Hits in auxiliary tracking detectors from secondaries created outside the tracking region do not keep truth association

[simonge]

By design, particles which are created outside of the cylindrical tracking region are not saved to the MCParticles collection in order to limit the number particles from calorimeter showers. The current solution only works for the central detector. Far-Forward and Far-Backward detectors will see several hits from e.g. interactions with the beampipe creating showers of secondaries. Currently all of the hits from these particles are associated to (usually) the generator particle giving no access to separation of the background from secondaries.

Relevant code snippets
https://dd4hep.web.cern.ch/dd4hep/reference/Geant4ParticleHandler_8cpp_source.html

https://dd4hep.web.cern.ch/dd4hep/reference/Geant4TCUserParticleHandler_8cpp_source.html

Some potential solutions and issues:

• Use the quality flag in the SimTrackerHit, contains a ProducedBySecondary bit - Requires no changes to code base but no way to collect together hits/tracks from the same secondary.
• Additionally keep all particles which have a tracker hit - Background secondaries which only interact with the auxiliary calorimeters will still not be treated correctly. I think this might be close to the default when userParticleHandler (tracking region) is turned off.
• Create a more involved userParticleHandler class for our purposes.

Steps to reproduce: (give a step by step account of how to trigger the bug)

1. Run simulation with low-q2 events
2. Some electrons interact with beampipe and generate secondary particles which create hits in the lowq2 tagger

Expected Result: (what do you expect when you execute the steps above)

Hits from produced secondaries are associated to a MCParticles entry for the secondary particle

Actual Result: (what do you get when you execute the steps above)

Hits from produced secondaries are associated to parent MCParticles which left the tracking region.