[Halld-offline] [EXTERNAL] new branch of hdgeant4 with ancestry in mcTrajectoryPt

[Richard Jones]

Teagan,

If you look at the HDGeant4 github repo, you will see that there is a new
branch,

   - store_trajectory_ancestry

that you can build and run. When you turn on the TRAJECTORIES card, and
look in the output hddm file, you will see that the mcTrajectoryPoint has
changed.

In the standard build of hdgeant4, every other mcTrajectoryPoint
represented the particle simply crossing a geometry boundary, so the
physics mechanism was mech="0". That gave me a free slot to introduce the
ancestry information. Every time the standard code would write mech="0" I
now write mech="-N" where -N is the negative of the trackID of the parent
particle. That is, N is the value of the mcTrajectoryPoint that is shown
for the parent particle in its "track" attribute. This injection of
ancestry information is uniquely separated from the standard mech="SSSS"
information because SSSS is guaranteed to be a positive integer.

This is ready to go on https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_JeffersonLab_HDGeant4&d=DwIBaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=Te_hCR4EUlJ6iCDYLJ8Viv2aDOR7D9ZZMoBAvf2H0M4&m=QzekzYWE3EHJAmXffnZvQcTitwJdeVNzOlqfKfCupVU&s=yKhaH-LBLBAVPfwCUnKwN4ZWyA-TWKCuLKMViwoyGHw&e= . In a few
minutes you will also see a similar feature appearing on a branch with the
same name in https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_JeffersonLab_halld-5Fsim&d=DwIBaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=Te_hCR4EUlJ6iCDYLJ8Viv2aDOR7D9ZZMoBAvf2H0M4&m=QzekzYWE3EHJAmXffnZvQcTitwJdeVNzOlqfKfCupVU&s=pYZ28gFL0M-jxTqS9_zQgD66YgZY5cEj4tO7k0-a6QY&e=  for hdgeant.

-Richard Jones
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