[Frost] Analysis Update

[Michael Dugger]

Dear Jo,

I have a few comments.

-------------------------

General comment:
It looks like you are making good progress. I think you might be the first 
person to analyze the linearly polarized photon data. The phi 
distributions look promising.

-------------------------
Scale factor:

I have a plot the phase space dependent scale factors for the reaction
gamma p -> pi+ X at

http://www.jlab.org/~dugger/g9/g9a/tmp/pipScaleFactors.gif
The y-axis is the scale factor I get and the x-axis is massX.

In your plot, it looks like you, essentially, use the massX region of 
~0.5 to ~0.9 MeV to get the scale factor. I estimate that your scale 
factor is only off by about 4 to 5%. This is fairly good considering that 
the current implementation of the phase space scale factors look to have a 
systematic uncertainty of about 3%.

For other reactions, using the direct scaling would be worse. For example, 
a plot of the scale factors for the reaction gamma p -> p pi+ pi- can be 
found at
http://www.jlab.org/~dugger/g9/g9a/tmp/pipiScaleFactors.gif
where the x-axis is massX^2.
Direct scaling would give an error of about 7.5 percent for the higher 
massX^2 states.

For the gamma p -> p X reaction, the scale factor has a much wider range:
http://www.jlab.org/~dugger/g9/g9a/tmp/proScaleFactors.gif
This reaction simply does not allow for the direct scaling to give 
reasonable results.

-------------------------

Dilution factor:

You have to be careful with regards to the dilution factor. Assuming 
that you scale away all of the Carbon, the Hydrogen contamination of the 
Carbon target should not be very important. The neutron yield would simply 
be less, than had the Carbon target not any Hydrogen contamination. 
The fractional amount of lost Hydrogen signal coming from the 
Carbon-subtracted Butanol target should not depend on the the beam or 
target polarization. Since you are measuring an asymmetry, this fractional 
loss of neutron yield should cancel out when you calculate the asymmetry.

This canceling effect of the Hydrogen contamination is very fortunate. It 
is extremely difficult to estimate the amount of Hydrogen contamination in 
the Carbon. If this could be done by fitting a function to the Carbon 
neutron peak, we would have no need for scale factors. (You could simply 
fit the Butanol neutron signal to extract yields.)

-------------------------

Overall, I think that your analysis is progressing nicely.

Take care,
Michael




On Fri, 17 Dec 2010, mcandrew at jlab.org wrote:

> Hi all,
>
> Apologies that I won't be able to come to the meeting today.  I thought
> I'd send an e-mail round to let you know that I've updated my analysis
> page:
>
> http://www2.ph.ed.ac.uk/~s0783525/index.htm
>
> I've corrected the timing problems and added some phi-distribution plots.
> (I'm working on the delta-beta cut at the moment).
>
> I have also created a page discussing my carbon background subtraction:
>
> http://www2.ph.ed.ac.uk/~s0783525/carbon_background.htm
>
> I've also updated the links to my analysis on the meeting page.  I should
> have obtained some initial asymmetries by next week.
>
> Please feel free to e-mail me with any comments or questions.
>
> Have a good weekend,
> Jo.
>
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