Use field-free media for the L3 magnet and the compensator

This changes the media of volumes that sit outside the field map, diagnosed with
the geometry doctor. 17 volumes were built from media that declare a magnetic
field, whereas the field there is always zero. This is now fixed by switching
five media in place, and by giving a field-free variant to the three volumes
whose medium is shared with volumes that do reach the field.

The new copper medium also needs its own line in simcuts_COMP.dat, because
Geant4 assigns cuts per material and it would otherwise take the default ones.

#15683

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

Author: sawenzel

Detectors/Passive/data/simcuts_COMP.dat

@@ -27,6 +27,8 @@ COMP 50 1.e-3 1.e-2 1.e-3 1.e-3
 *       GAM   ELEC  NHAD  CHAD  MUON  EBREM MUHAB  EDEL  MUDEL MUPA ANNI BREM COMP DCAY DRAY HADR LOSS MULS PAIR PHOT RAYL STRA
 *COMP 17 1.e-3 1.e-3 1.e-3 1.e-3 1.e-3 1.e-3 1.e-3 -1.    -1.   -1.  -1    -1    -1   -1   -1   -1   -1   -1   -1   -1   -1   -1
 COMP 17 1.e-1 1.e-1 1.e0 1.e0 1.e-2 1.e-1 1.e-1 -1.    -1.   -1.  -1    -1    -1   -1   -1   -1   -1   -1   -1   -1   -1   -1
+* field-free twin of the above, same cuts
+COMP 18 1.e-1 1.e-1 1.e0 1.e0 1.e-2 1.e-1 1.e-1 -1.    -1.   -1.  -1    -1    -1   -1   -1   -1   -1   -1   -1   -1   -1   -1
 * shielded
 COMP 37 1.e-3 1.e-2 1.e-3 1.e-3
 * very shielded

Detectors/Passive/src/Compensator.cxx

@@ -75,8 +75,10 @@ void Compensator::createMaterials()
 
   // --- Define the various materials + tracking media for GEANT ---
   //     Aluminum
+  //     ALU_C0 builds the coil supports only, and every one of them is placed
+  //     clear of the field, so they are tracked without one.
   matmgr.Material("COMP", 9, "ALUMINIUM0", 26.98, 13., 2.7, 8.9, 37.2);
-  matmgr.Medium("COMP", 9, "ALU_C0", 9, 0, isxfld1, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
+  matmgr.Medium("COMP", 9, "ALU_C0", 9, 0, isxfld2, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
   matmgr.Material("COMP", 29, "ALUMINIUM1", 26.98, 13., 2.7, 8.9, 37.2);
   matmgr.Medium("COMP", 29, "ALU_C1", 29, 0, isxfld1, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
   matmgr.Material("COMP", 49, "ALUMINIUM2", 26.98, 13., 2.7, 8.9, 37.2);
@@ -95,6 +97,12 @@ void Compensator::createMaterials()
   matmgr.Material("COMP", 37, "COPPER1", 63.55, 29., 8.96, 1.43, 15.1);
   matmgr.Material("COMP", 57, "COPPER2", 63.55, 29., 8.96, 1.43, 15.1);
   matmgr.Medium("COMP", 17, "Cu_C0", 17, 0, isxfld1, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
+  // The horizontal coils are the only copper that stays clear of the field. The
+  // vertical ones reach the beam axis, where the machine compensators are, so
+  // they keep Cu_C0. This medium needs its own line in simcuts_COMP.dat: cuts
+  // are assigned per material, so without one both media fall back to the
+  // default and the copper loses the cuts Cu_C0 relies on.
+  matmgr.Medium("COMP", 18, "Cu_C0_NF", 17, 0, isxfld2, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
   matmgr.Medium("COMP", 37, "Cu_C1", 37, 0, isxfld1, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
   matmgr.Medium("COMP", 57, "Cu_C2", 57, 0, isxfld1, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
 }
@@ -121,6 +129,7 @@ TGeoVolume* Compensator::createMagnetYoke()
   auto& matmgr = o2::base::MaterialManager::Instance();
   auto kMedAlu = matmgr.getTGeoMedium("COMP_ALU_C0");
   auto kMedCooper = matmgr.getTGeoMedium("COMP_Cu_C0");
+  auto kMedCooperNF = matmgr.getTGeoMedium("COMP_Cu_C0_NF");
   auto kMedIron = matmgr.getTGeoMedium("COMP_FE_C0");
 
   // we use a special optimized tracking medium for the inner part
@@ -170,7 +179,7 @@ TGeoVolume* Compensator::createMagnetYoke()
   }
 
   // Make the coils:
-  TGeoVolume* voCoilH = gGeoManager->MakeBox("voCoilH", kMedCooper, 12.64 / 2.0, 21.46 / 2.0, 310.5 / 2.0);
+  TGeoVolume* voCoilH = gGeoManager->MakeBox("voCoilH", kMedCooperNF, 12.64 / 2.0, 21.46 / 2.0, 310.5 / 2.0);
   TGeoVolume* voCoilV = gGeoManager->MakeBox("voCoilV", kMedCooper, 12.64 / 2.0, 35.80 / 2.0, 26.9 / 2.0);
 
   // Make the top coil supports:

Detectors/Passive/src/Magnet.cxx

@@ -66,6 +66,11 @@ void Magnet::createMaterials()
   Int_t isxfld = 2.;
   Float_t sxmgmx = 10.;
   o2::base::Detector::initFieldTrackingParams(isxfld, sxmgmx);
+
+  // The coils, the yoke and the crown sit outside the region the field map
+  // covers, so they are tracked without a field. The doors and the plugs are
+  // the exception: they reach the beam axis inside the solenoid and keep it.
+  Int_t isxfldNoField = 0;
   Float_t epsil, stmin, deemax, tmaxfd, stemax;
 
   // --- Define the various materials for GEANT ---
@@ -117,19 +122,20 @@ void Magnet::createMaterials()
   matmgr.Medium("MAG", 30, "FE_C1", 30, 0, isxfld, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
 
   //     ALUMINUM
-  matmgr.Medium("MAG", 9, "ALU_C0", 9, 0, isxfld, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
-  matmgr.Medium("MAG", 29, "ALU_C1", 29, 0, isxfld, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
+  matmgr.Medium("MAG", 9, "ALU_C0", 9, 0, isxfldNoField, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
+  matmgr.Medium("MAG", 29, "ALU_C1", 29, 0, isxfldNoField, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
 
   //     AIR
   matmgr.Medium("MAG", 15, "AIR_C0", 15, 0, isxfld, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
-  matmgr.Medium("MAG", 35, "AIR_C1", 35, 0, isxfld, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
+  matmgr.Medium("MAG", 35, "AIR_C1", 35, 0, isxfldNoField, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
 
   //    Steel
   matmgr.Medium("MAG", 19, "ST_C0", 19, 0, isxfld, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
   matmgr.Medium("MAG", 39, "ST_C1", 39, 0, isxfld, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
+  matmgr.Medium("MAG", 49, "ST_C1_NF", 39, 0, isxfldNoField, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
   matmgr.Medium("MAG", 59, "ST_C3", 59, 0, isxfld, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
   //    WATER
-  matmgr.Medium("MAG", 16, "WATER", 16, 0, isxfld, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
+  matmgr.Medium("MAG", 16, "WATER", 16, 0, isxfldNoField, sxmgmx, tmaxfd, stemax, deemax, epsil, stmin);
 }
 
 void Magnet::ConstructGeometry()
@@ -183,6 +189,7 @@ void Magnet::ConstructGeometry()
   auto medAlu = matmgr.getTGeoMedium("MAG_ALU_C1");
   auto medAluI = matmgr.getTGeoMedium("MAG_ALU_C0");
   auto medSteel = matmgr.getTGeoMedium("MAG_ST_C1");
+  auto medSteelNF = matmgr.getTGeoMedium("MAG_ST_C1_NF");
   auto medWater = matmgr.getTGeoMedium("MAG_WATER");
   //
   // Offset between LHC and LEP axis
@@ -282,7 +289,7 @@ void Magnet::ConstructGeometry()
   shYoke->DefineSection(0, -kLYoke, kRYokeInner, kRYokeOuter);
   shYoke->DefineSection(1, +kLYoke, kRYokeInner, kRYokeOuter);
   //
-  TGeoVolume* voYoke = new TGeoVolume("L3YO", shYoke, medSteel);
+  TGeoVolume* voYoke = new TGeoVolume("L3YO", shYoke, medSteelNF);
   voBMother->AddNode(voYoke, 1, new TGeoTranslation(0., 0., 0.));
 
   //
@@ -294,7 +301,7 @@ void Magnet::ConstructGeometry()
   shCrown->DefineSection(2, kLCrown2, kRCrownInner, kRCrownOuter);
   shCrown->DefineSection(3, kLCrown3, kRCrownInner, kRCrownOuter);
   //
-  TGeoVolume* voCrown = new TGeoVolume("L3CR", shCrown, medSteel);
+  TGeoVolume* voCrown = new TGeoVolume("L3CR", shCrown, medSteelNF);
 
   //
   // Door including "Plug"