[Lowq] Solenoid field map - follow up

[Eric Voutier]

Dear all,

The problems of the solenoid field map used for CLAS
analysis was discussed by the Saclay group, on the basis
of our analysis meeting minuts.

_Normalization factor_ :  the residual normalization factor
might have different origins, particularly the current
read-back is done at the power-supply level and not at
the magnet level which leaves room for power losses and
some 1-2 amps differences in the real current.  It might
also be that the final current setting was set a little bit
lower from the nominal for safety reasons... Jacques will
trace back the history of this factor to make sure of this
problem.

_Shift_ : the observed shift should indeed have some consequences
on slow particles, like DVCS protons. The effect might be small for
asymetries but can be critical for cross sections, typically for the
solid angle. The Saclay group is informed but it would be wise also
to inform all DVCS groups about the problem. The question raised
is about the real starting date of implementation of this map in
the CLAS software. Additionnally it would be desirable to identify
the person who did the implementation and should have all the
history in hands. The Saclay group will try to look in this. Eg6
on-site persons may also try to trace-back this issue.

Regards, Eric

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