feat: Synthetic space point event generator in Fatras

CMakeLists.txt

@@ -217,6 +217,7 @@ set_option_if(
 set_option_if(ACTS_BUILD_PLUGIN_JSON ACTS_BUILD_EXAMPLES)
 set_option_if(ACTS_BUILD_FATRAS ACTS_BUILD_EXAMPLES)
 set_option_if(ACTS_BUILD_FATRAS ACTS_BUILD_EXAMPLES_FASTJET)
+set_option_if(ACTS_BUILD_FATRAS ACTS_BUILD_UNITTESTS)
 set_option_if(ACTS_BUILD_PLUGIN_GNN ACTS_BUILD_EXAMPLES_GNN)
 set_option_if(ACTS_BUILD_PLUGIN_FPEMON ACTS_BUILD_EXAMPLES)
 set_option_if(ACTS_BUILD_PLUGIN_JSON ACTS_BUILD_PLUGIN_TRACCC)

Fatras/CMakeLists.txt

@@ -11,6 +11,15 @@ acts_add_library(
     src/Kernel/SimulationError.cpp
     src/Physics/BetheHeitler.cpp
     src/Physics/StandardInteractions.cpp
+    src/Synthetic/DetectorLayout.cpp
+    src/Synthetic/EventConfig.cpp
+    src/Synthetic/EventCsvWriter.cpp
+    src/Synthetic/EventGenerator.cpp
+    src/Synthetic/SeedingTruth.cpp
+    src/Synthetic/detail/ParticlePropagator.cpp
+    src/Synthetic/detail/PrimaryGenerator.cpp
+    src/Synthetic/detail/Propagation.cpp
+    src/Synthetic/detail/Sampling.cpp
     src/Utilities/LandauDistribution.cpp
     ACTS_INCLUDE_FOLDER include/ActsFatras
 )

Fatras/README.md

@@ -14,3 +14,23 @@ simulated using parametrized models:
     approximations.
 -   Ionisation loss is simulated using the Bethe-Bloch formalism.
 -   Radiation loss is simulated using the Bethe-Heitler formalism.
+
+## Synthetic space point events
+
+The `ActsFatras::Synthetic` subcomponent is the exception to everything above: it
+uses neither the propagator nor `Acts::TrackingGeometry`. Its geometry is a list
+of cylinders around the beam axis and discs normal to it as plain structs, i.e. a
+collider-style barrel and endcap detector, so a helix crosses every surface in
+closed form, and its output is an `Acts::SpacePointContainer` rather than hits on
+surfaces. That makes it fast enough to generate an ATLAS-like pile-up of 200 in
+tens of milliseconds, which is what a seeding benchmark needs and what the
+propagator-based simulation above cannot provide.
+
+It is deliberately coarse in its physics and is meant for throughput and
+combinatorics studies rather than physics performance. Its *geometry* is not:
+every disc holds its own rings and every barrel cylinder its own half-length, so
+a real endcap is described as the staggered discs and rings it is.
+
+An event configuration is fitted against a full simulation of one detector *and*
+one layout of it, so the two belong together. `docs/groups/fatras.md` documents
+what such a fit reproduces and what it cannot.