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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