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25 //
26 /*
27 * =============================================================================
28 *
29 * Filename: CexmcReimplementedGenbod.cc
30 *
31 * Description: reimplemented GENBOD
32 * (mostly adopted from ROOT TGenPhaseSpace)
33 *
34 * Version: 1.0
35 * Created: 08.09.2010 18:52:39
36 * Revision: none
37 * Compiler: gcc
38 *
39 * Author: Alexey Radkov (),
40 * Company: PNPI
41 *
42 * =============================================================================
43 */
44
45 #include <cmath>
46 #include <Randomize.hh>
47 #include <G4PhysicalConstants.hh>
48 #include <G4SystemOfUnits.hh>
49 #include "CexmcReimplementedGenbod.hh"
50 #include "CexmcException.hh"
51
52
53 namespace
54 {
55 G4int DoubleMax( const void * a, const void * b )
56 {
57 G4double aa( *( ( G4double * )a ) );
58 G4double bb( *( ( G4double * )b ) );
59
60 if ( aa > bb )
61 return 1;
62
63 if ( aa < bb )
64 return -1;
65
66 return 0;
67 }
68
69
70 G4double PDK( G4double a, G4double b, G4double c )
71 {
72 G4double x( ( a - b - c ) * ( a + b + c ) * ( a - b + c ) *
73 ( a + b - c ) );
74 x = std::sqrt( x ) / ( 2 * a );
75
76 return x;
77 }
78 }
79
80
81 CexmcReimplementedGenbod::CexmcReimplementedGenbod() : maxWeight( 0. ),
82 nmbOfOutputParticles( 0 )
83 {
84 }
85
86
87 G4double CexmcReimplementedGenbod::Generate( void )
88 {
89 // Generate a random final state.
90 // The function returns the weigth of the current event.
91 // Note that Momentum, Energy units are Gev/C, GeV
92
93 G4double te_minus_tm( totalEnergy - totalMass );
94 G4double rno[ maxParticles ];
95 rno[ 0 ] = 0;
96
97 if ( nmbOfOutputParticles > 2 )
98 {
99 for ( G4int i( 1 ); i < nmbOfOutputParticles - 1; ++i )
100 {
101 rno[ i ] = G4UniformRand();
102 }
103 qsort( rno + 1, nmbOfOutputParticles - 2, sizeof( G4double ),
104 DoubleMax );
105 }
106 rno[ nmbOfOutputParticles - 1 ] = 1;
107
108 G4double invMas[ maxParticles ];
109 G4double sum( 0 );
110
111 for ( int i( 0 ); i < nmbOfOutputParticles; ++i )
112 {
113 sum += outVec[ i ].mass / GeV;
114 invMas[ i ] = rno[ i ] * te_minus_tm / GeV + sum;
115 }
116
117 //
118 //-----> compute the weight of the current event
119 //
120 G4double wt( maxWeight );
121 G4double pd[ maxParticles ];
122
123 for ( int i( 0 ); i < nmbOfOutputParticles - 1; ++i )
124 {
125 pd[ i ] = PDK( invMas[ i + 1 ], invMas[ i ],
126 outVec[ i + 1 ].mass / GeV );
127 wt *= pd[ i ];
128 }
129
130 //
131 //-----> complete specification of event (Raubold-Lynch method)
132 //
133 outVec[ 0 ].lVec->setPx( 0. );
134 outVec[ 0 ].lVec->setPy( pd[ 0 ] );
135 outVec[ 0 ].lVec->setPz( 0. );
136 outVec[ 0 ].lVec->setE( std::sqrt( pd[ 0 ] * pd[ 0 ] +
137 outVec[ 0 ].mass / GeV *
138 outVec[ 0 ].mass / GeV ) );
139
140 G4int i( 1 );
141
142 while ( true )
143 {
144 outVec[ i ].lVec->setPx( 0. );
145 outVec[ i ].lVec->setPy( -pd[ i - 1 ] );
146 outVec[ i ].lVec->setPz( 0. );
147 outVec[ i ].lVec->setE( std::sqrt( pd[ i - 1 ] * pd[ i - 1 ] +
148 outVec[ i ].mass / GeV *
149 outVec[ i ].mass / GeV ) );
150
151 G4double cZ( 2 * G4UniformRand() - 1 );
152 G4double sZ( std::sqrt( 1 - cZ * cZ ) );
153 G4double angY( 2 * pi * G4UniformRand() );
154 G4double cY( std::cos( angY ) );
155 G4double sY( std::sin( angY ) );
156
157 for ( int j( 0 ); j <= i; ++j )
158 {
159 G4LorentzVector * v( outVec[ j ].lVec );
160 G4double x( v->px() );
161 G4double y( v->py() );
162 v->setPx( cZ * x - sZ * y );
163 v->setPy( sZ * x + cZ * y ); // rotation around Z
164 x = v->px();
165 G4double z( v->pz() );
166 v->setPx( cY * x - sY * z );
167 v->setPz( sY * x + cY * z ); // rotation around Y
168 }
169
170 if ( i == nmbOfOutputParticles - 1 )
171 break;
172
173 G4double beta( pd[ i ] / std::sqrt( pd[ i ] * pd[ i ] +
174 invMas[ i ] * invMas[ i ] ) );
175 for ( int j( 0 ); j <= i; ++j )
176 outVec[ j ].lVec->boost( 0, beta, 0 );
177
178 ++i;
179 }
180
181 for ( int j( 0 ); j < nmbOfOutputParticles; ++j )
182 *outVec[ j ].lVec *= GeV;
183
184 //
185 //---> return the weigth of event
186 //
187 return wt;
188 }
189
190
191 void CexmcReimplementedGenbod::ParticleChangeHook( void )
192 {
193 nmbOfOutputParticles = outVec.size();
194
195 if ( nmbOfOutputParticles < 2 || nmbOfOutputParticles > maxParticles )
196 throw CexmcException( CexmcKinematicsException );
197
198 SetMaxWeight();
199 }
200
201
202 void CexmcReimplementedGenbod::FermiEnergyDepStatusChangeHook( void )
203 {
204 SetMaxWeight();
205 }
206
207
208 void CexmcReimplementedGenbod::SetMaxWeight( void )
209 {
210 G4double te_minus_tm( totalEnergy - totalMass );
211
212 if ( fermiEnergyDepIsOn )
213 {
214 // ffq[] = pi * (2*pi)**(N-2) / (N-2)!
215 G4double ffq[] = { 0
216 ,3.141592, 19.73921, 62.01255, 129.8788, 204.0131
217 ,256.3704, 268.4705, 240.9780, 189.2637
218 ,132.1308, 83.0202, 47.4210, 24.8295
219 ,12.0006, 5.3858, 2.2560, 0.8859 };
220 maxWeight =
221 std::pow( te_minus_tm / GeV, nmbOfOutputParticles - 2 ) *
222 ffq[ nmbOfOutputParticles - 1 ] / ( totalEnergy / GeV );
223 }
224 else
225 {
226 G4double emmax( ( te_minus_tm + outVec[ 0 ].mass ) / GeV );
227 G4double emmin( 0. );
228 G4double wtmax( 1. );
229
230 for ( G4int i( 1 ); i < nmbOfOutputParticles; ++i )
231 {
232 emmin += outVec[ i - 1 ].mass / GeV;
233 emmax += outVec[ i ].mass / GeV;
234 wtmax *= PDK( emmax, emmin, outVec[ i ].mass / GeV );
235 }
236 maxWeight = 1 / wtmax;
237 }
238 }
239
240