4cout << " totalMass " << totalMass << "
101 << G4endl;
102 }
103
104 ComputeWeightScale(masses);
105 }
106
107
108 // Generate ordered list of random numbers
109
110 void G4HadPhaseSpaceGenbod::FillRandomBuffer()
111 if (GetVerboseLevel()>1) G4cout << GetName()
112
113 rndm.resize(nFinal-2,0.); // Final states ge
114 std::generate(rndm.begin(), rndm.end(), unif
115 std::sort(rndm.begin(), rndm.end());
116 if (GetVerboseLevel()>2) PrintVector(rndm, "
117 }
118
119
120 // Final state effective masses, min to max
121
122 void
123 G4HadPhaseSpaceGenbod::FillEnergySteps(G4doubl
124 const std::vector<G4double>& ma
125 if (GetVerboseLevel()>1) G4cout << GetName()
126
127 meff.clear();
128 pd.clear();
129
130 meff.push_back(masses[0]);
131 for (size_t i=1; i<nFinal-1; i++) {
132 meff.push_back(rndm[i-1]*massExcess + msum
133 pd.push_back(TwoBodyMomentum(meff[i], meff
134 }
135 meff.push_back(initialMass);
136 pd.push_back(TwoBodyMomentum(meff[nFinal-1],
137
138 if (GetVerboseLevel()>2) {
139 PrintVector(meff,"meff",G4cout);
140 PrintVector(pd,"pd",G4cout);
141 }
142 }
143
144
145 // Maximum possible weight for final state (us
146
147 void
148 G4HadPhaseSpaceGenbod::ComputeWeightScale(cons
149 if (GetVerboseLevel()>1)
150 G4cout << GetName() << "::ComputeWeightSca
151
152 weightMax = 1.;
153 for (size_t i=1; i<nFinal; i++) {
154 weightMax *= TwoBodyMomentum(massExcess+ms
155 }
156
157 if (GetVerboseLevel()>2) G4cout << " weightM
158 }
159
160
161 // Event weight computed as either constant or
162
163 G4double G4HadPhaseSpaceGenbod::ComputeWeight(
164 if (GetVerboseLevel()>1) G4cout << GetName()
165
166 return (std::accumulate(pd.begin(), pd.end()
167 std::multiplies<G4double>()));
168 }
169
170 G4bool G4HadPhaseSpaceGenbod::AcceptEvent() co
171 if (GetVerboseLevel()>1)
172 G4cout << GetName() << "::AcceptEvent? " <
173
174 return (G4UniformRand() <= ComputeWeight());
175 }
176
177
178 // Final state momentum vectors in CMS system,
179
180 void G4HadPhaseSpaceGenbod::
181 GenerateMomenta(const std::vector<G4double>& m
182 std::vector<G4LorentzVector>& finalState)
183 if (GetVerboseLevel()>1) G4cout << GetName()
184
185 finalState.resize(nFinal); // Preallocate v
186
187 for (size_t i=0; i<nFinal; i++) {
188 AccumulateFinalState(i, masses, finalState
189 if (GetVerboseLevel()>2)
190 G4cout << " finalState[" << i << "] " <<
191 }
192 }
193
194 // Process final state daughters up to current
195
196 void G4HadPhaseSpaceGenbod::
197 AccumulateFinalState(size_t i,
198 const std::vector<G4double>& masses,
199 std::vector<G4LorentzVector>& finalSt
200 if (GetVerboseLevel()>2)
201 G4cout << GetName() << "::AccumulateFinalS
202
203 if (i==0) { // First final state particl
204 finalState[i].setVectM(G4ThreeVector(0.,pd
205 return;
206 }
207
208 finalState[i].setVectM(G4ThreeVector(0.,-pd[
209 G4double phi = G4UniformRand() * twopi;
210 G4double theta = std::acos(2.*G4UniformRand(
211
212 if (GetVerboseLevel() > 2) {
213 G4cout << " initialized Py " << -pd[i-1] <
214 << " theta " << theta << G4endl;
215 }
216
217 G4double esys=0.,beta=0.,gamma=1.;
218 if (i < nFinal-1) { // Do not boost fina
219 esys = std::sqrt(pd[i]*pd[i]+meff[i]*meff[
220 beta = pd[i] / esys;
221 gamma = esys / meff[i];
222
223 if (GetVerboseLevel()>2)
224 G4cout << " esys " << esys << " beta " <
225 << G4endl;
226 }
227
228 for (size_t j=0; j<=i; j++) { // Accumulat
229 finalState[j].rotateZ(theta).rotateY(phi);
230 finalState[j].setY(gamma*(finalState[j].y(
231 if (GetVerboseLevel()>2) G4cout << " j " <
232 }
233 }
234