Where can I find what the fields in MCParticles mean?

[wdconinc]

• What do reason, mask, steps, secondaries, colorFlow, spare in MCParticles mean?
• What is the difference between mcparticles.vs.x and mcparticles.ve.x?
• I see properTime is interspersed with several "0" vaules. What does it mean?
• There are g4Parent and parents_begin and parents_begin.
• What are meaning of mcparticles_0 and mcparticles_1?

[wdconinc]

The explanation of the fields in the MCParticles collection of data type MCParticle is included in the edm4hep data model definition. It is reproduced here for version v00-05:

  edm4hep::MCParticle:
    Description: "The Monte Carlo particle - based on the lcio::MCParticle."
    Author : "F.Gaede, DESY"
    Members:
      - int32_t PDG                         //PDG code of the particle
      - int32_t generatorStatus             //status of the particle as defined by the generator
      - int32_t simulatorStatus             //status of the particle from the simulation program - use BIT constants below
      - float charge                    //particle charge
      - float time                      //creation time of the particle in [ns] wrt. the event, e.g. for preassigned decays or decays in flight from the simulator.
      - double mass                     //mass of the particle in [GeV]
      - edm4hep::Vector3d vertex              //production vertex of the particle in [mm].
      - edm4hep::Vector3d endpoint            //endpoint of the particle in [mm]
      - edm4hep::Vector3f momentum             //particle 3-momentum at the production vertex in [GeV]
      - edm4hep::Vector3f momentumAtEndpoint   //particle 3-momentum at the endpoint in [GeV]
      - edm4hep::Vector3f spin                 //spin (helicity) vector of the particle.
      - edm4hep::Vector2i colorFlow                //color flow as defined by the generator
    OneToManyRelations:
      - edm4hep::MCParticle parents // The parents of this particle.
      - edm4hep::MCParticle daughters // The daughters this particle.

Earlier versions of the output files (before the adoption of edm4hep) followed a format which followed the dd4pod data model definition, e.g. reproduced here for v1.0.0:

  dd4pod::Geant4Particle:
    Description: "Podio implementation of dd4hep's dd4hep::sim::Geant4Particle class"
    Author : "W. Armstrong, S. Joosten"
    Members:
      - int32_t                   ID                // unique identifier for this particle
      - int32_t                   g4Parent          // g4 parent particle
      - int32_t                   reason            // TODO document
      - int32_t                   mask              // TODO document
      - int32_t                   steps             // TODO document
      - int32_t                   secondaries       // TODO document
      - int32_t                   pdgID             // TODO document
      - int32_t                   status            // TODO document
      - std::array<int,2>         colorFlow         // TODO document
      - int32_t                   genStatus         // Generator status code
      - int32_t                   charge            // Particle charge
      - int32_t                   _spare            // extra int to have doubles properly aligned at 8 bytes
      - std::array<float,3>       spin              // Particle spin state
      - dd4pod::VectorXYZ         vs                // Start vertex [mm]
      - dd4pod::VectorXYZ         ve                // End vertex [mm]
      - dd4pod::VectorXYZ         ps                // 3-momentum at start vertex [GeV]
      - dd4pod::VectorXYZ         pe                // 3-momentum at end vertex [GeV]
      - double                    mass              // particle mass [GeV]
      - double                    time              // start vertex time [ns]
      - double                    properTime        // proper time
    VectorMembers:
      - int32_t                   parents           // parent IDs
      - int32_t                   daughters         // daughter IDs