RICH 2010, Cassis, May 2-7, 2010 Itzhak Tserruya The Hadron Blind Detector of the PHENIX Experiment at RHIC RICH 2010, Cassis, May 2-7, 2010 Itzhak Tserruya

Outline Motivation Detector Concept Design and Construction Operation Performance Summary Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

1. Motivation Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

combinatorial e+ e - pair Motivation (I) Electron pairs (or dileptons in general) are unique probes to study the hot and dense 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 created by e+e- pairs from copiously produced 0 Dalitz decay and  conversions.  e+ e - po   e+ e - combinatorial e+ e - pair Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

Baseline PHENIX detector: S/B ratio of 1/200 at m = 500 MEV/c2 Background sources in PHENIX Main sources contributing to the combinatorial background: 0  e+ e-   e+ e-  It often happens that only one electron is detected in PHENIX and the other is lost due to: limited geometrical acceptance low pT particle curling in the magnetic field or not reconstructed. ~12 m Baseline PHENIX detector: S/B ratio of 1/200 at m = 500 MEV/c2 Main goal of the HBD: recognize tracks from conversions and 0 Dalitz decays thereby considerably reducing the combinatorial background

2. Concept Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

Main task of the HBD: distinguish single vs double e-hit Upgrade Concept Strategy Exploit the fact that 0 Dalitz decays and  conversions have a very small opening angle to identify them. Create a field free region close to the vertex to preserve the opening angle of close pairs. Identify electrons in the field free region reject close pairs. Hardware realization * HBD in inner region * Inner coil  B0 for r  60cm Main task of the HBD: distinguish single vs double e-hit Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

HBD Concept HBD concept: ♣ windowless Cherenkov detector (L=50cm) ♣ CF4 as radiator and detector gas ♣ Proximity focus: detect circular blob not ring E hadron UV-photon 50 cm ~1 cm CF4 radiator detector element 5 cm beam axis Detector element: ♣ Triple GEM stack with pad readout ♣ CsI reflective photocathode evaporated on top face of top GEM ♣ RB mode of operation to repel ionization charge Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

Very attractive features… Unprecedented N0 Bandwidth 6 - 11.5 eV (100-200 nm)  N0 ≈ 800 cm-1 in an ideal detector with no losses Reflective photocathode  No photon feedback Hadron blind: Most of the ionization charge in the drift region is repelled away from the GEM stack and collected by the mesh Hexagonal pads with size (a=15.5 mm area =6.2 cm2 ) comparable to Cherenkov blob size (10.2 cm2) hadrons: single pad hit, electrons: 2-3 pads per hit Low granularity ~1000 pads per central arm acceptance Low gain primary charge of 5-10 e/pad  gain of 5x103 is enough …but many open questions  extensive R&D A. Kozlov et al, NIM A523 (2004) 345 Z. Fraenkel et al, NIM A546 (2005) 466,

3. Design and Construction Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

The Detector Detector designed and built at the Weizmann Institute Each arm has 12 (23x27cm2) triple GEM stacks: Mesh electrode Top gold plated GEM for CsI & two standard Gems Pad electrode Two identical arms All panels made of honeycomb & FR4 structure FEEs Readout plane with 1152 hexagonal pads made of Kapton in one single sheet to serve as gas seal Low material budget: vessel 0.92%, gas 0.54%, electronics ~1.5%  total under 3% X0. Side panel Readout plane ~ 350 gluing operations per arm Leak rate 0.12cc/min (~1 vol /year)! Mylar window HV terminals Low dead/inactive area of 6% (0.5 mm tolerance between adjacent modules) Service panel Triple GEM module with mesh grid Sealing frame

HBD Construction Jigs 6 active panels glued together Jig #1: Glue active panels together and to the PCB cathode board Jig #2: Glue the rest of the panels to complete the HBD box 6 active panels glued together

The people behind the detector

Detector assembly CsI evaporation and final detector assembly in clean tent at Stony Brook Laminar Flow Table for GEM assembly CsI Evaporator and quantum efficiency measurement (on loan from INFN) Can make up to 4 photocathodes in one shot High Vacuum GEM storage 6 men-post glove box, continuous gas recirculation & heating O2 < 5 ppm H2O < 10 ppm Class 10-100 ( N < 0.5 mm particles/m3) Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

CsI evaporation facility Absolute QE of CsI photocathodes Excellent reproducibility. Excellent stability 4 photocathodes produced per shot together with chicklets for QE monitorin For each GEM 3 measurements are taken at 160 nm across X axis

The SB plant the crew Jason Gabby Greg Ben Matt Ben-II

4. Operation Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

HBD – short history The HBD was commissioned during the 2007 RHIC run The detector encountered severe high voltage problems which ultimately damaged many GEMs: Minor GEM sparks induced larger, more damaging sparks due to large stored energy in filter capacitors Spark in one module would induce sparks in other modules by optical coupling Problem exacerbated by LeCroy 1471N PS that reapplied HV after a trip causing HV spurious HV spikes The main problem with GEM sparking was due to dust ! Detector rebuilt (minimum exposure to glove box, Zener diodes, relay box ) Detector successfully operated in Run-9 (pp collisions) and in Run-10 (Au+Au collisions – still ongoing) each one about 6m long. Fe55 x-ray UV lamp Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

כ"ב/כסלו/תשע"ט Readout and noise Excellent electronic noise in all modules. Typically  = 1.5 ADC counts corresponding to 0.15 fC or 0.2 p.e. at a gain of 5000. Allowed to operate the detector at a low gain of 3000 to 5000. The entire readout chain worked reliably and smoothly (BNL & Columbia Univ) Mean Pedestal rms vs HV Sigma Mean Sigma

Gain determination using scintillation hits כ"ב/כסלו/תשע"ט 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 an exp fctn: 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> = Zoom Reverse Bias Scintillation: unchanged Ionization: suppressed Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

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 for p/T windows. T1 T2 T3 CONFIG CHANGE APPLY HV CHANGE shown above IR ACCE SS RUNS PROCESSED: 288094 - 288900 ES3 WN5

Adjusting the Reverse Bias using scintillation light Requires high precision in adjusting the mesh voltage with wrt the GEM stack, to preserve the p.e. collection efficiency Requires setting ~ 4KV PS to ~ 5 V precision ( 0.1 %) Operating Point 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 Pulse height [ADC counts]

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% 29

5. Performance Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

Position Resolution כ"ב/כסלו/תשע"ט Hadron tracks reconstructed in central arms projected to HBD. Position resolution: z ≈  ≈ 1.5 cm Dictated by: pad (hexagon) size: a = 1.55 cm 2a/√12 = 0.9 cm vertex resolution ~ 1 cm Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

Hadron Blindness כ"ב/כסלו/תשע"ט Hadron suppression illustrated by comparing hadron spectra in FB and RB (same number of central tracks) Pulse height Hadron rejection factor Pulse height Strong suppression of hadron signal at reverse drift field Larger rejection by combining pulse height and cluster size Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

Electron - hadron separation in RB כ"ב/כסלו/תשע"ט HBD in RB mode Reconstructed tracks in Central Arms are projected to HBD Single electrons selected by Dalitz open pairs (m< 150 MeV/c2) 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

Single electron detection efficiency כ"ב/כסלו/תשע"ט 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 %

Single vs double electron separation כ"ב/כסלו/תשע"ט Fully reconstructed 0 Dalitz pairs (m < 150 MeV/c2) in the central arms Matched to HBD into two separate clusters or one single cluster. Double electron response (0 Dalitz close pairs) Single electron response (0 Dalitz open pairs) ~ 40 p.e. per two electron track ~ 22 p.e. per single electron track Agrees with expected yield taking into account p.e. collection efficiency and transmission loss in the gas Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

Combinatorial Background rejection כ"ב/כסלו/תשע"ט With the performance results shown in the previous slides we should be very close to the design values for the CB rejection. **Preliminary ** rejection numbers: - matching to HBD 7.1  2.2 - double hit cut - close hit cut - single pad cluster cut ~2 6.5

Summary A novel HBD detector has been developed, installed and is successfully being operated in the PHENIX set-up Analysis of data taken in pp and Au+Au collisions show: Clear separation between e and h Expected Hadron rejection factor Excellent electron detection efficiency Good separation of single vs double electron hits Considerable reduction of the combinatorial background Looking forward to the physics Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010

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 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 Weizmann Institute of Science Z. Fraenkel*, A. Kozlov, A. Milov, M. Naglis, I. Ravinovich, D. Sharma, I.Tserruya * deceased

Event display Itzhak Tserruya RICH 2010, Cassis, May 2-7, 2010