SpaceCharge in pp500
Joe Seele (MIT)
2009 STAR TPC Review
Outline
• pp500 environment
• New estimate of
spacecharge
•Current status
•Towards a data-driven
model of spacecharge
1
pp500 environment
• There are ~2.5e7 bunch crossings
‘worth’ of accumulated ionic charge in
TPC during any given event
• In pp500 have seen trigger rates on
O(1) minbias trigger per bunch
crossing
• But how much (average) charge is
that?
2
Charge Structure
1 timebucket = time between subsequent bunch crossings
3
Charge Structure
Electron drift direction
Any charge that could have
come from the triggered
event should be in the 2m
volume at the interaction
time (modulo some
generation time)
2m/elec drift vel = 330 tbs
2m
4
Charge Structure
“Charge Step”
“Normal” Charge
Both part of the 330 tbs
5
Charge Structure
“Charge Step”
“Postmembrane” Charge
“Normal” Charge
6
Postmembrane Charge/Hits
Electron drift direction
Any charge that could have
come from the triggered
event should be in the 2m
volume at the interaction
time (modulo some
generation time)
2m/elec drift vel = 330 tbs
The TPC is read out for ~30
tbs (~20cm in electron drift
distance) after the 330
“good” tbs.
2m
0.2m
If the hit’s z position is naively reconstructed from its drift,
it would look like it came from other half of TPC which is
separated by the membrane -> post-membrane hits
7
Postmembrane Charge/Hits
Electron drift direction
Any charge that could have
come from the triggered
event should be in the 2m
volume at the interaction
time (modulo some
generation time)
2m/elec drift vel = 330 tbs
The TPC is read out for ~30
tbs (~20cm in electron drift
distance) after the 330
“good” tbs.
2m
0.2m
But since charge doesn’t traverse the membrane.
This is charge that is unassociated with the
triggered event -> an unbiased estimate of
electronic charge in the TPC
8
Charge Structure
Charge associated with triggered event
9
Postmembrane Charge/Hits
• The PMB data gives an unbiased estimate
of electronic charge in the TPC
• Though there is really (2.5e7/30) ~ 8e5
times more ionic charge in TPC than in a
single ‘event’ of pmb charge
• But provided these are linearly related, the
pmb charge will give a good local (in time…
maybe space?) measure of the spacecharge
in the TPC
10
Postmembrane Charge/Hits
The older method which relates
the sc to the zdc coincidence
trigger rate (has done quite
well, but shows signs of
saturation at the pp500
luminosities)
The new method using the pmb
charge in each event. There is
some noise in the data, but this
is currently being investigated.
But still seems to track the
spacecharge quite well -> ~1
month till ready to use in data
11
Calibration using older method
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
12
But things are not bad at all
Gene was kind enough to take a quick look at the data
with some code and without much effort sees the K0
and the Lambda in the pp500 data
13
Towards a data-driven model of sc
• As has been said, it is important to
understand the nature of the spacecharge.
Need to test our models…
• STAR collects “zerobias” events (clock
triggers) at ~2Hz during data taking.
• The collected pmb data will also be quite
useful, especially in understanding the
fluctuations.
• With proper assembling, can get a model of
spacecharge, complete with fluctuations
and (some) structure.
14
Conclusions
• There is a lot of spacecharge in the
TPC at any given time.
• But even with the large amount of
charge we can still see our familiar
candles.
• Developing/studying new tools (“pmb
charge”) to understand and correct
for the dynamical distortions.
15
Backup Slides
16
Hits
“Charge Step”
“Postmembrane” Hits
“Normal” Hits
17