[Primex] the latest version of the prl paper

Sun Jul 11 09:42:27 EDT 2010

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.

>
>     Dear Aron,
>
>   We had a very active and exceptionally productive pi0 paper meeting on
>   Thursday, July 1th, (the minutes are on the website also) with a high
>   level representation of different groups in the PrimEx. I remember we had
>   long discussions of both questions you are raising in this letter.
>
>   If I am not mistaken, particular decisions (by simple majority agreements)
>   were made on those questions.
>
>   There we decided that it is not our task in this PRL paper
>   to judge about the work of others, including the PDG work also.
>   In particular we agreed to show the "QCD sum rules" based Ioffe
>   calculation, without questioning its value. We may do that in a
>   separate publication, if there are enough arguments for it.
>
>   About the Table I: we made a particular agreement/decision in that
>   meeting to have this table in this article. Also, I asked Dustin
>   to provide his coefficients for the table, there is no problem in
>   that at all.
>
>   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
> .............................................................
>
>
> On Fri, 9 Jul 2010, Aron Bernstein wrote:
>
>> Dear Ashot and all,
>>
>> 1)  Drop table 1. The values are from Ilya and not average as is  
>> stated in the text. Furthermore the values of C_NC are dependent on  
>> the form of the incoherent cross sections (Tulio or Sergey)  
>> although the value of Gamma is not.  The quantities in the table  
>> are not well defined in the text and their meaning is not clear.  
>> What is important is the relative contributions of each of the  
>> contributing cross sections. This is presented in the cross section  
>> figures and not commented on in the text. The main point of the PRL  
>> is to present the value of Gamma and the other quantities should be  
>> dealt with in a later, more detailed publication.
>>
>> 2)  Fig. 1. Include the particle data book average, as well as the  
>> e+e- result which is not included in the average for completely  
>> unknown reasons.
>>
>> I do not think that the QCD sum rule result should be shown in the  
>> figure. It is a model calculation and not on the same QCD level as  
>> the other chiral calculations. Furthermore the fact that it is  
>> lower then the other calculations is due to the fact that it does  
>> not properly include the eta-prime contribution. We have verified  
>> this by turning off this contribution and this is the result that  
>> is obtained in this case.  All of the authors of the theoretical  
>> papers were at the chiral dynamics meeting last summer in Bern and  
>> I discussed this issue with all of them. I am reporting the opinion  
>> of all of them. Clearly Ioffe is a respected theorist and his paper  
>> should be quoted and discussed.
>>
>> I would like to discuss these two points on Monday.
>>
>> Regards,
>> Aron
>>
>> PS. Please use my middle initial A.M. Bernstein in the list of  
>> authors and in ref. 6.
>>
>>
>> Aron M. Bernstein
>> Professor of Physics
>> MIT
>> Cambridge, MA. 02139
>> office: 26-419
>> phone: 617-253-2386
>>
>>
>>
>>
>>
>>
>> On Jul 9, 2010, at 12:45 PM, Ashot Gasparian wrote:
>>
>>>
>>>  Dear PrimEx Collaborators,
>>>
>>>  The latest version of the PRL draft is in the attachment.
>>>  It includes the most of suggestions done before yesterday
>>>  (Thursday) evening and it is formatted to fit 4 pages (it
>>>  was not an easy work).
>>>
>>>  This version does not include suggestions and comments received
>>>  from this morning (Friday). Many of them are very valuable
>>>  and we will try to implement them in the text.
>>>
>>>  Please keep commenting on the draft before Monday's Collaboration
>>>  meeting, where we intend to finalize the most part of the paper.
>>>
>>>  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
>>> .............................................................
>>> <pi0_prl_draft_10.pdf>_______________________________________________
>>> Primex mailing list
>>> Primex at jlab.org
>>> https://mailman.jlab.org/mailman/listinfo/primex
>>
>>
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