Question on Pythia Background Normalization

[Matthew Shepherd <mashephe@indiana.edu>]

Hi Richard, Eugene, (and whoever else might know the answer to this  
question),

I'm thinking about about this technique of using PYTHIA  to generate  
backgrounds for specific signal channels.  I'm afraid I may have  
overlooked a normalization detail in the past.

In the past, say for the calorimetry review, we took the total  
photoproduction cross section at 9 GeV is 120 mu-b.  Then we take some  
signal process with a cross section of 1 mu-b.  To create a mock data  
sample we then mix events from Eugene's bggen with signal in a 120 : 1  
ratio.

However, if I look at the control files and documentation for bggen I  
see that by default it appears to be pulling events from the full  
coherent brem spectrum (which makes sense!):

EPHLIM   0.15 12.       energy range in GeV

So I believe what we are getting then is not the 120 mu-b at 9 GeV,  
but instead the total photoproduction cross section integrated over  
the coherent brem energy distribution.  I'm not sure what this cross  
section is, but it will be larger than the 120 mu-b which assumes the  
beam energy distribution is a delta function at 9 GeV.  That means  
that if I take raw events from bggen the mixture with signal needs to  
be much higher than (for our example) 120 : 1.  In other words we have  
been underestimating our backgrounds.

Alternatively, I could setup bggen to only pull events from a narrow  
window around 9 GeV by adjusting the EPHLIM card above.  When I do  
this, then I may correctly mix the two samples at the ratio of cross  
sections at 9 GeV.

In this latter case, I'm neglecting things produced by photons outside  
of the peak of the coherent brem spectrum.  If these events cause  
pileup or background problems then I'm neglecting this background.   
One can imagine this latter case is equivalent to requiring that I tag  
the beam photon.  Then I'm strictly interested in signal / background  
separation for 9 GeV photons.

Do you agree that the logic above is correct?

-Matt