[b1_ana] Fwd: Uncertainties in measuring b1/Azz

Fri May 3 18:37:13 EDT 2013

The numbers look fine.

I suggest reducing them for the Al and coils. We do know that for SANE,
Al contributed 5% of the rate, but the NMR coil, which is Cu+Ni, is
about 4-8 times longer for ND3 than for NH3 and it's in the beam. So I
suggest using 0.95*f(x).

Cheers,

Oscar

Elena Long wrote:
> 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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