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25 //
26 //
27 // Multibody "phase space" generator using Kopylov's algorithm
28 //
29 // Author: Michael Kelsey (SLAC) <kelsey@slac.stanford.edu>
30
31 #include "G4HadPhaseSpaceKopylov.hh"
32 #include "G4LorentzVector.hh"
33 #include "G4Pow.hh"
34 #include "Randomize.hh"
35 #include <vector>
36 #include <algorithm>
37 #include <numeric>
38 #include <cmath>
39
40
41 // Generator
42
43 void G4HadPhaseSpaceKopylov::
44 GenerateMultiBody(G4double initialMass,
45 const std::vector<G4double>& masses,
46 std::vector<G4LorentzVector>& finalState) {
47 if (GetVerboseLevel()) G4cout << GetName() << "::GenerateMultiBody" << G4endl;
48
49 finalState.clear();
50
51 G4int N = (G4int)masses.size();
52 finalState.resize(N);
53
54 G4double mtot = std::accumulate(masses.begin(), masses.end(), 0.0);
55 G4double mu = mtot;
56 G4double PFragMagCM = 0.0;
57 G4double Mass = initialMass;
58 G4double T = Mass-mtot;
59 G4LorentzVector PFragCM(0.0,0.0,0.0,0.0);
60 G4LorentzVector PRestCM(0.0,0.0,0.0,0.0);
61 G4LorentzVector PRestLab(0.0,0.0,0.0,Mass);
62
63 for (G4int k=N-1; k>0; --k) {
64 mu -= masses[k];
65 T *= (k>1) ? BetaKopylov(k) : 0.;
66
67 G4double RestMass = mu + T;
68
69 PFragMagCM = TwoBodyMomentum(Mass,masses[k],RestMass);
70
71 // Create a unit vector with a random direction isotropically distributed
72 G4ThreeVector RandVector = UniformVector(PFragMagCM);
73
74 PFragCM.setVectM(RandVector,masses[k]);
75 PRestCM.setVectM(-RandVector,RestMass);
76
77 G4ThreeVector BoostV = PRestLab.boostVector();
78
79 PFragCM.boost(BoostV);
80 PRestCM.boost(BoostV);
81 PRestLab = PRestCM;
82 Mass = RestMass;
83 finalState[k] = PFragCM;
84 }
85
86 finalState[0] = PRestLab;
87 }
88
89
90 // Generate scale factor for final state particle
91
92 G4double G4HadPhaseSpaceKopylov::BetaKopylov(G4int K) const {
93 G4Pow* g4pow = G4Pow::GetInstance();
94
95 G4int N = 3*K - 5;
96 G4double xN = G4double(N);
97 G4double Fmax = std::sqrt(g4pow->powN(xN/(xN+1.),N)/(xN+1.));
98
99 G4double F, chi;
100 const G4int maxNumberOfLoops = 10000;
101 G4int loopCounter = 0;
102 do {
103 chi = G4UniformRand();
104 F = std::sqrt(g4pow->powN(chi,N)*(1.-chi));
105 } while ( ( Fmax*G4UniformRand() > F ) && ++loopCounter < maxNumberOfLoops ); /* Loop checking, 02.11.2015, A.Ribon */
106 if ( loopCounter >= maxNumberOfLoops ) {
107 G4ExceptionDescription ed;
108 ed << " Failed sampling after maxNumberOfLoops attempts : forced exit" << G4endl;
109 G4Exception( " G4HadPhaseSpaceKopylov::BetaKopylov ", "HAD_KOPYLOV_001", JustWarning, ed );
110 }
111
112 return chi;
113 }
114