MyEvent.cxx

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// Author: Bertrand Bellenot   22/08/02

/*************************************************************************
 * Copyright (C) 1995-2002, Bertrand Bellenot.                           *
 * All rights reserved.                                                  *
 *                                                                       *
 * For the licensing terms see the LICENSE file.                         *
 *************************************************************************/

#include <stdlib.h>

#include <TROOT.h>
#include <TPolyLine3D.h>
#include <TRandom.h>
#include <TParticle.h>
#include <TDecayChannel.h>
#include <TGenPhaseSpace.h>
#include "MyParticle.h"
#include "TVector3.h"
#include "MyEvent.h"
#include "RootShower.h"

//______________________________________________________________________________
//
// MyEvent class implementation
//______________________________________________________________________________

ClassImp(EventHeader)
ClassImp(MyEvent)

TClonesArray *MyEvent::fgParticles = 0;

//______________________________________________________________________________
MyEvent::MyEvent()
{
   // Create an Event object.
   // When the constructor is invoked for the first time, the
   // class static variables fgParticles and fgTracks is 0 and
   // the TClonesArray fgParticles is created.

   if (!fgParticles) fgParticles = new TClonesArray("MyParticle", 1000);
   fgParticles->BypassStreamer(kFALSE);
   fParticles = fgParticles;
   fNparticles = 0;
}

//______________________________________________________________________________
MyEvent::~MyEvent()
{
   // Destructor

   Clear();
}

//______________________________________________________________________________
void MyEvent::Init(Int_t id, Int_t first_particle, Double_t E_0, Double_t B_0)
{
   // Initialize event ...
   // creates detector and set initial values

   Char_t  strtmp[80];
   Int_t i;
   fId = id;
   fB = B_0;

   // generate array of energies threshold used
   // to give a track color related to the particle
   // energy
   for (i=0;i<16;i++)
      fEThreshold[i] = EMass + E_0 / (2 << i);

   Clear();
   Reset();

   if (!fgParticles) fgParticles = new TClonesArray("MyParticle", 1000);
   fParticles = fgParticles;
   fNparticles = 0;

   fTotalParticles = 0;
   fLast = 0;
   fAliveParticles = 1;
   fMatter = 0;

   fDetector.ClearELoss();

   TVector3 location(0.0,fDetector.GetMinY(),0.0);
   TVector3 momentum(0.0,E_0,0.0);

   AddParticle(0,first_particle, location, momentum);
   GetParticle(0)->GenerateTimeOfDecay();

   gTmpLTI = gEventListTree->AddItem(gBaseLTI, GetParticle(0)->GetName());
   gTmpLTI->SetUserData(GetParticle(0));
   sprintf(strtmp,"%1.2e GeV",GetParticle(0)->Energy());
   gEventListTree->SetToolTipItem(gTmpLTI, strtmp);
   gLTI[0] = gTmpLTI;

}

//______________________________________________________________________________
void MyEvent::Clear(Option_t *option)
{
   // Clear tracks and particles arrays

   fgParticles->Delete(option);
   fNparticles = 0;
   fMatter = 0;
}

//______________________________________________________________________________
void MyEvent::Reset(Option_t *option)
{
   // Static function to reset all static objects for this event

   fgParticles->Delete(option);
   fNparticles = 0;
   fMatter = 0;
}

//______________________________________________________________________________
void MyEvent::SetHeader(Int_t i, Int_t run, TDatime date, Int_t primary,
                        Double_t energy)
{
   // set event header with event identification and startup parameters
   
   fEvtHdr.Set(i, run, date, primary, energy);
}

//______________________________________________________________________________
MyParticle *MyEvent::AddParticle(Int_t id, Int_t pdg_code, const TVector3 &pos,
                                 const TVector3 &mom)
{
   // Add a new particle to the list of particles for this event.
   // To avoid calling the very time consuming operator new for each track,
   // the standard but not well know C++ operator "new with placement"
   // is called. If particle[i] is 0, a new MyParticle object will be created
   // otherwise the previous particle[i] will be overwritten.
   
   TClonesArray &parts = *fParticles;
   MyParticle *part = new(parts[fNparticles++])
                          MyParticle(id, pdg_code, CREATED, UNDEFINE, pos, mom);
   part->AddTrack(pos.x(), pos.y(), pos.z(), ParticleColor(id));
   //Save reference to last particle in the collection of particles
   fLastParticle = part;
   return part;
}

//______________________________________________________________________________
Int_t MyEvent::Action(Int_t id)
{
   // main event's action
   
   Int_t  nchild;
   CheckMatter(id);
   if (GetParticle(id)->GetDecayType() == UNDEFINE)
      DefineDecay(id);
   if (GetParticle(id)->GetPdgCode() == PHOTON) {
      // compute the step delta x to be covered by the particle
      TVector3 delta_x(GetParticle(id)->GetvMoment() *
               (CSpeed * fDetector.GetdT(fMatter) / GetParticle(id)->Energy()));
      // check if moved too far (out of detector's bouds)
      if (Move(id, delta_x) == DEAD)
         // set its status as dead
         DeleteParticle(id);
      else {
         // if distance covered is greater than particle's decay length,
         // apply pair production and check if particle is dead. If not,
         // increment total alive particles by the two created children,
         // then set the particle status as dead
         if (GetParticle(id)->GetPassed() >= GetParticle(id)->GetDecayLength()) {
            if (PairCreation(id) == DEAD) return(DEAD);
            else {
               fAliveParticles += 2;
               DeleteParticle(id);
            }
         }
      }
   }
   else if ((GetParticle(id)->GetPdgCode() == NEUTRINO_E) ||
           (GetParticle(id)->GetPdgCode() == NEUTRINO_TAU) ||
           (GetParticle(id)->GetPdgCode() == NEUTRINO_MUON) ||
           (GetParticle(id)->GetPdgCode() == ANTINEUTRINO_E) ||
           (GetParticle(id)->GetPdgCode() == ANTINEUTRINO_TAU) ||
           (GetParticle(id)->GetPdgCode() == ANTINEUTRINO_MUON) ) {
      // if current particle is a neutrino ( or antineutrino )
      // set its status as dead ( estimate its probability of
      // interaction as null )
      DeleteParticle(id);
   }
   else { // particle is not a photon or neutrino
      // if current particle is charged, apply magnetic field influence
      if (GetParticle(id)->GetPDG()->Charge() != 0)
         ScatterAngle(id);
      if ((fB != 0) && (GetParticle(id)->GetPDG()->Charge() != 0))
         MagneticField(id);
      // compute the step delta x to be covered by the particle
      TVector3 delta_x(GetParticle(id)->GetvMoment() * (CSpeed *
                       fDetector.GetdT(fMatter) / GetParticle(id)->Energy()));
      // check if moved too far (out of detector's bouds)
      if (Move(id, delta_x) == DEAD) {
         // set its status as dead
         DeleteParticle(id);
      }
      else {
         // check energy loss, and if too much energy loss
         // ( particle at rest ) set its status as dead
         if (DEDX(id) == DEAD) {
            DeleteParticle(id);
         }
         else {
            // if at end of particle's life time, decay it
            if (CheckDecayTime(id) == 1) {
               // if no child found
               if ((nchild = Decay(id)) == -1) {
                  return(DEAD);
               }
               else {
                  // else increment total alive particles by amount
                  // of particle's children
                  fAliveParticles += nchild;
                  DeleteParticle(id);
               }
            }
            // if not at end of particle's life time, check if distance
            // covered is greater than particle's decay length, apply
            // defined decay type and check if particle is dead. If not,
            // increment total alive particles by the two created children,
            // then set the particle status as dead
            else if (GetParticle(id)->GetPassed() >=
                     GetParticle(id)->GetDecayLength()) {
               switch (GetParticle(id)->GetDecayType()) {
                  case BREMS:
                     if (Bremsstrahlung(id) == DEAD) return(DEAD);
                     else {
                        fAliveParticles += 2;
                        DeleteParticle(id);
                     }
                     break;
                  case CONVERSION:
                     if (PairCreation(id) == DEAD) return(DEAD);
                     else {
                        fAliveParticles += 2;
                        DeleteParticle(id);
                     }
                     break;
               }
            }
         }
      }
   }
   return(ALIVE);
}

//______________________________________________________________________________
Double_t MyEvent::BremsProb(Int_t id)
{
   // Check if bremsstrahlung is allowed and generate
   // a random decay length related to detector's material
   // radiation length (X0)
   
   Double_t p, retval;

   if (GetParticle(id)->Energy() > GetParticle(id)->GetMass()) {
      p = gRandom->Uniform(0.0, 1.0);
      retval = (-fDetector.GetX0(fMatter))*TMath::Log(p);
      return (retval);
   }
   else return (-1.);
}

//______________________________________________________________________________
Int_t MyEvent::Bremsstrahlung(Int_t id)
{
    // compute bremsstrahlung for particle "id"
    
   Double_t  ratio;
   Int_t     d_num1,d_num2;
   Char_t    strtmp[80];
   MyParticle *part;

   // find two ids for children particles
   if ((FindFreeId(&d_num1) != DEAD) && (FindFreeId(&d_num2) != DEAD)) {
      // compute the particle's energy ratio...
      ratio = (GetParticle(id)->Energy() - GetParticle(id)->GetMass()) /
              (2 * GetParticle(id)->P());
      // create first child if fact, electron continues with less energy
      // and in a different direction. To that end the electron is added
      // to its own list of children, because otherwise it would vanish.
      part = AddParticle(d_num1, GetParticle(id)->GetPdgCode(),
                         GetParticle(id)->GetvLocation(),
                         GetParticle(id)->GetvMoment() * ratio);
      part->SetFirstMother(id);
      // as its first child is in fact the same particle,
      // keep the same decay time
      part->SetTimeOfDecay(GetParticle(id)->GetTimeOfDecay());
      GetParticle(id)->SetChild(0, d_num1);
      // add a track related to this particle

      // add a particle related list tree item to the event list tree
      gTmpLTI = gEventListTree->AddItem(gLTI[id], part->GetName());
      gTmpLTI->SetUserData(part);
      sprintf(strtmp,"%1.2e GeV",part->Energy());
      gEventListTree->SetToolTipItem(gTmpLTI, strtmp);
      gLTI[d_num1] = gTmpLTI;

      // create second child
      part = AddParticle(d_num2,PHOTON, GetParticle(id)->GetvLocation(),
                         GetParticle(id)->GetvMoment() * ratio);
      part->SetFirstMother(id);
      // generate time of decay (not used in this case, as it is a photon,
      // but to keep the same philosophy in every case...
      part->GenerateTimeOfDecay();
      GetParticle(id)->SetChild(1, d_num2);

      // add a track related to this particle

      // add a particle related list tree item to the event list tree
      gTmpLTI = gEventListTree->AddItem(gLTI[id], part->GetName());
      gTmpLTI->SetUserData(part);
      sprintf(strtmp,"%1.2e GeV",part->Energy());
      gEventListTree->SetToolTipItem(gTmpLTI, strtmp);
      gLTI[d_num2] = gTmpLTI;

      // increment number of children by the two created particles
      GetParticle(id)->SetNChildren(2);

      return(ALIVE);
   }
   else return(DEAD);
}

//______________________________________________________________________________
Int_t MyEvent::CheckDecayTime(Int_t id)
{
   // Check decay time for particle "id".

   if ( (GetParticle(id)->GetPdgCode() == PHOTON) ||
        (GetParticle(id)->GetPdgCode() == ELECTRON) ||
        (GetParticle(id)->GetPdgCode() == POSITRON ))
      return 0;
   Double_t timeofdecay = GetParticle(id)->GetTimeOfDecay();
   if (timeofdecay == 0.0)  return 0;
   Double_t distToDecay = timeofdecay * 0.996 * CSpeed;
   // check if actual particle life is greater than particle life time
   if (GetParticle(id)->GetPassed() >= distToDecay) {
      return 1;
   }
   return 0;
}

//______________________________________________________________________________
void MyEvent::CheckMatter(Int_t id)
{
   // Check material into which the particle "id" is. 

   TGeoNode *Node = gGeoManager->FindNode(
             GetParticle(id)->GetvLocation().x(),
             GetParticle(id)->GetvLocation().y(),
             GetParticle(id)->GetvLocation().z());
   if (Node) fMatter = Node->GetNumber();
}

//______________________________________________________________________________
Int_t MyEvent::Decay(Int_t id)
{
   // Decay the particle "id".

   Char_t   strtmp[80];
   Int_t    d_num[5];
   Int_t    n_daughters;
   Int_t    ptype[5];
   Double_t mass[5];
   Int_t    i, index;
   Double_t sumBR = 0.0;
   MyParticle *Particle[5];
   MyParticle *part;

   // compute total branching ratio
   for (i=0;i<GetParticle(id)->GetPDG()->NDecayChannels();i++) {
      sumBR += GetParticle(id)->GetPDG()->DecayChannel(i)->BranchingRatio();
   }
   // choose random decay in respect to the branching ratio
again:
   float r = gRandom->Uniform(sumBR);
   index = 0;
   while
      ((r -= GetParticle(id)->GetPDG()->DecayChannel(index)->BranchingRatio())
       > 0 && index < GetParticle(id)->GetPDG()->NDecayChannels()) index++;

   // set number of daughters
   n_daughters = GetParticle(id)->GetPDG()->DecayChannel(index)->NDaughters();
   for (i=0;i<n_daughters;i++) {
      // create temporary child particle to obtain its mass
      ptype[i] =
         GetParticle(id)->GetPDG()->DecayChannel(index)->DaughterPdgCode(i);
      if (TMath::Abs(ptype[i]) < 6) // it is a quark...do it again
         goto again;
      Particle[i] = new MyParticle(0,ptype[i], CREATED, UNDEFINE,
                                   GetParticle(id)->GetvLocation(),
                                   GetParticle(id)->GetvMoment());
      mass[i] = Particle[i]->GetMass();
      delete Particle[i];
   }

   // setup the decay
   TLorentzVector W(GetParticle(id)->GetvMoment(), GetParticle(id)->Energy());
   TGenPhaseSpace genPhaseSpace;
   if (!genPhaseSpace.SetDecay( W, n_daughters, mass ))
      return (-1);
   genPhaseSpace.Generate();

   // find ids for children
   for (i=0;i<n_daughters;i++) {
      if (FindFreeId(&d_num[i]) == DEAD) return -1;
   }

   TLorentzVector *p;
   for (i=0;i<n_daughters;i++) {
      p = genPhaseSpace.GetDecay(i);
      // create child
      part = AddParticle(d_num[i], ptype[i], GetParticle(id)->GetvLocation(),
                         p->Vect());
      part->SetFirstMother(id);
      // generate time of decay (may be useful in this case)
      part->GenerateTimeOfDecay();
      GetParticle(id)->SetChild(i, d_num[i]);
      // add a track related to this child

      // add a child related list tree item to the event list tree
      gTmpLTI = gEventListTree->AddItem(gLTI[id], part->GetName());
      gTmpLTI->SetUserData(part);
      sprintf(strtmp,"%1.2e GeV",part->Energy());
      gEventListTree->SetToolTipItem(gTmpLTI, strtmp);
      gLTI[d_num[i]] = gTmpLTI;
   }
   // increment number of children by the number of created particles
   GetParticle(id)->SetNChildren(n_daughters);
   return(n_daughters);
}

//______________________________________________________________________________
void MyEvent::DefineDecay(Int_t id)
{
   // Define decay type for particle "id", then check decay length for it

   Double_t idecay_length = -1.;
   Double_t iactual_length;
   Int_t    idecay_type = CONVERSION;

   if ( (GetParticle(id)->GetPdgCode() == ELECTRON) ||
        (GetParticle(id)->GetPdgCode() == POSITRON)) {
      // check if bremsstrahlung is allowed
      if ( (iactual_length = BremsProb(id)) > 0.) {
         if ( (idecay_length == -1) || (iactual_length < idecay_length) ) {
            idecay_length = iactual_length;
            idecay_type = BREMS;
         }
      }
   }
   else if (GetParticle(id)->GetPdgCode() == PHOTON) {
      // check if pair production is allowed
      if ( (iactual_length = PairProb(id)) > 0. ) {
         if ( (idecay_length == -1) ||
             (iactual_length < idecay_length) ) {
            idecay_length = iactual_length;
            idecay_type = CONVERSION;
         }
      }
   }
   if ( idecay_length > 0) {
      GetParticle(id)->SetDecayType(idecay_type);
      GetParticle(id)->SetDecayLength(idecay_length);
   }
   else {
      GetParticle(id)->SetDecayType(STABLE);
      GetParticle(id)->SetDecayLength(0.0);
   }
}

//______________________________________________________________________________
void MyEvent::DeleteParticle(Int_t id)
{
   // Delete the particle id

   // To be sure that the last track has at least two points
   if (GetParticle(id)->GetTrack(GetParticle(id)->GetNTracks()-1)->GetN() < 2) {
      GetParticle(id)->SetNextPoint(GetParticle(id)->GetTrack(GetParticle(id)->GetNTracks()-1)->GetLineColor());
   }
   // Add this particle's energy loss at the total
   // energy loss into the detector
   fDetector.AddELoss(GetParticle(id)->GetELoss());
   // Mark the particle's status as dead and decrement
   // the total alive particles
   GetParticle(id)->SetStatus(DEAD);
   fAliveParticles --;
}

//______________________________________________________________________________
Int_t MyEvent::DEDX(Int_t id)
{
   // Compute de/dx for particle "id" into detector material
   // for more infos, please refer to the particle data booklet
   // from which the formulas has been extracted

   Double_t gamma,abs_beta,abs_p,abs_loss,dX;

   // if particle's energy is equal to its mass, it is at rest,
   // so set its status as dead
   if (GetParticle(id)->Energy() <= GetParticle(id)->GetMass()) {
      return(DEAD);
   }
   else {
      // absolute value of momentum
      abs_p = GetParticle(id)->P();
      if (abs_p <= 0) {
         // if absolute value of momentum is less or equal to zero,
         // set it to the particle's mass (minimum allowed value for momentum)
         GetParticle(id)->SetMomentum(0.0, 0.0, 0.0,GetParticle(id)->GetMass());
      }
      else {
         // Compute energy loss in detector's material
         // cf Bethe Bloch formula
         TVector3 p_0(GetParticle(id)->GetvMoment() * (1 / abs_p));
         abs_beta = abs_p / GetParticle(id)->Energy();
         dX = fDetector.GetdT(fMatter) * CSpeed * abs_beta;
         abs_beta *= abs_beta;
         if (abs_beta < .9999999999) gamma = 1/TMath::Sqrt(1.0-abs_beta);
         else gamma = MAX_GAMMA;
         abs_loss = (fDetector.GetPreconst(fMatter) * dX / abs_beta) *
                    (TMath::Log(2.0 * GetParticle(id)->GetMass() *
                     gamma * gamma * abs_beta /
                     fDetector.GetI(fMatter)) - abs_beta);
         if (abs_loss < 0) abs_loss = -abs_loss;
         if (abs_loss >= (GetParticle(id)->Energy() -
                         GetParticle(id)->GetMass())) {
            // if energy loss leave less energy to the particle than
            // its mass, set its momentum equal to its mass
            // (minimum allowed value for momentum)
            GetParticle(id)->SetMomentum(0.0, 0.0, 0.0,
                             GetParticle(id)->GetMass());
         }
         else {
            // else decrease its energy by calculated energy loss
            GetParticle(id)->SetMoment(GetParticle(id)->GetvMoment(),
                GetParticle(id)->Energy() - abs_loss);
            abs_p = TMath::Sqrt((GetParticle(id)->Energy() *
                          GetParticle(id)->Energy()) -
                         (GetParticle(id)->GetMass() *
                          GetParticle(id)->GetMass()));
            GetParticle(id)->SetMoment(p_0 * abs_p);
            // Add calculated energy loss at total particle's energy loss
            GetParticle(id)->AddELoss(abs_loss);
         }
      }
      if (GetParticle(id)->Energy() > GetParticle(id)->GetMass()) {
         return(ALIVE);
      }
      else {
         return(DEAD);
      }
   }
}

//______________________________________________________________________________
Int_t MyEvent::FindFreeId(Int_t *FreeId)
{
   // Give next available particle's id.
   
   fTotalParticles++;
   *FreeId = fTotalParticles;
   if (fTotalParticles > fLast) fLast = fTotalParticles;
   return(ALIVE);
}

//______________________________________________________________________________
void MyEvent::MagneticField(Int_t id)
{
   // Extrapolate track in a constant field oriented along X axis
   // translated to C++ from GEANT3 routine GHELX3.
   
   Double_t sint, sintt, tsint, cos1t, sin2;
   Double_t f1, f2, f3, v1, v2, v3;
   Double_t pol = GetParticle(id)->GetPDG()->Charge() / 3.0;
   Double_t h4  = pol * 2.9979251e-04 * fB;
   Double_t tet = -h4 * CSpeed * fDetector.GetdT(fMatter) /
                  GetParticle(id)->P();
   if (TMath::Abs(tet) > 0.15) {
      sint  = TMath::Sin(tet);
      sintt = sint / tet;
      tsint = (tet - sint) / tet;
      sin2  = TMath::Sin(0.5 * tet);
      cos1t = 2.0 * sin2 * sin2 / tet;
   } else {
      tsint = tet * tet / 6.0;
      sintt = 1.0 - tsint;
      sint  = tet * sintt;
      cos1t = 0.5 * tet;
   }
   f1 = -tet * cos1t;
   f2 = sint;
   f3 = tet * cos1t * GetParticle(id)->Px();
   v1 = GetParticle(id)->Px() + (f1 * GetParticle(id)->Px() + f3);
   v2 = GetParticle(id)->Py() + (f1 * GetParticle(id)->Py() + f2 *
        GetParticle(id)->Pz());
   v3 = GetParticle(id)->Pz() + (f1 * GetParticle(id)->Pz() - f2 *
        GetParticle(id)->Py());
   TVector3 new_mom(v1, v2, v3);
   GetParticle(id)->SetMoment(new_mom);
}

//______________________________________________________________________________
Int_t MyEvent::Move(Int_t id, TVector3 &dist)
{
   // Move particle "id" by step dist, update the distance covered
   // then check if out of detector's bounds.

   GetParticle(id)->SetLocation(GetParticle(id)->GetvLocation() + dist);
   GetParticle(id)->SetPassed(GetParticle(id)->GetPassed() + dist.Mag());

   if ((GetParticle(id)->GetvLocation().x() > fDetector.GetMaxX()) ||
      (GetParticle(id)->GetvLocation().x() < fDetector.GetMinX()) ||
      (GetParticle(id)->GetvLocation().y() > fDetector.GetMaxY()) ||
      (GetParticle(id)->GetvLocation().y() < fDetector.GetMinY()) ||
      (GetParticle(id)->GetvLocation().z() > fDetector.GetMaxZ()) ||
      (GetParticle(id)->GetvLocation().z() < fDetector.GetMinZ())) {
      return(DEAD);
   }
   // If not out of bounds, set related Track's next point
   else {
      if ((GetParticle(id)->GetPdgCode() != PHOTON) &&
         (GetParticle(id)->GetPdgCode() != NEUTRINO_E) &&
         (GetParticle(id)->GetPdgCode() != NEUTRINO_MUON) &&
         (GetParticle(id)->GetPdgCode() != NEUTRINO_TAU) &&
         (GetParticle(id)->GetPdgCode() != ANTINEUTRINO_E) &&
         (GetParticle(id)->GetPdgCode() != ANTINEUTRINO_MUON) &&
         (GetParticle(id)->GetPdgCode() != ANTINEUTRINO_TAU) ) {
         GetParticle(id)->SetNextPoint(ParticleColor(id));
      }
      return(ALIVE);
   }
}

//______________________________________________________________________________
Double_t MyEvent::PairProb(Int_t id)
{
   // Check if pair production is allowed and generate
   // a random decay length related to detector's material
   // radiation length (X0).

   Double_t p;

   if (GetParticle(id)->Energy() > 2.0 * EMass) {
      p = gRandom->Uniform(0.0, 1.0);
      return ((-9.)*fDetector.GetX0(fMatter)*TMath::Log(p)/7.);
   }
   return (-1.);
}

//______________________________________________________________________________
Int_t MyEvent::PairCreation(Int_t id)
{
   // Compute the pair production for particle "id"

   Int_t    d_num1, d_num2;
   Char_t   strtmp[80];
   MyParticle *part;
   TGenPhaseSpace genPhaseSpace;

   // setup the decay
   TLorentzVector target(0.0, 0.0, 0.0, 0.00511);
   TLorentzVector beam(GetParticle(id)->GetvMoment(),
                       GetParticle(id)->Energy());
   TLorentzVector W = beam + target;
   Double_t masses[2] = { EMass, EMass } ;

   if (!genPhaseSpace.SetDecay( W, 2, masses ))
      return (DEAD);
   genPhaseSpace.Generate();

   TLorentzVector *p;
   // calculate the location of the decay vertex

   // find two ids for children particles
   if ((FindFreeId(&d_num1) != DEAD) && (FindFreeId(&d_num2) != DEAD)) {
      p = genPhaseSpace.GetDecay(0);
      // create child
      part = AddParticle(d_num1, POSITRON, GetParticle(id)->GetvLocation(),
                         p->Vect());
      part->SetFirstMother(id);

      // generate time of decay (not used in this case, as it is an electron
      // or a positron, but to keep the same philosophy in every case...
      part->GenerateTimeOfDecay();
      GetParticle(id)->SetChild(0, d_num1);
      // add a track related to this particle

      // add a particle related list tree item to the event list tree
      gTmpLTI = gEventListTree->AddItem(gLTI[id], part->GetName());
      gTmpLTI->SetUserData(part);
      sprintf(strtmp,"%1.2e GeV",part->Energy());
      gEventListTree->SetToolTipItem(gTmpLTI, strtmp);
      gLTI[d_num1] = gTmpLTI;

      // create second child
      p = genPhaseSpace.GetDecay(1);

      // create child
      part = AddParticle(d_num2, ELECTRON, GetParticle(id)->GetvLocation(),
                         p->Vect());
      part->SetFirstMother(id);
      // generate time of decay (not used in this case, as it is an electron
      // or a positron, but to keep the same philosophy in every case...
      part->GenerateTimeOfDecay();
      GetParticle(id)->SetChild(1, d_num2);
      // add a track related to this particle

      // add a particle related list tree item to the event list tree
      gTmpLTI = gEventListTree->AddItem(gLTI[id], part->GetName());
      gTmpLTI->SetUserData(part);
      sprintf(strtmp,"%1.2e GeV",part->Energy());
      gEventListTree->SetToolTipItem(gTmpLTI, strtmp);
      gLTI[d_num2] = gTmpLTI;

      // increment number of children by the two created particles
      GetParticle(id)->SetNChildren(2);

      return(ALIVE);
   }
   else return(DEAD);
}

//______________________________________________________________________________
Int_t MyEvent::ParticleColor(Int_t id)
{
   // Return color index related to particle's energy.
   //Int_t ctable[11] = {2,50,46,45,44,43,42,41,21,19,5};

   Int_t i;
   for (i=0;i<16;i++)
      if (GetParticle(id)->Energy() > fEThreshold[i]) break;
   if (i > 16) i = 16;
   return(gColIndex + i);
}

//______________________________________________________________________________
void MyEvent::ScatterAngle(Int_t id)
{
   // Compute scatter angle into the detector's material
   // for the current particle
   // for more infos, please refer to the particle data booklet
   // from which the formulas has been extracted :
   // Multiple scattering through small angles.
   
   Double_t alpha,beta;
   Double_t abs_p,p1,p2,r_2;
   Double_t fact1,fact2;

   do {
      p1 = gRandom->Uniform(-1.0, 1.0);
      p2 = gRandom->Uniform(-1.0, 1.0);
      r_2 = (p1 * p1) + (p2 * p2);
   } while (r_2 > 1.0);
   abs_p = GetParticle(id)->P();
   alpha = TMath::Sqrt(-2.0 * TMath::Log(r_2) / r_2) *
           fDetector.GetTheta0(fMatter) / abs_p;
   beta  = gRandom->Uniform(0.0, 2.0 * TMath::Pi());
   alpha *= p1;
   TVector3 x_0(GetParticle(id)->GetvMoment().Orthogonal());
   TVector3 p_0(GetParticle(id)->GetvMoment() * (1.0 / abs_p));
   TVector3 y_0(x_0.Cross(p_0));
   fact1 = TMath::Sin(alpha);
   fact2 = fact1 * TMath::Cos(beta);
   fact1 *= TMath::Sin(beta);
   TVector3 vtmp1(x_0 * fact1);
   TVector3 vtmp2(y_0 * fact2);
   TVector3 vtmp3(vtmp2 + p_0);

   GetParticle(id)->SetMoment(vtmp1 + vtmp3);
   GetParticle(id)->SetMoment(GetParticle(id)->GetvMoment() *
                             (abs_p/ GetParticle(id)->P()));
}

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