AliceO2Group · sawenzel · Sep 25, 2026 · Sep 25, 2026
@@ -29,7 +29,6 @@
 /// `G4.fastSimRegions` overrides the walk with an explicit space-separated list
 /// of media, for when a model should see less than a whole subtree.
 
-#include "TG4RunConfiguration.h"
 #include "TG4VUserFastSimulation.h"
 #include "TG4VUserPostDetConstruction.h"
 
@@ -53,15 +52,11 @@ class G4FastSimulation : public TG4VUserFastSimulation
   double mMinEnergy = 1.;
 };
 
-/// Supplies Geant4-VMC with the fast simulation models and their regions.
-/// Returns nullptr when no model is configured, so nothing is set up.
-class G4RunConfiguration : public TG4RunConfiguration
-{
- public:
-  using TG4RunConfiguration::TG4RunConfiguration;
-  TG4VUserFastSimulation* CreateUserFastSimulation() override;
-  TG4VUserPostDetConstruction* CreateUserPostDetConstruction() override;
-};
+/// The fast simulation for Geant4-VMC, or nullptr when `G4.fastSimModels` is empty.
+TG4VUserFastSimulation* createFastSimulation();
+
+/// The construction of the fast simulation regions, or nullptr when `G4.fastSimModels` is empty.
+TG4VUserPostDetConstruction* createFastSimRegionConstruction();
 
 } // namespace o2::fastsim
 

@@ -67,7 +67,7 @@ void G4FastSimulation::Construct()
 }
 
 //_____________________________________________________________________________
-TG4VUserFastSimulation* G4RunConfiguration::CreateUserFastSimulation()
+TG4VUserFastSimulation* createFastSimulation()
 {
   const auto& params = o2::conf::G4Params::Instance();
   auto models = split(params.fastSimModels, ',');
@@ -79,12 +79,12 @@ TG4VUserFastSimulation* G4RunConfiguration::CreateUserFastSimulation()
 }
 
 //_____________________________________________________________________________
-TG4VUserPostDetConstruction* G4RunConfiguration::CreateUserPostDetConstruction()
+TG4VUserPostDetConstruction* createFastSimRegionConstruction()
 {
   const auto& params = o2::conf::G4Params::Instance();
   auto models = split(params.fastSimModels, ',');
   if (models.empty()) {
-    return TG4RunConfiguration::CreateUserPostDetConstruction();
+    return nullptr;
   }
   std::vector<FastSimRegionConstruction::ModelRegions> wanted;
   wanted.reserve(models.size());

@@ -15,7 +15,7 @@ o2_add_library(G3Setup
 )
 
 o2_add_library(G4Setup
-               SOURCES src/G4Config.cxx
+               SOURCES src/G4Config.cxx src/G4RunConfiguration.cxx src/G4LocalFieldConstruction.cxx
                PUBLIC_LINK_LIBRARIES MC::Geant4VMC MC::Geant4 FairRoot::Base O2::SimulationDataFormat O2::Generators O2::SimSetup O2::FastSim
 )
 

@@ -65,7 +65,7 @@ R__LOAD_LIBRARY(libgeant4vmc)
 #include "G4ScoringManager.hh"
 #include "G4VScoringMesh.hh"
 #include <unistd.h>
-#include "FastSim/G4FastSimulation.h"
+#include "SimSetup/G4RunConfiguration.h"
 #endif
 #include "commonConfig.C"
 
@@ -119,12 +119,11 @@ void Config()
     LOG(fatal) << "Unsupported geometry navigation mode";
   }
 
-  // o2::fastsim::G4RunConfiguration differs from TG4RunConfiguration only in
-  // providing the fast-simulation hook; with G4.fastSimModels empty it behaves
-  // identically.
-  auto runConfiguration = new o2::fastsim::G4RunConfiguration(geomNavStr, physicsSetup,
-                                                              "stepLimiter+specialCuts",
-                                                              specialStacking, mtMode);
+  // o2::g4config::G4RunConfiguration adds the fast-simulation hook and the local
+  // magnetic fields; with neither configured it behaves like TG4RunConfiguration.
+  auto runConfiguration = new o2::g4config::G4RunConfiguration(geomNavStr, physicsSetup,
+                                                               "stepLimiter+specialCuts",
+                                                               specialStacking, mtMode);
   if (g4Params.g4scoring) {
     runConfiguration->SetUseOfG4Scoring();
     if (g4Params.g4fluenceweight) {

@@ -47,12 +47,33 @@
 /process/em/transportationWithMsc Disabled
 
 #
-# Adding extra lines for fixing tracking bias
-#
+# Field integration (O2-7198)
+# a tight deltaIntersection removes the 1/pT bias from chord-to-boundary placement (ALIROOT-7121)
 /mcMagField/setDeltaIntersection  1.0e-05 mm
-/mcMagField/setMinimumEpsilonStep 0.5e-05
-/mcMagField/setMaximumEpsilonStep 1.0e-05
+# the epsilons do not change the bias; minimumEpsilon sets the accuracy of long steps,
+# which only the muon spectrometer needs (it keeps the tight value as a local field)
+/mcMagField/setMinimumEpsilonStep 1.0e-04
+/mcMagField/setMaximumEpsilonStep 1.0e-03
 /mcMagField/printParameters
+# the muon spectrometer keeps tight epsilons as local fields (see G4LocalFieldConstruction)
+/mcDet/createMagFieldParameters YOUT1
+/mcMagField/YOUT1/stepperType NystromRK4
+/mcMagField/YOUT1/setConstDistance 1 mm
+/mcMagField/YOUT1/setDeltaIntersection 1.0e-05 mm
+/mcMagField/YOUT1/setMinimumEpsilonStep 0.5e-05
+/mcMagField/YOUT1/setMaximumEpsilonStep 1.0e-05
+/mcDet/createMagFieldParameters DDIP
+/mcMagField/DDIP/stepperType NystromRK4
+/mcMagField/DDIP/setConstDistance 1 mm
+/mcMagField/DDIP/setDeltaIntersection 1.0e-05 mm
+/mcMagField/DDIP/setMinimumEpsilonStep 0.5e-05
+/mcMagField/DDIP/setMaximumEpsilonStep 1.0e-05
+/mcDet/createMagFieldParameters YOUT2
+/mcMagField/YOUT2/stepperType NystromRK4
+/mcMagField/YOUT2/setConstDistance 1 mm
+/mcMagField/YOUT2/setDeltaIntersection 1.0e-05 mm
+/mcMagField/YOUT2/setMinimumEpsilonStep 0.5e-05
+/mcMagField/YOUT2/setMaximumEpsilonStep 1.0e-05
 
 # Change default parameters for killing looping particles
 #

@@ -0,0 +1,37 @@
+// Copyright 2019-2026 CERN and copyright holders of ALICE O2.
+// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+// All rights not expressly granted are reserved.
+//
+// This software is distributed under the terms of the GNU General Public
+// License v3 (GPL Version 3), copied verbatim in the file "COPYING".
+//
+// In applying this license CERN does not waive the privileges and immunities
+// granted to it by virtue of its status as an Intergovernmental Organization
+// or submit itself to any jurisdiction.
+
+#ifndef O2_SIMSETUP_G4LOCALFIELDCONSTRUCTION_H_
+#define O2_SIMSETUP_G4LOCALFIELDCONSTRUCTION_H_
+
+#include "TG4VUserPostDetConstruction.h"
+
+#include <memory>
+
+namespace o2::g4config
+{
+
+/// Gives every volume with its own field parameters (/mcDet/createMagFieldParameters <vol>)
+/// a local copy of the global field, so that the /mcMagField/<vol>/ settings take effect.
+class G4LocalFieldConstruction : public TG4VUserPostDetConstruction
+{
+ public:
+  /// next is another construction step run first (may be nullptr); it is owned
+  explicit G4LocalFieldConstruction(TG4VUserPostDetConstruction* next) : mNext(next) {}
+  void Construct() override;
+
+ private:
+  std::unique_ptr<TG4VUserPostDetConstruction> mNext;
+};
+
+} // namespace o2::g4config
+
+#endif // O2_SIMSETUP_G4LOCALFIELDCONSTRUCTION_H_
@@ -0,0 +1,31 @@
+// Copyright 2019-2026 CERN and copyright holders of ALICE O2.
+// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+// All rights not expressly granted are reserved.
+//
+// This software is distributed under the terms of the GNU General Public
+// License v3 (GPL Version 3), copied verbatim in the file "COPYING".
+//
+// In applying this license CERN does not waive the privileges and immunities
+// granted to it by virtue of its status as an Intergovernmental Organization
+// or submit itself to any jurisdiction.
+
+#ifndef O2_SIMSETUP_G4RUNCONFIGURATION_H_
+#define O2_SIMSETUP_G4RUNCONFIGURATION_H_
+
+#include "TG4RunConfiguration.h"
+
+namespace o2::g4config
+{
+
+/// The Geant4 VMC run configuration of O2: adds the fast simulation and the local magnetic fields.
+class G4RunConfiguration : public TG4RunConfiguration
+{
+ public:
+  using TG4RunConfiguration::TG4RunConfiguration;
+  TG4VUserFastSimulation* CreateUserFastSimulation() override;
+  TG4VUserPostDetConstruction* CreateUserPostDetConstruction() override;
+};
+
+} // namespace o2::g4config
+
+#endif // O2_SIMSETUP_G4RUNCONFIGURATION_H_
@@ -0,0 +1,87 @@
+// Copyright 2019-2026 CERN and copyright holders of ALICE O2.
+// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+// All rights not expressly granted are reserved.
+//
+// This software is distributed under the terms of the GNU General Public
+// License v3 (GPL Version 3), copied verbatim in the file "COPYING".
+//
+// In applying this license CERN does not waive the privileges and immunities
+// granted to it by virtue of its status as an Intergovernmental Organization
+// or submit itself to any jurisdiction.
+
+#include "SimSetup/G4LocalFieldConstruction.h"
+#include "SimConfig/G4Params.h"
+
+#include "TG4GeometryManager.h"
+
+#include <G4UIcommandTree.hh>
+#include <G4UImanager.hh>
+#include <TGeoManager.h>
+#include <TVirtualMC.h>
+#include <fairlogger/Logger.h>
+
+#include <string>
+#include <unordered_set>
+
+namespace o2::g4config
+{
+
+namespace
+{
+// First volume in the subtree of vol whose medium has no magnetic field (ifield = 0)
+const TGeoVolume* findZeroFieldVolume(const TGeoVolume* vol, std::unordered_set<const TGeoVolume*>& visited)
+{
+  if (!visited.insert(vol).second) {
+    return nullptr;
+  }
+  auto med = vol->GetMedium();
+  if (med && !vol->IsAssembly() && med->GetParam(1) == 0) {
+    return vol;
+  }
+  for (int i = 0; i < vol->GetNdaughters(); ++i) {
+    if (auto nf = findZeroFieldVolume(vol->GetNode(i)->GetVolume(), visited)) {
+      return nf;
+    }
+  }
+  return nullptr;
+}
+} // namespace
+
+void G4LocalFieldConstruction::Construct()
+{
+  if (mNext) {
+    mNext->Construct();
+  }
+  auto tree = G4UImanager::GetUIpointer()->GetTree()->FindCommandTree("/mcMagField/");
+  auto field = TVirtualMC::GetMC()->GetMagField();
+  if (!tree || !field) {
+    return;
+  }
+  int nattached = 0;
+  for (int i = 1; i <= tree->GetTreeEntry(); ++i) {
+    std::string path = tree->GetTree(i)->GetPathName(); // "/mcMagField/<vol>/"
+    auto name = path.substr(12, path.size() - 13);
+    auto vol = gGeoManager->GetVolume(name.c_str());
+    if (!vol) {
+      LOG(warn) << "local field: no volume " << name << "; its field parameters are unused";
+      continue;
+    }
+    // Geant4 VMC forces a local field onto all daughters, which would override zero-field media
+    std::unordered_set<const TGeoVolume*> visited;
+    if (auto nf = findZeroFieldVolume(vol, visited)) {
+      LOG(warn) << "local field: volume " << name << " contains the zero-field volume " << nf->GetName() << "; skipped";
+      continue;
+    }
+    vol->SetField(field);
+    LOG(info) << "local field: volume " << name << " uses the parameters in /mcMagField/" << name << "/";
+    ++nattached;
+  }
+  if (nattached > 0) {
+    TG4GeometryManager::Instance()->SetIsLocalField(true);
+    if (o2::conf::G4Params::Instance().navmode != o2::conf::EG4Nav::kTGeo) {
+      LOG(warn) << "local field: Geant4 VMC builds local fields only with TGeo navigation; the global field applies everywhere";
+    }
+  }
+}
+
+} // namespace o2::g4config
@@ -0,0 +1,30 @@
+// Copyright 2019-2026 CERN and copyright holders of ALICE O2.
+// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+// All rights not expressly granted are reserved.
+//
+// This software is distributed under the terms of the GNU General Public
+// License v3 (GPL Version 3), copied verbatim in the file "COPYING".
+//
+// In applying this license CERN does not waive the privileges and immunities
+// granted to it by virtue of its status as an Intergovernmental Organization
+// or submit itself to any jurisdiction.
+
+#include "SimSetup/G4RunConfiguration.h"
+#include "SimSetup/G4LocalFieldConstruction.h"
+#include "FastSim/G4FastSimulation.h"
+
+namespace o2::g4config
+{
+
+TG4VUserFastSimulation* G4RunConfiguration::CreateUserFastSimulation()
+{
+  return o2::fastsim::createFastSimulation();
+}
+
+TG4VUserPostDetConstruction* G4RunConfiguration::CreateUserPostDetConstruction()
+{
+  auto fastSimRegions = o2::fastsim::createFastSimRegionConstruction();
+  return new G4LocalFieldConstruction(fastSimRegions ? fastSimRegions : TG4RunConfiguration::CreateUserPostDetConstruction());
+}
+
+} // namespace o2::g4config