feat: add helix cut tolerance parameter to seeding config (#2472)

This adds an optional helix tolerance parameter to the seeding algorithm, which allows you to increase the minimum helix diameter allowed manually rather than automatically as determined in the usual way. This is useful for seeding with misaligned surfaces, where the seeds may not actually well represent a helix and thus many good misaligned seeds get thrown out at this stage of the finding. The parameter is set to 1 by default so as to not change the default behavior.

Core/include/Acts/Seeding/SeedFinder.ipp

@@ -723,7 +723,7 @@ inline void SeedFinder<external_spacepoint_t, platform_t>::filterCandidates(
 
       // sqrt(S2)/B = 2 * helixradius
       // calculated radius must not be smaller than minimum radius
-      if (S2 < B2 * options.minHelixDiameter2) {
+      if (S2 < B2 * options.minHelixDiameter2 * m_config.helixCut) {
         continue;
       }
 

Core/include/Acts/Seeding/SeedFinderConfig.hpp

@@ -101,6 +101,10 @@ struct SeedFinderConfig {
   // xyz
   float toleranceParam = 1.1 * Acts::UnitConstants::mm;
 
+  // Parameter which can loosen the tolerance of the track seed to form to a
+  // helix, useful for (e.g.) misaligned seeding
+  float helixCut = 1.;
+
   // Geometry Settings
   // Detector ROI
   // limiting location of collision region in z