[SBS_sim] Cluster plots

[Evaristo Cisbani]

Dear Ole,

the "barn door" distribution of x (and y) is correct. The Montecarlo is
configured to generate trajectory in a small aperture, around the center
of the tracker (which may have an offset).

If you cut out clusters with single hit, do you improve the reconstruction ?

Concerning the digitization of x and y, they are actually slightly
different (we tried to simulate the real strips which are different). I
cannot say that the difference you found are realistic; in any case the
size distribution look reasonable.

I checked the alignment of the generate hits, before digitization
(production g15, no background). In the first attached file (hit_track
...) you find the distribution of the sum of the distances (squared) of
the hits from the best fitted line. The values looks very small for most
of the tracks (about 89 %). No cuts applied.

In the second file (clus_track ...) I tried to figure out how the
digitization perturb the alignment: I used the size of the clusters (in
number of strip) and the position of the first strip in the cluster to get
the position of the cluster (no charge weighting). As before, the
distribution of the sum of the distances (squared) of the cluster center
from the best line in unit of strip. Apparently the distribution is
reasonable and compatible with the previous plot (the width of the
distribution is below 1 strip).

Do you use the charge information to get the cluster centroid ? If
positive, could you try without charge: assume the middle of the cluster
as cluster center, and see if it improve ?

Cheers,
Evaristo




> Hi all,
>
> just to illustrate the issue with the cluster position a little bit,
> attached are plots of the calculated cluster center position in the x1
> and y1 planes for the g15 0%-background production. My observations
> (already made in December):
>
> x distribution is wide as a barn door, about 8 mm wide. The incident
> tracks definitely do not seem to be precisely centered on the chamber.
> This may be fine.
>
> y distribution is narrower (~3 mm), but shows very pronounced peaks
> separated by the strip pitch. Such peaks occur if a found "cluster"
> consists of only one strip because in that case no intermediate position
> between strips can be calculated.
>
> The next two plots show the sizes of the clusters whose positions are
> shown in the other plots. As already suspected, there is a significant
> number of y-clusters with only one strip. For this coordinate, I do see
> the average cluster size of 1.8 that Vahe mentioned on the phone.
>
> Lastly, I include the plot of the residuals (in y1) that I mentioned I
> showed in December. This one is for the g11 production (10% background).
> All the other plots are for g15/0%.
>
> The qualitative features of the cluster plots do not change if I require
> exactly one good track or similar obvious cuts. Also, the cluster plots
> are essentially identical for all 6 x and y planes. For planes further
> in the back of the spectrometer, one sees a slight widening of the
> x-distribution, perhaps due to scattering effects.
>
> My suspicion at this point is that the single-strip "clusters" in y are
> unsuitable input for track reconstruction. They cause the hit position
> artificially to "snap" to the fixed strip positions, which may often be
> 100s of um away from the crossing point of the track.
>
> Evaristo: could you comment on the discrepancy in position distribution
> and cluster size in x and y? Shouldn't both coordinates have at least
> identical cluster sizes? Or is such an asymmetry an intrinsic feature of
> the readout planes?
>
> Ole
> _______________________________________________
> SBS_sim mailing list
> SBS_sim at jlab.org
> https://mailman.jlab.org/mailman/listinfo/sbs_sim
>


-- 
 Evaristo Cisbani                      office : +39 06 4990 2847
 Istituto Superiore di Sanita`         lab    : +39 06 4990 2347
 Department of Technology and Health   fax    : +39 06 4938 7075
 via Regina Elena 299 - 00161 Rome     web    : www.iss.infn.it/cisbani
 Italy
------------------------------------------------------------------------
-------------- next part --------------
A non-text attachment was scrubbed...
Name: hit_track_reco_1000_evt_892_in.pdf
Type: application/pdf
Size: 13532 bytes
Desc: not available
Url : https://mailman.jlab.org/pipermail/sbs_sim/attachments/20110209/dd43cb76/attachment-0002.pdf 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: clus_track_reco_1000_evt_892_in.pdf
Type: application/pdf
Size: 13226 bytes
Desc: not available
Url : https://mailman.jlab.org/pipermail/sbs_sim/attachments/20110209/dd43cb76/attachment-0003.pdf