[Halld-offline] choice of tagger hit / beam bucket for analysis

[Richard Jones]

Dear GlueX Offliners,

I was about to check in a change to hdgeant, but decided to solicit your
input on it first since it has the potential of complicating life for those
doing analysis studies. I think this complication is needed in preparation
for analyzing real data, but it needs to be anticipated and phased in
correctly.

Right now multiple tagger hits, both microscope and fixed array hodoscope
hits, are being recorded by hdgeant and saved in the output hddm event
record. The tag times are already locked to the accelerator RF, so the
values are integer multiples of 2ns. This means that, in principle, anyone
doing reconstruction should be looping over all tags present in the event
and computing all times measured in the detector (Start Counter, drift
times, Bcal and FCal hits, TOF, etc.) relative to the particular tag of
reference. Together with the tag hit time, there is also the tag energy
recorded in the hit record as well as the tagger channel number, for easy
reference.

However, I believe that essentially no one is doing this. If you just
naively assume that the simulated event is timed against the "correct" RF
bucket and let t=0 mark the time that the interacting photon bunch crosses
through the target midplane, you do get consistent answers at present. As a
result, I think we are not making any use of the tagger in the
reconstruction. Therefore I propose that the correct beam bucket for the
true tag be smeared in the simulation by assigning the event time reference
to a random beam bucket near but not fixed to t=0. I was thinking a RMS of
+/- 5 beam buckets would be a good estimate for the time resolution of the
level one trigger. We could store/fetch this rms trigger time in the ccdb,
to make it keep up with improvements in the online.

What do you think?

-Richard Jones
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