feat: Pythia8 label secondary vertices based on proximity in Examples

Examples/Algorithms/GeneratorsPythia8/ActsExamples/Generators/Pythia8ProcessGenerator.cpp

@@ -1,6 +1,6 @@
 // This file is part of the Acts project.
 //
-// Copyright (C) 2017-2019 CERN for the benefit of the Acts project
+// Copyright (C) 2017-2024 CERN for the benefit of the Acts project
 //
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
@@ -115,10 +115,15 @@ Pythia8Generator::operator()(RandomEngine& rng) {
     // only secondaries have a defined vertex position
     if (m_cfg.labelSecondaries && genParticle.hasVertex()) {
       // either add to existing secondary vertex if exists or create new one
-      // TODO can we do this w/o the manual search and position check?
-      auto it = std::find_if(
-          vertices.begin(), vertices.end(),
-          [=](const SimVertex& other) { return pos4 == other.position4; });
+
+      // check if an existing vertex is close enough
+      auto it =
+          std::find_if(vertices.begin(), vertices.end(),
+                       [&pos4, this](const SimVertex& other) {
+                         return (pos4.head<3>() - other.position()).norm() <
+                                m_cfg.spatialVertexThreshold;
+                       });
+
       if (it != vertices.end()) {
         particleId.setVertexSecondary(std::distance(vertices.begin(), it));
         it->outgoing.insert(particleId);

Examples/Algorithms/GeneratorsPythia8/ActsExamples/Generators/Pythia8ProcessGenerator.hpp

@@ -1,6 +1,6 @@
 // This file is part of the Acts project.
 //
-// Copyright (C) 2017-2019 CERN for the benefit of the Acts project
+// Copyright (C) 2017-2024 CERN for the benefit of the Acts project
 //
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
@@ -45,7 +45,9 @@ class Pythia8Generator : public EventGenerator::ParticlesGenerator {
     /// Turn on/off the labeling of secondary vertices
     /// TODO this is essentially broken as the current code will label any kind
     /// of decay as secondary
-    bool labelSecondaries = false;
+    bool labelSecondaries = true;
+    /// The spatial threshold to consider a particle originating from a vertex
+    double spatialVertexThreshold = 1.0 * Acts::UnitConstants::um;
   };
 
   Pythia8Generator(const Config& cfg, Acts::Logging::Level lvl);