[G8b_run] Tagger sag correction

[William J Briscoe]

I think that we should consider replacing the focal plane after removing it.
Age gets us all...

-----Original Message-----
From: g8b_run-bounces at jlab.org [mailto:g8b_run-bounces at jlab.org] On Behalf
Of Barry Ritchie
Sent: Thursday, August 06, 2009 10:39 PM
To: Michael Dugger; g8b
Subject: Re: [G8b_run] Tagger sag correction

I believe what Mike has uncovered here is a continuing increase in the
sag of the focal plane that is growing with age. (A sagging saga, so to
speak.) 

I think he has established that it is very important that EACH running
period with the tagger perform such an analysis to establish the
correction for that running period. (This means the first FROST running
period, too, of course.) 

It also probably would be quite valuable during the next FROST runs to
do a (greatly reduced intensity, of course)
accelerator-beam-on-tagger-focal-plane test just to tie the two parts of
energy definition together.   

Also, while I understand the tagger magnet will be left in place when
CLAS is upgraded, it might be a good idea to remove the focal plane.

---BGR

Professor Barry G. Ritchie
Department of Physics
Arizona State University
Tempe, AZ  85287-1504
 
Telephone: (480) 965-4707
Fax: (480) 965-7954

-----Original Message-----
From: g8b_run-bounces at jlab.org [mailto:g8b_run-bounces at jlab.org] On
Behalf Of Michael Dugger
Sent: Thursday, August 06, 2009 12:27 PM
To: g8b
Subject: [G8b_run] Tagger sag correction


Dear g8b collaborators,

Stuart showed an energy dependence in the photon energy correction (for 
g8b data) that was not a simple multiplicative factor. I decided to
verify 
Stuart's results. I found that there is an energy dependence in the
photon 
energy correction that is not a simple multiplicative factor. It appears

that the tagger sag correction applied to g8b data was not sufficient.

The method I had been using previously (massX^2 vs. incident photon 
energy) can not be used to determine an energy correction that is 
different from a common multiplicative factor. Therefore, I am now using

an iterative routine that capitalizes on the overdetermined reaction
gamma p -> p pi+ pi- (as is standard in this type of study). This 
iterative routine has been successfully tested against the known energy 
correction obtained by the CMU group for the g1c data set.

The g1c CMU energy correction plot (figure 9 on page 12):
http://www1.jlab.org/ul/Physics/Hall-B/clas/public/2004-017.pdf

compares well to the ASU iterative routine
http://www.jlab.org/Hall-B/secure/g8b/ASU/egCorr/g1cEgCorr.gif

The CMU energy correction has one free parameter. This parameter is an 
offset to the fractional photon energy (E_gamma/E_e). For g8b data, in 
addition to the standard CMU energy correction, I have included a term 
that is a simple multiplicative factor:
energy_correction -> CMU_correction + E_gamma*multiplicative_factor

The result of this 2 parameter fit can be found at
http://www.jlab.org/Hall-B/secure/g8b/ASU/egCorr/g8bEgCorr.gif

The three bump structure of the tagger sag is clearly seen.

The fit results:
multiplicative factor = -0.00351(6)
offset = 0.00134(3)

The function is very simple and a FORTRAN version of the g8b photon 
energy correction can be found at
http://www.jlab.org/Hall-B/secure/g8b/ASU/egCorr/cmuErgCor.f

I have decided not to produce a stack of slides fully describing the
steps 
taken to obtain the results shown in this email. Instead, I will write a

CLAS note that can be easily referenced from analysis notes. I hope to 
have the note completed in the next week or two.

Sincerely,
Michael

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