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25 //
26 //
27 // Multibody "phase space" generator using GENBOD (CERNLIB W515) method.
28 //
29 // Author: Michael Kelsey (SLAC) <kelsey@slac.stanford.edu>
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 passing to std::fill
45 G4double uniformRand() { return G4UniformRand(); }
46 }
47
48
49 // Constructor initializes everything to zero
50
51 G4HadPhaseSpaceGenbod::G4HadPhaseSpaceGenbod(G4int verbose)
52 : G4VHadPhaseSpaceAlgorithm("G4HadPhaseSpaceGenbod",verbose),
53 nFinal(0), totalMass(0.), massExcess(0.), weightMax(0.), nTrials(0) {;}
54
55
56 // C++ re-implementation of GENBOD.F (Raubold-Lynch method)
57
58 void G4HadPhaseSpaceGenbod::
59 GenerateMultiBody(G4double initialMass,
60 const std::vector<G4double>& masses,
61 std::vector<G4LorentzVector>& finalState) {
62 if (GetVerboseLevel()) G4cout << GetName() << "::GenerateMultiBody" << G4endl;
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 distribution
71 ++nTrials;
72 FillRandomBuffer();
73 FillEnergySteps(initialMass, masses);
74 } while ( (!AcceptEvent()) && nTrials < maxNumberOfLoops ); /* Loop checking, 02.11.2015, A.Ribon */
75 if ( nTrials >= maxNumberOfLoops ) {
76 G4ExceptionDescription ed;
77 ed << " Failed sampling after maxNumberOfLoops attempts : forced exit" << G4endl;
78 G4Exception( " G4HadPhaseSpaceGenbod::GenerateMultiBody ", "HAD_GENBOD_001", FatalException, ed );
79 }
80 GenerateMomenta(masses, finalState);
81 }
82
83 void G4HadPhaseSpaceGenbod::
84 Initialize(G4double initialMass, const std::vector<G4double>& masses) {
85 if (GetVerboseLevel()>1) G4cout << GetName() << "::Initialize" << G4endl;
86
87 nFinal = masses.size();
88 msum.resize(nFinal, 0.); // Initialize buffers for filling
89 msq.resize(nFinal, 0.);
90
91 std::partial_sum(masses.begin(), masses.end(), msum.begin());
92 std::transform(masses.begin(), masses.end(), masses.begin(), msq.begin(),
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 << " massExcess " << massExcess
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() << "::FillRandomBuffer" << G4endl;
112
113 rndm.resize(nFinal-2,0.); // Final states generated in sorted order
114 std::generate(rndm.begin(), rndm.end(), uniformRand);
115 std::sort(rndm.begin(), rndm.end());
116 if (GetVerboseLevel()>2) PrintVector(rndm, "rndm", G4cout);
117 }
118
119
120 // Final state effective masses, min to max
121
122 void
123 G4HadPhaseSpaceGenbod::FillEnergySteps(G4double initialMass,
124 const std::vector<G4double>& masses) {
125 if (GetVerboseLevel()>1) G4cout << GetName() << "::FillEnergySteps" << G4endl;
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[i]);
133 pd.push_back(TwoBodyMomentum(meff[i], meff[i-1], masses[i]));
134 }
135 meff.push_back(initialMass);
136 pd.push_back(TwoBodyMomentum(meff[nFinal-1], meff[nFinal-2], masses[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 (used with accept/reject)
146
147 void
148 G4HadPhaseSpaceGenbod::ComputeWeightScale(const std::vector<G4double>& masses) {
149 if (GetVerboseLevel()>1)
150 G4cout << GetName() << "::ComputeWeightScale" << G4endl;
151
152 weightMax = 1.;
153 for (size_t i=1; i<nFinal; i++) {
154 weightMax *= TwoBodyMomentum(massExcess+msum[i], msum[i-1], masses[i]);
155 }
156
157 if (GetVerboseLevel()>2) G4cout << " weightMax = " << weightMax << G4endl;
158 }
159
160
161 // Event weight computed as either constant or Fermi-dependent cross-section
162
163 G4double G4HadPhaseSpaceGenbod::ComputeWeight() const {
164 if (GetVerboseLevel()>1) G4cout << GetName() << "::ComputeWeight" << G4endl;
165
166 return (std::accumulate(pd.begin(), pd.end(), 1./weightMax,
167 std::multiplies<G4double>()));
168 }
169
170 G4bool G4HadPhaseSpaceGenbod::AcceptEvent() const {
171 if (GetVerboseLevel()>1)
172 G4cout << GetName() << "::AcceptEvent? " << nTrials << G4endl;
173
174 return (G4UniformRand() <= ComputeWeight());
175 }
176
177
178 // Final state momentum vectors in CMS system, using Raubold-Lynch method
179
180 void G4HadPhaseSpaceGenbod::
181 GenerateMomenta(const std::vector<G4double>& masses,
182 std::vector<G4LorentzVector>& finalState) {
183 if (GetVerboseLevel()>1) G4cout << GetName() << "::GenerateMomenta" << G4endl;
184
185 finalState.resize(nFinal); // Preallocate vectors for convenience below
186
187 for (size_t i=0; i<nFinal; i++) {
188 AccumulateFinalState(i, masses, finalState);
189 if (GetVerboseLevel()>2)
190 G4cout << " finalState[" << i << "] " << finalState[i] << G4endl;
191 }
192 }
193
194 // Process final state daughters up to current index
195
196 void G4HadPhaseSpaceGenbod::
197 AccumulateFinalState(size_t i,
198 const std::vector<G4double>& masses,
199 std::vector<G4LorentzVector>& finalState) {
200 if (GetVerboseLevel()>2)
201 G4cout << GetName() << "::AccumulateFinalState " << i << G4endl;
202
203 if (i==0) { // First final state particle left alone
204 finalState[i].setVectM(G4ThreeVector(0.,pd[i],0.),masses[i]);
205 return;
206 }
207
208 finalState[i].setVectM(G4ThreeVector(0.,-pd[i-1],0.),masses[i]);
209 G4double phi = G4UniformRand() * twopi;
210 G4double theta = std::acos(2.*G4UniformRand() - 1.);
211
212 if (GetVerboseLevel() > 2) {
213 G4cout << " initialized Py " << -pd[i-1] << " phi " << phi
214 << " theta " << theta << G4endl;
215 }
216
217 G4double esys=0.,beta=0.,gamma=1.;
218 if (i < nFinal-1) { // Do not boost final particle
219 esys = std::sqrt(pd[i]*pd[i]+meff[i]*meff[i]);
220 beta = pd[i] / esys;
221 gamma = esys / meff[i];
222
223 if (GetVerboseLevel()>2)
224 G4cout << " esys " << esys << " beta " << beta << " gamma " << gamma
225 << G4endl;
226 }
227
228 for (size_t j=0; j<=i; j++) { // Accumulate rotations
229 finalState[j].rotateZ(theta).rotateY(phi);
230 finalState[j].setY(gamma*(finalState[j].y() + beta*finalState[j].e()));
231 if (GetVerboseLevel()>2) G4cout << " j " << j << " " << finalState[j] << G4endl;
232 }
233 }
234