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RICH 2004 Playa del Carmen, Mexico Nov. 30 – Dec. 5, 2004

Cherenkov Counters in Heavy-Ion Physics. RICH 2004 Playa del Carmen, Mexico Nov. 30 – Dec. 5, 2004. Itzhak Tserruya. SPS. RHIC. PHENIX Central Arms. STAR TPC. Pb – Au  s NN = 17 GeV. Au – Au  s NN = 200 GeV. The Challenge: Huge multiplicities. CBM RICH CERES double RICH

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RICH 2004 Playa del Carmen, Mexico Nov. 30 – Dec. 5, 2004

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  1. Cherenkov Counters in Heavy-Ion Physics RICH 2004Playa del Carmen, MexicoNov. 30 – Dec. 5, 2004 Itzhak Tserruya

  2. SPS RHIC PHENIX Central Arms STAR TPC Pb – Au sNN = 17 GeV Au – Au sNN = 200 GeV The Challenge: Huge multiplicities

  3. CBMRICH CERESdouble RICH HADESRICH PHENIXRICH andHBD … ALICE RICH BRAHMS RICH PHENIX Aerogel STAR RICH … Existing detectors Planned or under construction Roles of Cherenkov counters in HI Physics Cherenkov counters, and RICH counters in particular, play a crucial role in particle identification: • RICH is the main instrument fore-idin HI, making it possible to measure electron pairs. • Identification of high momentum charged particles • Timing (100ps time resolution). … RICH 2004, Playa del Carmen

  4. Outline • Introduction • RICH counters for e-id: • Motivation • CERES double RICH spectrometer • PHENIX HBD • Cherenkov counters for High Momentum PID: • Motivation • PHENIX aerogel • BRAHMS RICH • ALICE (STAR) RICH • Summary RICH 2004, Playa del Carmen

  5. Electron pairs: Best probes for chiral symmetry restoration and thermal radiation RICH 2004, Playa del Carmen

  6. Best probe to search for Thermal Radiation QGP: q q  *  l + l - or HG: + -  *  l + l - Physics through dileptons • Best probe of Chiral Symmetry Restoration Chiral symmetry spontaneously broken in nature. Quark condensate is non-zero: < qbarq >  300 MeV3  0 at high T and/or high  Constituent mass  current mass Chiral Symmetry (approximately) restored. Meson properties (m,) expected to be modified Best candidates: -meson decay ( = 1.3fm/c) simultaneous measurement of   l+ l- and   K+ K- RICH 2004, Playa del Carmen

  7. CERES: Unconventional Design Original set-up TMAE 2 PPAC + MWPC Pad readout Radiator gas CH4 (γth = 28) Si drift chambers CaF2 window Carbon fiber mirror • First use of RICH detector in HI physics • Double RICH spectrometer – no real tracking • First use of Si radial drift chambers in an experiment • Unique features to cope with the high multiplicities: • High gamma threshold  tiny fraction of charged hadrons emit Cherenkov • UV detectors upstream of target  not traversed by huge flux of forward particles • Field free region in RICH1 for effective recognition of 0 Dalitz and γ conversions

  8. No enhancement in pp nor in pA 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) Most updated CERES result (from 2000 Pb run): Enhancement factor (0.2 <m <1.1 GeV/c2) 3.1 ± 0.3 (stat) RICH 2004, Playa del Carmen

  9. Huge combinatorial background PHENIX • PHENIX was designed with emphasis on electromagnetic probes e, γ, • PHENIX can measure electrons in the • central region (DC&PC for tracking • RICH & EMcal for e-id) Present set-up lacks the means to identify and reject the overwhelming electron yield from 0 Dalitz decays and  conversions

  10. RealandMixede+e- Distribution Real-Mixede+e- Distribution Monte Carlo Data  e+ e - po   e+ e - combinatorial pairs e+e- from light hadron decays e+e- pairs (real) total background S/B ~ 1/500 net e+e- e+e- pairs (mixed) charm background signal e+e- from charm (PYTHIA) charm signal PHENIX Performance: present set-up Low-mass pairs: (0.3 – 1.0 GeV/c2): S/B  1/100 -- 1/500! depending on pt cut and mass. Measurement of low-mass continuum practically impossible

  11. Upgrade Concept Hardware * Compensate magnetic field with inner coil B0 at r  50-60cm * Compact HBD in inner region Strategy * Identify electrons with pT > 200 MeV/c in outer PHENIX detectors (DC, PC, RICH, EMcal) * match to HBD * reject electron if there is a neighboring one in the HBD within opening angle < 200 mrad (for a 90% rejection). Specifications * Electron efficiency  90% * Double hit recognition  90% * Modest  rejection ~ 100 RICH 2004, Playa del Carmen Expect at least two orders of magnitude improvement in S/B

  12. ~1 cm detector element 50 cm CF4 radiator 5 cm beam axis • Bandwidth 6-11.5 eV, N0 ≈ 900 cm-1 Npe ≈ 40! • No photon feedback • Detect blob, pad size ≈ blob size (~10 cm2) • Low granularity (less than 2000 channels) • Relatively low gain (less than 104) HBD Concept HBD concept: ♣ Windowless Cherenkov detector (L=50cm) ♣ CF4 as radiator and detector gas ♣ CsI reflective photocathode ♣ Triple GEM with pad readout RICH 2004, Playa del Carmen

  13. 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 the PHENIX environment GEM foils of 3x3, 10x10 and 25x25 cm2 produced at CERN

  14. Fe55 x-ray UV lamp Gain Curve: Triple GEM with CsI in CF4:measured with Fe55 and UV lamp • Gains in excess of 104 are • easily attainable. • Gain increases by factor ~3 • for ΔV = 20V • Slopes are similar for CF4 • and Ar/CO2 but CF4 requires • ~140 V higher voltage. • Pretty good agreement • between gain measured • with Fe55 and UV lamp. RICH 2004, Playa del Carmen

  15. Unexpected Saturation effect in CF4 measured with Am241 Deviation from exponential growth when Q ≥ 107 <Q> saturates at ~4 x 107 below the Raether limit of 108 RICH 2004, Playa del Carmen

  16. 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, the discharge threshold is close • to the 108 Raether limit, whereas in CF4 the • discharge threshold depends on GEM • quality and occurs at VGEM 560-600V • CF4 more robust against discharges • than Ar/CO2 . • HBD expected to operate at gains < 104 • i.e. with comfortable margin below • the discharge threshold

  17. Hg lamp Absorber E=0 CsI GEM1 1.5mm 1.5mm GEM3 2mm PCB pA Ion back-flow to the CsI photocathode, a potential aging factor 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 In all cases, ion back-flow is of order 1! RICH 2004, Playa del Carmen

  18. 200 120 [nm] CsI absolute QE Previous measurements: 6.2 – 8 eV Present: 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) QE_CsI = QE_PMT x I_CsI /{I_PMTxC1xC2} Conservative extrapolation to 11.5 eV  N0 = 822 cm-1 RICH 2004, Playa del Carmen

  19. D ED (+) G ET T G pA T ET G I EI ED = 0 See also D. Mormann et al, NIM A478 (2002) 230 Hadron Blindness (I): Response to Electronsdetector response vs ED at fixed gain Efficient detection of photoelectrons even at negative drift fields RICH 2004, Playa del Carmen

  20. Hadron Blindness (II): Response to Hadrons Suppression of hadron signal at negative drift field

  21. Hadron Blindness (III): Response to Hadrons KEK 1 GeV/c  beam At ED ≈ 0: signal drops dramatically Landau fit Only the primary charge deposited in the region of ~150  above the first GEM is collected when the drift field polarity is reversed.

  22. Hadron Rejection Factor • Rejection factors of the order of 50 can be achieved with an amplitude cut of ~ 10 e. • Much higher rejection factors can be achieved by combining cuts on amplitude and hit size.

  23. Ar/CO2 CF4 Gain variations: ±10%, as in the lab Std. Conical, Segmented CERN Foils Active area of pads ~1.0x1.2cm2 The incorporation of a GEM detector in the inner region of PHENIX is quite feasible when considering how stable the GEMs’ performance was in such a high multiplicity environment. Triple-GEM detector in PHENIX IR 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 presence of the ambient magnetic field generated by the central magnet. RICH 2004, Playa del Carmen

  24. 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 ?)

  25. The HBD Detector HBD Gas Volume: Filled with CF4 Radiator (nCF4=1.000620, LRADIATOR = 50 cm) 5 cm 55 cm Space allocated for services Triple GEM detectors (8 panels per side) Beam Pipe Full scale prototype under construction Installation of final detector foreseen for RHIC run 6 in 2006 RICH 2004, Playa del Carmen

  26. PID at High pTMotivation :Jet Quenching RICH 2004, Playa del Carmen

  27. leading particle AA • In the colored medium, quarks • radiate energy (energy loss ~GeV/fm) • modify jet shape. q q leading particle Jets: A New Probe For High Density Matter • Jets from hard scattered quarks: - produced very early in the collision (τ <1fm/c) - expected to be significant at RHIC schematic view of jet production pp RICH 2004, Playa del Carmen

  28. p-p collision at √s = 200 GeV STAR PHENIX STAR RHIC events Au-Au central collision at √sNN = 200 GeV RICH 2004, Playa del Carmen

  29. leading particle AA • In the colored medium, quarks • radiate energy (energy loss ~GeV/fm) • modify jet shape. q q leading particle • Identify jet and its possible modifications through leading particles or correlations between the leading particles. leading particles correlations • Decrease their momentum  Suppression of high pT particles • “Jet Quenching” Jets: A New Probe For High Density Matter • Jets from hard scattered quarks: - produced very early in the collision (τ <1fm/c) - expected to be significant at RHIC schematic view of jet production pp • Not (yet) possible to observe jets directly in RHIC due to the large particle multiplicty. RICH 2004, Playa del Carmen

  30. 0-10% central Ncoll =975 ± 94.0 Central Au-Au collisions yield significantly suppressed relative to scaled pp yield p0 yield in AuAu vs. pp collisions 70-80% peripheral Ncoll =12.3 ± 4.0 Excellent agreement between measured π0’s in p-p and measured π0’s in Au-Au peripheralcollisions scaled by the number of collisions over ~ 5 decades

  31. Mesons are suppressed, baryons not. • Ф mesons are heavy, but follow 0, not p+pbar! • Indicates the absence of suppression of proton at intermediate pT is not a mass effect. RICH 2004, Playa del Carmen

  32. 118 cm 4 m Rφ Ｚ PHENIX PID extension: Aerogel (I): West Arm Panel PMT Integration Volume • 4.5 m from the vertex. • Coverage ; || < 0.35, 15o in . • Space available for increased coverage • Space available for new TOF (MRPC) Aerogel (11x22x11 cm3) PMT 10 x 16 Cells 3” Hamamatsu R6233

  33. 0 0 0 0 0 0 4 4 4 4 4 4 8 8 8 8 8 8 PHENIX PID Extension: Aerogel (II) Note:Aerogel together with TOF can extend the PID capability up to 10 GeV/c (without TOF, no K-proton separation at < 5 GeV/c) RICH 2004, Playa del Carmen

  34. PHENIX Aerogel: first results Emcal_TOF Emcal_TOF & Aerogel Veto Au+Au 200 GeV p_threshold of Aerogel: • pion : pth ~ 0.9 GeV/c • kaon : pth ~ 3.3 GeV/c • proton : pth ~ 6.2 GeV/c • Timing information: • Emcal Time-of-Flight RICH 2004, Playa del Carmen

  35. PHENIX Aerogel: first results work in progress • + with aerogel • x+ with TOF only • o0 RICH 2004, Playa del Carmen

  36. RICH for high momentum pid 2.3o – 15o Small solid angle (0.8 msr) forward spectrometer on rotating platforms 2.3o – 30o BRAHMS set-up RICH 2004, Playa del Carmen

  37. BRAHMS RICH Photon detector 26.5 x 21.2 cm2 Rsat = 9 cm 4 pixel PMT array (Hamamatsu R7600 03 M4F) C5F12 392 Torr C4F10 till 1.25 atm n-1 = 2030 x 10-6 RICH 2004, Playa del Carmen

  38. q q BRAHMS RICH: performance • Two magnetic field settings: • Full field setting p > 7 GeV/c • ¼ magnetic field setting Excellent , K, p separation up to ~25 GeV/c

  39. Covers ~5% of central barrel phase space • Extends identification of • /K to 3 GeV/c and K/p to 5 GeV/c ALICE HMPID RICH 2004, Playa del Carmen

  40. 10mm C6F14 radiator 5 mm 80 mm CH4 4 mm CsI photo- cathode MWPC ALICE HMPID (see talk of A. Gallas) • Proximity focusing RICH counters consisting of seven modules. • 15 mm thick liquid C6F14 radiator (n= 1.2988 at  = 175 nm) • 12 m2 of CsI photocathode deposited onto the pad cathode of a MWPC. ( talk of H. Hoedlmoser) proximity gap RICH 2004, Playa del Carmen

  41. ALICE HMPID prototype in STAR See talk of Nikolai Smirnov Prototype successfully tested and used in STAR

  42. Summary and Outlook • Cherenkov detectors, and RICH counters in particular, are crucial devices providing unique physics information in relativistic heavy-ion collisions. • RHI physics is witnessing a blossoming present with outstanding performance of RHIC machine and experiments and results still coming out of SPS • RHI physics has a promising future with FAIR, LHC and RHIC-upgrades in the horizon • Cherenkov detectors expected to continue playing crucial role in the field. RICH 2004, Playa del Carmen

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