Let the ZEM calorimeters be built without the far beam line

This adds two ZDCSimParam switches so a simulation can keep the ZEM geometry
without paying for transport to the ZN/ZP calorimeters at +-113 m.

- The ZDC geometry is the +-113 m beam line, its magnets, ZN, ZP and ZEM; the
  transport cost is the beam line, while ZEM sits at z = 7.6 m.
- buildBeamLine=false skips the beam line, the magnets and ZN/ZP; buildZEM
  controls the ZEM calorimeters.
- The ZN and ZP construction moves into createZNZP() and the ZEM construction
  into createZEM(), so createDetectors() shows what depends on the beam line.
- The PMC and PMQ medium ids move ahead of that branch: ProcessHits compares
  every step against them for ZEM too, so leaving them in the ZN block gave ZEM
  hits with no energy loss and no light yield.
- The ZN/ZP sensitive volume lookups are guarded so they no longer abort when
  the volumes are absent.
- 20 pythia8pp events with TGeant3: 276.9 s with the full ZDC, 141.6 +- 1.1 s
  with --skipModules ZDC, 136.9 +- 1.9 s with buildBeamLine=false.
- 50 events of 20 muons at 200 GeV into 4.6 < eta < 5.3 give 234 ZEM hits with
  2167 photoelectrons at buildBeamLine=false, against 230 hits and 2097 with the
  full ZDC.

https://its.cern.ch/jira/browse/O2-7158

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: sawenzel

Detectors/ZDC/simulation/include/ZDCSimulation/Detector.h

@@ -121,6 +121,8 @@ class Detector : public o2::base::DetImpl<Detector>
   void createCsideBeamLine();
   void createMagnets();
   void createDetectors();
+  void createZNZP();
+  void createZEM();
 
   // determine detector; sector/tower and impact coordinates given volumename and position
   void getDetIDandSecID(TString const& volname, math_utils::Vector3D<float> const& x,

Detectors/ZDC/simulation/include/ZDCSimulation/ZDCSimParam.h

@@ -24,6 +24,11 @@ namespace zdc
 struct ZDCSimParam : public o2::conf::ConfigurableParamHelper<ZDCSimParam> {
 
   bool continuous = true;                      ///< flag for continuous simulation
+  /// Build the +-113 m beam line, its magnets and the ZN/ZP calorimeters that sit
+  /// there. This is what the ZDC costs in transport time; turning it off leaves the
+  /// ZEM calorimeters at z ~ 7.6 m in the geometry.
+  bool buildBeamLine = true;
+  bool buildZEM = true;                        ///< build the ZEM calorimeters
   int nBCAheadCont = 1;                        ///< number of BC to read ahead of trigger in continuous mode
   int nBCAheadTrig = 3;                        ///< number of BC to read ahead of trigger in triggered mode
   bool recordSpatialResponse = false;          ///< whether to record 2D spatial response showering images in proton/neutron detector

Detectors/ZDC/simulation/src/Detector.cxx

@@ -217,47 +217,54 @@ void Detector::ConstructGeometry()
 
   createMaterials();
 
-  createAsideBeamLine();
-  createCsideBeamLine();
-  createMagnets();
+  if (ZDCSimParam::Instance().buildBeamLine) {
+    createAsideBeamLine();
+    createCsideBeamLine();
+    createMagnets();
+  } else {
+    LOG(info) << "ZDC: beam line, magnets and the ZN/ZP calorimeters are not built";
+  }
   createDetectors();
 }
 
 //_____________________________________________________________________________
 void Detector::defineSensitiveVolumes()
 {
   LOG(info) << "defining sensitive for ZDC";
-  auto vol = gGeoManager->GetVolume("ZNENV");
-  if (vol) {
-    AddSensitiveVolume(vol);
-    mZNENVVolID = vol->GetNumber(); // initialize id
-
-    AddSensitiveVolume(gGeoManager->GetVolume("ZNF1"));
-    AddSensitiveVolume(gGeoManager->GetVolume("ZNF2"));
-    AddSensitiveVolume(gGeoManager->GetVolume("ZNF3"));
-    AddSensitiveVolume(gGeoManager->GetVolume("ZNF4"));
-  } else {
-    LOG(fatal) << "can't find volume ZNENV";
-  }
-  vol = gGeoManager->GetVolume("ZPENV");
-  if (vol) {
-    AddSensitiveVolume(vol);
-    mZPENVVolID = vol->GetNumber(); // initialize id
-
-    AddSensitiveVolume(gGeoManager->GetVolume("ZPF1"));
-    AddSensitiveVolume(gGeoManager->GetVolume("ZPF2"));
-    AddSensitiveVolume(gGeoManager->GetVolume("ZPF3"));
-    AddSensitiveVolume(gGeoManager->GetVolume("ZPF4"));
-  } else {
-    LOG(fatal) << "can't find volume ZPENV";
+  TGeoVolume* vol = nullptr;
+  if (ZDCSimParam::Instance().buildBeamLine) {
+    vol = gGeoManager->GetVolume("ZNENV");
+    if (vol) {
+      AddSensitiveVolume(vol);
+      mZNENVVolID = vol->GetNumber(); // initialize id
+
+      AddSensitiveVolume(gGeoManager->GetVolume("ZNF1"));
+      AddSensitiveVolume(gGeoManager->GetVolume("ZNF2"));
+      AddSensitiveVolume(gGeoManager->GetVolume("ZNF3"));
+      AddSensitiveVolume(gGeoManager->GetVolume("ZNF4"));
+    } else {
+      LOG(fatal) << "can't find volume ZNENV";
+    }
+    vol = gGeoManager->GetVolume("ZPENV");
+    if (vol) {
+      AddSensitiveVolume(vol);
+      mZPENVVolID = vol->GetNumber(); // initialize id
+
+      AddSensitiveVolume(gGeoManager->GetVolume("ZPF1"));
+      AddSensitiveVolume(gGeoManager->GetVolume("ZPF2"));
+      AddSensitiveVolume(gGeoManager->GetVolume("ZPF3"));
+      AddSensitiveVolume(gGeoManager->GetVolume("ZPF4"));
+    } else {
+      LOG(fatal) << "can't find volume ZPENV";
+    }
   }
   // em calorimeter
   vol = gGeoManager->GetVolume("ZEM ");
   if (vol) {
     AddSensitiveVolume(vol);
     mZEMVolID = vol->GetNumber();
     AddSensitiveVolume(gGeoManager->GetVolume("ZEMF"));
-  } else {
+  } else if (ZDCSimParam::Instance().buildZEM) {
     LOG(fatal) << "can't find volume ZEM";
   }
 }
@@ -2068,6 +2075,22 @@ void Detector::createMagnets()
 }
 //_____________________________________________________________________________
 void Detector::createDetectors()
+{
+  // ProcessHits compares the medium of every step against these two, for ZEM as
+  // much as for ZN and ZP, so they have to be resolved whatever is built.
+  mMediumPMCid = getMediumID(kSiO2pmc);
+  mMediumPMQid = getMediumID(kSiO2pmq);
+
+  // ZN and ZP sit in the ZDCA/ZDCC mother volumes that the beam line builds, so
+  // they stand or fall with it. ZEM is at z = 7.6 m and is built either way.
+  if (ZDCSimParam::Instance().buildBeamLine) {
+    createZNZP();
+  }
+  createZEM();
+}
+
+//_____________________________________________________________________________
+void Detector::createZNZP()
 {
   // Create the ZDCs
 
@@ -2082,8 +2105,6 @@ void Detector::createDetectors()
 
   // -------------------------------------------------------------------------------
   //--> Neutron calorimeter (ZN)
-  mMediumPMCid = getMediumID(kSiO2pmc);
-  mMediumPMQid = getMediumID(kSiO2pmq);
 
   // an envelop volume for the purpose of registering particles entering the detector
   double eps = 0.1; // 1 mm
@@ -2300,8 +2321,17 @@ void Detector::createDetectors()
   TVirtualMC::GetMC()->Gspos("ZPBS", 3, "ZDCA", Geometry::ZPAPOSITION[0] + Geometry::ZPDIMENSION[0] + zpSupportWallside[0], Geometry::ZPAPOSITION[1] + 0.75, Geometry::ZPAPOSITION[2] + zpSupportWallside[2], 0, "ONLY");
   TVirtualMC::GetMC()->Gspos("ZPBS", 4, "ZDCA", Geometry::ZPAPOSITION[0] - Geometry::ZPDIMENSION[0] - zpSupportWallside[0], Geometry::ZPAPOSITION[1] + 0.75, Geometry::ZPAPOSITION[2] + zpSupportWallside[2], 0, "ONLY");
 
+}
+
+//_____________________________________________________________________________
+void Detector::createZEM()
+{
   // -------------------------------------------------------------------------------
   // -> EM calorimeter (ZEM)
+  if (!ZDCSimParam::Instance().buildZEM) {
+    LOG(warning) << "ZDC: the ZEM calorimeters are not built";
+    return;
+  }
   int32_t irotzem1, irotzem2;
   double rangzem1[6] = {0., 0., 90., 90., -90., 0.};
   double rangzem2[6] = {180., 0., 90., 45. + 90., 90., 45.};