[b1_ana] Fwd: Uncertainties in measuring b1/Azz

[Elena Long]

Good evening,

For f(x), an ideal number I'm getting (excluding aluminum windows, but 
including estimates on deuterium, nitrogen, and helium) is:

f(0.18) = 0.28
f(0.29) = 0.28
f(0.36) = 0.28
f(0.44) = 0.30

Take care,
Ellie

Elena Long, Ph.D.
Post Doctoral Research Associate
University of New Hampshire
elena.long at unh.edu
ellie at jlab.org
http://nuclear.unh.edu/~elong
(603) 862-1962

On 05/03/2013 05:10 PM, O. A. Rondon wrote:
> Hi,
>
> I think that the form of Azz in eq. (20) of the draft, or eq. (4) in the
> section we shared with Steve, obscures the method. He is still thinking
> of a difference, which indeed is the numerator.
>
> I think that if we use the form that I've been advocating, which the
> same that is given in the appendix with the derivation of the dilution
> factor, it should be clearer that it is a ratio - 1, not a difference.
>
> In any case, in addition to addressing Steve's comments, at least we
> should write the equation for Azz as
>
> Azz = 2/(f*Pzz)*(Np/Nu - 1),
>
> just as it's given in Werner's and Heinz's proposal (without the
> dilution factor, which they missed like we were before I found it).
>
> That proposal got an A-, (in spite of the missing f) because even though
> it has even more sources of systematic errors than ours, it was very
> clear. We should be able to clarify everything, too.
>
> For example, we are taking Q_pol/Q_unpol = 1. We need to keep them and
> propagate their errors. The same applies to the detector efficiencies.
>
> And we do need to specify f, maybe a plot f(x), at least a number
> (~0.26). Pzz is, of course, given, but it wasn't in what we gave Steve.
>
> Cheers,
>
> Oscar
>
> Patricia SOLVIGNON wrote:
>> Begin forwarded message:
>>
>>> *From: *"Stephen A. Wood" <saw at jlab.org <mailto:saw at jlab.org>>
>>> *Subject: **Uncertainties in measuring b1/Azz*
>>> *Date: *May 3, 2013 4:40:27 PM EDT
>>> *To: *Patricia Solvignon-Slifer <solvigno at jlab.org <mailto:solvigno at jlab.org>>
>>>
>>> Patricia:
>>>
>>> Thank you for showing me the document describing estimates of errors in
>>> measuring b1.
>>>
>>> The formula for estimating statistical errors (or rather time needed to
>>> achieve a given statistical precision) looks correct to me.
>>>
>>> In the discussion of systematic errors, the uncertainties discussed appear to
>>> mostly relate to the scale of b1 or Azz.  While important, these scale
>>> systematics are not the dominant systematics.  The main systematic will be
>>> unknown differences in the calibrations, efficiencies, luminosities etc
>>> between the two polarization states.
>>>
>>> This is similar to experiments with polarized beam, particularly parity
>>> experiments, where one worries that about unknown differences in rate between
>>> the two helicities that are not due to the physics of interest.  These
>>> differences could be due to fluctuations in target density, fluctuations in
>>> beam current measurement calibration etc.  In such experiments, these
>>> systematics are mitigated by reversing the beam polarization quickly so that
>>> efficiencies, target densities etc don't have time to drift.
>>>
>>> With a rate of switching polarization states of hours or days, there will be
>>> drifts in things, drifts that get magnified because this measurement is a
>>> subtraction of large numbers.  I don't know what the target (f * Pzz *
>>> delta-Azz) is, but it is 10^-something where something is certainly >= 4.  If,
>>> for example, the detector efficiency were to drift by 1% between the two
>>> polarization states, and this drift was unknown, it would be a complete killer.
>>>
>>> I think the main systematic drift effects will be
>>>
>>> 1.  Drifts in beam current measurement calibration
>>> 2.  Drifts in detector efficiency
>>> 3.  Drifts in luminosity
>>>
>>> I am sure there are others.
>>>
>>> There can be many subtle effects in any of these.  BCM efficiency could depend
>>> on ambient temperature which will have daily variations as well as several
>>> hour variations due to AC cycling.  The BCM calibration at different currents
>>> could be different, making
>>>
>>> Detector efficiency can drift for a variety of reasons, for example including
>>> fluctuations in gas quality, HV drift or drifts in spectrometer magnetic field.
>>>
>>> There can be difficult to know changes in luminosity.  As I understand it, the
>>> target is a set of discrete beads.  So, when the beam moves, the thickness of
>>> the target seen by the beam changes.  Or if the amount of helium seen by the
>>> beam changes, the rate changes.
>>>
>>> It may be that all of these systematics can be addressed, but it may not be
>>> possible to make a completely convincing case by the the of the PAC.  But I
>>> think it is important to acknowledge all the systematics you can think of and
>>> acknowledge the difficulty of beating them down. (Otherwise PAC members or the
>>> technical review will point them out.) There are things that can be pointed to
>>> such as the big effort that went into modernizing BCMs for g2p and Qweak.
>>>   Some of that effort may be useful in this case.  It would be good to sketch
>>> out a plan for studying the systematic effects.  There will be plenty of time
>>> to do parasitic systematic effects studies during the experimental program
>>> that will precede an eventual b1 run.
>>>
>>> It would be helpful if you could remind me of the expected numerical values of
>>> f and Pzz, and the desired deltaAzz.
>>>
>>> Steve
>>
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