G4TauLeptonicDecayChannel.cc

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// G4TauLeptonicDecayChannel class implementation
//
// Author: H.Kurashige, 30 May 1997
// --------------------------------------------------------------------
 
#include "G4TauLeptonicDecayChannel.hh"
 
#include "G4DecayProducts.hh"
#include "G4LorentzRotation.hh"
#include "G4LorentzVector.hh"
#include "G4ParticleDefinition.hh"
#include "G4PhysicalConstants.hh"
#include "G4SystemOfUnits.hh"
#include "G4VDecayChannel.hh"
#include "Randomize.hh"
 
G4TauLeptonicDecayChannel::G4TauLeptonicDecayChannel(const G4String& theParentName, G4double theBR,
                                                     const G4String& theLeptonName)
  : G4VDecayChannel("Tau Leptonic Decay", 1)
{
  // set names for daughter particles
  if (theParentName == "tau+") {
    SetBR(theBR);
    SetParent("tau+");
    SetNumberOfDaughters(3);
    if ((theLeptonName == "e-" || theLeptonName == "e+")) {
      SetDaughter(0, "e+");
      SetDaughter(1, "nu_e");
      SetDaughter(2, "anti_nu_tau");
    }
    else {
      SetDaughter(0, "mu+");
      SetDaughter(1, "nu_mu");
      SetDaughter(2, "anti_nu_tau");
    }
  }
  else if (theParentName == "tau-") {
    SetBR(theBR);
    SetParent("tau-");
    SetNumberOfDaughters(3);
    if ((theLeptonName == "e-" || theLeptonName == "e+")) {
      SetDaughter(0, "e-");
      SetDaughter(1, "anti_nu_e");
      SetDaughter(2, "nu_tau");
    }
    else {
      SetDaughter(0, "mu-");
      SetDaughter(1, "anti_nu_mu");
      SetDaughter(2, "nu_tau");
    }
  }
  else {
#ifdef G4VERBOSE
    if (GetVerboseLevel() > 0) {
      G4cout << "G4TauLeptonicDecayChannel:: constructor :";
      G4cout << " parent particle is not tau but ";
      G4cout << theParentName << G4endl;
    }
#endif
  }
}
 
G4TauLeptonicDecayChannel&
G4TauLeptonicDecayChannel::operator=(const G4TauLeptonicDecayChannel& right)
{
  if (this != &right) {
    kinematics_name = right.kinematics_name;
    verboseLevel = right.verboseLevel;
    rbranch = right.rbranch;
 
    // copy parent name
    parent_name = new G4String(*right.parent_name);
 
    // clear daughters_name array
    ClearDaughtersName();
 
    // recreate array
    numberOfDaughters = right.numberOfDaughters;
    if (numberOfDaughters > 0) {
      if (daughters_name != nullptr) ClearDaughtersName();
      daughters_name = new G4String*[numberOfDaughters];
      // copy daughters name
      for (G4int index = 0; index < numberOfDaughters; ++index) {
        daughters_name[index] = new G4String(*right.daughters_name[index]);
      }
    }
  }
  return *this;
}
 
G4DecayProducts* G4TauLeptonicDecayChannel::DecayIt(G4double)
{
  // this version neglects muon polarization
  //              assumes the pure V-A coupling
  //              gives incorrect energy spectrum for neutrinos
 
#ifdef G4VERBOSE
  if (GetVerboseLevel() > 1) G4cout << "G4TauLeptonicDecayChannel::DecayIt()";
#endif
 
  CheckAndFillParent();
  CheckAndFillDaughters();
 
  // parent mass
  G4double parentmass = G4MT_parent->GetPDGMass();
 
  // daughters'mass
  const G4int N_DAUGHTER = 3;
  G4double daughtermass[N_DAUGHTER];
  for (G4int index = 0; index < N_DAUGHTER; ++index) {
    daughtermass[index] = G4MT_daughters[index]->GetPDGMass();
  }
 
  // create parent G4DynamicParticle at rest
  G4ThreeVector dummy;
  auto parentparticle = new G4DynamicParticle(G4MT_parent, dummy, 0.0);
  // create G4Decayproducts
  auto products = new G4DecayProducts(*parentparticle);
  delete parentparticle;
 
  // calculate daughter momentum
  G4double daughtermomentum[N_DAUGHTER];
 
  // calculate lepton momentum
  G4double pmax = (parentmass * parentmass - daughtermass[0] * daughtermass[0]) / 2. / parentmass;
  G4double p, e;
  G4double r;
  const std::size_t MAX_LOOP = 10000;
  for (std::size_t loop_counter = 0; loop_counter < MAX_LOOP; ++loop_counter) {
    // determine momentum/energy
    r = G4UniformRand();
    p = pmax * G4UniformRand();
    e = std::sqrt(p * p + daughtermass[0] * daughtermass[0]);
    if (r < spectrum(p, e, parentmass, daughtermass[0])) break;
  }
 
  // create daughter G4DynamicParticle
  // daughter 0 (lepton)
  daughtermomentum[0] = p;
  G4double costheta, sintheta, phi, sinphi, cosphi;
  costheta = 2. * G4UniformRand() - 1.0;
  sintheta = std::sqrt((1.0 - costheta) * (1.0 + costheta));
  phi = twopi * G4UniformRand() * rad;
  sinphi = std::sin(phi);
  cosphi = std::cos(phi);
  G4ThreeVector direction0(sintheta * cosphi, sintheta * sinphi, costheta);
  auto daughterparticle =
    new G4DynamicParticle(G4MT_daughters[0], direction0 * daughtermomentum[0]);
  products->PushProducts(daughterparticle);
 
  // daughter 1 ,2 (nutrinos)
  // create neutrinos in the C.M frame of two neutrinos
  G4double energy2 = parentmass - e;
  G4double vmass = std::sqrt((energy2 - daughtermomentum[0]) * (energy2 + daughtermomentum[0]));
  G4double beta = -1.0 * daughtermomentum[0] / energy2;
  G4double costhetan = 2. * G4UniformRand() - 1.0;
  G4double sinthetan = std::sqrt((1.0 - costhetan) * (1.0 + costhetan));
  G4double phin = twopi * G4UniformRand() * rad;
  G4double sinphin = std::sin(phin);
  G4double cosphin = std::cos(phin);
 
  G4ThreeVector direction1(sinthetan * cosphin, sinthetan * sinphin, costhetan);
  auto daughterparticle1 = new G4DynamicParticle(G4MT_daughters[1], direction1 * (vmass / 2.));
  auto daughterparticle2 =
    new G4DynamicParticle(G4MT_daughters[2], direction1 * (-1.0 * vmass / 2.));
 
  // boost to the muon rest frame
  G4LorentzVector p4;
  p4 = daughterparticle1->Get4Momentum();
  p4.boost(direction0.x() * beta, direction0.y() * beta, direction0.z() * beta);
  daughterparticle1->Set4Momentum(p4);
  p4 = daughterparticle2->Get4Momentum();
  p4.boost(direction0.x() * beta, direction0.y() * beta, direction0.z() * beta);
  daughterparticle2->Set4Momentum(p4);
  products->PushProducts(daughterparticle1);
  products->PushProducts(daughterparticle2);
  daughtermomentum[1] = daughterparticle1->GetTotalMomentum();
  daughtermomentum[2] = daughterparticle2->GetTotalMomentum();
 
  // output message
#ifdef G4VERBOSE
  if (GetVerboseLevel() > 1) {
    G4cout << "G4TauLeptonicDecayChannel::DecayIt ";
    G4cout << "  create decay products in rest frame " << G4endl;
    products->DumpInfo();
  }
#endif
  return products;
}
 
G4double G4TauLeptonicDecayChannel::spectrum(G4double p, G4double e, G4double mtau, G4double ml)
{
  G4double f1;
  f1 = 3.0 * e * (mtau * mtau + ml * ml) - 4.0 * mtau * e * e - 2.0 * mtau * ml * ml;
  return p * (f1) / (mtau * mtau * mtau * mtau) / (0.6);
}