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Detector simulation

The simulation program ddsim is part of the DD4Hep package, which uses as main components the ROOT geometry package, which is used for construction and visualization of geometry, and the Geant4 simulation toolkit, which can be interfaced via DD4hep to perform detector simulation in complex detector designs.

To run ddsim with a specific configuration file the following command should be used:

ddsim --steeringFile <path_to_the_steering_file>.py

Configuration file is a simple python script that configures an instance of the DD4hepSimulation object. For example the path to the detector geometry, assigned to the compactFile parameter, is extracted from the $MUCOLL_GEO environment variable, which holds the path to the default geometry of the software release:

SIM.compactFile = os.environ.get('MUCOLL_GEO')

You can check the actual path of the geometry in the terminal:

echo $MUCOLL_GEO

Any parameter defined in the steering file can be overriden via a command-line parameter. To see a complete list of available command-line parameters:

ddsim --help

Output of a simulation job is stored in the LCIO format and contains several kinds of objects:

  • input MCParticles;
  • potential secondary MCParticles created during the interaction with detector material;
  • SimHits representing energy deposits of individual MCParticles in the detector.

A number of different SimHit classes are available to represent the most relevant information for each type of subdetector, such as SimTrackerHit, SimCalorimeterHit, etc. In order to check the collections in the simulation output file: 

anajob output_sim.slcio

It is also possible to visually check the simulated events with the event display: 

ced2go -d $MUCOLL_GEO output_sim.slcio

Simulating H -> bb events

First create a dedicated folder for the simulation stage:

mkdir sim_Hbb && cd sim_Hbb

To simulate 10 H -> bb events from the sample generated in the previous step using the baseline configuration:

ddsim --steeringFile ../mucoll-benchmarks/simulation/ilcsoft/steer_baseline.py \
--inputFile ../gen_Hbb/mumu_H_bb_3TeV.hepmc \
--numberOfEvents 10 \
--outputFile output_sim.slcio

Alternatively you can modify the relevant parameters directly in the steering file, and execute it without any additional command-line parameters:

ddsim --steeringFile ../mucoll-benchmarks/simulation/ilcsoft/steer_baseline.py

Simulating single particles

Instead of storing particles to a file and then simulating them, it is also possible to generate them directly in ddsim as configured in the SIM.gun component. To activate the particle-gun the following parameter has to be enabled:

SIM.enableGun = True

Type of the generated particles and their kinematic distributions are configured like this:

SIM.gun.particle = "mu-" # muon
SIM.gun.energy = 10.0*GeV  # fixed energy at 10 GeV
SIM.gun.distribution = "uniform"  # flat in theta distribution    

SIM.gun.isotrop = True # isotropic distribution in Phi
SIM.gun.multiplicity = 1 # one muon per event
SIM.gun.phiMax = None
SIM.gun.phiMin = None
SIM.gun.thetaMax = 180*deg
SIM.gun.thetaMin = 10*deg

You can also define parameters of the beamspot for your particle gun sample, where you provide vertex sigma in cm. To get the correct beamspot position for the COM energy you are trying to simulate, you can refer to this accelerator table: https://muoncollider.web.cern.ch/design/general-parameters 

SIM.vertexSigma = [0.0, 0.0, 0.0, 0.0]