1. Itzhak Tserruya IEEE 2007, October 29, 2007, Hawaii Itzhak Tserruya Weizmann Institute of Science, Rehovot, Israel for the HBD group: for the HBD group: BNL (Physics): B.Azmoun, A.Milov, R.Pisani, T.Sakaguchi, A.Sickles, S.Stoll, C.Woody J.Harder, P.O’Connor, V.Radeka, B.Yu BNL (Instrumentation): J.Harder, P.O’Connor, V.Radeka, B.Yu Columbia Univ : C-Y. Chi SUNY: W.Anderson, A.Drees, Z. Citron, M.Durham, T.Hemmick, R.Hutter, B.Jacak, J.Kamin Weizmann: A.Dubey, Z.Fraenkel, A. Kozlov, M.Naglis, I.Ravinovich, D.Sharma, I.Tserruya Construction, Commissioning and Performance of a Hadron Blind Detector for the PHENIX Experiment at RHIC

2. Itzhak Tserruya IEEE-NSS, Hawaii, October 29, 2007 2 Outline  Motivation  Concept  Construction  Performance

3. Motivation Electron pairs (or dileptons in general) are unique probes to study the matter formed in relativistic heavy ion collisions at RHIC: – best probe for chiral symmetry restoration and in-medium modifications of light vector mesons , ω and  – sensitive probe for thermal radiation: QGP qqbar   *  e + e - HG  +  -     *  e + e - Experimental challenge: huge combinatorial background arising from e + e - pairs from copiously produced from  0 Dalitz decay and  conversions.  e + e -    e + e - Both members of the pair are needed to reconstruct a Dalitz decay or a  conversion. Pair reconstruction limited by: – Low p T acceptance of outer PHENIX detector: ( p > 200MeV) – Limited geometrical acceptance of present PHENIX configuration

4. 4 Upgrade Concept Hardware * HBD in inner region * Inner coil (foreseen in original design)  B  0 for r  60cm Software * Identify electrons with p>200 MeV in PHENIX central arm detectors * Match to HBD * Reject if another electron is found in the HBD within opening angle < 200 mrad. Strategy Create a field free region close to the vertex to preserve opening angle of close pairs. Identify electrons in the field free region reject close pairs.

5. HBD Concept HBD concept: ♣ windowless Cherenkov detector (L=50cm) ♣ windowless Cherenkov detector (L=50cm) ♣ CF 4 as radiator and detector gas ♣ CF 4 as radiator and detector gas ♣ Proximity focus: ♣ Proximity focus: detect circular blob not ring detect circular blob not ring 50 cm ~1 cm CF 4 radiator detector element 5 cm beam axis  Why is it Hadron Blind? reverse bias between mesh and top GEM repels ionization charge away from multiplication area Sensitive to UV and blind to traversing ionizing particles E hadron UV-photon Detector element: ♣ CsI reflective photocathode ♣ CsI reflective photocathode ♣ Triple GEM with pad readout ♣ Triple GEM with pad readout

6. 6 All panels made of honeycomb & FR4 structure Mylar window Readout plane Service panel Triple GEM module with mesh grid The Detector  The detector fits under 3%X 0 (vessel 0.92%, gas 0.54%, electronics ~1.5%) and it is leak tight to keep water out 0.12cc/min (~1 volume per year)! Side panel Sealing frame HV terminals Detector designed and built at the Weizmann Institute FEEs  Readout plane with 1152 hexagonal pads is made of Kapton in a single sheet to serve as gas seal  Each side has 12 (23x27cm 2 ) triple GEM detectors stacks: Mesh electrode  Top gold plated GEM for CsI  Two standard GEMs  pads Two identical arms

7. Detector elements  GEM positioning elements are produced with 0.5mm mechanical tolerance.  Dead areas are minimized by stretching GEM foils on a 5mm frames and a support in the middle.  Detector construction involves ~350 gluing operations per box Jig for box assembly

8. Itzhak Tserruya IEEE-NSS, Hawaii, October 29, 2007 8 CsI evaporation and detector assembly in clean tent at Stony Brook” CsI Evaporator and quantum efficiency measurement (on loan from INFN) Can make up to 4 photocathodes in one shot 6 men-post glove box, continuous gas recirculation & heating O 2 < 5 ppm H 2 O < 10 ppm Laminar Flow Table for GEM assembly High Vacuum GEM storage Class 10-100 ( N < 0.5 mm particles/m 3 ) Detector assembly

9. 9 HBD Engineering Run  The HBD was commissioned during the 2007 RHIC run.  After overcoming an initial HV problem, the detector operated smoothly at a gas gain of 3000-6000 for several months.  The CF 4 recirculation gas system worked very smoothly. The oxygen and water content of the gas were monitored at the input as well as at the output of each vessel. In addition the gas transparency was monitored with a monochromator system. A reasonable transmittance of ~80% was achieved at a gas flow of 4 lpm.  The entire readout chain (both analog and digital) worked smoothly.  The excellent noise performance of the device (pedestal rms corresponding to 0.15 fC or 0.2 p.e. at a gain of 5000) allowed online implementation of a simple zero suppression algorithm to reduce the data volume.  A few billion minimum bias Au+Au collisions at √s NN = 200 GeV were collected and are presently being analyzed. Typical noise performance

10. Itzhak Tserruya IEEE-NSS, Hawaii, October 29, 2007 10 Tracking & position resolution Run 226502 ES4 at 3600V FB Position resolution:  z ≈   ≈ 1 cm Dictated by pad size: hexagon a = 1.55 cm (2a/√12 = 0.9 cm) Hadrons selected in central arm: Vertex +/- 20 cm < 50 tracks 3  matching to PC3 and EMCal n0 < 0 EMC energy < 0.5 Projected onto HBD:  Z in HBD +/- 2 cm  in HBD +/- 25 mrad

11. Itzhak Tserruya IEEE-NSS, Hawaii, October 29, 2007 11 Hadron Blindness & e-h separation Hadron suppression illustrated by comparing hadron spectra in FB and RB (same number of central tracks) Pulse height Hadron rejection factor Pulse height Electron - hadron separation (RB) Strong suppression of hadron signal while keeping efficient detection of photoelectrons at reverse drift field

12. 12 Electron detection efficiency  Identify e in central arm using RICH and EMCal  Project central arm track to HBD  Relative e-detection efficiency in HBD obtained by varying the charge threshold of the closest (matched) pad EN3 G ≈ 3300 (several runs at “nominal” voltage) (several runs at “nominal” voltage) All modules ≈ 6600 (Run 237092 “nominal” voltage + 100V) ~4 p.e.  Efficiency drop at pad threshold larger than about 4p.e. probably due to electrons converted in the gas near the GEMs. Needs further study.

13. Itzhak Tserruya IEEE-NSS, Hawaii, October 29, 2007 13Summary Low-mass e + e - pairs is a significant observable to diagnose the matter formed at RHIC.  A novel HBD detector has been constructed and installed in the PHENIX set-up  A commissioning run took place in spring 2007  Preliminary analysis of data show:  Clear separation between e and h  Hadron rejection factor  Good electron detection efficiency