[G8b_run] Tagger sag correction

[Michael Dugger]

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