**A Heavy Flavor Tracker for STAR**

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400670.doc

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**A Heavy Flavor Tracker\
for STAR**

Z. Xu

Brookhaven National Laboratory, Upton, New York 11973

Y. Chen, S. Kleinfelder, A. Koohi, S. Li

University of California, Irvine, California

H. Huang, A. Tai

University of California, Los Angeles, California 90095

V. Kushpil, M. Sumbera

Nuclear Physics Institute AS CR, 250 68 Rez/Prague, Czech Republic

C. Colledani, W. Dulinski, A. Himmi, C. Hu, A. Shabetai, M. Szelezniak,
I. Valin, M. Winter

Institut de Recherches Subatomique, Strasbourg, France

B. Surrow, G. Van Nieuwenhuizen

Massachusetts Institute of Technology, Cambridge, MA 02139

L. Greiner, Y. Lu, H.S. Matis, M. Oldenburg, H.G. Ritter, F. Retiere, A.
Rose, L. Ruan, K. Schweda, E. Sichtermann, J.H. Thomas, H. Wieman, N.
Xu, Y. Zhang

Lawrence Berkeley National Laboratory, Berkeley, California 94720

I. Kotov

Ohio State University, Columbus, Ohio 43210

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Contents

[1 Executive Summary 15](#executive-summary)

[1.1 Scientific Motivation 15](#scientific-motivation)

[1.2 Detector Concept 16](#detector-concept)

[1.3 R&D for an Advanced Detector 17](#rd-for-an-advanced-detector)

[1.4 Coordination with Other New STAR Detectors
17](#coordination-with-other-new-star-detectors)

[2 The Physics of the HFT 18](#the-physics-of-the-hft)

[2.1 Introduction 18](#introduction)

[2.2 Initial Heavy Quark Production 20](#initial-heavy-quark-production)

[2.3 The Need for Direct Topological Reconstruction of Open Charm
23](#the-need-for-direct-topological-reconstruction-of-open-charm)

[2.4 Probing Medium Thermalization: Charm Quark Re-interactions
24](#probing-medium-thermalization-charm-quark-re-interactions)

[2.4.1 Elliptic Flow 26](#elliptic-flow)

[2.4.2 Charm Hadro-Chemistry 28](#charm-hadro-chemistry)

[2.5 Probing the Density of the Medium: Heavy Quark Energy Loss
30](#probing-the-density-of-the-medium-heavy-quark-energy-loss)

[2.5.1 R~AA~ and Energy Loss 30](#raa-and-energy-loss)

[2.5.2 Charm Angular Correlations 32](#charm-angular-correlations)

[2.5.3 Baryon -- Meson Ratios 34](#baryon-meson-ratios)

[2.6 Measuring Vector Mesons with Dileptons
35](#measuring-vector-mesons-with-dileptons)

[2.7 Spin Measurements 39](#spin-measurements)

[3 HFT Overview and Simulation 40](#hft-overview-and-simulation)

[3.1 STAR and the HFT detector 40](#star-and-the-hft-detector)

[3.2 The Design of the HFT 42](#the-design-of-the-hft)

[3.3 Monte Carlo Simulations 43](#monte-carlo-simulations)

[3.3.1 Physics Simulation 44](#physics-simulation)

[3.3.2 Reconstruction 45](#reconstruction)

[3.3.3 Primary Track Reconstruction Performance
49](#primary-track-reconstruction-performance)

[3.3.4 Analytic Resolution Calculations and Expected Performance
51](#analytic-resolution-calculations-and-expected-performance)

[3.4 Open Charm Reconstruction Simulation
52](#open-charm-reconstruction-simulation)

[3.4.1 D^0^ reconstruction 54](#d0-reconstruction)

[3.4.2 D^+^ reconstruction 58](#d-reconstruction)

[3.5 Charm Elliptic Flow 60](#charm-elliptic-flow)

[3.6 Reconstruction of the Λ~c~ Baryon
63](#reconstruction-of-the-λc-baryon)

[3.7 Hit Density at the Front Surface of the HFT
65](#hit-density-at-the-front-surface-of-the-hft)

[3.7.1 Measured Hit Density at 6 cm Radius in STAR
66](#measured-hit-density-at-6-cm-radius-in-star)

[4 CMOS Sensors 67](#cmos-sensors)

[4.1 Introduction 67](#introduction-1)

[4.2 Main Features and Performance of CMOS Active Pixel Sensors
70](#main-features-and-performance-of-cmos-active-pixel-sensors)

[4.3 Thinning 72](#thinning)

[4.4 Additional R&D 72](#additional-rd)

[4.5 MIMOSTAR Sensor Design 75](#mimostar-sensor-design)

[4.6 The path to a CMOS detector for the HFT
76](#the-path-to-a-cmos-detector-for-the-hft)

[4.6.1 MIMOSTAR-3 -- a half sized chip
77](#mimostar-3-a-half-sized-chip)

[4.6.2 MIMOSTAR-4 -- a full sized chip
77](#mimostar-4-a-full-sized-chip)

[4.6.3 The ultimate sensor 78](#the-ultimate-sensor)

[5 Data Acquisition and Readout 80](#data-acquisition-and-readout)

[5.1 Prototype and Final Design 80](#prototype-and-final-design)

[5.2 Requirements and Prototype Design
80](#requirements-and-prototype-design)

[5.3 Architecture for the Prototype System
81](#architecture-for-the-prototype-system)

[5.4 Data Synchronization, Readout and Latency
85](#data-synchronization-readout-and-latency)

[5.5 Data Rates for the Prototype RDO
87](#data-rates-for-the-prototype-rdo)

[5.6 Requirements for the Ultimate Design
87](#requirements-for-the-ultimate-design)

[5.7 Architecture for the Ultimate System
88](#architecture-for-the-ultimate-system)

[5.8 Data Rates for the Ultimate RDO 89](#__RefHeading___Toc137569127)

[5.9 Prototypes 90](#prototypes)

[6 Integration with STAR 93](#integration-with-star)

[6.1 Mechanical Introduction 93](#mechanical-introduction)

[6.2 Support Carriage for Rapid Installation and Removal
93](#support-carriage-for-rapid-installation-and-removal)

[6.2.1 Position Alignment and Calibration
97](#position-alignment-and-calibration)

[6.3 Ladder Design and Fabrication 98](#ladder-design-and-fabrication)

[6.3.1 Detector Radiation Length 99](#detector-radiation-length)

[6.3.2 Expected Radiation Exposure 99](#expected-radiation-exposure)

[6.4 Ladder Mechanical Tests 100](#ladder-mechanical-tests)

[6.4.1 Load Distortion Tests 100](#load-distortion-tests)

[6.4.2 Thermal Distortion Tests 102](#thermal-distortion-tests)

[6.4.3 Cooling Measurements 102](#cooling-measurements)

[6.5 Cabling 103](#cabling)

[6.6 Beam Pipe 105](#beam-pipe)

[6.6.1 Minimum Beam Pipe Radius Consistent with Injected Beam
106](#minimum-beam-pipe-radius-consistent-with-injected-beam)

[6.6.2 Beam Pipe Radius - Vacuum Considerations
106](#beam-pipe-radius---vacuum-considerations)

[6.6.3 Supporting Section 107](#supporting-section)

[6.6.4 Central Beam Pipe Thin Window Section
108](#central-beam-pipe-thin-window-section)

[6.6.5 RF Background from the Wake Field
108](#rf-background-from-the-wake-field)

[6.6.6 Beam Pipe Insertion and Removal
108](#beam-pipe-insertion-and-removal)

[6.6.7 Bake Out 108](#bake-out)

[6.7 Compatibility with the SSD and other cone mounted detectors
109](#compatibility-with-the-ssd-and-other-cone-mounted-detectors)

[6.8 Compatibility with an Upgraded Inner Si Tracker Barrel
109](#compatibility-with-an-upgraded-inner-si-tracker-barrel)

[7 Preliminary Cost and Schedule Estimates
110](#preliminary-cost-and-schedule-estimates)

[7.1 Cost and Schedule 110](#cost-and-schedule)

[8 Summary 116](#summary)

[9 Appendix I -- MIMOSTAR II Specifications
117](#appendix-i-mimostar-ii-specifications)

[10 Appendix II -- MIMOSTAR II Users Guide
118](#appendix-ii-mimostar-ii-users-guide)

[11 Appendix III -- HFT Prototype Read Out
119](#appendix-iii-hft-prototype-read-out)

[12 Appendix IV -- Production Schedule
127](#appendix-iv-production-schedule)

[13 Appendix V -- Cost and Schedule Presentation
128](#appendix-v-cost-and-schedule-presentation)

[14 Appendix VI -- Report of the Technical Committee
129](#appendix-vi-report-of-the-technical-committee)

[15 References 133](#references)

List of Figures

[Figure 1: Masses of the six quark flavors. The current and additional
QCD masses are shown by blue- and yellow-boxes, respectively. The figure
is adapted from Ref. \[12\]. 20](#__RefHeading___Toc137569161)

[Figure 2: Contributions to charm production at RHIC energies include
the initial gluon fusion and *qq* annihilation phase (solid line), a
pre-thermal phase characterized by scattering between free-streaming
partons, and a thermal phase that assumes parton equilibration. The
distributions were calculated with an energy density of 3.2 GeV/fm^3^ at
the moment of thermal equilibration. As a reference, the faint dotted
line is the thermal production of charm due to an initial fully
equilibrated QGP. The figure is from Ref. \[15\].
22](#__RefHeading___Toc137569162)

[Figure 3: Total cc production cross-sections per nucleon-nucleon
collision vs. collision energy. The dashed line depicts a PYTHIA
calculation. The dot-dashed line depicts a NLO pQCD calculation with
MRST HO, m~c~ = 1.2 GeV/c^2^, µ~F~ = 2m~c~, µ~R~ = 2m~c~. The figure is
adapted from Ref. \[17, 19\]. 22](#__RefHeading___Toc137569163)

[Figure 4: Solid- and dashed-curves represent the charm- (red) and
beauty-hadron (blue) spectra from Blast-Wave and Pythia model
calculations, respectively. The corresponding heavy flavor decayed
electron spectra are shown as black curves. The data are the measured
single electron distributions measured in 10% central Au + Au collisions
at 130 GeV by the PHENIX collaboration. The figure is adapted from Ref.
\[20\]. 23](#__RefHeading___Toc137569164)

[Figure 5: Nuclear modification factor R~AA~ of D-mesons assuming a
hydro-dynamically inspired parameterization assuming a collective flow
velocity of \<β~r~\> = 0.4*c* and 0.6c for D-mesons. The corresponding
electron decay-spectra are shown by the blue dashed (1) and (2) lines.
24](#__RefHeading___Toc137569165)

[Figure 6: Elliptic flow of strange hadrons at RHIC as measured by the
STAR detector. The top panel demonstrates typical hydro-dynamical mass
ordering up to a momentum p~T~ \< 2GeV/c and saturation at larger
momentum. The bottom panel shows the scaling of elliptic flow with the
number n of constituent quarks in the saturation region (baryons, n=3;
mesons, n= 2). 27](#_Ref88041181)

[Figure 7: Non-photonic electron elliptic flow at RHIC. Open and closed
symbols are data of non-photonic electron v~2~ from PHENIX^35^
Statistical errors are shown as solid lines and the open boxes indicate
the size of the systematic errors. The solid-line represents the results
from Greco et al. where, in this coalescence calculation, similar v~2~
distributions for c- and u-quarks are assumed. 28](#_Ref88041446)

[Figure 8: The ratio of suppression factors in hot matter for charm (H)
and light (L) quarks. The solid line represents results from
calculations with unrestricted gluon radiation, while the dashed line is
based on calculations with a cut on gluon energies ω \> 0.5 GeV. The
size of the static medium traversed by the fast quark is assumed to be 5
fm. The figure is from Ref. \[49\]. 31](#__RefHeading___Toc137569168)

[Figure 9: Non-photonic electron R~AA~ in central Au + Au collisions
from STAR (open-symbols, top 5%) and PHENIX (filled-symbols, top 10%)
from Refs. \[56,57,58\]. Theoretical predictions for charmed-hadron and
charm and beauty-hadron decayed electrons are shown as blue- and
yellow-bands, respectively. In these calculations, the initial gluon
density was assumed to be unrealistically large: \~3500, see Ref \[61\]
. 32](#__RefHeading___Toc137569169)

[Figure 10: D-meson correlation functions for 200 GeV p + p collisions.
Default parameters in the Pythia model were used in these calculations.
A clear back-to-back correlation in the angular distribution of charmed
mesons is observed (shown by the open circles). The Solid-line and the
diamonds represent the results with angular smearing for σ~φ~ = π/4
and π/2, respectively. 33](#__RefHeading___Toc137569170)

[Figure 11: Left top: TPC dE/dx vs. p in d + Au collisions. Left bottom:
TPC dE/dx vs. p after TOFr PID selection of \| 1-β \| \< 0.03. Clean
electron identification is achieved. Right: dE/dx from the TPC after
TOFr PID selection for 1.0 \< p \< 1.5 GeV/c.
36](#__RefHeading___Toc137569171)

[Figure 12: Electron p~T~ spectra from γ conversions reconstructed by
requiring TPC tracking alone, a tack in the TPC and 2 HFT hits. The
rejection factor is about 20. 37](#_Ref86316844)

[Figure 13: Di-electron invariant mass spectrum. The solid black curve
is a prediction from R. Ralf Ref. \[74\] using the full STAR acceptance.
The red curve at the top is the total di-electron invariant mass
spectrum seen with the 2004 configuration of STAR but assuming electron
PID from Full TOF coverage. The gray curve is the charm e+e- pair
invariant mass distribution after a cut on the HFT DCA \< 80um. The dot
dashed line is the Dalitz decays from π0 and η after rejection from TPC.
38](#_Ref118890711)

[Figure 14: The STAR detector at RHIC. The primary detector elements at
mid-rapidity are shown in this cut-away view of the detector. For scale,
the TPC is 4.2 meters long and 4 meters in diameter. Heavy Ion beams
enter from the left and right while collisions take place in the center
of the detector. 40](#__RefHeading___Toc137569174)

[Figure 15: Left - The HFT is shown embedded inside the SSD. Right -
Schematic view of the HFT detector ladder arrangement with 6 ladders
forming the inner layer and 18 ladders forming the outer layer. The
width of the ladders (the dimension seen in this view) is 19 mm. The
shingled layers provide cross calibration of the ladders and provide an
open geometry for ease of installation. 43](#_Ref124519579)

[Figure 16: Simulation block flow diagram. Only small modifications were
needed to optimize the STAR reconstruction software for use with the
HFT. 46](#_Ref124506772)

[Figure 17: DCA of tracks to the Primary vertex. The geometric signed
distance of closest approach between all tracks (p~T~ \> 200MeV/c) and
the vertex is shown for two cases: a) DCA to the vertex defined by
TPC+SSD tracks. The non-zero mean of this distribution is used to
correct the vertex coordinate. b)  DCA distribution after iterative
correction using TPC+SSD+HFT tracks, with a significant improvement in
the mean and shape of the distribution.
47](#__RefHeading___Toc137569177)

[Figure 18: Mean and sigma of the geometric -- signed DCA distribution
as a function of the number of iterations used to correct the vertex.
With each iteration, the previously defined vertex is corrected with the
mean of the distribution of the track dca's. The first iteration uses
the TPC+SSD defined vertex as a vertex seed.
47](#__RefHeading___Toc137569178)

[Figure 19: Vertex resolution σ(MC Vertex Position -- Reconstructed
Vertex position) as a function of collision centrality. The HFT refit
vertex shows an order of magnitude improvement over the previously
reconstructed primary vertex. a) Vertex resolution in X vs. N~ch~ . b)
Vertex resolution along the beam direction. Problems in the vertex fit
in the z direction drive the distribution wider than in the transverse
direction. The highlighted area demarcates the centrality region with
minimum (or better) acceptable TPC+SSD vertex resolution. This
corresponds to 0%-60% most central events.
48](#__RefHeading___Toc137569179)

[Figure 20: The efficiency for finding tracks in central Au + Au
collisions in the STAR TPC, SSD, and the HFT. Accepted tracks have 15
TPC hits (or more), 1 SSD hit, and 2 HFT hits that match to a single
Monte Carlo track. The quoted efficiency is for pions in the detectors
acceptance; \|η\| \< 0.5 49](#_Ref124524795)

[Figure 21: Reconstructed tracks with false HFT hits. Left -- The number
of ghost tracks and reconstructed tracks passing through the pixels of
the HFT as a function of p~T~. Right - The ratio of ghost tracks to good
tracks. The increased ghosting rate at lower transverse momentum is a
direct consequence of multiple Coulomb scattering, and is the motivating
factor to make the detector as thin as possible.
50](#__RefHeading___Toc137569181)

[Figure 22: Efficiency (a) and ghost rate (b) of the HFT, using an
analytic calculations. The luminosity dependence is given in (c) and (d)
for 1-4 times the current peak luminosity. Finite and infinite search
windows are represented by solid- and dashed- line, respectively. The
efficiency and ghost rate for each layer of the HFT are calculated
separately and combined into that for full HFT. The vertex, SSD, HFT,
and TPC search radii are assumed to be 300 um, 60 um, 90 um, and 1.5 mm,
respectively. At nominal RHIC luminosity 1×10^27^/cm^2^s, the density of
background hits (hits/cm^2^) are summarized in Table 10. It will be
necessary to upgrade the sensors for the full RHICII luminosities.
52](#__RefHeading___Toc137569182)

[Figure 23: The distance of closest approach (dca) distributions for
D^0^ (triangles) and D^+^ (circles). The results of the dca distribution
for primary tracks are shown in squares.
54](#__RefHeading___Toc137569183)

[Figure 24: The decay topology for a D^0^ decaying to a kaon and a pion.
Isolation cuts to identify the D^0^ from the background tracks are
described in the text. 55](#__RefHeading___Toc137569184)

[Figure 25: Distribution of quantities used to distinguish signal from
background. Panels a1, a2, b1, and b2 compare the difference between
background events and D0 mesons for different variables. The yellow
shaded area shows the regions where we places cuts to enrich the D0 to
background ratio. The lower panels, c1, c2, and c2, show the pt
distribution for signal and background event tracks with a 100μm error
vertex (c1), 20μm error vertex (c2), and perfect vertex (c3). Central
Au + Au collisions are assumed for the background primary tracks.
56](#__RefHeading___Toc137569185)

[Figure 26: D^0^ invariant mass with different cut sets. The power of
the cuts to enhance the signal relative to the background is clear.
57](#__RefHeading___Toc137569186)

[Figure 27: The absolute yield of D^0^ into the TPC + SSD + HFT divided
by the input D^0^ yield. For comparison, a simulation with thick Si
detectors that are similar to the wafers used in the ALICE vertex
detector is also shown (1000 μm effective silicon thickness).
57](#_Ref535399427)

[Figure 28: The D^+^ decayed tracks decay-length, dca, and cos(θ)
distributions (triangles). The same distributions for primary tracks are
also shown (squares). 59](#__RefHeading___Toc137569188)

[Figure 29: Invariant mass distributions for D^+^ for several p~T~
windows. The lines are a polynomial (up to 2^nd^ order) + Gaussian fit.
59](#__RefHeading___Toc137569189)

[Figure 30: D-meson efficiencies as a function of transverse momentum,
together with fits. to the distributions. The D^0^ efficiency approaches
10%, while the D^+^ efficiency approaches .3%. 60](#_Ref535343174)

[Figure 31: D-meson rates estimates: (a) dN/dp~T~ distributions for
D-mesons. The integrated yield dN/dy = 0.03 as measured in p + p
collisions at 200 GeV \[Ref. 17\]. The number of binary collisions
(N~bin~ ) = 950, corresponds to the top 10% most central Au + Au
collisions at RHIC and is used to scale the Au + Au collisions; (b) 3-σ
significance D^0^ efficiency with TPC+SSD+HFT. The dashed-line is the
fit to the simulation results; (c) D^0^ meson rates from p + p and top
10% central Au + Au collisions at 200 GeV. 61](#_Ref124740173)

[Figure 32: D-meson v~2~ rate estimates: (a) dN/dp~T~ distributions for
D-meson. The integrated yield dN/dy = 0.03 as measured in p + p
collisions at 200 GeV \[Ref. 17\]. The number of binary collisions
N~bin~ = 290, corresponding to the minimum bias Au + Au collisions, is
used to scale the Au + Au collisions. A power-law shape from p + p
collisions is assumed; (b) Assumed v~2~ distributions for D-mesons. The
solid-line is the results of fit to the measured Kaon v~2~ \[Ref. 23\].
Both circle- and diamond-symbols are from \[Ref. 46\] for the case with
and without charm quark flow, respectively. Error bars shown are from
15% systematic errors; (c) 3-σ significance D^0^ efficiency with
TPC+SSD+HFT. The Dashed-line is the fit to the simulated result; (d)
D^0^ meson v~2~ rates from minimum bias Au + Au collisions at 200 GeV.
The small and large error bars are for 15% and 30% systematic errors,
respectively. For the v~2~ analysis, 12 bins in ϕ are used.
63](#_Ref124741045)

[Figure 33: Λ~c~ decay length, dca, and invariant mass distributions.
64](#__RefHeading___Toc137569193)

[Figure 34: Epitaxial Silicon used as a sensor. In this design, a
primary ionizing particle creates free charges in the epitaxialy grown
Si layer that is a few tens of microns thick and a few electrons in the
bulk layers. The liberated charges are then free to diffuse towards a
potential well structure at the top of the sensor where they are
extracted and readout into a DAQ system.
70](#__RefHeading___Toc137569194)

[Figure 35: Wafer of reticle size sensors (left) and zoomed-in view of
individual chips (right). 71](#__RefHeading___Toc137569195)

[Figure 36: An APS Sensors developed by the LBNL/UIC group. The picture
shows 16 separate test structures. Each structure has a 36 × 36 array of
20 µm pixels. 73](#__RefHeading___Toc137569196)

[Figure 37: This graph shows the results of one event taken with 1.5 GeV
electrons. Each bin represents one pixel and the height is proportional
to the measured charge. Several electron hitselectrons can be identified
in the plot. 73](#__RefHeading___Toc137569197)

[Figure 38: Ladder Layout - sketch of the readout-topology on a detector
ladder. This figure shows the ten APS and the corresponding current to
voltage conversion and driver electronics. The drivers will be located
out of the low mass region of the detector and may require additional
cooling. 82](#_Ref121896626)

[Figure 39: Prototype DAQ Layout: schematic of DAQ system for a single
MIMOSTAR4 ladder. Analog data is carried as differential current on the
low mass cable at 50 MHz. The signals are driven in parallel over short
(\~1m) twisted pair cables to the motherboard. Analog to digital
conversion, CDS and data reduction are performed in the Motherboard /
Daughter cards. The reduced hit data is transferred digitally to the SIU
and carried to Linux based readout PCs via an optical fiber. Control,
synchronization, and event ID tagging are accomplished in the
Synch/Trigger FPGA on the motherboard. 83](#_Ref136929565)

[Figure 40: A simple cluster finding algorithm for the HFT detector. ADC
data from two MIMOSTAR detector columns + 3 pixels are sent to a
high/low threshold discriminator. The resulting 2 bits are fed
sequentially in an 2-bit wide shift register. The center pixel of a 3 ×
3 pixel window is compared to a high threshold with each clock tick. If
the threshold is exceeded, the additional cluster identification
criteria are checked for the 3 x 3 pixel window. If the results meet the
critera for a cluster, the center pixel address is stored into a readout
FIFO. This method is extendable to allow for multiple simultaneous
thresholds and geometric pattern triggers.
84](#__RefHeading___Toc137569200)

[Figure 41: Efficiency versus accidentals for a cluster finding
algorithms run on cluster data from a MIMOSA5 detector. Note that some
parameter combinations of this algorithm are already over 98% efficient
with a accidentals rate of 1-2 hits / cm^2^.
85](#__RefHeading___Toc137569201)

[Figure 42: Multiple event FIFOs are fed in parallel from the cluster
finder. A separate Event FIFO is enabled for one frame upon the receipt
of a trigger from the TCD. The resulting separate complete frames are
then passed to STAR DAQ as they are completed in the Event FIFOs.
86](#__RefHeading___Toc137569202)

[Figure 43: Data rates at the various stages of the Prototype MIMOSTAR4
readout chain. 87](#__RefHeading___Toc137569203)

[Figure 44: Functional block diagram for Ultimate sensor based readout
system. 88](#__RefHeading___Toc137569204)

[Figure 45: Data rates in ULTIMATE HFT readout.
89](#__RefHeading___Toc137569205)

[Figure 46: A prototype ladder showing low mass PCB, MIMOSA5 detectors
and driver electronics bonded to a mechanical carbon fiber and
reticulated vitreous carbon foam based carrier. 90](#_Ref136929718)

[Figure 47: Functional component diagram of the prototype readout system
constructed for the readout on a MIMOSA5 based ladder.
91](#__RefHeading___Toc137569207)

[Figure 48: Early Prototype motherboard and daughter card used for
reading out MIMOSA5 detectors. 92](#__RefHeading___Toc137569208)

[Figure 49: The HFT is shown integrated with the STAR inner detectors
cone assembly. 94](#__RefHeading___Toc137569209)

[Figure 50: Close up view of the HFT ladders.
95](#__RefHeading___Toc137569210)

[Figure 51: Detector support structure with kinematic mounts to insure
repeatable detector positioning. 95](#__RefHeading___Toc137569211)

[Figure 52: Kinematic mount structure. 96](#__RefHeading___Toc137569212)

[Figure 53: The HFT is shown integrated with the STAR inner detectors
cone assembly. 97](#__RefHeading___Toc137569213)

[Figure 54: Ladder assembly. The current ladder is composed of a row of
10 thinned APS detectors on top of a matching row of thinned readout
chips and a three-layer aluminum Kapton cable. The silicon cable
structure is bonded to a carbon composite "v", closing the beam to make
a rigid structure. 99](#__RefHeading___Toc137569214)

[Figure 55: A cross-section of the prototype detector ladder showing its
structure and materials composition. 100](#__RefHeading___Toc137569215)

[Figure 56: A cross-section of the prototype detector ladder showing its
structure and materials composition. 101](#__RefHeading___Toc137569216)

[Figure 57: Measured bend of a silicon/carbon composite ladder test
structure and the calculated bend shape. The ladder was rigidly
supported at one end with a 10 gram weight placed on the other.
101](#__RefHeading___Toc137569217)

[Figure 58: TV Holography system viewing test ladder in a small
transparent wind tunnel. Inset shows diffraction pattern with color map
of the surface displacement. 102](#__RefHeading___Toc137569218)

[Figure 59: A prototype readout cable for the HFT.
103](#__RefHeading___Toc137569219)

[Figure 60: Mechanical Prototype with 4 MIMOSA-5 detectors glued to the
Kapton cable assembly. 104](#__RefHeading___Toc137569220)

[Figure 61: Beam pipe structure with outer support barrel in the center.
105](#__RefHeading___Toc137569221)

[Figure 62: View of the beampipe mid-section.
106](#__RefHeading___Toc137569222)

[Figure 63: End view showing the HFT ladders between spokes to the inner
beam pipe support. 107](#__RefHeading___Toc137569223)

[Figure 64: A Schematic view of the installation activities in each
year. Installation is typically done in the summer of each year.
110](#__RefHeading___Toc137569224)

[Figure 65: Schedule overview and roll-up of activities.
111](#__RefHeading___Toc137569225)

[Figure 66: Detail from the Gantt Chart that tracks the full enterprise
of HFT activities. 112](#_Ref137471196)

List of Tables

[Table 1: Cross section for the production of charm and beauty.
21](#__RefHeading___Toc137569227)

[Table 2: Charm quark fragmentation functions. The left column is from
Ref. \[44,45\]. The right column is from Ref. \[43\]. The D^+^ and D^0^
yields include feed-down from D^\*+^ and D^\*0^ decays.
29](#__RefHeading___Toc137569228)

[Table 3: The number of vector meson pairs which can be recorded by STAR
in one RHIC year. We assume 200 M central Au *+* Au events will be
recorded per RHIC year when DAQ 1000 is operational.
38](#__RefHeading___Toc137569229)

[Table 4: Selected HFT parameters for the detector we propose to build.
The simulated parameters are the same except that we have assumed that
the detector is 0.38% radiation lengths thick, per layer, in order to
allow for the possibility of an onboard readout chip.
43](#__RefHeading___Toc137569230)

[Table 5: Open charm hadron properties 53](#__RefHeading___Toc137569231)

[Table 6: The cuts for the D0 and D+ reconstruction and efficiency
analysis. The input D0 and D+ spectra followed an exponential
distribution, \<pt\>=1.3 GeV/c. 56](#__RefHeading___Toc137569232)

[Table 7: An estimate of the number of events required to achieve 10%
statistical errors for the spectra measurements of 200 GeV p + p
collisions and 0-10%/ 0-80% Au + Au collisions. A power-law shape p~T~
distribution from the p + p collision has been assumed with the
mid-rapidity dN/dy = 0.03 \[Ref. 17\]. The number of binary collisions
is from Glauber calculations \[Ref. 23\].
61](#__RefHeading___Toc137569233)

[Table 8: An estimate of the number events required for 10% statistical
errors for D^0^ v2 measurements of 200 GeV 0-80% Au + Au collisions
(N~bin~=290). A power-law shape p~T~ distribution from the p + p
collision has been assumed with the mid-rapidity dN/dy = 0.03 \[Ref.
17\]. The number of binary collisions is from the Glaubor calculations
\[Ref. 23\]. 62](#__RefHeading___Toc137569234)

[Table 9: Luminosity and other parameters that determine the particle
flux on the HFT. 65](#__RefHeading___Toc137569235)

[Table 10: Integrated hit loading on the HFT and associated pileup.
65](#__RefHeading___Toc137569236)

[Table 11: Hit loading on the HFT from Au + Au central collisions and
associated pileup. 66](#__RefHeading___Toc137569237)

[Table 12: Table -- Chart of APS chips that the IReS group has produced
in the past 5 years \[\]. 69](#__RefHeading___Toc137569238)

[Table 13: Comparison between MIMOSA-5 characteristics and MIMOSTAR-4
specifications. 78](#__RefHeading___Toc137569239)

[Table 14: Silicon requirements for maximum average Au+Au RHIC
luminosity of 7.0 × 10^27^ Hz/cm^2^ or 2.5 nb^-1^/week
79](#__RefHeading___Toc137569240)

[Table 15: Prototype Stage Requirement Summary - constraints for the
MIMOSTAR4 APS. 80](#__RefHeading___Toc137569241)

[Table 16: Data rate calculation parameters
87](#__RefHeading___Toc137569242)

[Table 17: Final Stage Specification Summary - constraints for the ULTRA
series APS. 88](#__RefHeading___Toc137569243)

[Table 18: Data rate calculation parameters
89](#__RefHeading___Toc137569244)

[Table 19: Materials in the beam pipe and the first detector layer with
their total thickness and radiation length. For details see Ref. \[\]
99](#__RefHeading___Toc137569245)

[Table 20: Estimated cost of the HFT components. The total cost for the
hardware components is \$3.75M. This does not include engineering labor
but it does include approximately \$1M in manufacturing labor to be
spent in the Mechanical shops at BNL and LBL. 114](#_Ref122927285)

[Table 21: Proposed Funding Profile for the HFT that appears in the BNL
Mid Term Plan^136^ 115](#__RefHeading___Toc137569247)

# []{.mark} 

# Executive Summary

We propose to construct a Heavy Flavor Tracker (HFT) for the STAR
experiment at RHIC. The HFT will bring new physics capabilities to STAR
and it will significantly enhance the physics capabilities of the
detector at central rapidities.

The HFT will use CMOS Active Pixel Sensors (APS) which are an innovative
new technology which have never been used before in a collider
experiment. Our APS sensors are only 50 μm thick and this opens up a new
realm of possibilities for physics measurements. In particular, a thin
detector (0.28% radiation length per layer) in STAR makes it possible to
do the direct topological reconstruction of open charm hadrons by the
identification of the charged daughters of the hadronic decay. So, for
example, we can directly identify the decays D^0^ → K^−^ π^+^ and
D~s~^+^ → K^−^ π^+^ K^+^ by identifying the kaons and pions with the
rest of STAR and then using the pointing resolution of the HFT to find
the daughters with a common decay-vertex but where this vertex is
displaced by 100 μm from the primary vertex.

## Scientific Motivation

The primary motivation for the HFT is to extend STAR's capability to
measure heavy flavor production by the measurement of displaced vertices
and to do the direct topological identification of open charm hadrons.
These are key measurements for the heavy ion and spin physics programs
at RHIC. Heavy quark measurements will facilitate the heavy ion program
as it moves from the discovery phase to the systematic study of the
dense medium created in heavy ion collisions as well as the nucleon spin
structure in polarized p + p collisions. The primary physics topics to
be addressed by the HFT include heavy flavor energy loss, flow and a
test of partonic thermalization at RHIC.

A precise measurement of the spectra of D meson states will shed light
on several open questions in heavy ion collisions. From the spectra and
the production ratios of D states we will be able to extrapolate to the
total yield for charm quark production. Furthermore, the open charm
production rate is high enough at RHIC that the coalescence process
becomes relevant for Charmonium production. Knowledge of the total
production cross section for charm quarks is also essential as a
baseline for J/ψ measurements. A meaningful answer to the question of
whether the J/ψ mesons are suppressed or enhanced at RHIC requires
knowledge of the charm production in heavy ion reactions.

The heavy quark can also be used to probe the properties of the medium
created in heavy ion collisions. The production of gluons is
kinematically suppressed for heavy flavors (due to the dead cone
effect). As a consequence, heavy flavors should lose less energy in the
dense medium.

An important measurement to be made with the HFT is R~AA~; the ratio of
charmed meson production in Au-Au collisions to the binary-scaled
production rate in p-p or d-Au collisions. (See section 2.5.1). Current
measurements using non-photonic electrons as a measure of the abundance
of charm and beauty hadrons, indicate that the rate of energy loss for
heavy quarks is unexpectedly high and inconsistent with our current
understanding in pQCD models. Based on the non-photonic electron data
presented at Quark Matter, the whole theory of heavy quark energy loss
is uncertain and may be completely wrong; especially in regards to
beauty.

Another important measurement to be made with the HFT is a measurement
of the elliptic flow of D mesons down to very low p~T~ values. (See
section 2.4.1). It is generally accepted that elliptic flow is
established in the partonic phase. If charm quarks, with a mass much
larger than the temperature of the system, undergo elliptic flow then it
has to arise from many collisions with the abundant light quarks. Thus,
flow of charm quarks can be taken as a probe for frequent re-scatterings
of light quarks and is an indication of thermalization that may be
reached in the early stages of heavy ion collisions at RHIC. We believe
that proof of thermalization constitutes the last step towards the
establishment of the QGP at RHIC and this measurement requires a very
thin detector to push the measurement down to the lowest momenta where
transverse elliptic flow is manifest.

The pixel detector offers the possibility of measuring e^+^e^-^ pairs
down to the vector meson mass region by removing the gamma conversion
background, while electrons and positrons are identified in the TPC and
TOF. (See section 2.6). Vector mesons reconstructed in the dilepton
channel may have decayed at any stage of the systems evolution, since
leptons are very unlikely to re-interact. Thus, studying the low mass
e^+^e^-^ spectrum will yield crucial insights into the nature of the
system before it reaches the low-density hadronic freeze-out stage.
Measuring the low-mass vector mesons through their leptonic decay
channel may also yield information about the onset of chiral symmetry
restoration.

## Detector Concept

The HFT detector brings extremely high precision tracking capabilities
to STAR with a resolution of 10 µm at the first layer of the detector,
over a large pseudo-rapidity range, and with complete azimuthal angular
coverage. The HFT will enable STAR to perform high precision
measurements of heavy-quark production over the broadest range of phase
space, colliding system sizes and energies. It will exploit all of
STAR's unique features including particle ID and tracking from the
lowest p~T~ to the highest. In these respects, the HFT is unique at
RHIC.

The HFT will sit inside the STAR TPC and it will surround the
interaction vertex. The HFT has two tracking layers composed of
monolithic CMOS pixel detectors using 30 μm × 30 μm square pixels. These
critical innermost tracking layers lie at radii of 1.5 cm and 5.0 cm,
respectively, and these layers are active over 20 cm in z and have
\~ 100 million pixels. The HFT will provide tracking information for
decaying particles that are displaced by 100 microns from the primary
event-vertex. The silicon chips for the detector will be thinned to 50
μm and will be mounted on low mass carbon fiber structures to minimize
pointing errors generated by multiple Coulomb scattering.

Such a thin detector requires a correspondingly thin beam pipe.
Therefore, we propose to build a new beam-pipe for the STAR detector
which is only 0.5 mm thick. The construction of such a thin beam-pipe is
challenging and it requires a unique design to enable the beampipe to be
handled during installation and bakeout.

## R&D for an Advanced Detector

The HFT is an advanced detector. It explores new technologies that have
never been used before at RHIC and therefore we propose to evaluate the
technology, carefully, with an extended R&D phase in the project plan.
For example, the new beampipe needs to be designed and tested in
collaboration with the BNL Collider Accelerator Divison. We have already
started an extensive program of R&D using LDRD funds from LBL, and
contributed labor from the IRES group in Strasbourg, France, to develop
the basic Si technology and chipset. Progress on this front is excellent
and it is described in section 4, however, more work on the chips is
required. The readout electronics must be compatible with the large data
flow from the Si detectors, and even the software for calibration and
tracking will be pushed to new limits. Finally, the mechanical systems
to hold the chips and ladders must meet very demanding mechanical
tolerances and so this will require advanced engineering prototyping and
design. We feel that these challenges are interesting, fun, and can be
surmounted but we propose to reduce the risk to the project by exploring
these topics with an extended program of R&D activities.

## Coordination with Other New STAR Detectors

The HFT is to be built in coordination with a new, high rate,
Intermediate STAR Tracker. The primary purpose of the IST is to extend
the tracks found in the TPC and to point them accurately at the outer
layer of the HFT. Pointing with an accuracy of 125 μm, or less, is
required in order to independently find hit points in the HFT without
resorting to a vertex constraint on the track. Using a vertex constraint
works in a high multiplicity environment and so ensures a baseline
physics program for the HFT. But, a vertex constraint doesn't work for
pp collisions or the most peripheral Au-Au interactions. So the IST is
needed in order to collect the low multiplicity data needed to complete
our flow measurement program and it is required in order to gather the
reference data in d-Au or p-p collisions for the denominator in our
R~AA~ measurements.

# The Physics of the HFT

## Introduction

An important goal of high-energy nuclear physics is to understand
Quantum Chromodynamics (QCD) at extreme temperatures and energy
densities. Under these extraordinary conditions, we believe that the
fundamental symmetries of QCD will reveal themselves: quarks and gluons
will be the relevant degrees of freedom, color will be deconfined and
chiral symmetry will be restored. Calculations within the framework of
regularized lattice QCD predicts a fast crossover from ordinary nuclear
matter into a deconfined and locally thermalized state of quarks and
gluons called the Quark-Gluon Plasma[^1] (QGP).

High-energy nuclear collisions can be characterized by three distinct
phases: the initial phase where hard interactions between the partons of
the incoming nuclei dominate, an intermediate phase where
re-interactions between the constituents in the matter result in
collectivity, and a final stage where hadronization, and chemical and
thermal freeze-out occur. The matter produced in high-energy nuclear
collisions can be investigated by studying the dynamics of the
collective expansion of the bulk of the produced particles and by
studying the interaction of the medium with penetrating probes such as
leading particles and jets. In particular, the measurement of large
elliptic flow[^2], and the observation of strong modifications of high
*p~T~* particle production as measured by the ratio R~AA~[^3], and the
disappearance of the away side jet[^4] have provided evidence for a high
density and strongly interacting state of matter at RHIC[^5]. The goal
of our research program is to elucidate the nature of this matter and to
determine if it is dominated by hadronic or partonic degrees of freedom.

Rare processes provide new ways to probe the medium generated in
high-energy nuclear collisions. Bjorken[^6] proposed that hard scattered
partons (quarks and gluons) drawn from the incoming nuclei will interact
with the medium in a density-dependent way. Bjorken's initial energy
loss mechanism (elastic scattering) did not provide effects large enough
to be observed but medium-induced radiation (gluonic bremsstrahlung) can
generate significant energy loss effects[^7]^,^[^8]. For example, hard
parton scatterings can be experimentally reconstructed in elementary
particle collisions (*e^+^ + e^−^*, *p* + *p*) because the outgoing
parton fragments into a collimated spray of energetic hadrons at large
transverse angles with respect to the beam.

The cluster of hadrons, from the parton fragmentation, is known as a
jet. Jets can also be identified in high energy nuclear collisions on a
statistical basis; the modification of their properties may signal novel
flavor dependent energy loss mechanisms in a dense medium by the parton
that initiated the jet. Full jet reconstruction in heavy ion collisions
is exceedingly difficult but leading hadrons (i.e. high *p*~T~ hadrons,
which typically carry a large fraction of the jet energy) and their
correlations with other hadrons may provide the essential signals of
partonic interactions in the medium.

Collective flow measurements have played a prominent role in
understanding the physics of nuclear collisions because the magnitude
and pattern of the collective motion is closely related to the equation
of state (EOS) of the produced matter.

If local equilibrium is achieved, we will be able to use hydrodynamic
models to study the EOS and understand the degrees of freedom relevant
for the basic constituents of the matter. Heavy quark (*c, b*)
production provides some of the most important observables. Due to their
large masses, *c* and *b* quarks are produced dominantly by the
interactions of the initial incoming partons whereas lighter quarks are
produced throughout the later stages of the evolution of a heavy ion
collision. Thus, the total yields of *c* and *b* quark production
provide a direct connection to the initial state.

Due to their heavy mass and presumably small hadronic cross sections,
charmed quarks are a sensitive probe for the frequency of interaction
and therefore the degree of thermalization with constituents before
hadronization. At high *p*~T~, heavy quarks may be less suppressed than
light quarks due to the \"dead cone\" effect and so high p~T~ probes are
an additional way to study partonic energy loss. If charmed quarks
participate sufficiently in re-scattering processes, they will develop
flow (i.e. transverse radial and elliptic flow), which can be observed
in charmed hadron momentum distributions. Finally, charmed quarks might
achieve thermal equilibrium with the surrounding medium leading to
statistical hadronization. In this case, the relative abundances of
charmed hadrons are significantly modified.

The study of the structure of the nucleon is an important frontier in
strong interaction physics. Despite considerable experimental and
theoretical progress over the past several decades, many open questions
remain. A striking example is the spin structure of the proton, which
has been of key interest ever since the European Muon Collaboration
published their data on the spin structure function
*g~1~(x,Q^2^)*[^9]^,^[^10]. The EMC data and subsequent data indicate
that the quark helicity contribution to the proton spin is remarkably
small. The natural question regarding the fraction of the nucleon spin
carried by gluon helicities remains basically unanswered. Unlike
previous nucleon spin measurements, STAR will observe processes where
the polarized gluon distribution function *∆G(x,Q^2^)* contributes at
leading order via a hadronic, not electromagnetic, probe. Heavy quark
production is expected to provide direct access to gluons in the proton
with a large analyzing power for longitudinal spins.

## Initial Heavy Quark Production 

Quarks are elementary particles and, depending on the energy scale,
there are two mechanisms that generate their masses with different
degrees of importance: current quark masses are generated by the
electroweak symmetry breaking mechanism (Higgs mass) and spontaneous
chiral symmetry breaking leads to the constituent quark masses in QCD
(QCD mass). The QCD interaction affects only the light quarks (u, d, s)
while the heavy quark masses(c, b, t) are determined by the Higgs
mechanism, see Figure 1. In high-energy nuclear collisions at RHIC,
heavy quarks are produced through gluon fusion and qq annihilation[^11].

Heavy quark production is also sensitive to the parton distribution
function. Unlike the light quarks, heavy quark masses are not modified
by the surrounding QCD medium[^12] (or the excitations of the QCD
medium) and the value of their masses is much higher than the initial
excitation of the system. It is these differences between light and
heavy quarks in a medium that make heavy quarks an ideal probe to study
the properties of the hot-dense medium created in high-energy nuclear
collisions.

![](media/image2.png){width="4.929861111111111in"
height="2.9993055555555554in"}

[]{#__RefHeading___Toc137569161 .anchor}Figure 1: Masses of the six
quark flavors. The current and additional QCD masses are shown by blue-
and yellow-boxes, respectively. The figure is adapted from Ref. \[Error:
Reference source not found\].

Understanding the yield of charmed hadrons in hadron-hadron collisions
requires a knowledge of the projectile and target parton distribution
functions, the cross section for parton-parton interactions which
generate charm quarks and the fragmentation functions for *c*(*b*)
quarks into charmed (bottom) hadrons. The parton distributions within
the proton can be extracted from electron-proton collisions while the
cross-sections for gluon fusion and *qq* annihilation are calculated in
a perturbative QCD framework up to next-to leading-order (NLO)[^13].
However, the parton and gluon distribution functions within the nucleus,
relevant to the charm and bottom quark production at RHIC energies, are
poorly understood[^14] and thus leave room for precise measurements of
charm cross sections in p + p, d + Au and Au + Au collisions.
Perturbative QCD predictions for the cross section σ(*cc*) and σ(*b*) in
p + p collisions at √s~NN~ = 200 GeV may be found in the
literatureError: Reference source not found and they are in reasonably
good agreement with the cross-sections measured at RHIC given that some
of the theoretical calculations are uncertain by as much as 50%.

+-----------------------------------------------------------------------+
| Experiment: σ~NN~ (*cc* ) = 900 - 1400 µb                             |
|                                                                       |
| Theory : σ~NN~ (*cc* ) = 289 - 445 µb                                 |
|                                                                       |
| 20 - 30 *cc* pairs per central Au + Au collision at √s~NN~ = 200 GeV  |
+-----------------------------------------------------------------------+
| Theory : σ~NN~ (*b*) = 1.64 - 2.16 µb                                 |
|                                                                       |
| 0.04 - 0.06 *b* pairs per central Au + Au collision at √s~NN~ = 200   |
| GeV                                                                   |
+-----------------------------------------------------------------------+

[]{#__RefHeading___Toc137569227 .anchor}Table 1: Cross section for the
production of charm and beauty.

The uncertainty in the theoretical cross-sections arises from a
reasonable variation of quark masses (*m~c~* = 1.2-1.8 GeV, *m~b~* =
4.5-5.0 GeV), factorization and renormalization scales (μ~R~ and μ~F~),
and parton distribution functions (MRST, CTEQ, GRV). The number of
underlying binary nucleon-nucleon collisions normalizes the cross
section values. (There are approximately 1000 binary nucleon-nucleon
scatterings for each central Au + Au collision.)

In heavy ion collisions, final state interactions may also enhance charm
production relative to the binary scaling of initial parton collisions
and may also lead to additional production of charm via "thermal"
processes. An analogous mechanism dominates strangeness production. The
heavy quark channels should be greatly suppressed by the thermal factor
due to the heavy quark mass (*m~c~* ≈ 1.2 to 1.8 GeV, *m~b~* ≈ 4.5 to
5.0 GeV)[^15], making heavy quark production rates primarily sensitive
to the dynamics of the initial collisions. This is especially true for
the b quark. Figure 2 shows the calculated contributions to the total
charm production at *y* = 0 for √s~NN~ = 200 GeV, with an energy density
of 3.2 GeV/fm^3^ at the moment of thermal equilibration.

STAR and PHENIX have made measurements of charm production in Au+Au,
d+Au, and p+p collisions at RHIC. The PHENIX data for Au + Au collisions
suggests that the cross-section for open charm production is consistent
with the expectations of pQCD. These results were derived from
non-photonic single electron spectra[^16] as shown in Figure 3. The STAR
results show that the cross-section for open charm production in d+Au
and Au+Au is consistent with binary collision scaling but the total
yield may be larger than NLO pQCD models. Note, since we only measure
the electrons and D^0^s to extract the total charm cross-section,
several assumptions such as the ratios of the neutral over charged
D-mesons have to be used. Both the p~T~ integrated yield of dN/dy and
the value of \<p~T~\> of the measured D-meson spectrum are larger than
the pQCD model predictions. The STAR results are obtained from a direct
reconstruction of the open charm hadrons[^17] as well as through single
electron spectra.

![](media/image3.jpeg){width="2.9923611111111112in"
height="3.254861111111111in"}

[]{#__RefHeading___Toc137569162 .anchor}Figure 2: Contributions to charm
production at RHIC energies include the initial gluon fusion and *qq*
annihilation phase (solid line), a pre-thermal phase characterized by
scattering between free-streaming partons, and a thermal phase that
assumes parton equilibration. The distributions were calculated with an
energy density of 3.2 GeV/fm^3^ at the moment of thermal equilibration.
As a reference, the faint dotted line is the thermal production of charm
due to an initial fully equilibrated QGP. The figure is from Ref.
\[Error: Reference source not found\].

![](media/image4.png){width="4.0in" height="2.892361111111111in"}

[]{#__RefHeading___Toc137569163 .anchor}Figure 3: Total cc production
cross-sections per nucleon-nucleon collision vs. collision energy. The
dashed line depicts a PYTHIA calculation[^18]. The dot-dashed line
depicts a NLO pQCD calculation with MRST HO, m~c~ = 1.2 GeV/c^2^, µ~F~ =
2m~c~, µ~R~ = 2m~c~[^19]. The figure is adapted from Ref. \[Error:
Reference source not found, Error: Reference source not found\].

## The Need for Direct Topological Reconstruction of Open Charm

When direct measurements of heavy flavor hadrons are not possible,
non-photonic electrons from heavy flavor decays can be used to study
charm production. However, there are serious limitations in such
situations. As shown by Batsouli, Kelly, Gyulassy and Nagle in Ref.
\[[^20]\], the decayed electron distributions are insensitive to the
intrinsic shape of the D-meson transverse momentum distribution. Due to
the decay kinematics and the light mass of the electrons and positrons,
the dynamical information in the primary spectrum is washed out. This
phenomenon is shown in Figure 4. There is a clear difference between the
zero mean free path hydrodynamic flow prediction (solid) and the
infinite mean free path pQCD Pythia calculations (dashed-line) for
D-mesons; but the resulting electron spectra are nearly
indistinguishable. In order to extract useful information about heavy
flavor production in heavy ion collisions, we have to measure the charm
and beauty-hadrons by direct topological reconstruction. Single electron
spectra are not sufficient.

![](media/image5.jpeg){width="4.610416666666667in"
height="3.997916666666667in"}

[]{#__RefHeading___Toc137569164 .anchor}Figure 4: Solid- and
dashed-curves represent the charm- (red) and beauty-hadron (blue)
spectra from Blast-Wave and Pythia model calculations, respectively. The
corresponding heavy flavor decayed electron spectra are shown as black
curves. The data are the measured single electron distributions measured
in 10% central Au + Au collisions at 130 GeV by the PHENIX
collaboration. The figure is adapted from Ref. \[Error: Reference source
not found\].

The same conclusion is reached when doing the analysis of R~AA~ for
heavy flavor spectra. Recall that R~AA~ is the ratio scaled by the
number of binary collisions of the charm yield measured in heavy ion
collisions relative to the yield in p-p or d-Au. The nuclear
modification factor, R~AA,~ for D-mesons is shown in Figure 5. Two
vastly different models are shown. The figure shows that there are large
differences in  the two D-meson R~AA~ ratio curves but the corresponding
decayed-electron R~AA~ curves  are essentially identical for all p~T~.
These are theoretical curves without error bars which suggests that the
decayed electron R~AA~ curve is impossible to measure; additionally, any
electron measurement will have large systematic uncertainties at low
momentum due to the large background from photonic electron
productionError: Reference source not
found^,Error:\ Reference\ source\ not\ found^~.~ At higher p~T~,
electrons from B-meson decays will become more abundant making the
electron measurement for charm physics even less realistic.  This
clearly calls for a  direct measurement of heavy flavor hadrons, i.e. D-
and B-mesons.

The proposed Heavy Flavor Tracker (HFT) will make these and other
measurements by the direct topological reconstruction of the various
charmed hadrons, including the D^+^, D^−^, D^0^, D~s~^+^ and possibly
Λ~c~^+^. Thus the HFT will enable us to dramatically reduce the
systematic uncertainties that are inherent in single electron spectra.

![](media/image6.png){width="4.330555555555556in"
height="3.1006944444444446in"}

[]{#__RefHeading___Toc137569165 .anchor}Figure 5: Nuclear modification
factor R~AA~ of D-mesons assuming a hydro-dynamically inspired
parameterization assuming a collective flow velocity of \<β~r~\> =
0.4*c* and 0.6c for D-mesons. The corresponding electron decay-spectra
are shown by the blue dashed (1) and (2) lines[^21].

## Probing Medium Thermalization: Charm Quark Re-interactions

RHIC data on light flavor hadrons strongly suggests that partonic
collectivity has been achieved in heavy ion collisions. The successful
measurement of partonic collectivity is a necessary step toward the
discovery of a QGP; however, it is not sufficient information to claim a
discovery until we address the issue of thermalization and collectivity,
together.

Collectivity in heavy ion reactions is addressed by studying flow. Many
important measurements of transverse radial and elliptic flow of
identified hadrons, containing light quark flavors (*q = u, d, s*), have
been performed at RHIC. The main conclusions from these studies are that
the systems created in ultra-relativistic Au + Au collisions exhibit
strong collective expansion. Compared to measurements at lower SPS
energies, the degree of collectivity is stronger. In addition,
measurements of the spectra of multi-strange hadrons (Ξ and Ω) at RHIC
suggest that they freeze-out at a higher temperature and lower
collective velocity than the lighter hadrons; however, a significant
amount of elliptic flow is also observed for multi-strange baryons. The
elliptic flow of the multi-strange baryons is comparable to the amount
of flow observed for the non-strange baryons and is in good accord with
the number of constituent quarks (ncq -- scaling) hypothesis that
describes the non-strange quark bearing mesons and baryons so well[^22].
These results have been interpreted as an indication that sizeable
partonic collectivity develops at RHIC. Details of these studies can be
found in the literatureError: Reference source not
found^,^[^23]^,^[^24]^,^[^25]^,^[^26].

The key question, then, is whether or not charm quarks flow. If the
elliptic flow of charm were comparable to the elliptic flow of the
lighter quarks, then this would be a clear indication of a thermalized
state of matter at RHIC because it takes many interactions with lighter
quarks, and gluons, to cause a heavy quark to acquire the collective
motion of the bulk matter.

Theoretical calculations indicate that thermalization may be reached at
RHIC at temperatures T~eq~ ≈ 0.3-0.5 GeV[^27] and that the duration of
the equilibrium period is on the order of 5-10 fm/c[^28]^,^[^29].
Thermal production of *cc* pairs is suppressed due to their large masses
(m~cc~ = 2.2 -- 2.3 GeV/*c*^2^), however, charmed hadrons may still be
produced in a thermalized fashion if the *c* and *c* quarks become
embedded in a thermalized bath of light quarks. The relative probability
of creating different charmed hadrons will be driven by the properties
of the medium which is providing the additional quarks necessary to form
the hadrons. The relative yield of various charmed hadrons is thus
sensitive to the properties (temperature and chemical potentials) of the
light quark medium and these hadron yields must be measured in order to
achieve a full understanding of the medium.

The transverse momentum distributions of these hadrons are particularly
important. The p~T~ distributions reflect the dynamical evolution of the
system and yield indirect information about different stages of the
collision. After hadronization is complete and inelastic collisions
cease, then the particle abundances are fixed: this is commonly referred
to as chemical freeze-out. Later when elastic interactions cease, the
particle momentum spectra become fixed; this is referred to as kinetic
freeze-out. The evolution of the system is recorded in the p~T~ spectra
because, for most particle species, transverse *radial* flow is
accumulated throughout the whole collision history while transverse
*elliptic* flow is believed to saturate at early times in the collision
sequence.

In summary, charmed hadron flow is potentially an indicator of
thermalization due to the heavy quark interactions with light quarks and
gluons. If thermalization takes place, it is reached during the partonic
stage of the collision. In order to test the question of thermalization
experimentally, we propose to measure the charmed hadron transverse
radial and elliptic flow using the STAR HFT detector. The combination of
the open charm spectra and v~2~ will allow us to determine the
collective properties of the charm quarks and the probable degree of
thermalization of the light quarks.

### Elliptic Flow

In non-central heavy ion collisions, the overlap of the colliding nuclei
is spatially deformed and has an ellipsoidal shape. Through many
re-scatterings, the pressure gradients will be built up in non-head on
high energy collisions. In this way the initial state spatial anisotropy
can be transformed into a final state momentum space anisotropy. Notice
that re-scattering is a sufficient condition for the development of
these anisotropies and thermalization is not required. Theoretically,
the largest momentum anisotropies are obtained in the hydrodynamic
limit[^30] where there is a zero mean free path; this leads to
instantaneous local thermal equilibrium.

These momentum space anisotropies lead to a dependence of the
transverse-momentum distribution on the emission angle relative to the
reaction plane. The anisotropy can be quantified by the coefficients of
a Fourier decomposition of the distribution in azimuth. The largest
contribution comes from the second Fourier coefficient v~2~(*p*~T~,
*y*), the *elliptic flow coefficient*. In the evolution of elliptic
flow, the strong and early spatial deformation decreases because the
matter begins to expand more rapidly in the direction of the shorter
axis of the ellipsoid[^31]. As the spatial deformation disappears, the
build-up of flow due to pressure gradients ceases and the elliptic flow
saturates. For this reason, elliptic flow is thought to be a signal that
develops in the early stages of a collision. RHIC dataError: Reference
source not found^,^[^32]^,^[^33] show that in semi-central Au + Au
collisions, elliptic flow reaches the hydrodynamic limit for transverse
momenta up to 2 GeV/c and this suggests early thermalizationError:
Reference source not found at a time of τ = 0.6 fm/c since the collision
began. Thus, information about the equation of state[^34] can be
determined. If all hadron species experience the same anisotropic flow,
their *v*~2~ coefficients should obey simple hydrodynamic
relationsError: Reference source not found and exhibit a characteristic
mass dependence. As an example, Figure 6 (top) shows the measured
elliptic flow of strange hadrons up to, and including, the multiply
strange Ξ baryon. At low momentum, all particles exhibit a linear rise
in *v*~2~ and a clear mass ordering appears from the lower mass kaon to
the heavier Ξ. The bottom plot in Figure 6 shows the measured elliptic
flow versus transverse momentum; where both axes are scaled with the
number of constituent quarks. Quark coalescence models predict a
universal scaling of *v*~2~/n versus *p*~T~/n at intermediate momentum
where quark recombination is (supposedly) the dominant hadron production
mechanism. In these models, collectivity -- the elliptic flow of
constituent quarks -- is intrinsically built in and supports the idea of
partonic collectivity. These analyses demonstrate that collectivity is
established at the partonic level.

![](media/image7.png){width="2.85in" height="4.022222222222222in"}

[]{#_Ref88041181 .anchor}Figure 6: Elliptic flow of strange hadrons at
RHIC as measured by the STAR detector. The top panel demonstrates
typical hydro-dynamical mass ordering up to a momentum p~T~ \< 2GeV/c
and saturation at larger momentum. The bottom panel shows the scaling of
elliptic flow with the number n of constituent quarks in the saturation
region (baryons, n=3; mesons, n= 2).

At RHIC energies, charm quarks are abundantly produced. Due to their
high mass and small interaction cross section, the strength of elliptic
flow of heavy flavor hadrons may be a good indicator of thermalization
occurring at the partonic level. If heavy flavor hadrons flow together
with the light flavor hadrons, this indicates frequent interactions
between the light and heavy quarks. Hence, thermalization of light
quarks is likely to have been reached through partonic re-scattering.

Figure 7 shows the first indication of charm particle elliptic flow at
RHIC measured in the inclusive electron channel[^35]. The predictions
assume elliptic flow for the light quarks as determined by fits to
experimental data. Presently, the data support the idea that the heavy
charm quarks flow. As shown in the figure, the uncertainties are rather
large especially at low transverse momentum where hydrodynamic behavior
should occur. A precise measurement of directly reconstructed open charm
hadrons to low momentum is essential to confirm and further quantify
elliptic flow of the charm quarks.

![](media/image8.jpeg){width="5.238194444444445in"
height="3.745138888888889in"}

[]{#_Ref88041446 .anchor}Figure 7: Non-photonic electron elliptic flow
at RHIC. Open and closed symbols are data of non-photonic electron v~2~
from PHENIXError: Reference source not found Statistical errors are
shown as solid lines and the open boxes indicate the size of the
systematic errors. The solid-line represents the results from Greco[^36]
et al. where, in this coalescence calculation, similar v~2~
distributions for c- and u-quarks are assumed.

### Charm Hadro-Chemistry

Hadronic yields and their ratios have been measured from AGS to RHIC
energies and have been successfully described by statistical
models[^37]^,^[^38]. The relevant parameters are the chemical freeze-out
temperature T~ch~, and the chemical potentials μ~i~ for conserved
quantum numbers, i.e. net strangeness, charge, and baryon number
conservation[^39]. The extracted chemical freeze-out temperature T~ch~
is higher than the kinetic freeze-out temperature T~fo~ extracted from
momentum spectra[^40] and this suggests that chemical freeze-out occurs
before kinetic freeze-out, as expected.

Charm quarks are dominantly produced in initial parton-parton
scatteringsError: Reference source not found and the thermal production
of charm quarks is suppressed due to their large mass. In the case of
sufficient re-scatterings, initially produced charm quarks might
thermally (but not chemically) equilibrate with the surrounding medium.
This means their momentum distribution can be described by a temperature
parameter consistent with the spectra of light quarks, while the total
abundance of charm quarks is determined by the initial parton collision
dynamics.

These arguments lead to the idea of statistical hadronization of charm
quarks[^41]^,^[^42]. Statistical coalescence scenarios predict large
changes in open (regarding relative abundances, e.g. the D^0^ /
D~s~*^+^* ratio) and hidden charm production with a strong centrality
dependence[^43]. Statistical coalescence implies that charm quarks
travel over significant distances, e.g. in a QGP. Therefore, we believe
that a consistent description of precision data by these models would be
an indication of deconfinementError: Reference source not found.

Measuring the total charm and bottom yields requires measuring the
yields of several different hadrons. Indeed, charm quarks may fragment
into a variety of hadrons as shown in the first column of Table 2. These
fragmentation ratios have recently been compiled[^44]^,^[^45] using
*e-p* and *e^+^-e^-^* collision data. The ratios are found to be
independent of the collision energy and the collision system (e-p or
e^+^-e^-^). They are likely to be the same in p-p collisions at
mid-rapidity where the HFT can be used to detect charmed particles at
RHIC. Note that the ratios calculated using PYTHIA[^46], shown in the
third column of Table 2, differ significantly from the measured ratios.
To avoid any uncertainties in the charge yield measurement, these ratios
will have to be measured in *p + p* collisions at RHIC energies.

+-----------------+------------------+----------------+---------------+
|                 | *e*-*p* and      | Pythia         | Statistical   |
|                 | *e*^+^-*e*^−^    |                | coalescence   |
|                 |                  |                |               |
|                 | average          |                |               |
+-----------------+------------------+----------------+---------------+
| ƒ(c → *D*^+^)   | 0.232            | 0.162          | 0.21          |
+-----------------+------------------+----------------+---------------+
| ƒ(c → *D*^0^)   | 0.549            | 0.639          | 0.483         |
+-----------------+------------------+----------------+---------------+
| ƒ(c →           | 0.101            | 0.125          | 0.182         |
| *D~s~^+^*)      |                  |                |               |
+-----------------+------------------+----------------+---------------+
| ƒ(c → Λ~c~^+^)  | 0.076            | 0.066          | 0.080         |
+-----------------+------------------+----------------+---------------+
| ƒ(c → J/ψ)      |                  | 0.006          | 0.057         |
+-----------------+------------------+----------------+---------------+

[]{#__RefHeading___Toc137569228 .anchor}Table 2: Charm quark
fragmentation functions. The left column is from Ref. \[Error: Reference
source not found,Error: Reference source not found\]. The right column
is from Ref. \[Error: Reference source not found\]. The D^+^ and D^0^
yields include feed-down from D^\*+^ and D^\*0^ decays.

The effect of statistical coalescence is shown in the last column of
Table 2 (see Ref. Error: Reference source not found). The calculation
assumes that charm quarks statistically coalesce with the lighter
quarks, (i.e. according to the temperature and chemical potential of the
light-quark system) so the temperature and chemical potential are set so
that the light hadron yields measured at RHIC are reproduced. In
addition, the number of charm quarks present in the system is set by
pQCD calculations and this is reflected by a charm-chemical potential in
the statistical coalescence model. The results of the model show that
statistical coalescence increases the yield of the D~s~*^+^* meson by
80% and the J/ψ yield by a factor of 10 compared to PYTHIA while the
yield of D^0^ and D^+^ decrease slightly. Thus, the ratios D~s~^+^/
D^0^, D~s~^+^/D^+^ and J/ψ / D^0^ are very sensitive probes of thermal
charm hadron production.

Experimentally, we will be able to precisely measure the ratio of
D~s~*^+^* to D*^+^* yields because most of the systematic errors in the
individual spectra cancel out when they are reconstructed in very
similar decay channels: D*^+^* → K^−^ π^+^ π^+^ and D*~s~^+^* → K^−^
π^+^ K^+^.

A precise measurement of the total charm production (mostly carried by
open charm mesons) will also serve as a baseline for J/ψ
enhancement/suppression measurements, which have been suggested as a
possible signature for QGP formation[^47].

## Probing the Density of the Medium: Heavy Quark Energy Loss

In order to develop collective flow in heavy ion collisions, there must
be frequent interactions between the constituents of the medium. These
interactions will cause energy loss for the energetic partons that are
traversing the medium and the amount of energy loss will depend on the
distance traveled in the medium.

Results reported at the recent QM05 conference on the nuclear
modification factor R~AA~ indicate that the rate of energy loss for
heavy-quarks in central Au + Au collisions is surprisingly similar to
that for the light-quarks (u, d, s). This experimental observation
contradicts our early understanding of the pQCD interactions of
energetic partons in a hot and dense medium where much less energy loss
was expected for heavy-quarks compared to the light
ones[^48]^,^[^49]^,^[^50]. Since there have been no directly
reconstructed heavy-quark hadron distributions from RHIC experiments so
far, non-photonic electrons in the transverse momentum range 4 \< p~T~
\< 10 GeV/c were used for these heavy-quark studies. When analyzing the
data with only non-photonic electron p~T~ distributions, the data are
complicated by an unknown mixture of charm and beauty contributions.
Therefore, the only way to disentangle these effects is by a direct
topological reconstruction of charmed-hadron distributions and a
measurement of charmed hadron angular correlations at RHIC.

### R~AA~ and Energy Loss 

The discovery of a factor of 4 to 5 suppression of high *p~T~* hadrons
(5 \< *p~T~* \< 10 GeV/c) produced in Au + Au collisions at RHIC and the
disappearance of the away-side jet has been interpreted as evidence for
jet quenchingError: Reference source not found^,^[^51]^,^[^52]^,^[^53].
This effect was predicted to occur due to radiative energy loss of high
energy partons that propagated through a dense and strongly interacting
medium[^54]. Heavy quarks are also predicted to lose energy as they
traverse the medium, however, their energy loss is predicted to be
significantly less because of a suppression of gluon radiation at angles
Θ \< *M~Q~ /E;* where M~Q~ is the heavy quark mass and *E* is the heavy
quark energy[^55]. This kinematic effect is known as the "dead cone"
effect. The suppression of small angle radiation has the advantage that
the heavy quark fragmentation function and the spectrum of light
particles produced in association with the heavy quarks can be
calculated perturbatively.

Figure 8 shows the result for the ratio of charm (H) to light (L) quark
suppression from QCD calculations assuming a size of about 5 fm for the
static medium traversed by the fast quark. For transverse momentum p~T~
\> 7.5 GeV/c this ratio is predicted to be about 2.0 due to the smaller
energy loss of the heavy quark. In the case where heavy quarks suffer
the same amount of energy loss as light quarks, this ratio would be 1.0.
This ratio is exponentially sensitive to the density of color charges in
the medium, and so the measurement of nuclear modification factors of
open charm mesons at large p~T~ is a promising tool for the study and
further characterization of QCD matter at RHIC.

![](media/image9.png){width="4.00625in" height="2.6805555555555554in"}

[]{#__RefHeading___Toc137569168 .anchor}Figure 8: The ratio of
suppression factors in hot matter for charm (H) and light (L) quarks.
The solid line represents results from calculations with unrestricted
gluon radiation, while the dashed line is based on calculations with a
cut on gluon energies ω \> 0.5 GeV. The size of the static medium
traversed by the fast quark is assumed to be 5 fm. The figure is from
Ref. \[Error: Reference source not found\].

Figure 9 shows R~AA~ for non-photonic electrons[^56]^,^[^57]^,^[^58]
from STAR and PHENIX. The data extend up to p~T~ \~ 10 GeV/c. The figure
shows that the suppression factor for single electron spectra is in the
range of 0.2-0.3 which is almost exactly the same as the suppression
factor observed for charged hadrons and pions[^59]^,^[^60] (i.e. light
quarks). Using an unrealistically large initial gluon density of
dN~g~/dy = 3500, Djordjevic, Gyulassy, Vogt and Wicks have done a pQCD
calculation[^61], including gluon radiative energy-loss, which can
barely reproduce the electron R~AA~ (see the blue-line in Figure 9). The
model ignores the contributions from beauty-hadrons, and when beauty is
added to the model, the resulting prediction for R~AA~ cannot describe
the data at all (see the yellow band in Figure 9 ); the model with
beauty over-predicts the data by a factor of 2-3. Note that for
light-quark hadrons, like pions, a gluon density of \~1000 has been used
to reproduce the observed R~AA~. These results raise a serious challenge
to our understanding of both the mechanism for heavy-quark production
and the mechanism for energy-loss in a hot and dense medium. In order to
resolve these important issues, we have to do direct topological
reconstruction of open charm. It is experimentally and theoretically too
difficult to make definite conclusions from the single-electron
measurements due to the complications from the mixing of the electrons
from D-meson decay and electrons from bottom hadron decay as well as the
smearing of the decay kinematics.

![](media/image10.jpeg){width="5.53125in" height="3.998611111111111in"}

[]{#__RefHeading___Toc137569169 .anchor}Figure 9: Non-photonic electron
R~AA~ in central Au + Au collisions from STAR (open-symbols, top 5%) and
PHENIX (filled-symbols, top 10%) from Refs. \[Error: Reference source
not found,Error: Reference source not found,Error: Reference source not
found\]. Theoretical predictions for charmed-hadron and charm and
beauty-hadron decayed electrons are shown as blue- and yellow-bands,
respectively. In these calculations, the initial gluon density was
assumed to be unrealistically large: \~3500, see Ref \[Error: Reference
source not found\] .

### Charm Angular Correlations

Correlations between charmed hadrons are another way to separate charm
and beauty physics at RHIC. Let the correlation between D mesons be
defined as the normalized pair distribution C(∆φ) = N(p~1~,p~2~)/N~0~
where p~1~ and p~2~ are the momenta of the charmed hadrons, and N~0~ is
the total number of pairs.

Similar to jet production, heavy-quark production requires a large
momentum transfer and so we expect a distinct back-to-back topology for
the quarks (and mesons) as shown by the open-circles in Figure 10. In
this calculation, the Pythia (v6.2) event generator was used with the
default sets of parameters. As one can see in the figure, there is a
clear back-to-back correlation for the D-mesons. We propose to utilize
this distinct correlation to study the charm-quark energy loss in
high-energy nuclear collisions.

![](media/image11.png){width="4.999305555555556in"
height="3.8270833333333334in"}

[]{#__RefHeading___Toc137569170 .anchor}Figure 10: D-meson correlation
functions for 200 GeV p + p collisions. Default parameters in the Pythia
model were used in these calculations. A clear back-to-back correlation
in the angular distribution of charmed mesons is observed (shown by the
open circles). The Solid-line and the diamonds represent the results
with angular smearing for σ~φ~ = π/4 and π/2, respectively.

Now lets consider a few potential observables. For example, when a
charm-quark interacts with the medium, it will lose energy and the
original angular correlation pattern, as shown in the figure, will be
modified. We have tried angular smearings of \<σ~φ~\> = π/4 and π/2. In
addition, the changes in the angular correlation depends on the nature
of the interaction. Most of the (semi)elastic scatterings are directed
in a narrow cone in the forward direction[^62] while the inelastic
scatterings, such as the gluon radiative energy lossError: Reference
source not found will lead to a much wider smearing in the final
correlation. In the inelastic scattering scenarioError: Reference source
not found^,Error:\ Reference\ source\ not\ found^, the energy-loss
occurs deep inside the plasma and the final correlation function
reflects the hot/dense properties of the medium. On the other hand, the
resonant scattering happens near
T~cError:\ Reference\ source\ not\ found~. Although both scenarios lead
to sizable energy-loss, the angular correlations may allow us to
distinguish these two different mechanisms in high-energy nuclear
collisions. In order to perform the measurement, a large acceptance for
the reconstructed charmed hadrons is required. The proposed HFT, plus
STAR TPC and TOF, will be necessary for this study.

Recently, the measurements on charm production by the Belle
Collaboration[^63] show a surprisingly large cross-section for J/ψ
production in √s = 10.6 GeV e^+^+e^-^ collisions. Even more surprising
is the fact that more than half of the observed J/ψ's are accompanied by
cc pairs. This result contradicts our current understanding for J/ψ
production in the pQCD framework, such as those discussed in Refs.
\[Error: Reference source not found,[^64],[^65],[^66]\], and implies a
different production mechanism for heavy-quarks in elementary
collisions[^67]. As proposed in Ref. \[Error: Reference source not
found\], gluon fragmentation is increasingly important for collisions at
higher bombarding energies and so RHIC energies are very interesting. In
elementary collisions, the main difference between the new and
conventional processes lies in the angular correlation of the produced
charmed hadrons. With the proposed HFT and STAR EMC (Electro-Magnetic
Calorimeter), we will be able to study the correlation of D mesons to
further understand pQCD in p + p collisions. We will also study the
correlation of D mesons in high-energy nucleus-nucleus collisions, were
the gluon density is high[^68]^,^[^69], and this might enhance the
effects observed in elementary p + p collisions. These studies will
certainly shed light on the production mechanisms for charm and
charmonium at RHIC.

As mentioned earlier, heavy-quark production leads to a back to back
correlation between particle and anti-particles. This correlation is
also reflected in their decay products, such as the electron pairs. In
this case, it causes the background in the intermediate mass region[^70]
1 \< m~ee~ \< 3 GeV/c^2^ to have a correlation, too. (Here m~ee~ is the
invariant mass of the electron pair.) This creates a significant
background for a low mass vector meson analysis. Using the HFT to
measure the correlated electron pairs will greatly reduce the background
for vector meson and charm measurements via non-photonic electron
spectra.

### Baryon -- Meson Ratios

In the intermediate p~T~ region 2 \< p~T~ \< 6 GeV/c, light-quark baryon
yields are known to be enhanced compared to meson yields in Au + Au
collisions at RHIC. The enhancement is found to be proportional to the
collision centrality. These observations have been confirmed by both
R~AA~ and v~2~ measurements. The results are usually explained by a
hadronization mechanism involving collective multi-parton coalescence
rather than by independent vacuum fragmentation . The success of the
coalescence approach implies deconfinement and possibly thermalization
of the light quarks prior to hadronization. Since Λ~c~ is the lightest
charmed-baryon and its mass is not far from the other D-mesons, it will
be very interesting to measure the R~AA~ of Λ~c~ and compare it with the
R~AA~ of the other charmed mesons in order to see if there is a
meson-baryon difference. Theories about heavy quark deconfinement and
collectivity can be tested with these comparisons. In addition, due to
the different branching ratios for the semi-leptonic decays, the
measurement of Λ~c~ spectra will help us understand the surprising
suppression observed in the non-photonic electrons. In this case, even
if charmed quark production scales with the number of binary collisions,
an increase in the ratio Λ~c~/D-mesons similar to that seen for the
Λ/Kaon ratio will lead to a \~20% suppression in non-photonic electrons
( 1 \< p~T~ \< 5 GeV/c ) for central Au + Au collisions[^71].

## Measuring Vector Mesons with Dileptons 

In addition to the direct measurement of open-charm hadrons, the HFT
will serve as a powerful device to discriminate primordial electrons
from background electrons in the measurement of electromagnetic probes.

Photons and leptons emitted from the early stages of heavy ion
collisions suffer few interactions with their surroundings since they
interact electromagnetically instead of via the strong force. Therefore,
electromagnetic probes are ideal tools to study the properties of matter
created by relativistic heavy ion collisions.

On the other hand, processes, which produce photons and leptons, are
rare and are overwhelmed by photons and leptons from electromagnetic
decays of hadrons and subsequent γ conversions to leptons. In addition,
the final state electrons and positrons are the integrated yield of
radiation over the whole evolution of the colliding system. Despite all
the experimental difficulties, the low and intermediate mass dileptons
have been measured at the CERN-SPS and an excess of radiation above the
hadron cocktail has been observed in the invariant mass region of 0.2 \<
M~ee~ \< 0.6 GeV/c^2^ in semi-central Pb + Au collisions[^72]^,^[^73].
Also, new and exciting results from the first three RHIC runs indicate
that jets have lost a large fraction of their energy in dense matter and
the hadronic phase is relatively short. We believe that these
observations at RHIC favor experimental measurements of photons and
dileptons due to thermal radiation and will result in a clearer
signature than from lower energy heavy ion collisions.

At STAR, electron identification is made possible by a combination of
two measurements: a measurement of the energy lost by charged particles
due to ionization in the TPC gas (dE/dx), and a velocity measurement
with the time-of-flight system. The relativistic rise of the electron
dE/dx separates the electron dE/dx from those of hadrons except at the
crossovers with pions at a momentum of \~ 0.2 GeV/c, kaons at \~ 0.6
GeV/c, protons at \~ 1.1 GeV/c and deuterons at \~ 1.5 GeV/c. A
time-of-flight measurement, using a requirement that \|1-β\| \< 0.03,
eliminates slow hadrons and cleans up the crossovers. This results in
clean electron identification, as shown in Figure 11. The top plot on
the left shows the conventional dE/dx measurement in the TPC gas as a
function of particle momentum. The bottom plot on the left shows the
same dE/dx measurement but now requiring \|1-β\| \< 0.03 from the TOF
detector. The upper electron band is clearly separated from the lower
pion band. Heavier particles (e.g. kaons and protons) are completely
removed by the TOF requirement. The right hand plot of Figure 11 shows a
dE/dx projection in the transverse momentum range p~T~ = 1.0-1.5 GeV/c.
Even at this rather large momentum, the distribution of electrons is
clearly separated from the distribution of charged hadrons.

![](media/image12.jpeg){width="2.7944444444444443in"
height="2.7180555555555554in"}
![](media/image13.jpeg){width="2.9298611111111112in"
height="2.7055555555555557in"}

[]{#__RefHeading___Toc137569171 .anchor}Figure 11: Left top: TPC dE/dx
vs. p in d + Au collisions. Left bottom: TPC dE/dx vs. p after TOFr PID
selection of \| 1-β \| \< 0.03. Clean electron identification is
achieved. Right: dE/dx from the TPC after TOFr PID selection for 1.0 \<
p \< 1.5 GeV/c.

These electrons are, to a large extent, from photons converting into
electron-positron pairs γ → e^+ ^e^-^ in the detector material. The HFT
detector will reduce the background of electrons and positrons from
these γ conversions. By requiring hits in the HFT, electrons from photon
conversion outside the HFT, i.e. the SSD, and TPC inner field cage, are
rejected. Figure 12 shows the resulting yield from pure Monte Carlo
simulations (no tracking involved) of conversion electrons when
requiring hits in the TPC only (open circles), hits in the TPC and hits
in the TPC+SSD+HFT (closed circles). The requirement of hits in the HFT
reduces the detected yield of conversion electrons by a factor of 50.
Full simulations with appropriate pile-up events show that the rate of
random ghost tracks is 5%. This means that we can achieve a maximum γ
rejection by a factor of 20. To estimate the signal-to-background ratio
in the vector meson measurements, we take a reasonably conservative
approach and assume that the HFT can reject γ conversions by a factor of
10. Another source of electron background comes from the semi-leptonic
decays of heavy quark hadrons. The dominant source at intermediate
mass[^74]^,^[^75] is from the semi-leptonic decay of open charm. Due to
the large charm yield at RHIC energies, the number of e^+^e^-^ pairs
from charm-anti-charm decays is comparable to that from γ conversion and
π and η Dalitz decays after the HFT rejection factor, described above,
is applied. Due to the lifetime of \~ 100 μm of heavy-flavor hadrons,
this charm semi-leptonic decay background can be greatly reduced by
measuring the displaced decay vertex with high resolution as can be
provided by the HFT detector. Detailed simulations show that HFT is
capable of rejecting 75% of e^+^e^-^ pairs from the D^0^ decay while
preserving 50% of the primordial e^+^e^-^ pairs. This increases the
signal-to-background ratio by a factor of 2.

![](media/image14.png){width="3.592361111111111in"
height="3.5305555555555554in"}

[]{#_Ref86316844 .anchor}Figure 12: Electron p~T~ spectra from γ
conversions reconstructed by requiring TPC tracking alone, a tack in the
TPC and 2 HFT hits. The rejection factor is about 20.

The large reduction in photonic-electron background will enable us to
observe electromagnetic decays of short-lived vector meson, e.g. φ, ω →
*e*^+^*e*^−^, and intermediate mass dileptons with a few hundred
thousand central Au + Au events in STAR. The rejection of π^0^ and η
Dalitz decays by a factor of 3 (single track) can be achieved by
measuring both electrons, of a pair, which is possible because of the
large acceptance of the STAR TPC.

Figure 13 shows the di-electron invariant mass distribution for various
signals plus the background. The signals for vector mesons and thermal
radiation, are shown as a black curveError: Reference source not found
and have been calculated using the full acceptance of the STAR detector.
The red curve, at the top, is the total di-electron invariant mass yield
using the STAR configuration in 2004. This is obtained from single
inclusive electron spectrum measurements in 200 Au + Au collisions with
the assumption of electron PID from Full TOF coverage. The gray curve is
the e^+^e^-^ pair invariant mass distribution for charm after a cut on
the HFT DCA \< 80 μm. The dot dashed line show the Dalitz decays from
π^0^ and η after rejection from the TPC. The standard method of dealing
with the residual background is by a mixed-event method. This method has
been used in CERES and NA52/NA60 at the SPS, and will be used in both
PHENIX and STAR.

![](media/image15.jpeg){width="5.16875in" height="3.5in"}

[]{#_Ref118890711 .anchor}Figure 13: Di-electron invariant mass
spectrum. The solid black curve is a prediction\
from R. Ralf Ref. \[Error: Reference source not found\] using the full
STAR acceptance. The red curve at the top is the total di-electron
invariant mass spectrum seen with the 2004 configuration of STAR but
assuming electron PID from Full TOF coverage. The gray curve is the
charm e+e- pair invariant mass distribution after a cut on the HFT DCA
\< 80um. The dot dashed line is the Dalitz decays from π0 and η after
rejection from TPC.

Our simulations show that we can extract a 3 σ vector meson signal from
the data with only 800 K events in the underlying spectrum. If we had to
do this without the HFT, then the same 3σ signal would require 8 M
events in the underlying spectrum. The inclusion of the SSD and HFT
increases the available statistics by a factor of 10. Table 3 shows an
estimate of the number of vector meson pairs which can be observed in
one RHIC year.

  --------------------------- --------- ------- --------------------------
  Detectors                   ω         φ       Comments

  TPC+TOF+SSD+HFT             22 K      6 K     Pairs per RHIC year
  --------------------------- --------- ------- --------------------------

[]{#__RefHeading___Toc137569229 .anchor}Table 3: The number of vector
meson pairs which can be recorded by STAR in one RHIC year. We assume
200 M central Au *+* Au events will be recorded per RHIC year when DAQ
1000 is operational.

For these simulations we assumed a 50% efficiency for matching tracks
between the TPC and the HFT, and we assumed a 50% efficiency for TOF +
TPC tracking and PID. An online trigger was not employed. The results
shown in Table 3 results are comparable to the NA60 In + In run results.

## Spin Measurements

The European Muon CollaborationError: Reference source not found
discovered that the spin of the quarks only contributes a small fraction
to the total spin of the proton and this has motivated, and continues to
motivate, considerable experimental and theoretical activity to further
unravel the mystery of the nucleon spin structure. At present, only the
total quark spin contribution is known with reasonably good precision.
The most prominent unknown is the spin-dependent gluon density, ∆G.

The main goal of the RHIC spin program[^76] is to precisely determine ∆G
by measuring double spin asymmetries in longitudinally polarized
proton-proton collisions for a series of complementary reaction channels
and over a broad range of the gluon momentum fraction x~g~ and
transverse momenta p~T~. One of the promising reaction channels in STAR
is the production of prompt photons and jets with high transverse
momenta.

The production of charm and bottom quarks proceeds, in leading order in
the strong coupling constant α~s~, through gluon-gluon fusion and quark
anti-quark annihilation. The gluon-gluon fusion process is known to be
dominant in the spin averaged case for all experimentally relevant
kinematic regions. In the spin dependent case both processes have large
analyzing powers, as measured by the parton-parton spin asymmetry a~LL~,
which approaches -1 for asymptotically large transverse momenta. The
masses of the heavy quarks lead to large modifications of the
parton-parton spin asymmetry in the kinematic region accessible with
STAR[^77]. Next-to-leading order corrections are available and predict
relatively sizable changes in the observed asymmetry as compared to
leading order expectations[^78].

These properties make charm and bottom production a good test to advance
the current understanding of the nucleon spin (unless ∆G is found to be
vanishingly small).

As is the case for the study of heavy quark energy loss described in
this proposal, the relative smallness of the production cross section at
large transverse momenta requires triggering with the STAR
electromagnetic calorimeters. In spin measurements, the trigger serves
to relate the event to the RHIC beam bunch crossing, which determines
the spin orientations of the colliding protons. Commonly used
parameterizations of the deep-inelastic scattering data[^79] lead us to
expect measurable, and negative, double longitudinal spin asymmetries at
the level of --5x10^-3^ in heavy flavor production in STAR. These
asymmetries originate from gluon polarization at intermediate momentum
fractions x~g~ and are expected to grow in size with increasing p~T~ of
the decay electron or positron.

In order to make these measurements, fast external pointing with
high-rate capability is required to distinguish collisions from
different bunch crossings and, hence, different spin orientations. The
HFT is an ideal component in this program, however, a high-rate
intermediate tracker (IST) is also required.

# HFT Overview and Simulation

## STAR and the HFT detector

STAR[^80] is one of the two large detectors currently operating at RHIC.
It was designed to make measurements of hadron production over a large
solid angle, and it features detector systems for high precision
tracking, momentum analysis and particle identification (see Figure 14).
It is the only experiment at RHIC which measures the full azimuth in φ
out to ±1.5 units in rapidity, and tracks particles from 100 MeV/*c* to
20 GeV/*c*, so it is well suited for event-by-event characterizations of
heavy ion collisions and for the detection of hadron jets.

By adding the Heavy Flavor Tracker (HFT) to STAR, we will be able to
measure displaced vertices that are displaced 100 microns, or less, from
the primary vertex and thereby identify neutral and charged particles
with very short lifetimes can be distinguished from the primary
particles which originate at the collision vertex. The addition of the
HFT will extend STAR's unique capabilities even further by providing
particle identification for hadrons containing charm and beauty and
electrons decaying from charm and beauty hadrons. Thus, the HFT is the
enabling technology for making direct charm and beauty measurements at
STAR.

![](media/image16.jpeg){width="5.9527777777777775in"
height="3.623611111111111in"}

[]{#__RefHeading___Toc137569174 .anchor}Figure 14: The STAR detector at
RHIC. The primary detector elements at mid-rapidity are shown in this
cut-away view of the detector. For scale, the TPC is 4.2 meters long and
4 meters in diameter. Heavy Ion beams enter from the left and right
while collisions take place in the center of the detector.

STAR currently has several tracking detectors at midrapidity; we will
focus on the Silicon Strip Detector (SSD) and the Time Projection
Chamber (TPC). The SSD is installed at a radius of 23 cm, while the TPC
tracking begins at a radius of approximately 60 cm. The TPC tracking
system extends out to a radius of 190 cm and yields a total of 45
position measurements along the track of a particle. At the time of HFT
installation, a complete Time of Flight detector will surround the TPC
barrel (φ = 2π). This detector will replace the Central Trigger Barrel
(CTB), which is a simple array of scintillators without TOF information.
The new TOF detector will surround the outer field cage of the TPC and
will extend the PID region where kaons, pions, and protons can be
separated. Also by the time the HFT is installed in STAR, the existing
Silicon Vertex Detector (SVT) will be decommissioned and it will be
replaced by a new Intermediate Silicon Tracker (IST) that is capable of
running at the highest data acquisition rates envisioned for STAR. But
before the IST is ready, there will be an opportunity to do significant
R&D with a prototype HFT detector and so the simulations presented in
this chapter (unless specified) use only the TPC and the SSD and do not
utilize an intermediate tracking device at radii less than 23 cm.

Tracking without an intermediate tracking device, such as the SVT, is
not as severe a problem as it may sound. The purpose of the intermediate
tracking device is to increase the tracking precision at the outer layer
of the HFT, in order to increase the probability of matching the correct
HFT hits. It turns out that a tight vertex constraint also enables us to
do the track matching with high efficiency. All of our simulations in
this chapter are done using a 100 μm vertex constraint instead of an
intermediate tracker. A vertex resolution of 100 μm is easily achieved
in a high multiplicity heavy ion collision at RHIC ( \> 90 tracks per
event). When the IST becomes available in the future, a vertex
constraint will not be necessary and the physics results presented here
will only get better; especially for low multiplicity events such as p-p
and peripheral collisions. Thus we strongly encourage the collaboration
to add a high resolution pointing device between the HFT and the SSD,
however, there is a very exciting baseline physics program that we can
accomplish with the HFT detector operating in stand-alone mode and so
this baseline program is what we will discuss in the remainder of this
chapter.

At the present time, the STAR detector has modest capabilities to
measure secondary decay vertices that are displaced from the primary
vertex. To make these measurements, we use the TPC and the SSD to
determine the position of the primary vertex to a precision of
approximately 100 μm in the central collisions. Thus, by identifying
decay vertices that are displaced from the primary vertex, we can
identify secondary decays of neutral and charged particles.

For example, is possible to use this technique to identify the *K~S~*^0^
because a *K~S~*^0^ decays into two pions with a mean lifetime of 0.896
× 10^-10^ seconds and it has a characteristic decay distance of 2.68 cm.
Since its decay distance is so much greater than the STAR vertex
resolution, this particle can easily be identified. Similarly, strange
particle decays such as the Ω baryon can also be identified.

By improving the single track pointing accuracy of STAR by a factor of
50, the HFT allows us to measure very short-lived particles. The HFT
will be a low mass detector, and it will be located very close to the
beam pipe. It will have two layers of silicon pixel detectors; one at
1.5 cm average radius and the other at 5.0 cm average radius. Each layer
will be composed of 30 μm × 30 μm pixels and the pointing accuracy of
the system (including multiple coulomb scattering in the beam pipe and
Si layers) will be better than 60 μm for 1 GeV/c tracks.

## The Design of the HFT

The HFT is shown schematically in Figure 15 and its parameters are
summarized in Table 4. The innermost layer of the detector lies at a
mean radius of 1.5 cm radius and it is composed of 6 detector ladders.
The outer layer lies at a mean radius of 5.0 cm and it is composed of 18
ladders. Each ladder contains a row of 10 monolithic CMOS detector chips
and each ladder has an active area of 19.2 cm × 1.92 cm. Each CMOS
detector chip contains a 640 × 640 array of 30 μm square pixels and is
thinned down to a thickness of 50 μm to minimize multiple coulomb
scattering (MCS) in the detector. The effective thickness of each ladder
is 0.28% of a radiation length. For the purpose of Monte Carlo
simulations, however, we have used an effective thickness of 0.38%, per
layer, to allow for the possibility of adding an onboard readout chip.

The HFT will achieve maximum vertex resolution by lying as close as
possible to the interaction point without residing inside the RHIC beam
pipe. To bring the detector to 1.5 cm radius will require a new,
smaller, beam pipe and this also gives us the opportunity to make the
beam pipe thinner (0.5 mm). The new beam pipe, which will reduce the MCS
scattering in front of the HFT, is an essential part of the proposal and
its design will be discussed in section 6.6.

![](media/image17.jpeg){width="2.65625in"
height="2.6013888888888888in"}![](media/image18.jpeg){width="3.0770833333333334in"
height="2.6506944444444445in"}

[]{#_Ref124519579 .anchor}**Figure 15: Left - The HFT is shown embedded
inside the SSD. Right - Schematic view of the HFT detector ladder
arrangement with 6 ladders forming the inner layer and 18 ladders
forming the outer layer. The width of the ladders (the dimension seen in
this view) is 19 mm. The shingled layers provide cross calibration of
the ladders and provide an open geometry for ease of installation.**

  ---------------------------------------- ------------------------------
  Number of pixels                         98,304,000

  Pixel dimension                          30 μm × 30 μm

  Detector Chip active area                19.2 mm × 19.2 mm

  Detector Chip pixel array                640 × 640

  Number of ladders                        24

  Ladder active area                       192 mm × 19.2 mm

  Number of barrels                        2

  Inner barrel (6 ladders)                 r = 1.5 cm

  Outer barrel (18 ladders)                r = 5.0 cm

  Frame read time                          4 ms

  Pixel read rate, after zero suppression  63 MHz

  Ladder (w/Al cable) % X~0~               0.28% (0.38%)

  Beam Pipe Thickness                      0.5 mm or 0.14% X~0~
  ---------------------------------------- ------------------------------

[]{#__RefHeading___Toc137569230 .anchor}Table 4: Selected HFT parameters
for the detector we propose to build. The simulated parameters are the
same except that we have assumed that the detector is 0.38% radiation
lengths thick, per layer, in order to allow for the possibility of an
onboard readout chip.

## Monte Carlo Simulations

The STAR experiment makes use of an extensive simulation framework to
accurately predict the performance of the physical detector. This
framework involves many software packages working in tandem to produce
the final results. We will give a general overview of the simulation
chain in order to place the results and predictions contained in this
document in a broader context.

The philosophy of the simulation is to use as much standard STAR
software as possible. For instance, we use the standard GSTAR simulation
package with modifications for the new beam pipe design and the
introduction of the HFT. For tracking, we use the new STAR tracking
software package ITTF. During the process of writing this proposal, we
needed a stable platform, so we froze the ITTF software on February 5,
2005. To that package, we added tracking in the HFT layers. Since ITTF
was never optimized for the high precision requirements of the HFT, we
have tuned it for this new configuration and added new support software,
as required. We will indicate where this new software is used.

The simulation framework has three stages:

1.  The simulation of the physics under study: for example a Au-Au
    interaction at √s = 200 GeV.

2.  The simulation of produced particles as they interact with the
    detector and physical structures of the experiment; and finally

3.  The simulation and digitization of signals in the detector.

The goal of the simulation software package is to use the same analysis
methods on both the simulated and real data, to predict performance and
optimize analysis methods.

### Physics Simulation

The simulations presented in this chapter separate the charmed mesons
and baryons from the rest of the event. This has the benefit of greatly
increased speed for the analysis of events and for the development of
algorithms. This method has been used previously by the ALICE
collaboration[^81]. For charm production, events with one 'signal'
particle were simulated independently from the background. The
background for these studies were simulated as a cocktail of pion, kaon,
and protons at the maximum multiplicity expected for central collisions,
and produced with the MEVSIM package developed for use in STAR[^82]. The
'signal' and 'background' events are analyzed using the same
reconstruction and analysis code, with the same cuts and with the same
tracking parameters. (For further details see section 3.7, Table 9, and
Table 10).

The physical interaction between daughter particles and the material of
the detector, as well as signal digitization, are simulated using the
STAR implementation of the GEANT simulation package[^83] This package is
used in STAR and is a standard analysis tool which includes a detailed
understanding of the TPC response function; including dead areas and
realistic detector resolutions and responses.

For both the SSD and HFT detectors, the exact crossing point of the
particle through the detector is reported by GEANT to the track
reconstruction package. Realistic detector resolutions are used to smear
the perfect position information, and the resulting simulated hits are
used in tracking. For the SSD, the hits were smeared by 20 µm in φ and
750 µm in z, in agreement with the SSD specifications[^84]. For the HFT,
the hits were smeared by 6 µm in both φ and z. We ignore the effects of
cluster overlap because we expect these effects to be minimal in the
case of the HFT as the occupancy, even in a central Au + Au event, is
relatively low.

For these simulations, the event reconstruction algorithms traditionally
used in STAR were modified to optimize the HFT hit finding accuracy and
vertex reconstruction performance. These modifications and performance
improvements are detailed in the next section.

### Reconstruction

Reconstruction of the simulated events is handled by the usual framework
implemented in STAR to unite the various tracking detectors. We use this
package, along with a detailed GEANT simulation, to provide an estimate
of the expected performance of the HFT in the full suite of STAR
detectors.

As mentioned the introduction to this chapter, we use a vertex
constraint in conjunction with the TPC and SSD detectors to provide
efficient reconstruction and track matching purity. In this algorithm,
the STAR vertex finder package uses tracks defined with only the TPC and
SSD to determine a primary vertex. If no vertex is found for an event,
the event is lost. Since the 'signal' events contain one signal particle
per event, the Monte Carlo vertex is used, but smeared with a resolution
typical for events of these centralities (100μm).

With the current STAR algorithm, the tracker follows a series of hits in
the TPC and SSD, refitting after every new hit is added. These tracks
(called "global" tracks) are then passed to the vertex finder, which
reconstructs the event vertex. The global tracks are extended to the
resulting reconstructed vertex, and, if the fit is successful, the new
track is stored and labeled a primary track.

The new algorithm simply takes these primary tracks and refits the
track, this time with the HFT hits available. HFT hits that meet the
tracking selection criteria are added. The Kalman fitter updates the
track fit as the new HFT hits are added. The tracks with added HFT hits
are then stored See Figure 16 for a block diagram of this algorithm. The
success of the algorithm depends on the precision of finding the primary
vertex, which is centrality dependent.

![](media/image19.png){width="5.71875in" height="3.501388888888889in"}

[]{#_Ref124506772 .anchor}Figure 16: Simulation block flow diagram. Only
small modifications were needed to optimize the STAR reconstruction
software for use with the HFT.

The dramatic improvement of the track fit at this stage allows a more
accurate primary vertex fit. A simplified vertex algorithm was adopted
for this refit. The distribution of the distance of closest approach for
each HFT track to the TPC + SSD primary vertex shows a distinct offset
from zero, as well as a non-gaussian shape (see Figure 17). The offset
(calculated in three dimensions) is used as a correction to the primary
vertex position. We found that iterating on this procedure produced the
best resolution (see Figure 18). A final vertex position resolution of 8
µm was achieved for the highest multiplicity events, while in the most
general case the vertex resolution follows the functional form:

$$\sigma = \frac{\text{380}\mu m}{\sqrt{N_{\text{ch}}}}$$

where N~ch~ is the multiplicity of charged particles in the detector
acceptance. The vertex resolution functional form follows Poisson
statistics, as expected. This functional form is drawn as a line to
guide the eye in Figure 19. The highlighted region in Figure 19
demarcates the centrality region of TPC + SSD vertex resolution that is
useful and allows the tracking code to achieve the necessary hit purity
in the HFT. The highlighted region corresponds to 0%-60% centrality in
Au-Au collisions or, equivalently, to a multiplicity of N~ch~ \> 90.

![](media/image20.jpeg){width="5.997222222222222in"
height="2.9256944444444444in"}

[]{#__RefHeading___Toc137569177 .anchor}Figure 17: DCA of tracks to the
Primary vertex. The geometric signed distance of closest approach
between all tracks (p~T~ \> 200MeV/c) and the vertex is shown for two
cases: a) DCA to the vertex defined by TPC+SSD tracks. The non-zero mean
of this distribution is used to correct the vertex coordinate. b)  DCA
distribution after iterative correction using TPC+SSD+HFT tracks, with a
significant improvement in the mean and shape of the distribution.

![](media/image21.jpeg){width="5.031944444444444in"
height="3.5006944444444446in"}

[]{#__RefHeading___Toc137569178 .anchor}Figure 18: Mean and sigma of the
geometric -- signed DCA distribution as a function of the number of
iterations used to correct the vertex. With each iteration, the
previously defined vertex is corrected with the mean of the distribution
of the track dca's. The first iteration uses the TPC+SSD defined vertex
as a vertex seed.

![](media/image22.png){width="5.995833333333334in"
height="3.1326388888888888in"}

[]{#__RefHeading___Toc137569179 .anchor}Figure 19: Vertex resolution
σ(MC Vertex Position -- Reconstructed Vertex position) as a function of
collision centrality. The HFT refit vertex shows an order of magnitude
improvement over the previously reconstructed primary vertex. a) Vertex
resolution in X vs. N~ch~ . b) Vertex resolution along the beam
direction. Problems in the vertex fit in the z direction drive the
distribution wider than in the transverse direction. The highlighted
area demarcates the centrality region with minimum (or better)
acceptable TPC+SSD vertex resolution. This corresponds to 0%-60% most
central events.

Application of a vertex constraint before adding detector hits is a new
tool for the STAR software chain, and this technique requires minor
modifications of the STAR reconstruction code. The performance of this
algorithm has been studied in terms of the efficiency and the expected
rate of false hit associations.

The efficiency of the reconstruction package to find and correctly
associate HFT hits is defined as:

![](media/image23.wmf)

where the reconstructed tracks are matched to their corresponding Monte
Carlo track at the hit level. Tracks that do not match, or which only
partially match, to a Monte Carlo track , do not contribute to the
efficiency. Hit matching requirements are set to 33% in the TPC (15
hits), 100% in the SSD (1 hit) and 100% in the HFT (2 hits). Tracks with
one or more non-matched HFT hit are designated as 'ghost' tracks. The
measured efficiency as a function of p~T~ is \~50% for p~T~ \> 1 GeV/c
(see Figure 20) and the level of ghosting is 14% over the same range.

![](media/image24.jpeg){width="5.999305555555556in"
height="3.560416666666667in"}

[]{#_Ref124524795 .anchor}Figure 20: The efficiency for finding tracks
in central Au + Au collisions in the STAR TPC, SSD, and the HFT.
Accepted tracks have 15 TPC hits (or more), 1 SSD hit, and 2 HFT hits
that match to a single Monte Carlo track. The quoted efficiency is for
pions in the detectors acceptance; \|η\| \< 0.5

### Primary Track Reconstruction Performance

The performance of the HFT was simulated using tracks from central Au +
Au collisions at √s~NN~ = 200 GeV which were generated by the "MEVSIM"
code and simulated using single particle tracks. The simulated tracks
and events were processed through GEANT and, finally, a detailed
response function for the Time Projection chamber (TPC) and a response
function for the Silicon Drift Detector (SSD) were applied to the data.
These response functions take into account physical effects in the
detector and the subsequent electronics. For the HFT, we smeared the
Monte Carlo hits by a Gaussian function with a width of σ=6 μm in the
x-y-plane and in the z-direction. The 4 ms read-out speed of the 1^st^
generation HFT will lead to pile-up of 120 Au + Au collisions in the HFT
at four times the RHIC design luminosity. We account for this by
randomly uploading hits on top of the Monte Carlo generated hits in the
HFT. The topic of pile-up is discussed in detail in section 3.7

Tracks with at least 15 hits in the TPC, 1 hit in the SSD and 2 hits in
the HFT detector were reconstructed with the STAR inner-tracker code.
Figure 20 shows the efficiency for tracking pions with \|*y*\| \< 0.5 in
various detector configurations. The TPC results are designated by the
open triangles, TPC + SSD by squares, and the TPC + SSD + HFT detector
by filled dots (red). Note that the TPC tracking efficiency limits the
efficiency of the SSD at low transverse momentum and, in addition, the
efficiency of the HFT at low momentum is affected by ghost tracks and
event pileup. But at high momentum, the acceptance of the detectors is
quite uniform. Of the TPC and SSD tracks above 700 MeV/c traced to the
HFT detector volume, about 70% have correctly matched HFT hits. The
absolute efficiency for STAR tracking with the HFT (using the TPC, SSD
and HFT) is 50% or above at a transverse momentum larger than 700 MeV/c.
Simulations using mono-energetic pions show that the efficiency stays
approximately constant up to 20 GeV/c.

In the high multiplicity environment of a central Au + Au collision,
there can be ambiguities in finding the next hit on a track while
following a track along its path. A ghost track is defined to be a
reconstructed track with at least one hit from a different Monte Carlo
track. With the present tracking algorithm, the number of ghost tracks
in the TPC is less than 1% compared to all reconstructed tracks. The HFT
is more restrictive, however, because it is continuously read out;
events will pile up between successive reads of the Si chip, which occur
every 4 ms.

Figure 21 shows the number of ghost tracks observed in the full HFT
tracking system. At low momentum, ghost tracks dominate due to multiple
Coulomb scattering. The bulk of the ghost tracks appear below 0.5
GeV/*c* and the effect of these miss-identified tracks can be seen in
Figure 20 because the tracking efficiency for the HFT drops steeply
below 0.5 GeV/*c*. The ratio of ghost tracks to good tracks is 20% at
0.5 GeV/*c* and saturates at 14% above 0.7 GeV/*c* (see the right hand
panel of Figure 21).

![](media/image25.jpeg){width="5.996527777777778in"
height="2.1277777777777778in"}

[]{#__RefHeading___Toc137569181 .anchor}Figure 21: Reconstructed tracks
with false HFT hits. Left -- The number of ghost tracks and
reconstructed tracks passing through the pixels of the HFT as a function
of p~T~. Right - The ratio of ghost tracks to good tracks. The increased
ghosting rate at lower transverse momentum is a direct consequence of
multiple Coulomb scattering, and is the motivating factor to make the
detector as thin as possible.

### Analytic Resolution Calculations and Expected Performance

To verify our detailed STAR software simulation, we have made several
cross checks. One method is to do a simple analytic calculation of
efficiency and ghosting versus the transverse momentum.

The probability to pick up the correct hit can then be described by the
following equation:

$$\int_{0}^{a}\frac{r}{\sigma^{2}}e^{- r^{2}\left( \text{πρ} + \frac{1}{2\sigma^{2}} \right)}\text{dr}$$

The probability to pick up the wrong hit is described by the equation:

$\int_{0}^{a}\frac{r}{\sigma^{2}}e^{- \frac{r^{2}}{2\sigma^{2}}}\left( 1 - e^{- {\pi r}^{2}\rho} \right)\text{dr}$

where σ is the uncertainty in the track extrapolation from the outer
detectors to the target detector, ρ is the background hits density, and
'a' (the upper limit of the integral) is the search cone radius. The
analytic calculation of the quantity σ, includes contributions from
multiple Coulomb scattering[^85], the detector resolution, and the
uncertainty in the position of the track due to the quality of the fit
given by the tracking software.[^86]. The quantity ρ includes
contributions from the pile-up, other tracks in the current event, and
ultra-peripheral collision (UPC) electrons.

We discovered that track fitting errors in the standard STAR software
are the dominant term in the calculation of σ while the detector
resolution drops out due to the extremely high resolution of the
detectors. So in the discussion presented here, we have set σ equal to
the search radius used by the tracking software at each detector layer.
This suggests that the tracking software can be improved and future
simulations may someday be better than what we present here.

Figure 23 shows the efficiency and ghost rate results using these
equations. The vertex, SSD, HFT, and TPC search radii are assumed to be
300 um, 60 um, 90 um, and 1.5 mm, respectively. There is excellent
agreement in sub-panels (a), (b), (c) and (d) between efficiency and
ghost rate; the analytic calculations match the characteristics and
shape of the distributions found with the full Monte Carlo simulations.
The larger the efficiency, the smaller the ghost rate. The analytic
calculations can reproduce the shape of the distribution as a function
of p~T.~ At high p~T~, the efficiency is limited by the detector
resolution. The efficiency decreases quickly below 1 GeV/c due to
multiple Coulomb scattering, which causes a large rise in the track
projection errors at low p~T~. The efficiency decreases consistently
with an increase of the luminosity.

![](media/image26.png){width="5.763194444444444in"
height="4.007638888888889in"}

[]{#__RefHeading___Toc137569182 .anchor}Figure 22: Efficiency (a) and
ghost rate (b) of the HFT, using an analytic calculations. The
luminosity dependence is given in (c) and (d) for 1-4 times the current
peak luminosity. Finite and infinite search windows are represented by
solid- and dashed- line, respectively. The efficiency and ghost rate for
each layer of the HFT are calculated separately and combined into that
for full HFT. The vertex, SSD, HFT, and TPC search radii are assumed to
be 300 um, 60 um, 90 um, and 1.5 mm, respectively. At nominal RHIC
luminosity 1×10^27^/cm^2^s, the density of background hits (hits/cm^2^)
are summarized in Table 10. It will be necessary to upgrade the sensors
for the full RHICII luminosities.

## Open Charm Reconstruction Simulation

As previously discussed, particles containing charm and beauty quarks
are the probes relevant for the main thrust of our physics objectives.
Charm and beauty quarks occur in a wide variety of hadrons, and these
hadrons decay into a large number of different channels. To demonstrate
the power of the HFT, we have simulated several specific charm meson
decay channels, including D^0^ → K^−^ π^+^ and D~s~^+^ → K^−^ π^+^ K^+^,
because of their large relative branching ratios. Table 5 displays some
of the properties of these channels.

+---------+-------------------+-------------------+-------------------+
| **par   | **Daughters**     | **Cτ (μm)**       | **Mass (GeV)**    |
| ticle** |                   |                   |                   |
+---------+-------------------+-------------------+-------------------+
| *       | K^-^π^+^ (3.8%)   | 123               | 1.8646            |
| *D^0^** |                   |                   |                   |
+---------+-------------------+-------------------+-------------------+
| *       | K^-^π^+^π^+^      | 312               | 1.8694            |
| *D^±^** | (9.2%)            |                   |                   |
+---------+-------------------+-------------------+-------------------+
| *       | K^+^K^-^π^+^      | 147               | 1.9683            |
| *D~s~** | (4.4%)            |                   |                   |
|         |                   |                   |                   |
|         | π^+^ π^+^ π^-^    |                   |                   |
|         | (1.0%)            |                   |                   |
+---------+-------------------+-------------------+-------------------+
| *       | p K^-^π^+^ (5.0%) | 59.9              | 2.2849            |
| *Λ~c~** |                   |                   |                   |
+---------+-------------------+-------------------+-------------------+

[]{#__RefHeading___Toc137569231 .anchor}Table 5: Open charm hadron
properties

Signal and background events are generated separately. The signal
consists of one D^0^ or D~s~^+^ per event. The transverse momentum
distribution of the charmed hadrons follows a Boltzman distribution
which reproduces the \<*p~T~*\> of D-mesons as measured by STAR in d +
Au collisions at √s~NN~ = 200 GeV and the rapidity distribution
suggested by perturbative QCD calculations applying the program code
PythiaError: Reference source not found. The background is simulated
using the MevSim event generator parameterized to reproduce the
experimentally measured particle multiplicities in Au + Au collisions at
√s~NN~ = 200 GeV. Our parameterization is accurate for particles below 3
GeV/c. It may underestimate the background above this momentum. The
distributions of reconstructed D-meson signal and background were scaled
to match the expected D-meson production per central Au + Au
collisionError: Reference source not found. Also, the higher track
reconstruction efficiency in single (D^0^ signal) events compared to
central Au +Au collisions (background) was taken into account.

Analysis of the reconstructed tracks to identify the 'signal' particle
is done separately for the signal and background events. We use a direct
reconstruction approach, where the daughter particles are identified as
originating from a vertex displaced from the primary vertex. The
invariant mass of the parent particle is then calculated from the
kinematics of the daughters. This technique is superior to techniques
depending on calculating all track combinations in an event both in
signal to noise and analysis speed. The distributions from the analysis
of signal and background events are then combined for the finished
invariant mass distributions.

The heavy flavor tracker is designed to allow us to directly reconstruct
mesons containing charm quarks. If this is true, we should see
differences between the charm meson daughter tracks and the background
and the primary tracks in several important variables. The distance of
closest approach between the tracks and the vertex is an important
example of this; see Figure 23. The distributions of reconstructed D^0^
and D^+^ daughters is compared to the primary track background. The
charm meson tracks clearly have a broader distribution, driven by the
decay length of the charm mesons. Cutting on the track DCA, then, will
improve the signal to noise in the analyses.

![](media/image27.png){width="4.401388888888889in"
height="3.498611111111111in"}

[]{#__RefHeading___Toc137569183 .anchor}Figure 23: The distance of
closest approach (dca) distributions for D^0^ (triangles) and D^+^
(circles). The results of the dca distribution for primary tracks are
shown in squares.

### D^0^ reconstruction

The HFT detector allows us to identify a D^0^ decay-vertex by
reconstructing the trajectory of its two daughters. Figure 24 shows the
topology of the decay.

The following selection criteria were used to separate the D^0^ signal
from background:

-   The decay length *l*; with *l* the distance between the primary
    vertex and the D-meson vertex

-   The difference, ∆*m,* between the reconstructed invariant mass and
    the D^0^ rest mass

-   The distance of closest approach DCA~πK~, between the two daughter
    tracks

-   Isolation cuts on cos(θ), with θ **being the angle between the D^0^
    momentum (vector sum of the two daughter momenta) and the vector
    joining the primary vertex to the D-meson decay vertex

-   Isolation cuts on cos(θ\*), with θ\* being the angle between the
    kaon in the D-meson center of mass frame and the D-meson momentum

![](media/image1.jpeg){width="5.0in" height="4.070138888888889in"}

[]{#__RefHeading___Toc137569184 .anchor}Figure 24: The decay topology
for a D^0^ decaying to a kaon and a pion. Isolation cuts to identify the
D^0^ from the background tracks are described in the text.

Charged decay daughters were identified by their specific energy loss in
the TPC gas and the time of flight measured in the TOF. For example, to
identify pions, we used a 3σ cut around the expected band for pions. The
width of this distribution is momentum dependent, and was extracted from
experimental data. With this method, pions can be separated from kaons
up to 700 MeV/c. The TOF detector extends this range up to 1.6 GeV/c, as
demonstrated from existing STAR data with a small version of the full
TOF detector. Tracks with a momentum above the range of these PID
methods can still be used, but they must be entered into the
reconstruction algorithm twice, once under a pion mass assumption, once
assuming a kaon mass.

The selection of proper cuts is very important in order to reduce the
background. The applied cuts for D^0^ reconstruction were determined by
using the program MINUIT. To achieve the best sensitivity, we selected
the cuts to optimize the signal (D^0^ yield, S) to noise (N) ratio =
S/√(S+N). See Table 6 for the values that were determined this way.
Comparisons of signal and background distributions for various cut
variables is shown in Figure 25 (and Figure 28 for the D^+^), along with
the final cut values. Figure 26 demonstrates that the significance, of
course, depends critically on the proper choice of cuts.

  ----------------------------------------------------- --------------------------- --------------------
  **Cuts**                                              **D^0^**                    **D^+^**

  nFit \>                                               15                          15

  \|η\| \<                                              1.0                         1.0

  HFT hits =                                            2                           2

  dca(global) ≥                                                                     100 μm

  dca(V~0~) ≤                                           35 μm                       100 μm

  decay length ≥                                        150 μm                      150 μm

  ![](media/image28.wmf){width="0.4861111111111111in"   0.996                       0.85
  height="0.18055555555555555in"} \>                                                

  ∆m ≤                                                  40 MeV                      
  ----------------------------------------------------- --------------------------- --------------------

[]{#__RefHeading___Toc137569232 .anchor}**Table 6: The cuts for the D0
and D+ reconstruction and efficiency analysis. The input D0 and D+
spectra followed an exponential distribution, \<pt\>=1.3 GeV/c.**

![](media/image29.png){width="5.129166666666666in"
height="4.999305555555556in"}

[]{#__RefHeading___Toc137569185 .anchor}Figure 25: Distribution of
quantities used to distinguish signal from background. Panels a1, a2,
b1, and b2 compare the difference between background events and D0
mesons for different variables. The yellow shaded area shows the regions
where we places cuts to enrich the D0 to background ratio. The lower
panels, c1, c2, and c2, show the pt distribution for signal and
background event tracks with a 100μm error vertex (c1), 20μm error
vertex (c2), and perfect vertex (c3). **Central Au + Au collisions are
assumed for the background primary tracks.**

![](media/image30.png){width="4.1097222222222225in" height="4.0in"}

[]{#__RefHeading___Toc137569186 .anchor}Figure 26: D^0^ invariant mass
with different cut sets. The power of the cuts to enhance the signal
relative to the background is clear.

![](media/image31.png){width="4.561111111111111in" height="3.3in"}

[]{#_Ref535399427 .anchor}Figure 27: The absolute yield of D^0^ into the
TPC + SSD + HFT divided by the input D^0^ yield. For comparison, a
simulation with thick Si detectors that are similar to the wafers used
in the ALICE vertex detector is also shown (1000 μm effective silicon
thickness).

Once we have made these cuts, we can study the reconstruction
efficiency. The overall efficiency takes into account acceptance, single
track efficiency and D^0^ reconstruction efficiency. We have studied the
D^0^ detection efficiency for three cases: knowledge of the exact
location of the vertex, a vertex that is known to 20 µm, and an ALICE
like detector. Figure 27 shows that the overall reconstruction
efficiency increases with increasing momentum and saturates above 3
GeV/c at a value of about 10%. The efficiency of a 1000 µm detector is
about an order of magnitude less.[^87]

Since the single track efficiency of the HFT is 50%, the maximum D^0^
reconstruction efficiency can be estimated to be 0.5 × 0.5 (tracks) =
25%, not taking into account the D^0^ acceptance effects. This
efficiency directly impacts the feasability of a D^0^ measurement for
topics such as charm flow. Full estimates of the expected rate for
detecting D^0^'s and the data needed to accomplish the flow measurement
are detailed in section 3.5 In the next section, we will describe a
related analysis; the reconstruction of the D^+^ meson.

### D^+^ reconstruction

The dca-distribution, cos(θ), and decay-length distribution for both
primary tracks (open-squares) and D+ decayed tracks are shown in Figure
28. Clearly the decayed tracks are well separated from the primary
track. For D^+^ reconstruction, a slightly different method was used
compared to that of the D^0^. The 'signal event' and 'background event'
were mixed together. The distribution of the invariant mass from a K and
two π tracks is then formed. The number of tracks used in the background
events is consistent with the top 10% central Au + Au collisions at
RHIC. The resulting invariant mass distributions for several p~T~ bins
are shown in Figure 29. For the p~T~ bins studied so far, the
significance S/√(S+B) is better than 3.

![](media/image32.png){width="6.039583333333334in"
height="2.8986111111111112in"}

[]{#__RefHeading___Toc137569188 .anchor}**Figure 28: The D^+^ decayed
tracks decay-length, dca, and cos(θ) distributions (triangles). The same
distributions for primary tracks are also shown (squares).**

![](media/image33.png){width="5.995833333333334in"
height="2.9916666666666667in"}

[]{#__RefHeading___Toc137569189 .anchor}Figure 29: Invariant mass
distributions for D^+^ for several p~T~ windows. The lines are a
polynomial (up to 2^nd^ order) + Gaussian fit.

The final reconstruction efficiencies for D^0^ and D^+^ mesons are shown
in Figure 30. Below p~T~ \~ 0.5 GeV/c for the Ds, the efficiency is very
low since the slow charged daughter particles do not make it into the
TPC. Above p~T~ \~ 1 GeV/c, both efficiencies increases as a function of
the transverse momentum. The large difference between D^0^ and D^+^
efficiencies is caused by the topological cuts used for the
reconstruction. Although the numbers are not yet fully optimized, these
efficiencies are already usable for data analysis.

![](media/image34.jpeg){width="4.079166666666667in"
height="3.6034722222222224in"}

[]{#_Ref535343174 .anchor}Figure 30: D-meson efficiencies as a function
of transverse momentum, together with fits. to the distributions. The
D^0^ efficiency approaches 10%, while the D^+^ efficiency approaches
.3%.

## Charm Elliptic Flow

As stated in Section 2, a measurement of charm elliptic flow is one of
the main goals of the HFT. The measurement is necessary to understand
the degree of thermalization in the partonic phase of a relativistic
collision. To study the sensitivity of the HFT for such a measurement,
we will focus on the capability of the HFT to measure the flow of D^0^
mesons. The simulated D^0^ transverse momentum distribution was
generated from 200 GeV p + p collisions in PythiaError: Reference source
not found. The shape follows a power-law, and the integrated yield is
fixed to the yield measured at mid-rapidity at RHICError: Reference
source not found, or dN/dy = 0.03 . The default value of dN/dy in Pythia
is about a factor of 3 lower.

For the 10% most-central and 0-80% minimum bias Au + Au collision
events, the number of binary collisions are 950 and 290,
respectively[^88]. The resulting distributions together with the
efficiencies described previously are shown in Figure 31. The final
measured particle yield versus p~T~ is also shown. Assuming 10%
statistical errors in each p~T~ bin, the number of events required can
be easily calculated, and the results are given in Table 7. At p~T~ \~
10 GeV/c, a good R~AA~ measurement requires about 7.5 billion p + p
events and 10 million Au + Au 10% central events. These numbers are
achievable with the planned RHIC luminosity^,^[^89], 60% duty factor,
and the current STAR detector system in a week of RHIC running.

![](media/image35.png){width="4.999305555555556in"
height="4.204861111111111in"}

[]{#_Ref124740173 .anchor}**Figure 31: D-meson rates estimates: (a)
dN/dp~T~ distributions for D-mesons. The integrated yield dN/dy = 0.03
as measured in p + p collisions at 200 GeV \[Ref. Error: Reference
source not found\]. The number of binary collisions (N~bin~ ) = 950,
corresponds to the top 10% most central Au + Au collisions at RHIC and
is used to scale the Au + Au collisions; (b) 3-σ significance D^0^
efficiency with TPC+SSD+HFT. The dashed-line is the fit to the
simulation results; (c) D^0^ meson rates from p + p and top 10% central
Au + Au collisions at 200 GeV.**

+-----------+------------+-----------+---------------+---------------+
| **p~T~    | **∆p~T~    | **\# of   | **\# of       | **\# of       |
| (GeV/c)** | (GeV/c)**  | Events**  | Events**      | Events**      |
|           |            |           |               |               |
|           |            | **(p +    | **0-10% Au +  | **0-80% Au +  |
|           |            | p)**      | Au**          | Au**          |
|           |            |           |               |               |
|           |            |           | (N~bin~ =     | (N~bin~ =     |
|           |            |           | 950)          | 290)          |
+-----------+------------+-----------+---------------+---------------+
| 1.0       | 0.5        | 44 ×      | 0.45 × 10^6^  | 1.75 × 10^6^  |
|           |            | 10^6^     |               |               |
+-----------+------------+-----------+---------------+---------------+
| 2.0       | 0.5        | 70 ×      | 0.45 × 10^6^  | 1.75 × 10^6^  |
|           |            | 10^6^     |               |               |
+-----------+------------+-----------+---------------+---------------+
| 3.5       | 1.0        | 70 ×      | 0.45 × 10^6^  | 1.75 × 10^6^  |
|           |            | 10^6^     |               |               |
+-----------+------------+-----------+---------------+---------------+
| 5.5       | 1.0        | 350 ×     | 0.75 × 10^6^  | 3 × 10^6^     |
|           |            | 10^6^     |               |               |
+-----------+------------+-----------+---------------+---------------+
| 7.5       | 1.0        | 1200 ×    | 3.5 × 10^6^   | 11 × 10^6^    |
|           |            | 10^6^     |               |               |
+-----------+------------+-----------+---------------+---------------+
| 10.5      | 1.5        | 7500 ×    | 9 × 10^6^     | 30 × 10^6^    |
|           |            | 10^6^     |               |               |
+-----------+------------+-----------+---------------+---------------+

[]{#__RefHeading___Toc137569233 .anchor}Table 7: An estimate of the
number of events required to achieve 10% statistical errors for the
spectra measurements of 200 GeV p + p collisions and 0-10%/ 0-80% Au +
Au collisions. A power-law shape p~T~ distribution from the p + p
collision has been assumed with the mid-rapidity dN/dy = 0.03 \[Ref.
Error: Reference source not found\]. The number of binary collisions is
from Glauber calculations \[Ref. Error: Reference source not found\].

In order to make a rate estimate relevant for the charm *v~2~*
measurement, we need to make further assumptions about the p~T~
dependence of the anisotropy. Models of D^0^ flow with and without charm
flowError: Reference source not found, represent the extreme cases and
thus the expected measurement range. The corresponding distributions are
shown in Figure 32. In the same figure, the K~0~ v~2~ is shown as a
line. With these assumptions and the known efficiency, the estimated
rates per event can be estimated. Assuming the desired statistical
uncertainties are less than 10%, the resulting required number of
minimum bias events for the D^0^ *v~2~* measurement is provided in Table
8 for various p~T~ bins. As one can see, with 500 million events, the
D^0^ v~2~ (p~T~) can be measured up to p~T~ \~ 5 GeV/c, using the
proposed STAR HFT.

+--------------+----------------+----------------+--------------------+
| **p~T~       | **∆p~T~        | **\# of        | **\# of Events**   |
| (GeV/c)**    | (GeV/c)**      | Events**       |                    |
|              |                |                | **q~c~ does not    |
|              |                | **q~c~ does    | flow**             |
|              |                | flow**         |                    |
+--------------+----------------+----------------+--------------------+
| 0.6          | 0.2            | 260 × 10^6^    | 525 × 10^6^        |
+--------------+----------------+----------------+--------------------+
| 1.0          | 0.5            | 70 × 10^6^     | 140 × 10^6^        |
+--------------+----------------+----------------+--------------------+
| 2.0          | 0.5            | 53 × 10^6^     | 125 × 10^6^        |
+--------------+----------------+----------------+--------------------+
| 3.0          | 1.0            | 105 × 10^6^    | 175 × 10^6^        |
+--------------+----------------+----------------+--------------------+
| 5.0          | 1.0            | 210 × 10^6^    | 440 × 10^6^        |
+--------------+----------------+----------------+--------------------+

[]{#__RefHeading___Toc137569234 .anchor}Table 8: An estimate of the
number events required for 10% statistical errors for D^0^ v2
measurements of 200 GeV 0-80% Au + Au collisions (N~bin~=290). A
power-law shape p~T~ distribution from the p + p collision has been
assumed with the mid-rapidity dN/dy = 0.03 \[Ref. Error: Reference
source not found\]. The number of binary collisions is from the Glaubor
calculations \[Ref. Error: Reference source not found\].

As mentioned earlier, the assumptions in the modelError: Reference
source not found are extreme limits. In order to test the thermodynamic
behavior of the D-meson, the most important region is p~T~ \< 3 GeV/c
because at higher p~T~, other dynamical effects will become important
(e.g. jet correlations). The proposed STAR HFT detector will be able to
make precise v~2~ measurements in the transverse momentum region 0.7 \<
p~T~ \< 3 GeV/c. As we have done with the hadrons from the light-flavor
sectorError: Reference source not found, the combined analysis of the
D-meson spectra, v~2~ distributions and ratios will allow us to
determine the charm quark collectivity and the thermodynamic nature of
the medium created in heavy ion collisions at RHIC.

![](media/image36.png){width="5.554166666666666in"
height="4.653472222222222in"}

[]{#_Ref124741045 .anchor}Figure 32: D-meson v~2~ rate estimates: (a)
dN/dp~T~ distributions for D-meson. The integrated yield dN/dy = 0.03 as
measured in p + p collisions at 200 GeV \[Ref. Error: Reference source
not found\]. The number of binary collisions N~bin~ = 290, corresponding
to the minimum bias Au + Au collisions, is used to scale the Au + Au
collisions. A power-law shape from p + p collisions is assumed; (b)
Assumed v~2~ distributions for D-mesons. The solid-line is the results
of fit to the measured Kaon v~2~ \[Ref. Error: Reference source not
found\]. Both circle- and diamond-symbols are from \[Ref. Error:
Reference source not found\] for the case with and without charm quark
flow, respectively. Error bars shown are from 15% systematic errors; (c)
3-σ significance D^0^ efficiency with TPC+SSD+HFT. The Dashed-line is
the fit to the simulated result; (d) D^0^ meson v~2~ rates from minimum
bias Au + Au collisions at 200 GeV. The small and large error bars are
for 15% and 30% systematic errors, respectively. For the v~2~ analysis,
12 bins in ϕ are used.

## Reconstruction of the Λ~c~ Baryon

The Λ~c~ is the lowest lying heavy flavored baryon. Roughly 10% of the
charm quarks end up in the baryon sector[^90] following a heavy ion
collision. As has been mentioned in chapter 2, the measurement of the
Λ~c~ is the best way for us to study heavy-flavored baryon/meson
differences. It is important ingredient in our understanding of the
hadro-chemistry and hadronization involving heavy-flavors. The branching
ratio of Λ~c~ to the non-resonant decay is about 5% with a cτ = 60 μm.
With a similar reconstruction algorithm used in the previous sections of
this proposal, we have simulated the reconstruction efficiencies for
Λ~c.~

Plots (a), (b) and (c) in Figure 33 are the dca distributions (distance
of closet approach of the Λ~c~ to the primary vertex), decay length (the
distance from primary vertex to the decay vertex), and cos(θ)
distributions (the angle between reconstructed Λ~c~ momentum vector and
the direction of the primary vertex), respectively. (See Figure 24.) The
red-lines in Figure 33 are for primary charged tracks and the circles
are for tracks from Λ~c~ decay. The cuts on dca, decay length and cos(θ)
are 35 \< dca \< 300 μm, 50 \< decay length \< 350 μm, and cos(θ) \>
0.92. Plots (d) and (e) are the invariant mass plots in the momentum
windows: 2.0 \< p~T~ \< 2.2 GeV/c and 4 \< p~T~ \< 4.5 GeV/c,
respectively. The reconstructed efficiency is shown in plot (f).
Additional work to further optimize the cuts, especially for the low pT
region, is underway.

![](media/image37.png){width="5.424305555555556in"
height="3.6944444444444446in"}

[]{#__RefHeading___Toc137569193 .anchor}Figure 33: Λ~c~ decay length,
dca, and invariant mass distributions.

In this study, an average of 0.8 Λ~c~ per event was used. Below p~T~ \<
2 GeV/c, the signal over background ratio S/(S+B) is less than 3. 50 M
central events (top 10%) are needed in order to measure a Λ~c~ p~T~
distribution from 200 GeV Au+Au collisions. Assuming binary scattering
scaling for the heavy flavor production at 200 GeV, we find that 1000 M
peripheral events (60-80%) will be required to measure the nuclear
modification factor R~AA~(p~T~) with an error of about 20% up to p~T~ \~
5 GeV/c. This can then be used to compare with other heavy-flavor mesons
and light-flavor hadrons.

## Hit Density at the Front Surface of the HFT

There are several issues that may affect the performance of the proposed
HFT detector. The HFT is not triggered and is continually active,
recording all particles that pass through it. This means that in
tracking the events of interest there will be extraneous hits and pileup
in the detector due to other collisions at an earlier or later time,
tracks from beam gas showers, and other background sources. The areal
density of extraneous hits that can be tolerated depends on the tracking
precision projected onto the HFT surfaces and on details of the particle
reconstruction algorithms; while the extraneous hit density depends on
the HFT frame readout speed, beam luminosity, interaction cross sections
and background rates from additional sources. Reasonable limits on these
processes have been included in the simulations. The focus of this
section is to discuss and quantify these and other background issues
that go into the simulations.

To set the scale of the problem, consider the extraneous hit density
from normal interactions. The parameters used in the calculation are
shown in Table 9 and an estimate of the hit density is shown in Table
10.

  --------------------------------------------- -------------------------
  Au + Au Luminosity                            1 × 10^27^ cm^-2^s^-1^

  dN/dη (min bias)                              170

  Min bias cross section                        10 barns

  Interaction diamond size, σ                   30 cm
  --------------------------------------------- -------------------------

[]{#__RefHeading___Toc137569235 .anchor}Table 9: Luminosity and other
parameters that determine the particle flux on the HFT.

  ------------------------------------- ----------------- ----------------
                                        HFT Outer Layer   HFT Inner Layer

  Radius                                5.0 cm            1.5 cm

  Hit Flux                              5,600 Hz/cm^2^    28,750 Hz/cm^2^

  Hit Density 4 ms Integration          22.5/cm^2^        115/cm^2^

  Projected Tracking Window Area        0.6 mm^2^         0.15 mm^2^

  HFT Hit Resolving Area                0.001 mm^2^       0.001 mm^2^

  Probability of HFT Pileup             0.3%              1%
  ------------------------------------- ----------------- ----------------

[]{#__RefHeading___Toc137569236 .anchor}Table 10: Integrated hit loading
on the HFT and associated pileup.

As shown in the table, the accumulated hit density from the integrated
Au + Au collisions is 22.5/cm^2^ in the outer layer and 115/cm^2^ in the
inner layer of the HFT[^91]. This produces a pileup probability in the
track search window of 14% and 17% at the inner and outer HFT barrels,
respectively. The area of the track search window in this case is based
on the projection resolution for 1 GeV/c pions as determined from GEANT
simulations. For comparison, the unresolved hit pileup intrinsic to the
HFT detector is much less, 0.28%, and 1%, for the outer and inner barrel
respectively.

The luminosity applied in this case is the maximum instantaneous
luminosity that was recorded during the Au + Au run in 2004 (1 × 10^27^
cm^-2^s^-1^). This is 5 times the nominal "design luminosity". Design
luminosity is defined to be 2 × 10^26^ cm^-2^s^-1^ averaged over a 10
hour fill. The STAR Experiment measures interaction rates through the
zero-degree calorimeter detector, and a RHIC instantaneous luminosity of
2×10^26^ cm^-2^s^-1^ is equivalent to an instantaneous ZDC coincidence
rate of 2000 Hz. It should be noted that the RHIC beam luminosity starts
high and drops with time over the course of a fill. During the RHIC II
era, the most important improvements in RHIC luminosity will be longer
decay times during the fill. However, we do expect an increase in peak
luminosity by about a factor of 4, which will have the largest impact on
our program. This is important, and so under RHIC II conditions we will
have to improve the readout time of the HFT in order to limit pileup in
the tracking search window to about 10%.

Table 11 gives the same loading information resulting from a single
central Au + Au collision. The numbers for a single collision is a
factor of 10 less then the integrated load, which demonstrates that an
isolated collision is easily tracked by the HFT.

  -------------------------------------- --------------- ----------------
                                         HFT Outer Layer HFT Inner Layer

  Radius                                 5.0 cm          1.5 cm

  Hit Density Au + Au Central Collision  1.8/cm^2^       7.4/cm^2^

  Projected Tracking Window Area         0.6 mm^2^       0.15 mm^2^

  HFT Hit Resolving Area                 0.001 mm^2^     0.001 mm^2^

  Probability of HFT Pileup              0.02%           0.09%
  -------------------------------------- --------------- ----------------

[]{#__RefHeading___Toc137569237 .anchor}Table 11: Hit loading on the HFT
from Au + Au central collisions and associated pileup.

### Measured Hit Density at 6 cm Radius in STAR

We have used the STAR SVT to actually measure the hit density that will
contribute to pileup in the HFT. The SVT is the innermost detector
currently available in STAR and it gives the best-hit density
information closest to the interaction point. This is of interest
because it can identify potential background contributions including
very low momentum tracks. As it turned out the measured hit density at
the SVT inner layer for central collision triggers is 1 hit/cm^2^. This
is consistent with the expected track density for central Au + Au
collisions with dN/dη = 700, so we see no evidence of additional
background beyond the pileup of normal Au + Au events[^92]. This
measurement was done at B = 0.25 T, half the normal STAR magnet field,
to open the acceptance to lower momentum particles. It would be
desirable to use SVT events with a zero bias trigger to have a measure
of other background sources such as beam gas showers, but so far, our
noise suppression filters for the SVT are not sufficient for this task.

Although we do not have a direct measure of beam showering, we do know
from studies of space charge distortions in the STAR TPC that at larger
radii, 65 cm to 200 cm, the ionization density due to normal Au + Au
interactions is larger than the contributions from other sources such as
beam gas[^93] interactions.

# CMOS Sensors

## Introduction

The STAR heavy flavor physics program requires a thin, fast, detector
that can operate in a relatively high radiation environment. Our goal is
to measure charmed hadron decays and to measure flow in the charm
sector. Since flow is a soft process, the HFT must measure transverse
momenta down to 150 MeV/c. To reach this low threshold requires a very
thin detector in order to maintain precise tracking without degradation
by multiple Coulomb scattering. It is difficult to meet all of these
requirements using the "usual" techniques currently employed in high
energy physics experiments.

One possible technology choice for a vertex detector is Charged Coupled
Devices (CCD). They provide the necessary granularity and can be thinned
down to the desired thickness, but these detectors are slow, radiation
sensitive and complex to operate. On the other hand, Hybrid Pixel
Sensors, and other sensor concepts, are fast and radiation tolerant but
suffer from modest granularity problems; also, they are usually too
thick for our application.

The emergence of CMOS sensor technology offers a new perspective on high
precision charged particle tracking and vertex finding. This technology
can provide the performance parameters required by the HFT. Recent
developments have shown that CMOS technology is capable of excellent
spatial resolution and charge collection efficiency, together with
satisfactory radiation tolerance.

1.  **Choice of technology**

Lets look at these technology choices in more detail. At this time,
there are four good technologies that can be used for a thin vertex
detectors: Charge Coupled Devices (CCD), Active Pixel Sensors (APS),
Hybrid Pixels, and DEPleted Field Effect Transistor structure (DEPFET).
Each of these devices has strengths and weaknesses. In the process of
preparing this proposal, we have evaluated each and selected APS as our
choice for the HFT detector.

0.  **CCDs**

At SLAC, the SLD collaboration built and successfully operated a pixel
vertex detector[^94], VXD3, based on CCD technology.[^95] But since
silicon is damaged by radiation and CCDs require that the charge be
transferred from one pixel to another, CCDs are more susceptible to
radiation than other vertex devices. Charge in the end row of a CCD
chip, for example a 1000 × 1000 array, must be transferred through more
than 1000 pixels before being digitized. Therefore, any small loss in
charge transfer produces large signal losses and signal sharing. The SLD
vertex detector ran at a relatively low radiation intensity because that
is the nature of the SLAC Linear electron Collider (SLC), so the SLC CCD
could tolerate the radiation environment. In addition, the complexity of
the clocking makes the readout slow. This was suitable for the SLC where
it only had to be operated at 2 Hz.

CCDs require significant power to clock the charge around the chip
because high capacitance electrode structures covering the whole chip
must be voltage switched. This becomes a power versus speed trade off
with consequences in the mass budget because liquid cooling is usually
required. In any case, cooling is a complication for CCD operation. For
instance, VXD3 used LN~2~ gas to cool the device but because the ladders
were operated far below room temperature, elaborate mechanical and
alignment systems were needed to achieve the excellent resolution of
about 30 µm for determining the impact parameter. An outer heated jacket
was needed to prevent condensation and the cooling system added extra
complication and mass.

Finally, to fabricate CCDs requires a very special process. This process
requires specialized knowledge, is expensive, and requires a long
learning curve to become familiar with its details. Currently, there is
a group studying whether the limitations of the process can be overcome
to use this technology it at a future electron collider.[^96]

1.  **Hybrid Pixels**

The radiation environment at the LHC is much higher. For this reason,
and the need to have the vertex detector in the trigger, CCDs are not an
appropriate choice at the LHC (Large Hadron Collider)[^97] at CERN,
which has a much higher radiation background than SLC. At the LHC, the
three major experiments decided to use a hybrid technology where the
sensor is bump bonded to a read-out chip. The hybrid technology has the
disadvantage that the pixel size is much greater than a CCD pixel and
two chips have to be layered on top of each other. The two chips and
their interconnection are much thicker than can be done in CCD
technology.

2.  **DEPFET**

The MPI Semicondcutor Laboratory has recently invented[^98] and has been
the pioneering leader in the use of DEPFETs.[^99] This concept is based
on the combination of the sideward depletion, as used in a semiconductor
drift detector with a field effect transistor to collect the charge. The
MPI Group has a conceptual design for a detector for the Linear Collider
but much development is needed to make a realistic device. DEPFETs
require a very special process and MPI is the only producer of this
device so any development must be done within that institute. (This has
both the advantage and disadvantage that its process is controlled by
physicists.)

+------------+----+----------+-----+----+----+----------------------+---+
| **Chip**   | ** | *        | *   | *  | ** | **Comments**         |   |
|            | Ye | *Process | *Ep | *P | Pi |                      |   |
|            | ar | (µm)**   | i.\ | it | xe |                      |   |
|            | ** |          | (µm | ch | ls |                      |   |
|            |    |          | )** | (µ | ** |                      |   |
|            |    |          |     | m) |    |                      |   |
|            |    |          |     | ** |    |                      |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA-1   | 19 | AMS 0.6  | 14  | 20 | 4k | thick epitaxial      |   |
|            | 99 |          |     |    |    | layer                |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA-2   | 20 | MIETEC   | 4.2 | 20 | 4k | thin epitaxial layer |   |
|            | 00 | 0.35     |     |    |    |                      |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA-3   | 20 | IBM 0.25 | 2   | 8  | 3  | deep sub-mm          |   |
|            | 01 |          |     |    | 2k |                      |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA-4   | 20 | AMS 0.35 | no  | 20 | 4k | low dopant substrate |   |
|            | 01 |          |     |    |    |                      |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA-5   | 20 | AMS 0.6  | 14  | 17 | 1M | real scale 1M pixels |   |
|            | 01 |          |     |    |    |                      |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA-6   | 20 | AMIS     | 4.2 | 28 | 3k | fast column parallel |   |
|            | 02 | 0.35     |     |    |    | readout\             |   |
|            |    |          |     |    |    | internal data        |   |
|            |    |          |     |    |    | sparsification       |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA-7   | 20 | AMS 0.35 | no  | 25 | 1k | fast column parallel |   |
|            | 03 |          |     |    |    | readout\             |   |
|            |    |          |     |    |    | internal data        |   |
|            |    |          |     |    |    | sparsification.      |   |
|            |    |          |     |    |    | (photoFET)           |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA-8   | 20 | TSMC     | \~8 | 25 | 4k | fast column parallel |   |
|            | 03 | 0.25     |     |    |    | readout\             |   |
|            |    |          |     |    |    | internal data        |   |
|            |    |          |     |    |    | sparsificaton        |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA-9   | 20 | AMS      | \   | 2  | 7k | tests                |   |
|            | 04 | 0.35\    | ~14 | 0\ |    | diodes/pitch/leakage |   |
|            |    | opto     |     | 3  |    | current              |   |
|            |    |          |     | 0\ |    |                      |   |
|            |    |          |     | 40 |    |                      |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA-9\  | 20 | AMS      | no  | 2  | 7k | tests                |   |
| (no epi)   | 04 | 0.35\    |     | 0\ |    | diodes/pitch/leakage |   |
|            |    | opto     |     | 3  |    | current              |   |
|            |    |          |     | 0\ |    |                      |   |
|            |    |          |     | 40 |    |                      |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA-10\ | 20 | TSMC     | \~8 | 30 | 1  | first prototype for  |   |
| (M         | 04 | 0.25     |     |    | 6k | STAR HFT             |   |
| IMOSTAR-1) |    |          |     |    |    |                      |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA-11  | 20 | AMS      | \   | 30 | 7k | radiation tolerant   |   |
|            | 05 | 0.35\    | ~14 |    |    | structure            |   |
|            |    | opto     |     |    |    |                      |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA-12\ | 20 | AMS      | no  | 35 | 0. | multi-memory pixels  |   |
| (Mosaic 1) | 05 | 0.35\    |     |    | 6k | (FAPS)               |   |
|            |    | high     |     |    |    |                      |   |
|            |    | r        |     |    |    |                      |   |
|            |    | esistive |     |    |    |                      |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA-13\ | 20 | AMS      | no  | 20 | 1. | fast column readout  |   |
| (Mosaic 2) | 05 | 0.35\    |     |    | 4k |                      |   |
|            |    | high     |     |    |    |                      |   |
|            |    | r        |     |    |    |                      |   |
|            |    | esistive |     |    |    |                      |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA-14  | 20 | AMS      | no  | 30 | 1  | second prototype     |   |
|            | 05 | 0.35\    |     |    | 6k | STAR HFT             |   |
| (M         |    | opto     |     |    |    |                      |   |
| IMOSTAR-2) |    |          |     |    |    |                      |   |
+------------+----+----------+-----+----+----+----------------------+---+
| MIMOSA15   | 20 | AMS      | \   | 2  | 7k | multi-purpose        |   |
|            | 05 | 0.35\    | ~14 | 0\ |    | tracker-imager       |   |
|            |    | opto     |     | 30 |    |                      |   |
+------------+----+----------+-----+----+----+----------------------+---+
| SUC 1      | 20 | AMIS     | 4.2 | 2  | 4k | radiation tolerant   |   |
|            | 03 | 0.35     |     | 5\ |    | structure\           |   |
|            |    |          |     | 35 |    | (SUCIMA project)     |   |
+------------+----+----------+-----+----+----+----------------------+---+
| SUC 2      | 20 | AMS 0.35 | no  | 40 | 2k | low dopant           |   |
|            | 03 |          |     |    |    | substrate\           |   |
|            |    |          |     |    |    | (SUCIMA project)     |   |
+------------+----+----------+-----+----+----+----------------------+---+
| SUC 3      | 20 | AMIS     | 4.2 | 20 | 8k | radiation tolerant   |   |
|            | 03 | 0.35     |     |    |    | structure\           |   |
|            |    |          |     |    |    | (SUCIMA project)     |   |
+------------+----+----------+-----+----+----+----------------------+---+
| SUC 4      | 20 | AMS 0.35 | 14  | 1  | 1  | Hadron therapy/beam  |   |
| Mtera      | 04 |          |     | 50 | 2. | monitor.             |   |
|            |    |          |     |    | 5k |                      |   |
|            |    |          |     |    |    | (SUCIMA project)     |   |
+------------+----+----------+-----+----+----+----------------------+---+
| SUC 5      | 20 | AMIS     | 4.2 | 30 | 6  | proton dosimetry\    |   |
|            | 04 | 0.35     |     |    | 5k | (SUCIMA project)     |   |
+------------+----+----------+-----+----+----+----------------------+---+

[]{#__RefHeading___Toc137569238 .anchor}Table 12: Table -- Chart of APS
chips that the IReS group has produced in the past 5 years \[[^100]\].

3.  **Active Pixel Sensors**

APS devices have been used as photon detectors since late 1960s.[^101]
They have recently surpassed CCDs in the photography market because of
their lower cost and lower demand for power. Power consumption is
important in a particle detector application because a detector that can
be air cooled is thinner and lower mass, by definition, than a detector
that requires water cooling. The IReS group in Strasbourg France has
done a great deal of research on these detectors; taking them from small
prototypes to large arrays of successful detector elements. Table 12
(above) presents a detailed listing of the CMOS sensor program.

APS technology is our preferred technology for the HFT and we are
working with the Strasbourg group to design and utilize this technology
in our detector. The APS technology will be described in more detail in
the next section.

## Main Features and Performance of CMOS Active Pixel Sensors

CMOS sensors are manufactured using industry-standard CMOS technology.
This offers low fabrication costs and fast turn-around times in their
development. The key element of this technology, for our purposes, is
the use of an n-well/p-epi diode to collect the charge, through thermal
diffusion, which is generated by the impinging particles in the thin
epitaxial layer underneath the read-out electronics[^102] (See Figure
34). An attractive feature of these sensors is that they allow
fabrication of System-on-Chips (SoC) by integrating signal processing
micro-circuits (amplification, pedestal correction, digitization,
discrimination, etc.) on the detector substrate. Moreover, a CMOS
substrate can be thinned down to a few tens of microns because the
active region is less than 20 μm thick.

![](media/image38.png){width="2.4027777777777777in" height="3.5in"}

[]{#__RefHeading___Toc137569194 .anchor}Figure 34: Epitaxial Silicon
used as a sensor. In this design, a primary ionizing particle creates
free charges in the epitaxialy grown Si layer that is a few tens of
microns thick and a few electrons in the bulk layers. The liberated
charges are then free to diffuse towards a potential well structure at
the top of the sensor where they are extracted and readout into a DAQ
system.

The ability of these sensors to provide charged particle tracking is now
well established.[^103] The IReS group, at Strasbourg, has built a
series of these sensors, which they called MIMOSA.[^104] Similarly, the
LBL/UCI group has also built and successfully tested these sensors. The
MIMOSA line of detectors has explored different CMOS fabrication
processes and key parameters of the charge sensing system, and the
results demonstrate that a detection efficiency of \~ 99% and a single
point resolution of \~2 μm can be achieved using a pixel pitch of 20 μm.
The prototypes also show that digitizing the charge with a small number
of ADC bits does not degrade the resolution significantly (the measured
reduction was \~ 2.5--3 μm) while the double hit resolution is \~ 30 μm.

The radiation tolerance of the sensors to bulk damage[^105] was also
investigated. No significant performance loss was observed up to
fluences close to 10^12^ n~eq~ cm^-2^. As far as ionizing radiation
damage is concerned, the real potential of this technology is still
being explored, but it is already established that it stands up to more
than 100 kRad.

Most of the R&D at IReS was performed with small prototypes (a few
mm^2^) containing a few thousand pixels. Figure 35 shows a full size
prototype (i.e. \~3.5 cm^2^) called MIMOSA-5. It is composed of \~ 1
million pixels per chip, and it was fabricated on a 6 inch wafer, as
shown in the figure. The wafers were thinned down to 120 μm before the
chip was cut and diced into individual, reticle sized, detector
elements.

Tests at the CERN-SPS confirmed that MIMOSA-5 performed as well as the
smaller prototypes: a 99% detection efficiency was observed with \~ 2 μm
single point resolution. The prototypes were operated with a read-out
time of 25 ms, which was limited by the maximum operation frequency of
the read-out board (i.e. 10 MHz). The chip was actually designed for a 4
times faster read-out speed. The LBL group has tested several of these
chips at the LBL ALS and have measured the expected Landau spectrum on
each.

![](media/image39.jpeg){width="2.717361111111111in"
height="1.9909722222222221in"}![](media/image40.jpeg){width="3.2152777777777777in"
height="1.9868055555555555in"}

[]{#__RefHeading___Toc137569195 .anchor}Figure 35: Wafer of reticle size
sensors (left) and zoomed-in view of individual chips (right).

After MIMOSTAR-5, several fabrication processes were explored, aiming to
find the process providing the smallest leakage current. In general,
several parameters underlying the sensor performance parameters depend
on features specific to each fabrication process and so the process
specific characteristics need to be explored in parallel with the
development of the chip architecture.

For instance, a new fabrication technology, relying on a lightly doped
substrate but exhibiting no epitaxial layer was investigated with two
different prototypes chips. Further IReS tests show that a detection
efficiency of 99.9% can be achieved with this technology, as well as a
single point resolution of about 2.5 μm[^106]. A major advantage of this
technology is that a large signal can be generated because the charges
are collected from several tens of microns of Si instead of from \~ 10
µm in an epitaxial layer. The extra charge makes it well suited to
applications with substantial electronic noise.

## Thinning

Up to now, tested MIMOSA-5 chips have been thinned down to 120 μm
thickness. This operation was successful and we have not found any
degradation of the sensor parameters.

In the HFT, the MIMOSTAR thickness will be 50 μm. Thinning a 6 or 8 inch
wafer down to this thickness is not expected to be a problem, since the
process is an industry standard technique. We have demonstrated that 50
µm sensors are mechanically stable and can be assembled into ladders. We
have used low-yield MIMOSA-5 wafers to make these investigations and
have successfully thinned detectors to 50 µm[^107].

Our colleagues at the LBL light source have just characterized several
MIMOSA-5 chips in the ALS's 1.5 GeV/c electron beam and have
subsequently thinned them. Tests of the thinned chips are completed and
we find no degredation of signal from thinned chips[^108].

## Additional R&D

CMOS sensors have been developed in Strasbourg since 1999 for various
applications, which range from vertex detectors for subatomic physics,
to bio-medical imaging (e.g. beam monitoring for oncotherapy, dosimeters
for brachyotherapy) and operational dosimetry (e.g. control of ambient
radon and neutron radiation levels in nuclear plants).

Several application domains call for SoCs providing fast read-out speed
(meaning signal treatment and data flow reduction integrated on the
chip), high radiation tolerance, minimal material budget and low power
dissipation. Developments for the STAR upgrade will thus benefit from
the synergy with the R&D for other applications, in terms of fabrication
process exploration, development of fast signal processing
architectures, radiation tolerance investigations, and improvements.
More information on the activities and achievements of the Strasbourg
research team are available in Ref. \[[^109]\].

Starting with a IReS design, the LBNL/UCI group has built several
generations of APS sensors. These devices were built in the 0.25 µm
process at TSMC. These ICs have been tested using different sources,
i.e. ^55^Fe, 1.5 GeV accelerator electrons and a scanning electron
microscope. Figure 36 shows one of the sensors that has 16 different
test structures.

![](media/image41.jpeg){width="3.011111111111111in"
height="2.765277777777778in"}

[]{#__RefHeading___Toc137569196 .anchor}Figure 36: An APS Sensors
developed by the LBNL/UIC group. The picture shows 16 separate test
structures. Each structure has a 36 × 36 array of 20 µm pixels.

Our measurements have confirmed the IReS results that APS sensors can
measure charged particles with excellent spatial
resolution[^110]^,^[^111]**.** Figure 37 shows several 1.5 GeV electrons
recorded at LBNL's Advanced Light Source.

![](media/image42.jpeg){width="3.498611111111111in"
height="3.3506944444444446in"}

[]{#__RefHeading___Toc137569197 .anchor}Figure 37: This graph shows the
results of one event taken with 1.5 GeV electrons. Each bin represents
one pixel and the height is proportional to the measured charge. Several
electron hitselectrons can be identified in the plot.

To study the effects of radiation, we have exposed the chips to protons
at LBNL's 88" Cyclotron. Afterwards, we measured the change in leakage
current and pulse height. These results show that there was a modest
change with an irradiation of 300 krad. What was most interesting is
that after 6 months, the device self annealed. Therefore, the effect of
gradual radiation is much less than that of an acute exposure[^112].
These radiation measurements are complementary to the neutron exposures
measured by the IReS group.

We also completed a study on the effects of varying the pixel pitch. We
built and tested a sensor with 5, 10, 20 and 30 µm spacing. Our tests
show that to first order the charge collection was identical when
comparing the central pixel to the charge collected on its
neighbors.[^113] As the pixel spacing decreases, more total charge is
collected. This observation can be explained by the fact that as pixel
spacing becomes smaller, the charge is collected by the diode faster, so
there is less time for it to recombine. This result implies that we can
easily extrapolate our measurements at 20 µm spacing to the selected
spacing of 30 µm for MIMOSTAR.

Our group has been looking at several other techniques to improve APS
sensors. It is clearly desirable to speed up the readout as well as
reduce the signal spreading to multiple pixel diodes. Concentrating the
signal onto a single diode would improve signal to noise.

We can increase the fraction of charge collected by a single pixel using
the photo-gate technology. In principle, this technology allows us to
use a large area photo-gate for charge collection, without increasing
the capacitance, because the charge is transferred from the photogate
region to a low capacitance diode. Even though we have demonstrated the
sensitivity of the photo-gate structure to X-rays, we have not achieved
the expected performance of the device. The transfer time required for
moving electrons from the photo-gate to the drain appears to be very
long (several ms). We believe this undesirable signal delay is probably
consistent with surface traps at the Si0~2~ silicon boundary. Studies to
find a way to avoid this delay are in an SBIR proposal that is currently
being considered.

Correlated double sampling (CDS) is a standard technique that is used to
remove the fixed pattern noise and KTC noise introduced by the reset
transistor. Its main drawback is the required read out and storage of a
full frame of data. To avoid doing CDS, we have produced a clamp
circuit[^114] that reduces the reset noise by a factor of 3**.**

Two generations of "active reset chips" have been fabricated and tested.
In this approach the pixel voltage is reset to the empty level with a
feed back amplifier potentially reducing fixed pattern noise and the KTC
noise associated with a passive reset switch. Preliminary testing shows
some noise reduction, but not the full potential improvement. We will be
making some changes to this circuit and fabricating this in a new sensor
design.

To explore how charge is collected on a sensor as a function of
position, we tested a device at LBNL's National Center for Electron
Microscopy. Using the scanning electron microscope, we have been able to
position the beam on a pixel with precision of about 1 µm. This allows
us to explore algorithms for determining the position of an incoming
particle by relating how charge is shared among neighboring pixels. The
results show that we can easily obtain the position of the incoming
electron by weighting the charge collected from nearby pixels.

We have also been collaborating with colleagues at UC San Diego on the
suitability of using an APS sensor in an electron microscope[^115]. We
have been studying the response of electrons from energies of 100 keV to
300 keV. Results show that single electrons can be detected with a good
signal to noise ratio. This technique is well suited for this
application as it overcomes problems for traditional CCDs that suffer
radiation damage. Excellent pictures of proteins have been taken. Using
APS devices are very appealing for this different type of application.

We have several ideas on how to further improve our device. We have
fabricated a sensor with different diode sizes. We have used this device
to understand the effect of diode size on signal to noise. The results
show that the smallest diode that meets the design rules produces the
best signal to noise.

Recently, we submitted an IC design without an epi-layer. We have
received the sensor and in the process of testing it. Simulations show
that we should be able to collect more charge from the process.

In addition, we are studying ways to reduce the sampling time. In the
current MIMOSTAR design, the sensor is always sensitive to radiation. We
will study a sample-and-hold circuit to see if we can reduce the time
window of sensitivity. If we could gate such a sensor, then pile-up
effects of out of time interactions would be significantly reduced.

## MIMOSTAR Sensor Design

Based on our experience with CMOS technology, a new series of chips,
MIMOSTAR, have been fabricated. The first chip in the series is also
called MIMOSTAR-10, which indicates its place in the evolution tree of
CMOS sensors. Its most significant design parameters are:

-   Pixel pitch: 30 μm.

```{=html}
<!-- -->
```
-   Passive forward bias diode in place of reset switch

-   Additional details can be found in Ref. \[[^116]\].

The goal of the MIMOSTAR series is to provide for a full-scale prototype
that is suitable for evaluating the performance of a CMS sensor in a
collider environment.

Since the read-out speed requirement for this prototype is modest, the
chip's architecture is based on relatively slow signal processing at the
pixel level. This low speed requirement enables us to quickly implement
this sensor, while at the same time we are independently developing a
full-featured sensor for RHIC II. Its design favors moderately low noise
and modest power consumption. On the other hand, high speed signal
processing is needed at the chip level (i.e. after amplification and
multiplexing) due to the need for short integration times and limited
shot noise.

The chip includes JTAG based remote control functionality (e.g. bias
setting, test settings, etc.). The power dissipation of this
architecture is estimated to be slightly more than 50 mW (i.e. less than
15 mW/cm^2^), which can easily be cooled with air.

The IReS group has studied MIMOSTAR-1, which was fabricated in the TSMC
0.25 µm process. They showed that the conventional controls work (via
the JTAG controller), and that the bias circuits work and exhibit linear
response via their DACs. The analog performance of the chips has been
tested and the gain on the pre-amplifiers was about 3.5 at the two 10
MHz outputs. However, due to the inherent properties of the TSMC
process, the charge collected on the n-diode could not hold the
collected charge long enough for it to be read out. Our group at
Berkeley Lab observed a similar effect with another chip designed in
collaboration with UCI, which also was fabricated in the TSMC 0.25
process.

Consequently, we decided to switch to the AMS 0.35 OPTO process, in
which the passive forward bias diode reset circuit has already been
demonstrated to work. A new chip, MIMOSTAR-2, has been fabricated,
passed ^55^Fe tests, and then tested in DESY's 5 GeV electron beam. A
radiation tolerant design and a standard design showed an efficiency
greater than 99.7% at a temperature of 40 C. The radiation tolerant
design showed a very small increase in noise at 40 C when it was exposed
to 23 kRad of ^60^Co.

The specifications used to design the chip are shown in *Appendix I --
MIMOSTAR II Specifications* and a detailed guide to using the chip has
been completed and is included in *Appendix II -- MIMOSTAR II Users
Guide.*

## The path to a CMOS detector for the HFT

With this research and development done at IReS and LBL, we are
confident that we can build a CMOS sensor appropriate for RHIC II
luminosities. To achieve this goal, we have identified a series of steps
that must be taken before we can design and build the final HFT sensor.
In the next subsections, we will describe the different R & D steps
necessary to achieve this ultimate goal.

### MIMOSTAR-3 -- a half sized chip

MIMOSTAR-3, which is half the size of the final sensor, continues the
work done on MIMOSTAR-2. It is made up of 640 × 320 pixels. The
fabrication will rely on an engineering run in AMS 0.35 OPTO technology.
The chip has two analog outputs implemented on the same side of the
sensor. Each output runs in parallel at 50 MHz so the total time for
readout is 2 ms.

Once these chips have been tested, they will be incorporated on a ladder
for a full function test in STAR. As a ladder will be half the final
size, significant tests will be able to be done.

### MIMOSTAR-4 -- a full sized chip

Using the experience gained from building and testing MIMOSTAR-3, we
will design and manufacture a full sized sensor that can both be used in
a full sized ladder prototype. This chip will be twice the size of
MIMOSTAR-3, so it will be 640 × 640 pixels. It will have 2 analog
outputs on one side running at the same speed of MIMOSTAR-3, so it will
take 4 ms to read out all of the pixels. The fabrication of this chip
will be done in a production run.

Table 13 shows how the measured performance of the MIMOSA-5 sensor
compares to the specifications of MIMOSTAR-4. Most of the requirements
(granularity, radiation tolerance, thinning, read-out speed, power
dissipation, and sensor size) have already been demonstrated with
MIMOSA-5. Some effort is still needed to achieve a higher read-out speed
and higher yield during thinning. Moreover, since the sensors will be
operated at room temperature, special attention will be given to the
magnitude of the leakage current in order to keep the corresponding shot
noise at an acceptable level.

  ------------------------- ---------------------- -----------------------
  Parameter                 MIMOSTAR-4             MIMOSA-5 performance
                            specifications         

  Detection efficiency      \> 98% at 30 -- 40° C  \~99% ≤ 20° C

  Single point resolution   \< 10 μm               \~2 μm

  Granularity (pixel pitch) 30 μm                  17 μm

  Read-out time             4 ms                   24 ms (\< 20 ms
                                                   possible)

  Ionizing radiation        3.7. kRad/yr[^117]     \> 100 kRad
  tolerance                                        

  Fluence tolerance         2 × 10^10^ n~eq~/cm^2^ ≤ 10^12^ n~eq~/cm^2^

  Power dissipation         \< 100 mW/cm^2^        \~10 mW/cm^2^

  Chip size                 \~2 × 2 cm^2^          1.9 × 1.7 cm^2^

  Chip thickness            ∼50 μm                 120 μm
  ------------------------- ---------------------- -----------------------

[]{#__RefHeading___Toc137569239 .anchor}Table 13: Comparison between
MIMOSA-5 characteristics and MIMOSTAR-4 specifications.

### The ultimate sensor

A next generation CMOS IC is needed to meet the requirements of the high
intensity at RHIC II. At these higher luminosities, we need to read-out
the chip to match the 1 ms readout time of the TPC and to have the chip
sensitive for a much shorter time to reduce the effect of pileup. The
goal of our R&D program is to find a sensor that can meet the
specifications listed in Table 14.

This sensor will be the same size as MIMOSTAR-4. It will have the same
pitch, 30 µm, and pixel array, 640 x 640 µm. It also will be thinned to
50 µm. A specific architecture needs to be developed for this design.
Based on our experience, we have a good description for this sensor. The
device has a active time period much shorted than the readout time. The
active period of time is strobed by the STAR trigger, followed by a 1 ms
period to read out the full number of pixels.

The general organization of the chip relies on columns processed in
parallel. The chip operation includes a continuous cycling over the
array with an integration time of 100 to 200 µs. There will be in-pixel
storage of the integrated charges, but at this time it is not clear
whether CDS can be done "on chip".

The details of the signal processing, i.e. ADC or double threshold
discrimination for zero suppression are still open and need to be
studied with real data. Studies on this chip will commence this year.

  --------------------------------------------------- -------------------
  Ladder active area                                  2 cm × 20 cm

  Pixel size                                          30 μm × 30 μm

  \~Pixel mapping on the ladder                       640 × 6400

  Minimum operating distance from beam                1.5 cm

  Power                                               ≤200 mW/cm^2^

  Operating temperature                               ≥30 °C

  Integration time[^118]                              ≤0.2 ms

  Mean silicon thickness                              ≤100 μm

  Readout time                                        ≤1 ms

  Efficiency (min I)[^119]                            ≥98%

  Accidental cluster density                          ≤22/cm^2^

  Binary readout, number of threshold bits[^120]      1 or 2

  Radiation tolerance [^121]                          ≥171 kRad

  Number of conductors supporting the ladder (10      ≤140
  chips/ladder)[^122]                                 

  Triggered readout, maximum trigger delay[^123]      2 μs
  --------------------------------------------------- -------------------

[]{#__RefHeading___Toc137569240 .anchor}**Table 14: Silicon requirements
for maximum average Au+Au RHIC luminosity of 7.0 × 10^27^ Hz/cm^2^ or
2.5 nb^-1^/week**

# Data Acquisition and Readout

## Prototype and Final Design

We are designing the readout and data acquisition system for the HFT in
two stages. These stages follow the development of the silicon sensors
in our project. The initial prototype readout system is designed to read
out the MIMOSTAR4 detectors which have analog outputs and a 4 ms readout
time. The second stage is for use with the final Ultimate series sensors
which have a digital output and a 1 ms readout time but only a 200 µs
integration time. The goal of the first stage is to develop much of the
infrastructure for doing cluster finding and data sparsification, the
interfaces to trigger and DAQ and the mechanical readout structures and
assemble a working prototype detector with the MIMOSTAR4 sensors. The
second stage would make use of the development that we have done with
the prototype detector and integrate the Ultimate series sensors with a
developed readout system.

## Requirements and Prototype Design

The requirements for the prototype and final readout system are very
similar. They include:

-   Triggered detector system fitting into existing STAR infrastructure
    and interfaces to the existing Trigger and DAQ systems.

-   Deliver full frame events to STAR DAQ for event building at
    approximately the same rate as the TPC.

-   Reduce the total data rate of the detector to a manageable level (\<
    TPC rate)

We have designed the prototype data acquisition system to read out the
large body of data from the individual MIMOSA4 sensors, to digitize the
signals, to perform data compression, and to deliver the sparsified data
to an event building and storage device. A summary of the specifications
and requirements is provided in Table 15.

  ---------------------------------------------------- ------------------
  Total number of pixels                               1.35 × 10^8^

  Number of pixels per chip                            640 × 640

  Pixel Readout rate (analog output)                   2 × 50 MHz / chip

  Readout time per frame                               4 ms

  Frame integration time                               4 ms

  Noise after Correlated Double Sampling               10 *e^−^*

  Maximum signal                                       900 *e^−^*

  Dynamic range after Correlated Double Sampling       8 bits

  Total sensor power consumption (24 33 ladders)       90 132 W

                                                       
  ---------------------------------------------------- ------------------

[]{#__RefHeading___Toc137569241 .anchor}Table 15: Prototype Stage
Requirement Summary - constraints for the MIMOSTAR4 APS.

Digitizing the analog signal on each pixel into a 12 bit digital signal
yields approximately 1.2 Gb/s per sensor chip when read out in 4 ms.
Thus, the total front end data rate is \~50.7 GB/s. Clearly, the volume
of data must be reduced before being passed to the DAQ event builder and
written to storage.

Data compression is achieved by performing correlated double sampling
(CDS) i.e. subtraction of two consecutive frames followed by zero
suppression. CDS cancels out fixed pattern and reset noise and reduces
1/f noise. The fixed pattern noise corresponds to the spread of the
baseline voltage in all pixels. It has been measured on the MIMOSA-5
chip to be 2000 electrons. The noise remaining after CDS must be on the
order of 14 *e^−^* to guarantee an efficiency of greater than 98%. The
maximum signal is estimated from dE/dx calculation and by measuring how
the charge spreads over pixels. The signal can be truncated above 900
*e^−^* without compromising either the efficiency or the position
resolution, so 8 bits is a sufficient dynamic range for signal storage.
A synchronous cluster finding algorithm and the reduction of the data to
addresses of cluster center pixels reduce the data to a manageable rate.

## Architecture for the Prototype System

The basic flow of a ladder data path starts with the APS sensors. An HFT
ladder has 10 MIMOSTAR4 APS chips each with a 640 by 640 pixel array.
Each chip is divided in half with two sectors each containing a separate
analogue, differential current output buffer. The chips are continuously
clocked at 50 MHz and the data is read out, running serially through all
the pixels connecting them to the output buffer. This operation is
continuous during the operation of the MIMOSTAR detectors on the HFT
ladder. Analog data is carried from the two 50 MHz outputs in each
sensor in parallel on a low mass ladder flex printed circuit board to
discrete electronics at the end of the ladder and out of the low mass
detector region. This electronics performs current to voltage conversion
and contains buffers and drivers for the clocks and other control
signals needed for ladder operation.

Each MIMOSTAR detector requires a JTAG connection for configuration of
the chip, power, ground and a 50 MHz readout clock. These signals and
power as well as the analog outputs and synchronization and marker
signals from the detectors are carried via a low mass twisted pair cable
from the discrete electronics at the end of the ladder to the readout
electronics located about 1 meter from the HFT ladders. Each ladder and
associated readout board are the same and there is one readout board per
HFT ladder. A functional diagram of an HFT ladder and a description of
the data flow are shown in Figure 38.

![](media/image43.wmf){width="5.427083333333333in"
height="1.6041666666666667in"}

[]{#_Ref121896626 .anchor}Figure 38: Ladder Layout - sketch of the
readout-topology on a detector ladder. This figure shows the ten APS and
the corresponding current to voltage conversion and driver electronics.
The drivers will be located out of the low mass region of the detector
and may require additional cooling.

The readout electronics currently consist of a motherboard and daughter
card configuration. A functional block diagram is shown in Figure 39.
There are 3 daughter cards per motherboard and each daughter card is
capable of servicins 4 of the MIMOSTAR sensors on the ladder. The analog
signals are carried to the daughter cards where they are digitized with
a012-bit ADC at 50 MHz. Following digitization, ADC values are passed
synchronously to an FPGA for CDS. Performing CDS requires that a data
sample be stored for each pixel of the detector. This requirement drives
the need for external RAM on the daughter cards. After CDS and pedestal
subtraction, 8 bits is used to represent the data. The data is then
transferred to the next stage for hit finding and data reduction.

![](media/image44.wmf){width="5.99375in" height="3.245138888888889in"}

[]{#_Ref136929565 .anchor}Figure 39: Prototype DAQ Layout: schematic of
DAQ system for a single MIMOSTAR4 ladder. Analog data is carried as
differential current on the low mass cable at 50 MHz. The signals are
driven in parallel over short (\~1m) twisted pair cables to the
motherboard. Analog to digital conversion, CDS and data reduction are
performed in the Motherboard / Daughter cards. The reduced hit data is
transferred digitally to the SIU and carried to Linux based readout PCs
via an optical fiber. Control, synchronization, and event ID tagging are
accomplished in the Control/Trigger FPGA on the motherboard.

The 8-bit data data exiting the CDS stage is resorted on the fly to be a
traditional raster scan through the pixels of the sector. This stream of
rasterized data can then be passed to the cluster finder. We are
currently investigating methods of hit finding and data reduction for
use on the motherboard. A simple readout of the address of a center
pixel high threshold hit with the surrounding 8 pixels meeting
additional cluster selection criteria such as at least 1 cell over the
low threshold is our default approach. This can be implemented in an
FPGA and run as a pipeline filling the output buffer with center pixel
address values. A simple example of an FPGA logic diagram that
accomplishes this can be found in Figure 40. We are also investigating a
number of cluster selection methods including summing algorithms around
different thresholds and center pixel determination by geometric pattern
with high and low thresholds. A preliminary study of some simple and
FPGA implementable cluster finding algorithms shows promising results
for efficiency and noise rejection. A sample of these results can be
seen in Figure 41. An implementation document using this method of hit
finding saving only the center pixel address of the cluster is available
as an appendix.

![](media/image46.wmf){width="5.991666666666666in"
height="4.511111111111111in"}

[]{#__RefHeading___Toc137569200 .anchor}Figure 40: A simple cluster
finding algorithm for the HFT detector. ADC data from two MIMOSTAR
detector columns + 3 pixels are sent to a high/low threshold
discriminator. The resulting 2 bits are fed sequentially in an 2-bit
wide shift register. The center pixel of a 3 × 3 pixel window is
compared to a high threshold with each clock tick. If the threshold is
exceeded, the additional cluster identification criteria are checked for
the 3 x 3 pixel window. If the results meet the critera for a cluster,
the center pixel address is stored into a readout FIFO. This method is
extendable to allow for multiple simultaneous thresholds and geometric
pattern triggers.

![](media/image47.jpeg){width="5.996527777777778in"
height="5.103472222222222in"}

[]{#__RefHeading___Toc137569201 .anchor}Figure 41: Efficiency versus
accidentals for a cluster finding algorithms run on cluster data from a
MIMOSA5 detector. Note that some parameter combinations of this
algorithm are already over 98% efficient with a accidentals rate of 1-2
hits / cm^2^.

The reduced data is then buffered and transferred to the STAR DAQ system
over a high-speed bi-directional fiber link. We intend to use the Source
Interface Unit (SIU) and Readout Receiver Cards (RORC) developed for
ALICE as our optical link hardware to transfer data to and from the STAR
DAQ system. These links have been chosen as the primary readout
connections for the new STAR TPC FEE. Leveraging existing hardware and
expertise in STAR allows for a faster and more reliable design than
developing our own custom solution. The complete system consists of a
parallel set of ladder readouts consisting of 33 separate chains.

## Data Synchronization, Readout and Latency

The readout of the prototype HFT sensors is continuous and hit and
cluster finding is always in operation during the normal running of the
detector. The receipt of a trigger initiates the saving of the found
clusters into a FIFO for 1 frame (204,800 pixels). The HFT detector as a
whole will be triggered via the standard STAR TCD module. Since 4 ms are
required to read out the complete frame of interest, the data will be
passed to DAQ for event building \~ 4 ms after the trigger is received.
We will provide multiple buffers that will allow the capture of
temporally overlapping complete frames. This will allow us to service
multiple triggers within the 4 ms readout time A functional block
diagram of this system is shown in Figure 42. In this system, the
cluster data is fanned out to 5 Event FIFOs. A separate Event FIFO is
enabled for the duration of one frame upon the receipt of a trigger from
the TCD. Subsequent triggers enable additional Event FIFOs until all of
the event FIFOs are full and the system goes busy. The resulting
separate complete frames are then passed to STAR DAQ as they are
completed in the Event FIFOs. This multiple stream buffering gives a
system that can be triggered up to the expected rate of the STAR TPC
(approximately 1 KHz) after the DAQ1K upgrade. This will result in the
duplication of some data in frames that overlap in time, but our data
rate is low and the duplication of some data allows for contiguous event
building in the STAR DAQ, which greatly eases the offline analysis. In
addition, synchronization between the ladders/boards must be maintained.
The HFT will receive a clock via the standard STAR TCD and will derive
its internal clocks from the RHIC strobe. We will provide functionality
to allow the motherboards to be synchronized at startup and any point
thereafter.

[]{#__RefHeading___Toc137569202 .anchor}Figure 42: Multiple event FIFOs
are fed in parallel from the cluster finder. A separate Event FIFO is
enabled for one frame upon the receipt of a trigger from the TCD. The
resulting separate complete frames are then passed to STAR DAQ as they
are completed in the Event FIFOs.

## Data Rates for the Prototype RDO

  --------------------------------------------------------- --------------------------
  [Item]{.underline}                                        [**N**umber]{.underline}

  Bits/address                                              18

  Inner ladders (r=2.5 cm)                                  9

  Middle ladders (r=6.5 cm)                                 12

  Outer ladders (r=7.5 cm)                                  12

  MIMOSTAR sensors per ladder                               10

  Average hits/sensor, inner, L = 10^27^                    212

  Average hits/sensor, middle, L = 10^27^                   43

  Average hits/sensorr, outer, L = 10^27^                   35

  Trigger rate                                              1 KHz
  --------------------------------------------------------- --------------------------

[]{#__RefHeading___Toc137569242 .anchor}Table 16: Data rate calculation
parameters

The ADC data rate from each 640 × 640 MIMOSTAR detector is approximately
4.9 Mb / frame or 1.2 Gb / sec. The total rate of raw ADC data entering
the processing chain in the detector is thus approximately 50.7 GB/sec.
After CDS, the data can be represented by 8 bits. Pixel addressing
within a sector requires 18 bits. The sector-in-ladder address will be
accomplished as address words in the data stream. Ladder address will be
added at the DAQ receivers. This covers the address space to map the
detector pixel space. Each cluster word stored in the FIFO contains the
18 address bits of a cluster central pixel. Combining this with the
occupancy per layer, the overhead for event format and headers and the
readout rate of 1 KHz (with data duplication in the event FIFOs) gives
an event size of 114 KB and data rate from the detector of 114 MB/sec.
Figure 43 shows this graphically.

![](media/image48.wmf){width="5.999305555555556in"
height="1.0972222222222223in"}

[]{#__RefHeading___Toc137569203 .anchor}Figure 43: Data rates at the
various stages of the Prototype MIMOSTAR4 readout chain.

## Requirements for the Ultimate Design

The Ultimate series of APS detector will incorporate several changes
from the previous MIMOSTAR versions. The primary changes include on
pixel CDS and a two level programmable discriminator applied to the CDS
output for each chip. The Ultimate chip will be read out digitally in 2
bit words / pixel through 4 LVDS outputs / chip. The control functions
for the chip are still via the JTAG interface. A summary of the new
specifications is provided in Table 17. The basic requirements are as
indicated previously.

[Physical]{.underline}

-   640 x 640 pixels with 30um pitch.

-   Minimum of 3 fiducial marks / chip for optical survey purposes.

-   All bonding pads located along 1 side of chip

-   Two bonding pads per I/O of the chip to facilitate probe testing
    before sensor mounting.

[Electronic]{.underline}

-   Continuous cycling 100-200 µs integration time.

-   Trigger input reads out the previous 200 µs integration window.

-   On chip CDS for the pixels.

-   On chip JTAG programmable discriminator thresholds, biases, etc.

-   4 LVDS readout channels / chip with a full readout time of less than
    1 ms.

-   JTAG programmable testing modes.

-   Appropriate flags for readout synchronization, e.g. first.pixel
    marker

-   Synch input to start chip function.

-   Signal / Noise \> 9 at operating temperature of 30 degrees C.

-   Maximum average power dissipation of 100 mW / cm2.

[]{#__RefHeading___Toc137569243 .anchor}Table 17: Final Stage
Specification Summary - constraints for the ULTRA series APS.

This system readout is a bit different than the previous MIMOSTAR4 based
readout but most components are the same.

## Architecture for the Ultimate System

In this system, the much of the functionality of the daughter cards has
been moved into the Ultimate sensors themselves. The correlated double
sampling and dual level discriminator functionality are now integrated
onto the sensor and there are 4 LVDS readout lines / chip. The rest of
the system, however, remains substantially the same.. A revised
functional block diagram is shown below in Figure 44.

![](media/image50.wmf){width="5.99375in" height="4.8909722222222225in"}

[]{#__RefHeading___Toc137569204 .anchor}Figure 44: Functional block
diagram for Ultimate sensor based readout system.

**[Data Flow]{.underline}**

The full HFT ultimate sensor readout system is a parallel system of 33
ladders, each with the same readout, thus the readout system for a
single ladder is presented as the basic building block of the system. In
this readout design, the sensors are as described above and are
triggered. Triggers, clock, JTAG, Synchronization and markers are
provided from the Readout board. After receiving a trigger, the sensors
perform a high / low threshold compare to the CDS value for each pixel
and send a serial stream of the logic values results of this threshold
comparison through LVDS buffers located at the end of the ladder (out of
the low mass region) to the Readout board via low mass twisted pair
cable. On the Readout board the data is transferred into high speed
memory located in an FPGA. This memory now contains the full contents of
each pixel on the sensor represented by a 2 bit string with the hot
pixel map implemented as a mask. The sensor threshold data is then run
through a cluster finder with the same form as previously described in
our readout system and the resulting center address is passed to a
cluster FIFO. The contents of these cluster FIFOs are built into an
event with header and event ID information and the resulting event is
passed to DAQ via the DDL.

**[Timing and Implementation]{.underline}**

The proposed time structure of the readout is shown in the diagram
below.

![](media/image52.wmf){width="5.99375in" height="3.279861111111111in"}

Figure 45 - Proposed time structure of the Mimostar Ultimate readout. To
meet the 1 KHz trigger requirement, we require two memory buffers to
allow for immediately retriggered readout of the sensor.

The requirement of servicing a 1 KHz trigger combined with the 1 ms
readout time of the sensors drives us to implementing two full sensor
readout buffers in a ping-pong configuration. Data resorting, if
required, will be implemented in the memory controller to give a raster
scan data image in the memory. The hot pixel map will also be
implemented as a mask in the memory. Unlike the synchronous analog
readout cluster finder, which operates on the readout clock (50 MHz),
the cluster finding for the ultimate sensor can run at implementation
speed in the FPGA which is significantly faster. The event building
readout into the event buffer similarly will run at FPGA implementation
speed. Data transfer through the DDL is fast and additional event FIFO
buffers can be implemented if needed.

It is important to note that current FPGAs are fast and have significant
memory block resources. The Xilinx Virtex-5 can be purchased in
configurations that have 11.664 Mbit of block memory and 550MHz internal
clock distribution[^124]. We could implement the entire RDO system for
one ladder on two Xilinx Virtex-5 FPGAs in the current technology. The
projected time frame for producing this readout system is 3 years away,
during which time we expect to see further improvements in FPGA
technology.

[]{#__RefHeading___Toc137569127 .anchor}Data Rates for the Ultimate RDO

  --------------------------------------------------------- -------------
                                                            

                                                            

                                                            

                                                            

                                                            

                                                            

                                                            
  --------------------------------------------------------- -------------

[]{#__RefHeading___Toc137569244 .anchor}Table

We use the same parameters as used in the prototype system data rate
calculations but scale the occupancy to reflect the much smaller
integration time. We obtain the following rates. Note that the rates are
still for the 10^27^ luminosity. Please scale as appropriate.

The data rate from each 640 × 640 Ultimate detector is 102 MB / sec. The
total rate of raw data entering the processing chain in the detector is
thus approximately 3.38 GB/sec. Pixel addressing within a sector still
requires 18 bits and is padded to 32 bits in our event format. Combining
this with the occupancy per layer and the readout rate of 1 KHz gives an
event size of 22.8 KB and data rate from the detector of 22.8 MB/sec.
Figure 46 shows this graphically.

![](media/image53.wmf){width="4.59375in" height="1.25in"}

[]{#__RefHeading___Toc137569205 .anchor}Figure 46: Data rates in
ULTIMATE HFT readout.

## Prototypes

Several different prototype readout electronics boards have been
constructed and tested that are very similar to the proposed prototype
readout electronics described above. In Figure 47 one can see the
prototyping results of a low mass flex PCB on a prototype ladder with
MIMOSA5 detectors. This prototype flex cable has the signal trace
configuration that we expect to use (except in copper conductor rather
than aluminum) for the final readout for both the prototype and ultimate
ladder readout.

![](media/image55.jpeg){width="6.0in" height="2.1375in"}

[]{#_Ref136929718 .anchor}Figure 47: A prototype ladder showing low mass
PCB, MIMOSA5 detectors and driver electronics bonded to a mechanical
carbon fiber and reticulated vitreous carbon foam based carrier.

A prototype readout system for reading the MIMOSTAR2 sensors was also
constructed. The MIMOSTAR2 sensor is a preliminary prototype for the
MIMOSTAR4 and contains the basic interface and functionality that we
expect to have in the first stage prototype detector[^125]. We are
developing a three MIMOSTAR2 sensor telescope for test during the 2007
beam run at RHIC. Our goal is to test functionality of a prototype
MIMOSTAR2 detector in the environment at STAR in the 2006-2007 run at
STAR. We anticipate gathering information on:

-   Charged particle environment near the interaction region in STAR.

-   Performance of our cluster finding algorithm.

-   Performance of the MIMOSTAR2 sensors.

-   Functionality of our tested interfaces to the other STAR subsystems.

-   Performance of our hardware / firmware as a system.

-   The noise environment in the area in which we expect to put the
    final HFT.

To realize this goal we are assembling a prototype telescope and the
mechanical infrastructure and positioning system to allow us to place a
3 MMIMOSTAR2 detector telescope into the STAR magnet near the
interaction region. This plan presents challenges in the mechanical,
electronics, firmware and infrastructure aspects of assembling and
integrating a prototype system into the STAR environment. The basic
design of our system is shown below.

[Electronics Hardware]{.underline}

Configuration: 3 separate MIMOSTAR2 chips on individual mounts in a
telescope configuration. This system is controlled and read out into a
motherboard mounted to a Stratix development board[^126]. Trigger and
clock come from the STAR TPC detector subsystem cables as does power.
This system is meant to be a functional test of our intended prototype
HFT readout system. A block diagram of the test system is shown below.

![](media/image56.wmf){width="5.99375in" height="5.013194444444444in"}

Figure 48: Block diagram for the HFT MIMOSTAR2 three detector telescope
to be tested in the STAR 2007 beam run.

In this test, the basic data flow is as follows. Triggers are received
from the STAR triggering system into the motherboard and implemented in
the FPGA on the daughtercard. MIMOSTAR2 Configuration (JTAG), latch up
protected power, clock, sync and reset are provided via shielded CAT-5
cables from the motherboard[^127]. In our configuration, the MIMOSTAR2s
will be running in parallel with multidrop LVDS clock and only 1 marker
signal is brought out for synchronization. The analog signals and marker
signals are transferred from the MIMOSTAR2 detectors mounted on the flex
PCBs[^128] via the same CAT-5 cables back to the motherboard and routed
to the daughtercard[^129] for ADC conversion, data resorting, CDS and
cluster finding. Cluster addresses or full frame data (raw or CDS) is
then routed from the daughtercard through the motherboard into the
Stratix development board and to our data acquisition linux PC via the
CERN ALICE DDL / RORC modules. From the DAQ linux PC, the events are
built into the STAR event structure and/or into our own event files.

[Hardware]{.underline}

The hardware used in this test consists of 3 MIMOSTAR2 detectors mounted
on 25 micron kapton flex PCBs, a motherboard, daughtercard, Stratix
development board, DDL, DAQ linux PC and configuration PC. Photographs
of the assembled motherboard, daughtercard, Stratix development board
and DDL, and of the assembled cable are shown below.

![](media/image57.jpeg){width="5.998611111111111in"
height="4.498611111111111in"}

Figure 49: Photograph of HFT prototype readout system containing
motherboard, daughter card, Microtronix Stratix development board and
CERN DDL fiber optic interface module.

![](media/image58.jpeg){width="5.988888888888889in"
height="2.9090277777777778in"}

Figure 50: 25 µm Kapton flex cable with MIMOSTAR2 sensor mounted and
bonded. Three of these will be stacked to form a telescope.

[Firmware]{.underline}

The firmware required for this test includes most all of the
functionality of the final firmware needed for the full detector. The
following basic functionality is required:

*Implemented on daughtercard*

-   Accept and respond appropriately to triggers in the STAR trigger
    system.

-   SRAM memory controller

-   50MHz 12bit ADC serial interface

-   Data re-sorter

-   CDS

-   Full frame readout

-   Cluster finding

-   Cluster FIFO building

*Implemented on Stratix board*

-   Event building

-   JTAG configuration of the chained MIMOSTAR2 chips from the Stratix
    board.

-   Interface to control shell

-   DDL control and interface

-   Latch-up detection and remote reset

[Integration]{.underline}

Some significant effort is required to insert this telescope into the
STAR experiment close to the interaction point. An insertion system,
electronics control box and cooling system have been designed and
fabricated. The basic system is shown below:

![](media/image59.jpeg){width="6.291666666666667in"
height="4.486805555555556in"}

Figure 51: Solid model of telescope insertion system and electronics box
position. The large blue toroid is the magnet endcapthe insertion tube
is shown below the beampipe.

In this diagram, the insertion tube that carries our telescope to a
point near the interaction region is shown with respect to the STAR
magnet pole tip. The electronics box is mounted in the hole in the pole
tip above the beam pipe. Cooling is provided by a vacuum system that
draws air past the MIMOSTAR2 detectors through a tube that also carries
our signal cables. The MIMOSTAR2 detectors on their flex cables will be
mounted into a positioning assembly (shown below) that will hold
position and protect the telescope as it is slid into the insertion
tube.

![](media/image60.jpeg){width="5.999305555555556in"
height="2.8409722222222222in"}

Figure 52: Telescope mechanical housing and positioning head. The
MIMOSTAR2 flex bacles will be glued to the aluminum support and
positioning brackets.

One can see that this test contains most all of the required firmware
development and experiment interfaces required for both generations of
detector.

# Integration with STAR

## Mechanical Introduction

Simulations have shown that for good D meson identification in the high
track density environment of Au + Au collisions, it is important to
minimize the scattering thickness of the beam pipe and the first
detection layer. This requirement has been the primary driver of the
current design. By using APS technology we can use thinned silicon with
modest connection requirements to minimize the support electronics and
cable thickness. In addition, the low power nature of these devices
allows a mechanical design that is air-cooled which helps to minimize
scattering material in the track path. Several conceptual designs have
been considered. We present the design that is currently the focus of
our investigations.

In recognition of difficulties encountered in a variety of experiments,
we are adopting design requirements for rapid insertion and removal of
the vertex detector, rapid calibration and calibration transfer and
multiple detector copies. By addressing these issues early in the design
cycle the requirements can be met without major cost impact.

Our mechanical design makes significant use of carbon composite
material, which has nearly the same radiation length as beryllium. This
allows us to take advantage of the extensive work going on at LBNL for
the ATLAS pixel detector. This will be a significant advantage when we
move into the production phase of the project.

Since the design of the original STAR detector system, there has been
significant progress in tools available for mechanical work. Very
powerful low cost 3D CAD programs are now available which allow complex
modeling with moving parts and direct interfacing to CNC machines and
rapid prototyping. These tools provide the means to tackle the more
complex mechanical designs required for rapid insertion and alignment.
The tools are much easier to learn than older software, but because of
their newness, there is still a limited base of experienced users to
talk to. The R&D effort to develop fabrication and fixturing methods is
allowing us to gain the required expertise with these new tools.

## Support Carriage for Rapid Installation and Removal

We have started to evaluate our design with test structures, detector
carrier support elements, and digitizing and readout electronics boards.

A conceptual design has been developed for the support carriage that
will permit rapid insertion and removal without moving other detector
components. This system is shown in Figure 55 as it will be positioned
for operation in the STAR detector system. The HFT is a small detector
at the center of the STAR system. An additional layer of tracking (not
shown), the SSD, lies out side of the HFT. Room is left for another
detector between the SSD and the HFT that can be added at a later date
for improved performance. As shown, the mechanical support and
electronic service for the HFT is located on one end only. The light
weight guide track and support bridge are permanent elements of the HFT
design. The detector system permits shown in the figure allows for rapid
removal and replacement while maintaining reproducible position through
the use of fully defined kinematic mounts. For simplicity several
existing STAR structures such as the SSD and one of the cone supports
that is integral to its support are not shown

![](media/image63.jpeg){width="6.0in" height="4.027083333333334in"}

[]{#__RefHeading___Toc137569209 .anchor}Figure 55: The HFT is shown
integrated with the STAR inner detectors cone assembly.

A close up view in Figure 56 shows the HFT ladder arrangement. There are
two tracking layers: one at 1.5 cm radius and the other at 5.0 cm
radius. The ladder design shown in the figure consists of a single thin
gull shaped carbon fiber beam with a flex PC cable plus multiple thinned
silicon APS detector chips. Other two layer carbon fiber laminates are
being developed which are simpler to construct but pose additional
challenges in terms of mechanical integration.

![](media/image64.jpeg){width="5.0in" height="3.0in"}

[]{#__RefHeading___Toc137569210 .anchor}Figure 56: Close up view of the
HFT ladders.

The detector support structure illustrated in Figure 57 shows elements
of the kinematic position control assembly. The detector ladders are
arranged in 6 modules with 3 outer layer ladders and 1 inner ladder on
each module. The module is supported on a long structure, shown in
green. The long green structure allows stable support of the ladder
while avoiding contact with cabling and other structures that are part
of the intermediate tracking system The brass colored structures provide
common tie points for coupling the detector system in a reproducible
manner. A more detailed view of this support system appears in Figure 58
showing the three identical frames (brass colored) that are
kinematically joined around the beam pipe using ball on cylinder
contacts.

![](media/image65.jpeg){width="5.2875in" height="2.7090277777777776in"}

[]{#__RefHeading___Toc137569211 .anchor}Figure 57: Detector support
structure with kinematic mounts to insure repeatable detector
positioning.

The system provides three point kinematic mounts for each of the
detector module stalks. This allows opening of the ladders to pass over
the large radius region of the beam pipe during detector insertion and
removal. The three multi coupling structures are joined with each other
(a total of 6 contact points joining the three structures) again
providing a defined reproducible assembly in lockdown while allowing
separation for disassembly from around the beam pipe. Finally there is
an additional 3 point kinematic mount on the 3 fold multi coupler to
lock it to the STAR support cone. All the contacts use commercially
available balls on cylinder pairs. Releasable capture mechanisms are
required to load the kinematic mounts. These devices are not shown in
the illustrations.

![](media/image66.jpeg){width="5.325in" height="3.8625in"}

[]{#__RefHeading___Toc137569212 .anchor}Figure 58: Kinematic mount
structure.

The common tie point joining the HFT detector system to the STAR support
cone is illustrated in Figure 59. The blue-grey piece is the final
common tie point for critical mating parts. It supports the beam pipe
wheel structure, the kinematic detector frame, the roll-in rails and
joins the HFT detector assembly as a whole to the existing STAR support
cone. This design will allow preassembly and testing of the critical
matching pieces prior to final installation into the STAR system. The
final installation requires simply securing the blue-grey piece to the
STAR support cone using existing bolt holes. Note, the card barrel is
loosely coupled to the detector cluster giving freedom for the kinematic
mount to define final precision positioning.

![](media/image67.jpeg){width="5.325in" height="3.99375in"}

[]{#__RefHeading___Toc137569213 .anchor}Figure 59: The HFT is shown
integrated with the STAR inner detectors cone assembly.

For simplicity several existing STAR structures such as the SSD and the
cone support are not shown.

### Position Alignment and Calibration

The required position resolution of better than 10 µm is a significant
challenge for calibration and alignment. The detector system is small
and so it permits a design with a single kinematic support to define a
reproducible position centered on the STAR system. With the proposed
mechanical design the detector system can be moved as a unit between the
visual coordinate measuring system and the docking support in the STAR
experiment without disturbing the relative HFT ladder positions. The
brass colored structures shown in Figure 58 can be separated and
reassembled without changing their relative alignment. This opening of
the structure is necessary both for installation around the beam pipe
and for visually accessing the inner layer detector surfaces with the
visual coordinate measuring machine. The detector silicon will be
patterned with a fiducial grid in the top aluminum connection layer.
This fine-grained grid will be mapped in 3D with a visual coordinate
measuring machine.

The pixel-to-pixel mapping will be preserved through transfer and
docking in the middle of the STAR detector system. The calibration
burden by track matching will be limited to determining the 6 parameters
defining the location of the vertex detector unit within the other STAR
detectors. It is expected that success of this approach will require
careful temperature control. Further analysis and measurements will
quantify this requirement.

## Ladder Design and Fabrication

One of the ladder designs currently under evaluation is shown in Figure
60. This structure must be stiff enough to be supported at one end,
only, and maintain reliable position stability to better than 10 µm. The
ladder is made up of a 150 µm two ply carbon composite open beam, which
is closed with the detector sandwich to make a rigid structure. The
bottom layer of the detector sandwich is an aluminum Kapton cable. Wire
bonds will provide connections between the APS chip and the cable.

It was found with test structures that it is relatively straightforward
to make wire bonds on these thinned devices where vacuum chucks maintain
the sandwich as a flat firm surface. Alternate carrier designs are also
being investigated.

A method to bond the silicon sandwich has been developed using DuPont
Pyralux LF thermally activated acrylic sheet adhesive. Advantages of
using sheet adhesive include fixed bondline thickness and ease of
handling. Other bonding methods including low viscosity epoxies are also
under investigation.

A complete ladder using silicon-sandwiches has not been produced yet but
we have tested fabrication methods that use a uni-layer silicon
structure. We have used vacuum chuck fixturing to butt-join chips side
by side against a straight edge and have then bonded to Kapton with
Pyralux. This bonded structure was then bonded at room temperature to
the carbon beam using Hysol EA9396 aerospace epoxy. 75 µm glass spacer
beads were added to the epoxy for the initial test structure to control
the bondline thickness. Prior to bonding, both the sandwich and carbon
beam are quite flexible and their shape is defined during bonding with
vacuum chucks.

A few µm protective polymer, Parylene, will be used to protect the
exposed wire bonds and to control the spread of carbon dust. The
Parylene coating process is available in the electronics industry. The
coating material is applied at the molecular level by a vacuum
deposition process at ambient temperature. The thickness is well
controlled and it is uniform without pinholes, so protection can be
achieved without compromising detector thickness. Application at room
temperature avoids introducing stresses that distort the ladder shape.

![](media/image68.jpeg){width="5.992361111111111in"
height="2.1319444444444446in"}

[]{#__RefHeading___Toc137569214 .anchor}Figure 60: Ladder assembly. The
current ladder is composed of a row of 10 thinned APS detectors on top
of a matching row of thinned readout chips and a three-layer aluminum
Kapton cable. The silicon cable structure is bonded to a carbon
composite "v", closing the beam to make a rigid structure.

### Detector Radiation Length

Multiple scattering in the beam pipe and the first inner layer of the
vertex detector sets the intrinsic limit of precision for vertex
resolution. The radiation thickness for an inner ladder and the beam
pipe is given in Table 19. The detector ladder is placed at an angle and
is quite close to the interaction point so particles pass through the
material at varying angles. This results in path length through the
materials that is greater than the thickness. Table 19 shows the
thickness and radiation length of a ladder and the beampipe.

  ------------------------ ------------ ------------------------- ---------
  Material                 Si           Material thickness (μm)   \% X~0~
                           equivalent                             
                           (µm)                                   

  Beryllium beam pipe      133          500                       0.14

  Silicon                  50           50                        0.053

  Adhesive                 13.4         50                        0.014

  Cable Assembly           83.9         125                       0.089

  Adhesive                 13.4         50                        0.014

  Carbon Composite         103          3200                      0.11

  Total for **one ladder** 264          3475                      0.282
  ------------------------ ------------ ------------------------- ---------

[]{#__RefHeading___Toc137569245 .anchor}Table 19: Materials in the beam
pipe and the first detector layer with their total thickness and
radiation length. For details see Ref. \[[^130]\]

### Expected Radiation Exposure

RHIC luminosity projected to 2008 with the assumptions of a 21 week
running period and a 60% operating efficiency for Au + Au give a
radiation dose of approximately 3.7 kRad/year. This calculation may be
found on the web[^131]. The materials that we anticipate using in the
HFT detector are the same as used in much higher radiation environments
like the Atlas pixel detector. One exception is the acrylic adhesive,
which exhibits excellent radiation resistance (Radiation Index
\~5)[^132]. Our structure should not exhibit any mechanical degradation
from the absorbed dose.

## Ladder Mechanical Tests

### Load Distortion Tests

Figure 60, and Figure 62 show a prototype of the ladder and detector
carrier that we have built. Figure 61 shows a cross-sectional view of
the ladder. The mechanical tests have shown that the stiffness and
bending characteristics of the assembly are acceptable. For these and
similar tests[^133], the ladder's mechanical structure was supported on
one end and the surface contours were measured using the vision
measuring machine at LBNL with and without a 10 gram end load. The
deflection profile is shown in Figure 63 along with the calculated
deflection for a simple triangular closed beam. The stiffness is within
20% of expectations from an engineering model. The measured fundamental
frequency of the ladder is 140 Hz while the engineering model gives a
resonant frequency of 135 Hz[^134]. We have also measured the
vibrational environment at the STAR detector and it is dominated by low
frequencies, \~ 1 Hz, and so we anticipate that the ladder will maintain
its position to better than a few microns.

![](media/image69.jpeg){width="5.992361111111111in" height="1.29375in"}

[]{#__RefHeading___Toc137569215 .anchor}Figure 61: A cross-section of
the prototype detector ladder showing its structure and materials
composition.

![](media/image70.jpeg){width="4.852777777777778in" height="3.5in"}

[]{#__RefHeading___Toc137569216 .anchor}Figure 62: A cross-section of
the prototype detector ladder showing its structure and materials
composition.

![](media/image71.jpeg){width="5.080555555555556in"
height="4.007638888888889in"}

[]{#__RefHeading___Toc137569217 .anchor}Figure 63: Measured bend of a
silicon/carbon composite ladder test structure and the calculated bend
shape. The ladder was rigidly supported at one end with a 10 gram weight
placed on the other.

### Thermal Distortion Tests

We are in the process of measuring thermally generated deflections using
TV holography[^135]. This tool, available to us, courtesy of the ATLAS
Pixel project, provides a rapid visual distortion map on the sub micron
scale. This is proving to be a useful tool for identifying and
understanding different design parameters. Measurements are done on
structures thermally isolated in a transparent box (see Figure 64). This
tool will also be used to test the stability of the final detector.

![](media/image72.jpeg){width="5.991666666666666in"
height="3.495833333333333in"}

[]{#__RefHeading___Toc137569218 .anchor}Figure 64: TV Holography system
viewing test ladder in a small transparent wind tunnel. Inset shows
diffraction pattern with color map of the surface displacement.

### Cooling Measurements

In the interest of low mass, the detector system is being designed to
use air-cooling for the detector structure in the active tracking
volume. A small wind tunnel has been constructed (see Figure 64) to
evaluate the cooling capacity of air and the design of the system.
Measurements with resistive heated ladder test structures and
thermocouple readouts show that air velocities on the order of 1 m/s are
sufficient to handle power levels of 150 mW/cm^2^. This setup, with its
thermal camera, will be used to check operating silicon-ladder-devices
to evaluate localized heating issues. In addition to cooling tests the
wind tunnel is being used to evaluate the vibration stability of the
ladder design under the required wind flow conditions. The amplitude of
cooling air driven vibrations in the ladder is measured with non-contact
capacitive probes[^136] and if necessary specific vibration modes can be
monitored with the TV holography system. The vibration induced by the
air cooling gives an acceptable positional location σ of 1.6 µm. The low
mass ladder structure is the critical design element requiring vibration
testing, but the full detector structure will be evaluated with this
system as well[^137]. Additional information on the cooling studies is
available on the web[^138].

## Cabling

Connection to the detector readout chips will be done with an aluminum
Kapton cable. The cable is part of the ladder and is located underneath
the APS detector. It will carry power, clock, analogue signals and
control. We have recently found a commercial vendor that makes aluminum
cables with vias. Fabrication tests have been done with 0.35 mil
aluminum with 1 mil Kapton plus two 1 mil acrylic adhesive layers. Wire
bonding tests with this material have been successful.

Figure 65 shows the prototype for a HFT readout cable. The prototype is
an active cable with a buffer and differential amplifier for each sector
of 10 MIMOSA detectors for use in testing the MIMOSA-5 detectors. The
final detector readout cable will not have the components shown but will
narrower and will be sized to be slightly wider than the detectors
themselves to allow wire bonding. The cable in the figure uses a copper
conductor on a 25 µm Kapton insulator. The final cable will be a 4-layer
low-mass aluminum conductor cable with a radiation length equivalent of
84 µm of Si. The prototype has been successfully tested and is working
in our prototype ladder readout system. Additional and more recent
information on cable and ladder developments may be found on the
web[^139].

![](media/image73.jpeg){width="5.993055555555555in"
height="1.5659722222222223in"}

[]{#__RefHeading___Toc137569219 .anchor}Figure 65: A prototype readout
cable for the HFT.

Figure 66 shows a mechanical Prototype with 4 MIMOSA-5 detectors glued
to the Kapton cable assembly with a thin acrylic film adhesive. The
adhesive has a bond line of 50 µm. The MIMOSA is relatively flat in
horizontal direction but has a "dish" shape in vertical (y) direction.
The magnitude of these distortions is, however, quite small. The largest
deviation from a linear fit in y is only ± 6 µm. This low rate of
deformation will allow us to make a minimum number of measurements of
the detector positions on the ladders in order to locate the individual
pixel position to the required accuracy by a parameterized location
function[^140].

![](media/image74.jpeg){width="5.989583333333333in"
height="2.9784722222222224in"}

[]{#__RefHeading___Toc137569220 .anchor}Figure 66: Mechanical Prototype
with 4 MIMOSA-5 detectors glued to the Kapton cable assembly.

## Beam Pipe

The vertex detector extends the measurement radius to 15 mm from the
interaction axis and will require a new small radius beam pipe for STAR.
The design concept for this pipe is shown in Figure 67.

![](media/image75.jpeg){width="6.0in" height="3.73125in"}

[]{#__RefHeading___Toc137569221 .anchor}Figure 67: Beam pipe structure
with outer support barrel in the center.

The beam pipe is constructed from 60 mm diameter aluminum tubes that
taper down to a 29 mm diameter beryllium beam pipe with 1 mm thick
walls. The central region where the HFT is located is a 29 mm diameter
beryllium pipe with a 0.5 mm thick wall. An exoskeleton made up of an
outer tube and the two spoked wheels carries the support load, isolating
the thin central tube such that it only has to carry vacuum load.
Central support of the beam pipe system against gravity is provided by
securing the exoskeleton structure. The central portion with the
exoskeleton structure is shown more clearly in Figure 68. The outer
shell has been made transparent in the figure to show the inner
structure.

![](media/image76.jpeg){width="6.0in" height="4.5in"}

[]{#__RefHeading___Toc137569222 .anchor}Figure 68: View of the beampipe
mid-section.

### Minimum Beam Pipe Radius Consistent with Injected Beam

We have chosen a minimum radius for the beam pipe which, now that the
RHIC optics have stabilized, is reasonable conservative. From the
standpoint of beam pipe interaction with beam optics, the beam pipe wall
should, according to accelerator physicists at BNL, have a minimum
radius of 6 σ while 10 σ is very conservative[^141]. σ is the beam
envelope at injection. For STAR at injection the beam emittance is 15 π
mrad⋅mm, β^\*^ is 10 m, βγ is 10.52 which gives σ = 1.5 mm plus a 5 mm
beam offset at injectionError: Reference source not
found^,^[^142]^,^[^143]^,^[^144]. For protons injection σ is smaller and
therefore less of a constraint. We have chosen an inner beam pipe radius
of 14.5 mm or \~ 7 σ. This is satisfactory for RHIC operation, however,
depending on focusing details this could be the limiting aperture,
leaving the HFT vulnerable to uncontrolled beam dumps135.

### Beam Pipe Radius - Vacuum Considerations

The dimensions of the beam pipe sets limits on pumping speed and the
expected pressure at the center of the interaction region in STAR. The
1600 mm long central section with a 28 mm diameter joins a larger 58 mm
diameter pipe, which extends 3000 mm to the pumping section. A simple
analysis was performed[^145]^,^[^146]^,^[^147] which gives a pressure
increase at the center in the interaction region of \~ 10^-11^ torr.
This is the pressure increase above the pressure at the pumping station
and it is based on a surface out-gassing rate of 5.3 × 10^-11^ W/m^2^
and a conservative estimate of the pipe conductanceError: Reference
source not found. This is well within the requirements. The greatest
uncertainty in this estimate is the out-gassing rate, but a factor of 10
greater value is still tolerable. If an Active NEG coating is used the
inner section will be a pumping surface rather than an out gassing
surface.

### Supporting Section

The central supporting section will be attached to the existing carbon
cone in STAR and will provide the support for the center portion of the
beam pipe. The outer cylinder and spoke section, made of either carbon
composite or beryllium, is designed to support the pipe sections
extending from either end and constrain them sufficiently that a very
thin beam pipe structure can be used in the center region. This outer
cylinder with spokes plus flex sections will isolate the thin central
pipe from outside forces so that the central pipe only has to support
the vacuum load. The spokes that couple the extended beam pipe to the
outer cylinder are shaped to allow free insertion of the vertex detector
into the enclosed center region. The end view in Figure 69 shows how the
detector ladders mesh with the support spokes.

![](media/image77.jpeg){width="5.2in" height="4.115277777777778in"}

[]{#__RefHeading___Toc137569223 .anchor}Figure 69: End view showing the
HFT ladders between spokes to the inner beam pipe support.

### Central Beam Pipe Thin Window Section

The current plan for the thin central region is to use a beryllium pipe
with a 0.5 mm wall thickness. It is believed that this will provide the
minimum coulomb scattering while maintaining a reasonable structural
safety margin. Preliminary analysis indicates a 50:1 safety margin for
buckling[^148]. There is a structure failure limit imposed by the degree
of roundness of the cylinder. A stress analysis indicates that a 1 mm
deviation from a cylinder (1 mm difference between major and minor axis
of an ellipse) sets a 20:1 safety margin for material failure.

### RF Background from the Wake Field

It is believed that the beam pipe in STAR will be sufficient to suppress
wake field signals in the detector to well below the expected signal.
The evidence for this belief comes from experience with the gas jet
polarimeter and carbon strip polarimeter located at the 12 o'clock
intersection region of RHIC. These polarimeters have silicon detectors
located inside the beam pipe close to the beam and it has been found
that a few microns of conductor are sufficient to shield these detectors
from the wake field[^149].

The HFT will be located outside the beam pipe and will benefit from the
RF shielding this pipe provides. The pipe will be constructed from at
least 500 microns of beryllium. The maximum beam in RHIC will be 1 ×
10^9^ gold ions per bunch. This gives a wake field current of 110 mA
(counting both beams), which with a 1 µm skin depth will generate a
resistive potential drop of only 7 mV. A more sophisticated calculation
should include induction, rise time and skin depth and detector
filtering, since our bandwidth cutoff will be closer to the wake field
GHz frequency than the polarimeter detector cutoff.

### Beam Pipe Insertion and Removal

We are currently investigating the removal and insertion of the
beampipe. We anticipate replacing the current center section of the STAR
beampipe with the components described above. This will likely entail a
period of refit and installation to install the new beampipe after which
removal and reinsertion should not be necessary during the normal course
of running and detector repair.

### Bake Out

Beam pipe bake out is still under discussion. It is expected that the
same matter NEG coating will be used and there are two possibilities in
how this will be treated. It may either be a minimum bake out to remove
primarily water as it is done now with a hot dry nitrogen scrub gas at
150 C to 110 C or it could be a full activation of the NEG coating
turning it into a pumping surface[^150]. This later case requires baking
at 150 C to 200 C with vacuum. This could impose additional constraints
on beam pipe materials and would require additional thermal isolation
from the other detectors. If it is decided that the less aggressive bake
out procedure is adequate then carbon composites can be used in the
construction of the beam pipe exoskeleton. In either case the HFT will
be withdrawn during bake out from the operating location to isolate it
from the heated beam pipe.

## Compatibility with the SSD and other cone mounted detectors

The HFT will be supported and installed from one end only. There will be
no other detectors requiring support or access at this end. Other
detectors requiring cone support will be outside of the HFT space. The
FTPC on one end will have to be removed to accommodate the HFT and
support electronics as shown in Figure 55.

## Compatibility with an Upgraded Inner Si Tracker Barrel

If the SVT is replaced with a new inner Si Tracker barrel and forward
disks for the tracking outside of the SVT then the support structure for
the new tracker will be installed from one end of STAR and the HFT will
be supported and installed from the opposite end. This approach is being
further developed by our group and the MIT Si barrel group.

# Preliminary Cost and Schedule Estimates

## Cost and Schedule

The HFT is a very demanding technical project and it requires a
substantial amount of R&D in order to ensure that it will be ready to
take data when RHIC is running. The enclosed cost and schedule estimates
reflects the need to develop and test the pixel sensors over a period of
several years. We envision at least four generations of chips before we
have a full sized, production quality, chip design. The cost and
schedule also reflects the need for replacement parts so that the
detector can be easily repaired during a run.

Synchronizing with the RHIC run schedule is beyond the scope of this
document, but it is an important topic none-the-less because a
successful HFT program requires several years of running, including (as
a minimal set) a high statistics top energy Au-Au run, a p-p run, and a
d-Au run. Since the RHIC schedule is hard to predict, it is very
important that the HFT be ready on time, and be reliable during each
run. The number of opportunities to run top energy Au-Au beams are very
limited. Therefore, we have designed the HFT to be easily repaired
during a run and even replaced, if necessary.

A schematic overview of the proposed R&D and construction schedule is
shown in Figure 70 with particular emphasis on the installation dates.
Note, for example, that the figure includes a milestone for the
installation of a detector telescope based on MimoSTAR II chips in the
summer of '06. The detector telescope will take data in FY07 with the
goal of testing the APS chip technology under working conditions at the
STAR experiment. Additional installation activities are shown in each
year. A more detailed roll-up of the projects activities is shown in
Figure 71.

![](media/image78.wmf){width="5.9944444444444445in"
height="1.4569444444444444in"}

[]{#__RefHeading___Toc137569224 .anchor}Figure 70: A Schematic view of
the installation activities in each year. Installation is typically done
in the summer of each year.

![](media/image79.png){width="9.67986111111111in" height="5.85in"}

[]{#__RefHeading___Toc137569225 .anchor}Figure 71: Schedule overview and
roll-up of activities.

A full Micro-Soft-Project model for the HFT project has been developed
and the files are available on the WWW. The files are too complex to be
included in this document but an extract of one of these files is shown
in Figure 72.

![](media/image80.png){width="6.0in"
height="2.24375in"}[]{#_Ref137471196 .anchor}

**Figure 72: Detail from the Gantt Chart that tracks the full enterprise
of HFT activities.**

The HFT requires a substantial amount of R&D work due to the innovative
new technology that is an integral part of the project. The items
needing further development include the Si chips, the readout and DAQ
electronics, the mechanical arms to hold and insert the detector, the
beam pipe, and the calibration and tracking software. The R&D activities
will begin in FY06 but the bulk of the R&D activities will take place in
FY07 and FY08. During FY06, the engineering labor will be contributed by
the Berkeley Lab, while in FY07 and 08, we anticipate that one
mechanical engineer and one electrical engineer will be funded by the
R&D dollars associated with the HFT project.

The contributed engineering estimates shown in the tables, below, have
not been negotiated with LBL or BNL management and are subject to
further refinement and the availability of funds.

Significant R&D activities to be studied in FY06 include:

-   QA and test of MimoSTAR II chips from Strasbourg

-   Conceptual design of readout boards for the Strasbourg chips

-   Prototype ladders to support the chips

-   DAQ interface prototype

-   Develop Al clad cable technology

-   Conceptual design of the kinematic mounts to hold the ladders

-   Live beam tests in STAR using MimoSTAR II chip

-   R&D for the MimoSTAR III chip

Estimated expenses in FY06:

-   \~150 K Procurements

-   \~150 K Engineering

-   \~500 K Contributed Engineering

Significant R&D activities to be studied in FY07 include:

-   QA and test of MimoSTAR III chips from Strasbourg

-   Prototype readout boards for the Strasbourg chips

-   Prototype ladders to support the chips

-   Conceptual design of the kinematic mounts to hold the ladders

-   Live beam tests in STAR using MimoSTAR II chips

-   Collaborative design of the beam pipe

Estimated R&D expenses in FY07:

-   \~670 K Procurements

-   \~250 K Engineering

-   \~1000 K Contributed Engineering

Significant activities to be studied in FY08 include:

-   QA and test of the MimoSTAR IV chips from Strasbourg

-   Live beam tests in STAR using MimoSTAR III and prototype kinematic
    mounts

-   Integration studies for the support of the HFT and Cone
    modifications

-   Development of alignment and calibration techniques

-   Design and test of the thin walled beam pipe and analysis of risk

-   Develop and test interface to STAR DAQ

-   R&D for the UltraSTAR Chip

Estimated R&D expenses in FY08:

-   \~800 K Procurements

-   \~900 K Engineering

-   \~1000 K Contributed Engineering

> ![](media/image81.wmf)

[]{#_Ref122927285 .anchor}

> Table 20: Estimated cost of the HFT components. The total cost for the
> hardware components is \$3.75M. This does not include engineering
> labor but it does include approximately \$1M in manufacturing labor to
> be spent in the Mechanical shops at BNL and LBL.

The estimated costs for the HFT components and hardware are shown in
Table 20. The cost table includes only purchases and manufacturing
expenses. The total hardware cost is 3.75 M\$; this includes \~1M\$
manufacturing labor but does not include the engineering labor or the
contributed labor.

The electronic costs shown in Table 20 are primarily for procurements
from outside vendors while mechanical fabrication costs are primarily
for LBNL and BNL shops. All costs shown include overhead multipliers and
contingency multipliers. The contingency multiplier for the detector
silicon is 1.5, and for all other items the multiplier is 1.75. The cost
for detector silicon and ladders reflect the plan to make 2 copies of
the prototype detector (with MimoSTAR IV chips) and 4 copies of the
final HFT detector (with UltraSTAR chips). The pricing of the readout
electronics includes 100% spares.

The MS-Project files yield a rolled-up labor summary that consumes 13.5
FTEs of engineering labor over the life of the project, 7.5 FTEs of
technical labor, and 3 FTEs of management and management support. The
labor costed to the project is 3 M\$ , and the re-directed labor
required is estimated to cost 1 M\$ at BNL and 2 M\$ at LBL. The
activities at BNL include, for example, technicians to install the
support cones and beam pipe. The cost for labor at BNL and LBL includes
75% contingency on wages.

The total estimated cost of the project is 7 M\$ for R&D and
construction activities. The re-directed labor adds an additional 3 M\$
to the total project cost.

Studying the profile of funds for the HFT project is the next step.
Currently, the profile of funds shown in the Micro-Soft-Project cost
model is not a perfect match to the availability of funds. Refining this
cost model is homework for the future and will appear in our Conceptual
Design Report. Our goal is to match the profile of funds, and the
completion date for the project, which was proposed by Brookhaven
National Laboratory in the "Mid-Term Strategic Plan for the Relativistic
Heavy Ion Collider"[^151]. See Table 21.

  ----------------- ----------------- ----------------- -----------------
  FY 2007           FY 2008           FY 2009           FY 2010

  1 M               0.8 M + 0.3 M     2.5 M             2.5 M

  R&D               R&D + PED         Construction      Construction
  ----------------- ----------------- ----------------- -----------------

[]{#__RefHeading___Toc137569247 .anchor}Table 21: Proposed Funding
Profile for the HFT that appears in the BNL Mid Term PlanError:
Reference source not found

The HFT project was recently reviewed by the Technical Advisory
Committee for RHIC detector upgrades. A copy of the cost and schedule
talk that was presented to the committee is attached in "Appendix V --
Cost and Schedule Presentation" and the report of the committee is
attached in "Appendix VI -- Report of the Technical Committee".

# Summary

Probing charm quark flow and thermal equilibration at RHIC may prove to
be the final step towards the discovery of a Quark Gluon Plasma.
Furthermore, measuring the energy lost by high transverse momentum heavy
(c,b) quarks while traversing the medium will help disentangle between
energy loss scenarios in cold nuclear matter and in partonic matter. The
HFT is designed to tackle both tasks by precisely measuring open charm
hadron yields, spectra and elliptic flow (v~2~) as well as tagging the
electrons produced by high transverse momentum beauty hadrons. The
design requirements are fulfilled by having two thin ( ≤ 0.28% radiation
length) layers of Active Pixel Sensors (APS) with a resolution of 10 μm
at the front surface of the detector. APS technology is the only option
that fits these requirements without compromising the efficiency or the
readout speed. Indeed, an APS can be thinned down to 50 μm and their low
power consumption allows us to use air-cooling. The mechanical support
will be carefully designed so that the detector can be easily retracted.
This feature allows the detector to be externally aligned, repaired and
upgraded. By combining cutting edge sensor and readout technologies with
a flexible and robust mechanical design, the HFT will provide the high
precision data on heavy flavor hadrons that are crucial to understand
the nature of the medium formed in Au + Au collisions at RHIC.

# Appendix I -- MIMOSTAR II Specifications

  -------------------------------- --------------------------------------
  Technology                       AMS 0.35 opto

  Metal layers                     4

  Epitaxial effective thickness    \~ 12 µm

  Diode size                       1.7 µm × 1.7 µm\
                                   2.4 µm × 2.4 µm

  Diode pitch                      30 µm

  Signal (5 GeV electrons)         700 -- 800 e^-^

  Noise                            10 e^-^

  Pixels                           128 × 128

  Control                          JTAG

  Readout rate                     2 MHz and 10 MHz

  Operational temperature          \< 40 C
  -------------------------------- --------------------------------------

# Appendix II -- MIMOSTAR II Users Guide

This page intentionally left blank. See the next page.

# Appendix III -- HFT Prototype Read Out

The design for the STAR prototype HFT read out is based on the daughter
board designed by Fred Bieser and Robin Gareus and the synchronous
cluster identifier and data compression scheme proposed by Leo Greiner.
The advantages of this design are:

-   The daughter board hardware will provide the full data rate and
    functionality required for the prototype 4 ms HFT.

-   Much of the required VHDL firmware is running, tested and
    understood.

-   The cluster identifier runs at the same speed as digitization
    providing immediate data compression. Only cluster center addresses
    are passed on for data storage.

-   Data digitization and compression takes the same 4 ms for all events
    independent of data.

-   The cluster identifier and data compression fits well in the FPGA
    environment. It requires few resources and can be implemented with
    simple straightforward VHDL coding.

-   The design is triggered and fits the standard DAQ design.

-   All the hits for an event are stored directly for that event. There
    are no complications with frame boundaries or hits for an event
    located in different frames. This is important because file handling
    software used in STAR data analysis does not have to be altered to
    accommodate the HFT.

-   The latency is 4 ms, but the dead time is 1 ms matching the new TPC
    system.

-   The HFT data size for an event is 90 kB, significantly less than the
    2 MB for TPC central collision event.

This document first provides a brief description of the system. Then,
this section is followed with a diagram and a list, which describes the
elements of the design. Finally, a section on a proposed data storage
structure with expected data loads is discussed.

**System Design**

An HFT ladder has 10 MIMOSTAR-4 chips with 640 × 640 pixels on each
chip. Each chip is divided in half with two parallel analogue,
differential current output buffers. The chips are continuously clocked
at 50 MHz, rastering repeatedly through all the pixels connecting them
to the output buffer. Current to voltage buffers at the end of the
ladder drive the analogue signals over twisted pairs (about a meter
length) that are connected eventually to the daughter boards. The
daughter boards contain an 8 channel ADC, two SRAM chips and an FPGA.
The daughter boards will handle digitization, zero suppression and data
compression. For each event, they will generate a list of hits giving
the address of the pixels at the cluster centers. They will also be able
to operating in slow diagnostic mode providing amplitudes for all the
pixels. The daughter board with its 8 ADC channels can handle 4
MIMOSTAR-4 chips. Using 3 daughter boards per ladder leaves 4 channels
unused.

The daughter boards function in the following manner. A common 50 MHz
clock that drives the MIMOSTAR-4 chip drives the ADCs at 50 MSPS
digitizing one pixel after the other. Each ADC channel writes a 10 bit
digital amplitude to a circular memory buffer repeatedly cycling through
the 640 × 320 pixels on half of a MIMOSTAR-4 chip. The ADCs have 12-bit
capability, so we may use more than 10 bits.

The baseline voltage of the pixels varies significantly from pixel to
pixel -- much more than the amplitude of the minimum ionizing signal. To
extract a signal the pixel must be digitized twice, once before and once
after the particle hits the pixel. The signal is obtained from the
difference in the two values -- a method called Correlated Double
Sampling (CDS). CDS is accomplished in this design by taking the
difference between the old value saved on the circular buffer and the
current ADC value. This is done before overwriting the circular buffer
with the new ADC value. The CDS value or hit amplitude is then checked
against a high and low threshold and the result is clocked into a shift
register, again with the same common clock.

The shift register is long enough to contain 2 rows of the MAPS plus an
additional 3 pixels into the third row. The purpose of the shift
register is to provide a simple fast way to slide a 3 by 3 window over
every pixel of the MAPS to test for clusters. Ports on the nine desired
shift register cells are permanently connected to the cluster sensor
(see Figure 1). Demanding a high threshold in the center pixel plus at
least one low threshold in at least one neighbor pixel should be
sufficient to find the center reliably. This has been checked with
existing data and found to be a suitable approach. In addition to
finding the center pixel, the purpose is to be able to detect minimum
ionizing hits with 98% efficiency while limiting false hits from noise
to a few 10s of hits per cycle through the detector. Since the noise is
essentially random, requiring two pixels above threshold instead of just
one significantly reduces accidentals. This will also be a good filter
against single hot pixels. However, if a hot pixel map is required, it
can be stored in another bit on the circular buffer and this can be
interrogated during CDS.

If the cluster test is positive, then the address of the pixel gets
recorded into a FIFO whenever the trigger enables the FIFO. By timing
from the trigger, a fixed width acceptance window with a fixed delay is
generated. The window width is just the time required to cycle through
all the pixels once. This allows all the hits associated with the
trigger event to be recorded into the FIFO. With this continuous raster
scan view a time window provides the simple selection of event hits.
There is no complication of event hits spanning separate frames.

Once all of the hits for the event are recorded into the FIFO then the
FIFOs for all the channels are read, assembled, and cleared by the
mother board and sent over the RORC/DLL to the STAR DAQ system. The
event acceptance window for an event is the same for all channels and
can be generated at a single location. As soon as the acceptance window
is closed the FIFOs are ready for reading.

**Block Diagram**

The system shown in the Figure 1 will have a dead time of a little more
than 4 ms, but this can be significantly reduced with a reactively
simple addition. Instead of one FIFO per channel, we will use 5. Each
FIFO for the channel will have a separate trigged acceptance window
generator. This approach will result in some data duplication, but it
has the advantage that each event will be self contained, greatly
simplifying file handling during data analysis. Data duplication is not
an issue because the data volume is relatively small.

![](media/image82.png){width="6.7in" height="8.125in"}

Fig. 1. Schematic for the readout system

A.  MIMOSTAR MAPS, 640 × 640 pixels. Continuous parallel raster scan
    readout of the two halves at 50 MHz pixel rate.

B.  Two SRAM memory chips, 72 bits wide and 256 kb deep, rated at
    150-333 MHz, operated as a circular buffer addressed by the pixel
    counter looping over 320 × 640 = 204,800 locations. Up to 18 bits
    available in the memory for each pixel, but only 10 are required for
    the ADC value plus a possible 11^th^ bit used as a hot pixel marker.
    Two memory chips to handle 4 MIMOSTAR chips. VHDL code for memory
    control currently operating. GSI Technology
    <http://gsitechnology.com/18MbBurst.htm>

C.  ADC chip with 8 parallel channels of 12 bit ADCs. Each ADC channel
    continuously digitizes the signals from one half of a MIMOSTAR chip
    at 50 MSPS. The output of each channel is serial LVDS. The ADCs are
    currently operating on the daughter board, but not at the required
    data rate. The firmware used was adapted by Robin Gareus from code
    for another FPGA and is based on work appearing in:
    <http://direct.xilinx.com/bvdocs/appnotes/xapp774.pdf>. Higher speed
    grades of the FPGA may provide speed sufficient for deserialization
    at our desired data rate. The ADC on the daughter board is an
    ADS5270 capable of 40 MSPS. The ADS5271 operates at 50 MSPS -- see
    Texas Instruments' web site
    <http://focus.ti.com/docs/prod/folders/print/ads5270.html>

D.  Correlated Double Sampling (CDS) is accomplished at full readout
    speed by reading the old pixel ADC value from the circular buffer
    and subtracting it from the current new ADC value. The new ADC value
    is written back to the circular buffer overwriting the old value.

E.  A digital discriminator viewing the CDS value returns either 0, over
    low threshold or over high threshold and delivers the result to a
    shift register.

F.  A cluster detector following Leo Greiner's design uses shift
    registers to check for clusters centered on each pixel at the full
    chip readout speed. The signal to noise is supposed to be sufficient
    to identify clusters just by a single pixel over high threshold. So,
    a simple center pixel over high threshold with an adjacent vertical
    or horizontal neighbor over low threshold may provide more than
    enough sensitivity to cleanly identify cluster centers. The goal is
    to be 98% efficient for minimum ionizing with less than a few tens
    of false hits per half chip. All 9 pixels (the center pixel and all
    its adjacent pixels) are available for a more complicated sensor
    algorithm, but the proposed simple scheme should be adequate. There
    may be sufficient filtering to forgo using a hot pixel map. Note,
    however, the proposed daughter board architecture lends its self to
    using a hot pixel map. There is just the added complication of
    uploading the map to the circular buffer. In the figure the shift
    registers are shown as 3 separate items, but in practice they may
    well be implemented as a single register ported as required. The
    cluster sensor/ shift registers, digital discriminator (E), CDS (D),
    and ADC channel (C) are repeated 8 times. This should be well within
    the resources of the Xilinx Vertex-II FPGA, XC2V1000 current used on
    the daughter board
    --<http://www.xilinx.com/products/silicon_solutions/fpgas/virtex/virtex_ii_platform_fpgas/product_table.htm>.
    The selected FPGA has a total of 720 kb Block RAM and 160 kb
    distributed RAM. The shift registers require only 10 kb.

G.  When the cluster sensor detects a cluster it loads (if trigger
    enabled) the center pixel address (the pixel counter + a fixed
    offset) into a FIFO for export. This is the only data that gets
    exported. The FIFO shown is 18 bits wide as required to contain the
    pixel address for half the chip. Note the selected data loaded into
    the FIFO is the same as the address used to access the circular
    buffer. The expected number of hits on the inner ladders at 10^27^
    Hz/cm^2^ luminosity is \~200 for half a chip, so the 2k deep FIFO is
    generous. This FIFO is repeated 8 times for a total Block RAM
    requirement of 288 kb. This is well within the 720 kb available on
    the current daughter board. The system shown will operate triggered
    with a 4 ms dead time. As discussed, a faster 1 ms dead time system
    will require 5 times as many FIFOs or 1440 kb which is over the
    XC2V1000 limit. A bigger FPGA, the XC2V3000, however would suffice
    or the FIFO depth could be reduced by half which is still 5 times
    the expected load.

H.  The trigger, signal, which arrives 1 µs after the event collision of
    interest, is delayed and stretched setting the recording window for
    storing cluster center addresses into the FIFO. The delay is set to
    322 clock cycles less the 1 micro second trigger delay. This total
    delay is the time required for the first hit pixel of the event to
    ripple through the shift register to the center of the cluster
    sensor. The stretch time (acceptance window width) is 204,800 clock
    cycles, the time required for the last potentially struck pixel of
    the event to reach the center of the cluster sensor. These numbers
    can be changed to exclude pixels in the edge rows being counted as
    cluster centers. In this scheme all the clusters for the triggered
    collision are recorded. The dead time is roughly 4 ms the time
    required to read through all the pixels once. Some additional dead
    time may result in transferring the FIFO data down to the STAR DAQ
    system. Additional collisions occur during the 4 ms read time and
    these background clusters are included. A simple variation on this
    scheme can be used to reduce the dead time so that the HFT can be
    included with every TPC trigger. This is done by adding four more
    FIFOs and trigger stretchers for each ADC/cluster finder channel.
    When a FIFO is accepting cluster addresses from one event and a new
    trigger occurs, the next available FIFO/trigger stretcher can
    process data. The acceptance windows of the two FIFOs will partially
    overlap, but each event will carry all the clusters associated with
    the event. The addition greatly simplifying STAR data analysis.

In the design outlined some event building and trigger handling gets
done on the mother board. This differs from the current ladder prototype
design where there are no FPGAs on the mother board. All data
communication is handled by the daughter board FPGA. The advantage in
this approach of using the daughter board FPGA for all the functions is
the simplification of VHDL code development. Having all the code in one
place for debugging and maintenance is certainly desirable. More thought
will be given to following this example. Perhaps the trigger functions
and event building can also be accomplished in the daughter board FPGA.

In any case additional, work is required on for the data connection to
the outside. Currently the connection is via Robin Gareus' PCI
connection protocol and his slow connection code scsn. We want to use
instead the RORC/DLL connection that has become the STAR standard. We
are considering implementing a USB connection to LabVIEW, which provides
a more portable operation for testing and debugging.

**Data rates, zero suppression, data structure**

The data reduction achieved in the daughter board with this design is
significant. For a half a chip, data rate is 65 MB/s before CDS and 50
MB/s after CDS. After cluster identification, the rate depends on hit
density and luminosity. At a luminosity of 10^27^ Hz/cm^2^, the data
rate for the ½ chip in the inner radius is 0.5 MB/s which translates to
an event rate of 1 kHz. This is a reduction of over 100 for the inner,
high exposure chips. The reduction is more for the outer layer.

18 b wide

+------------------------------+---------------------------------------+
| Event/Header                 | range 0-23                            |
|                              |                                       |
|                              | range 0-19                            |
|                              |                                       |
|                              | range 0-204,799                       |
+------------------------------+---------------------------------------+
| ladder address 0             |                                       |
+------------------------------+---------------------------------------+
| half chip address 0          |                                       |
+------------------------------+---------------------------------------+
| pixel address 157,921        |                                       |
+------------------------------+---------------------------------------+
| pixel address 159,203        |                                       |
+------------------------------+---------------------------------------+
| .                            |                                       |
|                              |                                       |
| .                            |                                       |
|                              |                                       |
| .                            |                                       |
|                              |                                       |
| .                            |                                       |
|                              |                                       |
| .                            |                                       |
|                              |                                       |
| .                            |                                       |
+------------------------------+---------------------------------------+
| pixel address 142,888        |                                       |
+------------------------------+---------------------------------------+
| pixel address 148,321        |                                       |
+------------------------------+---------------------------------------+
| half chip address 1          |                                       |
+------------------------------+---------------------------------------+
| pixel address 155,423        |                                       |
+------------------------------+---------------------------------------+
| pixel address 155,231        |                                       |
+------------------------------+---------------------------------------+
| .                            |                                       |
|                              |                                       |
| .                            |                                       |
|                              |                                       |
| .                            |                                       |
|                              |                                       |
| .                            |                                       |
|                              |                                       |
| .                            |                                       |
+------------------------------+---------------------------------------+
| ladder address 1             |                                       |
+------------------------------+---------------------------------------+
| half chip address 0          |                                       |
+------------------------------+---------------------------------------+
| .                            |                                       |
+------------------------------+---------------------------------------+
| .                            |                                       |
+------------------------------+---------------------------------------+

Fig. 2. Data structure for HFT

The total data volume depends on the data structure used. A proposed
data structure with example population is shown in Fig. 2. Half chip
addresses and ladder addresses are distinguished from pixel addresses by
adding constants to make them larger than the maximum pixel address of
204,799. If it is desirable to format the data into 3 bytes instead of
18 bits, then the additional bits may be used to distinguish the 3
address types directly.

As calculated in Table 1, the HFT event size of 90 kB is significantly
smaller than the TPC, which has an event size of 2 MB for central Au +
Au. This proposed data format of ordered hits can be compared with one
that saves the hit addresses. The event storage size for the address
case is 12 MB, over 100 times larger than the proposed scheme.

  ----------------------------------------------------- -----------------
  **Item**                                              **Value**

  bits/address                                          18

  inner ladders                                         6

  outer ladders                                         18

  half chips per ladder                                 20

  ave hits/half chip, inner, L = 10^27^ Hz/cm^2^        200

  ave hits/half chip, outer, L = 10^27^ Hz/cm^2^        40

  Event Size                                            90 kB

  Data Rate at 1 KHz event rate                         90 MB/sec
  ----------------------------------------------------- -----------------

Table 1. Readout numbers for the HFT

# Appendix IV -- Production Schedule

  ----------------------- ----------------------- -----------------------
  **Sensor**              **Item**                **Date**

  MIMOSTAR-1              Begin design            Summer 2003

                          Production              2004

  MIMOSTAR-2              Beam Test #1 at DESY    November 2005

                          Beam Test #2 at DESY    June/July 2006

                          Install in STAR         Fall 2006

  MIMOSTAR-3              Sent to production      June 2006

                          Probe tests             September 2006

                          Dicing and thinning     October 2006

                          Mounting on a ladder    December 2006

  MIMOSTAR-4              Back from foundry       August 2007

                          Dicing and thinning     September 2007

                          Testing of chips        October 2007 to Early
                                                  2008

  Ultimate                Early exploration       2006

                          Production version      2009
  ----------------------- ----------------------- -----------------------

From "Summary of the STAR meeting at IReS-Strasbourg on January 18-20,
2008", M. Winter

# Appendix V -- Cost and Schedule Presentation 

Cost and Schedule estimates presented to the Technical Advisory
Committee for RHIC detector upgrades on March 15, 2006.

# Appendix VI -- Report of the Technical Committee

**\[ *Excerpts from the report of the Technical Advisory Committee* \]**

**Report of the Technical Advisory Committee for RHIC Detector
Upgrades**

**March 14-16, 2006**

*Committee members: M. Cooper (LANL), C. Haber (LBNL), B. Mecking
(JLab), J.* *Proudfoot (ANL), V. Radeka (BNL), R. VanBerg (U. Penn, not
present at the review), J.  Va'vra (SLAC)*

**Introduction**

This committee was convened by BNL to provide advice to the Laboratory
on four proposed upgrades to the PHENIX and STAR detectors:

-   PHENIX Forward Vertex Tracker (FVTX)

-   PHENIX Nose Cone Calorimeter (NCC)

-   STAR Heavy Flavor Tracker (HFT)

-   STAR Integrated Tracking System (INT)

These upgrades play a prominent role in the future physics program for
RHIC, as described in the "Mid-Term Strategic Plan for the Relativistic
Heavy Ion Collider", submitted by BNL to DOE on Feb. 14, 2006. The
committee was asked to assess each in the light of the proposed physics
goals, and advise the Laboratory as to their feasibility and status of
development, the soundness of the proposed costs and schedules, and
their readiness to proceed as DOE construction projects on the proposed
time scales.

The proposed upgrades are not all at the same stage of development. The
two PHENIX upgrades (FVTX and NCC) are well advanced in their scientific
and technical development, and are being proposed as Major Item of
Equipment (MIE) projects to be funded in FY2008, with a Total Project
Cost of less than \$5M each. The two STAR projects are in a more
developmental stage. The HFT relies on a newly-emerging technology for
silicon pixel detectors, requiring substantial R&D effort in the years
2006-2008. The INT involves an optimization of the overall tracking
configuration in STAR that is presently still under study. The STAR
collaboration is aiming for both of these upgrades to begin construction
as MIE projects in FY 2009.

The committee notes that both PHENIX and STAR have specified thin-walled
beryllium beam pipes with a diameter of 3 cm in conjunction with the
proposed vertex detectors. We recommend that this challenging aspect of
the upgrade program be undertaken as a joint effort with the experts in
the Collider-Accelerator Department, and with oversight by the
Laboratory.

The committee's findings and recommendations for the four proposals are
given in the following sections.

**STAR Heavy Flavor Tracker and Integrated Tracking**

STAR has proposed a precision vertex tracking device, the Heavy Flavor
Tracker, using a new technology for fine-grained pixel detectors. This
uses CMOS Active Pixel Sensors (APS) technology, with the potential for
30 μm pixel size on silicon sensors only 50 μm thick. The technology is
being developed at the IReS Laboratory in Strasbourg, France, as well as
at LBNL. The proposed HFT would utilize two layers of such sensors, at 2
cm and 5 cm radii from the collision axis. The readout rate is
relatively slow (4 msec per frame, with possible upgrade to 0.2 msec),
resulting in multiple-event pile-up, but the occupancy is very small, as
the detector will have 108 pixels. The detector would rely on additional
tracking information to make the connection from TPC tracks to the
correct hit segments in the pixel layers. The existing Silicon Strip
Detector (SSD) may be used for an intermediate space point. However, as
part of its Integrated Tracking upgrade (INT), STAR is proposing a new
3-layer set of silicon strip detectors to surround the HFT.

In addition to the intermediate layers of silicon strip detectors to
surround the HFT, the Integrated Tracking upgrade aims to provide new
tracking capability in the forward direction, at angles subtended by the
End Cap Electromagnetic Calorimeter, in the pseudorapidity range η\~
1-2, where the TPC tracking is not adequate to resolve the charge sign
of high-energy electrons from W± decays. In the present proposal, the
forward tracking elements would consist of four discs of silicon strips
close to the collision region, and larger-area detectors downstream
using gas detectors with segmented readout based on GEM technology. The
collaboration is still in the process of simulating the physics
measurements and optimizing the configuration for the Integrated
Tracking upgrades.

**Heavy Flavor Tracker**

[Physics Motivation]{.underline}

The proposal to add a heavy-flavor tracker (HFT) to the STAR detector
will significantly enhance the capabilities of STAR in the mid-rapidity
range. The detection of a displaced secondary vertex will cleanly
identify the production of heavy flavors from the topology of the event
alone. For example, D-meson decays can be identified without the need
for kaon identification and without combinatorial background.

Heavy flavor measurements are important for the RHIC program to clarify
the properties of the dense medium created in heavy-ion collisions.
Important issues to be addressed are the questions regarding the extent
to which thermalization, flow, and energy loss for heavy quarks differ
from what has been observed for the light flavors.

In addition to its role in heavy--flavor identification, the low-mass
HFT will also play an important role in reducing the background in
vector meson mass spectra (measured in e+e- decays) due to contributions
from photon conversions.

[Detector Concept and Technology]{.underline}

The proposed APS technology is the most promising choice for high
granularity, low radiation length, and low power dissipation vertex
tracking in an environment such as RHIC.

The proposed design configuration maximizes the solid angle coverage
while keeping the number of detector layers and the area of silicon at a
minimum. It is assumed that the small beam pipe diameter will be
consistent with reliable machine operation. The overall concept and
implementation plan is well thought out.

This is a cutting edge technology, and it will be used for the first
time on a fairly large scale in a large physics experiment. If
successful, it will have a significant impact on future experiments.

[Technical Issues]{.underline}

The basic sensor cell (pixel), while based on "standard" CMOS process,
depends critically on some properties of the process usually not
specified by the foundry. For example, the dark current at the femto-amp
level is not important for most CMOS applications. While the broad-based
and outstanding R&D program at IRES is commendable, one will have to
capture a good point in the continuing development progress and an
available CMOS process to focus on the design of the device to be used
for HFT.

The decision on how thin the sensors need to be should be based on the
yield, and ease of handling of the sensors, as well as on minimizing
multiple scattering, taking into account other materials necessary in
the detector assembly.

The mechanical design concept makes use of advanced composite materials.
The present design is mostly motivated by the desire to make possible
installation and removal of the detector in a short time. Design
variations which would improve the position stability of the detector
components with respect to each other might be considered. A well
thought out design for the readout was presented.

[Cost, Schedule, Manpower]{.underline}

-   This is a well planned project which recognizes that it has
    limitations in the available manpower and funding, particularly with
    respect to mechanical engineering.

-   The overall listed manpower (\~25 FTE integrated over a five year
    period) will not be sufficient.

-   Given the cutting edge technology, the proposed schedule does not
    contain sufficient float.

-   The cost estimate has been made with realistic contingency figures.

-   Silicon fabrication costs are not a major part of the project cost,
    and allowing for four detector copies is justified.

[Recommendations]{.underline}

1.  This R&D effort should proceed. The proposed schedule for realizing
    a working detector is very tight, and, as the STAR project manager
    noted at this review, the funding profile shown in BNL's Mid-Term
    Plan for RHIC is not well matched to the current plan for R&D
    activities. BNL and STAR should work with DOE to make realistic
    plans for this valuable project.

2.  The STAR/HFT team should consider a third detector layer in
    coordination with the Intermediate Tracker, and analyze various
    aspects of an integrated design concept.

#  

#  References

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[^3]: J. Adams et al., *Phys. Rev. Lett.* 91, 172302 (2003).

[^4]: C. Adler et al., *Phys. Rev. Lett.* 90, 082302 (2003).

[^5]: J. Adams et al. (STAR Collaboration), Nucl. Phys. A 757, 102
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[^6]: J. D. Bjorken, *FERMILAB-Pub-82/59-THY*.

[^7]: X. N. Wang, M. Gyulassy, I. Vitev, and B. Zhang,
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[^8]: D. Schiff, R. Baier, and B. Zakharov, *Ann. Rev. Nucl. Part.
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[^9]: J. Ashman et al., *Phys. Lett.* B206, 364 (1988).

[^10]: J. Ashman et al., *Nucl. Phys.* B328, 1 (1989).

[^11]: B.L. Combridge, *Nucl. Phys.* B151, 429 (1979).

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[^108]: **A Study of monolithic CMOS pixel sensors back-thinning and
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[^110]: H. S. Matis, F. Bieser, S. Kleinfelder, G. Rai, F. Retiere, H.G.
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[^114]: S. Kleinfelder, F. Bieser, Y. Chen, R. Gareus, H.S. Matis, M.
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[^115]: N.H. Xuong, A.-C. Milazzo, P. LeBlanc, F. Duttweiler, J. Bouwer,
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[^116]: Technical report,
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[^117]: Estimate based on RHIC achieving an average Au + Au luminosity
    of 1.0 × 10^27^ cm-^2^s^-1^ for 21 weeks at 60% efficiency. T.
    Roser, W. Fischer, A. Drees, H. Huang, V. Ptitsyn, "RHIC Collider
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[^118]: The time that a pixel is sensitive to tracks, this determines
    amount of pileup.

[^119]: Efficiency after cluster filter on binary threshold information
    from the detector

[^120]: To satisfy both the efficiency and accidental requirements it is
    expected that some off chip cluster analysis will be required.
    Depending on signal to noise either one or two thresholds will be
    needed.

[^121]: 4 year operation at maximum RHIC luminosity for a running period
    of 21 weeks/year see: W. Fisher, T. Roser, I. Ben-Zvi, A Fedotov,
    16-Mar-2005\
    http://www.phenix.bnl.gov/phenix/WWW/publish/leitch/rhicii-forward/RHIC_II_Luminosity_Roser.pdf

[^122]: This requirement addresses the radiation thickness of the flex
    cable that is part of the thin ladder structure. The current
    proto-type ladder that uses the MIMOSA5 chips has this many
    conductors. At the end of the ladder there can be additional mass
    for cables, drivers and cooling.

[^123]: The short integration time allows operation of the pixel
    detector like any other STAR detector, namely one frame associated
    with one interaction event only. The STAR trigger is delivered 1.6
    μs following the collision.

[^124]: Please see
    <http://direct.xilinx.com/bvdocs/publications/ds100.pdf>

[^125]: Mimostar2 Manual, http://www.lbnl.leog.org/rdo/MimoStar2_1_1.pdf

[^126]: User Guide
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[^127]: Schematic,
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[^128]: Schematic,
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[^129]: Schematic, http://www.lbnl.leog.org/pdf/hft-daughterboard.pdf

[^130]: Technical report,
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[^131]: [[http://www-rnc.lbl.gov/\~wieman/radiationdoseinapsDulinskiValue.htm]{.underline}](http://www-rnc.lbl.gov/~wieman/radiationdoseinapsDulinskiValue.htm)

[^132]: http://doc.cern.ch/yellowrep/2001/2001-006/p1.pdf

[^133]: <http://www.lbnl.leog.org/writeup.htm>

[^134]: Technical report,
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[^135]: TV Holography System at LBNL, courtesy of Eric Andersen and the
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[^136]: Technical report,
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[^137]: Technical report, <http://www.lbnl.leog.org/writeup.htm>

[^138]: Technical report, [http://www.lbnl.leog.org/ir prelim
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[^139]: Technical report, [http://www.lbnl.leog.org/cable
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[^140]: Technical report,
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[^141]: Steve Peggs and Steve Tepikian, Private Communication.

[^142]: S. Tepikian,. Twiss functions and beam sizes of the RHIC
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[^143]: Technical report,
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[^144]: Technical report,
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[^145]: Technical report,
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[^146]: John O'Hanlon, *A User's Guide to Vacuum Technology*. John Wiley
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[^147]: Seth Wieman, Private Communication.

[^148]: Technical report,
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[^149]: Sandro Bravar, Private Communication.

[^150]: Dan Weiss, Private Communication 30-Nov-2004.

[^151]:
    #####  "Mid-Term Strategic Plan: 2006-2011 for the Relativistic Heavy Ion Collider at Brookhaven National

    #####  Laboratory", Prepared by Brookhaven National Laboratory with the RHIC Scientific Community

    #####  for the U.S. Department of Energy February 14, 2006.