How HDGeant defines time-zero for physics events

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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 time the particle emerges from the gun is defined to be zero. The other two cases to consider are:

  1. the internal coherent bremsstrahlung beam generator: time0 is defined as the instant the beam photon being simulated passes through the midplane of the GlueX target, or would have if it had not interacted upstream of that plane.
  2. 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 GlueX 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.

How?

In Geant3 the clock that maintains the current time during tracking is the variable TOFG in common block /GCTRAK/. When the user code calls GSVERT followed by GSKINE for each of the final-state particles to be simulated for a given event, each particle is pushed onto the tracking stack together with a copy of the value that was in TOFG at the time GSVERT/GSKINE was called. Thereafter as the simulation proceeds for each track, its clock advances from the start time that was initialized when the event was declared. Most users assume that it starts from zero, but it is better to explicitly set it. The computation of the value of TOFG at event startup is centralized in a single-line fortran function called settofg(vertex,t0), which has the following header.

      subroutine settofg(vertex,time0)
      real vertex(3)        ! cm
      real time0            ! ns
*
* Sets the Geant variable TOFG which determines the start time of the
* tracking for subsequent particles placed on the primary stack.  The
* start time is determined assuming a beam photon is being generated.
* It is set so that the photon will cross the reference time plane at
* TOF=t0 if it makes it that far.

The first argument gives the position of the photon interaction that produces the event, so that the time offset can be computed to its passage through the target midplane. Only the third component is used at present, but it is a 3-vector because an argument of this type is needed for the call to GSVERT that is needed anyway to initialize the event. The second argument is there to provide an additional time offset, in case the interacting photon is from a different beam bucket than the one that generated the trigger for the event. Most of the time the call is made with time0=0. For a description of an exception, see the next section. The formula used by settofg is

      TOFG=2*int(time0/beam_bucket_period_ns+0.5)*1e-9
     +    +(vertex(3)-reference_time_plane_z)/CLIGHT
where the meaning of parameters
beam_bucket_period_ns
,
reference_time_plane_z
and
CLIGHT
are self-evident.

Where?

Why?