Slightly silly question:
How are we going to set 60 angle with a non-functioning hall probe in
the target?
Vitaliy
John Calarco wrote:
>Since it seems that people are lobbying by making "public" statements
>about their choice of target spin angle, I seem to have no choice but
>to reply.
>
>Facts:
>
>(1) It is not true that Ge/Gm is insensitive to the choice of spin
>angle. Chris' analysis of the recent 32 and 45 deg data shows that
>the properly normalized errors are reduced by a factor of about 0.7
>in going from 32 to 45 deg. This is equivalent to a factor of 2 in
>beam time. MC shows even further improvement if we get the equivalent
>target polarization at a spin angle of 60 deg.
>
>(2) Ge/Gm is the PAC approved experiment and the one we have promised
>to the DOE review committee to complete by the end of 2004.
>
>(3) The parasitic experiments are NOT the PAC approved physics.
>
>It is very clear that the studies in the Delta would prefer the smaller
>spin angle, but they are not what is driving the choice. If the upcoming
>test at a spin angle of 60 deg (after the 40 kC at 32) shows that the
>target polarization is still high, the spokespersons of the approved
>program will request to run at that angle. It does not seem reasonable
>to compromise the results of an approved program in order to try to
>extract physics which we may or may not be able to.
>
>
>
>
>On Tue, 2 Nov 2004, Tancredi Botto wrote:
>
>
>
>>Please see the attached MAID200 result for the S_lt' and S_tt' partial
>>cross section in inclusive e,e'pi+.
>>This only to illustrate the strong sensitivity to the delta and the C2
>>quadrupole. There is less sensitivity to E2 since that always comes in
>>the form ( M1^2 + E2*M1 )
>>
>>The asymmetry goes as A ~ [ cos(th*) S_tt' + sin(th*) S_lt'], with S_tt'
>>of 90 ub and S_lt' - 12.8 ub on top of the delta, Q2=0.15, pi+ channel.
>>
>>A 30 deg holding field angle (th* of 60 for eleft, th* of 0 for e-right)
>>is by far the optimal by a factor of almost 3 in the error bar for s_lt'
>>(compared to 45 deg).
>>
>>Yesterday I actually did only propagate the error due to the knowledge of
>>the spin angle. But the asymmetry changes rapidly with spin angle and so
>>does the relative error in the asymmetry measurament. This is actually the
>>driving term. In fact by the looks of the above expression, th* of 90 deg
>>would look even better when one tries to get at S_lt'. But in reality that
>>would not be the case since - although the angle is maximal - the lt'
>>bearing asymmetry has dropped considerably (0.051 vs 0.136) and the
>>sensitivity to c2 is significantly worsened.
>>
>>So, I am evermore convinced that 30 deg is by far better for N-Delta. I do
>>not think the Ge/Gm signal changes that much with spin angle since G_e and
>>G_m are almost of the same scale. Here the problem is mixing a delicate
>>term (s_lt', ~1) with a monster one (s_tt', ~9).
>>
>>regards
>>-- tancredi
>>
>>P.S.
>>Zilu: I was to quick in my reply. Of course I should know that to measure
>>E2 you need that "third eye in the sky" or OOPS.
>>
>>________________________________________________________________________________
>>Tancredi Botto, phone: +1-617-253-9204 mobile: +1-978-490-4124
>>research scientist MIT/Bates, 21 Manning Av Middleton MA, 01949
>>^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>
>>
>>
>
>
>
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