[Halld-offline] {SPAM?} memory usage for bggen events

[Justin Stevens]

Hi Richard, All,

On Feb 10, 2014, at 6:01 AM, Richard Jones wrote:

> I cannot think of any study that is going to fail because we generated a factor 2-3 fewer events, but there are plenty of things that can go wrong if we fail to simulate realistic backgrounds in the detector.  I see no advantage to generating even a part of the DC sample with less than 10^7 g/s background rates and the full 1us gate.

I completely agree that we have to face the background issue head-on, but it seems to me that a sub-sample without background could still be useful for a comparison.  We have some understanding of the event selection techniques that have been developed so far without background, and being able to connect with those using the updated tracking, etc. in the software since DC 1 would be useful in my opinion.  Also being able to compare an analysis done on MC with and without background may provide some insight into what features of the background are the most problematic and how to reduce those effects in the analysis.

> With more time and effort, we could probably come up with a way to generate a library of background events and overlay random entries from the library onto the bggen events at the desired rate.  This is not trivial to do because the superposition must be carried out at the hits accumulation level inside Geant before hits aggregation, but given the computing cost it is likely the way to go.  

In the other experiment I'm involved with (STAR) this superposition is done with background events from data (once it's available), so the superposition is done by combining raw hits for the physics event from Geant and raw hits for the background event from data.  Is there something about the GlueX simulation which requires that "the superposition must be carried out at the hits accumulation level inside Geant before hits aggregation"?  I was (maybe naively) assuming the mechanism that would be developed for this in GlueX would occur outside of Geant for superimposing with data as well.  I'm not suggesting we wait for this for DC 2, just trying to understand the issue.

Thanks,
Justin