When determining truth information during extraction from i3 files, if the frame doesn't contain MCInIcePrimary, the code assumes that the first item in the MC tree is the in-ice primary (this line). For NuGen, this is not always the case - the injected neutrino is injected anywhere in the atmosphere, and may interact before reaching the detector. In this case, the in-ice neutrino that produces the interaction whose detector response is simulated may be a different particle (at a different place in the MC tree).
When determining truth information during extraction from i3 files, if the frame doesn't contain
MCInIcePrimary, the code assumes that the first item in the MC tree is the in-ice primary (this line). For NuGen, this is not always the case - the injected neutrino is injected anywhere in the atmosphere, and may interact before reaching the detector. In this case, the in-ice neutrino that produces the interaction whose detector response is simulated may be a different particle (at a different place in the MC tree).