RICH 2004 Playa del Carmen, Mexico Nov. 30

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

• Itzhak Tserruya

2
The Challenge Huge multiplicities
3
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 for e-id in HI,
• making it possible to measure electron pairs.
• Identification of high momentum charged particles
  
• Timing (100ps time resolution).

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

5

• Electron pairs
• Best probes for chiral symmetry
• restoration and thermal radiation

6
Physics through dileptons

• Best probe of Chiral Symmetry Restoration

Chiral symmetry spontaneously broken in nature.
Quark condensate is non-zero lt qbarq gt
? 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-
7
CERES Unconventional Design
Original set-up
TMAE 2 PPAC MWPC Pad readout
Radiator gas CH4 (?th 28)
Si drift chambers
Carbon fiber mirror
CaF2 window

• 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 ? conversions

8
CERES RICH event
Pb Au ?sNN 17 GeV
9
Low-mass Dileptons Main CERES Result
Strong enhancement of low-mass ee- pairs in A-A
collisions (wrt to expected yield from known
sources)
Most updated CERES result (from 2000 Pb run)
Enhancement factor (0.2 ltm lt1.1 GeV/c2) 3.1 0.3
(stat)
10
PHENIX

• PHENIX was designed with emphasis on
  electromagnetic probes e, ?, ?
• PHENIX can measure electrons in the
• central region (DCPC 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
11
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
12
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 gt
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 lt 200 mrad (for
a 90 rejection).
Specifications Electron efficiency ? 90
Double hit recognition ? 90 Modest ?
rejection 100
Expect at least two orders of magnitude
improvement in S/B
13
HBD Concept

• HBD concept
• ? Windowless Cherenkov detector (L50cm)
• ? CF4 as radiator and detector gas
• ? CsI reflective photocathode
• ? Triple GEM with pad readout

14
HBD Concept

• HBD concept
• ? Windowless Cherenkov detector (L50cm)
• ? CF4 as radiator and detector gas
• ? CsI reflective photocathode
• ? Triple GEM with pad readout

• Bandwidth 6-11eV, N0 940cm-1 Npe 40!
• No photon feedback
• Detect blob, pad size blob size
• Low granularity, relatively low gain

15
HBD Concept

• HBD concept
• ? Windowless Cherenkov detector (L50cm)
• ? CF4 as radiator and detector gas
• ? CsI reflective photocathode
• ? Triple GEM with pad readout

16
RD 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
17
Gain Curve Triple GEM with CsI in CF4measured
with Fe55 and UV lamp

• Gains in excess of 104 are
• easily attainable.
• Gain increases by factor 3
• for ?V 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.

18
Unexpected Saturation effect
in CF4 measured with Am241
Deviation from exponential growth when Q 107
? ltQgt saturates at 4 x 107 below the Raether
limit of 108
19
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 lt 104
• i.e. with comfortable margin below
• the discharge threshold

20
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!
21
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
22
Hadron Blindness (I) Response to
Electronsdetector response vs ED at fixed gain
Efficient detection of photoelectrons even at
negative drift fields
23
Hadron Blindness (II) Response to Hadrons
Suppression of hadron signal at negative drift
field
24
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.
25
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.
26
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.

27
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.

28
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 ?)

29
The HBD Detector
HBD Gas Volume Filled with CF4 Radiator
(nCF41.000620, 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 Installati
on of final detector foreseen for RHIC run 6 in
2006
30

• PID at High pT Motivation Jet Quenching

31
Jets A New Probe For High Density Matter

• Jets from hard scattered quarks
• - produced very early in the collision (t
  lt1fm/c)
• - expected to be significant at RHIC

schematic view of jet production
pp
32
RHIC events
Au-Au central collision at vsNN 200 GeV
33
Jets A New Probe For High Density Matter

• Jets from hard scattered quarks
• - produced very early in the collision (t
  lt1fm/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.

34
p0 yield in AuAu vs. pp collisions
70-80 peripheral Ncoll 12.3 4.0
Excellent agreement between measured p0s in p-p
and measured p0s in Au-Au peripheral collisions
scaled by the number of collisions over 5
decades
35
Mesons are suppressed, baryons not.

• ? mesons are heavy, but follow ?0, not ppbar!
• Indicates the absence of suppression of proton at
  
• intermediate pT is not a mass effect.

36
PHENIX PID extension Aerogel (I) West Arm Panel
PMT
Integration Volume
Aerogel (11x22x11 cm3)
PMT
10 x 16 Cells
3 Hamamatsu R6233

• 4.5 m from the vertex.
• Coverage ? lt 0.35, 15o in ?.
• Space available for increased coverage
• Space available for new TOF (MRPC)

37
PHENIX PID Extension Aerogel (II)
Pion-Kaon separation Kaon-Protonseparation
TOF s100 ps 0 - 2.5 - 5
RICH n1.00044 gth34 5 - 17 17 -
Aerogel n1.011 gth6.8 1 - 5 5 - 9
Note Aerogel together with TOF can extend the
PID capability up to 10 GeV/c (without TOF, no
K-proton separation at lt 5 GeV/c)
38
PHENIX Aerogel first results
AuAu 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

39
PHENIX Aerogel first results
40
BRAHMS set-up
41
BRAHMS RICH
42
BRAHMS RICH performance

• Two magnetic field settings
• Full field setting p gt 7 GeV/c
• ¼ magnetic field setting

Excellent ?, K, p separation up to 25 GeV/c
43
ALICE HMPID

• Covers 5 of central barrel phase space
• Extends identification of
• ?/K to 3 GeV/c and K/p to 5 GeV/c

44
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
45
ALICE HMPID prototype in STAR
See talk of Nikolai Smirnov
Prototype successfully tested and used in STAR
46
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.