[Frost] pi0 mass

[Michael Dugger]

Hi,

I am seeing differences in the pi0 mass when I plot the pi0 yield as a 
function of massX as opposed to massX^2.

Because to CLAS resolution issues, the best way to plot the pi0 yield 
distribution is how Sung has (massX^2).

For the g1c and g8b data, we extract pi0 yields using massX^2. We do this 
to make sure we are not losing pi0 events due to the yield being "pinched" 
off at massX<0. I did not think that this "pinching" effect would change 
the mass centroid of the pi0 peak by much. In the plots using massX, the 
pi0 mass is being shifted upwards by ~10 MeV for Butanol, and ~4 MeV for 
CH2.

Plots:
massX CH2:       http://www.jlab.org/~dugger/g9/g9a/tmp/mXCH2.gif
massX^2 CH2:     http://www.jlab.org/~dugger/g9/g9a/tmp/mXsqCH2.gif
massX Butanol:   http://www.jlab.org/~dugger/g9/g9a/tmp/mXButanol.gif
massX^2 Butanol: http://www.jlab.org/~dugger/g9/g9a/tmp/mXsqButanol.gif

Notes:
The only correction applied is eloss
BLACK-> no Carbon subtraction;
RED-> scaled Carbon;
Blue-> Carbon subtracted

Results:
massX from massX CH2 plot = 142 MeV
massX from massX^2 CH2 plot = 138 MeV
massX from massX Butanol plot = 150 MeV
massX from massX^2 Butanol plot = 140 MeV

This is good news :) The pi0 mass situation is not as bad as we 
have previously thought. Instead of the masses being different for the 
Butanol and CH2 targets by 8 MeV, they are only different by about 2 MeV.

The difference between the pi0 mass as measured by massX distributions as 
opposed to massX^2 distributions is being driven by CLAS momentum 
resolution. This might explain why Hideko sees such large differences in 
the pi0 mass as a function of CLAS angle.

For E_gamma/E_e, Sung is seeing pi0 masses for the Butanol and CH2 targets 
that are within 5 MeV of each other using photon energy corrections that 
(look) to be similar for each target. There is a good chance that the 5 
MeV might be attributable to differences in the Carbon subtraction, the 
lack of background contributions being part his pi0 fit routine, and 
differences in the photon energy correction.

Since we can not have a target dependence in the photon energy corrections 
it would be good to see the pi0 mass for the Butanol and CH2 targets when 
the photon energy correction is made to be identical to each target.

Sung, can you make your preliminary energy and momentum corrections 
available to fellow FROST collaborators? It would be good to check the 
consistency of pi0 mass results between different groups.

Take care,
Michael