rich in the phenix experiment at rhic hideki hamagaki center for nuclear study graduate school of...

RICH in the PHENIX Experimentat RHIC

Hideki HamagakiCenter for Nuclear Study

Graduate School of Science, University of Tokyo

2006/3/6 International CBM-RICH workshop at GSI

2RHIC

• Brookhaven National Laboratory, USA

• The first heavy-ion collider on the earth

• Two independent rings with circular length = 3.83 km

• Energy at CMS p+p 500 GeV Au+Au 200 A ・ GeV

• Luminosity– Au-Au: 2 x 1026 cm-2 s-1

– p-p : 2 x 1032 cm-2 s-1

-• Construction: 1991~1999 • Experimental runs since

2000

2006/3/6 International CBM-RICH workshop at GSI

3PHENIX Detector System

Central ArmsCoverage (E&W) -0.35< y < 0.35 30o <||< 120o

A complex apparatus to measure: Hadrons, Muons, Electrons, Photons

Muon ArmsCoverage (N&S) -1.2< |y| <2.3 - < <

2006/3/6 International CBM-RICH workshop at GSI

4Central Arms of the PHENIX experiment

• hadron, electron and photon measurement• detector components

– tracking chambers (DC + PC1,2,3 + TEC)– PID (TOF + RICH)– EM calorimeter (EMCAL)

2006/3/6 International CBM-RICH workshop at GSI

5Guiding Principles of PHENIX-RICH

• Primary purpose– electron identification under huge particle flux

environment

• Type of Cherenkov detector– gas radiator with relatively large index of refraction– ring imaging with a spherical mirror

• it measures incident direction of particles

• Small radiation length– low-Z radiator gas & materials

• Photon detectors should not face particle flux– protect them from the flux with the iron of central magnet

2006/3/6 International CBM-RICH workshop at GSI

6PHENIX-RICH overview

Acceptance coverage• || < 0.35 ; d = 90 degrees x

2

Three major components• Gas vessel

– C2H6 (th ~ 25) or CO2(th ~ 35)

– eID pt range : 0.2 ~ 4 GeV/c

• Gull-wing shaped spherical mirrors

• Photon detection by PMTs– 5,120 channels in 2 arms

Pixel size• 2-D angles (,) of electron

tracks– ~1 degree x ~1 degree

BNL, FSU, KEK, SUNY/SB, NIAS, ORNL, U.Tokyo, Waseda

Cerenkov photons are detected by array of PMTs

mirrorMost hadrons do not emit Cerenkov light

Electrons emit Cerenkov photons

Central Magnet

RICH

PMT arrayPMT array

2006/3/6 International CBM-RICH workshop at GSI

7RICH Specifications

• Vessel– Weight: 7250 kg/arm– Gas volume: 40 m3/arm– Radiator length: 0.9 - 1.5

m

• Mirror system – Radius : 403 cm– Surface area: 20 m2 / arm

• Photon detector– 2560 PMTs / arm

• Radiation length = 2.14%– Gas: 0.41 % (ethane)– Windows: 0.2%– Mirror panels: 0.53%– Mirror support: 1.0%

2006/3/6 International CBM-RICH workshop at GSI

8Gas Vessel

Dimensions• Width in Z

– 6000 mm

• Distance from top to bottom corner of vessel

– 5798.3 mm

• Entrance and Exit window– R(ent) = 2575 mm– R(exit) = 4100 mm– S(ent) = 8.9 m2

– S(exit) = 21.6 m2

– 125 m Kapton with supporting graphite-epoxy beams

– black vinyl coated polyester for light shield

2006/3/6 International CBM-RICH workshop at GSI

9RICH PMT

Hamamatsu H3171S• Cathode Diameter: 25 mm• Tube Diameter: 29 mm• Cathode: Bialkali• Gain: > 107

• Operation Voltage: - 1400 ~ -1800 V• Quantum efficiency: > 19% at 300 nm > 5 % at 200 nm• Rise Time: < 2.5n• Transit Time Spread: < 750ps

A Winstone cone shaped conical mirror• Entrance: 50 mm, Exit: 25 mm, Angular cut off = 30

Magnetic shielding case• FERROPERM (NKK); high susceptibility + large saturation

field

2006/3/6 International CBM-RICH workshop at GSI

10RICH PMT Arrays

• Supermodules– assembly of 32 PMTs

(2x16) – PMTs are grouped by its

gain• 8 tubes share the same HV

• Supermodules are installed in RICH vessel to form a tightly packed PMT array

– 40 super-modules (1280 PMTs) per one side of a RICH vessel

2006/3/6 International CBM-RICH workshop at GSI

11

Rohacell foam core (1.25 cm thick)

Gel-coat (0.05 mm thick)

4 ply graphite-epoxy (0.7 mm thick)

Mirror• Segmented spherical mirror

– R = 403 cm, L = 81.2 cm, W = 43.2 ~ 50.5 cm

– 48 panels/arm; 2 (side) x 2 (z) x 12 ()

• Structure of panel panels– substrates (by ARDCO)

• steel master• 1.25 cm Rohacell foam + 4 layers of graphite

epoxy (~0.7 mm) at each side• gel coat layer (0.05 mm)

– reflection surface (by OPTICON)• replication of aluminum surface using a glass

master (surface roughness rms ~ 2.5 nm)• epoxy thickness = ~150 m

• Performance– weight: 1.2 ~ 1.3 kg– reflectivity = 83% at 200 nm; 90% at 250

nm– R = 401.1 cm, with variation of 2.2 cm– surface roughness = ~+-1.5 mrad

2006/3/6 International CBM-RICH workshop at GSI

12Mirror Support Structure

• Frame bars– graphite fiber– 1 % of radiation length

(ave.)• Mirror panels are mounted by

adjustable 3 point mounts on the frame bars

2006/3/6 International CBM-RICH workshop at GSI

13Mirror Alignment

• After mirror installation, the RICH vessel is rotated up in the same orientation as on PHENIX carriage

• Positions of optical targets placed on mirror surface were surveyed with a computerized theodolite system (MANCAT).

2006/3/6 International CBM-RICH workshop at GSI

14Front End Electronics (FEE)

• Readout Signals from 5120 PMTs:– 0 ~ 10 Photon; max = 160 pC (with x10 Pre-Amp)– Time resolution = ~200 ps (for background rejection)

• Very fast operation– 9.6 MHz RHIC beam clock– Average Trigger Frequency 25 KHz

• Transfer to Data Collection Module (DCM) using G-LINK• Compactness -> 640 PMT signals per crate

2006/3/6 International CBM-RICH workshop at GSI

15Readout ModuleController Module

• Heap Management of AMU• Controls FEE synchronous to Master Timing System• Controls Burst Transfer• Generate AMU Write / TAC Stop Timing• Slow Serial Control using ARCNET

DSU/ALM

BTSK2

Readout FIFO

G-LinkTransmitter

ROC

PhaseShifter

• Transferring Data to PHENIX-DCM usingG-LINK at the maximum speed of 800 Mbps• G-LINK Transfer asynchronous to BUS Transfer inside RICH-FEE using four FIFOs (Depth: 9 events)

Analog Processing (AMU/ADC) Module

Inte

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TA

C (

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H)

Chi

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AM

U/A

DC

Chi

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AM

U/A

DC

Bur

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ontr

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• 64 Inputs, 64 Charge and TAC Outputs/Board• Trigger Sum: 16 Trigger Sum Outputs/Board (4 PMT Signal Sum)• Burst Transfer to Readout Module in 20-40MHz• Serial Controllable ASICs on Board

• 8 RICH Chips (Integrator+VGA+LED+CFD+TAC+Trigger Sum)• 8 Inputs, 8 CHARGE and TAC outputs/Chip, and 4 TriggerSum/Chip

• 4 AMU/ADC Chips (Analog Memory Unit and ADC); 32 Inputs/Chip

2006/3/6 International CBM-RICH workshop at GSI

16Number of Photo-electrons

• resolution for 1pe/MPH(1pe) ~ 0.26

• number of photo-electrons per ring ~ 10.8

– N0 ~ 120 (Npe = N0 sin2C)

2006/3/6 International CBM-RICH workshop at GSI

17Association of RICH Hits to Tracks

• reconstruction of rings is difficult– due to coarse segmentation of photon detectors

• association method– associate the photon hits to a charged track

2006/3/6 International CBM-RICH workshop at GSI

18Performance Limitation

• Association is not seen well in high multiplicity events

• Still works great to enrich electrons

2006/3/6 International CBM-RICH workshop at GSI

19Rejection Power of RICH (CO2 Gas)

• Data– Class defined by PC1 Hit:

• Peripheral: (PC1 Hit) < 150• Central: (PC1 Hit) > 400

– e+ e- and + - identified with TOF in the momentum range from 0.3 GeV to 0.4 GeV

– Number of PMTs are counted for hits with 3.4cm < r < 8.4cm

– Ring shape cut is applied– Errors are statistical only

• Simulation– additional shielding materials in

• effect of a factor of ~2

– the shielding materials are in after this run

Black: Simulation for Au+Au CentralBlue: Real data for Au+Au PeripheralRed: Real data for Au+Au Central

2006/3/6 International CBM-RICH workshop at GSI

20E/p Plot

• Pion rejection factor with RICH only

– ~100 for high multiplicity events

– 10000 for single pions

• With RICH + EMCAL– another factor of ...,

depending on the momentum

2006/3/6 International CBM-RICH workshop at GSI

21J/ in Au-Au collisions at RHIC

• In Run-4, 240 b-1 recorded with improved detector performance

– ~ 100 times more J/signals expected than in Run-2

• Outstanding J/ peak in e+e- invariant mass spectrum in 200 AGeV Au + Au central collisions

2006/3/6 International CBM-RICH workshop at GSI

22Some results on J/

• Nuclear modification factor of J/ yield for d + Au collisions

• for Au + Au and Cu + Cu collisions

|y|<0.350.2

0.4

0.6

0.8

1.0

1.2

RdAu

Rapidity

2006/3/6 International CBM-RICH workshop at GSI

23Thermal radiation and low-mass vector mesons

• Still missing at RHIC

• Experimentally, combinatorial background is very large and must be subtracted properly

• Large physics background comes from charm.

– Charm production is measured with ~15% accuracy by single electron measurements.

A prediction

R. Rapp, nucl-ex 0204003

2006/3/6 International CBM-RICH workshop at GSI

24Combinatorials is overwhelming

• Combinatorial background is determined with ~1% accuracy in Run3 and Run2 using a mixed event method

• Higher statistics from Run4 data should help, but it may not be enough for low-mass vector mesons

Real and Mixed e+e- Distribution

Run2 AuAuMinimum Bias

0

Real - Mixed with systematic errors

1 2 3 [GeV/c2] 0 1 2 3 [GeV/c2]

2006/3/6 International CBM-RICH workshop at GSI

25How to measure low-mass pairs

• Efforts are on-going to develop a Dalitz rejecter, based on HBD (hadron Blind Detector).

– UV photon detector• with CsI cathode• CF4 gas radiator• Ne(Cherenkov) > Ne(ionization)

S/B ~ 1/500

“combinatorial pairs”

total background

Irreducible charm backgroundall signal

charm signal

2006/3/6 International CBM-RICH workshop at GSI

26For Better Performance

• PHENIX-RICH– coarse segmentation of photon detector

• difficult to reconstruct Cherenkov rings• angular resolution is not great

-> false electron candidates from miss-association

• Recommendations– fine segmentation of photon detector– large number of photo-electrons preferablefor precise position determination

Experiment: PHENIX

detector part requirements solution advantages disadvantages

mirror •Lrad ~ 0.5%•precise R < 1/200•small surface roughness (< 3 nm)

•segmented structure

•Rohacell + graphite-epoxy

•steel master• replication

• light weight~1300 g per mirror segment

• fine adjustment is needed; once for all

photodetector •sensitive to ~200 nm

•reasonable segmentation

•adoption of PMT + light collection funnel

•high stability -> easy operation

•small concern to gas quality

•high cost: ~400$ per PMT

• low segmentation

radiator •> 10 PE for ~1 particles

• low scintillation

• CO2–original choice was C2H6, and its use is pended due to safety reason

• non-flammable• small Lrad

• small N(PE)• small ring radius

optical layout, adjustment, monitoring

• spherical mirror + photon detector

• gain monitor

• photon detector behind the magnet iron

• LED + fiber for gain monitoring

• low flux in the detectors