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PHENIX Collaboration meeting, Ames, July 11, 2010 Itzhak Tserruya

HBD in Run-10. PHENIX Collaboration meeting, Ames, July 11, 2010 Itzhak Tserruya. Outline. Operation Performance Central collisions Summary. 1. Operation. HBD – short reminder.

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PHENIX Collaboration meeting, Ames, July 11, 2010 Itzhak Tserruya

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  1. HBD in Run-10 PHENIX Collaboration meeting, Ames, July 11, 2010Itzhak Tserruya

  2. Outline • Operation • Performance • Central collisions • Summary PHENIX CM, Ames, July 11, 2010

  3. 1. Operation PHENIX CM, Ames, July 11, 2010

  4. HBD – short reminder • In Run-9 both East and West arms of the HBD performed very well. But the West was more stable than the East : • The difference was attributed to the different assembly procedures used for the West and the East arm. • After Run-9 we decided to refurbish the East HBD using the fast assembly procedure that was used for the West and proved to be superior • This turned out to be a right decision. In Run-10, both arms performed in a very stable mode (except for one module EN2 that was problematic already at the assembly phase and was disabled early in the run). • The run started with a 95% active detector and the run ended with a 95% active detector PHENIX CM, Ames, July 11, 2010

  5. Readout, pedestal and noise The entire readout chain worked reliably and smoothly Some DAQ issues: needed frequent feeds; occasional hits on disabled module Electronic noise was excellent in all modules and stable during the entire duration of the run. Typically  = 1.5 ADC counts corresponding to 0.15 fC or 0.2 p.e. at a gain of 5000.

  6. Event size reduction Simple zero suppression was implemented early in the run: sample(8) – sample(0) > n. However, event size remained large due to the scintillation light A new readout scheme implemented in run10 reduced the event size by a factor of 3: (12 samples in 2 words instead of 6 words) PHENIX CM, Ames, July 11, 2010

  7. Zoom Gain determination using scintillation hits Exploit scintillation hits (identified as single pad hits not belonging to tracks) to determine the gain on a pad by pad basis. Forward Bias Scintillation Ionization • Gain determination: • Fit scintillation component with exp function: • 1/slope = G . <m> • <m> = avrg nr of scintillation photons in a fired pad • In pp collisions <m>  1 • In Au +Au collisions, assuming the nr of scintillation photons per pad follows a Poisson distribution: • <m> = Reverse Bias Scintillation: unchanged Ionization: suppressed PHENIX CM, Ames, July 11, 2010

  8. Gain equilibration and monitoring Before: rms = 0.52 • Gain equilibration: gain is non-uniform over the GEM area due to variations of the hole diameter coming from the GEM production process. • Determine the gain on a pad by pad basis using scintillation hits and apply correction factor to equilibrate the gain to the average value of each module. <G> = Gi/96 aiai <G> / Gi • Determine correction factor using run #303603. Check the effect on different run. • Gain monitoring: gain is calculated for each module and each run. After: rms = 0.11

  9. Correction for P/T Variation • Gain monitoring: gain is calculated for each module and each run. • Gain variations due to p/T changes, automatically compensated by varying the operating HV in 5 pre-determined p/T windows. RUNS PROCESSED: 288094 - 288900 T3 IR ACCESS CONFIG CHANGE T2 APPLY HV CHANGE shown above ES3 WN5 T1

  10. Adjusting the Reverse Bias using scintillation light • Adjust the mesh voltage wrt the GEM stack, as close to zero as possible to preserve the p.e. collection efficiency • Requires setting ~ 4KV PS to ~ 5 V precision ( 0.1 %) Electrons hadrons • Exploit the scintillation that remains unchanged when switching from FB to RB • Scan voltage between mesh and top GEM for each module • Method invented in Run-9 • Simple, fast and precise ES1 =-10V Red (+5V), black (0V)‏ green(-5V),blue(-10V), rest(-15V and lower)‏ Hits per event Operating Point Pulse height [ADC counts]

  11. GasTransmission Continuously monitored using a monochromator system – Recent scan on April 20 Gas flowing in recirculation mode, with scrubbing at 4.5 lpm Gas transmission stable over the entire run Input gas upper 90% Output gas lower 90%

  12. CsI quantum efficiency preserved Measurements done with the scintillation cube, that illuminates one single pad in each arm, show that the quantum efficiency of the West arm CsI photocathodes was preserved during the entire period of about 2 years. There is every reason to believe that the same holds true for the East arm in Run 9 and in Run-10

  13. 2. Performance PHENIX CM, Ames, July 11, 2010

  14. Position Resolution  resolution • Hexagonal pad: • a = 1.55 cm 2a/√12 = 0.9 cm • For electrons c.o.g gives •  = 0.8 mrad = 0.48 cm • Run-10 peripheral events (ncentral <10) - electron tracks from low-mass pairs (m < 150 MeV/c2) reconstructed in central arms projected to HBD. Z resolution • Position resolution: quadratic sum of intrinsic resolution • (~0.5 cm) and z-vertex resolution (~1cm) •  ≈ 1.05 cm

  15. Hadron rejection factor Pulse height HadronBlindness Hadron suppression illustrated by comparing hadron spectra in FB and RB (same number of central tracks) Pulse height • Strong suppression of hadron signal at reverse drift field • Large rejection by combining pulse height and cluster size PHENIX CM, Ames, July 11, 2010

  16. Electron - hadron separation in RB • HBD in RB mode • Tracks reconstructed in Central Arms are projected to HBD • Single electrons selected by Dalitz open pairs (m< 150 MeV/c2) Hadrons Electrons Hadrons dE/dx signal in the small (~ 100 mm) region above the top GEM and in the first transfer gap very small wrt electron Cherenkov signal

  17. Single electron detection efficiency 1. Single electron efficiency using a sample of open Dalitz decays (V cut rejects conversions; not very effective for conversions in the radiator gas ) :   90 % 2. Single electron efficiency derived from the J/psi region ( m > 3.5 GeV/c2 after background subtraction):  = 90.6  9.9 %

  18. Single vs double electron separation Fully reconstructed 0Dalitz pairs (m < 150 MeV/c2) in the central arms Matched to HBD into two separate clusters or one single cluster. Single electron response (0Dalitz open pairs) Double electron response (0Dalitz close pairs) Run-9 Run-10 (online production) ~ 22 p.e. per single electron track ~ 40 p.e. per two electron track

  19. HBD figure of merit • Cherenkov counters are characterized by the figure of merit N0 defined as the folding of the quantum efficiency of the detector over the sensitive bandwidth. • The number of photo-electrons is then given by: • Npe = N0 . L / th2

  20. 3. Central Au+Au collisions PHENIX CM, Ames, July 11, 2010

  21. The issue and how we are coping with it • Large scintillation yield in CF4 leads to large detector occupancy breakdown of the cluster finding algorithm • rejection limited by random matching • Two venues are pursued: • Subtract a centrality dependent average signal from each pad • Subtract average background from each pad determined on an event by event basis by average charge in surrounding area • Two evaluation tools: • Accumulate pp events to simulate Au+Au data • Embed MC generated single electrons in real Au+Au data

  22. Occupancy vs centrality Raw event Central arm tracks HBD fired cells Fired cells vs nCentral

  23. Average background subraction define the average charge per fired pad: where a[pad] normalizes the pad area • Look at the <ch>pp as a function of centrality (nCentral) • Subtract charge from each pad according to nCentral • Evaluate performance with pp accumulator • Large

  24. Occupancy vs centrality Raw event After background subtraction Central arm tracks HBD fired cells Fired cells vs nCentral

  25. Compare Au+Au to pp accumulated data 120 accumulated, <nCentral>=236 240 accumulated, <nCentral>=471 20 accumulated, <nCentral>=39

  26. Compare Au+Au to pp accumulated data • Average charge per fired pad vs centrality • Excellent agreement between Au+Au data and the pp accumulator

  27. Electron efficiency vs centrality(relative to pp) • Electrons are reconstructed in central Au+Au collisions • but both cluster size and charge grow with centrality

  28. Local background subtraction • Form preclusters defined as compact triplets with a charge larger than 5 pe and smaller than 50 pe after subtracting the local background • Local background defined by the average charge of the first and second neighbors of the triplet: mem=triplet member fn=first neighbor, sn=second neighbor a=area, q=number of photoelectrons w= weight, for now set to 0.5 • Merge preclusters sharing at least one pad and redefine the background for the merged clusters.

  29. Justification of background estimation • Basic assumption of the method • Scintillation background varies continuously over HBD surface • Background in any compact group of pads can be estimated from the average charge in its neighboring pads 1st vs 2nd neighbors Triplet vs. background

  30. Efficiency and rejection Efficiency at a given cut value dcut can be calculated as Rejection of electrons produced behind the HBD defined by the random matching probability at a given cut value dcut Next slides: efficiency and rejection for 10% centrality bins

  31. Efficiency vs random matching rejection 0-10% 10-20% 40-50% 20-30%

  32. Summary • The HBD performed very well in Run-10 • Analysis of data from pp and Au+Au peripheral collisions show: • Clear separation between e and h • Good Hadron rejection • Excellent electron detection efficiency • Good separation of single vs double electron hits • Considerable reduction of the combinatorial background • Good progress in dealing with the high occupancy of central Au+Au events • Looking forward to the physics

  33. HBD Team • Brookhaven National Lab B. Azmoun, R. Pisani, T. Sakaguchi, A.Sickles, S. Stoll, C. Woody • Columbia University (Nevis Labs) C-Y. Chi • Stony Brook University E.T. Atomssa, B. Bannier, Z. Citron, M. Connors, M. Durham, T. Hemmick, J. Kamin, B. Lewis, V. Pantuev, M. Prossl, J.Sun • University of California Riverside A. Iordanova, S. Rolnick • University of Tokyo Y. Komatsu, K. Ozawa, Y. Watanabe • Weizmann Institute of Science Z. Fraenkel*, A. Kozlov, M. Makek, A. Milov, M. Naglis, I. Ravinovich, D. Sharma, I.Tserruya * deceased

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