fix: Fix Geant4 initial particle translation (#2492)

Observed problem: Particles put into G4 will not have the same properties after `BeanOn` is called. We relied on this in order to translate them back into Acts.

The following image shows how pt=1GeV looked like in `particles_initial` after a G4 forth and back.

![image](https://github.com/acts-project/acts/assets/487211/41c83669-0f3d-45ab-8b24-bcfe5d639680)

Solution: Put particles into the initial collection on translation and adopt generic translation code so it will accept already translated particles.

Examples/Algorithms/Geant4/src/Geant4Simulation.cpp

@@ -199,10 +199,6 @@ ActsExamples::Geant4Simulation::Geant4Simulation(const Config& cfg,
     runManager().SetUserAction(primaryGeneratorAction);
   }
 
-  // Please note:
-  // The following two blocks rely on the fact that the Acts
-  // detector constructions cache the world volume
-
   // Particle action
   {
     // Clear tracking action if it exists
@@ -251,6 +247,10 @@ ActsExamples::Geant4Simulation::Geant4Simulation(const Config& cfg,
   // Get the g4World cache
   G4VPhysicalVolume* g4World = m_detectorConstruction->Construct();
 
+  // Please note:
+  // The following two blocks rely on the fact that the Acts
+  // detector constructions cache the world volume
+
   // Set the magnetic field
   if (cfg.magneticField) {
     ACTS_INFO("Setting ACTS configured field to Geant4.");
@@ -360,6 +360,21 @@ ActsExamples::Geant4MaterialRecording::Geant4MaterialRecording(
     runManager().SetUserAction(primaryGeneratorAction);
   }
 
+  // Particle action
+  {
+    // Clear tracking action if it exists
+    if (runManager().GetUserTrackingAction() != nullptr) {
+      delete runManager().GetUserTrackingAction();
+    }
+    ParticleTrackingAction::Config trackingCfg;
+    trackingCfg.eventStore = m_eventStore;
+    trackingCfg.keepParticlesWithoutHits = true;
+    // G4RunManager will take care of deletion
+    auto trackingAction = new ParticleTrackingAction(
+        trackingCfg, m_logger->cloneWithSuffix("ParticleTracking"));
+    runManager().SetUserAction(trackingAction);
+  }
+
   // Stepping action
   {
     // Clear stepping action if it exists

Examples/Algorithms/Geant4/src/ParticleTrackingAction.cpp

@@ -44,24 +44,25 @@ void ActsExamples::ParticleTrackingAction::PreUserTrackingAction(
     ACTS_WARNING("Hit buffer not empty after track");
   }
 
-  auto particleId = makeParticleId(aTrack->GetTrackID(), aTrack->GetParentID());
+  auto barcode = makeParticleId(aTrack->GetTrackID(), aTrack->GetParentID());
 
-  // There is already a warning printed in the makeParticleId function
-  if (not particleId) {
+  // There is already a warning printed in the makeParticleId function if this
+  // indicates a failure
+  if (not barcode) {
     return;
   }
 
-  auto [it, success] =
-      eventStore().particlesInitial.insert(convert(*aTrack, *particleId));
+  auto particle = convert(*aTrack, *barcode);
+  auto [it, success] = eventStore().particlesInitial.insert(particle);
 
   // Only register particle at the initial state AND if there is no particle ID
   // collision
   if (success) {
-    eventStore().trackIdMapping[aTrack->GetTrackID()] = *particleId;
+    eventStore().trackIdMapping[aTrack->GetTrackID()] = particle.particleId();
   } else {
     eventStore().particleIdCollisionsInitial++;
     ACTS_WARNING("Particle ID collision with "
-                 << *particleId
+                 << particle.particleId()
                  << " detected for initial particles. Skip particle");
   }
 }
@@ -72,6 +73,8 @@ void ActsExamples::ParticleTrackingAction::PostUserTrackingAction(
   // collision
   if (eventStore().trackIdMapping.find(aTrack->GetTrackID()) ==
       eventStore().trackIdMapping.end()) {
+    ACTS_WARNING("Particle ID for track ID " << aTrack->GetTrackID()
+                                             << " not registered. Skip");
     return;
   }
 
@@ -132,7 +135,7 @@ ActsExamples::ParticleTrackingAction::makeParticleId(G4int trackId,
   // or we are making a final particle state)
   if (eventStore().trackIdMapping.find(trackId) !=
       eventStore().trackIdMapping.end()) {
-    return eventStore().trackIdMapping.at(trackId);
+    return std::nullopt;
   }
 
   if (eventStore().trackIdMapping.find(parentId) ==

Examples/Algorithms/Geant4/src/SimParticleTranslation.cpp

@@ -150,6 +150,7 @@ void ActsExamples::SimParticleTranslation::GeneratePrimaries(G4Event* anEvent) {
     // Add the primary to the vertex
     pVertex->SetPrimary(particle);
 
+    eventStore().particlesInitial.insert(part);
     eventStore().trackIdMapping[particle->GetTrackID()] = part.particleId();
 
     ++pCounter;

Examples/Python/tests/root_file_hashes.txt

@@ -3,7 +3,7 @@ test_fatras__particles_final.root: d3a900a7151623109f2e22e11beb477635eb549d5e611
 test_fatras__particles_initial.root: 0e2d9974fdd4aa5549c5c05817ec54fe6c7d9cbe7969647f227956167e31110d
 test_fatras__hits.root: d3ec06cd12e3737b5c783302ca29a532c885dc35930901d86b2f6d17041037c6
 test_geant4__particles_final.root: abf91622e96c0723651e864e050c7fdeee84b230aacb1c261ad81ff4e5df5f2c
-test_geant4__particles_initial.root: 5be86aa4a2fc369093746451f37085e2c93f80f079b66023f5fe4b67e20100f6
+test_geant4__particles_initial.root: cfe3d50a9c38b45eefc2e0c0948d5b0899ced6332dd183e1d19cb0b766f76363
 test_geant4__hits.root: 70b592a546fd362c9341d87c9068b1f8fed657e0036e62e7262c01fa5eb2d469
 test_seeding__estimatedparams.root: f11b6bdc2b9b13e84f974f2dfbd9f409255f4f0eab212fe8581c352b3f2cfc27
 test_seeding__performance_seeding.root: 992f9c611d30dde0d3f3ab676bab19ada61ab6a4442828e27b65ec5e5b7a2880