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0001 # Fatras
0002
0003 Fatras is an ACTS implementation for fast track simulation which was primarily built to easily obtain hits from a tracking geometry.
0004 Note that it is not meant for detailed physical simulations or studies.
0005
0006 Fatras is using the {class}`Acts::Propagator` as a backend for the track extrapolation. The simulation is just an **Actor** in the terminology of the ACTS propagation.
0007
0008 :::{tip}
0009 A dedicated description of the propagation can be found [here](propagation_impl).
0010 :::
0011
0012 Hits on sensitive surfaces are recorded and tagged with the particle passing through the surface.
0013 Later, this hit information can be used by a digitization algorithm to mimic a detector response.
0014
0015 ACTS Fatras is fully capable of handling secondary particles even though the current set of interactions is not utilizing this to a big extent.
0016 The simulation will propagate each particle until it reaches the end of the detector or until a specified path length is reached.
0017 Afterwards, the first secondary particle that might have been generated in the process will be propagated.
0018 This continues until all particles are transported.
0019
0020 ## Supported interactions
0021
0022 Fatras implements a few interactions
0023 - Bethe-Bloch energy loss for charged particles (see `ActsFatras::BetheBloch`)
0024 - Bethe-Heitler energy loss for electrons (and positrons) (see `ActsFatras::BetheHeitler`)
0025 - Photon conversion for pair production (see `ActsFatras::PhotonConversion`)
0026 - Scattering for charged particles (see `ActsFatras::GenericScattering`)
0027
0028 These interactions are meant to be physical accurate within their boundaries but are far from sufficient for a full simulation.
0029
0030 ## Use-cases
0031
0032 The primary use-case for ACTS Fatras is early algorithm validation.
0033 It is fast enough to simulate thousands of small to medium events (about 100 tracks) on consumer hardware in a few seconds.
0034 This provides a quick turnaround time for development.
0035
0036 Fatras does not replace a full detector simulation like Geant4 as only a few interactions are implemented and the geometry is highly simplified.
