Hi genya,
I believe we already discussed this issue at the collaboration meeting.
As you say, given the fact the hydrogen has a lower critical field than d2
the magnitude of the holding field could be a very significant parameter.
In any consideration about optimizing the spin angle (i.e. 32 or 45) one
can not forget that the polarization might be different. At this time we
do not have numbers to discuss !
1) I believe the proper way to arrive to a decision is measure the target
polarization in both configurations, and compare the numbers. This is
what I remember was the decision taken less than one month ago at the
collaboration meeting (so I don't get why we are re-iterating once more).
2) In N-D the q-direction is indeed very forward and the spin orientation
of 32 degrees is required to separate resonant vs non-resonant amplitudes
in spin perpendicualr vs spin paralllel kinematics. On the other hand for
the Ge/Gm super-ratio measurament there is little sensitivity to the spin
angle at intermediate Q2 while it is the larger Q2 that prefers the more
backward orientation. If I understand it correctly this entirely driven by
the BATS but the error bar there will be large (there is no 1,000 hrs of
beam time available).
Given the little time left for the h2 run I would think we need a critical
evaluation of the impact of the added running on the Ge/Gm error bar.
This of course should only be based on the measured values of the target
and re-united that with the cumulated statistics of previouos runs.
As far as N-D, the extra H2 running will make a significant impact since
the present asymmetry error bars are still large and can improve rapidly.
In my opinion N-D - which per se is also a thesis project - is an
important added bonus to the BLAST program and it can be achieved with
existing set up. It yields directly to the low-Q2/pion cloud physics we
are addressing and represents a big prize via (demonstrated) measurament
of *new* observables (double pol asymmetries in the pi+ channel), which
have great potential for a good paper.
3) Finally, h2 is also important check of the d2 running and my instinct
tells me not to change configuration when checking your apparatus on a new
target.
In 3/4 days of running we should have a good idea of what to expect for
hydrogen.
-- tancredi
________________________________________________________________________________
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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On Thu, 21 Oct 2004, Genya wrote:
> Dear collaborators,
> We need to make a prompt decision about the direction of magnetoc
> holding field for
> upcominh hydrogen run. From the ABS point of view, the optimal is to
> keep the field at
> max, which means the angle is the same as for deuterium, 32 degrees.
> I don't have a good feeling about the optimal angle for different H
> experiments... and
> we must make the decision in a couple of days. Please voice up your
> considerations.
> Genya
>
>
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