G4HadPhaseSpaceKopylov.cc

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// $Id$
//
// Multibody "phase space" generator using Kopylov's algorithm
//
// Author:  Michael Kelsey (SLAC) <kelsey@slac.stanford.edu>

#include "G4HadPhaseSpaceKopylov.hh"
#include "G4LorentzVector.hh"
#include "G4Pow.hh"
#include "Randomize.hh"
#include <vector>
#include <algorithm>
#include <numeric>
#include <cmath>


// Generator

void G4HadPhaseSpaceKopylov::
GenerateMultiBody(G4double initialMass,
          const std::vector<G4double>& masses,
          std::vector<G4LorentzVector>& finalState) {
  if (GetVerboseLevel()) G4cout << GetName() << "::GenerateMultiBody" << G4endl;

  finalState.clear();

  size_t N = masses.size();
  finalState.resize(N);

  G4double mtot = std::accumulate(masses.begin(), masses.end(), 0.0);
  G4double mu = mtot;
  G4double PFragMagCM = 0.0;
  G4double Mass = initialMass;
  G4double T = Mass-mtot;
  G4LorentzVector PFragCM(0.0,0.0,0.0,0.0);
  G4LorentzVector PRestCM(0.0,0.0,0.0,0.0);
  G4LorentzVector PRestLab(0.0,0.0,0.0,Mass);

  for (size_t k=N-1; k>0; --k) {
    mu -= masses[k];
    T *= (k>1) ? BetaKopylov(k) : 0.;
    
    G4double RestMass = mu + T;
    
    PFragMagCM = TwoBodyMomentum(Mass,masses[k],RestMass);
    
    // Create a unit vector with a random direction isotropically distributed
    G4ThreeVector RandVector = UniformVector(PFragMagCM);
    
    PFragCM.setVectM(RandVector,masses[k]);
    PRestCM.setVectM(-RandVector,RestMass);

    G4ThreeVector BoostV = PRestLab.boostVector();
    
    PFragCM.boost(BoostV);
    PRestCM.boost(BoostV);
    PRestLab = PRestCM;
    Mass = RestMass;
    finalState[k] = PFragCM;
  }
  
  finalState[0] = PRestLab;
}


// Generate scale factor for final state particle

G4double G4HadPhaseSpaceKopylov::BetaKopylov(G4int K) const {
  G4Pow* g4pow = G4Pow::GetInstance();

  G4int N = 3*K - 5;
  G4double xN = G4double(N);
  G4double Fmax = std::sqrt(g4pow->powN(xN/(xN+1.),N)/(xN+1.)); 

  G4double F, chi;
  do {
    chi = G4UniformRand();
    F = std::sqrt(g4pow->powN(chi,N)*(1.-chi));      
  } while ( Fmax*G4UniformRand() > F);  
  return chi;
}