How HDGeant defines time-zero for physics events
Normally Geant simulations produce hit times that are referenced to the instant of the interaction in the target. This is not the case for HDGeant, whenever there is an incident photon present or implied) for the event. The only exception to this rule is the trivial case of the particle gun, where the user specifies the location of the starting point for the single-track vertex via the SCAP input card, and the start time is defined to be zero. The other two cases to consider are:
- the internal coherent bremsstrahlung beam generator: time0 is defined as the instant the beam photon being simulated passes through the midplane of the target, or would have if it had not interacted upstream of that plane.
- events generated by an external Monte Carlo generator (eg. pythia, genr8): time0 is defined as the instant that the (implied) beam photon, whose interaction produced the MonteCarlo-specified final state, passed through the midplane of the target, or would have if it had lived that long.
That is the executive summary. If you only need to know the answer to what questions, then you have read far enough. If you want to know how, where, and (above all) why this behavior, then read on.
Case 1 above is covered in