[Gdh_lowq2] EG4 meeting

Tue Feb 18 14:48:31 EST 2014

I may be wrong, but I thought Xiaochao's analysis does NOT include detection of the neutron in the pi+ case. Then, you otherwise valid exclusivity argument is not quite as convincing - you may get a reasonably good measurement at the 20% level, but not as good as the dedicated Ae measurements Volker mentions.

- Sebastian

On Feb 18, 2014, at 2:12 PM, bosted at jlab.org wrote:

> Hi all,
>   Yes, there is some mixture in our measurement of Ae from
> A>2 nuclei like nitrogen. But, in the fully exclusive
> topologies, like e p -> e pi+ n (all particles detected),
> the exclusivity cuts reduce the A>2 contribution to a very
> small level (less than 10% for eg1-dvcs), due to the ability
> to cut on the neutron angle relative to that expected for
> a stationary target. Similarly for pi0 with the proton and
> both pi0 decay photons detected.
>   If CLAS had published Ae for exclusive pion production
> already, then it would be interesting to compare with
> pure proton results. However, as far as I can tell, almost
> nothing is published. If there are results for Ae
> for exclusive pi+ and/or pi0, can somebody please give
> me the references? In fact, the eg1-dvcs Exclusive Analysis
> Note Review committee asked for such a comparison, so
> I am vitally interested!
>   Yours, Peter
> 
> 
>> Hi Xiaochao,
>> 
>> thanks for the update. I know that Peter is always interested in Ae - I'm
>> a bit puzzled by that. As you know, what we measure in experiments like
>> EG4 is a rather haphazard average of Ae on a mix of proton, Helium,
>> Nitrogen and other elements. As a rough check (does it agree with
>> precision data on the proton alone) it may be useful (especially for pi+
>> which will be totally dominated by protons). So, I agree, the EXPERIMENTAL
>> systematic uncertainty is small (no dilution factor or Pt needed), but I'm
>> not sure what one can really learn.
>> 
>> - Sebastian
>> 
>> On Feb 18, 2014, at 2:06 AM, Xiaochao Zheng <xiaochao at jlab.org> wrote:
>> 
>>> Dear Alexandre and all:
>>> 
>>> Just another weekly update:
>>> 
>>> My student has converted all exclusive pi+/NH3 ntuple results to plain
>>> text files. I emailed these files to both Igor and Peter.  Peter has
>>> made some plots on the 3 GeV data. I believe he is looking at other
>>> energies right now. And we will keep discussing about the plots.
>>> 
>>> Igor is taking shifts at Mainz and he said he will look at the data
>>> carefully when he comes back. He mentioned that if we need to combine
>>> bins to get higher statistics, he prefer that I do it, so that the
>>> combining process can be done properly.
>>> 
>>> Both Igor and Peter have expressed interest in the single-beam asymmetry
>>> Ae. I didn't expect this, and need to look at the systematic uncertainty
>>> of Ae carefully. At the first thought, the uncertainty of Ae should come
>>> only from the beam polarization, which would be from Moller alone. At
>>> the moment I am taking a simple average of the Moller error bars (for
>>> example if there are two Moller results during the 3GeV run period, I am
>>> taking the average of the two relative Moller uncertainties).  If Moller
>>> is indeed the only dominate error for Ae, then I plan to calculate the
>>> statistics-averaged value from multiple Moller measurement.  Please
>>> comment.
>>> 
>>> If eventually Ae is provided as results from EG4, should I add these to
>>> the analysis note too?
>>> 
>>> I also found I have misused the uncertainty of (f*Pb*Pt), with f the
>>> dilution factor, as the systematic uncertainty for both Aet and At. The
>>> correct way should be to use the error of (f*Pb*Pt) for Aet alone. For
>>> At, the uncertainty should come from (f*Pt), not Pb. But since the
>>> elastic analysis provides the product of Pb*Pt, the correct way for At
>>> would be to add the relative uncertainty of Pb (Moller) to that of
>>> (f*Pb*Pt) in quadrature, to get a conservative estimate of the
>>> systematic uncertainty for (f*Pt) for At. The resulting relative
>>> uncertainty isn't too much higher than that of Aet because Moller
>>> typically have <2% of relative uncertainty.
>>> 
>>> I will keep everyone posted on the progress. Thanks,
>>> 
>>> Xiaochao
>>> 
>>> ----- Original Message -----
>>> From: "Xiaochao Zheng" <xiaochao at jlab.org>
>>> To: "Alexandre Deur" <deurpam at jlab.org>
>>> Cc: "gdh lowq2" <gdh_lowq2 at jlab.org>
>>> Sent: Monday, January 27, 2014 3:07:13 PM
>>> Subject: Re: [Gdh_lowq2] EG4 meeting
>>> 
>>> Dear Alexandre and all:
>>> 
>>> I am never able to attend this meeting so it might help to provide an
>>> update on the exclusive analysis by email:
>>> 
>>> As some of you know, we received the review on our analysis note last
>>> fall. Reading through the review, the major concern is what physics
>>> conclusion can we draw from these results?  I discussed this with the
>>> exclusive group and we think the best way is to send the results to Igor
>>> and have him look at it. He will be able to compare the new data with
>>> all existing ones and come up with ideas.
>>> 
>>> Igor requested the data in plain text form, while my data were stored in
>>> ntuples.  I asked one of my 2nd-year students to work on the format
>>> conversion. It has been about two weeks and he has been making some
>>> progress. Once he completes for one kinematics, I will do a cross check
>>> and send the plain-text data to Igor, and we will proceed from there.
>>> 
>>> Thanks for your attention.
>>> 
>>> Xiaochao
>>> 
>>> PS is today already Jan. 28th?
>>> 
>>> 
>>> ----- Original Message -----
>>> From: "Alexandre Deur" <deurpam at jlab.org>
>>> To: "gdh lowq2" <gdh_lowq2 at jlab.org>
>>> Sent: Monday, January 27, 2014 1:33:57 PM
>>> Subject: [Gdh_lowq2] EG4 meeting
>>> 
>>> Dear EG4 collaborator,
>>> 
>>> We will have our EG4 analysis meeting today (Jan 28th) at
>>> 9:30am (EST) in room F227. Instructions to call-in can be found at:
>>> 
>>> http://www.jlab.org/Hall-B/secure/eg4/deur/call_in_procedure.html
>>> 
>>> Best regards,
>>> 
>>> Alexandre
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>> 
>> 
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