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
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
-- John R. Calarco Dept. of Physics Univ. of New Hampshire Durham, NH 03824 phone: (603)862-2088 FAX: (603)862-2998 email: calarco@unh.edu
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