Re: Question on Pythia Background Normalization

[Richard Jones <richard.t.jones@uconn.edu>]

Matt,

I am coming into this conversation a bit late, but maybe should give an
overview of the purpose of the BGRATE / BGGATE settings in Geant,
within the context of both hadronic and electromagnetic backgrounds. 
In this discourse, we are using the term "background" in two different
ways.

1. background = undesired events in the detector that somehow pass the trigger conditions and get into the event stream, but which did not come from a tagged hadronic photoproduction in the target.
   
2. background = tracks or track stubs, em showers, and other hits that are not associated with the primary event that satisfied the trigger.

I suggest that we continue to use the term "background" for item #1,
and that we switch to "contamination" for item #2.  I claim that
hadronic interactions by the untagged part of the beam are responsible
for essentially all of item #1, and that electromagnetic interactions
are responsible for essentially all of #2 that we care about in the
detector [see PS below].  I set up the BGRATE / BGGATE facility of
HDGeant to enable us to study contamination.  If you accept my above
assertions, then you will agree that Pythia generator is the correct
way to study "background".  There is no facility within Geant that can
do a convincing job of simulating these high-energy photohadronic
interactions.  Eugene has set it up to be properly weighted with the
flux energy profile of the photon beam, I believe.  For the
normalization, you can use the following rule of thumb.

• at 10^7 tags/s rate, 36KHz of total gamma,p hadronic rate from pion threshold to the endpoint, which corresponds to 1.1e9 Geant events running with card "BEAM 12. 9.0 0.12".
  
• at 10^8 tags/s rate, 360KHz of total gamma,p hadronic rate from pion threshold to the endpoint, which corresponds to 1.1e10 Geant events running with card "BEAM 12 9.0 0.12".

Matt, something in your earlier message puzzled me.  There was some
funny card "EPHLIM" you mentioned.  Is that something you added
yourself?  There is no such card in the standard svn release.  The way
you control the photon beam generator is through the BEAM card, as
described above.

Richard Jones

PS. The claim that "contamination" comes from EM processes might be
oversimplified in the case of rare decay searches, where relatively
rare hadronic pileup effects might be able to fake the signature of a
rare event.  I haven't thought much about the specifics of this, but it
might be worth talking about.  Right now, in HDGeant I do not have the
capability of simulating this.  You might get a crude estimate of the
effect offline just by merging the hits from two different events, but
this is not accurate enough to be reliable because the relative timing
between the two pileup events will not be random.  If this is a real
concern then we need to add the capability to do this in HDGeant.