[b1_ana] b1 higher twist

[Oscar Rondon-Aramayo]

Hi Simonetta,

Thank you for checking that Jaffe's formulas refer to the electron beam. It
would be more difficult to measure things with the field along the q vector.

We don't need to do two separate measurements (in the sense of measuring
first one cross section and then the other). We can have two target cells in
the beam at the same time, one polarized, the other unpolarized. They would
essentially have the same acceptance. Corrections for the residual
difference in acceptances can be investigated following the method used 25
years ago by the EMC experiment that found the proton spin crisis.

The EMC had to take data on two targets of opposite polarizations, because
they could not flip the helicity of their muon beam. Nevertheless, they
managed to control the systematic effects of their acceptances, etc. A
discussion of their method is posted on my b1 page
http://twist.phys.virginia.edu/~or/b1/EMC-piegaia_thesis.pdf

The method I suggested is based on the EMC approach,
http://twist.phys.virginia.edu/~or/b1/b1_method.pdf

with the difference  being one cup polarized, the other not, and alternating 
them during the run, just like the EMC did. Narbe has already done 
simulations that show we can cleanly separate the events from each cup, see
http://twist.phys.virginia.edu/~or/b1/zbeam2.eps
(He has many more examples, I'll send links to his plots folder later)

So, my view is that I think the method can achieve the necessary statistical
precision, and I'm not yet convinced that the systematics cannot be 
controlled to the corresponding precision.

For example, in the outline of the method I proposed, the \delta pf that is 
introduced before eq. (10) is a relative fraction, should really be dpf/pf. 
So in eq. (11), the contribution of the dilution factor is suppressed by the 
relative error on the packing fraction, which is about 4% for current 
experiments (SANE, RSS) and can be made smaller by using targets shaped as 
disks, instead of irregular fragments, etc. For an error in f itself ~ 
0.05*f (RSS achieved 0.047*f), and f=0.3, we have a systematic error on b1 
due to f and pf of  0.05*0.3*0.04 = 6E-4. Remember that f and pf are proxies 
for the target thickness and unpolarized contributions.

The same kind of relative errors are involved in the propagation of the 
differences in charge and acceptance. We need to estimate them 
conservatively, following the EMC approach, before reaching conclusions.

Finally, there is the method of taking the ratio sigma_pol/sigma_unpol 
discussed in Anklin's and Boeglin's proposal, which could be an alternative 
to the EMC method, and could be directly applied to the data taken 
simultaneously on polarized+unpolarized cups, as I propose.

Cheers,

Oscar

On Thu, 18 Apr 2013 18:21:54 -0400
  "Simonetta  Liuti" <sl4y at cms.mail.virginia.edu> wrote:
> Hi All,
> 
> even if higher twists are negligible, I am skeptical of the Jaffe et al. 
>formulae for extracting  b_1 (Chapter 6).
> 
> In fact, I checked that the observable is OK in the sense that they are 
>referring to the electron beam (and not the virtual photon).
> 
> However, I expect systematic errors to be very big. The "polarized cross 
>sections" observables can be obtained with two separate measurements. A 
>polarized spin 1 target one, and an unpolarized one. The two measurements 
>have to be carried out separately, just to be clear. And this is going to 
>impact the systematics. 
> This is exactly why Hermes came up with A_zz which only involves target 
>polarization. 
> I do not see the advantage of going back to Section 6 of Jaffe et al., 
>while one could do something new and exciting  measuring deuteron DVCS 
>asymmetries.
> 
> However...please let me know what you think.
> Simonetta 
> 
> On Thu, 18 Apr 2013 16:06:07 -0400
> "O. A. Rondon" <or at virginia.edu> wrote:
>>Hi,
>>
>>There is a paper by Hoodbhoy, Jaffe and Sather, which says that higher
>>twist (twist-4) effects on b1 at Q^2 = 1 GeV^2 are only 5%. So they
>>would seem negligible at the kinematics of our proposal or HERMES. See
>>
>>http://dx.doi.org/10.1103/PhysRevD.43.3071
>>
>>Cheers,
>>
>>Oscar
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>>b1_ana mailing list
>>b1_ana at jlab.org
>>https://mailman.jlab.org/mailman/listinfo/b1_ana
> 
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