[Primex] pi0 prl article

[tulio at if.usp.br]

Dear Ashot and PrimEx members

Following a democratic policy, I would appreciate to have a feedback  
of my messages, since I am also a co-author of the pi_0 prl. I have  
sent an e-mail (below) more than one month ago (July- 11) and did not  
receive any response by now.

So, once again I forward my comments to the pi_0 prl (below) with the  
following additional information:

(1) Ref. [20] is now obsolete; it does not reflect the tables that I  
have provided for the PrimEx collaboration, including my event  
generator for the NI process. My recent results for pi_0  
photoproduction are all included in my new article which was accepted  
in PRC and will be available online quite soon. You can use Ref. [20]  
by now but it would be necessary to replace it by the new one, once it  
is available. We can do this during any resubmission of the pi_0 prl  
(if any) or during the analysis of the proofs (after acceptance);

(2) Table I looks really strange for me. It is stated in the text that  
the fitting parameters reflect the weighted average between two  
groups, but I am afraid this is not the case. The latest results from  
Dustin that I have acknowledge show that the NC fitting parameters are  
close to 1.8 for C and 1.1 for Pb  
(http://www.jlab.org/primex/collab_meetings/2009_06/mcnulty_pi0width_final1.pdf, slide 9). If Table I reflects the average, then we can infer that Ilia?s NC parameters are negative for C and close to zero for Pb, since the averages are 0.83 and 0.69, respectively. Obviously that this is not the case and I would appreciate your feedback, since I consider this Table crucial in the  
publication.

With my best regards,
-Tulio



Message sent on July-11 by T.E. Rodrigues:

Dear Ashot and all,

Just few comments to the pi_0 PRL.

1. There is a hierarchy problem with the two models used to fit the NI  
part. It seems
that the cascade model was used only for double checking Sergey?s  
calculations and this
is not in accordance with the historical facts and the current status  
of the PrimEx
analysis. The cascade model (first version) was published more than 5  
years ago and shed
a light into the subject of incoherent photoproduction. It is also  
effectively used in
one of the analyses (although it was agreed that two independent  
models for the NI cross
section should be used in all the analysis) and it should be quoted in  
the paper in the
same foot as Sergey?s calculations. So I would suggest that the  
corresponding paragraph
(p. 3, column 2, line 30) is modified to explicitly mention that two  
models were used to
fit the data, not only to double check the model uncertainty.  The  
updated version of the
cascade model is about to be published in PRC and if times permit I  
will send the
reference to be included in the pi_0 PRL.

2. I agree with Aron that the fitting parameters of the cross sections  
should be
addressed in much more detail in another paper. It is clear that Table  
I does not reflect
the weighted average between the analyses and a more complete table  
with further
explanation is required. I personally believe that this discussion  
should be left for the
near future since the parameters (maybe statistically inconsistent for  
C and Pb) would
raise questions not directly related with the goal of the PrimEx; the  
pi_0 decay width.
For instance, I have just found that an energy cut of about 100 MeV  
was used in the
analysis (p. 3, column 1, line 1) and I was working with a pi_0  
elasticity of about 0.92.
Consequently, the NI cross sections (mainly for Pb) are expected to be  
a little bit
smaller and the fitting parameters even closer to unit. I found at 2.5  
degrees 10 microb
for Carbon and 80 microb for Lead (numbers quite consistent with Figs.  
3 and 4 of the
pi_0 PRL) assuming 0.92 of elasticity and these numbers should  
decrease considering 0.98
of elasticity (1.8 % of energy difference between the tagged photon  
and calorimeter). So,
I would propose a more detailed work with the fitting parameters of  
the cross sections in
the near future that would provide a more comprehensive interpretation  
of the angular
distributions (with energy cuts) from PrimEx.
Unfortunately I won?t be able to join tomorrow?s meeting.

With my best regards,

-Tulio.

PS. I prefer my name with two initials if possible: T. E. Rodrigues.











Citando "Ashot Gasparian" <gasparan at jlab.org>:

>
>   Dear pi0 prl authors,
>
>  The latest version of the pi0 paper is in the attachment.
>  Please read one more time and send my your correction(s)
>  about the text if any.
>
>  We will send this article to the journal from this Friday.
>  So all corrections shoul be sent to me by that time.
>
>  Thank you for your contribution to this experiment.
>
>  Best regards,
>  Ashot
>
>
>
> .............................................................
> Ashot Gasparian                    Phone:(336)285-2112 (NC A&T)
> Professor of Physics
> Physics Department                       (757)-269-7914 JLab
> NC A&T State University              Fax:(757)-269-6273 JLab
> Greensboro, NC 27411               email: gasparan at jlab.org
> .............................................................
>



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