Experimental background at the presence of pressure rise in RHIC

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Experimental background at the presence of
pressure rise in RHIC
Angelika Drees, Ubaldo Iriso-Ariz

• RHIC
• How to measure collision rates
• Vacuum and collision data
• Summary

eCloud 04, Apr 22, 2004
Angelika Drees
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RHIC
The STAR Experiment
b1m 4m Be 2x4m Al
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New records reached ?
Best 7 days delivered 179 (mb)-1 to Phenix
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The cross section to measure luminosity
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The ZDC detector
Located behind DX magnets

• ZDCs are common to all IRs
• Shielded by the DX magnets (small backgrounds)
• Small acceptance (small backgrounds)
• Sensitive to neutrons only (small backgrounds)

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Expected collision rates
RZDC (Hz) s ZDC L Cross section has been
calculated and measured to better than /- 10
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Fill pattern in 04
STAR and PHENIX had both b1m For most of the
run the abort gap was aligned such that it
crossed in PHENIX and BRAHMS Needs to be
corrected for to compare experiments gt 7 less
expected collision rates in STAR
Yellow gap
Blue gap
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Automated Luminosity Steering
LISA commissioning version used for Vernier Scans
in Run 3. Used for automated collision steering,
using feedback from ZDC at the beginning of every
store.
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Vacuum history at STAR during 04
difference between good and bad lt x5
bad vacuum periods
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Background conditions in STAR
Backgrounds during period of high vacuum
Blue bkgd Yellow bkgd
ZDC coincidence
Background during period of good vacuum
ZDC coincidence
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Background and pressure after rebucketing
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Different vacuum conditions
good
bad
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Pressure evolution during a store in STAR and
PHOBOS
Evolution in store is very different (?). Causes
unacceptable backgrounds in PHOBOS and high
trigger rates in STAR
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Fill 4471 ZDC coincidence, pressure and
backgrounds
No excess rate at STAR
Decent background
Small pressure rise
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Fill 4420 ZDC coincidence and vacuum
STAR should show 7 less coincidence rates than
PHENIX
Excess rate
ZDC coinc.
Use STAR-PHENIX as a measure for accidental
collisons due to beam-gas.
Pressure _at_ STAR
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Beam-gas cross section
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Calculating Accidental Collision rates
gt Racc.coin p2
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Accidental Collisions vs. Pressure (I)
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Accidental Collisions vs. Pressure (II)
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Predicted vacuum distribution
Instantaneous pressure distribution as created by
EC
Without beam (VACCALC)
Ping He, BNL
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Summary

• The ZDCs are a detector with very small
  background contributions
• Due to automatic luminosity optimization
  expected coincidence are very well known
• Pressure rise due to EC is a problem not only at
  PHOBOS
• Pressure evolution is quite different at PHOBOS
  and STAR
• Accidental collision rate is consistent with
  beam-gas and can be calculated to benchmark
  different models for EC simulations

eCloud 04, Apr 2004