1. 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

2. RHIC eCloud 04, Apr 2004 The STAR Experiment  *=1m 4m Be 2x4m Al

3. maximum projection minimum projection physics target Best 7 days: delivered 179 (  b) -1 to Phenix New records reached

4. The cross section to measure luminosity eCloud 04, Apr 2004

5. The ZDC detector eCloud 04, Apr 2004 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)

6. Expected collision rates eCloud 04, Apr 2004 R ZDC (Hz) =  ZDC * L Cross section has been calculated and measured to better than +/- 10%

7. Fill pattern in 04 eCloud 04, Apr 2004 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

8. 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.

9. Vacuum history at STAR during 04 eCloud 04, Apr 2004 “bad” vacuum periods difference between “good” and “bad” < x5

10. Background conditions in STAR eCloud 04, Apr 2004 Backgrounds during period of high vacuum ZDC coincidence Blue bkgd Yellow bkgd Background during period of “good” vacuum

11. Background and pressure after rebucketing

12. Different vacuum conditions eCloud 04, Apr 2004 “good” “bad”

13. 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

14. Fill 4471 ZDC coincidence, pressure and backgrounds eCloud 04, Apr 2004 No excess rate at STAR Decent background Small pressure rise

15. Fill 4420 ZDC coincidence and vacuum eCloud 04, Apr 2004 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.

16. eCloud 04, Apr 2004 Beam-gas cross section

17. Calculating Accidental Collision rates eCloud 04, Apr 2004 => R acc.coin ~ p 2

18. Accidental Collisions vs. Pressure (I) eCloud 04, Apr 2004

19. Accidental Collisions vs. Pressure (II) eCloud 04, Apr 2004

20. Predicted vacuum distribution eCloud 04, Apr 2004 Without beam (VACCALC)* Instantaneous pressure distribution as created by EC * Ping He, BNL

21. Summary eCloud 04, Apr 2004  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