Run the FAst TRAck Simulation

Prerequisites

Acts must be build with the ACTS_BUILD_EXAMPLES_PYTHIA8=on option to activate building the Fatras simulation. This enables the generic detector. To be able to run e.g. DD4hep-base detectors, also ACTS_BUILD_EXAMPLES_DD4hep=on must be set.

In the following, it is assumed that Acts was build as described above in a separate build directory <build>.

Generate TrackML-like datasets

There exists one simulation executable for each type of detectors. For TrackML-like datasets, the Generic detector must be used. This does not require any additional settings.

The fast simulations generates the initial particles of the truth event and then propagates them through the detector. At each surface, particle interactions are simulated using parametric models and potential hits are registered.

There are two options for generating the initial particles: a particle gun or the Pythia8 generator. The following command generates 100 events using the particle gun, in a 2T magnetic field, and writes the generated events into the sim-pg directory:

$ <build>/bin/ActsSimFatrasGeneric \
    --output-dir=sim-gun \
    --output-csv=1 \
    --evg-input-type=gun \
    --bf-value=0 0 2 \
    --events=100

For each event, the following files will be created

  • event<number>-particles_generated.csv

  • event<number>-particles_initial.csv

  • event<number>-particles_final.csv

  • event<number>-truth.csv

  • event<number>-hits.csv

  • event<number>-cells.csv

where <number> is the event number. The first three files contain the generated particle states, initital and final states of simulated particles. The simulated particles differ from the generated particles: particles could not be simulated due to kinematic cuts or additional particles might have been generated due to interactions. The truth contains the true intersection with all surfaces, while the hits and the cells describe the simulated detector readout.

When generating particles with Pythia8, the event is build from a hard scatter interaction and additional pileup interactions. The following commands will generate top-pairs as the hard scatter interaction and uses the default soft QCD process for the additional 140 (on average) pileup interactions. The detector setup is unchanged.

$ <build>/bin/ActsSimFatrasGeneric \
    --output-dir=sim-pythia8 \
    --output-csv=1 \
    --evg-input-type=pythia8 \
    --evg-hard-process=Top:qqbar2ttbar=on \
    --evg-pileup=140 \
    --bf-value=0 0 2 \
    --events=100

The output file structure will be the same as discussed above.