Revise how GenParticle status is set for particles created during the simulation
HepMC3 does not natively support the barcode property for GenParticle or GenVertex.
For the initial migration from HepMC2 to HepMC3, then the solution was to add barcode
as an IntAttribute. This worked, but had performance penalties.
The longer term solution is to migrate to using a combination of GenParticle/Vertex::id()
and GenParticle/Vertex::status() to hold the information currently held in the barcode
attribute.
The first step of this is to use the new status code scheme to encode origin information.
- Origin information currently held in barcodes:
- a) Particle produced during Evgen or Simulation (barcode > 200k)
- b) Link back to original parent Particle when particles survive interactions (barcode%1M)
- c) Number of interactions survived by particle (int(barcode/1M))
- d) Vertex produced during Evgen or Simulation (barcode < -200k)
Sketch of how GenParticle status should be updated for the new scheme.
| Scenario | Old status | Old barcode | New status |
|---|---|---|---|
| Nth Truth particle created+decayed during evgen | 2 | N | 2 |
| Nth Truth Particle created, but not decayed during evgen | 1 | N | 1 |
| Nth Secondary particle created by an interaction/decay during sim | 1 | 200000 + N | 20001 |
| Generator created particle with has survived M interactions during sim (B = original barcode) | 1 | B + M*1000000 | 1 + M*100000 |
| Secondary particle created during sim, which has survived M interactions (B = original barcode) | 1 | B + M*1000000 | 20001 + M*100000 |
Sketch of how GenVertex status should be updated for the new scheme.
| Scenario | Old status | Old barcode | New status |
|---|---|---|---|
| Nth Truth vertex created during evgen | <1000 (generator specific) | -N | <1000 (same as old scheme) |
| Nth Truth vertex created during sim | 1000 + G4 process | -200000 - N | 20000 + 1000 + G4 process |
The new status scheme can be used to obtain (a), (c) and (d), but (b) will be determined by stepping back through the tree.
This merge request migrates the transient GenEvent classes to use the new status scheme. Barcodes remain unchanged. Persistient GenEvent classes remain unchanged and xAOD::Truth classes remain unchanged.
A more detailed discussion can be found here.
The next step will be to implement the new helper function for (b).