sPHENIX tracking discussion

[Thomas K Hemmick <hemmick AT skipper.physics.sunysb.edu>]

Hi everyone

After our C&S rehearsal yesterday, I spent some time chatting with Jamie Dunlop regarding space charge, including quantitative discussions based upon figures from the talk given by (and hallway discussions with) Nikolai Smirnov during the Santa Fe workshop.  Jamie was kind enough to spend the rest of the evening and this morning making some quick calculations of the space-charge-induced fluctuations and limits they impose on TPC resolution (i.e. the uncertainty in the correction).  He agreed that I could share these mails with the whole tracking group and this is the first of a set of forwarded mails (one loses attachments by only fwd-ing the last one).

For nomenclature sake, since the electron drift is fast, and the ion drift is slow, the positive ions coming from the avalanche stage due to a single collision are effectively stationary pancakes of charge whose position depends upon the past history of avalanches in the TPC.  Hence the title of the thread "5000 pancakes".

Tom

---------- Forwarded message ----------
From: James Dunlop <dunlop AT bnl.gov>
Date: Thu, Oct 29, 2015 at 5:47 PM
Subject: 5000 pancakes
To: thomas.hemmick AT stonybrook.edu


OK, just taking the clearing time of star, about 1 s, and bringing that down an order of magnitude, you still have 0.1 s clearing time.  At 50 khz interaction rate that's 5000 pancakes in 80 cm, or something like 150 um on average between the pancakes.   Now there are large fluctuations in the charge of these pancakes, both due to loopers and the underlying humpbacked multiplicity distribution, but I bet that knowing fine details of the spatial distribution of the charge really doesn't matter much.  Should be a simple Monte Carlo with a quick 2d calculation of the kick per charge for a Delta function folded in with a charge per pancake uniform in charge^1/4, spaced randomly, as compared to the same average amount of charge spread randomly.  I bet the rms is actually quite small.  5000-folding gets you pretty far on the central limit theorem.