1. Technical Specifications
Basic Questions
Are the physics-driven requirements of HFT understood?
Will the proposed detector/mechanics/electronics meet those requirements?
Is HFT capable of making the proposed physics measurements? In what luminosity environment?
Spiros/Jana(Gerrit)

2. What HFT wants to do?
Essentially measure D-mesons. Two and three charge particle decay modes (Kp, KpK, Kpp)
Plus, some peripheral benefits for other things due to its good pointing ability

3. D-meson reconstruction facts
Mean life-time D+=320m D0=125m
Small impact parameter (few tens of m)
MCS similar (2mrads or 30m DCA from first layer for a 0.5 GeV pion in 0.3% X0 thick ’layer’)
Going to higher pt (Lorentz boost) doesn’t help due to forward focusing (for fixed lifetime)
Small rates (~10-3), in-detector acceptance
Huge Combinatorial Background
Scales as (event multiplicity)^(2 or 3) [for 2 or 3 charged particles in the final state

4. HFT overall philosophy
Low Mass
Use extremely thin Silicon (and beam pipe)
50 micron
Ultra light support
Most of it out of the (tracking) way
Ladder total = 0.26% X0
Good pointing
Close to vertex
Non-drifting (pixel) technology
Smaller beam-pipe
First layer at 1.5 cm

5. Comments on HFT overall philosophy
PLUSES
Overall Approach is sound
Modulo technology issues
Ultra-light, non-drifting, next-to-vertex detector
MINUSES
First-time implementation in experiment
Serial Read-out -> Slow -> Pile-up->Tracking Degradation + Ghosting
Passive charge collection – Low S/B ratio?

6. Simulation work
MEVSIM (Au+Au) + Geant + Response Simulators for TPC/SVT [i.e. Realistic]
Hit resolution of 6m might be a bit optimistic for real operations.
Tracks are claimed to be ‘reconstructed’ with the ‘inner-tracker’ code?!
Is it a real tracker/track-fitter?
What is it? STAR’s tracker/ITTF/what?
Tracking efficiency at 4-times RHIC luminosity for >0.5 GeV tracks, independent of momentum(?) [probably dominated by acceptance requirements of 15 TPC + 1 SVT + 2 HFT hits]
Poor low momentum performance due to pile-up + other reasons

7. In D0 reconstruction exercise (full V0 reco.)
Ghost tracks (>=1 wrong hit) are about 10% for 0.5GeV and about 4% for higher momenta
This is for 4-times the RHIC designed luminosity. Assuming that HFT will be there for RHIC-II (10x the luminosity), it appears that pile-up is a show-stopper. As proposal also says need new chips.
In D0 reconstruction exercise (full V0 reco.)
Signal/Background generated/reconstructed separately (not good but necessary). What exactly was done to account for higher tracking efficiency of D0 tracks? Real or simple scaling?
Cuts seemed reasonable
Invariant mass plot (to justify Dm=+-40MeV) is missing
Fig 16 doesn’t(?) [cannot] include branching ratio. What about rapidity cuts? What ‘acceptance’ means?
Comparison with ALICE pixels includes pile-up/luminosity effects?
Fig 17 has an order of magnitude difference between left/right plot
Not real justification for shooting at 6m resolution. It seems that twice that (12m, factor of four less/larger pixels?) does equally well. It appears that anything below 20m will do the job.

8. In D0s and Flow reconstruction exercise
Similar to D0 reconstruction comments plus:
Fig 20. How many events are used here? What is ‘one year of data taking? At what luminosity?
Otherwise it appears that HFT will be able to do both measurements with the quoted caveats.

9. PILE-UP, a closer look
To be frank I am at a loss with the hit density numbers. I can’t reproduce them for inner layer. See also:
www-rnc.lbl.gov/~wieman/HitDensityMeasuredLuminosity.htm
Example-1: SVT first layer at ~7cm. Eta=1=40degrees. Effective area A=2*p*R*Dz, with R=7cm and Dz=R*sin(90-40)=8.3cm, I get A=365cm^2. For average dN/deta in central Au+Au of about 600 charged particles I should see about hit/cm^2 if I include decays and garbage. It is quoted (3.4.1) 1hit/cm^2, a factor of two difference (O.K.).
Example-2: Hit density scales with the inverse square of the radial distance. Then Table-8 is not self-consistent as a density of should be and it is quoted as 7.4. Similar problem with Table-7.

10. Example-3: If I use 1hit/cm^2@7cm then I get 2@5cm (O. K
Example-3: If I use then I get (O.K. with Table-8) and for central Au+Au. These numbers double for (my estimate)
Example-4: If I use the quoted (3.3) number of 120 min. bias design luminosity with an average dN/deta~140tracks (~17Ktracks) and multiply by 0.25 to take into account the gaussian diamond 30cm size, I would expect about 23 (close to 17 in Table-7) and 250 NOT 72!
COMMENTS: 1) I have done simulations confirming the naïve expectation of inverse radius square dependence of hit densities. 2) I looked at Howard’s web note and I located the area where this is calculated. I find discrepancies. Still waiting to get him on the phone. 3) If it turns out that I am correct the picture will change dramatically (tracking window pileup shoots to ~40% from 10%)

11. Thorny Issues Good Intermediate Tracker is crucial but missing!
HFT needs modification for high Luminosity
Is total final radiation thickness close to design
Total hit density in inner layer?
Tracking efficiency is low and sensitive to hit density
Details on HFT bunch-to-bunch operation in p-p spin collisions are not given.
Details on pile-up vs conversion-electrons rejection are not given but an issue.