Gerrit J. van Nieuwenhuizen

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Experience Upgrade
Gerrit J. van Nieuwenhuizen For the PHOBOS
collaboration
RHIC future detectors RD Workshop Brookhaven
National Laboratory November 13-14, 2001
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PHOBOS Detector Overview
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Silicon Sensor Tasks

• Measuring charged particle multiplicity with
  Octagon/Rings
• Determining the interaction point with Vertex
  Detector
• Do 3d-tracking with Spectrometer Arms
• Use dE/dx for particle identification

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Silicon Pad Technology
Double Metal, Single sided, AC coupled,
polysilicon biased detectors produced by ERSO,
Taiwan
bias bus
signal lines
1.2um ONO
vias
0.2um ONO
p Implant
300um 5kOhm nSi
Polysilicon Drain Resistor

n
AC coupled Pad (p-implant metal 1
pad) polisilicon bias resistor metal 2 readout
line contact hole metal 1- metal 2
Conservative design, but 9 types
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Spectrometer
21 sensors x 256 ch.
66 sensors x 256 ch
18 sensors x 500 ch.
28 sensors x 512 ch.
8 sensors x 1536 ch.
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Multiplicity detector
Octagon Vertex
Ring counter 6
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Events in 2001 and 2000
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First RHIC Results in 2000 and 2001
Charged Particle Multiplicity at MidRapidity
3 weeks after first 2000 collisions
PRL 85 3100 (2000)
nucl-ex/0108009
10 days after first 2001 collisions
PRL in press
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Signal Response Stability
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Signal to Noise Ratios
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Tracking
dE/dx resolution 6.9
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Latchup Protection Circuitry

• Trying to protect VA chips by quickly switching
  off strings of chips when current gradients too
  high
• With current RHIC running this happens every 1-2
  hours
• Highly dependent on beam quality
• If not too many strings are affected then a quick
  reset is possible, otherwise the silicon system
  has to be switched off/on
• Not a single chip lost until....

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Dead Channels after Beam Blast
Just after the beam blasted the detector
After readjusting the VA chip operating parameters
RingP
Octagon
RingN
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Broken Calibration and Dead Channels
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Damage report

• Gain Calibrations is broken for many channels
• Most of the channels exhibit same mip peak after
  applying the old gain on beam data after blast
• Amount of dead channels went from 1 to 2 (2800
  out of 137000)
• Most likely huge hit signals doing damage, not
  the integrated radiation dose
• We survived the first blast and the second and
  the third and.....

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Performance Conclusions

• Well understood signal response of detector (down
  to few level) leads to quick and reliable
  physics results
• Not zero suppressing was crucial for offline
  analysis of detector response
• Very stable system until the beam gets dumped in
  it (1 extra dead)
• Low maintenance system, turn-key operation
• Happy Silicon Happy PHysicists

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Charm Upgrade Motivation

• Charm cross-section in AuAu_at_ RHIC uncertain
• Hidden charm is major part of RHIC program
• Total charm production essential normalization
• Hidden charm/open charm is O(0.01)
• Gap in RHIC experimental program
• no true micro-vertexing so far (3-inch beampipe)

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Charm facts

• dN/dy (cc) in central AuAu _at_ RHIC approx. 1
  (PRL 81(1998) 1801)
• Decay length
• 0.13mm for D0 (m1.86 GeV)
• 0.32mm for D/- (m1.87GeV)
• Channels (BR O(5))
• D -gt Kpi
• Full kinematic reconstruction of D mesons
• Needs large acceptance
• D -gt (di-)muon(s) X
• D -gt electron X
• Best suited for small acceptance near mid-rapidity

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Statistics (resolution, hadron rejection)
1 RHIC year, PHOBOS acc.

• Assume gt103 pion rejection at 90 e/- efficiency
• Assume lt20 micron point resolution in y,z
• O(104) electrons from charm per RHIC year (after
  IDacceptance cuts)

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uVertex module

• Plenty of space for
• Hybrids
• Mechanics
• Cooling

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uVertex option I

• Use the existing PHOBOS Silicon Pad technology
• Advantages
• Well known sensor technology
• Identical hybrids/chips(rad.tol.)
• Identical readout system
• Disadvantages
• Granularity limited by chip density
• 50um x 6.5mm strips
• 2 back-to-back sensors needed to get sufficient
  spatial resolution
• 2 different sensor types

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uVertex option II

• Use the ALICE Pixel Detectors
• Advantages
• 50um x 500um pixels
• 1 sensor gives 1 spacepoint
• Well established design
• Disadvantages
• Different sensor technology
• Assembly different from current design
• 'Out of House' production
• Different readout system

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The PHOBOS Advantage

• Complementary to charm capabilities of other
  experiments
• Timely and cost-effective implementation
  (2003-2004)
• Smooth transition from existing PHOBOS apparatus
  and physics program
• Leverage existing facilities and detectors
  within extended collaboration