[SBS_sim] Cluster plots

[Ole Hansen]

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
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