[Frost] phi offset

[Michael Dugger]

Eugene,

Will a small shift in beam position matter much in determination of 
momentum? It look like (my very rough estimate) that the shift in phi is 
much less than a degree. It only shows up as something that looks large 
when we look at the lambda pulls because those are being divided by the 
lambda resolution.

-Michael

On Sun, 7 Jul 2013, Eugene Pasyuk wrote:

> We know that the beam was off the CLAS center in g9a and g9b. I suspect that beam offset might not properly accounted for in tracking. Could it be a problem in translation from sector coordinates to CLAS coordinates? May be instead of translating to CLAS coordinates we need to translate to "beam" coordinates?
>
> -Eugene
>
> ----- Original Message -----
>> From: "Michael Dugger" <dugger at jlab.org>
>> To: "frost FROST" <frost at jlab.org>
>> Sent: Friday, July 5, 2013 3:20:49 PM
>> Subject: [Frost] phi offset
>>
>>
>> Hi,
>>
>>> From CLAS-Note 2003-017 (Mike William's kinematic fitting
>>> CLAS-Note), the
>> lambda resolution of CLAS from the TBER bank (using data with
>>  0.5*B_max)
>> is about 2.5 mrad. The value of lambda used in the kinematic fitter
>> was
>> 1.5 times as that given in TBER.
>>
>> This gives us a resolution of about 0.21 degrees for lambda. Priya's
>> pull
>> distributions for lambda of the proton (momentum < 1.0 GeV) have the
>> pull
>> maxing out at about 0.5 for angles near -90 degrees.
>>
>> If I take the standard deviation for lambda to be 0.21 degrees and
>> assume
>> a transverse path-length of 1 meter, I can get the same sort of
>> results as
>> Priya if the coordinate system of CLAS was displaced from the "true"
>> coordinate system by about 1.8 mm in the x-direction.
>>
>> It is starting to look like the lambda pulls might be explainable by
>> simply shifting the reaction vertex by a small amount.
>>
>> Another way of looking at this, is that our systematic error in phi
>> is
>> about 0.1 degrees.
>>
>> Take care,
>> Michael
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>