[d2n-analysis-talk] Positron dilution

[Diana Parno]

Hi Matt,

> The two dilution factors differ quite a bit at low x and agree at  
> higher x.

You would expect pair production to be more and more negligible an  
effect as you move to higher and higher x, as you see in both your  
plots. The absolute difference between (1-D) and (1-D') should thus  
get very small at large x as a consequence of (1-D) and (1-D') getting  
very small. What does the relative difference  show?

> The first thing that I did was to use a run set (runs
> 2199-2212) where the BigBite magnet was in positive polarity mode,
> meaning we are detecting positrons. I applied our good electron cuts  
> to
> this positive polarity set and the cuts look fine. I then went ahead
> and computed a dilution factor.
>
>  D = 1 - (N+/N-)*(Q-/Q+),
> where N+ are the total events counted with BB set to positive polarity
>       Q+ is the total charge for the run set with BB set to positive
> polarity
>       N- are the total events counted with BB set to negative polarity
>       Q- is the total charge for the run set with BB set to negative
> polarity

I'm not very clear on how this worked, exactly. First, a nitpick: we  
detect both positrons and electrons in both positive- and negative- 
polarity mode. In positive-polarity mode, though, it's the positrons  
that bend up and thereby pass a charge==-1 cut. Now, my point of  
confusion. N+ and Q+ come from 2199-2212, according to your  
description -- but where do N- and Q- come from? What's the run set  
used "with BB set to negative polarity" in this part of the analysis?

In case the pion photoproduction process that gives rise to electron- 
positron pairs is spin-dependent, it would probably be a good idea to  
use runsets with a consistent target polarization direction, at least  
until we've seen how large (or small) the asymmetry in the positron  
data is.

> For this
> I used the BB negative polarity setting corresponding to runs  
> 2024-2195.
> I chose electrons using the BB.optics.charge[]==-1 cut and positrons
> using BB.optics.charge[]==1 cut.


I can think of one potential lurking problem here: I'm not sure how  
much weight we should put on our positive-optics calibration. In our  
present code, I took Xin's positive-optics calibration (which came  
from our data) and removed all the look-up tables and low-momentum  
corrections that made the reconstructed momentum spectrum look so  
nonsensical. However, the basic momentum reconstruction (from the  
first-order optics model) is not necessarily right, which may affect  
the way that the E/p cut is applied to the positron sample. Since our  
cut on E/p is so tight, I think it's worth a look to see whether the E/ 
p positron and electron spectra match -- with the Cerenkov and  
preshower cuts, most of our positive-charge data should be positrons.  
Gregg and I had been hoping that this calculation could be done simply  
with the positive-polarity data, so that there would be no need to  
revisit the positive BB optics.

Best,
Diana

On Apr 20, 2011, at 4:34 PM, posik at jlab.org wrote:

> Hi all,
>
>   So I spent a fair amount of today trying to get a positron dilution
> factor. The first thing that I did was to use a run set (runs
> 2199-2212) where the BigBite magnet was in positive polarity mode,
> meaning we are detecting positrons. I applied our good electron cuts  
> to
> this positive polarity set and the cuts look fine. I then went ahead
> and computed a dilution factor.
>
>  D = 1 - (N+/N-)*(Q-/Q+),
> where N+ are the total events counted with BB set to positive polarity
>       Q+ is the total charge for the run set with BB set to positive
> polarity
>       N- are the total events counted with BB set to negative polarity
>       Q- is the total charge for the run set with BB set to negative
> polarity
>
> This dilution factor can be seen  as a function of x below
> (pos_outrun_dilution.eps)
>
> After talking to Zein-Eddine, because we only have one positive  
> polarity
> setting at 4.7GeV, and I could not find any at 5-pass, he suggested
> looking at the positrons and electrons with in a common run set. For  
> this
> I used the BB negative polarity setting corresponding to runs  
> 2024-2195.
> I chose electrons using the BB.optics.charge[]==-1 cut and positrons
> using BB.optics.charge[]==1 cut. This time for the Dilution factor,  
> since
> the total charge for all these runs is the same for positrons and
> electrons, the dilution factor is:
>
>  D' = 1-N+/N-
> A plot of D' can be see below (pos_inrun_dilution.eps)
>
> The two dilution factors differ quite a bit at low x and agree at  
> higher x.
> I suspect that the shape change at lower x is due to the BB field  
> bending
> positrons away from the detectors when it is in negative polarity  
> mode.
> I am uncertain what dilution factor should be included in the
> asymmetries? Or if there is another way to go about getting the  
> positron
> contamination?
>
> Thanks
> Matt 
> < 
> pos_outrun_dilution 
> .eps 
> > 
> < 
> pos_inrun_dilution.eps>_______________________________________________
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