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o2-sim: Improvements for media initialization order and strong track-seeding for Geant4 - #15663
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Fixing a long standing error message about "No magnetic field found" when initializing tracking media. Fixed by reordering of initializations.
SimCutParams.trackSeed was so far a no-op with Geant4 because seeding only occurred in the Geant3 stack-pop path. Add the missing seeding hook in O2MCApplicationBase::PreTrack(), which Geant4 invokes for both primary and secondary tracks on their first step. Geant3 continues to seed at stack-pop time. This preserves existing behaviour and avoids moving seeding to a later hook, which measurably reduces reproducibility. The PreTrack seed is derived from the engine track state rather than Stack::GetCurrentTrack(), which is only reliable for Geant4 primaries. Also add diagnostics to detect missing seed propagation by counting successful seed applications and warning at the end of the event if track seeding was requested but never performed. Verified on both Geant3 and Geant4: per-track seeding now works as intended and existing Geant3 behaviour remains unchanged.
alibuild
commented
Aug 5, 2026
Collaborator
Removed redundant comments about Geant3 seeding comparison.
alibuild
commented
Aug 5, 2026
Collaborator
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Fixing a long standing bug in the order of initialization between magnetic field and media (harmless).
Providing strong track seeding and better reproducibility for Geant4 (so far only worked for Geant3).