UserParticleHandler: fix logic for long lived particles with pre-assigned...

UserParticleHandler: fix logic for long lived particles with pre-assigned decays that end in the calorimeter

fixes an issue where the MonteCarlo history becomes inconsistent with the history from the generator because the original particle entry is not updated

DDG4/plugins/Geant4TCUserParticleHandler.cpp

@@ -93,8 +93,12 @@ void Geant4TCUserParticleHandler::end(const G4Track* /* track */, Particle& p)
   double z_prod = std::fabs(p.vsz);
   bool starts_in_trk_vol = ( r_prod <= m_rTracker && z_prod <= m_zTracker )  ;
 
-  // created in tracking volume but below energy cut
-  if( starts_in_trk_vol && ! (p.reason&G4PARTICLE_ABOVE_ENERGY_THRESHOLD) )  {
+  DD4hep::ReferenceBitMask<int> reason(p.reason);
+
+  if( reason.isSet(G4PARTICLE_PRIMARY) ) {
+    //do nothing
+  } else if( starts_in_trk_vol && ! reason.isSet(G4PARTICLE_ABOVE_ENERGY_THRESHOLD) )  {
+    // created in tracking volume but below energy cut
     p.reason = 0;
     return;
   }
@@ -103,15 +107,21 @@ void Geant4TCUserParticleHandler::end(const G4Track* /* track */, Particle& p)
   double z_end  = std::fabs(p.vez);
   bool ends_in_trk_vol =  ( r_end <= m_rTracker && z_end <= m_zTracker ) ;
 
-  // created and ended in calo
-  if( !starts_in_trk_vol ) {
-    
-    if( !ends_in_trk_vol ){  
-      p.reason = 0;
+  // created and ended in calo but not primary particle
+  //
+  // we can have particles from the generator only in the calo, if we have a
+  // long particle with preassigned decay, we need to keep the reason or the
+  // MChistory will not be updated later on
+  if( not reason.isSet(G4PARTICLE_PRIMARY) ) {
+    if( !starts_in_trk_vol ) {
+      if( !ends_in_trk_vol ){
+	p.reason = 0;
+      }
+      //fg: dont keep backscatter that did not create a tracker hit
+      else if( ! reason.isSet(G4PARTICLE_CREATED_TRACKER_HIT) ) {
+	p.reason = 0;
+      }
     }
-    //fg: dont keep backscatter that did not create a tracker hit
-    else if( !( p.reason & G4PARTICLE_CREATED_TRACKER_HIT  ) )
-      p.reason = 0;
   }
 
   // Set the simulator status bits