Fatras is an ACTS implementation for fast track simulation which was primarily built to easily obtain hits from a tracking geometry. Note that it is not meant for detailed physical simulations or studies.

Fatras is using the Acts::Propagator as a backend for the track extrapolation. The simulation is just an Actor in the terminology of the ACTS propagation.


A dedicated description of the propagation can be found here.

Hits on sensitive surfaces are recorded and tagged with the particle passing through the surface. Later, this hit information can be used by a digitization algorithm to mimic a detector response.

ACTS Fatras is fully capable of handling secondary particles even though the current set of interactions is not utilizing this to a big extent. The simulation will propagate each particle until it reaches the end of the detector or until a specified path length is reached. Afterwards, the first secondary particle that might have been generated in the process will be propagated. This continues until all particles are transported.

Supported interactions

Fatras implements a few interactions

  • Bethe-Bloch energy loss for charged particles (see ActsFatras::BetheBloch)

  • Bethe-Heitler energy loss for electrons (and positrons) (see ActsFatras::BetheHeitler)

  • Photon conversion for pair production (see ActsFatras::PhotonConversion)

  • Scattering for charged particles (see ActsFatras::GenericScattering)

These interactions are meant to be physical accurate within their boundaries but are far from sufficient for a full simulation.


The primary use-case for ACTS Fatras is early algorithm validation. It is fast enough to simulate thousands of small to medium events (about 100 tracks) on consumer hardware in a few seconds. This provides a quick turnaround time for development.

Fatras does not replace a full detector simulation like Geant4 as only a few interactions are implemented and the geometry is highly simplified.