ITS: distinguish inactive layers from track holes

Add an InactiveLayerMask to TrackingParameters to represent detector layers that are absent for a given input or configuration. Inactive layers remain skippable, but they do not inflate the effective seed length or consume the ordinary hole budget.

Guard the multiple-scattering helper against zero material so inactive layers can be configured with no material budget.

Author: mpuccio

Detectors/ITSMFT/ITS/tracking/GPU/cuda/TrackerTraitsGPU.cxx

@@ -308,7 +308,7 @@ void TrackerTraitsGPU<NLayers>::findRoads(const int iteration)
   const bool extendTop = this->mTrkParams[iteration].PassFlags[IterationStep::TrackFollowerTop];
   const bool extendBot = this->mTrkParams[iteration].PassFlags[IterationStep::TrackFollowerBot];
   const bool extendTracks = extendTop || extendBot;
-  const auto nonCountingLayers = ~this->mTrkParams[iteration].getSeedingLayerMask();
+  const auto nonCountingLayers = this->mTrkParams[iteration].InactiveLayerMask | ~this->mTrkParams[iteration].getSeedingLayerMask();
   for (int startLevel{this->mTrkParams[iteration].CellsPerRoad()}; startLevel >= this->mTrkParams[iteration].CellMinimumLevel(); --startLevel) {
     bounded_vector<TrackSeed<NLayers>> trackSeeds(this->getMemoryPool().get());
     for (int startCellTopologyId{0}; startCellTopologyId < hostTopology.nCells; ++startCellTopologyId) {

Detectors/ITSMFT/ITS/tracking/include/ITStracking/Configuration.h

@@ -87,6 +87,7 @@ struct TrackingParameters {
   int MinTrackLength = 7;
   int MaxHoles = 0;
   LayerMask HoleLayerMask = 0;
+  LayerMask InactiveLayerMask = 0;
   LayerMask SeedingLayers = 0;
   float NSigmaCut = 5;
   float PVres = 1.e-2f;

Detectors/ITSMFT/ITS/tracking/include/ITStracking/MathUtils.h

@@ -122,6 +122,9 @@ GPUhdi() constexpr float SqDiff(float x, float y)
 
 GPUhdi() float MSangle(float mass, float p, float xX0)
 {
+  if (xX0 <= 0.f) {
+    return 0.f;
+  }
   float beta = p / o2::gpu::CAMath::Hypot(mass, p);
   return 0.0136f * o2::gpu::CAMath::Sqrt(xX0) * (1.f + 0.038f * o2::gpu::CAMath::Log(xX0)) / (beta * p);
 }

Detectors/ITSMFT/ITS/tracking/src/Configuration.cxx

@@ -59,6 +59,9 @@ std::string TrackingParameters::asString() const
   if (MaxHoles) {
     str += std::format(" MaxHoles:{} HoleMask:{}", MaxHoles, HoleLayerMask.asString());
   }
+  if (!InactiveLayerMask.empty()) {
+    str += std::format(" InactiveMask:{}", InactiveLayerMask.asString());
+  }
   if (!SeedingLayers.empty()) {
     str += std::format(" SeedingLayers:{}", SeedingLayers.asString());
   }

Detectors/ITSMFT/ITS/tracking/src/TrackerTraits.cxx

@@ -759,7 +759,7 @@ void TrackerTraits<NLayers>::findRoads(const int iteration)
     unsortedClusters[iLayer] = mTimeFrame->getUnsortedClusters()[iLayer].data();
   }
   const auto topology = mTimeFrame->getTrackingTopologyView();
-  const auto nonCountingLayers = ~mTrkParams[iteration].getSeedingLayerMask();
+  const auto nonCountingLayers = mTrkParams[iteration].InactiveLayerMask | ~mTrkParams[iteration].getSeedingLayerMask();
   for (int startLevel{mTrkParams[iteration].CellsPerRoad()}; startLevel >= mTrkParams[iteration].CellMinimumLevel(); --startLevel) {
 
     const track::TrackSeedSelector<NLayers> seedFilter{1.e3f, mTrkParams[iteration].MaxChi2NDF * ((startLevel + 2) * 2 - 5), mTrkParams[iteration].MaxHoles, mTrkParams[iteration].MinTrackLength, mTrkParams[iteration].HoleLayerMask, nonCountingLayers};