G4NuTauNucleusCcModel.cc

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// $Id: G4NuTauNucleusCcModel.cc 91806 2015-08-06 12:20:45Z gcosmo $
//
// Geant4 Header : G4NuTauNucleusCcModel
//
// Author : V.Grichine 12.2.19
//  
 
#include <iostream>
#include <fstream>
#include <sstream>
 
#include "G4NuTauNucleusCcModel.hh"
// #include "G4NuMuNuclCcDistrKR.hh" 
 
// #include "G4NuMuResQX.hh" 
 
#include "G4SystemOfUnits.hh"
#include "G4ParticleTable.hh"
#include "G4ParticleDefinition.hh"
#include "G4IonTable.hh"
#include "Randomize.hh"
#include "G4RandomDirection.hh"
// #include "G4Threading.hh"
 
// #include "G4Integrator.hh"
#include "G4DataVector.hh"
#include "G4PhysicsTable.hh"
/*
#include "G4CascadeInterface.hh"
// #include "G4BinaryCascade.hh"
#include "G4TheoFSGenerator.hh"
#include "G4LundStringFragmentation.hh"
#include "G4ExcitedStringDecay.hh"
#include "G4FTFModel.hh"
// #include "G4BinaryCascade.hh"
#include "G4HadFinalState.hh"
#include "G4HadSecondary.hh"
#include "G4HadronicInteractionRegistry.hh"
// #include "G4INCLXXInterface.hh"
#include "G4QGSModel.hh"
#include "G4QGSMFragmentation.hh"
#include "G4QGSParticipants.hh"
*/
#include "G4KineticTrack.hh"
#include "G4DecayKineticTracks.hh"
#include "G4KineticTrackVector.hh"
#include "G4Fragment.hh"
#include "G4NucleiProperties.hh"
#include "G4ReactionProductVector.hh"
 
#include "G4GeneratorPrecompoundInterface.hh"
#include "G4PreCompoundModel.hh"
#include "G4ExcitationHandler.hh"
 
 
#include "G4TauMinus.hh"
#include "G4TauPlus.hh"
#include "G4Nucleus.hh"
#include "G4LorentzVector.hh"
 
using namespace std;
using namespace CLHEP;
 
#ifdef G4MULTITHREADED
    G4Mutex G4NuTauNucleusCcModel::numuNucleusModel = G4MUTEX_INITIALIZER;
#endif     
 
 
G4NuTauNucleusCcModel::G4NuTauNucleusCcModel(const G4String& name) 
  : G4NeutrinoNucleusModel(name)
{
  fData = fMaster = false;
  fMtau = 1.77686*GeV;
  theTauMinus = G4TauMinus::TauMinus();
  theTauPlus = G4TauPlus::TauPlus();
  InitialiseModel();  
}
 
 
G4NuTauNucleusCcModel::~G4NuTauNucleusCcModel()
{}
 
 
void G4NuTauNucleusCcModel::ModelDescription(std::ostream& outFile) const
{
 
    outFile << "G4NuTauNucleusCcModel is a tau neutrino-nucleus (charge current)  scattering\n"
            << "model which uses the standard model \n"
            << "transfer parameterization.  The model is fully relativistic\n";
 
}
 
/////////////////////////////////////////////////////////
//
// Read data from G4PARTICLEXSDATA (locally PARTICLEXSDATA)
 
void G4NuTauNucleusCcModel::InitialiseModel()
{
  G4String pName  = "nu_mu";
  // G4String pName  = "anti_nu_mu";
  
  G4int nSize(0), i(0), j(0), k(0);
 
  if(!fData)
  { 
#ifdef G4MULTITHREADED
    G4MUTEXLOCK(&numuNucleusModel);
    if(!fData)
    { 
#endif     
      fMaster = true;
#ifdef G4MULTITHREADED
    }
    G4MUTEXUNLOCK(&numuNucleusModel);
#endif
  }
  
  if(fMaster)
  {  
    const char* path = G4FindDataDir("G4PARTICLEXSDATA");
    std::ostringstream ost1, ost2, ost3, ost4;
    ost1 << path << "/" << "neutrino" << "/" << pName << "/xarraycckr";
 
    std::ifstream filein1( ost1.str().c_str() );
 
    // filein.open("$PARTICLEXSDATA/");
 
    filein1>>nSize;
 
    for( k = 0; k < fNbin; ++k )
    {
      for( i = 0; i <= fNbin; ++i )
      {
        filein1 >> fNuMuXarrayKR[k][i];
        // G4cout<< fNuMuXarrayKR[k][i] << "  ";
      }
    }
    // G4cout<<G4endl<<G4endl;
 
    ost2 << path << "/" << "neutrino" << "/" << pName << "/xdistrcckr";
    std::ifstream  filein2( ost2.str().c_str() );
 
    filein2>>nSize;
 
    for( k = 0; k < fNbin; ++k )
    {
      for( i = 0; i < fNbin; ++i )
      {
        filein2 >> fNuMuXdistrKR[k][i];
        // G4cout<< fNuMuXdistrKR[k][i] << "  ";
      }
    }
    // G4cout<<G4endl<<G4endl;
 
    ost3 << path << "/" << "neutrino" << "/" << pName << "/q2arraycckr";
    std::ifstream  filein3( ost3.str().c_str() );
 
    filein3>>nSize;
 
    for( k = 0; k < fNbin; ++k )
    {
      for( i = 0; i <= fNbin; ++i )
      {
        for( j = 0; j <= fNbin; ++j )
        {
          filein3 >> fNuMuQarrayKR[k][i][j];
          // G4cout<< fNuMuQarrayKR[k][i][j] << "  ";
        }
      }
    }
    // G4cout<<G4endl<<G4endl;
 
    ost4 << path << "/" << "neutrino" << "/" << pName << "/q2distrcckr";
    std::ifstream  filein4( ost4.str().c_str() );
 
    filein4>>nSize;
 
    for( k = 0; k < fNbin; ++k )
    {
      for( i = 0; i <= fNbin; ++i )
      {
        for( j = 0; j < fNbin; ++j )
        {
          filein4 >> fNuMuQdistrKR[k][i][j];
          // G4cout<< fNuMuQdistrKR[k][i][j] << "  ";
        }
      }
    }
    fData = true;
  }
}
 
/////////////////////////////////////////////////////////
 
G4bool G4NuTauNucleusCcModel::IsApplicable(const G4HadProjectile & aPart, 
                            G4Nucleus & )
{
  G4bool result  = false;
  G4String pName = aPart.GetDefinition()->GetParticleName();
  G4double energy = aPart.GetTotalEnergy();
  
  if(  pName == "nu_tau"  // || pName == "anti_nu_tau" )  
        &&
        energy > fMinNuEnergy                                )
  {
    result = true;
  }
 
  return result;
}
 
/////////////////////////////////////////// ClusterDecay ////////////////////////////////////////////////////////////
//
//
 
G4HadFinalState* G4NuTauNucleusCcModel::ApplyYourself(
         const G4HadProjectile& aTrack, G4Nucleus& targetNucleus)
{
  theParticleChange.Clear();
  fProton = f2p2h = fBreak = false;
  fCascade = fString  = false;
  fLVh = fLVl = fLVt = fLVcpi = G4LorentzVector(0.,0.,0.,0.);
 
  const G4HadProjectile* aParticle = &aTrack;
  G4double energy = aParticle->GetTotalEnergy();
 
  G4String pName  = aParticle->GetDefinition()->GetParticleName();
 
  if( energy < fMinNuEnergy ) 
  {
    theParticleChange.SetEnergyChange(energy);
    theParticleChange.SetMomentumChange(aTrack.Get4Momentum().vect().unit());
    return &theParticleChange;
  }
 
  SampleLVkr( aTrack, targetNucleus);
 
  if( fBreak == true || fEmu < fMtau ) // ~5*10^-6
  {
    // G4cout<<"ni, ";
    theParticleChange.SetEnergyChange(energy);
    theParticleChange.SetMomentumChange(aTrack.Get4Momentum().vect().unit());
    return &theParticleChange;
  }
 
  // LVs of initial state
 
  G4LorentzVector lvp1 = aParticle->Get4Momentum();
  G4LorentzVector lvt1( 0., 0., 0., fM1 );
  G4double mPip = G4ParticleTable::GetParticleTable()->FindParticle(211)->GetPDGMass();
 
  // 1-pi by fQtransfer && nu-energy
  G4LorentzVector lvpip1( 0., 0., 0., mPip );
  G4LorentzVector lvsum, lv2, lvX;
  G4ThreeVector eP;
  G4double cost(1.), sint(0.), phi(0.), muMom(0.), massX2(0.), massX(0.), massR(0.), eCut(0.);
  G4DynamicParticle* aLept = nullptr; // lepton lv
 
  G4int Z = targetNucleus.GetZ_asInt();
  G4int A = targetNucleus.GetA_asInt();
  G4double  mTarg = targetNucleus.AtomicMass(A,Z);
  G4int pdgP(0), qB(0);
  // G4double mSum = G4ParticleTable::GetParticleTable()->FindParticle(2212)->GetPDGMass() + mPip;
 
  G4int iPi     = GetOnePionIndex(energy);
  G4double p1pi = GetNuMuOnePionProb( iPi, energy);
 
  if( p1pi > G4UniformRand()  && fCosTheta > 0.9  ) // && fQtransfer < 0.95*GeV ) // mu- & coherent pion + nucleus
  {
    // lvsum = lvp1 + lvpip1;
    lvsum = lvp1 + lvt1;
    // cost = fCosThetaPi;
    cost = fCosTheta;
    sint = std::sqrt( (1.0 - cost)*(1.0 + cost) );
    phi  = G4UniformRand()*CLHEP::twopi;
    eP   = G4ThreeVector( sint*std::cos(phi), sint*std::sin(phi), cost );
 
    // muMom = sqrt(fEmuPi*fEmuPi-fMtau*fMtau);
    muMom = sqrt(fEmu*fEmu-fMtau*fMtau);
 
    eP *= muMom;
 
    // lv2 = G4LorentzVector( eP, fEmuPi );
    // lv2 = G4LorentzVector( eP, fEmu );
    lv2 = fLVl;
 
    // lvX = lvsum - lv2;
    lvX = fLVh;
    massX2 = lvX.m2();
    massX = lvX.m();
    massR = fLVt.m();
    
    if ( massX2 <= 0. ) // vmg: very rarely ~ (1-4)e-6 due to big Q2/x, to be improved
    {
      fCascade = true;
      theParticleChange.SetEnergyChange(energy);
      theParticleChange.SetMomentumChange(aTrack.Get4Momentum().vect().unit());
      return &theParticleChange;
    }
    fW2 = massX2;
 
    if(  pName == "nu_tau" )         aLept = new G4DynamicParticle( theTauMinus, lv2 );  
    // else if( pName == "anti_nu_tau") aLept = new G4DynamicParticle( theTauPlus,  lv2 );
    else
    {
      theParticleChange.SetEnergyChange(energy);
      theParticleChange.SetMomentumChange(aTrack.Get4Momentum().vect().unit());
      return &theParticleChange;
    }
    if( pName == "nu_tau" ) pdgP =  211;
    // else                   pdgP = -211;
    // eCut = fMpi + 0.5*(fMpi*fMpi-massX2)/mTarg; // massX -> fMpi
 
    if( A > 1 )
    {
      eCut = (fMpi + mTarg)*(fMpi + mTarg) - (massX + massR)*(massX + massR);
      eCut /= 2.*massR;
      eCut += massX;
    }
    else  eCut = fM1 + fMpi;
 
    if ( lvX.e() > eCut ) // && sqrt( GetW2() ) < 1.4*GeV ) // 
    {
      CoherentPion( lvX, pdgP, targetNucleus);
    }
    else
    {
      fCascade = true;
      theParticleChange.SetEnergyChange(energy);
      theParticleChange.SetMomentumChange(aTrack.Get4Momentum().vect().unit());
      return &theParticleChange;
    } 
    theParticleChange.AddSecondary( aLept, fSecID );
 
    return &theParticleChange;
  }
  else // lepton part in lab
  { 
    lvsum = lvp1 + lvt1;
    cost = fCosTheta;
    sint = std::sqrt( (1.0 - cost)*(1.0 + cost) );
    phi  = G4UniformRand()*CLHEP::twopi;
    eP   = G4ThreeVector( sint*std::cos(phi), sint*std::sin(phi), cost );
 
    muMom = sqrt(fEmu*fEmu-fMtau*fMtau);
 
    eP *= muMom;
 
    lv2 = G4LorentzVector( eP, fEmu );
    lv2 = fLVl;
    lvX = lvsum - lv2;
    lvX = fLVh;
    massX2 = lvX.m2();
 
    if ( massX2 <= 0. ) // vmg: very rarely ~ (1-4)e-6 due to big Q2/x, to be improved
    {
      fCascade = true;
      theParticleChange.SetEnergyChange(energy);
      theParticleChange.SetMomentumChange(aTrack.Get4Momentum().vect().unit());
      return &theParticleChange;
    }
    fW2 = massX2;
 
    if(  pName == "nu_tau" )         aLept = new G4DynamicParticle( theTauMinus, lv2 );  
    // else if( pName == "anti_nu_tau") aLept = new G4DynamicParticle( theTauPlus,  lv2 );
    else
    {
      theParticleChange.SetEnergyChange(energy);
      theParticleChange.SetMomentumChange(aTrack.Get4Momentum().vect().unit());
      return &theParticleChange;
    }
    theParticleChange.AddSecondary( aLept, fSecID );
  }
 
  // hadron part
 
  fRecoil  = nullptr;
  
  if( A == 1 )
  {
    if( pName == "nu_tau" ) qB = 2;
    // else                   qB = 0;
 
    // if( G4UniformRand() > 0.1 ) //  > 0.9999 ) // > 0.0001 ) //
    {
      ClusterDecay( lvX, qB );
    }
    return &theParticleChange;
  }
    /*
    // else
    {
      if( pName == "nu_tau" ) pdgP =  211;
      else                   pdgP = -211;
 
 
      if ( fQtransfer < 0.95*GeV ) // < 0.35*GeV ) //
      {
    if( lvX.m() > mSum ) CoherentPion( lvX, pdgP, targetNucleus);
      }
    }
    return &theParticleChange;
  }
  */
  G4Nucleus recoil;
  G4double ratio = G4double(Z)/G4double(A);
 
  if( ratio > G4UniformRand() ) // proton is excited
  {
    fProton = true;
    recoil = G4Nucleus(A-1,Z-1);
    fRecoil = &recoil;
    if( pName == "nu_tau" ) // (++) state -> p + pi+
    { 
      fMt = G4ParticleTable::GetParticleTable()->FindParticle(2212)->GetPDGMass()
          + G4ParticleTable::GetParticleTable()->FindParticle(211)->GetPDGMass();
    }
    else // (0) state -> p + pi-, n + pi0
    {
      // fMt = G4ParticleTable::GetParticleTable()->FindParticle(2212)->GetPDGMass()
      //     + G4ParticleTable::GetParticleTable()->FindParticle(-211)->GetPDGMass();
    } 
  }
  else // excited neutron
  {
    fProton = false;
    recoil = G4Nucleus(A-1,Z);
    fRecoil = &recoil;
    if( pName == "nu_tau" ) // (+) state -> n + pi+
    {      
      fMt = G4ParticleTable::GetParticleTable()->FindParticle(2112)->GetPDGMass()
          + G4ParticleTable::GetParticleTable()->FindParticle(211)->GetPDGMass();
    }
    else // (-) state -> n + pi-, // n + pi0
    {
      // fMt = G4ParticleTable::GetParticleTable()->FindParticle(2112)->GetPDGMass()
      //     + G4ParticleTable::GetParticleTable()->FindParticle(-211)->GetPDGMass();
    } 
  }
  // G4int       index = GetEnergyIndex(energy);
  G4int nepdg = aParticle->GetDefinition()->GetPDGEncoding();
 
  G4double qeTotRat; // = GetNuMuQeTotRat(index, energy);
  qeTotRat = CalculateQEratioA( Z, A, energy, nepdg);
 
  G4ThreeVector dX = (lvX.vect()).unit();
  G4double eX   = lvX.e();  // excited nucleon
  G4double mX   = sqrt(massX2);
  // G4double pX   = sqrt( eX*eX - mX*mX );
  // G4double sumE = eX + rM;
 
  if( qeTotRat > G4UniformRand() || mX <= fMt ) // || eX <= 1232.*MeV) // QE
  {  
    fString = false;
 
    G4double rM;
    if( fProton ) 
    {  
      fPDGencoding = 2212;
      fMr =  proton_mass_c2;
      recoil = G4Nucleus(A-1,Z-1);
      fRecoil = &recoil;
      rM = recoil.AtomicMass(A-1,Z-1);
    } 
    else // if( pName == "anti_nu_tau" ) 
    {  
      fPDGencoding = 2112;
      fMr =   G4ParticleTable::GetParticleTable()->
    FindParticle(fPDGencoding)->GetPDGMass(); // 939.5654133*MeV;
      recoil = G4Nucleus(A-1,Z);
      fRecoil = &recoil;
      rM = recoil.AtomicMass(A-1,Z);
    } 
    // sumE = eX + rM;   
    G4double eTh = fMr + 0.5*(fMr*fMr - mX*mX)/rM;
 
    if( eX <= eTh ) // vmg, very rarely out of kinematics
    {
      fString = true;
      theParticleChange.SetEnergyChange(energy);
      theParticleChange.SetMomentumChange(aTrack.Get4Momentum().vect().unit());
      return &theParticleChange;
    }
    // FinalBarion( fLVh, 0, fPDGencoding ); // p(n)+deexcited recoil
    FinalBarion( lvX, 0, fPDGencoding ); // p(n)+deexcited recoil
  }
  else // if ( eX < 9500000.*GeV ) // <  25.*GeV) // < 95.*GeV ) // < 2.5*GeV ) //cluster decay
  {  
    if     (  fProton && pName == "nu_tau" )      qB =  2;
    // else if(  fProton && pName == "anti_nu_tau" ) qB =  0;
    else if( !fProton && pName == "nu_tau" )      qB =  1;
    // else if( !fProton && pName == "anti_nu_tau" ) qB = -1;
 
 
      ClusterDecay( lvX, qB );
  }
  return &theParticleChange;
}
 
 
/////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////
 
/////////////////////////////////////////////////
//
// sample x, then Q2
 
void G4NuTauNucleusCcModel::SampleLVkr(const G4HadProjectile & aTrack, G4Nucleus& targetNucleus)
{
  fBreak = false;
  G4int A = targetNucleus.GetA_asInt(), iTer(0), iTerMax(100); 
  G4int Z = targetNucleus.GetZ_asInt(); 
  G4double e3(0.), pMu2(0.), pX2(0.), nMom(0.), rM(0.), hM(0.), tM = targetNucleus.AtomicMass(A,Z);
  G4double Ex(0.), ei(0.), nm2(0.);
  G4double cost(1.), sint(0.), phi(0.), muMom(0.); 
  G4ThreeVector eP, bst;
  const G4HadProjectile* aParticle = &aTrack;
  G4LorentzVector lvp1 = aParticle->Get4Momentum();
 
  if( A == 1 ) // hydrogen, no Fermi motion ???
  {
    fNuEnergy = aParticle->GetTotalEnergy();
    iTer = 0;
 
    do
    {
      fXsample = SampleXkr(fNuEnergy);
      fQtransfer = SampleQkr(fNuEnergy, fXsample);
      fQ2 = fQtransfer*fQtransfer;
 
     if( fXsample > 0. )
      {
        fW2 = fM1*fM1 - fQ2 + fQ2/fXsample; // sample excited hadron mass
        fEmu = fNuEnergy - fQ2/2./fM1/fXsample;
      }
      else
      {
        fW2 = fM1*fM1;
        fEmu = fNuEnergy;
      }
      e3 = fNuEnergy + fM1 - fEmu;
 
      if( e3 < sqrt(fW2) )  G4cout<<"energyX = "<<e3/GeV<<", fW = "<<sqrt(fW2)/GeV<<G4endl;
    
      pMu2 = fEmu*fEmu - fMtau*fMtau;
 
      if(pMu2 < 0.) { fBreak = true; return; }
 
      pX2  = e3*e3 - fW2;
 
      fCosTheta  = fNuEnergy*fNuEnergy  + pMu2 - pX2;
      fCosTheta /= 2.*fNuEnergy*sqrt(pMu2);
      iTer++;
    }
    while( ( abs(fCosTheta) > 1. || fEmu < fMtau ) && iTer < iTerMax );
 
    if( iTer >= iTerMax ) { fBreak = true; return; }
 
    if( abs(fCosTheta) > 1.) // vmg: due to big Q2/x values. To be improved ...
    { 
      G4cout<<"H2: fCosTheta = "<<fCosTheta<<", fEmu = "<<fEmu<<G4endl;
      // fCosTheta = -1. + 2.*G4UniformRand(); 
      if(fCosTheta < -1.) fCosTheta = -1.;
      if(fCosTheta >  1.) fCosTheta =  1.;
    }
    // LVs
 
    G4LorentzVector lvt1  = G4LorentzVector( 0., 0., 0., fM1 );
    G4LorentzVector lvsum = lvp1 + lvt1;
 
    cost = fCosTheta;
    sint = std::sqrt( (1.0 - cost)*(1.0 + cost) );
    phi  = G4UniformRand()*CLHEP::twopi;
    eP   = G4ThreeVector( sint*std::cos(phi), sint*std::sin(phi), cost );
    muMom = sqrt(fEmu*fEmu-fMtau*fMtau);
    eP *= muMom;
    fLVl = G4LorentzVector( eP, fEmu );
 
    fLVh = lvsum - fLVl;
    fLVt = G4LorentzVector( 0., 0., 0., 0. ); // no recoil
  }
  else // Fermi motion, Q2 in nucleon rest frame
  {
    G4Nucleus recoil1( A-1, Z );
    rM = recoil1.AtomicMass(A-1,Z);   
    do
    {
      // nMom = NucleonMomentumBR( targetNucleus ); // BR
      nMom = GgSampleNM( targetNucleus ); // Gg
      Ex = GetEx(A-1, fProton);
      ei = tM - sqrt( (rM + Ex)*(rM + Ex) + nMom*nMom );
      //   ei = 0.5*( tM - s2M - 2*eX );
    
      nm2 = ei*ei - nMom*nMom;
      iTer++;
    }
    while( nm2 < 0. && iTer < iTerMax ); 
 
    if( iTer >= iTerMax ) { fBreak = true; return; }
    
    G4ThreeVector nMomDir = nMom*G4RandomDirection();
 
    if( !f2p2h || A < 3 ) // 1p1h
    {
      // hM = tM - rM;
 
      fLVt = G4LorentzVector( -nMomDir, sqrt( (rM + Ex)*(rM + Ex) + nMom*nMom ) ); // rM ); //
      fLVh = G4LorentzVector(  nMomDir, ei ); // hM); //
    }
    else // 2p2h
    {
      G4Nucleus recoil(A-2,Z-1);
      rM = recoil.AtomicMass(A-2,Z-1)+sqrt(nMom*nMom+fM1*fM1);
      hM = tM - rM;
 
      fLVt = G4LorentzVector( nMomDir, sqrt( rM*rM+nMom*nMom ) );
      fLVh = G4LorentzVector(-nMomDir, sqrt( hM*hM+nMom*nMom )  ); 
    }
    // G4cout<<hM<<", ";
    // bst = fLVh.boostVector();
 
    // lvp1.boost(-bst); // -> nucleon rest system, where Q2 transfer is ???
 
    fNuEnergy  = lvp1.e();
    // G4double mN = fLVh.m(); // better mN = fM1 !? vmg
    iTer = 0;
 
    do // no FM!?, 5.4.20 vmg
    {
      fXsample = SampleXkr(fNuEnergy);
      fQtransfer = SampleQkr(fNuEnergy, fXsample);
      fQ2 = fQtransfer*fQtransfer;
 
      // G4double mR = mN + fM1*(A-1.)*std::exp(-2.0*fQtransfer/mN); // recoil mass in+el
 
      if( fXsample > 0. )
      {
        fW2 = fM1*fM1 - fQ2 + fQ2/fXsample; // sample excited hadron mass
 
        // fW2 = mN*mN - fQ2 + fQ2/fXsample; // sample excited hadron mass
        // fEmu = fNuEnergy - fQ2/2./mR/fXsample; // fM1->mN
 
        fEmu = fNuEnergy - fQ2/2./fM1/fXsample; // fM1->mN
      }
      else
      {
        // fW2 = mN*mN;
 
        fW2 = fM1*fM1; 
        fEmu = fNuEnergy;
      }
      // if(fEmu < 0.) G4cout<<"fEmu = "<<fEmu<<" hM = "<<hM<<G4endl;
      // e3 = fNuEnergy + mR - fEmu;
 
      e3 = fNuEnergy + fM1 - fEmu;
 
      // if( e3 < sqrt(fW2) )  G4cout<<"energyX = "<<e3/GeV<<", fW = "<<sqrt(fW2)/GeV<<G4endl;
    
      pMu2 = fEmu*fEmu - fMtau*fMtau;
      pX2  = e3*e3 - fW2;
 
      if(pMu2 < 0.) { fBreak = true; return; }
 
      fCosTheta  = fNuEnergy*fNuEnergy  + pMu2 - pX2;
      fCosTheta /= 2.*fNuEnergy*sqrt(pMu2);
      iTer++;
    }
    while( ( abs(fCosTheta) > 1. || fEmu < fMtau ) && iTer < iTerMax );
 
    if( iTer >= iTerMax ) { fBreak = true; return; }
 
    if( abs(fCosTheta) > 1.) // vmg: due to big Q2/x values. To be improved ...
    { 
      G4cout<<"FM: fCosTheta = "<<fCosTheta<<", fEmu = "<<fEmu<<G4endl;
      // fCosTheta = -1. + 2.*G4UniformRand(); 
      if( fCosTheta < -1.) fCosTheta = -1.;
      if( fCosTheta >  1.) fCosTheta =  1.;
    }
    // LVs
    // G4LorentzVector lvt1  = G4LorentzVector( 0., 0., 0., mN ); // fM1 );
 
    G4LorentzVector lvt1  = G4LorentzVector( 0., 0., 0., fM1 ); // fM1 );
    G4LorentzVector lvsum = lvp1 + lvt1;
 
    cost = fCosTheta;
    sint = std::sqrt( (1.0 - cost)*(1.0 + cost) );
    phi  = G4UniformRand()*CLHEP::twopi;
    eP   = G4ThreeVector( sint*std::cos(phi), sint*std::sin(phi), cost );
    muMom = sqrt(fEmu*fEmu-fMtau*fMtau);
    eP *= muMom;
    fLVl = G4LorentzVector( eP, fEmu );
    fLVh = lvsum - fLVl;
 
    // if( fLVh.e() < mN || fLVh.m2() < 0.) { fBreak = true; return; }
 
    if( fLVh.e() < fM1 || fLVh.m2() < 0.) { fBreak = true; return; }
 
    // back to lab system
 
    // fLVl.boost(bst);
    // fLVh.boost(bst);
  }
  //G4cout<<iTer<<", "<<fBreak<<"; ";
}
 
//
//
///////////////////////////