Star-fst-l mailing list

["Jason C. Webb" <jwebb AT rcf.rhic.bnl.gov>]

Hi Zhenyu,

I really don't recall the full context of the conversation with Daniel.  But 
let me clarify my
understanding of the two big bullet points:  segmentation and stereo 
angles...  and then I'll
address the issue w/r to the global track plots which you mentioned...


1) Radial segmentation:  going back to some the earliest studies:

Silicon only (see ~page 8) --
https://drupal.star.bnl.gov/STAR/system/files/FTS%20Tracking%20Plots%203_0.pdf

Silicon + sTGC (see ~ page 87) --
https://docs.google.com/presentation/d/1skB2Uhg8keu2BX1Eg5ijP7vjo0tNE2x3IzFtb6bsVGU/edit?usp=sharing

Increased radial segmentation showed modest improvement in efficiency at the 
most
extreme track densities.  But there was not really a make-or-break issue with 
radial segmentation.
In the Si+sTGC studies, you can look at efficiency vs centrality with 
different radial vs phi
segmentation ~page 103.

Bottom line: I think the radial segmentation of ~8 will work fine (but more 
segmentation can
improve matters somewhat in more central collisions).


2) Rastering:  this was always discussed in terms of the Si-only tracker

Silicon see pages 10 and 14
https://drupal.star.bnl.gov/STAR/system/files/FTS%20Tracking%20Plots%203_0.pdf

Shows a class of failed fits.  Believed to be due to small curvature tracks 
passing
through the same phi strip, leading to a degenerate (straight line) fit.

Rastering was introduced (slide 21), by placing the center of the disks off 
the beamline by 1mm.
This solved the issue of the failed fits by effectively introducing a stereo 
angle between
each Si disk.

Bottom line: stereo angles between Si planes are needed in order to 
reconstruct small curvature tracks.  This
can be introduced via rastering, but could also be built into the detector 
design.


3) "Jason, I was also told by Daniel that the global track efficiency for 
those plots is
Cheers,
 wrong. Can you please tell me what is wrong with them?"

"Those plots" refer to slide 42 of the proposal...
https://drupal.star.bnl.gov/STAR/system/files/ForwardUpgrade.v20.pdf

There was nothing wrong with them per se, but they were produced on a very 
short time scale and there were
open questions at the time which were not fully addressed.  "Those plots" 
originated on slide 23 here--
https://drupal.star.bnl.gov/STAR/system/files/FTS%20Tracking%20Plots%203_0.pdf

The open question was why (at high pT) we saw global track efficiencies > 
100%.  The reason was that the
global tracks were contaminated by combinatorics.  So when I say the global 
track plots were wrong, I mean
that they had a source of contamination which wasn't properly accounted for.  
Primaries, on the other hand,
had the additional requirement that they match the MC vertex.  This seemed to 
help suppress the conbinatorics
in the study.

Bottom line here... in those plots on page 42 of the proposal, trust the 
primary tracks as more reliable than
the globals.


Hope this helps,
Jason





On 6/26/19 10:29 AM, Zhenyu Ye wrote:
> Hi Daniel,
>
> I think I was confused with your statements "The study from Jason was later changed though 
> - he is one of the S&C people that made the point to me that more r segmentation was needed. 
> I talked with him/Gene about this last 1.5 week. Also I know that the global track efficiency on 
> those plots is wrong, also from talking with Jason." "OK i talked with Gene about the 
> hit rasterization in R just 1 or 2 ago”.
>
> I guess I know better now what was said to you after reading your email below 
> but still confused to some extent. So is it correct that Gene (and Jason?) 
> pointed out to you the importance of r rasterization, and Jason pointed out 
> to you the need of more r segmentations?
>
> Thanks,
> Zhenyu
>
> > On Jun 26, 2019, at 8:46 AM, Daniel Brandenburg <dbrandenburg.ufl AT gmail.com> 
> > wrote:
> >
> > Hi all,
> >
> > maybe just a little clarification:
> >
> > Gene, in our meeting a week ago about the forward tracking we discussed 
> > difficulty with the silicon. Specifically I talked about the resolution in R 
> > and you guys mentioned that rasterizing the hits was study and found to be 
> > important.
> >
> > Jason, we talked about the global efficiency some time ago now. We did 
> > several studies of the lambda decays with the older fast sim (with different 
> > resolution etc.) and with the newer ones for Si and sTGCs. We found that the 
> > lambda daughters (true global tracks, not just primary tracks without the 
> > primary vertex included in fit) had a very low reconstruction efficiency ( 
> > maybe 10%, I dont remember exactly) - we talked about this and how the 
> > tracker does the track following etc.
> > So my comment is not so much that those plots are wrong but that it is 
> > definitely not the performance we see with the current detector resolutions. 
> > I hope that is clear.
> >
> > Since i was not active with the forward tracking before last year I 
> > understand that a lot was done before I was active. I want to make sure we 
> > fully understand this issue before we get further along.
> >
> > Best,
> > Daniel
> >
> >
> > On Wed, Jun 26, 2019 at 9:16 AM Zhenyu Ye <yezhenyu2003 AT gmail.com> wrote:
> > Hi Jason, Gene,
> >
> > I was told by Daniel Brandenburg that you made the point to him very recently 
> > that more radial segmentations are absolutely needed for Forward Silicon 
> > Tracker. This would contradict to the statement in page 41 of the proposal 
> > https://drupal.star.bnl.gov/STAR/system/files/ForwardUpgrade.v20.pdf
> >
> > “In addition, we increased the radial segmentation from 8 to 16, 32, and 64 
> > and found no significant improvement on the momentum resolution or tracking 
> > efficiency.”
> >
> > Can you please help me understand why you believe more radial segmentations 
> > are needed for FST?
> >
> > Jason, I was also told by Daniel that the global track efficiency for those 
> > plots is wrong. Can you please tell me what is wrong with them?
> >
> > Thanks,
> > Zhenyu

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