[b1_ana] Fwd: Uncertainties in measuring b1/Azz

[J. P. Chen]

Hi Dustin,

Good that this is very small.
Looks like all the effects Steve mentioned are being discussed and 
considered.
Cheers.

Jian-ping

On 5/3/2013 5:55 PM, Dustin Keller wrote:
> 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
>>