**A Stand-alone Heavy Flavor Tracker for STAR**

![](media/image1.jpeg){width="6.158333333333333in"
height="5.002083333333333in"}

496920.doc

This page intentionally left blank

**A Stand-alone 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

M. Miller, B. Surrow, G. Van Nieuwenhuizen

Massachusetts Institute of Technology, Cambridge, MA 02139

L. Greiner, H.S. Matis, M. Oldenburg, H.G. Ritter, F. Retiere, A. Rose,
K. Schweda, E. Sichtermann, J.H. Thomas, H. Wieman, N. Xu, and more
STARs!

Lawrence Berkeley National Laboratory, Berkeley, California 94720

I. Kotov

Ohio State University, Columbus, Ohio 43210

Contents

[1 Executive Summary 13](#executive-summary)

[1.1 Scientific Motivation 13](#scientific-motivation)

[1.2 Detector Concept 14](#detector-concept)

[2 The Physics of the HFT 16](#the-physics-of-the-hft)

[2.1 Introduction 16](#introduction)

[2.2 Initial Heavy Quark Production 18](#initial-heavy-quark-production)

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

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

[2.3.1 Elliptic Flow 24](#elliptic-flow)

[2.3.2 Charm Hadro-Chemistry 26](#charm-hadro-chemistry)

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

[2.4.1 R~AA~ and Energy Loss 28](#raa-and-energy-loss)

[2.4.2 Charm Angular Correlations 30](#charm-angular-correlations)

[2.4.3 Baryon -- Meson Ratios 32](#baryon-meson-ratios)

[2.4.4 B Jet Tagging 33](#b-jet-tagging)

[2.5 Measuring Vector Mesons with Dileptons
34](#measuring-vector-mesons-with-dileptons)

[2.6 Spin Measurements 38](#spin-measurements)

[3 HFT Conceptual Design and Performance
40](#hft-conceptual-design-and-performance)

[3.1 Overview of the Experiment 40](#overview-of-the-experiment)

[3.2 Design of the HFT 41](#design-of-the-hft)

[3.3 Expected HFT Performance 42](#expected-hft-performance)

[3.4 Monte Carlo Simulations 44](#monte-carlo-simulations)

[3.5 Simulated HFT Performance 49](#simulated-hft-performance)

[3.5.1 An example: Open Charm and Beauty Reconstruction
52](#an-example-open-charm-and-beauty-reconstruction)

[3.7 Additional Issues that Relate to the Performance of the HFT
66](#additional-issues-that-relate-to-the-performance-of-the-hft)

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

[4 CMOS Sensors 69](#cmos-sensors)

[4.1 Introduction 69](#introduction-1)

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

[4.3 MIMOSTAR Sensor Design 74](#mimostar-sensor-design)

[4.3.1 Thinning 76](#thinning)

[4.4 Additional R&D 76](#additional-rd)

[5 Data Acquisition and Readout 81](#data-acquisition-and-readout)

[5.1 Requirements 81](#requirements)

[5.2 Architecture 82](#architecture)

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

[5.4 Data Rates 86](#data-rates)

[5.5 Prototypes 87](#prototypes)

[6 Integration with the Experiment 90](#integration-with-the-experiment)

[6.1 Mechanical Introduction 90](#mechanical-introduction)

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

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

[6.3 Ladder Design and Fabrication 95](#ladder-design-and-fabrication)

[6.3.1 Detector Radiation Length 96](#detector-radiation-length)

[6.3.2 Expected Radiation Exposure 96](#expected-radiation-exposure)

[6.4 Ladder Mechanical Tests 97](#ladder-mechanical-tests)

[6.4.1 Load Distortion Tests 97](#load-distortion-tests)

[6.4.2 Thermal Distortion Tests 99](#thermal-distortion-tests)

[6.4.3 Cooling Measurements 99](#cooling-measurements)

[6.5 Cabling 100](#cabling)

[6.6 Beam Pipe 102](#beam-pipe)

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

[6.6.2 Supporting Section 104](#supporting-section)

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

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

[6.6.5 Beam Pipe Insertion and Removal
105](#beam-pipe-insertion-and-removal)

[6.6.6 Bake Out 105](#bake-out)

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

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

[7 Cost, Schedule and Manpower 107](#cost-schedule-and-manpower)

[7.1 Cost and Schedule 107](#cost-and-schedule)

[8 Summary 108](#summary)

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

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

[11 References 110](#references)

List of Figures

[Figure 1: Masses of the six quark flavors. The current and constituent
masses are shown by blue- and yellow-boxes, respectively. The figure is
adapted from Ref. \[12\]. 18](#__RefHeading___Toc122543483)

[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. \[15\].
20](#__RefHeading___Toc122543484)

[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. \[17, 19\]. 20](#__RefHeading___Toc122543485)

[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.
\[20\]. 22](#__RefHeading___Toc122543486)

[Figure 5: Nuclear modification factor R~AA~ of D-mesons assuming the
measured spectrum from d+Au collisions scales as the light charged
hadrons in central Au+Au collisions (blue line, I) and from a
hydro-dynamically inspired parameterization assuming a collective flow
velocity of \<β~r~\> = 0.4*c* for D-mesons (red line, II); the
corresponding electron decay-spectra are shown by the blue dashed (I)
and red dashed (II) lines. 22](#__RefHeading___Toc122543487)

[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). 25](#__RefHeading___Toc122543488)

[Figure 7: Non-photonic electron elliptic flow at RHIC. Blue and Red
symbols are data of non-photonic electron v~2~ from PHENIX and STAR.
Only statistical errors are shown for both results. The
dashed-dotted-line represents the input D-meson v~2~ distributions,
assuming the scaling of constituent quarks for meson v~2~. The
corresponding decayed electron v~2~ are shown as the hatched band. The
dashed-line represents the results from Greco et al. In this coalescence
calculation, similar v~2~ distributions for c- and u-quarks are assumed.
26](#__RefHeading___Toc122543489)

[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. \[51\]. 29](#__RefHeading___Toc122543490)

[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. \[58,59,60\]. 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 \[63\] .
30](#__RefHeading___Toc122543491)

[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. 31](#__RefHeading___Toc122543492)

[Figure 11: Distance of the B^+^ decay position from the primary vertex.
Full circles are from high-momentum simulated B^+^ → e^+^ + X decays,
open triangles show the distribution of combinatorial background. The
arrow indicates a cut at 250 μm optimizing the signal to background
ratio. 34](#__RefHeading___Toc122543493)

[Figure 12: 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.
35](#__RefHeading___Toc122543494)

[Figure 13: Electron p~T~ spectra from γ conversions reconstructed by
requiring TPC tracking alone, a track in the TPC and 2 SVT hits, or a
TPC track and 2 SVT hits and 2 HFT hits. The rejection factor with an
additional SVT hit (2 hits) is 3 and with an additional HFT hit is 50.
36](#__RefHeading___Toc122543495)

[Figure 14: Di-electron invariant mass spectrum. The solid black curve
is a prediction from R. Ralf Ref. \[78\] 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](#__RefHeading___Toc122543496)

[Figure 15: 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 at the
centerline under the Silicon trackers. 40](#__RefHeading___Toc122543497)

[Figure 16: Left - 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. Right - The HFT is
shown embedded inside the SVT+SSD. 42](#__RefHeading___Toc122543498)

[Figure 17: Probability of getting the correct hit on the outer HFT
layer, given two different SVT geometries and a search window of 640 μm
43](#__RefHeading___Toc122543499)

[Figure 18: HFT efficiency and ghost rate: (a) projected track error.
The projection starts from TPC (radius from 60 cm to 189 cm) through SSD
(23 cm) to the second layer of HFT (5 cm). (b) and (d) are p~T~
dependence of HFT efficiency and ghost rate, respectively. (c) and (e)
are luminosity dependence for efficiency and ghost rate, respectively.
Finite and infinite search window are represented by solid- and dashed-
line, respectively. The vertex, SSD, TPC resolution are 100um, 20um,
575um, respectively. In the hand calculation, 3σ detector resolutions
are used. At nominal RHIC luminosity 1×10^27^/cm^2^s, pile-up hits
density is 22.5 hits/cm^2^. 44](#__RefHeading___Toc122543500)

[Figure 19: Simulation block flow diagram.
47](#__RefHeading___Toc122543501)

[Figure 20: DCA of tracks to the Primary vertex. Geometric signed
distance of closest approach between all tracks (pt\>200MeV/c) and the
vertex for two cases. a) DCA to the vertex defined by TPC+SSd tracks;
the non-zero mean is used to correct the vertex coordinate. b) Vertex
after iterative correction using TPC+SSD+HFT tracks, with a significant
improvement in the mean of the offset and the distribution shape.
48](#__RefHeading___Toc122543502)

[Figure 21: Mean and sigma of the geometric -- signed DCA distribution
as a function of the number of iterations used to correct the vertex.
48](#__RefHeading___Toc122543503)

[Figure 22: 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~ .
Resolution follows a/sqrt(N), expected from fitting statistics. 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. 49](#__RefHeading___Toc122543504)

[Figure 23: The efficiency for finding tracks in central Au+Au
collisions in the STAR TPC, SSD, and the HFT.
51](#__RefHeading___Toc122543505)

[Figure 24: 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. 51](#__RefHeading___Toc122543506)

[Figure 25: 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. 54](#__RefHeading___Toc122543507)

[Figure 26: 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.
55](#__RefHeading___Toc122543508)

[Figure 27: The absolute yield of D^0^ into the TPC+SSD+HFT divided by
the input D^0^ yield. In the SVT-1 configuration, the innermost layer of
the SVT has been removed in order to allow more room for the HFT. And
finally, Tthe Alice geometry assumes thick Si detectors that are similar
to the wafers used in the ALICE vertex detector. The D^0^ reconstruction
cuts were optimized using the program MINUIT.
57](#__RefHeading___Toc122543509)

[Figure 28: The number of central Au+Au events needed as a function of
the Si ladder thickness and the position resolution of the detector
(left). Results from Monte Carlo Simulations and full tracking in all
sub-detectors including TOF and the proposed HFT (right).
57](#__RefHeading___Toc122543510)

[Figure 29: Left: SVT-1 configuration without TOF. Right: SVT-1
configuration plus TOF. The estimated statistical uncertainties are
shown on the top line. 58](#__RefHeading___Toc122543511)

[Figure 30: The D^+^ decayed tracks decay-length, dca, and cos(θ)
distributions (triangles). The same distributions for primary tracks are
also shown (squares). 59](#__RefHeading___Toc122543512)

[Figure 31: The absolute yield of D^0^ into the TPC+SSD+HFT divided by
the input D^0^ yield. In the SVT-1 configuration, the innermost layer of
the SVT has been removed in order to allow more room for the HFT. And
finally, Tthe Alice geometry assumes thick Si detectors that are similar
to the wafers used in the ALICE vertex detector. The D^0^ reconstruction
cuts were optimized using the program MINUIT.
60](#__RefHeading___Toc122543513)

[Figure 32: Invariant mass distributions for D^+^ for several p~T~
windows. Lines are the Polynomial (up to 2^nd^ order)+ Gaussian fit.
60](#__RefHeading___Toc122543514)

[Figure 33: D-meson efficiencies as a function of transverse momentum.
Dashed-lines are the fits. 61](#__RefHeading___Toc122543515)

[Figure 34: Ds reconstruction efficiency as a function of transverse
momentum 62](#__RefHeading___Toc122543516)

[Figure 35: Flow results if charmed quarks flow, or do not flow.
63](#__RefHeading___Toc122543517)

[Figure 36: D-meson rates 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 \[Ref. 17\]. Number of binary N~bin~ = 950, corresponding to
the top 10% most central Au+Au collisions, is used to scaled the Au+Au
collisions; (b) 3-σ significance D^0^ efficiency with TPC+SSD+HFT.
Dashed-line is the fit to the simulated result; (e) D^0^ meson rates
from p+p and top 10% central Au+Au collisions at 200 GeV.
64](#_Ref122544609)

[Figure 37: 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 \[Ref. 17\]. Number of binary N~bin~ = 290, corresponding to
the minimum bias Au+Au collisions, is used to scaled the Au+Au
collisions. Power-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 \[Ref. 48\] 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. 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. 65](#__RefHeading___Toc122543519)

[Figure 38: 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 from
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.
72](#__RefHeading___Toc122543520)

[Figure 39: Wafer of reticle size sensors (left) and zoomed-in view of
individual chips (right). 73](#__RefHeading___Toc122543521)

[Figure 40: An APS Sensors developed by the LBNL/UIC group. The picture
shows 16 separate test structures. Each structure has a 36 x 36 array of
20 µm pixels. 77](#__RefHeading___Toc122543522)

[Figure 41: 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 electrons can be identified in the plot.
77](#__RefHeading___Toc122543523)

[Figure 42: 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](#__RefHeading___Toc122543524)

[Figure 43: DAQ Layout: schematic of DAQ system for a single 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](#__RefHeading___Toc122543525)

[Figure 44: A simple cluster finding algorithm for the HFT detector. ADC
data from two MIMOSTAR detector columns + 3 pixels are stored
sequentially in an 8-bit wide shift register. The senter pixel of a 3 x
3 pixel window is compared to a high threshold with each clock. If the
threshold is exceeded, the ADC data for the 3 x 3 pixel and the address
are stored as a cluster word into a readout FIFO. This method is
extendable to allow for multiple simultaneous thresholds and geometric
pattern triggers. 84](#__RefHeading___Toc122543526)

[Figure 45: 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.
85](#__RefHeading___Toc122543527)

[Figure 46: Data rates at the various stages of the HFT readout chain.
87](#__RefHeading___Toc122543528)

[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.
87](#__RefHeading___Toc122543529)

[Figure 48: Functional component diagram of the prototype readout system
constructed for the readout on a MIMOSA5 based ladder.
88](#__RefHeading___Toc122543530)

[Figure 49: Prototype motherboard and daughter card used for reading out
MIMOSA5 detectors. 89](#__RefHeading___Toc122543531)

[Figure 50: The HFT is shown integrated with the STAR inner detectors
cone assembly. 91](#__RefHeading___Toc122543532)

[Figure 51: Close up view of the HFT ladders.
92](#__RefHeading___Toc122543533)

[Figure 52: Detector support structure with kinematic mounts to insure
repeatable detector positioning. 92](#__RefHeading___Toc122543534)

[Figure 53: Kinematic mount structure. 93](#__RefHeading___Toc122543535)

[Figure 54: The HFT is shown integrated with the STAR inner detectors
cone assembly. 94](#__RefHeading___Toc122543536)

[Figure 55: 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. 96](#__RefHeading___Toc122543537)

[Figure 56: A cross-section of the prototype detector ladder showing its
structure and materials composition. 97](#__RefHeading___Toc122543538)

[Figure 57: A cross-section of the prototype detector ladder showing its
structure and materials composition. 98](#__RefHeading___Toc122543539)

[Figure 58: 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.
98](#__RefHeading___Toc122543540)

[Figure 59: TV Holography system viewing test ladder in a small
transparent wind tunnel. Inset shows diffraction pattern with color map
of the surface displacement. 99](#__RefHeading___Toc122543541)

[Figure 60: A prototype readout cable for the HFT.
100](#__RefHeading___Toc122543542)

[Figure 61: Mechanical Prototype with 4 MIMOSA-5 detectors glued to the
Kapton cable assembly. 101](#__RefHeading___Toc122543543)

[Figure 62: Beam pipe structure with outer support barrel in the center.
102](#__RefHeading___Toc122543544)

[Figure 63: View of the beampipe mid-section.
103](#__RefHeading___Toc122543545)

[Figure 64: End view showing the HFT ladders between spokes to the inner
beam pipe support. 104](#__RefHeading___Toc122543546)

List of Tables

[Table 1: Cross section for the production of charm and beauty.
19](#__RefHeading___Toc122543547)

[Table 2: Charm quark fragmentation functions. The left column is from
Ref. \[46,47\]. The right column is from Ref. \[45\]. The D^+^ and D^0^
yields include feed-down from D^\*+^ and D^\*0^ decays.
27](#__RefHeading___Toc122543548)

[Table 3: Number of central Au *+* Au events required to observe a 3-σ
signal for ω and φ in their leptonic decay channels under different
detector configurations. 37](#__RefHeading___Toc122543549)

[Table 4: Selected HFT parameters. 42](#__RefHeading___Toc122543550)

[Table 5: Open charm hadron properties 52](#__RefHeading___Toc122543551)

[Table 6: Cuts applied for D-meson reconstruction
53](#__RefHeading___Toc122543552)

[Table 7: The cuts for D^0^ and D^+^ reconstructions and efficiency
analysis. The exponential p~T~ spectrum, with \<p~T~\> = 1.3 GeV/c, was
used as input. 56](#__RefHeading___Toc122543553)

[Table 8: Cuts applied for D~s~^+^ reconstruction
58](#__RefHeading___Toc122543554)

[Table 9: Cuts applied for D~s~^+^ reconstruction
61](#__RefHeading___Toc122543555)

[Table 10: Number of estimated events for 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 the Glaubor calculations \[Ref.
23\]. 64](#__RefHeading___Toc122543556)

[Table 11: Number of estimated events for 10% statistical errors for D0
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 \# of binary
collisions is from the Glabour calculations \[Ref. 23\].
66](#__RefHeading___Toc122543557)

[Table 12: Luminosity and other parameters that determine the particle
flux on the HFT. 67](#__RefHeading___Toc122543558)

[Table 13: Integrated hit loading on the HFT and associated pileup.
67](#__RefHeading___Toc122543559)

[Table 14: Hit loading on the HFT from Au+Au central collisions and
associated pileup. 68](#__RefHeading___Toc122543560)

[Table 15: Table -- Chart of APS chips that the IReS group has produced
\[\] 71](#__RefHeading___Toc122543561)

[Table 16: Comparison between MIMOSA-5 characteristics and HFT
specifications. 74](#__RefHeading___Toc122543562)

[Table 17: Requirement Summary - constraints for the APS, due to signal
expectation, available design process, and mechanics
81](#__RefHeading___Toc122543563)

[Table 18: Data rate calculation parameters
86](#__RefHeading___Toc122543564)

[Table 19: Materials in the beam pipe and the first detector layer with
their radiation length fractions. For details see Ref. \[\]
96](#__RefHeading___Toc122543565)

# []{.mark}  {#section .unnumbered}

# 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 STAR
detector at central rapidities.

## Scientific Motivation

The primary motivation for the HFT is to extend STAR's capability to
measure heavy flavor production down to very low transverse momentum by
the measurement of displaced vertices. This is a key measurement for the
continuing 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 open charm measurements, thermalization, flow, and heavy quark
energy loss.

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 most exciting prospective measurement with the pixel detector is to
perform a measurement of the elliptic flow of D mesons down to very low
p~T~ values. It is a generally accepted fact that elliptic flow is
established in the partonic phase. If charm quarks, with a mass much
larger than the temperature of the system, 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. Proof of
thermalization requires additional measurements such as thermal photons
and low mass dilepton spectra. 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.

Re-scattering with light quarks is an important process because it will
affect the D meson transverse momentum spectrum. Re-scattering at the
hadronic level, mainly π-D interactions, will also affect the D meson
transverse momentum spectrum. We will measure the effects of
re-scattering by comparing transverse mass spectra measured in d+Au and
Au+Au collisions.

The HFT detector will allow us to measure the ratio of different D meson
states with high precision. This measurement can be used to test if the
different states are produced in the same ratio as in elementary
particle collisions or if the ratios are established according to
chemical equilibrium. 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) and as a consequence heavy flavors should lose less energy
in the dense medium.

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. 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
may 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.

With the current STAR detector configuration, heavy quark production has
been measured in p+p and d+Au collisions through the observation of
single electrons and the use of the event-mixing technique. These
methods are limited in their accuracy due to the systematic
uncertainties resulting from large background subtractions. Furthermore,
these methods require large amounts of data to overcome a poor signal to
background ratio. The HFT detector will bring extremely high precision
tracking capabilities to STAR with a point resolution of less than 10
µm, over a large pseudo-rapidity range, and complete azimuthal angle
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.

## Detector Concept

The proposed HFT detector will sit inside the STAR TPC and it will
surround the interaction vertex. It will exploit all of STAR's unique
features including full azimuthal coverage and tracking from the lowest
to the highest p~T~. 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 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 only a few tens of microns from
the primary 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. In
this respect, the STAR HFT is unique. No other Si detector at RHIC
combines this kind of extreme pointing accuracy and is effective for all
particles down to 150 MeV/c.

# The Physics of the HFT

## Introduction

The 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 cross-over 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 are 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="4.9222222222222225in"
height="2.9993055555555554in"}

[]{#__RefHeading___Toc122543483 .anchor}Figure 1: Masses of the six
quark flavors. The current and constituent 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%.

+-----------------------------------------------------------------------+
| Theory: σ~NN~ (*cc* ) = 900 - 1400 µb                                 |
|                                                                       |
| Experiment: 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___Toc122543547 .anchor}Table 1: Cross section for the
production of charm and beauty.

The uncertainty in the theoretical cross-sections arise 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 cross section
values are normalized by the number of underlying binary nucleon-nucleon
collisions. (Approximately 1000 binary nucleon-nucleon scatterings per
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. 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="3.428472222222222in"
height="3.7381944444444444in"}

[]{#__RefHeading___Toc122543484 .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="4.416666666666667in"
height="3.142361111111111in"}

[]{#__RefHeading___Toc122543485 .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

The first direct measurements of D-meson spectra were seen in d+Au
collisionsError: Reference source not found at RHIC by reconstructing
the D meson from its π -- K decay topology in the STAR TPC.  These were
challenging measurements due to the large multiplicity of pions, kaons,
and protons and the rather small production cross section for charmed
hadrons. The combinatorial background in the invariant mass
distributions is roughly 1000 times larger than the signal. This large
combinatorial background led to systematic uncertainties on the
extracted charm-hadron yields of the order of 30%. In the future,
extending the particle identification by using the new STAR time of
flight barrel (due to be completed in 2008) will improve the statistical
significance by a factor of five. This leaves room for optimism, but for
comparison, elliptic flow modulates particle yields with respect to the
reaction plane on the order of 10% so further improvements are needed.

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 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.

The same conclusion is reached when doing the analysis of R~AA~ for
heavy flavor spectra. Recall that R~AA~ is the ratio 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 RAA 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.  In order to
avoid these difficulties, and for a precise measurement of charm quark
flow, a direct reconstruction of D → π + K is required. This clearly
calls for a  direct measurement of heavy flavor hadrons, i.e. D- and
B-mesons.

![](media/image6.jpeg){width="3.7708333333333335in"
height="3.248611111111111in"}

[]{#__RefHeading___Toc122543486 .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 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/image7.png){width="5.677083333333333in"
height="4.064583333333333in"}

[]{#__RefHeading___Toc122543487 .anchor}Figure 5: Nuclear modification
factor R~AA~ of D-mesons the from a hydro-dynamically inspired
parameterization assuming a collective flow velocity of \<β~r~\> =
0.4*c* (solid-black line) and 0.6c (dashed-black line). The
corresponding electron decay-spectra are shown by the blue dotted- and
blue-dashed 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 is 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. The build-up of
pressure and pressure gradients cause collective motion of the
constituents of the medium if the matter produced in the reaction zone
re-scatters sufficiently, and 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 cascade 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 cascade. 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/image9.png){width="2.85in" height="4.022222222222222in"}

[]{#__RefHeading___Toc122543488 .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 channelError: Reference source
not found^,Error:\ Reference\ source\ not\ found^. 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/image10.jpeg){width="6.002777777777778in"
height="3.748611111111111in"}

[]{#__RefHeading___Toc122543489 .anchor}Figure 7: Non-photonic electron
elliptic flow at RHIC. Blue and Red symbols are data of non-photonic
electron v~2~ from PHENIX[^35] and STAR[^36]. Only statistical errors
are shown for both results. The dashed-dotted-line represents the input
D-meson v~2~ distributions, assuming the scaling of constituent quarks
for meson v~2~. The corresponding decayed electron v~2~ are shown as the
hatched band[^37]. The dashed-line represents the results from
Greco[^38] et al. 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[^39]^,^[^40]. 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[^41]. The extracted chemical freeze-out temperature T~ch~
is higher than the kinetic freeze-out temperature T~fo~ extracted from
momentum spectra[^42] 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[^43]^,^[^44]. 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[^45]. 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[^46]^,^[^47] 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[^48], 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___Toc122543548 .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[^49].

## 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 which 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[^50]^,^[^51]^,^[^52]. 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 no-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^,^[^53]^,^[^54]^,^[^55].
This effect was predicted to occur due to radiative energy loss of high
energy partons that propagated through a dense and strongly interacting
medium[^56]. 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[^57]. 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/image11.png){width="4.00625in" height="2.6805555555555554in"}

[]{#__RefHeading___Toc122543490 .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[^58]^,^[^59]^,^[^60]
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[^61]^,^[^62] (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[^63], 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/image12.png){width="5.35625in" height="3.8159722222222223in"}

[]{#__RefHeading___Toc122543491 .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/image13.png){width="4.999305555555556in"
height="3.8270833333333334in"}

[]{#__RefHeading___Toc122543492 .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[^64]^,^[^65] 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[^66] 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.
\[Error: Reference source not found,[^67],[^68],[^69]\], and implies a
different production mechanism for heavy-quarks in elementary
collisions[^70]. 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[^71]^,^[^72], 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[^73]
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[^74].

### B Jet Tagging

It is also interesting to note that a significant fraction of B (\~10%)
and D (6-17%) meson decays include an electron or positron in the final
state. The STAR electromagnetic calorimeter will allow us to trigger on
these high transverse momentum electromagnetic showers. Since the only
source of high transverse momentum electrons and positrons are D and B
decays, the D and B transverse momentum distribution can be directly
inferred from the data above a carefully chosen momentum threshold
applied to the observed electron and positron spectra.

If an electromagnetic shower can be matched to a track that is
reconstructed in the TPC, it will be tagged as an electron or positron.
But there are many background processes that can decrease the hadron
rejection power of the EMC and at high pT, the EMC+TPC hadron rejection
power is not sufficient to cleanly identify electrons. The only
measurable difference between an electron or positron from D and B decay
and a high transverse momentum pion is their spatial origin near the
vertex and so a high p~T~ charged pion will come directly from the
primary vertex while electrons and positrons from heavy meson decays
will come from the decay vertex which is typically displaced from the
primary vertex by a few hundred μm. Thus, these different decay
locations can be selected by the HFT.

The CDF collaboration has shown that B jet tagging can be achieved by
associating the electron or positron with one of the other charged
hadron decay products[^75]. Figure 11 shows a feasibility study where we
applied a topological technique to RHIC data, similar to what can be
done with the HFT, that imitates the CDF technique to remove background.
The channels they studied include
$B^{+} \rightarrow {\overline{D}}^{0} + e^{+} + \nu$ and
${\overline{D}}^{0} \rightarrow K^{+} + \pi^{-}$. The positron is
matched with either a kaon or a pion from the D meson decay, but only if
the distance of closest approach is less than 150 microns. The B decay
vertex is then located at the mid-point between both points of closest
approach on each helix. The variable shown in Figure 11 is the distance
between the B and the primary vertex as found by the HFT. The sign is
given by the scalar product of the electron momentum with the vector
constructed from the primary vertex to the reconstructed decay vertex.
The background is simulated by embedding high transverse momentum tracks
into events that match the momentum distribution measured in 200 GeV
central Au+Au collisions below 3 GeV/c. The relative normalization
between the background and B decay distribution is arbitrary, since the
charge-exchange cross-section is not well known. By applying a
signed-decay-length cut of more than 250 μm, the background is
suppressed by a factor 200 while keeping 60% of the signal.

Thus, adding the decay vertex information from the HFT will
significantly reduce the background in the high transverse momentum
electron/positron spectra and substantially increase the statistical
significance of the measurements. We note that the charm and bottom
decay lengths differ substantially, thus improving the statistical
significance of the measurements based on decay length.

![](media/image14.jpeg){width="3.9868055555555557in"
height="3.4972222222222222in"}

[]{#__RefHeading___Toc122543493 .anchor}Figure 11: Distance of the B^+^
decay position from the primary vertex. Full circles are from
high-momentum simulated B^+^ → e^+^ + X decays, open triangles show the
distribution of combinatorial background. The arrow indicates a cut at
250 μm optimizing the signal to background ratio.

## 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[^76]^,^[^77].
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 12. 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 12 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/image15.jpeg){width="2.7944444444444443in"
height="2.7180555555555554in"}
![](media/image16.jpeg){width="2.9298611111111112in"
height="2.7055555555555557in"}

[]{#__RefHeading___Toc122543494 .anchor}Figure 12: 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 13 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[^78]^,^[^79] 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/image17.jpeg){width="4.563194444444444in"
height="3.0993055555555555in"}

[]{#__RefHeading___Toc122543495 .anchor}Figure 13: Electron p~T~ spectra
from γ conversions reconstructed by requiring TPC tracking alone, a
track in the TPC and 2 SVT hits, or a TPC track and 2 SVT hits and 2 HFT
hits. The rejection factor with an additional SVT hit (2 hits) is 3 and
with an additional HFT hit is 50.

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. Table 3 shows our estimate of central
Au+Au collisions needed in order to observe a signal with a statistical
significance of 3σ. Compared to measurements incorporating only the TPC
and TOF, the HFT will enhance the sensitivity to electro-magnetic vector
meson decay by a factor 20-40.

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

  TPC+TOF                                     8M              2M

  TPC+TOF+SVT+HFT                             200K            100K
  ------------------------------------------- --------------- ------------

[]{#__RefHeading___Toc122543549 .anchor}Table 3: Number of central Au
*+* Au events required to observe a 3-σ signal for ω and φ in their
leptonic decay channels under different detector configurations.

Figure 14 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 pink dashed
line is the e^+^e^-^ pair invariant mass spectrum for charm
semi-leptonic decays derived from the non-photonic single electron
spectra measured in Au+Au collisions. The gray curve is the e^+^e^-^
pair invariant mass distribution for charm after a cut on the HFT DCA \<
80μm. 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/image18.jpeg){width="5.995138888888889in"
height="4.065972222222222in"}

[]{#__RefHeading___Toc122543496 .anchor}Figure 14: 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.

## 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[^80] 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[^81]. Next-to-leading order corrections are available and predict
relatively sizable changes in the observed asymmetry as compared to
leading order expectations[^82].

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[^83] lead us to
expect measurable, and negative, double longitudinal spin asymmetries at
the level of --5\*10^-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.

# HFT Conceptual Design and Performance

## Overview of the Experiment

STAR is a suite of detectors that surround the 6 o'clock collision
region at RHIC (see Figure 15). The primary goal of the experiment is to
provide momentum analysis and particle identification over a wide range
of rapidities, over the full azimuth, and over a broad range of momenta.
In this respect, STAR is unique. It is the only experiment at RHIC whose
primary detectors cover the full azimuth in φ, cover ±1.5 units in
rapidity, and track particles from 100 MeV/*c* to 20 GeV/*c*. The Heavy
Flavor Tracker (HFT) will extend these unique capabilities even further
by providing particle identification for hadrons containing charm and
beauty.

![](media/image19.jpeg){width="5.9527777777777775in"
height="3.623611111111111in"}

[]{#__RefHeading___Toc122543497 .anchor}Figure 15: 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 at the centerline under the Silicon
trackers.

The important tracking elements at mid-rapidity will be the Silicon
Strip Detector (SSD) and the Time Projection Chamber (TPC). The SSD is
installed at a radius of 23 cm around the center of the beam pipe 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. 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 an array of scintillators. The TOF surrounds the outer
field cage of the TPC. By the time the HFT is installed in STAR, the
existing Silicon Vertex Detector (SVT), will be decommissioned.

The goal of the STAR detector is to start measuring a particle's
trajectory as close as possible to the interaction vertex and then to
follow it continuously to a large radius as it traverses a uniform
magnetic field up to 0.5 T. The STAR tracking philosophy, on the other
hand, is to start from the outside where the hit density is lowest and
to follow the track inwards. This design allows us to measure the
collision vertex with great precision. Using this information we can
identify a short-lived particle whose decay is displaced from the vertex
by a significant distance. The *K~S~*^0^ is one example. It 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. It is relatively easy to
identify this non-vertex decay using the TPC, alone, because the TPC has
a vertex pointing accuracy of approximately 5 mm.

After including the SSD in the analysis, the vertex resolution is
approximately 2 mm, which substantially improves the efficiency for
finding topologically complex strange decays such as the Ω baryon.

The HFT improves upon these capabilities even further because it will be
placed between the beam pipe and the SSD detector. It will have two
layers of silicon pixel detectors; one at 1.5 cm radius and the other at
5 cm 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 then be 25 μm for 1 GeV/c tracks.
The primary reason we need the HFT is because the charmed mesons have a
characteristic decay length on the order of 125 μm. Thus, the HFT is the
enabling technology for making direct charm and beauty measurements at
STAR.

## Design of the HFT

The parameters for the HFT are summarized in Table 4 and the detector is
shown schematically in Figure 16. 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 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. We will thin down each chip to a total thickness of 50 μm to
minimize the multiple coulomb scattering (MCS) between layers of the
detector. The HFT is designed to be as close as possible to the
interaction point. This will require a new, smaller radius, beam pipe.
The beam pipe must also be made thinner (0.5 mm) to reduce scattering in
front of the HFT. This smaller, thinner, beam pipe is an essential part
of the proposal and is discussed in section 6.6.

  ---------------------------------------- ------------------------------
  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~               0.282 %

  Pointing Accuracy, σ                     14 μm + 12 μm (GeV/*c*) / *p*

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

[]{#__RefHeading___Toc122543550 .anchor}Table 4: Selected HFT
parameters.

![](media/image20.jpeg){width="3.0770833333333334in"
height="2.6506944444444445in"} ![](media/image21.jpeg){width="2.65625in"
height="2.6013888888888888in"}

[]{#__RefHeading___Toc122543498 .anchor}Figure 16: Left - 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. Right - The HFT is shown embedded inside the SVT+SSD.

## Expected HFT Performance

The physics program outlined previously defines clear criteria for the
Heavy Flavor tracker. The requirement for a thin detector positioned
close to the interaction dominates the design considerations.

1.  **Pixel size, resolution,** **MSC**

Hand calc of efficiency and ghosting vsusu momentum. These results are
in good agreement with hand calculations of the same quantities. The
analytical estimate of the tracking efficiency for various geometries
has been studied. The pile-up hit density, ρ, is set to 30 hits/cm^2^.
The probability to pick up the correct hit is described by the following
equation:

![](media/image22.pct)

where σ is the uncertainty in the track extrapolation from the SSD to
the HFT layer and 'a' is the search cone radius. The quantity σ includes
contributions from multiple Coulomb scattering and the position
uncertainty in the hits used on the track. The calculation matches the
data fairly well at momenta \> 1 GeV/c. While the calculations can
reproduce the shape of the distribution, the efficiency below 1 GeV/c is
systematically overestimated. This is most likely due to details in the
full tracking reconstruction algorithm, which are not considered in
these hand calculations. The calculation uses a straight line technique
to estimate the projection error, where only the two closest points on
the track are used to determine σ when extrapolating to the HFT. The
uncertainties from previous measurements on the track, therefore, are
not accounted for in the calculation and may account for differences
between Monte-Carlo results and the calculation.

The overall efficiency of the tracker and the number of ghost tracks
affects the efficiency for finding charmed mesons with the HFT. We have
performed a complete set of simulations of the TPC+SSD+HFT tracking
system and the results are given in the next section.

![](media/image24.png){width="5.364583333333333in"
height="5.270833333333333in"}

[]{#__RefHeading___Toc122543500 .anchor}Figure 17: HFT efficiency and
ghost rate: (a) projected track error. The projection starts from TPC
(radius from 60 cm to 189 cm) through SSD (23 cm) to the second layer of
HFT (5 cm). (b) and (d) are p~T~ dependence of HFT efficiency and ghost
rate, respectively. (c) and (e) are luminosity dependence for efficiency
and ghost rate, respectively. Finite and infinite search window are
represented by solid- and dashed- line, respectively. The vertex, SSD,
TPC resolution are 100um, 20um, 575um, respectively. In the hand
calculation, 3σ detector resolutions are used. At nominal RHIC
luminosity 1×10^27^/cm^2^s, pile-up hits density is 22.5 hits/cm^2^.

## 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. Details of the design and use of this package are
available elsewhere, but a general overview is useful to place the
results and predictions contained in this document in a broader context.

The simulation framework has three stages: first, the simulation of the
physics under study, for example a Au-Au interaction; second, the
simulation of the interaction of the produced particles with the
detector and physical structures of the experiment, and finally the
simulation and digitization of signals in the detector. The goal of such
a package is to use the same analysis package on both simulated and real
data, both to predict performance and optimize analysis methods.

For the event generation stage, the simulations presented here separate
the charm mesons and baryons from the rest of the event. This has the
benefit of greatly increased speed on the analysis side in algorithm
development. This method has been used previously (ref: ALICE). For
charm production, events with a single signal particle were simulated.
The background for these measurements is simulated as a cocktail of
pion, kaon, and protons at the multiplicity expected from the highest
top 10% centrality collisions, produced with the MEVSIM package
developed for use in STAR (ref: MEVSIM STAR note). The 'signal' and
'background' events are analyzed using the same reconstruction and
analysis code, with the same cuts and tracking parameters.

Physics interactions between daughter particles and the material of the
detector, as well signal digitization, are simulated using the
implementation of the GEANT simulation package (ref: GEANT CERN writeup,
papers) for the STAR experiment. This package is used by STAR as a
standard analysis tool. A detailed understanding of the TPC is included
in these simulations including dead areas and realistic detector
resolutions and response.

For both 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 with 20um in φ and 750 um
in z, in agreement with the SSD specifications (ref: SSD Nim). For the
HFT, hits were smeared with 6 μm in both φ and z. Such a method neglects
the effects of cluster overlap. We expect such effects to be minimal in
the case of the HFT.

For these simulations, a new algorithm was added to our analysis
software. With the original algorithm, the tracker follows a series of
hits in the TPC and SSD, refitting after every 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 (weighted with the vertex) and refits
the track, this time with the HFT hits available. The tracks with added
HFT hits are then stored. The success of the algorithm depends on the
precision of finding the primary vertex, which is centrality dependent
(see fig. 22).

Figure XX: Vertex Reconstruction with Simulated Events. The vertex
reconstruction resolution over a range of centralities in global x,y,
and z.

Figure YY: Track Residue width at the surface of the inner HFT Layer.
The track residue as various layers of detectors are added. The TPC and
SSD track error is clearly not sufficient to reliably determine HFT hits
(see hand calcs., previous section). Addition of the vertex information
to the track aids in determination of the correct HFT hits.

2.  **Reconstruction**

Reconstruction of the generated event, after simulation of the STAR
detector response, is handled through the tracking framework currently
implemented in STAR to unite the various tracking detectors. We use this
package, along with a detailed GEANT simulation, to provide an estimate
to the expected performance of the real detector.

As demonstrated in the analytic calculations, a vertex constraint used
in conjunction with the TPC and SSD detectors should provide acceptable
reconstruction efficiency and purity. Such an algorithm was implemented.
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 a cocktail 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).

Tracks identified as belonging to the primary vertex are again tracked
through the detector tree, and HFT hits are identified and added at this
time (see figure 3.3.2.1). A refit of the track, with the added HFT
hits, is performed by the Kalman fitter as the new hits are added.

![](media/image25.png){width="5.499305555555556in"
height="3.245833333333333in"}

[]{#__RefHeading___Toc122543501 .anchor}Figure 18: Simulation block flow
diagram.

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

$$s = \frac{\text{380}mm}{\sqrt{N_{\text{ch}}}}$$

![](media/image26.jpeg){width="2.935416666666667in"
height="3.2472222222222222in"}![](media/image27.png){width="2.9381944444444446in"
height="3.245833333333333in"}

[]{#__RefHeading___Toc122543502 .anchor}Figure 19: DCA of tracks to the
Primary vertex. Geometric signed distance of closest approach between
all tracks (pt\>200MeV/c) and the vertex for two cases. a) DCA to the
vertex defined by TPC+SSd tracks; the non-zero mean is used to correct
the vertex coordinate. b) Vertex after iterative correction using
TPC+SSD+HFT tracks, with a significant improvement in the mean of the
offset and the distribution shape.

![](media/image28.png){width="5.7125in" height="3.7402777777777776in"}

[]{#__RefHeading___Toc122543503 .anchor}Figure 20: Mean and sigma of the
geometric -- signed DCA distribution as a function of the number of
iterations used to correct the vertex.

![](media/image29.png){width="5.598611111111111in" height="2.6125in"}

[]{#__RefHeading___Toc122543504 .anchor}Figure 21: 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~ . Resolution follows a/sqrt(N), expected from
fitting statistics. 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.

Application of a vertex constraint before adding detector hits is new in
STAR, and required minor modification of the STAR reconstruction code.
The performance of this algorithm has been studied in terms of the
efficiency and the expected rate of false associations.

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

$$\varepsilon = \frac{N_{\text{reconstructed}}}{N_{\text{MC},\text{Accepted}}}$$

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 'ghost' tracks. The
determined efficiency as a function of p~T~ is about 50% for p~T~
\>1GeV/c (see fig. XXX). The level of ghosting is 12% in the same range.

## Simulated HFT Performance

The performance of the HFT has been simulated using tracks from √s~NN~ =
200 GeV central Au+Au collisions generated by the "MEVSIM" code. We have
also studied the performance of the detector by using single particle
tracks. The simulated tracks and simulated 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 carefully 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.
This simulates the finite hit resolution we expect for the HFT. The
limited read-out speed of the 1^st^ generation HFT will lead to pile-up
of up to 120 Au+Au collisions in the HFT at four times the RHIC design
luminosity. We account for that 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 3.6.

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 22 shows the efficiency for tracking pions with \|*y*\| \< 0.5 in
various detector configurations. The TPC results are designated by the
open circles (red), TPC+SSD by triangles (green), and the TPC+SSD+HFT
detector by filled dots (blue). 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 and the overall efficiency for the HFT
tracking system is 50% or above at a transverse momentum larger than 500
MeV/c. Simulations using mono-energetic pions show that the efficiency
stays approximately constant up to 20 GeV/c.

![](media/image30.jpeg){width="5.620138888888889in"
height="3.9972222222222222in"}

[]{#__RefHeading___Toc122543505 .anchor}Figure 22: The efficiency for
finding tracks in central Au+Au collisions in the STAR TPC, SSD, and the
HFT.

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 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, however, is more
complicated because it is continuously read out and events can pile up
between successive reads of the pixel chips which occur every 4 ms. So
we have studied the influence of the ghost tracks in the HFT detector by
using simulated central Au+Au collisions in the TPC+SSD+SVT+HFT. At low
momentum, ghost tracks dominate due to multiple Coulomb scattering.

[]{#__RefHeading___Toc122543506 .anchor}Figure 23: 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.

Figure 23 shows the number of ghost tracks observed in the full HFT
tracking system. 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 22;
the tracking efficiency for the HFT is steeply dropping below 0.5
GeV/*c*. The ratio of ghost tracks to good tracks is 10% at 0.5 GeV/*c*
and saturates at 4% above 2 GeV/*c* (see the right hand panel of Figure
23).

### An example: Open Charm and Beauty Reconstruction

Charm and beauty quarks occur in a wide variety of hadrons and these
hadrons decay into a number of different channels. We have investigated
a few specific channels, including: D^0^ → K^−^ π^+^ and
D~s~^+^ → K^−^ π^+^ K^+^. These decays represent a crucial sample of the
physics capabilities that can be achieved with the HFT detector.

+---------+-------------------+-------------------+-------------------+
| **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___Toc122543551 .anchor}Table 5: Open charm hadron
properties

Signal and background events for this study were generated separately.
The signal consists of one D^0^ or D~s~^+^ per event. The transverse
momentum distribution of the charmed hadrons follow a Boltzmann
distribution which reproduce 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.

The HFT detector allows us to identify a D^0^ decay-vertex by
reconstructing the trajectory of its two daughters. The following
selection criteria were used to separate the D^0^ signal from
background, also see Figure 24.

-   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

Table 6 lists the values of the cuts applied for D-meson reconstruction.

  -------------------------------------- --------------------------------
  Decay length *l*                       \> 200 μm

  ∆*m*                                   ± 40 MeV/*c*^2^

  DCA~πK~                                \< 50 μm

  cos(θ)                                 θ \< 5.0 degrees

  cos(θ\*)                               θ\* \> 37.0 degrees
  -------------------------------------- --------------------------------

[]{#__RefHeading___Toc122543552 .anchor}Table 6: Cuts applied for
D-meson reconstruction

Charged decay daughters were identified by their specific energy loss in
the TPC gas; for example, to identify pions, we used a 3 σ cut around
the expected band for pions with a momentum dependent width extracted
from experimental data. With that method, pions can be separated from
kaons up to 700 MeV/c. In addition, a time of flight PID selection
criteria was applied. We assume that the pions and kaons can be
separated up to 1.8 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 the selected PID method entered the reconstruction
algorithm twice, once under a pion mass assumption, once assuming a
kaon.

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

[]{#__RefHeading___Toc122543507 .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.

The applied cuts for D^0^ reconstruction were determined by using the
program MINUIT in order to achieve the highest statistical significance
of the reconstructed D^0^ yield. This is a tradeoff between the
reconstructed D^0^ yield and background.

![](media/image31.png){width="5.918055555555555in"
height="4.822916666666667in"}

[]{#__RefHeading___Toc122543508 .anchor}Figure 25: 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.

1.  **D^0^ Efficiencies**

To demonstrate the performance of the proposed STAR HFT, we will show
the reconstruction efficiencies for D^0^ (cτ \~ 123) and D^+^ (cτ \~
312) via the hadronic decay channels K^±^π^±^ and K^-^π^+^π^+^,
respectively. Because of the TPC large acceptance for secondary charge
particles, the STAR detector system is ideal for the measurement. The
cuts used in the study are shown in Table 7.

  ---------------------- --------------------------- --------------------
  **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

  cos(θ) \>              0.996                       0.85

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

[]{#__RefHeading___Toc122543553 .anchor}Table 7: The cuts for D^0^ and
D^+^ reconstructions and efficiency analysis. The exponential p~T~
spectrum, with \<p~T~\> = 1.3 GeV/c, was used as an input distribution.

The resulting yield of D^0^s divided by the input D^0^ yield, as a
function of transverse momentum is shown in Figure 26. This is the
overall reconstruction efficiency taking into account acceptance, single
track efficiency and D^0^ reconstruction efficiency. At low momentum,
the selected D^0^ decay length of \~100 μm dominates, leading to a
rather small number of reconstructed D^0^s. The overall reconstruction
efficiency increases with increasing momentum and seems to saturate
above 3 GeV/c at a value of about 10%. From the single track efficiency
of the HFT, the maximum D^0^ reconstruction efficiency can be estimated
to 0.5\*0.5 (tracks) = 25%, not taking into account the D^0^ acceptance
effects, i.e. at least one of the decay daughters does not reach the TPC
volume. Replacing the thinned Si wafers of the HFT by sensor similar to
those used in the ALICE detector (3 times thicker) results in the D^0^
reconstruction efficiency dropping by a factor 8, independent of
momentum. This would require an 8 times longer data taking period.. As a
result, we have exclude the ALICE detector technology from
consideration.

![](media/image32.png){width="5.998611111111111in"
height="4.886805555555555in"}

[]{#__RefHeading___Toc122543509 .anchor}Figure 26: The D^0^
reconstructionefficiecncy with the TPC+SSD+HFT configuration. The blue-
and red-circvles represent the efficiencies with and witout the smearing
of the primary vertex resolution, respectively. The open-triangles show
the efficiency with a much thicker Si-detector that are similar ones
used in the ALICE vertex detecotor. The D^0^ reconstruction cuts were
optimized using the program MINUIT.

We have studied the influence of different HFT hit resolutions and Si
ladder thicknesses. As a figure of merit we use the number of central
Au+Au collisions required to measure a D^0^ signal with a statistical
significance of 3σ. Figure 27 shows the number of central Au+Au
collisions needed to make the D^0^ measurement as a function of the HFT
hit resolution for different Si ladder thicknesses. The curve is almost
flat between 0 and 10 μm and then rises linearly. With increasing ladder
thickness, multiple-Coulomb scattering leads to larger distortions of
the reconstructed tracks and therefore requires more and more events. In
the following, we choose a position resolution of 6 μm and a total
ladder thickness of 240 micron Si-equivalent (0.3% X~0~) and with these
assumptions we present projected statistical errors on D-meson spectra
and elliptic flow.

![](media/image34.jpeg){width="2.953472222222222in"
height="2.152083333333333in"}![](media/image35.jpeg){width="2.953472222222222in"
height="2.152083333333333in"}

[]{#__RefHeading___Toc122543510 .anchor}Figure 27: The number of central
Au+Au events needed as a function of the Si ladder thickness and the
position resolution of the detector (left). Results from Monte Carlo
Simulations and full tracking in all sub-detectors including TOF and the
proposed HFT (right).

For the first year of data taking with the HFT, we assume a total of 50
M central Au+Au collisions at 200 GeV. (A similar number of events were
taken by STAR during the RHIC Run IV in 2004.) shows the anticipated
D^0^ spectrum from pQCD calculations (no flow) and a Blast wave
prediction (0.4 β~T~). The main differences between the spectra fall
below 3 GeV/c. The top line shows the estimated statistical uncertainty
in the measured spectrum with dE/dx PID (left) and additional TOF PID
(right). At D^0^ momenta below 1 GeV/c, the estimated uncertainties are
similar while at higher momentum the TOF measurement of D^0^ daughters
leads to smaller uncertainties. This is due to the reduced background
due to PID at larger momentum with the TOF. Even without the TOF
measurements (the TOF detectors are expected to be fully installed
before the HFT is complete) we can clearly distinguish flow from the
non-flow case in the D^0^ spectrum.

2.  **The strange open charm meson** **D~s~^+^**

We performed a similar study for the D~s~^+^ → φ + π^+^ and φ → K^+^ +
K^-^. Here the goal was to measure the D~s~ yield and integrate over all
p~T~ to extract the dN/dy for hadro-chemistry studies. The D~s~^+^ has a
lifetime of \~ 100 μm while the φ-meson decays immediately at the
secondary decay vertex. Table 8 lists the cuts used in our studies.
These cuts have been optimized using the program MINUIT analogous to the
D^0^ studies.

  ------------------------------ ----------------------------------------
  *L*                            \> 200 μm

  ∆*m*, D~s~^+^                  ± 50 MeV/*c*^2^

  ∆*m*, φ                        ± 20 MeV/*c*^2^

  *DCA*~πΚΚ~                     \< 60 μm

  cos(θ)                         θ \< 5.7 degrees

  cos(θ\*)                       θ\* \> 44.0 degrees
  ------------------------------ ----------------------------------------

[]{#__RefHeading___Toc122543554 .anchor}Table 8: **Cuts applied for
D~s~^+^ 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
K and two π tracks is then formed. The number of tracks used in the
background events is consistent with that of top 10% central Au+Au
collisions at RHIC. The resulting invariant mass distributions for
several p~T~ bins are shown in Figure 29. For all studied p~T~ bins, the
figure of merit S/(S+B) are better than 3.

![](media/image38.png){width="6.039583333333334in"
height="2.8986111111111112in"}

[]{#__RefHeading___Toc122543512 .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).

The acceptance for D~s~ reconstruction is shown in . Due to the large
background in a 3-body decay reconstruction, Strange D-mesons can be
detected only above 1.0 GeV/c with a saturation in efficiency of 2%
starting at 4 GeV/c. The rather low efficiency compared to D^0^ is also
due to the larger acceptance losses of the three daughter particles.

![](media/image40.png){width="5.995833333333334in"
height="2.9916666666666667in"}

[]{#__RefHeading___Toc122543514 .anchor}Figure 29: Invariant mass
distributions for D^+^ for several p~T~ windows. Lines are the
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, the efficiency is very low since
slow charged particles do not make it to 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
yet to be optimized in the near future, these efficiencies are already
usable for data analysis.

![](media/image41.png){width="5.9840277777777775in"
height="4.7027777777777775in"}

[]{#__RefHeading___Toc122543515 .anchor}Figure 30: D-meson efficiencies
as a function of transverse momentum. Dashed-lines are the fits.

We performed a similar study for the D~s~^+^ → φ + π^+^ and φ → K^+^ +
K^-^. Here the goal was to measure the D~s~ yield and integrate over all
p~T~ to extract the dN/dy for hadro-chemistry studies. The D~s~^+^ has a
lifetime of \~ 100 μm while the φ-meson decays immediately at the
secondary decay vertex. Table 9 lists the cuts used in our studies.
These cuts have been optimized using the program MINUIT analogous to the
D^0^ studies.

  ------------------------------ ----------------------------------------
  *L*                            \> 200 μm

  ∆*m*, D~s~^+^                  ± 50 MeV/*c*^2^

  ∆*m*, φ                        ± 20 MeV/*c*^2^

  *DCA*~πΚΚ~                     \< 60 μm

  cos(θ)                         θ \< 5.7 degrees

  cos(θ\*)                       θ\* \> 44.0 degrees
  ------------------------------ ----------------------------------------

[]{#__RefHeading___Toc122543555 .anchor}Table 9: **Cuts applied for
D~s~^+^ reconstruction**

The acceptance for D~s~ reconstruction is shown in Figure 31. Due to the
large background in a 3-body decay reconstruction, Strange D-mesons can
be detected only above 1.0 GeV/c with a saturation in efficiency of 2%
starting at 4 GeV/c. The rather low efficiency compared to D^0^ is also
due to the larger acceptance losses of the three daughter particles.

![](media/image39.jpeg){width="4.236805555555556in"
height="2.877083333333333in"}

[]{#__RefHeading___Toc122543516 .anchor}Figure 31: Ds reconstruction
efficiency as a function of transverse momentum

The momentum range 1.0-3.0 covers roughly 60% of the total D~s~ yield.
The statistical uncertainty of the measured yield is estimated to be
about 25%. However, extrapolation of the D~s~ spectrum in the momentum
range not covered by measurement is model dependent and introduces
systematic uncertainties. Studies to further optimize D~s~
reconstruction especially at lower momentum are a work in progress.

1.  **Rate Estimation**

In the following, we make estimations of the needed number of events for
both D-meson spectra and v~2~ measurements. We start with the D-meson
transverse momentum distribution generated from 200 GeV p+p collisions
in PythiaError: Reference source not found. The shape is power-law like.
The integrated yield is fixed as the measured at mid-rapidity at
RHICError: Reference source not found: dN/dy = 0.03 . The default value
of dN/dy in Pythia is about a factor of 3 lower.

For the 10% central and 0-80% minimum bias Au+Au collisions, the number
of binary collisions 950 and 290 were used as the scaling factors[^84],
respectively. The resulting distributions are shown in Figure 32(a). The
D~0~ efficiency with the fit is shown in (b) and the final counts versus
p~T~ are shown in (c). Assuming 10% statistic errors at given p~T~ bin,
the number of required events are shown in Table 10. At p~T~ \~ 10
GeV/c, a decent R~AA~ measurement requires about 7.5 billion p+p events
and 10 million Au+Au 10% central events. Those number of events are
achievable with the designed RHIC luminosity\*^,^[^85] and current STAR
detector system within a week of RHIC running.

\* A 60% duty factor is assumed here.

![](media/image43.png){width="5.18125in" height="4.314583333333333in"}

[]{#_Ref122544609 .anchor}**Figure 32: D-meson rates 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 \[Ref. Error: Reference source not
found\]. Number of binary N~bin~ = 950, corresponding to the top 10%
most central Au+Au collisions, is used to scaled the Au+Au collisions;
(b) 3-σ significance D^0^ efficiency with TPC+SSD+HFT. Dashed-line is
the fit to the simulated result; (e) 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+p)** | **0-10%       | **0-80%       |
|           |             |           | Au+Au**       | Au+Au**       |
|           |             |           |               |               |
|           |             |           | (N~bin~ =     | (N~bin~ =     |
|           |             |           | 950)          | 290)          |
+-----------+-------------+-----------+---------------+---------------+
| 1.0       | 0.5         | 44 x10^6^ | 0.45 x10^6^   | 1.75 x10^6^   |
+-----------+-------------+-----------+---------------+---------------+
| 2.0       | 0.5         | 70 x10^6^ | 0.45 x10^6^   | 1.75 x10^6^   |
+-----------+-------------+-----------+---------------+---------------+
| 3.5       | 1.0         | 70 x10^6^ | 0.45 x10^6^   | 1.75 x10^6^   |
+-----------+-------------+-----------+---------------+---------------+
| 5.5       | 1.0         | 350       | 0.75 x10^6^   | 3 x10^6^      |
|           |             | x10^6^    |               |               |
+-----------+-------------+-----------+---------------+---------------+
| 7.5       | 1.0         | 1200      | 3.5 x10^6^    | 11 x10^6^     |
|           |             | x10^6^    |               |               |
+-----------+-------------+-----------+---------------+---------------+
| 10.5      | 1.5         | 7500      | 9 x10^6^      | 30 x10^6^     |
|           |             | x10^6^    |               |               |
+-----------+-------------+-----------+---------------+---------------+

[]{#__RefHeading___Toc122543556 .anchor}Table 10: Number of estimated
events for 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 the Glaubor calculations \[Ref.
Error: Reference source not found\].

In order to make any rate estimation for v~2~ measurement, we need to
make further assumptions about the p~T~ dependence of the anisotropy. We
choose them from Ref. Error: Reference source not found in which the
authors considered two extreme cases: charm quark do / do not flow. The
corresponding distributions are shown in Figure 33 (b). In the same
figure, the K~0~ v~2~ is shown as the line. With these assumptions and
the know efficiency (Figure 33(c)), the estimated rates per event are
shown in plot-(d). To keep the statistical uncertainties within 10%, the
required number of minimum bias events for the D^0^ v~2~ measurement is
listed in Table 11 for given p~T~ bins. As one can see, within 500
million events, D^0^ v~2~ (p~T~) can be well measured up to p~T~ \~ 35
GeV/c, with the proposed STAR HFT.

![](media/image44.png){width="5.554166666666666in"
height="4.653472222222222in"}

[]{#__RefHeading___Toc122543519 .anchor}Figure 33: 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 \[Ref. Error:
Reference source not found\]. Number of binary N~bin~ = 290,
corresponding to the minimum bias Au+Au collisions, is used to scaled
the Au+Au collisions. Power-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. 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.

+--------------+----------------+----------------+--------------------+
| **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 x10^6^     | 525 x10^6^         |
+--------------+----------------+----------------+--------------------+
| 1.0          | 0.5            | 70 x10^6^      | 140 x10^6^         |
+--------------+----------------+----------------+--------------------+
| 2.0          | 0.5            | 53 x10^6^      | 125 x10^6^         |
+--------------+----------------+----------------+--------------------+
| 3.0          | 1.0            | 105 x10^6^     | 175 x10^6^         |
+--------------+----------------+----------------+--------------------+
| 5.0          | 1.0            | 210 x10^6^     | 440 x10^6^         |
+--------------+----------------+----------------+--------------------+

[]{#__RefHeading___Toc122543557 .anchor}Table 11: Number of estimated
events for 10% statistical errors for D0 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 \# of binary
collisions is from the Glabour calculations \[Ref. Error: Reference
source not found\].

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

## Additional Issues that Relate to the Performance of the HFT

This section addresses several issues that may affect the performance of
the proposed 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 arial
density of extraneous hits that can be tolerated depends on the tracking
precision projected onto the HFT surface 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 12 and an estimate of the hit density is shown in Table
13.

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

  dN/dη (min bias)                              170

  Min bias cross section                        10 barns

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

[]{#__RefHeading___Toc122543558 .anchor}Table 12: Luminosity and other
parameters that determine the particle flux on the HFT.

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

  Radius                                5 cm              1.5 cm

  Hit Flux                              4,300 Hz/cm^2^    18,000 Hz/cm^2^

  Hit Density 4 ms Integration          17/cm^2^          72/cm^2^

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

  Probability of Tracking Window Pileup 10 %              10 %

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

  Probability of HFT Pileup             0.14%             0.58%
  ------------------------------------- ----------------- ----------------

[]{#__RefHeading___Toc122543559 .anchor}Table 13: 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 17/cm^2^ in the outer layer and 72/cm^2^ in the
inner layer of the HFT[^86]. This produces a pileup probability in the
track search window of 10% at both the inner and outer HFT barrels.. 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, 0.14% and 0.58%,%, for the outer and inner barrel
respectively. This meets our specifications.

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. And for future reference, an 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, but we also expect an increase in
peak luminosity by about a factor of 4. 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 14 gives the same loading information resulting from a single
central Au+Au collisionError: Reference source not found. 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 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^

  Probability of Tracking Window Pileup  1%              1.1%

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

  Probability of HFT Pileup              0.014%          0.06%
  -------------------------------------- --------------- ----------------

[]{#__RefHeading___Toc122543560 .anchor}Table 14: 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 SVT data 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 close 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[^87]. 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 other contributions such as beam gas[^88].

# CMOS Sensors

## Introduction

The future STAR physics program requires a fast, thin, detector that can
operate in a relatively high radiation dose environment. Our goal is to
measure rare charmed decays and to measure flow in the charmed sector.
Since flow is a soft process, the HFT will measure transverse momenta
down to 150 MeV/c and this 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 and 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**

At this time, there are three good technologies that can be used for a
thin vertex detectors: Charge Coupled Devices (CCD), Active Pixel
Sensors (APS), 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 first generation detector.

1.  **CCDs**

At SLAC, the SLD collaboration built and successfully operated a pixel
vertex detector[^89], VXD3, based on CCD technology.[^90] As 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 loss and signal sharing. As the SLD vertex
detector ran at the relatively low radiation intensity of the SLAC
Linear electron Collider (SLC), it 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.

Because of the high radiation environment and need to have the vertex
detector in the trigger, CCDs were not the appropriate choice at the LHC
(Large Hadron Collider)[^91] 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 that two chips have to be assembled.
The two chips and their interconnection are much thicker than can be
done in CCD technology.

CCDs require significant power to clock the charge around the chip.
Consequently, liquid cooling has been required. For instance, VXD3 used
LN~2~ gas to cool the device. Because of the ladders operated at a much
lower temperature than they were constructed, 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. This cooling system adds 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.[^92]

2.  **DEPFET**

The MPI Semicondcutor Laboratory has recently invented[^93] and has been
the pioneering leader in the use of DEPFETs.[^94] Its concept is based
on the combination of the sideward depletion, as used in the
semiconductor drift detector with a field effect transistor to collect
the charge. The MPI Group has a concept 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 with that institute. It has
both the advantage and disadvantage that its process is controlled by
physicists. There has been significant testing in the last years. This
technology is very promising and will be carefully considered for the
design of the silicon upgrade to the HFT for RHIC II.

3.  **Active Pixel Sensors**

APS devices have been used as photon detectors since late 1960s.[^95]
They have recently surpassed CCDs in the photography market because of
their lower cost and less demand for power. Power consumption is
important in a particle detector applications because a detector that
can be air cooled is thinner, 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. See Table
15. APS technology is our preferred technology for the HFT and we
propose to work with the Strasbourg group to design and build the
detector. The technology will be described in more detail in the next
section.

+------------+----+----------+-----+----+----+----------------------+---+---+
| **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  | hadrontherapy/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___Toc122543561 .anchor}Table 15: Table -- Chart of APS
chips that the IReS group has produced \[[^96]\]

## Main Features and Performance of CMOS Active Pixel Sensors

CMOS sensors are manufactured in 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[^97] (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/image45.png){width="2.3986111111111112in"
height="3.504166666666667in"}

[]{#__RefHeading___Toc122543520 .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 from 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[^98] and several prototypes, called MIMOSA[^99], have
been built by the IReS group in Strasbourg. The LBL/UCI group has also
built and successfully tested these sensors. The prototype detectors
have explored different fabrication processes and key parameters of the
charge sensing system, and the results demonstrated 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)
and the double hit resolution is \~ 30 μm.

The radiation tolerance of the sensors to bulk damage[^100] 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 30 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 Figure 35. 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
about 4 times faster read-out speed. We have recently tested the chip at
the ALS and have measured the expected Landau spectrum.

![](media/image46.jpeg){width="2.717361111111111in"
height="1.9909722222222221in"}![](media/image47.jpeg){width="3.2152777777777777in"
height="1.9868055555555555in"}

[]{#__RefHeading___Toc122543521 .anchor}Figure 35: Wafer of reticle size
sensors (left) and zoomed-in view of individual chips (right).

Table 16 shows how the measured performance of the MIMOSA-5 sensors
compare to the specifications of the HFT detector for STAR.

Most of the requirements (granularity, radiation tolerance, thinning,
read-out speed, power dissipation, and sensor size) are already
satisfied 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 has to
be given to the magnitude of the leakage current in order to keep the
corresponding noise at an acceptable level. This issue has consequences
for the read-out time and on the choice of the fabrication technology.

  ------------------------- ---------------------- -----------------------
  Parameter                 HFT specifications     MIMOSA-5 performance

  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[^101]     \> 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___Toc122543562 .anchor}Table 16: Comparison between
MIMOSA-5 characteristics and HFT specifications.

## MIMOSTAR Sensor Design

Based on the results of MIMOSA-5, a new series of chips, MIMOSTAR, have
been fabricated Its most significant design parameters are summarized
below:

-   Pixel pitch: 30 μm.

-   Pixel array: 640 × 640, i.e. active area of 19.2 × 19.2 mm^2^.

-   Side band devoted to FEE: \~ 1.5--1.7 mm wide.

-   Chip dimension: \~ 21 × 20 mm^2^.

-   Additional details can be found in Ref. \[[^102]\].

Since the read-out speed requirement is modest, the chip's architecture
is based on relatively slow signal processing at the pixel level. This
feature 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. In order to achieve these
goals, the chip is subdivided into 10 groups of 64 × 640 pixels. Each
group is split into two equal size sub-groups, and each of these
sub-groups is connected to a common amplifier, which is placed on the
outer edge of the chip. The 2 × 10 amplifiers operate in parallel in
order to achieve a pixel read-out frequency of 2.5 MHz. A fast
multiplexer, driven at a frequency of 50 MHz, will receive the outputs
of each of the 10 groups. As a result, the chip read-out time is
approximately 8 ms.

A more ambitious version of the chip is under consideration, where the
read-out and multiplexing frequencies are 5 and 100 MHz respectively,
thus allowing us to readout the chip in only 4 ms.

The chip is foreseen to include 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 flow.

Last year MIMOSTAR-1 was fabricated in the TSMC 0.25 µm process. The
Strasbourg group has shown that the conventional controls work (via the
JTAG controller), and that the bias circuits work and exhibit linear
response via their DACs. Finally, the analog performance of the chips
has been tested and the gain on the pre-amplifiers is 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 hold the collected
charge long enough for it to be read out. We 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 process, in which
this Active Reset Circuit has already been demonstrated to work. The 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

We plan to learn from the MIMOSTAR architecture and there will be at
least one more prototype run before trying to produce the final chips to
be used in the HFT.

Meanwhile, several fabrication processes will be 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 of the chips 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. The IReS tests show that a detection efficiency of
99.9% can be achieved, as well as a single point resolution of about 2.5
μm[^103]. 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. This makes
it well suited to applications where one experiences 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 design, 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[^104].

Our colleagues at the LBL light source have just characterized several
MIMOSA-5 chips in the ALS's 1.5 GeV/c electron beam. These chips will
shortly be thinned. Afterwards, they will be compared with the original
test results.

.

## 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. \[[^105]\].

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/image48.jpeg){width="3.011111111111111in"
height="2.765277777777778in"}

[]{#__RefHeading___Toc122543522 .anchor}Figure 36: An APS Sensors
developed by the LBNL/UIC group. The picture shows 16 separate test
structures. Each structure has a 36 x 36 array of 20 µm pixels.

Our measurements have confirmed the IReS results that APS sensors can
measure charged particles with excellent spatial
resolution[^106]^,^[^107]**.** Figure 37 shows several 1.5 GeV electrons
recorded at LBNL's Advanced Light Source.

![](media/image49.jpeg){width="4.013194444444444in"
height="3.8444444444444446in"}

[]{#__RefHeading___Toc122543523 .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
electrons 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[^108].
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..[^109] 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.

While the current design of MIMOSTAR is suitable for this proposal, our
group has been looking at several other techniques for a next generation
detector. It is clearly desirable to speed up the readout and reduce the
signal spread.

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, while not increasing
the capacitance, because the charge is collected through a small size
gate. 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 introduced by the reset transistor. Its
main drawback is to require reading out at least two frames, hence
doubling the power consumption. To avoid doing CDS, we have produced a
clamp circuit[^110] 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[^111]. 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
single to noise ratio. This technique is well suited for this
application overcoming problems with traditional CCDs that suffer
radiation damage. 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 should be receiving it shortly. 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 eliminated.

Table X Silicon requirements for maximum average Au + Au RHIC luminosity
of 7.0 × 10^27^ Hz/cm^2^ or 2.5 nb^-1^/week

  --------------------------------------------------- -------------------
  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                                               ≤100 mW/cm^2^

  Operating temperature                               ≥30 °C

  Integration time[^112]                              ≤0.2 ms

  Mean silicon thickness                              ≤100 μm

  Readout time                                        ≤1 ms

  Efficiency (min I)[^113]                            ≥98%

  Accidental cluster density                          ≤50/cm^2^

  Binary readout, number of threshold bits[^114]      1 or 2

  Radiation tolerance [^115]                          ≥171 kRad

  Number of conductors supporting the ladder (10      ≤140
  chips/ladder)[^116]                                 

  Triggered readout, maximum trigger delay[^117]      2 μs
  --------------------------------------------------- -------------------

These new APS designs are promising but still speculative. These R&D
studies will be done concurrently with the construction of the HFT
detector. If any of these ideas leads to significant improvements, then
they can be added to a second-generation vertex detector.

A second generation IC is needed to meet the requirements of RHIC II. At
the higher lumonosities, we need to read-out the chip much faster and
have the chip sensitive for a much shorter time. The goal of our current
R&D is to find a sensor that can meet the specifications listed in Table
X.

# Data Acquisition and Readout

## Requirements

The data acquisition system is designed to read out the large body of
data from the individual 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 requirements is provided
in Table 17.

  ---------------------------------------------------- ------------------
  Total number of pixels                               98 × 10^6^

  Number of pixels per chip                            640 × 640

  Pixel Readout rate                                   10 ns (100 MHz) 2
                                                       x 50MHz / chip

  Readout time per frame                               4 ms

  Dynamic range of the ADC                             10 bits

  Raw data from one sensor using a 10 bit ADC          1 Gb/s

  Fixed pattern noise                                  2000 *e^−^*

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

  Maximum signal                                       900 *e^−^*

  Dynamic range after Correlated Double Sampling       8 bits

  Total power consumption                              90 W
  ---------------------------------------------------- ------------------

[]{#__RefHeading___Toc122543563 .anchor}Table 17: Requirement Summary -
constraints for the APS, due to signal expectation, available design
process, and mechanics

Digitizing the analog signal on each pixel into a 10 bit digital signal
yields approximately 1 Gb/s per sensor chip when read out in 4 ms. Thus,
the total front end data rate is \~240 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) and then leakage current subtraction, i.e. subtraction of two
consecutive frames followed by zero suppression. CDS cancels out fix
pattern and reset noises 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 10 *e^−^* to guarantee an
efficiency of greater than 99%. 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. Hit finding and the reduction of the
data to addressed clusters then reduces the data to a manageable rate.
The use of forced air-cooling on the low mass region of the ladder
limits the total power dissipation to about 100 - 150 mW/cm^2^.

## Architecture

The basic flow of a ladder data path starts with the APS sensors. An HFT
ladder has 10 MIMOSTAR MAPS chips with 640 by 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. 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 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 is shown in Figure 38.

![](media/image50.wmf){width="5.427083333333333in"
height="1.6041666666666667in"}

[]{#__RefHeading___Toc122543524 .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 consist of a motherboard and daughter card
configuration. A functional block diagram is shown in Figure 39. There
are 5 daughter cards per motherboard and each daughter card services 2
of the MIMOSTAR sensors on the ladder. The analog signals are carried to
the daughter cards where they are digitized with a 10 bit ADC at 50 MHz.
Following digitization, the 10 bit ADC values are passed synchronously
to an FPGA for CDS. Performing CDS and pedestal subtraction requires a
data sample to be stored for each pixel of the detector. This drives the
need for external RAM on the daughter cards. After CDS and pedestal
subtraction, the data can be represented by 8 bits. The data is then
transferred to the motherboard for hit finding and data reduction.

![](media/image51.wmf){width="5.996527777777778in" height="3.25625in"}

[]{#__RefHeading___Toc122543525 .anchor}Figure 39: DAQ Layout: schematic
of DAQ system for a single 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.

We are currently investigating methods of hit finding and data reduction
for use on the motherboard. A simple high threshold hit read out with
the surrounding 8 pixels is our default approach. This can be
implemented in an FPGA and run as a pipeline filling the output buffer
with ADC and address values. A simple example of an FPGA logic diagram
that accomplishes this can be found in Figure 40. Additional methods
currently under investigation include summing algorithms around
different thresholds and centroid determination by geometric pattern
with high and low thresholds. An implementation document using this
method of hit finding but saving only the center pixel address of the
cluster is attached as an appendix.

![](media/image52.wmf){width="5.997916666666667in"
height="4.504166666666666in"}

[]{#__RefHeading___Toc122543526 .anchor}Figure 40: A simple cluster
finding algorithm for the HFT detector. ADC data from two MIMOSTAR
detector columns + 3 pixels are stored sequentially in an 8-bit wide
shift register. The senter pixel of a 3 x 3 pixel window is compared to
a high threshold with each clock. If the threshold is exceeded, the ADC
data for the 3 x 3 pixel and the address are stored as a cluster word
into a readout FIFO. This method is extendable to allow for multiple
simultaneous thresholds and geometric pattern triggers.

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 24 separate chains.

## Data Synchronization, Readout and Latency

The readout of the 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. In order to
service multiple triggers within that 4 ms readout time we will provide
multiple buffers that will allow the capture of temporally overlapping
complete frames. A functional block diagram of this system is shown in
Figure 41. In this system, the cluster data is fanned out to 5 Event
FIFOs. A separate Event FIFO is enabled for a 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 it's internal clocks from the RHIC
strobe. We will provide functionality to allow the motherboards to be
synchronized at startup and any point thereafter.

![](media/image53.pct){width="5.309722222222222in"
height="3.502083333333333in"}

[]{#__RefHeading___Toc122543527 .anchor}Figure 41: 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

  --------------------------------------------------------- ----------------------
  [Item]{.underline}                                        [number]{.underline}

  bits/address                                              18

  inner ladders                                             6

  outer ladders                                             18

  MIMOSTAR sectors per ladder                               20

  ave hits/sector, inner, L = 10^27^                        200

  ave hits/sector, outer, L = 10^27^                        40
  --------------------------------------------------------- ----------------------

[]{#__RefHeading___Toc122543564 .anchor}Table 18: Data rate calculation
parameters

We have determined that 10 bits of dynamic range is required to give us
sufficient signal to noise given the expected signal sizes and
noise.[^118] The data rate from each 640 x 640 MIMOSTAR2 detector is
thus approximately 1 Gb / sec. The total rate of raw data entering the
processing chain in the detector is thus approximately 240 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 9 x
8-bit ADC values + 18 address bits for a total of 90 bits. Combining
this with the occupancy per layer and the readout rate of 1 KHz gives an
event size of 450KB and data rate from the detector of 450 Mb/sec. This
is shown graphically in Figure 42. After the ADC data with center
cluster addresses for each pixel are read into the DAQ structure,
further reductions in the data rate are possible. Centroid fitting on
the DAQ receiver processors can significantly reduce the rate at which
data is saved to RCF. If the centroid is fit, the ADC data can be
discarded and the saved data can be reduced to just a geometric point,
reducing the data rate to the order of 90 MB/sec Each HFT frame sent to
DAQ for event building will have the data rate shown below.

![](media/image54.wmf){width="6.0in" height="3.5625in"}

[]{#__RefHeading___Toc122543528 .anchor}Figure 42: Data rates at the
various stages of the HFT readout chain.

## Prototypes

Several different prototype readout electronics boards have been
constructed and tested that are very similar to the proposed readout
electronics described above. In Figure 43 one can see the prototyping
results of a low mass flex PCB on a prototype ladder with MIMOSA5
detectors.

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

[]{#__RefHeading___Toc122543529 .anchor}Figure 43: 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 MIMOSA5 detectors was also
constructed and used. A functional schematic is shown below in Figure
44.

![](media/image56.wmf){width="5.997916666666667in"
height="5.624305555555556in"}

[]{#__RefHeading___Toc122543530 .anchor}Figure 44: Functional component
diagram of the prototype readout system constructed for the readout on a
MIMOSA5 based ladder.

One can see the great similarity between what we have constructed for
this test and the final readout design. The basic data flow is the same.
The details of hit finding and final readout to DAQ are not present in
this system but the motherboard / daughter card concept and the ADC FPGA
and SDRAM for CDS are present and, for the most part, implemented and
working. A photograph showing the motherboard with a daughter card
attached is shown in Figure 45.

![](media/image57.jpeg){width="5.986805555555556in"
height="3.0909722222222222in"}

[]{#__RefHeading___Toc122543531 .anchor}Figure 45: Prototype motherboard
and daughter card used for reading out MIMOSA5 detectors.

This system is working and the basic elements of FPGA based control and
de-serialization of the ADC outputs, SDRAM memory interface and CDS are
implemented on the daughter cards seen above. These VHDL elements will
be used in our final design. The daughter card design above is quite
flexible and the same physical boards may be suitable for the HFT
readout system.

# Integration with the Experiment

## 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 46 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.

![](media/image58.jpeg){width="6.0in" height="4.5in"}

[]{#__RefHeading___Toc122543532 .anchor}Figure 46: The HFT is shown
integrated with the STAR inner detectors cone assembly.

For simplicity several existing STAR structures are not shown. The SSD
and one of the cone supports that is integral to its support is not
shown

A close up view in Figure 47 shows the HFT ladder arrangement. There are
two tracking layers: one at 1.5 cm radius and the other at 5 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/image59.jpeg){width="5.0in" height="3.0in"}

[]{#__RefHeading___Toc122543533 .anchor}Figure 47: Close up view of the
HFT ladders.

The detector support structure illustrated in Figure 47 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 49
showing the three identical frames (brass colored) that are
kinematically joined around the beam pipe using ball on cylinder
contacts.

![](media/image60.jpeg){width="5.2875in" height="2.7090277777777776in"}

[]{#__RefHeading___Toc122543534 .anchor}Figure 48: 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/image61.jpeg){width="5.325in" height="3.8625in"}

[]{#__RefHeading___Toc122543535 .anchor}Figure 49: Kinematic mount
structure.

The common tie point joining the HFT detector system to the STAR support
cone is illustrated in Figure 50. 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/image62.jpeg){width="5.325in" height="3.99375in"}

[]{#__RefHeading___Toc122543536 .anchor}Figure 50: The HFT is shown
integrated with the STAR inner detectors cone assembly.

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

### Position Alignment and Calibration

The required position resolution of better than 10 microns 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 49 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 the
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
51. 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/image63.jpeg){width="5.992361111111111in"
height="2.1319444444444446in"}

[]{#__RefHeading___Toc122543537 .anchor}Figure 51: 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. The average effective
thickness is represented in Table 19.

  ---------------------------- ---------- ----------------------- ---------
  Material                     X~0~ (cm)  Material thickness (μm) \% X~0~

  Beryllium beam pipe          35         500                     0.14

  Aluminum                     9.0        10                      0.007

  Silicon                      9.4        55                      0.06

  Kapton                       35         170                     0.05

  Adhesive                     35         85                      0.024

  Carbon Composite             28         174                     0.062

  Total for **one ladder**                490                     0.20
  ---------------------------- ---------- ----------------------- ---------

[]{#__RefHeading___Toc122543565 .anchor}Table 19: Materials in the beam
pipe and the first detector layer with their radiation length fractions.
For details see Ref. \[[^119]\]

### 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[^120]. 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)[^121]. Our structure should not exhibit any mechanical degradation
from the absorbed dose.

## Ladder Mechanical Tests

### Load Distortion Tests

Figure 51, and Figure 53 show a prototype of the ladder and detector
carrier that we have built. Figure 52 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[^122], 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 54 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[^123]. 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/image64.jpeg){width="5.992361111111111in" height="1.29375in"}

[]{#__RefHeading___Toc122543538 .anchor}Figure 52: A cross-section of
the prototype detector ladder showing its structure and materials
composition.

![](media/image65.jpeg){width="4.852777777777778in" height="3.5in"}

[]{#__RefHeading___Toc122543539 .anchor}Figure 53: A cross-section of
the prototype detector ladder showing its structure and materials
composition.

![](media/image66.jpeg){width="5.080555555555556in"
height="4.007638888888889in"}

[]{#__RefHeading___Toc122543540 .anchor}Figure 54: 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[^124]. 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 55). This
tool will also be used to test the stability of the final detector.

![](media/image67.jpeg){width="5.991666666666666in"
height="3.495833333333333in"}

[]{#__RefHeading___Toc122543541 .anchor}Figure 55: 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 55) 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[^125] 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[^126]. Additional information on the cooling studies is
available on the web[^127].

## Cabling

Connection to the detector readout chips will be done with an aluminum
Kapton cable. 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.

The cable is part of the ladder and is located underneath the APS
detector.. It will carry power, clock, analogue signals and control.

Figure 56 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[^128].

![](media/image68.jpeg){width="5.993055555555555in"
height="1.5659722222222223in"}

[]{#__RefHeading___Toc122543542 .anchor}Figure 56: A prototype readout
cable for the HFT.

Figure 57 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[^129].

![](media/image69.jpeg){width="5.989583333333333in"
height="2.9784722222222224in"}

[]{#__RefHeading___Toc122543543 .anchor}Figure 57: 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 58.

![](media/image70.jpeg){width="6.0in" height="3.73125in"}

[]{#__RefHeading___Toc122543544 .anchor}Figure 58: 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 59. The outer
shell has been made transparent in the figure to show the inner
structure.

![](media/image71.jpeg){width="6.0in" height="4.5in"}

[]{#__RefHeading___Toc122543545 .anchor}Figure 59: 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[^130]. 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^,^[^131]^,^[^132][^133]. 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 dumps125.

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[^134]^,^[^135]^,^[^136] 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 60 shows how the
detector ladders mesh with the support spokes.

![](media/image72.jpeg){width="5.875in" height="4.40625in"}

[]{#__RefHeading___Toc122543546 .anchor}Figure 60: 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[^137]. 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[^138].

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[^139]. 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 46.

## 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.

# Cost, Schedule and Manpower

## Cost and Schedule

An estimate has been made for the cost and the technically-driven
schedule for the HFT.

# 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 ( ≤ 0.36% radiation
length) layers of Active Pixel Sensors (APS) with a position resolution
better than 10 μm. APS technology is the only option that fits these
requirements while compromising neither the efficiency nor the readout
speed. Indeed, an APS can be thinned down to 50 μm and their low power
consumption allows using air-cooling. Matching the readout speed
required at RHIC will be achieved in two steps, first by building two
separate chips, one readout chip mounted on a purely analog sensor, and
then by designing one chip combining both analog and digital features.
The mechanical support has been 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

Attached.

# Appendix II -- MIMOSTAR II Users Guide

Attached.

#  References

[^1]: F. Karsch,. *Nucl. Phys.* A699, 199c (2002).

[^2]: K.H. Ackermann et al., *Phys. Rev. Lett.* 86, 402 (2001).

[^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
    (2005).

[^6]: J. D. Bjorken, *FERMILAB-Pub-82/59-THY*.

[^7]: X. N. Wang, M. Gyulassy, I. Vitev, and B. Zhang,
    *nucl-th/0302077*.

[^8]: D. Schiff, R. Baier, and B. Zakharov, *Ann. Rev. Nucl. Part.
    Sci.*, 50, 37 (2000).

[^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).

[^12]: B. Mueller, nucl-th/0404015.

[^13]: R. Vogt, hep-ph/0203115.0203151.

[^14]: K.J. Eskola, V.J. Kolhinen and R. Vogt.

[^15]: P. Levai, B. Mueller, and X.N. Wang, *Phys. Rev.* C51, 3326
    (1995) : P. Levai and R. Vogt, Phys. Rev. C56, 2707 (1997).).

[^16]: K. Adcox et al., *Phys. Rev. Lett.* 88, 192303 (2002).

[^17]: J. Adams et al. (STAR Collaboration), Phys. Rev. Lett. 94, 062301
    (2005); nucl-ex/0407006.

[^18]: T. Sjostrand et al., Computer Physics Commun. 135, 238 (2001).
    PYTHIA 6.152 was used with the parameter settings: MSEL=1, CTEQ5M1.
    Bottom contribution was estimated to be 20-30% in p~T~=2-3 GeV/c,
    40-50% in 3-4 GeV/c to the total non-photonic electrons, and
    negligible at lower p~T~.

[^19]: R. Vogt, Private Communication.

[^20]: Batsouli et al., Phys. Lett. B557, 26 (2003). nucl-th/0212068.

[^21]: X. Dong, "Open Charm Production at RHIC" QM2005, nucl-ex/0509038.

[^22]: J. Adams et al. (STAR Collaboration), Phys. Rev. Lett. 95, 122301
    (2005).

[^23]: J. Adams et al. (STAR Collaboration)., Phys. Rev. Lett. 92,
    052302 (2004).

[^24]: K. Schweda et al., J. Phys. G 30, S693 (2004).

[^25]: J. Castillo et al., J. Phys., G 30, S1207 (2004).

[^26]: J. Adams et al., (STAR Collaboration) Phys. Lett. B 616, 8 (2005)

[^27]: E. Shuryak, Phys. Lett. B78, 150 (1978).

[^28]: R. Rapp and J. Wambach, Adv. Nucl. Phys. 25, 1 (2000).

[^29]: R. Rapp, Phys. Rev. C 63**,** 054907 (2001); R. Rapp,
    hep-ph/0201101; R. Rapp and J. Wambach, hep-ph/9909229.

[^30]: U. Heinz and P.F. Kolb, Nucl. Phys. A702**,** 269c (2002).

[^31]: H. Sorge, Phys. Rev. Lett. 78, 239 (1997); H. Sorge, Phys. Rev.
    Lett. 8**2,** 2048 (1999).

[^32]: C. Adler *et al.,* (STAR Collaboration), Phys. Rev. Lett. 87,
    262301 (2001).

[^33]: C. Adler *et al.,* (STAR Collaboration), Phys. Rev. Lett. 89,
    132301 (2002).

[^34]: P. Huovinen *et al.*, Phys. Lett. B503, 58 (2001).

[^35]: M. Kaneta, J. Phys. G: Nucl. Part. Phys. 30, S1217 (2004).

[^36]: F. Laue et al. (STAR Collaboration), nucl-ex/0411007, (2004).

[^37]: X. Dong et al., Phys. Lett.  B597, 328(2004)

[^38]: Greco, Ko, Rapp, Phys. Lett. B595, 202(2004)

[^39]: J. Stachel, Nucl. Phys. A654, 119c (1999).

[^40]: P. Braun-Munzinger, D. Magestro, K. Redlich, and J. Stachel,
    Phys. Lett. B518**,** 41 (2001).

[^41]: P. Braun-Munzinger, K. Redlich, and J. Stachel, nucl-th/0304013.

[^42]: E. Schnedermann, J. Sollfrank, and U. Heinz, Phys. Rev. C48**,**
    2462 (1993).

[^43]: P. Braun-Munzinger and J. Stachel, Phys. Lett. B490**,** 196
    (2000).

[^44]: P. Braun-Munzinger and J. Stachel, Nucl. Phys. A690**,** 119c
    (2001).

[^45]: A. Andronic, P. Braun-Munzinger, K. Redlich, and J. Stachel,.
    *Phys. Lett.* B571, 36 (2003).

[^46]: <http://www-h1.desy.de/psfiles/confpap/EPS2003/H1prelim-02-076.ps>

[^47]: <http://www-zeus.desy.de/physics/phch/conf/eps03/564/paper.pdf>

[^48]: T.Sjöstrand,. *Comp. Phys. Commun.* 82, 74 (1994).

[^49]: A. Andronic et al., Phys. Lett. B571, 36 (2003).

[^50]: [M.
    Djordjevic](http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+Djordjevic,+Magdalena),
    [M.
    Gyulassy](http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+Gyulassy,+Miklos),
    and [S.
    Wicks](http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+Wicks,+Simon),
    Phys. Rev. Lett. [94]{.underline}, 112301(2005); hep-ph/0410372.

[^51]: Y.L. Dokshitzer and D.E. Kharzeev, Phys. Lett. B519, 199(2001).

[^52]: [N.
    Armesto](http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+Armesto,+Nestor),
    [A.
    Dainese](http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+Dainese,+Andrea),
    [C.A.
    Salgado](http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+Salgado,+Carlos+A.),
    [U.
    Wiedemann](http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+Wiedemann,+Urs+Achim),
    Phys. Rev. D71, 054027(2005); hep-ph/0501225

[^53]: S. S. Adler et al (STAR Collaboration),., *Phys. Rev. Lett.* 91,
    072301 (2003).

[^54]: S. S. Adler et al (STAR Collaboration)., *Phys. Rev. Lett.* 91,
    241803 (2003).

[^55]: C. Adler et al (STAR Collaboration)Phys. Rev.., *Lett.* 90,
    082302 (2003).

[^56]: M. Gyulassy and M. Plümer, *Nucl. Phys.* A527, 641c (1991).

[^57]: Y. L. Dokshitzer and D. E. Kharzeev, *Phys. Lett.* B519, 199
    (2001).

[^58]: H. Buesching, (Phenix Collaboration), QM05 proceedings.

[^59]: J. Dunlop, (STAR Collaboration), QM05 proceedings.

[^60]: X. Dong, charm production overview, QM05 proceedings.

[^61]: J. Adams, et al. (STAR Collaboration), Phys. Rev. Lett. 91 (2003)
    072304.

[^62]: S.S. Adler et al. (Phenix Collaboration), Phys. Rev. Lett. 91,
    072301(2003).

[^63]: M. Djordjevic, M. Gyulassy, R. Vogt, and S. Wicks,
    nucl-th/0507019.

[^64]: [H. van
    Hees](http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+van+Hees,+Hendrik),
    [V.
    Greco](http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+Greco,+Vincenzo),
    and [R.
    Rapp](http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+Rapp,+Ralf),
    nucl-th/0508055.

[^65]: \[Tai1 -- ask Nu Xu\]

[^66]: K. Abe, et al. (Belle Collaboration), Phys. Rev. Lett. 89,
    142001(2002).

[^67]: S. Baek, P. Ko, J. Lee, and H.S. Song, J. Korean Phys. Soc. 33,
    97 (1998).

[^68]: P.L.Cho and A.K. Leibovich, Phys. Rev. D54, 6690(1996).

[^69]: F. Yuan, D.F. Aiao, and K.T. Chao, Phys. Rev. D56, 321(1997).

[^70]: B.L. Ioffe and D.E. Kharzeev, Phys. Rev. D69, 014016(2004).

[^71]: [M.
    Djordjevic](http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+Djordjevic,+Magdalena),
    [M.
    Gyulassy](http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+Gyulassy,+Miklos),
    [R.
    Vogt](http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+Vogt,+Ramona),
    [S.
    Wicks](http://www.slac.stanford.edu/spires/find/wwwhepau/wwwscan?rawcmd=fin+Wicks,+Simon),
    nucl-th/0507019.

[^72]: D.E. Kharzeev and K. Tuchin, Nucl. Phys. A753, 316(2005).

[^73]: R. Rapp, Phys. Rev. C63, 054907(2001).

[^74]: X. Dong, P. Sorensen, N. Xu, in preparation, Sept. 2005.

[^75]: T. Affolder et al., Phys. Rev. D 64, 032002 (2001).

[^76]: A. Marín et al., J. Phys. G: Nucl. Part. Phys. 30, S709 (2004).

[^77]: D. Adamova et al. (CERES Collaboration), Phys. Rev. Lett. 91
    (2003) 042301

[^78]: R. Rapp, *Phys. Rev.* C63, 054907 (2001).

[^79]: R. Rapp, *Pramana* 60, 675 (2003).

[^80]: G. Bunce, N. Saito, J. Soffer, and W. Vogelsang, Annu. Rev. Nucl.
    Part. Sci. 49, 303 (1999).

[^81]: M. Karliner and R.W. Robinett, Phys. Lett. B 324, 209 (1994).

[^82]: I. Bojak and M. Stratmann Phys. Rev. D 67, 034010 (2003).

[^83]: M. Gluck, E. Reya, M. Stratmann, and W. Vogelsang Phys. Rev. D
    63, 094005 (2001).

[^84]: http://www.star.bnl.gov/protected/spectra/macross/glauber/flow/FlowGlauberCalculations.htm

[^85]: W. Fischer, RHIC-II workshop,
    [http://www.bnl.gov/panic05%5Fsatellite/](http://www.bnl.gov/panic05_satellite/)

[^86]: Technical report,
    [http://www-rnc.lbl.gov/*\~*wieman/HitDensityMeasuredLuminosity.htm](http://www-rnc.lbl.gov/~wieman/HitDensityMeasuredLuminosity.htm)

[^87]: Technical report,
    [http://www-rnc.lbl.gov/*\~*wieman/TrackDensityApr2004.htm](http://www-rnc.lbl.gov/~wieman/TrackDensityApr2004.htm)

[^88]: Jim Thomas, Private Communication.

[^89]: K. Abe, A. Arodzero, C. Baltay, J.E. Brau, M. Breidenbach, P.N.
    Burrows *et al.,* "Design and performance of the SLD vertex
    detector: a 307 Mpixel tracking system," *Nucl. Instrum. Methods,*
    vol. A400, pp. 287-343, 1997.

[^90]: J. Janesick, *Scientific Charge-Coupled Devices,* SPIE Press,
    Bellingham, WA, 2001.

    C.J.S. Damerell, "Charge coupled devices as particle tracking
    detectors," *Rev. Sci. Instr.,* vol. 69, pp. 1549-1573, 1998.

[^91]: C.J.S. Damerell, "Charge coupled devices as particle tracking
    detectors," *Rev. Sci. Instr.,* vol. 69, pp. 1549-1573, 1998.

[^92]: K.D. Stefanov, "CCd-based vertex detector for the future linear
    collider", Nucl. Instru. and Meth. A 549, 93 (2005).

[^93]: J. Kemmer and G. Lutz, Nucl. Instr. and Meth. A 253, 365 (1987).

[^94]: G. Lutz, "DEPFET development at the MPI semiconductor
    laboratory", Nucl. Instr. and Meth. A 549, 103, (2005).

[^95]: E. R. Fossum, "CMOS Image Sensors: Electronic Camera-On-A-Chip",
    IEEE Trans. On Electron Devices, 44, 1689 (1977).

[^96]: http://ireswww.in2p3.fr/ires/recherche/capteurs/

[^97]: R. Turchetta et al., *Nucl. Inst.* and *Meth*. A458, 677 (2001).

[^98]: M. Winter et al., *Proc. of the 8th ICATPP, Como Italy* (2003).

[^99]: MIMOSA: Standing for Minimum Ionizing particle MOS Active pixel
    sensor.

[^100]: M. Devaux et al., *Nucl. Inst.* and *Meth*. A512, 71 (2003).

[^101]: 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
    Projections (FY2006-FY2008). July 19, 2005.
    http:///www.ags**rhic**home.bnl.gov/AP/RHIC2004/RhicProjections.pdf

[^102]: Technical report,
    [http://www-lepsi.in2p3.fr/*\~*huch/MimoStar181203.pdf](http://www-lepsi.in2p3.fr/~huch/MimoStar181203.pdf)

[^103]: W. Dulinski et al., *Proc. IEEE Nuclear Science Symposium,
    Portland* OR. Trans. Nucl. Sci. (2003).

[^104]: http://www.lbnl.leog.org/thinned_mimosa_testing.htm

[^105]: Technical report,
    <http://ireswww.in2p3.fr/ires/recherche/capteurs/index.html>

[^106]: H. S. Matis, F. Bieser, S. Kleinfelder, G. Rai, F. Retiere, H.G.
    Ritter, K. Singh, S.E. Wurzel, H.H. Wieman, and E. Yamamoto, IEEE
    Trans. on Nucl. Sci. 50, 1020 (2003).

[^107]: S. Kleinfelder, H. Bichsel, F. Bieser, H. S. Matis, G. Rai, F.
    Retiere, H. Weiman, and E. Yamamoto, Proceedings of the SPIE, Hard
    X-Ray and Gamma Ray Detector Physics IV, July 2002, Vol. 4784, pp
    208.

[^108]: H.S. Matis, F. Bieser, Y. Chen, R. Gareus, S. Kleinfelder, M.
    Oldenburg, F. Retiere, H.G. Ritter, H.H. Wieman, S.E. Wurzel, E
    Yamamoto, Nucl. Instr. And Meth. A549, 130 (2005).

[^109]: S. Kleinfelder, S. Li, F. Bieser, R. Gareus, L. Greiner, J.
    King, J. Levesque, H.S. Matis^∗^, M. Oldenburg,, H.G. Ritter, F.
    Retiere, A. Rose, K. Schweda, A. Shabetai, E. Sichtermann, J.H.
    Thomas, H.H. Wieman, H. Bichsel, LBNL 59121. Submitted to the
    Proceedings of Pixel 2005, Bonn, Germany (2005).

[^110]: S. Kleinfelder, F. Bieser, Y. Chen, R. Gareus, H.S. Matis, M.
    Oldenburg, F. Retiere, H.G. Ritter, H.H. Wieman, E. Yamamoto, IEEE
    Trans. Nucl. Sci. 51, 2328 (2004).

[^111]: N.H. Xuong, A.-C. Milazzo, P. LeBlanc, F. Duttweiler, J. Bouwer,
    S. Peltier, M. Ellisman, P. Denes, F. Bieser, H.S. Matis, H. Wieman,
    S. Kleinfelder, In "Sensors and Camera Systems for Scientific,
    Industrial, and Digital Photography Applications V"SPIE 5301 242,
    (2004).

[^112]: The time that a pixel is sensitive to tracks, this determines
    amount of pileup.

[^113]: Efficiency after cluster filter on binary threshold information
    from the detector

[^114]: 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.

[^115]: 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

[^116]: 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.

[^117]: 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
    micro seconds following the collision.

[^118]: http://www-rnc.lbl.gov/\~wieman/ADCrange.htm

[^119]: Technical report,
    [http://www-rnc.lbl.gov/*\~*wieman/RadiationLengthsProposal.htm](http://www-rnc.lbl.gov/~wieman/RadiationLengthsProposal.htm)

[^120]: [[http://www-rnc.lbl.gov/\~wieman/radiationdoseinapsDulinskiValue.htm]{.underline}](http://www-rnc.lbl.gov/~wieman/radiationdoseinapsDulinskiValue.htm)

[^121]: http://doc.cern.ch/yellowrep/2001/2001-006/p1.pdf

[^122]: <http://www.lbnl.leog.org/writeup.htm>

[^123]: Technical report,
    [http://www-rnc.lbl.gov/*\~*wieman/StiffnessLadder1.htm](http://www-rnc.lbl.gov/~wieman/StiffnessLadder1.htm)

[^124]: TV Holography System at LBNL, courtesy of Eric Andersen and the
    ATLAS project.

[^125]: Technical report,
    <http://www.lionprecision.com/modular/probes.html>

[^126]: Technical report, <http://www.lbnl.leog.org/writeup.htm>

[^127]: Technical report, [http://www.lbnl.leog.org/ir prelim
    writeup.htm](http://www.lbnl.leog.org/ir prelim writeup.htm)

[^128]: Technical report, [http://www.lbnl.leog.org/cable
    constraints.htm](http://www.lbnl.leog.org/cable constraints.htm)

[^129]: Technical report,
    <http://www.lbnl.leog.org/crossing_lines_measurement_first_look.htm>

[^130]: Steve Peggs and Steve Tepikian, Private Communication.

[^131]: S. Tepikian,. Twiss functions and beam sizes of the RHIC
    intersection. *RHIC/AP25* (1994).

[^132]: Technical report,
    [http://www-rnc.lbl.gov//*\~*wieman/BeamPipeRadiusLimitsRHICAP25.htm](http://www-rnc.lbl.gov/~wieman/BeamPipeRadiusLimitsRHICAP25.htm)

[^133]: Technical report,
    http://www-rnc.lbl.gov/\~wieman/WolframFisherApertureNote.htm

[^134]: Technical report,
    [http://www-rnc.lbl.gov/*\~*wieman/BeamPipeVacuumShortAns.htm](http://www-rnc.lbl.gov/~wieman/BeamPipeVacuumShortAns.htm)

[^135]: John O'Hanlon, *A User's Guide to Vacuum Technology*. John Wiley
    and Sons, second edition (1989).

[^136]: Seth Wieman, Private Communication.

[^137]: Technical report,
    <http://www-rnc.lbl.gov/~wieman/TopThinPipeAnalysis.htm>

[^138]: Sandro Bravar, Private Communication.

[^139]: Dan Weiss, Private Communication 30-Nov-2004.