PhysicalVolumeInfoCollection
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Actually, the definition of a data product is slightly broader than this.
The art::Run object is just an art::RunId object plus a collection of data products;
similarly for the art::SubRun object. An example of a data product in the Run record
is PhysicalVolumeInfoCollection. When we intialize Geant4, we do it
in the beginRun method of Mu2eG4/src/G4_plugin.cc. At the end of Geant4
initialization we
create a std::vector of all physical volumes known to G4 and we put this
information into the art::Run object as a PhysicalVolumeInfoCollection.
The SimParticle class holds two indices into this collection; one describes the
volume in which the particle was born and the other describes the volume in which
the particle stopped.
The components are:
- PhysicalVolumeInfo - a volume number and text name of the volume
- PhysicalVolumeInfoCollection - a vector of PhysicalVolumeInfo
- PhysicalVolumeInfoMultiCollection - a vector of PhysicalVolumeInfoCollection
The PhysicalVolumeInfoMultiCollection is needed because different stages of a simulation might use completely different geometries, which have different numbers for the volumes.