This page very briefly summarizes the "negative process ID" issue in GEANT.

The questions

• Once in a while, processes that should have two daughters in the final state, have only one left in the GEANT KINE bank. What's the reason for that?
• When looking at Monte Carlo events, one notices that sometimes the process ID is negative. Why?

The short answer

The slightly longer answers

• In general, there is a cutoff parameter in GEANT, which determines whether an electron is kept in the KINE bank or not. Soft electrons (which are obviously copious) at some point are dropped, while the energy they deposited is still accounted for. That explains the case when a e+e- pair was apparently created but only e+ (or e-) is actually found in the GEANT output.
• Now, the negative process ID issue. The rationale for that (and quite a bit of technicality) can be found in the "Notes on new shower track philosophy" page which is dated but relevant nevertheless.

  In summary, we are being selective in picking GEANT tracks to be saved. We ignore the history of particles generated in the showers (while of course the energy of the hits is fully accounted for). On the other hand, if the track in question does satisfy our criteria (which is typically to leave hits in a tracking detector such as TPC) we obviously need to save it. This leads to the situation where the information about the parent of the saved track, is effectively being lost, and we have a "disconnected vertex". This is flagged by the negative process ID.