[Gdh_lowq2] EG4 meeting

Tue Feb 18 12:12:10 EST 2014

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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