[Frost] missing mass plot of missing proton in the double-pion photoproduction (updated)

[Michael Dugger]

Sung,

I would not be too concerned about the pi- momentum distribution 
comparison for the different topologies. There could be a problem, but 
there might not be. It is difficult to tell from the plots shown.

The momentum distribution you have for the case when all of the particles 
are seen looks similar to what Brian V. gets (see 
http://www.andrew.cmu.edu/user/bvernars/FROST_update_2010-10-22.pdf). The 
"knee" of the pi- momentum distribution at about 300 MeV/c is noticeable in 
both yours, and Brian's distribution. When you don't see the pi+, (e.g. 
(pi+) pi- p) the "knee" gets enhanced to the extent of making the 
distribution look bimodal, while the missing pi+ develops a "knee" like 
feature.

You should keep in mind that the pi- track will be bent towards the beam 
line while the proton and pi+ are bent outward. This means that the 
different topologies will have very different acceptances/efficiencies. 
For the case where you are missing the pi+, you are looking at momentum 
distributions that are weighted towards high pi- and proton 
acceptance/efficiency while also weighting towards low pi+ efficiency.

To get a better comparison you might want to look at the momentum 
distributions binned in angles. I would expect that the momentum 
distributions for a fixed angle bin to look much more consistent for each 
topology.

If the differences in the momentum distribution still concerns you after 
looking at fixed angle bins, the next step would be to Monte Carlo the 
reaction and see if the experimental distributions compare well with the 
Monte Carlo.

Missing mass of the proton: When I require that I see all of the particle 
(pi+ pi- proton) but leave the proton out of consideration (i.e. create 
mass X from gamma p -> pi+ pi- X) I get a massX^2 distribution that looks 
OK, and can be seen at
http://www.jlab.org/~dugger/tmp/mPro2Sq.gif

When I remove the restriction that a proton was seen in the event, I get 
a massX^2 distribution that look much more like yours. This can be found 
at
http://www.jlab.org/~dugger/tmp/mProSq.gif

The massX^2 distributions rely on the CLAS momentum resolution of the 
pions, these pion resolutions appear to be better where there is a higher 
chance of seeing the proton. I think you might need to live with the 
idea that the the proton missing-mass distributions are not that great. 
There is a chance, however, that additional corrections to the CLAS 
momentum will make the proton missing-mass distributions better.

Take care,
Michael

On Mon, 8 Nov 2010, Sungkyun Park wrote:

> Hi all,
>
> Here is the update of my last meeting presentation.
>  http://hadron.physics.fsu.edu/~skpark/research/research_nov0710.html
>
> I add missing mass plots in the missing proton and several momentum range.
> I also add the momentum and missing momentum of each topology.
>
> There is some problem in the pi- momentum in topology gamma p -> p pi- (pi+).
> The condition of the missing mass peak of proton in topology gamma p -> pi+ pi- ( p ) is not good.
> Do you have any idea what are the reasons?
>
> Sung
> Florida State University
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