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FlavorTagDiscriminants: Use HepMC::UNDEFINED_ID rather than -2 for "no truth...

John Derek Chapman requested to merge jchapman/athena:FlavorTagDiscriminantsBarcodeMigrationPrep_main into main

This merge request is part of the preparation for migrating Athena away from using GenParticle barcodes (removed in HepMC3) as a unique identifier for truth particles to GenParticle::id() instead.

Currently barcode decorations in the FlavorTagDiscriminants package use -2 to indicate "no truth particle associated to a track", this is because the value of -1 which was already reserved for padded tracks in our fixed dimension array in the FTAG ntuple dumper

In the current version we switch to using HepMC::UNDEFINED_ID(=0) rather than -2 for "no truth particle associated to a track".

An alternative option would be to switch using -2 or -999 for the padding and use HepMC::INVALID_PARTICLE_ID(=-1) to indicate "no truth particle associated to a track".

Leaving as draft for now while we discuss.

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