[b1_ana] b1 higher twist
Simonetta Liuti
sl4y at cms.mail.virginia.edu
Fri Apr 19 09:45:08 EDT 2013
Hi All,
just to make sure I understand:
if you can measure both polarized and unpolarized targets like the EMC did and that has little bearing on the systematics, then that seems the best thing to do with a solid target to get to b_1.
That would avoid going to a CLAS12 type of setting to measure azymuthal asymmetries. I am however interested in this aspect as well, i.e. getting out the total angular momentum sum rule for the spin 1 system. Because of the new developments concerning the comparison of longitudinal spin and transverse spin from Ji, Xiong and Yuan, the deuteron plays an important role. I understand the May deadline, however, is anyone interested in proposing this additionally?
Cheers
Simonetta
On Fri, 19 Apr 2013 00:37:59 -0400
"Oscar Rondon-Aramayo" <or at cms.mail.virginia.edu> wrote:
>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
>>>_______________________________________________
>>>b1_ana mailing list
>>>b1_ana at jlab.org
>>>https://mailman.jlab.org/mailman/listinfo/b1_ana
>>
>> **************************************************************
>> **************************************************************
>> Prof. Simonetta Liuti telephone (434) 982-2087
>> Department of Physics FAX (434) 924-4576
>> University of Virginia home (434) 973 9593
>> 382 McCormick Rd.
>> PO Box 400714
>> Charlottesville, VA 22904-4714
>>
>
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**************************************************************
**************************************************************
Prof. Simonetta Liuti telephone (434) 982-2087
Department of Physics FAX (434) 924-4576
University of Virginia home (434) 973 9593
382 McCormick Rd.
PO Box 400714
Charlottesville, VA 22904-4714
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