Hi john,
we are of course well aware of the problem you mention
> So my question is: is the factor of 3 in "effective" momentum resolution
> due to uncertainty in angular resolution? Or at least part of it?
some numbers have been quoted with this correction taken into account,
some without. The difference is not large and can be estimated with a
simple kinematics calculator. Please refer to the february collaboration
meeting.
About the z-resolution: it would not be surprising that problems there are
coupled with the problems in the angle, however "vertex-z" is measured
independently of angle.
To quote a "z-accuracy" we just look at Z-left - Z-right so there is no
explicit angle, momentum contribution. Also note that our approach so
far has been that of "track finding" not optics, therefore these correlations
are somewhat hidden
regards,
-- tancredi
________________________________________________________________________________
Tancredi Botto, phone: +1-617-253-9204 mobile: +1-978-490-4124
research scientist MIT/Bates, 21 Manning Av Middleton MA, 01949
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On Fri, 4 Jun 2004, John Calarco wrote:
>
>
> Let me toss out a general question. It is really aimed at Doug, Chris,
> and Chi, but anyone can put in their 2-cents worth.
>
> At UNH, we have been discussing Adrian's ep elastic analysis vis-a-vis
> this ongoing question of momentum resolution. It suddenly dawned on me
> that we are not measuring the "intrinsic" momentum resolution (probably
> everybody else already saw this) but rather some convolution of that
> with angular resolution.
>
> What is used by Adrian as well as Chris and Chi (as far as I understand)
> is a comparison of the measured momentum with that expected from a
> kinematics calculation based on the beam energy and measured track
> angle. Thus the calculated value has a spread due to the uncertainty
> in angle, and the sigma from the Gaussian fit to the difference between
> the measured and calculated momenta then clearly is a convolution of
> the intrinsic sigma for momentum and the sigma for angular resolution.
>
> I note that Adrian is getting about 41 MeV, and I think Chi (or was
> it Chris?) was saying 38 MeV. Those are close enough that the small
> difference is probably due to differences in cuts.
>
>
> Coupled to this is very poor vertex reconstruction. True ep elastic
> events appear to come from vertices with a spread of several centimeters.
> Given the distance to the chambers (of order a meter or so), the angular
> definition is quite poor, leading me to wonder whether the bulk of the
> apparent momentum resolution is actually an angular resolution problem.
>
> Perhaps someone can enlighten me.
>
> p.s. I apologize in advance for missing the next 2 Wednesday group
> and analysis meetings (I will be in CA for my daughter's graduation
> and visits with other family).
>
> John
>
>
>
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