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

Angelika Drees eCloud 04, Apr 22, 2004

RHIC

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The STARExperiment

*=1m4m Be2x4m Al

maximumprojection

minimumprojection

physics target

Best 7 days: delivered 179 (b)-1 to Phenix

New records reached

The cross section to measure luminosity

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The ZDC detector

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Located behindDX magnets

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

Expected collision rates

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RZDC (Hz) = ZDC * LCross section has been calculated and measured to better than +/- 10%

Fill pattern in 04

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STAR and PHENIX had both *=1m

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

=> ~7% less expected collision rates in STAR

Blue gap

Yellow gap

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.

Vacuum history at STAR during 04

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“bad” vacuum periods

difference between “good” and “bad” < x5

Background conditions in STAR

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Backgrounds during period of high vacuum

ZDC coincidence

ZDC coincidence

Blue bkgdYellow bkgd

Background during period of “good” vacuum

Background and pressure after rebucketing

Different vacuum conditions

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“good”

“bad”

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

Fill 4471 ZDC coincidence, pressure and backgrounds

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No excess rate at STAR

Decent background

Small pressure rise

Fill 4420 ZDC coincidence and vacuum

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ZDC coinc.

Pressure @ STAR

Excess rate STAR should show 7% less coincidence rates than PHENIX

Use STAR-PHENIX as a measure for accidental collisons due to beam-gas.

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Beam-gas cross section

Calculating Accidental Collision rates

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=> Racc.coin ~ p2

Accidental Collisions vs. Pressure (I)

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Accidental Collisions vs. Pressure (II)

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Predicted vacuum distribution

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Without beam (VACCALC)*Instantaneous pressuredistribution as created by EC

* Ping He, BNL

Summary

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The ZDCs are a detector with very small background contributionsDue to automatic luminosity optimization expected coincidence are very well knownPressure rise due to EC is a problem not only at PHOBOSPressure evolution is quite different at PHOBOS and STARAccidental collision rate is consistent with beam-gas and can be calculated to benchmark different models for EC simulations