[Clascomment] CLAS paper color transparency

[GUIDAL Michel (57321)]

Hi Kawtar,

I have one main question on your CT analysis. I wonder why you
have rho peaks (i.e. M_pi+pi- spectra) so "clean" (your fig.4), 
especially compared to the ones that were obtained in the analysis
with a "simple" proton target (e1-6 analysis, published in 
Eur.Phys.J.A39:5-31,2009; look at particularly at Figs. 4 and 11).

It looks to me that the cuts you used are not so different
from the ones used in the e1-6 analysis (for instance,
I think that you use W>2 while W>1.8 was used in e1-6;
you also make a cut to select low t's but these low t's
are also part of the e1-6 analysis and since low t's dominate
the cross section, the integrated spectra (over t) shoud largely
reflect the low t spectra).

In the e1-6 analysis, non-resonant 2-pion contributions
(which have plenty of sources: Delta++pi-, Delta0pi-,
incoherent ppi+pi-, etc...) under the rho0 peak could sometimes
reach 50%. And actually a systematic uncertainty of ~20% was 
associated with this "background" subtraction since actually
no existing model on the market coud satisfactorily reproduce
all the dependences (xB, Q2, t,...) of these M_pi+pi- spectra.

So, my question is why the situation (shape, normalization,...
of the 2-pion spectra) is so different in your case ?
And, as a corrolary, how can you be so confident in your non-resonant
2-pion contribution subtraction under the rho0 peak since I am
not aware of any fully reliable model in this energy region
for this process (and what systematic error did you attribute to it) ?

Amities,

Michel

***
Michel GUIDAL
Institut de Physique Nucleaire
Bat 100 - M052
91406 ORSAY Cedex
Tel: (33) 01 69 15 73 21
Fax: (33) 01 69 15 64 70
E-mail: guidal at ipno.in2p3.fr
***