[Frost] phi offset

[Eugene Pasyuk]

The beam offset for g9 a and b was of the order 1-3 mm. It is in caldb on the run number basis. I don't think that such an offset would affect momentum reconstruction.

-Eugene

----- Original Message -----
> From: "Michael Dugger" <dugger at jlab.org>
> To: "Eugene Pasyuk" <pasyuk at jlab.org>
> Cc: "frost FROST" <frost at jlab.org>
> Sent: Monday, July 8, 2013 9:37:44 AM
> Subject: Re: [Frost] phi offset
> 
> 
> 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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> >> Frost at jlab.org
> >> https://mailman.jlab.org/mailman/listinfo/frost
> >>
> >
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