[b1_ana] Fwd: Uncertainties in measuring b1/Azz

[Dustin Keller]

this is a very very small effect, but it is the reason to
keep the temperature the same in each case.

On Fri, 3 May 2013, J. P. Chen wrote:

> Dustin,
>
> I guess Steve also worried the change/drift in target
> thickness/density. Due to beam heating and He4 cooling,
> target can be slightly different at different time period.
>
> Cheers.
>
> Jian-ping
>
> On 5/3/2013 5:42 PM, Dustin Keller wrote:
>> I not sure why he lists luminosity as an additional issue.  We have a good
>> handle on acceptance differences so its really just beam current
>> calibration differences and drift in detector efficiency.
>> 
>> dustin
>> 
>> 
>> 
>> 
>> On Fri, 3 May 2013, Patricia SOLVIGNON wrote:
>> 
>>> 
>>> Begin forwarded message:
>>> 
>>>> From: "Stephen A. Wood" <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>
>>>> 
>>>> 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
>>> 
>> _______________________________________________
>> b1_ana mailing list
>> b1_ana at jlab.org
>> https://mailman.jlab.org/mailman/listinfo/b1_ana
>