Re: Question on Pythia Background Normalization

[Alexander Somov <somov@jlab.org>]

Hello Matt,

Here are some thoughts:

I think that the ration of 120:1, given that the
signal xsection is 1 mu-b for 9 GeV, is about correct.
We assume that Level1,3 triggers are able to select
(reconstruct the  beam energy) events in the energy range
8.4 -9 GeV and reject low-energy beam interactons.

But you have to generate hadronic background in the range
between  8.4 and 9.0 GeV (where the trigger efficiency is
expected to be almost flat). If you genererate background
for the entire photon energy spectrum (0.15 GeV < E), the ratio
should indeed be larger.

(Ideally, it would be nice to tune the signal event generator
to allow for event generation in some specific energy range rather
than for a fixed beam energy)

Cheers,
       Sascha



On Thu, 31 Jul 2008, Matthew Shepherd wrote:

>
> 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
>
>
>