[b1_ana] Axx

[Karl Slifer]

Hi,

While it would be easier to run with only positive Pzz, there is no
technical or theoretical reason that I know of that prevents us from using
negative pol.  This will require target development to achieve large
negative as well as positive tensor polarizations, along with careful study
of the systematics in extracting these values.  I thought we all agreed on
this yesterday....And I also see no technical or theoretical reason (other
than it is difficult and will require R&D) which limits the enhanced tensor
polarization to 10%.  I believe Don, Chris, Josh would all agree with this,
atleast they all did when I talked to them within the last 6 months.

So to be clear, I believe we can propose an experiment where we enhance the
m=0 population (via rf saturation or by using two independent microwave
sources) and measure N_0 unpolarized electrons inclusively scattered while
in this state, and then we deplete the m=0 state to obtain a positive
polarization and measure N_1 unpolarized electrons scattered while in this
state.  Then we form the asymmetry (with appropriate numerical factors).
The Pzz will not be the maximal positive or negative value in either state,
but we can correct for this by the relative Pzz in each state.    One
significant concern is that this introduces time dependent systematics
since it will likely require some time to switch between the two states.
 This has to be studied, but I do not see it as a fundamental limitation.

After careful study of the systematics, its possible that the cross section
difference method may well turn out to be the best way to do the
experiment, but I suspect we will struggle mightily to convince a very
skeptical PAC in 30 mins that we really can control all the systematic
effects to the level needed for a cross section measurement.

With this in mind, I think it is reasonable to aim for conditional approval
based on demonstration of the target performance to the level needed (+-20%
tensor pol with about 5% relative uncertainty).  I believe we can defend
these as reasonable goals, although we should get something in the way of a
support statement from Don or Chris.  And I believe conditional approval is
a highly desirable state, since the target groups will not be able to
commit serious R&D to this without an approved experiment for motivation.
 In addition, it opens the door to attract more theory support and start
consideration of several other possible experiments.

my few further cents,

 Karl










---
Karl J. Slifer
Assistant Professor
University of New Hampshire
Telephone : 603-722-0695


On Fri, Apr 26, 2013 at 11:04 AM, Dustin Keller <dustin at jlab.org> wrote:

> As I mentioned in the meeting using the notation Axx can be
> mis-leading especially in the case of DIS where azimuthal
> control is not obvious.  However the relationship for
> sigma^{+/-} for m=+1,-1 is sigma^{+/-}=sigma^u(1+(1/2)AzzPzz).
> If you believe that then Axx=Azz, and the conclusion is the same.
> If you call it Axx or Azz in either case you just measure the
> ratio of polarized and unpolarized cross sections.  This will lead
> to a target tensor polarization of about 10%.  There are certainly
> other systematic concerns but this is the best we can do target wise.
>
> dustin
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