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Spin Physics with the PHENIX Silicon Vertex
Tracker
Junji TojoRIKEN
for the PHENIX Collaboration
Advanced Studies Institutes - Symmetries and SpinJuly 27 - August 3, 2005, Prague
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Silicon Vertex Tracker (VTX) Upgrade for PHENIX
• The PHENIX baseline detector was completed since RHIC-Run3 (2003).
• The detector upgrade program has been developed to enhance the physics capabilities.→ M. Grosse Perdekamp’s talk
• The VTX upgrade in the PHENIX Central Arm Spectrometers has been proposed for :– Precise measurement of heavy-quark (charm & bottom) production w/
displaced decay vertices,– Large acceptance tracking especially for jet production.
• The VTX has to be operated is in high luminosity and high multiplicity environment for polarized pp, p(d)A, and AA collisions in the future RHIC running.
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Spin Physics with the VTX Detector• Gluon polarization
meas. w/ longitudinal double-spin asymmetry ALL for – heavy-quark
production– photon-jet production
• Precise determination by extending x-rangein different channels beyond the baseline
( ) ( )
( ) ( )LLA
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Gluon Polarizationwith Heavy-Quark Production
• Dominated by gluon-gluon fusion :Clean process with wide xg-range
• Limitation in the baseline detector– Indirect meas. w/ inclusive electrons– Huge background from π0 Dalitz decay and photon conversion
• Precise vertexing w/ VTX allows direct meas. by detecting displaced decay vertices thr. distance of closest approach (DCA).
1 2
1 2
( ) ( )ˆ ( )
( ) ( )LL LL
g x g xA a gg QQ
g x g x
Parton level asymmetryfrom pQCD calculation
gg
Q
Q
m (GeV) cμmD0 1865 125D± 1869 317B0 5279 464B± 5279 496
D, B
e
Primary event vertex
Displaced vertex
DCA
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Gluon Polarizationwith Photon-Jet Production
• Dominated by gluon Compton process :Golden channel for precise measurement
• Limitation in the baseline detector– Recoil jet reconstruction is desirable for better parton kinematics constr
aint, but difficult due to limited acceptance.
• Large acceptance tracking w/ VTX makes it possible to determine recoil jet axis and thus improve xg determination.
1
( )ˆ( ) ( )
( )g p
LL q LLg
g xA A x a gq q
g x
Parton level asymmetryfrom pQCD calculation
g
γq
qMeasured inpol DIS
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• Specifications– 4 layers with large acceptance ( & < 1.2)– Displaced vertex measurement : < 40 m– Charged particle tracking : p/p ~ 5% p at high pT
– Working detector for both of heavy ion and pp collisions• Technology Choice
– Hybrid pixel detectors in 2 inner layers– Stripixel sensors w/ SVX4 readout chip in 2 outer layers
The VTX Detector
Pixel layersr=5.0 cm, Δz~±10 cmr=2.5 cm, Δz~±10 cm
Strip layersr=10.0 cm, Δz~±16 cmr=14.0 cm, Δz~±19 cm
Beam pipe
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The VTX Detector in PHENIX
VTXVTX
|η| < 1.2
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Hybrid Silicon Pixel Detector
Solder bump ~ 15 μm
• Hybrid pixel detector– Technology developed by CERN/ALICE– Bump-bonding btw R/O chip
and sensor• R/O chip : ALICE1/LHCB chip
– 0.25μm process, rad hard ~ 30Mrad– Matrix of 32 x 256=8,192 cells– Pixel size 50 x 425 m2
– Active area 12.8 x 13.6 mm2
– 150μm thickness– Operation at 10 MHz– Power consumption ~ 1 W/chip
• Pixel sensor– p+/n/n+ structure– Same pixel size as R/O chip– Bump-bonded to 4 R/O chip– 200μm thickness
SensorR/O Chip
Pixel sensor
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Hybrid Silicon Pixel Detector• Half-module
– 2 bump-bonded ladder– High density Al/Kapton bus
• 70μm pitch & 5 layers• R&D on-going
– 1 pilot module : R/O & control• 2 half-modules makes 1 ladder
Bus
2 bump-bonded ladder2 bump-bonded ladder
Bus
Support+cooling
• Production of bump-bonded ladder and ladder test w/ 90Sr are on-going
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Silicon Strip Detector• Strip sensor
– BNL’s new “stripixel” concept : single-sided sensor w/ 2-D position sensitivity
– Charge sharing by 2 spirals in one pixel (80 μm ×1000 μm)& projective x/u-strip readout
– Pre-production sensor (Hamamatsu)
• p+/n/n+ structure• 3.5×6.4 cm2
• 625/500 μm thickness• Spiral : 5/3μm line/gap• Pixels : 384×30×2=23,040• Strips : 384×2×2=1,536
– Probing tests are on-going for evaluation
Z. Li, NIMA518, 738 (2004)
x3’ u1’ u2’
x2’ x1’ u1’u2 u3 x3
u1 x1 x2
0.00E+00
1.00E-12
2.00E-12
3.00E-12
4.00E-12
5.00E-12
6.00E-12
7.00E-12
8.00E-12
9.00E-12
1.00E-11
0 50 100 150 200 250 300 350 400
V_bias [V]
C [
F]
0.00E+00
5.00E-10
1.00E-09
1.50E-09
2.00E-09
2.50E-09
3.00E-09
3.50E-09
0 50 100 150 200 250 300 350 400
V_bias [V]
I [A
]CVIV
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• Readout chip– FNAL/LBNL’s SVX4 chip
• 0.25μTSMC process, radiation hard > 20 Mrad
• 128ch pipelined mixed signal chip• 128ch parallel 8-bit ADC• “Dead timeless operation”• Multi-event buffering (4 events)• Selectable on-chip 0-suppression• Power consumption ~ 0.4W/chip
– 12 SVX4s and control chip(RCC) per sensor
• R&D is on-going– SVX4 3-chip hybrids– Readout control board for 4 hybrids– Sensor+hybrid test will start soon.
Silicon Strip DetectorReadout Control Board
Hybrid
Hybrid
Hybrid Hybrid
Hybrid
Sensor
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Detector Integrationin the VTX Region
±40 cm
±34 cm
42 cm
Magnet
Calorim
eter
Magnet
Calorim
eter
VTX Endcapextension
VTX Endcapextension
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Expected Performance :Occupancy
• Layer-by-layer occupancy calculation– Maximum occupancy estimated from simulated central
Au-Au collisions at 200 GeV– Au-Au collisions generated with HIJING event generato
r– The VTX Detector implemented in a GEANT-based PHE
NIX detector simulation (PISA)
The VTX Detector will work for both heavy ionand polarized pp collisions.
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Expected Performance :Heavy-Quark Detection w/ Displaced Vertices
• Displaced vertex meas. using the distance of closest approach (DCA)
• DCA resolution of 36μm simulated w/ two inner-most pixel layers & 2 GeV/c pions : consistent w/ rφ pitch (50 μm) and detector geometry
DCA (m)
200μmDCA cut
• Electron DCA distribution from- π0 Dalitz decay- Charm semi-leptonic decay- Bottom semi-leptonic decay
Charm S/N improvementw/ VTX
Bottom S/N improvementw/ VTX
Significant S/N improvement
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Expected Performance :Jet axis Determination w/ Large Acc. Tracking
• Reconstruction of recoil jet axis (η jet, φjet ) in direct photon event• Simple cone algorism : pT-weighted ave. of charged tracks’ (ηi ,
φi) w/ > 1 GeV/c within VTX & corn radius < 0.5
Photon pT = 6.0-7.0 GeV/c•η-distribution of - Recoil quark- Recoil quark within VTX -η(recons. jet)-η(quark)
Strong constraint of jet axis
No jet info. w/ jet axis reconst.
Improved parton kinematics determination
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Summary
• Silicon vertex tracker (VTX) upgrade was proposed to enhance the physics capability of the PHENIX detector.
• Intensive R&Ds and production are on going.• Expected performance showed significant improv
ement especially for spin physics capabilities w/ heavy-quark & photon+jet production.
• Installation and operation of the VTX is planned in RHIC-Run8 (2007/2008).