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[Hugo Pereira Da Costa <hugo.pereira-da-costa AT cea.fr>]

Hello Jamie,

(also adding the sphenix calibration list in cc for furher insight)

On 2/8/21 7:39 PM, Jamie Nagle wrote:

Hello Tracking Experts,

I have been only somewhat following the progress on understanding track distortions, corrections, and momentum resolution and its relation to the TPOT proposal.    In looking back through recent presentations, I am missing part of the big picture -- which is certainly my fault.

I thought I would ask a few questions to see what quantitative answers exist.

* As a starting point, until September 2020,  the sPHENIX plan  was to have 150 kHz of AuAu  minimum bias collisions - a large fraction which would be outside the z  vertex, i.e. |z| < 10 cm.   For pp collisions the highest rates were 10 MHz or more, and now with the  plan only having pp 200 GeV and a crossing angle this is down to ~ 2 MHz.

* Only in September 2020 with  the new sPHENIX Beam Use Proposal did we give up  these high rates for calorimeter measurements only, and settle that the highest AuAu minimum bias rates would be approximately 50 kHz or so.  

* What was the original performance spec for the TPC to handle distortions at and achieve the benchmark Upsilon mass resolution?

If I understand the question right, the plan was to be able to reconstruct the distortions with a precision << the spacial resolution of the GEMs. That would meain ~ 100um at most in phi, and some 200-300um in z. (not clear about the accuracy you need in the radial direction, which we don't measure, since it affects momentum measurement only at second order).

What has been achieved so far: we are able to reconstruct the distortions induced by 50 kHz Au-Au collisions, averaged over timescale of the order 1/2h, to this level of accuracy, using  tracks, provided that we have Micromegas detectors in a fraction of the acceptance. With such reconstructed distortions, we are able to recover the inv. mass resolution down to < 80 MeV in low multiplicity events (this has not been tested with upsilons embeded in MB + Pileup yet). For completeness: the inv. mass resolution you get for the upsilon without correcting for these 50kHz beam-induced distortions is ~700 MeV (preventing us to do any physics at all). There are some plots on this (as well as on the momentum resolution) at https://indico.bnl.gov/event/10568/contributions/45112/attachments/32417/51575/talk.pdf, slides 9 and 10.

Reconstructing the shorter timescale fluctuations of the distortions on top of this time average is still a work in progress and cannot be achieved with tracks. However such fluctuations are expected to be much smaller than the average (few 100um instead of few millimeters). The impact of those have not been fully quantified yet either.

What happens without the Micromegas: we would essentially be incapable of reconstructing these distortions using tracks, due to the too poor extrapolation precision in the TPC provided by the MVTX and INTT only. This means that we would have to rely entirely on the other available methods at hand: the directed and diffuse laser, as well as the digital currents read on the GEM. Using these methods is still a work in progress, and so is demonstrating that they allow to recover the appropriate inv. mass resolution. In any case, not being able to use tracks to reconstruct the distortions takes away a lot of redundancy in any procedure we can derive, and would seriously impact our ability to quantify e.g. systematic uncertainties associated with it.

To my knowledge, there has been no study done yet about which maximum collision rate would generate distortions small enough that we can achieve the required upsilon mass without having to deal with them. However, assuming that distortions scale with collision rate, and since 50kHz beam-induced distortions are of the order of a couple millimeters, I guess you would need at least a 10x, 20x smaller rate for this to happen. (so 2 to 5 kHz).

Note that beam induced distortions are not the full story though. One can also expect "static" distortions, due to E and B field inhomogeneities and the fact that they might not be parallel. From Ross studies it turns out that these distortions are about 10x bigger than the beam-induced one (so cm scale). However, we expect to be able to correct for them using the directed lasers (although this is still work in progress). If we have Micromegas in place, those could also be adressed using tracks (from either beam, or cosmics).

Hoping this helps. Others in particular from the calibration TF might have additional information.

Best,

Hugo

  

* Some of the current momentum resolution / upsilon resolution plots look like that program is lost (or never was).    It is possible to produce a figure of (1) charged particle momentum resolution versus p up to 50 GeV, and (2) upsilon 3-state mass resolution for the following equivalent rates: 1 kHz, 10 kHz, 25 kHz, 50 kHz.     At what rate do the effects of degraded resolution  really kick in?

* How much of the issue  with fluctuations in IBF and corrections is CPU time versus simply a  loss of information (i.e. impossible to recover)?     Where do the outer MMGs come into play here in both cases?

Some of this information may be critical in potentially re-thinking the run plan to make sure a "good" data set, though smaller, can be assured, while testing at higher rates.   Also, I thought there were ideas with regards to how small in radius we have active in the TPC that influences things -- is that now set in stone or still being discussed?    Same with the gain and gas question.

Thanks for any  help in this direction.

Sincerely,

Jamie

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|| James L. Nagle   
|| Professor of Physics, University of Colorado Boulder

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