[b1_ana] b1 higher twist
O. A. Rondon
or at virginia.edu
Fri Apr 19 13:46:58 EDT 2013
Hi Simonetta,
As Karl said at the meeting, there are multiple physics problems that
can be investigated with a tensor polarized deuterium target. But there
may be some perception that tensor polarization is a big obstacle, so at
least one experiment would need to first be approved by the PAC, and
then put on the beam schedule, to open the way for later projects.
An experiment with compelling physics, that puts the least demands on
the lab would have the best chances. b1 by Jaffe's method may be it,
because, so far, it looks like doesn't need any special beam lines
(longitudinal target field), no special detectors (base Hall equipment
for single arm measurement), or special target requirements (solenoid
coil, only vector-optimized P_zz > 0). It could be the proverbial nose
of the camel under the tent, for future more advanced measurements.
Another possible experiment would be Axx, previously approved, but
officially "Dropped", so it's open to whoever would like to pick it up,
http://www.jlab.org/exp_prog/generated/6GeV/hallc.html
but it's too late for this PAC, and it's a coincidence experiment,
definitely harder than single arm.
And of course, it would be great to explore other applications, like the
polarized DVCS you suggest, but of course, it would need even more
development work (both theory and experiment) before it could be proposed.
Cheers,
Oscar
Simonetta Liuti wrote:
> 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
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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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>
> **************************************************************
> **************************************************************
> 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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