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HBD Concept: Advancing Dilepton Measurements at RHIC

This document outlines the physics motivation, detector design, R&D results, and performance of the Hadron Blind Detector (HBD) concept. The HBD aims to measure low-mass electron pairs as a sensitive probe for chiral symmetry restoration and thermal radiation in heavy-ion collisions.

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HBD Concept: Advancing Dilepton Measurements at RHIC

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  1. HBD CDR DC/EC Meeting, BNLMarch 9, 2005 Itzhak Tserruya

  2. Outline • Physics Motivation • HBD Concept • Summary of R&D results • Detector Design • Performance • Conclusion

  3. Physics Motivation

  4. Electron pairs: the case • e.m. probes  long mean free path  no fsi, carry information about the medium • Best probe for chiral symmetry restoration  in-medium modification of vector mesons, the  meson in particular ( = 1.3 fm) • Most sensitive probe for thermal radiation: QGP qqbar  *  e+e- HG +-   *  e+e- • Produced all along the history of the collision • richness but also a physics challenge • Exclusive niche of PHENIX. No other experiment at RHIC can perform this measurement

  5. Intermediate Low High Electron pairs: the landscape HG thermal radiation CSR In-medium modification of  (,)   e+e-  K+K- QGP thermal radiation Charm • Physics potential fully confirmed: • All dilepton experiments have produced very interesting and • intriguing results: DLS, KEK-E235, CERES, HELIOS3, NA38/50 • A few others in the horizon: HADES, NA60, CBM

  6. Low-mass Dileptons: Main CERES Result • Strong enhancement of low-mass e+e- pairs in A-A collisions • (wrt to expected yield from known sources) • No enhancement in p-Be and p-Au • Quantitative explanation requires: • thermal radiation from high-density • with in-medium modification of the • intermediate  meson HG+-    *  e+e- Spectral shape broadening dropping  meson mass free  meson hadron cocktail

  7. Low-mass e+e- Pairs: Prospects at RHIC R. Rapp nucl-th/0204003  interpretation of SPS data rely on a high baryon density at mid rapidity. Baryon density is almost the same at RHIC and SPS • Strong enhancement of low-mass pairs persists at RHIC

  8. RealandMixede+e- Distribution Real-Mixede+e- Distribution e+e- from light hadron decays e+e- pairs (real) net e+e- e+e- pairs (mixed) e+e- from charm (PYTHIA) PHENIX Performance: Run-2 data (the experimental challenge) Low-mass pairs: (0.3 – 1.0 GeV/c2): S/B  1/100 -- 1/500 depending on pt cut and mass. Need an upgrade to make possible the measurement of the low-mass pair continuum

  9. HBD Concept

  10. Upgrade Concept Hardware * Inner coil (foreseen in original design)  B0 for r  60cm * HBD in inner region Strategy * Identify electrons with p>200 MeV in outer PHENIX detectors * Match to HBD * Reject if another electron is found in the HBD within opening angle < 200 mrad.

  11. Principle Monte Carlo at the Ideal Detector Level: Beating the Dalitz and conversions CB (Monte Carlo: Signal  meson, Background  conversions and 0 Dalitz ) • Inner detector: * perfect e-id  = 100 % • * perfect dhr = 0 mrad • *  rejection =  • * plus veto area Pair Signal  Backgd tracks The number of tracks from 0 Dalitz and conversions is reduced by almost a factor of 20.

  12. HBD Concept • The principle Monte Carlo allowed us to determine • the system specifications (see TN391): • e-id with at least 90% efficiency • double hit recognition with at least 90% efficiency • moderate  rejection factor. of the order of 100 • veto area HBD concept: ♣ windowless Cherenkov detector (L=50cm) ♣ CF4 as radiator and detector gas ♣ CsI reflective photocathode ♣ Proximity focus: detect blob not ring ♣ Triple GEM with pad readout

  13. Very attractive features… • Unprecedented N0 Bandwidth 6 - 11.5 eV  N0 ≈ 800 cm-1  ~35 pe in a 50 cm radiator • Reflective photocathode  no photon feedback • Pad size comparable to blob size (~10 cm2) hadrons: single pad hit, electrons: more than one pad hit • Low granularity ~1000 pads to cover central arm acceptance • Low gain primary charge of at least 10 e/pad  gain of 5 103 is enough

  14. …But many open questions • CF4 has never been used as detector gas • Stable operation of triple GEM in CF4 at gain of 104 had to be demonstrated • Aging effects of CsI photocathode and GEM foils in pure CF4 had to be studied • Ion back-flow effects on the CsI. • No data on CsI QE beyond 8.3 eV • CF4 is a scintillator • HBD response to electrons and hadrons had to be studied and optimized Extensive R&D was needed

  15. Summary of R&D Results • A. Kozlov et al., Nucl. Instr. and Meth. A523, 345 (2004) • Z. Fraenkel et al., Nucl. Instr. and Meth. in press.

  16. New powering scheme R&D Set-up Stainless steel box Pumped to 10-6 before gas filling Measurements: * UV lamp, Fe55 x-rays, Am241  source *  (e) beam at KEK * Test in PHENIX environment GEM foils of 3x3, 10x10 and 24x25 cm2 produced at CERN

  17. Fe55 x-ray UV lamp Gain Curve: Triple GEM with CsI in CF4:measured with Fe55 and with UV lamp • Gains in excess of 104 are • easily attainable. • Voltage for CF4 is ~140 V • higher than for Ar/CO2 but • slopes are similar for both • gases. • Gain increases by factor ~3 • for ΔV = 20V • Pretty good agreement • between gain measured • with Fe55 and UV lamp.

  18. Saturation of preamp. Charge saturation effect Deviation from exponential growth when Q ≥ 107 <Q> saturates at ~4 x 107 below the Raether limit of 108

  19. Small GEMs: 3x3 cm2 ΔVGEM Segmented GEMs 10x10 cm2 ΔVGEM Discharge Probability • Stability of operation and absence of • discharges in the presence of heavily ionizing particles is crucial for the operation of the HBD. • Use Am241 to simulate heavily ionizing particles. • In Ar-CO2, discharges increase sharply when total charge is close to the Raether limit of 108. • In CF4 discharges do not depend on • the presence of  particles. It seems that • local defects are responsible for the • discharges • CF4 more robust against discharges • than Ar/CO2 . • HBD expected to operate at gains < 104 • i.e. with comfortable margin below • the discharge threshold

  20. Ar/CO2 CF4 Gain variations: ±10%, as in the lab Std. Conical, Segmented CERN Foils Active area of pads ~1.0x1.2cm2 Triple-GEM detector in PHENIX IR (during full luminosity Au-Au run4 PHENIX IR • The triple GEM detector performed smoothly within the PHENIX IR using both Ar/CO2 (70/30) and CF4 working gases and exhibited no sparking or excessive gain instabilities. • The operation of the GEM and the associated electronics were not hindered by the ambient magnetic field.

  21. Calibrated PMT Bandwidth: 6.2 – 10.3 eV PMT and CsI have same solid angle C1 optical transparency of mesh (81%) C2 opacity of GEM foil (83.3%) All currents are normalized to I(PMT-0) CsI on GEM QE(CsI) = QE(PMT) x I(CsI) / [ I(PMT) x C1 x C2 ] CsI absolute QE • Many measurements of CsI QE in 6-8 eV range • No data beyond 8.3 eV • Measurements extended to 10.3 eV confirm ~linear behavior of QE. • Extrapolation to 11.5 eV: N0 ≈ 820 cm-1

  22. CF4 transparency to UV photons CF4 is transparent

  23. Hadron blindness (I): Response to hadrons (KEK beam test) Pulse Height Charge Collection Landau fit Rejection Factor • Pulse height strongly suppressed when • ED switches polarity • Charge collected from 150μ layer above GEM • Rejection factor limited by Landau tail

  24. D ED (+) G ET T G pA T ET G I EI ED = 0 Hadron Blindness (II): Response to Electronsdetector response vs ED at fixed gain Efficient detection of photoelectrons even at negative drift fields RICH 2004, Playa del Carmen

  25. Hg lamp Absorber E=0 CsI GEM1 1.5mm 1.5mm GEM3 2mm PCB pA Ion back-flow (Potential aging factor of CsI) Independent of gas Mesh GEM2 Independent of Et Depends only on Ei (at low Ei some charge is collected at the bottom face of GEM3) Fraction of ion back-flow defined here as: Iphc / IPCB Ions closely follow the electric lines. In all cases, ion back-flow is of order 1!

  26. 3x3 cm2 10x10 cm2 Aging Tests Test both GEM and CsI photocathode: • Continuous UV irradiation • Operate triple GEM at gain ~ 104 • Measure DC current to PCB • Monitor gain periodically with Fe55 source • No significant aging effects of either GEM or CsI photocathode • up to ~ 150 μC/cm2 (~ 10 years at RHIC) • Behavior during initial phase not yet understood. • (Possible charging effect in GEM foils ?)

  27. Detector Design

  28. Full scale Prototype design Complete set of engineering drawings available at: https://www.phenix.bnl.gov/phenix/WWW/p/draft/ravini/hbd_drawings/prototype

  29. Prototype exploded view

  30. Full-scale prototype in reality

  31. Final HBD design • Same design as full-scale • prototype but: • larger acceptance • || ≤0.36 || ≤0.45 • =110o=135o • smaller pad size • a =16.7  15.6 mm • Number of channels • 1368  2304 • Number of detector modules • 16  24 • GEM size • 26 x 24  23 x 27 cm2

  32. HBD design parameters Acceptance nominal location (r=5cm) || ≤0.45, =135o retracted location (r=22 cm) || ≤0.36, =110o GEM size (,z) 23 x 27 cm2 Number of detector modules per arm 12 Frame 5 mm wide, 0.3mm cross Hexagonal pad size a = 15.6 mm Number of pads per arm 1152 Dead area within central arm acceptance 6% Radiation length within central arm acceptance box: 0.92%, gas: 0.54% Weight per arm (including accessories) <10 kg

  33. System Performance • Full PISA simulation of prototype design • Reconstruction and tracking in Fun4All framework

  34. Single particle response: Hit amplitude & size give very large hadron rejection factor. HBD e-id: cluster size > 1 pad, cluster amplitude > 20 pe  e efficiency 92.2% (mainly due to dead areas. Can be improved).

  35. GEANT hits Reconstructed e HIJING event display HIJING central events (top 2%) dNch/dy = 940: - 400 charged particles traverse the HBD - 19 reconstructed electrons: 5.7 from vertex, 13.5 not from vertex. • Excellent prospects for using the HBD as an electron trigger. • Plans to derive a high pT charged hadron trigger (HBD+RICH+PC3)

  36. Combinatorial background rejection 73K central HIJING events: Pattern recognition • central arm alone spectrum • Central Arm electron tracks matched to HBD with 3 p-dependent cuts. • Reject “conversions” tracks with amplitude cut (>60 e) • Reject Dalitzes with close hit cut (< 200 mrad) With 25% reduction of N0

  37. Signal to Background (I) Embedd e+e- in each HIJING event. Signal: Relative S, B and S/B

  38. Signal to Background (II) (scale the previous results to represent the background for dNch/dy = 650) Signal: • At this level of rejection, the quality of the low-mass pair measurement is not anymore limited by the combinatorial background from Dalitz decays and conversions but by the combinatorial background from open charm decays. • Improvement in the S/B ratio in the intermediate mass region not yet quantified.

  39. Conclusion We are ready for the HBD construction

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