[Solid_tracking] some initial Qsq / alignment studies

[Zhiwen Zhao]

hi, Bob

I am CCing this to solid_tracking at jlab.org because only a few of us are 
on the list.
also because the emaillist will keep a copy of email in archive.

The fake hits in space where there's no GEM plane is because current 
GEMC use the hit processing "flux" to record the hits I gave you.
If one low energy tracks go forward, then bend back by field and thus 
pass a GEMC plane twice or more, the hit position will be average out 
between all hits.
"flux" is not the real GEM digitization, so just ignore those tracks for 
now.

I didn't find the comment about pt on 201 page of latest pCDR, but it 
should be proportional for sure.

So when you fit the helix from the hits on GEM1,2,3 to determine radius 
of curvature and thus Pt, is field a fitting parameter or is it given as 
a constant?
See can see the field variation from plots in pCDR

The file I gave you have particles vertex distributed evenly along 40cm 
target length.
When you determine theta by fit to hit on GEM 1,2,3 in r VS z, how did 
you use the vertex information?

Does thetadiff.gif have 0.36 degree shift you mentioned already? it 
still doesn't sit at 0 now.

How do you get "systematics are about 3% in Qsq"?

I am not sure if qsq_compare.gif is best way to compare.
how about Qsq different VS p and theta in 2D plot?

I guess is the final question is
if GEM alignnment tolerance gives Qsq error larger than tracking error, 
we don't need very good tracking method for the study.
if GEM alignnment tolerance gives Qsq error smaller than tracking error, 
we have push tracking error by using better method in order to see the 
other effect.

Thanks

Zhiwen

On 6/10/2015 2:57 PM, Robert Michaels wrote:
> I didn't use the solid_tracking list because I couldn't
> figure out who is on that list, and I don't think this
> email needs to go the entire world.  Too many questions.
>
> The goal of this study is to find how the GEM alignnment tolerance
> affects the systematic error in four-momentum squared Qsq.
> At this point, I have constructed Qsq, and I mainly need to
> tweak the GEM positions to measure how it affects the errors.
> Here is an update.
>
> I use the GEM hits to reconstruct Qsq using two things:
>
> 1. scattering angle theta:
>     tan(theta) = slope of R vs Z for GEM hits, where R = sqrt(X^2+Y^2).
>     I restrict the fit to the first 3 GEM chambers where B is uniform.
>
> 2. Transverse momentum Pt is proportional to the radius of curvature
>     of the helix.  Using the constraint that the beam goes through
>     the origin, I derived an expression for R based on the GEM hits.
>     This results in the following correlation, which is admittedly
>     imperfect:
>
> http://userweb.jlab.org/~rom/solid/ptvsrad.gif
>
> One really silly problem is that the Monte Carlo gives me hits
> in GEMs for radii like 20 cm and 15 m, but I think these are outside
> the physical size of the GEM, so there must be a misunderstanding.
>
> Nevertheless, based on 1 and 2, I tried to reconstruct Q^2 and
> compare to MC as shown below.
>
> First, the scattering angle compared to MC.
>
> http://userweb.jlab.org/~rom/solid/thetadiff.gif
>
> In order to align the peak at zero I had to adjust the reconstructed
> angle by 0.36 degrees; perhaps because something is unknown about
> the geometry.
>
> Reconstructed Qsq vs MC Qsq
> http://userweb.jlab.org/~rom/solid/qsq_compare.gif
>
> The systematics are about 3% in Qsq at the moment.
>
> For the purpose of the study (of GEM alignment) I don't need
> zero error, I mainly need to look for deviations that occur with
> misalignments.  Of course, it would be nice if the initial systematics
> (presently 3%) would be smaller, but I assume these will reduce
> when we use better track fitting.
>
> I've made an initial study of the sensitivity to the GEM
> chamber position, but I don't fully trust the results yet,
> so I'll inform you about that later.
>
> Also, I don't undestand the comment on page 201 of the pre-CDR that
> the radius is inversely proportional to Pt.  Isn't it proportional,
> not inversely proportional ?  Am I missing some elementary physics
> here ?
>
> For example, see this treatise about charge particles moving in a
> uniform B field:
> http://www.worldscientific.com/doi/pdf/10.1142/9789812798657_bmatter
>
> -------------------------------------------------------
> Robert W. Michaels, Staff Scientist
> http://userweb.jlab.org/~rom    (757) 269 7410
> Thomas Jefferson National Accelerator Facility
> 12000 Jefferson Ave, Newport News, VA 23606 USA
>
>
>