**A Heavy Flavor Tracker for STAR**

Contents

[1 Executive Summary 8](#executive-summary)

[1.1 Scientific Motivation 8](#scientific-motivation)

[1.2 Detector Concept 10](#detector-concept)

[1.3 Cost and Schedule 10](#cost-and-schedule)

[2 The Physics of the HFT 11](#__RefHeading___Toc94556037)

[2.1 Introduction 11](#introduction)

[2.2 Initial Heavy Quark Production 12](#initial-heavy-quark-production)

[2.3 Probing medium density: Heavy Quark Energy Loss
15](#probing-medium-density-heavy-quark-energy-loss)

[2.4 Probing medium thermalization: charm quark re-interaction
17](#probing-medium-thermalization-charm-quark-re-interaction)

[2.4.1 Transverse Momentum Distributions
18](#transverse-momentum-distributions)

[2.4.2 Elliptic Flow 20](#elliptic-flow)

[2.4.3 Charm Hadro-Chemistry 22](#charm-hadro-chemistry)

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

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

[3.1 Overview of the Experiment 28](#overview-of-the-experiment)

[3.2 Design of the HFT 29](#design-of-the-hft)

[3.3 Simulated HFT Performance 30](#simulated-hft-performance)

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

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

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

[3.4.2 Calculated background from Ultra Peripheral Collisions
41](#calculated-background-from-ultra-peripheral-collisions)

[4 CMOS Sensors 43](#cmos-sensors)

[4.1 Introduction 43](#introduction-1)

[4.2 Main Features and Established Performance
43](#main-features-and-established-performance)

[4.3 MIMOSTAR-1 Sensor Design 46](#mimostar-1-sensor-design)

[4.3.1 Thinning 47](#thinning)

[4.4 Additional R&D 48](#additional-rd)

[5 Data Acquisition 51](#data-acquisition)

[5.1 Requirements 51](#requirements)

[5.2 Architecture 52](#architecture)

[5.2.1 Readout Chip 53](#readout-chip)

[5.3 Data Synchronization 55](#data-synchronization)

[5.4 Alternate Read-out Strategy 56](#alternate-read-out-strategy)

[6 Integration with the Experiment 57](#integration-with-the-experiment)

[6.1 Mechanical Introduction 57](#mechanical-introduction)

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

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

[6.3 Ladder Design and Fabrication 62](#ladder-design-and-fabrication)

[6.3.1 Detector Radiation Length 64](#detector-radiation-length)

[6.4 Ladder Mechanical Tests 65](#ladder-mechanical-tests)

[6.4.1 Load Distortion Tests 65](#load-distortion-tests)

[6.4.2 Thermal Distortion Tests 67](#thermal-distortion-tests)

[6.4.3 Cooling Measurements 67](#cooling-measurements)

[6.5 Cabling 68](#cabling)

[6.6 Beam Pipe 69](#beam-pipe)

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

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

[6.6.3 Supporting Section 72](#supporting-section)

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

[6.6.5 RF Background from the Wake Field
72](#__RefHeading___Toc94556082)

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

[6.6.7 Bake Out 73](#bake-out)

[6.7 Compatibility with the SVT and FTPCs
73](#__RefHeading___Toc94556085)

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

[7 Cost, Schedule, Manpower and Funding 75](#cost-schedule-and-manpower)

[7.1 Cost and Schedule - 01/25/2005 75](#cost-and-schedule---01252005)

[7.2 Sensor 77](#section-1)

[7.2.1 Estimated Costs 78](#__RefHeading___Toc94556090)

[7.2.2 Manpower 78](#__RefHeading___Toc94556091)

[7.3 DAQ 78](#__RefHeading___Toc94556092)

[7.4 Trigger 79](#__RefHeading___Toc94556093)

[7.5 Mechanical Systems 79](#__RefHeading___Toc94556094)

[8 Summary 80](#summary)

[9 Appendix I -- MIMOSTAR Specifications
81](#appendix-i-mimostar-specifications)

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

[11 Appendix III -- Inner Tracker Interface Specification
81](#appendix-iii-inner-tracker-interface-specification)

[12 Appendix IV -- SVT Interface Specification
81](#appendix-iv-svt-interface-specification)

[13 References 82](#references)

List of Figures

[Figure 1: Contributions to charm production at RHIC energies. The
distributions were calculated with an energy density of 3.2 GeV/fm^3^ at
the moment of thermal equilibration. The figure is from Ref. \[12\].
14](#__RefHeading___Toc94556101)

[Figure 2: cc production cross-sections per nucleon-nucleon collision
vs. the collision energy. The dashed line depicts a PYTHIA calculation.
The solid and dot-dashed lines depict two NLO pQCD calculations with
MRST HO, m~c~ = 1.2 GeV/c^2^, µ~F~ = 2m~c~, µ~R~ = m~c~ and 2m~c~,
respectively. The figure is from Ref. \[\].
15](#__RefHeading___Toc94556102)

[Figure 3: The ratio of suppression factors in hot matter for charm (H)
and light (L) quarks. The solid line represent results from calculations
with unrestricted gluon radiation, while the dashed line is based on
calculations with a cut on gluon energies \> 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. \[23\]. 16](#__RefHeading___Toc94556103)

[Figure 4: Separation of high-p~T~ B-meson decay from combinatorial
background. The arrow indicates a cut at 250 m optimizing the signal to
background ratio. 17](#__RefHeading___Toc94556104)

[Figure 5: B and D meson spectra at RHIC. The data points are from Ref.
\[\] showing 0-10% central Au + Au collisions at √S~NN~ = 130 GeV at
RHIC. 20](#__RefHeading___Toc94556105)

[Figure 6: Elliptic flow of strange hadrons at RHIC. The top panel
demonstrates mass ordering, the bottom panel shows constituent quark
scaling. 21](#__RefHeading___Toc94556106)

[Figure 7: A hint of charmed 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. 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 \[ref.1\]. Dashed-line represents the
results from Greco et al. \[ref.2\] . In this coalescence calculation,
similar v~2~ distributions for c- and u-quarks are assumed. The figure
is adopted from Ref. \[\]. 22](#__RefHeading___Toc94556107)

[Figure 8: Left top: TPC dE/dx vs p in d+Au collisions. Left bottom: TPC
dE/dx vs p after TOFr PID selection of \| 1- \| \< 0.03. Clean electron
identification is achieved. Right: dE/dx from TPC after TOFr PID
selection (left bottom panel) for 1.0 \< p \< 1.5 GeV/c.
26](#__RefHeading___Toc94556108)

[Figure 9: Electron p~T~ spectra from conversions reconstructed by
requiring TPC tracking, a TPC hit and 2 SVT hits, or a TPC hit and 2 SVT
hits and 2 HFT hits. The rejection factor with an additional SVT hit (2
hits) is 3 and with an additional pixel hit is 50.
27](#__RefHeading___Toc94556109)

[Figure 10: 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. 28](#__RefHeading___Toc94556110)

[Figure 11: 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. 30](#__RefHeading___Toc94556111)

[Figure 12: The efficiency for finding tracks in central Au+Au
collisions in the STAR TPC, SSD, SVT, and the HFT. In this simulation,
the beam luminosity is assumed to be four times the RHIC design
luminosity. 31](#__RefHeading___Toc94556112)

[Figure 13: 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. 32](#__RefHeading___Toc94556113)

[Figure 14: Probability of getting the correct hit on the outer pixel
layer, given two different SVT geometries and a search window of 640 m
33](#__RefHeading___Toc94556114)

[Figure 15: 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. 35](#__RefHeading___Toc94556115)

[Figure 16: The absolute yield of D^0^s into the TPC+SVT+HFT divided by
the input D^0^ yield. The classical configuration includes all three
layers of the SVT. 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, the Alice geometry assumes thick Si detectors that are similar
to the wafers used in the ALICE vertex detector. For all three detector
configurations shown the D^0^ reconstruction cuts were optimized using
the program MINUIT. 36](#__RefHeading___Toc94556116)

[Figure 17: The number of central Au+Au events needed as a function of
the Si ladder thickness and the position resolution of the detector.
37](#__RefHeading___Toc94556117)

[Figure 18: Left: SVT-1 configuration without TOF. Right: SVT-1
configuration plus TOF. The estimated statistical uncertainties are
shown on the top line. 37](#__RefHeading___Toc94556118)

[Figure 19: We need a D~s~ figure, here.
38](#__RefHeading___Toc94556119)

[Figure 20: Flow results 39](#__RefHeading___Toc94556120)

[Figure 21: 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. 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
bulk silicon does not participate in the detection process.
44](#__RefHeading___Toc94556121)

[Figure 22: Wafer of reticle size sensors (left) and zoomed-in view of
individual chips (right). 45](#__RefHeading___Toc94556122)

[Figure 23: DAQ Layout: schematic of DAQ system. Depending on effective
data rate, multiple ladders can be combined to one optical link. Using
PCI and gnu/Linux, allows flexible and transparent access to complex
test setups while minimizing changes from prototype to the final
detector. 53](#__RefHeading___Toc94556123)

[Figure 24: ROC block diagram: preliminary schematic/floor-plan of
readout chip. Internal design is subject to recent development.
54](#__RefHeading___Toc94556124)

[Figure 25: Readout Layout: schematic sketch of the readout-topology on
a detector ladder. This figure shows one of ten APS and the
corresponding readout chip layout. You can likewise add more APS chips.
(Note: this plot does not aim to be a complete view of the interfaces.
The readout chips will be mounted piggy-back on the APS.)
55](#__RefHeading___Toc94556125)

[Figure 26: Data flow and processing stages in the readout chip. Each
stage can be bypassed to allow raw/unprocessed data to be routed through
the DAQ. 56](#__RefHeading___Toc94556126)

[Figure 27: The HFT is shown integrated with the STAR inner detectors
cone assembly. 58](#__RefHeading___Toc94556127)

[Figure 28: Close up view of the HFT ladders.
59](#__RefHeading___Toc94556128)

[Figure 29: Detector support structure with kinematic mounts to insure
repeatable detector positioning. 60](#__RefHeading___Toc94556129)

[Figure 30: Kinematic mount structure. 61](#__RefHeading___Toc94556130)

[Figure 31: Precision tie points coupling the HFT system to the STAR
support cone 61](#__RefHeading___Toc94556131)

[Figure 32: Ladder assembly. The current ladder is composed of a row of
10 thinned APS detector 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. 63](#__RefHeading___Toc94556132)

[Figure 33: Close up view of the ladder showing APS detector chips
mounted on top of the readout chip. The chips are mounted and wire
bonded on a three layer aluminum kapton cable.
64](#__RefHeading___Toc94556133)

[Figure 34: A cross-section of the prototype detector ladder showing its
structure and materials composition. 65](#__RefHeading___Toc94556134)

[Figure 35: The prototype ladder on the testing apparatus.
66](#__RefHeading___Toc94556135)

[Figure 36: 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 gm weight placed on the other.
66](#__RefHeading___Toc94556136)

[Figure 37: TV Holography system viewing test ladder in a small
transparent wind tunnel. Inset shows diffraction pattern with color map
of the surface displacement. 67](#__RefHeading___Toc94556137)

[Figure 38: A prototype readout cable for the HFT.
69](#__RefHeading___Toc94556138)

[Figure 39: Mechanical Prototype with 4 MIMOSA5 detectors glued to the
kapton cable assembly. 69](#__RefHeading___Toc94556139)

[Figure 40: Beam pipe structure with outer support barrel in the center.
70](#__RefHeading___Toc94556140)

[Figure 41: View of the beampipe mid-section.
71](#__RefHeading___Toc94556141)

[Figure 42: End view showing the HFT ladders between spokes to the inner
beam pipe support. 73](#__RefHeading___Toc94556142)

[Figure 43: Estimated time line for major tasks in the building of the
HFT detector. 75](#__RefHeading___Toc94556143)

[Figure 44: Estimated cost projection for the HFT project showing
accumulated cost during the time of design and construction. The final
total is 4.1 M\$. 76](#__RefHeading___Toc94556144)

List of Tables

[Table 1: Charm quark fragmentation functions. The left column is from
Ref. \[49, 50\]. The right column is from Ref.\[48\]. The D^+^ and D^0^
yields include feed-down from D^\*+^ and D^\*0^ decays.
24](#__RefHeading___Toc94556145)

[Table 2: Number of central Au+Au events required to observe a 3- signal
for and in their leptonic decay channels under different detector
configurations. 27](#__RefHeading___Toc94556146)

[Table 3: Selected HFT parameters. 30](#__RefHeading___Toc94556147)

[Table 4: Luminosity and other parameters that determine the particle
flux on the HFT. 40](#__RefHeading___Toc94556148)

[Table 5: Integrated hit loading on the HFT and associated pileup.
40](#__RefHeading___Toc94556149)

[Table 6: Hit loading on the HFT from *Au+Au* central collisions and
associated pileup. 41](#__RefHeading___Toc94556150)

[Table 7: Comparison between MIMOSA-5 characteristics and HFT
specifications. 46](#__RefHeading___Toc94556151)

[Table 8: Requirement Summary: constraints for the APS, due to signal
expectation, available design process, and mechanics
51](#__RefHeading___Toc94556152)

[Table 9: Materials in the beam pipe and the first detector layer with
their radiation length fractions. For details see Ref. \[\]
65](#__RefHeading___Toc94556153)

[Table 10: Estimated manpower requirements for the HFT project
77](#__RefHeading___Toc94556154)

[Table 11: Tentative sensor design schedule.
77](#__RefHeading___Toc94556155)

[Table 12: Tentative chip production cost estimate.
78](#__RefHeading___Toc94556156)

[Table 13: Tentative sensor design schedule.
79](#__RefHeading___Toc94556157)

#   {#section .unnumbered}

# Executive Summary

The proposed HFT detector is an integral part of the STAR Inner Tracking
Upgrade. This upgrade will bring new physics capabilities to STAR and
will ensure that STAR will be able to take data at the highest
luminosity attainable throughout the proposed RHIC II phase. The
tracking upgrade consists of three projects. Each of the projects has
its own, independent physics justification and natural time scale. The
three projects are:

-   The Heavy Flavor Tracker is the subject of this proposal. It will
    bring full D-meson capability to STAR with the goal to be at least
    partially operational for the next full energy Au+Au run.

-   The Silicon Barrel detector will replace the SVT in the high
    luminosity era and will serve as a pointing detector for the HFT.

-   The Forward Tracker will enable W measurements for the 500 GeV
    polarized proton run.

## Scientific Motivation

We propose to construct a Heavy Flavor Tracker (HFT) for the STAR
experiment at RHIC. The HFT will significantly enhance the physics
capabilities of the STAR detector at central rapidities. The primary
motivation for the HFT is to provide precise measurements of heavy quark
production in *p+p*, *d+Au* and *Au+Au* collisions, including polarized
*p+p*. These are key measurements 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 addressed by the HFT are as follows:

A precise measurement of the spectra of several 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. The
total production cross section for charm quarks is also essential to
know as a baseline for J/ measurements. A meaningful answer to the
question of whether the J/ are suppressed or enhanced at RHIC and to the
question if we observe Debye screening of the *cc* pairs will need
precise 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
pT 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, acquire elliptic flow then it
has to come 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 thus as an indication of thermalization reached at
the early stages of heavy ion collisions at RHIC. Proof of
thermalization requires further measurement like thermal photons and low
mass di-lepton spectra. Proof of thermalization constitutes the last
missing step towards establishment of the QGP.

The D meson transverse momentum spectra will also be affected by
re-scattering with light quarks, as well as re-scattering at the
hadronic level, mainly pion-D interactions. The effect of re-scattering
will be probed by comparing transverse mass spectra measured in d-Au and
Au+Au collisions.

The pixel 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 as a probe to study the properties of
the medium created in heavy ion collisions. The production of gluons is
kinematically suppressed for heavy flavors (dead cone effect) and as a
consequence heavy flavors should loose less energy in the dense medium.

The pixel detector offers the possibility of measuring the low mass
e^+^e^-^ spectrum down to the vector meson region by removing the gamma
conversion background, while electrons and positrons are identified in
the TPC and TOF. Vector mesons reconstructed in the di-lepton channel
may have decayed at any stage of the systems evolution, since leptons
are very unlikely to re-interact. Thus, studying 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 by the systematic uncertainties
resulting from large background subtractions. Furthermore, these methods
require large amounts of data to overcome the 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 microns,
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 has two tracking layers composed of monolithic
CMOS pixel detectors with 30 m 30 m square pixels. These critical
innermost tracking layers at \~1.5 cm radius and \~4.5 cm radius will
provide our best identification of tracks that are displaced a few 10s
of microns from the primary vertex. The detector chips will be thinned
to 50 m and mounted on low mass carbon fiber structures to minimize
pointing errors generated by multiple coulomb scattering. The two layer
barrel of detectors is active over 20 cm in z and has \~100 Million
pixels.

## Cost and Schedule

The projected cost of this detector, less contributed labor, is 4.1 M\$
and will be constructed and installed over a two year period (2006 and
2007) in time for the next full energy Au + Au running at RHIC.

[]{#__RefHeading___Toc94556037 .anchor}The Physics of the HFT

## Introduction

The goal of high-energy nuclear collisions is to understand Quantum
Chromodynamics (QCD) at extreme temperatures and energy densities over
large volumes. Under these extraordinary conditions, we believe that the
fundamental entities and 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 may 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, 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
jets. The measurement of large elliptic flow[^2] and the observation of
strong modifications of high *p~T~* particle production[^3]^,^[^4] have
provided evidence for a high density and strongly interacting state of
matter at RHIC. The goal of our research is to elucidate the nature of
this matter, in particular whether it is dominated by hadronic or
partonic degrees of freedom.

Rare processes provide additional ways to probe the medium generated in
high-energy nuclear collisions. Bjorken[^5] 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)
could generate significant energy loss effects[^6]^,^[^7]. Hard partons
are indirectly observable in elementary particle collisions (*e^+^+e*,
*p*+*p*) due to their fragmentation into collimated sprays of energetic
hadrons (jets) at large transverse angle to the beam. Jets can also be
identified in high energy nuclear collisions on a statistical basis; the
modification of their properties could signal novel 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 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,
the EOS will be driven by the degrees of freedom relevant for the basic
constituents which make up the matter. By studying flow it is possible
to determine key information about the nature of the matter and when it
is produced.

Heavy quark production (*c,b*) provides 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 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 amount of interactions and
therefore the degree of thermalization among constituents before
hadronization. At high *p*~T~, heavy quarks may be less suppressed than
light quarks due to the \"dead cone\" effect, giving an additional way
to study partonic energy loss. If charmed quarks participate
sufficiently in re-scatterings, they will develop flow, i.e. transverse
radial and elliptic flow which can be observed in charmed hadrons
momentum distributions. Finally, charmed quarks might reach thermal
kinetic equilibrium in the surrounding medium leading to statistical
hadronization. In that case, relative abundances of charmed hadrons are
significantly modified.

## Initial Heavy Quark Production 

Heavy quarks are produced through gluon fusion and *qq*
annihilation[^8]. Heavy quark production is sensitive to the parton
distribution function and the mass of the heavy quark. Therefore, the
production of heavy quarks in nuclear collisions should scale with the
number of binary nucleon-nucleon collisions. For heavy-quarks, their
masses are not modified by the surrounding QCD medium[^9] and the values
of their masses are much higher than the initial excitation of the
system, they constituentconstitute ideal probes to study the properties
of the hot-dense medium created in high-energy nuclear collisions.

Understanding the charm hadron yield in hadron-hadron collisions
requires a knowledge of the projectile and target parton distribution
functions, the cross sections for gluon fusion ( *gg* *cc* ), the
cross-section for quark anti-quark annihilation ( *qq* *cc* ), and the
fragmentation functions for *c* (*b*) quarks into charm (bottom)
hadrons. These cross-sections are usually calculated in a perturbative
QCD framework up to next-to leading-order (NLO)[^10]. The parton
distributions within the proton are extracted by parameterizing the data
from electron-proton collisions. We note that the parton and gluon
distribution functions within the nucleus, relevant to the charm and
bottom quark production at RHIC energies, are poorly understood[^11].
Thus, precise measurements of the charm cross-section in *p+p*, *d+Au*
and *Au+Au* collisions will provide a handle on the gluon distribution
function within the proton or in a gold nucleus.

Pertubative QCD predictions for the cross section (*cc* ) and (*b*) in
*p+p* collisions at √S~NN~ = 200 GeV may be found in the
literatureError: Reference source not found.

+-----------------------------------------------------------------------+
| (*cc* ) = 289 - 445 µb                                                |
|                                                                       |
| 8 - 13 *cc* pairs per central *Au+Au* collision at √S~NN~ = 200 GeV   |
+-----------------------------------------------------------------------+
| (*b*) = 1.64 - 2.16 µb                                                |
|                                                                       |
| 0.04 - 0.06 *b* pairs per central *Au+Au* collision at √S~NN~ = 200   |
| GeV                                                                   |
+-----------------------------------------------------------------------+

The uncertainty arises from a reasonable variation of quark masses
(*m~c~* = 1.2-1.4 GeV, *m~b~* = 4.5-5.0 GeV), factorization scale (/m =
0.5, 1, 2), and parton distribution functions (MRST, CTEQ, GRV). The
number of pairs per central *Au+Au* collision was obtained by
multiplication with the nuclear overlap integral *T~AA~* (29.3/mb),
corresponding to \~ 1150 binary nucleon-nucleon scatterings per *Au+Au*
collision. The cross section values are normalized per participating
nucleon-pair, thus in the calculation of yield for central *Au+Au*
collisions, a factor of N~binary~ = 350 must be taken into account.

Final state interactions may enhance charm production relative to the
binary scaling of initial parton collisions and could lead to thermal
production. An analogous mechanism dominates strangeness production. The
heavy quark mass (*m~c~* ≈ 1.2 to 1.8 GeV, *m~b~* ≈ 4.5 to 5.0 GeV),
however, should greatly suppress this channel[^12], making heavy quark
production rates sensitive to the dynamics of the early partonic stage
of the collision. Figure 1 shows the calculated contributions to the
total charm production at *y* = 0 at √S~NN~ = 200 GeV, with an energy
density of 3.2 GeV/fm^3^ at the moment of thermal equilibration.

![](media/image1.jpeg){width="3.5277777777777777in"
height="3.9930555555555554in"}

[]{#__RefHeading___Toc94556101 .anchor}Figure 1: Contributions to charm
production at RHIC energies. The distributions were calculated with an
energy density of 3.2 GeV/fm^3^ at the moment of thermal equilibration.
The figure is from Ref. \[Error: Reference source not found\].

In Au-Au collisions, a large thermal enhancement with respect to the
expectations of pQCD is ruled out by the recent single electron spectra
measured by the PHENIX collaboration[^13]. On the other hand, the STAR
collaboration has reported an unexpectedly large cross-section for open
charm in d-Au collisions by doing a direct reconstruction of the open
charm hadrons[^14]. The data are shown in Figure 2. In general, single
electron spectra only provide an indirect measurement of the charmed
(bottom) hadron yield because the secondary decays of charm and bottom
decay products account for a significant fraction of the electron yield
at high momentum (above 2-3 GeV/c for charm hadrons and above 5 GeV/c
for bottom hadrons). Large systematic errors are associated with the
extraction of these charmed (bottom) quark yields because of the large
background subtraction required at low momentum and the assumptions that
need to be made on the charm (bottom) hadron composition and spectra.

In contrast, the HFT will directly measure the spectra of the various
charmed hadrons, including the D^+^, D, D^0^, D~s~^+^, and possibly
~c~^+^, thus overcoming these systematic problems.

![](media/image2.jpeg){width="4.997916666666667in"
height="3.535416666666667in"}

[]{#__RefHeading___Toc94556102 .anchor}Figure 2: cc production
cross-sections per nucleon-nucleon collision vs. the collision energy.
The dashed line depicts a PYTHIA calculation[^15]. The solid and
dot-dashed lines depict two NLO pQCD calculations with MRST HO, m~c~ =
1.2 GeV/c^2^, µ~F~ = 2m~c~, µ~R~ = m~c~ and 2m~c~, respectively[^16].
The figure is from Ref. \[[^17]\].

## Probing medium density: Heavy Quark 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 quenching[^18]^,^[^19]^,^[^20]^,^[^21]. This effect was predicted to
occur due to radiative energy loss of high energy partons that propagate
through a dense and strongly interacting medium[^22]. Heavy quarks are
also predicted to lose energy as they traverse the medium. The energy
loss, however, is predicted to be significantly less because of a
suppression of gluon radiation at angles \< *M~Q~ /E*, where *M~Q~* is
the heavy quark mass and *E* is the heavy quark's energy[^23]. This
kinematic effect is known as the "dead cone" effect. This 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 3 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. Above a transverse momentum
p~T~ \> 7.5 GeV/c this ratio is predicted to be about two 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 unity. This ratio is exponentially sensitive to the density of color
charges in the medium. Therefore, the measurement of nuclear
modification factors of open charmed mesons at large transverse momentum
is a promising tool to study and further characterize QCD matter at
RHIC.

![](media/image3.png){width="4.00625in" height="2.6805555555555554in"}

[]{#__RefHeading___Toc94556103 .anchor}Figure 3: The ratio of
suppression factors in hot matter for charm (H) and light (L) quarks.
The solid line represent results from calculations with unrestricted
gluon radiation, while the dashed line is based on calculations with a
cut on gluon energies \> 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. \[Error: Reference source not found\].

Measuring high transverse momentum charm and beauty quark production
requires a trigger to take full advantage of the large RHIC luminosity
because their production cross section is very small. A significant
fraction of B (\~10%) and D (6-17%) meson decays include an
electron/positron in the final state. The STAR electromagnetic
calorimeter allows triggering on high transverse momentum
electromagnetic showers, which include electrons and positrons. The only
source of high transverse momentum electrons and positrons are D and B
decay. Thus, the D and B transverse momentum distribution can be
inferred above a certain momentum from the electron and positron
spectra.

If an electromagnetic shower can be matched to a track reconstructed in
the TPC, it is tagged as an electron/positron, otherwise it is a photon.
Background to the reconstructed electron comes from charged pions going
through a charge-exchange process, i.e. becoming neutral pions, in the
calorimeter. The neutral pions decay immediately in two photons, which
produces an electromagnetic shower identical to an electron shower. The
charge-exchange cross-section is poorly known, which leads to large
uncertainties in the estimates for such processes. The only measurable
difference between an electron/positron from D and B decay and a high
transverse momentum pion producing an electromagnetic shower is their
spatial origin. High pT charged pions come from the primary vertex while
electrons and positrons come from the B and D decay vertex, displaced
from the primary vertex by typically a few hundred m.

The CDF collaboration has shown that B jet tagging can be achieved by
associating the electron/positron to one of the other charged hadrons
decay products[^24]. Figure 4 shows a feasibility study applying a
similar reconstructing technique to remove background using the HFT. The
B channel studied is
$B^{+} \rightarrow {\overline{D}}^{0} + e^{+} + \nu$ and
${\overline{D}}^{0} \rightarrow K^{+} + \pi^{-}$. The positron is
matched with either the kaon or pion from the D meson decay, if their
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 helices. The variable shown in figure 7 is the distance
between the B and the primary vertices. The sign is given by 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 central Au-Au collisions at
200 GeV below 3 GeV/c. The relative normalization between the background
and B decay distribution is arbitrary as the charge-exchange
cross-section is not well known. By applying a signed-decay-length cut
of more than  250m, the background is suppressed by a factor 200 while
keeping 60% of the signal. Thus, adding the decay vertex information
will help significantly remove the background in the high transverse
momentum electron/positron spectra and substantially increase
statistical significance for such a measurement.

![](media/image4.jpeg){width="4.759722222222222in"
height="3.3020833333333335in"}

[]{#__RefHeading___Toc94556104 .anchor}Figure 4: Separation of high-p~T~
B-meson decay from combinatorial background. The arrow indicates a cut
at 250 m optimizing the signal to background ratio.

## Probing medium thermalization: charm quark re-interaction

A measurement of transverse radial and elliptic flow of identified
hadrons consisting of light quark flavors (*q = u,d,s*) has been
performed at RHIC. The main conclusions from these studies are: the
system created in ultra-relativistic *Au+Au* collisions at RHIC exhibits
strong collective expansion. Compared to measurements at lower SPS
energies, the degree of collectivity is stronger. Also multi-strange
hadrons ( and ) appear to freeze-out at a higher temperature and lower
collective velocity than lighter hadrons. This has 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^,^[^25]^,^[^26]^,^[^27]^,^[^28]. The successful
measurement of partonic collectivity of light flavor hadrons is an
important step towards the discovery of a QGP. However, this is not
sufficient. It is also important to address the issue of thermalization
together with collectivity.

At RHIC energies, theoretical calculations indicate that thermalization
may be reached at temperatures T~eq~ 0.3-0.5 GeV[^29] and that the
duration of the equilibrium period is on the order of 5-10
fm/c[^30]^,^[^31]. Thermal production of *cc* pairs is therefore
suppressed (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
property of the medium providing the additional quarks necessary to form
the hadrons. The relative yield of various charmed hadrons is thus
sensitive to the collective properties (temperature and chemical
potentials) of the light quark medium.

The strength of the charm hadron flow is potentially an indicator of
thermalization occurring at the partonic level. If charm quarks flow
together with the light flavor quarks, there must be copious
interactions among light and charmed quarks. Therefore, thermalization
is likely be reached through parton re-scattering. In order to test the
question of thermalization experimentally, we propose to measure the
charmed hadron transverse radial (spectra) and elliptic flow (v~2~) with
the STAR HFT detector. The combination of the open charm spectra and
v~2~ will allow us to determine the collective properties of the charmed
quark and therefore the probable degree of thermalization of light
quarks connected with it.

### Transverse Momentum Distributions

The development of collectivity at the partonic level (among quarks and
gluons) in high-energy nuclear collisions and the degree of
thermalization are closely related to the underlying equation of state
of partonic matter. Theoretical arguments show that different aspects of
the measured final hadronic momentum distributions are determined at
different stages of the collision history. Hence, hadronic observables
and their corresponding parameterizations characterizing the freeze-out
distributions constrain the dynamical evolution and thus yield indirect
information about different stages of the collision. After
hadronization, inelastic interactions changing the particle species
cease and particle abundances are fixed: this is commonly referred to as
chemical freeze-out. Later, when elastic interactions cease, the
particle momentum spectra are fixed at kinetic freeze-out. For most
particles, transverse radial flow is accumulated throughout the whole
collision history while transverse elliptic flow is believed to saturate
earlier.

The separation of thermal motion and collective flow in the analysis of
soft-hadron spectra requires a thermal model analysis. Measured
transverse mass spectra can be fit with a hydrodynamically motivated
approach using a kinetic freeze-out temperature *T*~fo~ and a mean
collective flow velocity \<~r~\>. This type of analysis does not take
into account the abundances of particles and thus only tests the
*kinetic* thermal equilibrium and is not sensitive to chemical
equilibrium.

The spectra of low mass particles, like pions, are dominated by the
temperature parameter, while the spectra of heavier mass particles are
dominated by the flow velocity. Transverse radial flow modifies the
particle spectrum by depopulating the yield at low momentum. The result
is a characteristic shoulder in the low-momentum region, strongly
depending on particle mass. Hence, the measurement of particles to the
lowest possible momentum is essential for spectral analysis in the charm
sector. Also, good momentum coverage is necessary to reliably extract
integrated yields. This minimizes systematic uncertainties in
extrapolating yields especially at low momentum where theoretically the
particle spectrum is not well known.

Figure 5 shows the invariant yield, as a function of transverse
momentum, for *D* and *B* mesons, as predicted for p+p collisions (solid
lines) and assuming the existence of flow (dashed lines). Clearly, the
development of flow modifies the spectra when compared to p+p
collisions. The largest differences, about 30%, occur at momenta below 2
GeV/c. Therefore, the study of charmed flow requires precise
measurements of charmed hadron spectra at low momentum. The filled
squares represent the inclusive measurement of heavy flavor production
in the electron decay channel, *D* e+X. The systematic errors are large
at lower momentum due to the large experimental background from photonic
electron production. To reduce these large uncertainties, a direct
reconstruction of D   π + K is essential. This also minimizes the
ambiguities due to the decay kinematics in the electron channel and
helps disentangle contributions from *D* and *B* mesons. As one can see
in the figure, the difference seen in the D-meson spectra between
hydrodynamic behavior (solid-line) or pQCD behavior (dotted line) are
indistinguishable in the decayed-electron spectra. From measured
electron distributions alone, these differences are unlikely to be
disentangled because at the higher p~T~ region, decays from B-meson
become more and more important.

![](media/image5.png){width="4.520138888888889in" height="4.0in"}

[]{#__RefHeading___Toc94556105 .anchor}Figure 5: B and D meson spectra
at RHIC. The data points are from Ref. \[[^32]\] showing 0-10% central
Au + Au collisions at √S~NN~ = 130 GeV at RHIC.

### Elliptic Flow

The build-up of pressure and pressure gradients, in a nuclear collision,
leads to collectivity. In non-central collisions, the initial overlap
zone between the colliding nuclei is spatially deformed. If the matter
produced in the reaction zone re-scatters sufficiently, then this
spatial anisotropy is transferred into a momentum space anisotropy. The
development of anisotropies does not require thermalization, only
re-scattering. The largest momentum anisotropies are obtained in the
hydrodynamic limit[^33], where zero mean free path length and therefore
infinitely fast re-scattering is assumed; this leads to instantaneous
local thermal equilibrium.

The 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
an azimuthal Fourier decomposition of the distribution.

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 short
direction[^34]. As the spatial deformation disappears, the build-up of
flow due to pressure gradients ceases and the elliptic flow saturates.
Therefore, elliptic flow is considered to be a signal that develops in
the early stages of a collision. RHIC data[^35]^,^[^36]^,^[^37] show
that in semi-central Au+Au collisions the measured elliptic flow reaches
the hydrodynamic limit for transverse momenta up to 2 GeV/c, suggesting
early thermalization and pressure buildupError: Reference source not
found. Thus, information about the equation of state[^38] 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 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. These measurements demonstrate that collectivity is
established at the partonic level. The bottom plot of 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.

![](media/image6.png){width="2.85in" height="4.022222222222222in"}

[]{#__RefHeading___Toc94556106 .anchor}Figure 6: Elliptic flow of
strange hadrons at RHIC. The top panel demonstrates mass ordering, the
bottom panel shows constituent quark scaling.

At RHIC energies, charmed 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 a first indication of charm particle elliptic flow at
RHIC measured in the inclusive electron channel. The predictions assume
elliptic flow for the light quarks as determined by fits to experimental
data. The solid line results from elliptic flow of the charmed quarks,
whereas the dashed line represents zero elliptic flow for the charmed
quarks. Presently, the data support the idea that the heavy charmed
quarks flow. However, 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
charmed quarks.

![](media/image7.jpeg){width="6.002777777777778in"
height="3.748611111111111in"}

[]{#__RefHeading___Toc94556107 .anchor}Figure 7: A hint of charmed
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. 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 \[ref.1\]. Dashed-line represents the results from Greco et al.
\[ref.2\] . In this coalescence calculation, similar v~2~ distributions
for c- and u-quarks are assumed. The figure is adopted from Ref.
\[[^39]\].

### Charm Hadro-Chemistry

Measured hadronic yields and their ratios, from AGS to RHIC energies,
have been successfully described with statistical models[^40]^,^[^41].
The relevant parameters are the chemical freeze-out temperature *T*~ch~,
and the chemical potentials ~i~ for conserved quantum numbers, i.e. net
strangeness, charge, and baryon number conservation[^42]. The extracted
chemical freeze-out temperature *T*~ch~ turns out to be higher than the
kinetic freeze-out temperature *T*~fo~ extracted from momentum
spectra[^43].

It has been claimed, at the SPS and RHIC[^44], that for most particles
the chemical freeze-out temperature coincides with the critical
temperature *T*~c~ predicted from lattice QCD. Wide resonances might be
an exception[^45]. Obviously, this critical temperature *T*~c~ serves as
an upper limit for hadronic systems. Larger temperatures from partonic
systems might be observed when measuring thermal spectra of penetrating
electro-magnetic probes such as dileptons and photons.

Charmed quarks are dominantly produced in initial parton-parton
scatteringsError: Reference source not found and the thermal production
of charmed quarks is suppressed due to their large mass. In case of
sufficient re-scattering, initially produced charmed 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 charmed quarks is conserved.

These arguments lead to the idea of statistical hadronization of charmed
quarks[^46]^,^[^47]. Statistical coalescence scenarios predict large
changes in open (regarding relative abundances, e.g. the D^0^ /
D~s~*^+^* ratio) and hidden charm production with strong centrality
dependence[^48]. Statistical coalescence implies that charmed quarks
travel over significant distances, e.g. in a QGP. And so 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, charmed quarks may fragment
into a variety of hadrons as shown in the first column of Table 1. These
fragmentation ratios have recently been compiled by experiments H1[^49]
and ZEUS[^50] at HERA, using *e-p* and *e^+^-e^-^* collision data. These
ratios are found to be independent of the collision energies 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 will detect charmed
particles, even though the ratios calculated using PYTHIA[^51], shown in
the second column of Table 1, differ significantly from the measured
ratios. To avoid any uncertainties on 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          |                |               |
+-----------------+------------------+----------------+---------------+
| (c *D* ^+^ )    | 0.232            | 0.162          | 0.21          |
+-----------------+------------------+----------------+---------------+
| (c *D* ^0^ )    | 0.549            | 0.639          | 0.483         |
+-----------------+------------------+----------------+---------------+
| (c *D~s~^+\ ^*) | 0.101            | 0.125          | 0.182         |
+-----------------+------------------+----------------+---------------+
| (c ~c~^+^ )     | 0.076            | 0.066          | 0.080         |
+-----------------+------------------+----------------+---------------+
| (c J/ )         |                  | 0.006          | 0.057         |
+-----------------+------------------+----------------+---------------+

[]{#__RefHeading___Toc94556145 .anchor}Table 1: 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 1. This calculation assumes that charmed quarks statistically
coalesce with lighter quarks, i.e. according to the temperature and
chemical potential of the light-quark system. The temperature and
chemical potential are set so that the light hadron yields measured at
RHIC are reproduced. The number of charmed quarks present in the system
is set by pQCD calculations; this is reflected by a charm-chemical
potential in the statistical coalescence model. Statistical coalescence
increases the yield of the D~s~*^+^* meson by 80 % and the J/ 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/ / D^0^ are the most sensitive probes of thermal charm hadron
production.

Measuring the D~s~^+^/D^0^ and D~s~^+^/D^+^ ratios with the precision
required to separate direct and thermal production sets stringent
requirements on the HFT detector performance. Neglecting the errors on
the D^0^ yield, the D~s~*^+^* yield needs to be measured with a 2%
precision for separating the two scenarios. Such small statistical and
systematic errors are very hard to achieve. To separate both scenarios
using the D~s~^+^/D*^+^* ratio, the yields of both D*^+^* and D~s~*^+^*
need to be measured with a precision of about 7%. When comparing
D~s~*^+^* and D*^+^* yields, most of the systematic errors cancel out
because both are reconstructed in very similar decay channels: D*^+^* K
π^+^ π^+^ and D*~s~^+^* K π^+^ K^+^. If heavy quarks become embedded
within partonic matter, they provide a unique tool to study the
properties of such matter. Indeed, their yield can be accurately
extrapolated from p-p measurements. Any change in the charm and bottom
hadron behavior between proton-proton and heavy-ion collisions will
provide information about the matter these hadrons traverse. Thermal
kinetic equilibration and subsequent statistical coalescence of the
charmed quarks would be strong evidence for partonic matter formation
since the pion-charm hadron cross-section is small[^52]. Furthermore,
exotic phenomena such as the formation of charmed penta-quarks[^53] or
enhancement of the B~c~ meson yield[^54] may also take place. A precise
measurement of the total charm production (mostly carried by open charm
mesons) will also serve as a baseline for J/ enhancement/suppression
measurements suggested as a possible signature for QGP formation.

## Measuring Vector Mesons with Dileptons 

Besides the direct measurement of open charm hadrons, the HFT will serve
as a powerful device to discriminate background 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 via the electromagnetic force instead of the strong one.
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 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 at 0.2 \<
M~ee~ \< 0.6 GeV/c^\^2^ in semi-central Pb+Au collisions\[ref\]. 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 a
measurement of the energy lost by charged particles due to ionization of
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 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-\| \< 0.03, eliminates slow hadrons and cleans up
the crossovers. This results in clean electron identification, as shown
in Figure 8. The plot on the top left shows the conventional dE/dx
measurement in the TPC gas as a function of particle momentum. The lower
plot shows the same dE/dx measurement but now requiring at TOF of
\|1-\| \< 0.03. 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
8 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 Gaussian-shaped
distribution of electrons are clearly separated from the distribution of
charged pions at the lower energy losses.

![](media/image8.jpeg){width="2.964583333333333in"
height="2.7569444444444446in"}![](media/image9.jpeg){width="2.939583333333333in"
height="2.745833333333333in"}

[]{#__RefHeading___Toc94556108 .anchor}Figure 8: Left top: TPC dE/dx vs
p in d+Au collisions. Left bottom: TPC dE/dx vs p after TOFr PID
selection of \| 1- \| \< 0.03. Clean electron identification is
achieved. Right: dE/dx from TPC after TOFr PID selection (left bottom
panel) for 1.0 \< p \< 1.5 GeV/c.

These electrons are to a large extent from photons converting into
electron-positron pairs, e^+ ^e^-^ in the detector material. The HFT
detector will reduce the background electrons and positrons from these
conversions. By requiring hits in the HFT, electrons from photon
conversion outside the HFT, i.e. the SVT,SSD, TPC inner field cage are
rejected. Figure 9 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 SVT
(open triangles) and hits in the TPC+SVT+HFT (closed circles). The
requirement of hits in the HFT reduces the detected yield of conversion
electrons by a factor of 50. Another source of electron background stems
from the semi-leptonic decays of heavy quarks, charm and bottom. The
dominant source at intermediate mass[^55]^,^[^56] is from the
semi-leptonic decays of open charm. Due to the lifetime of \~100m of
heavy-flavor hadrons, this background can be largely reduced by
measuring the displaced decay vertex with high resolution as provided by
the HFT detector. Detailed simulations are in progress.

![](media/image10.jpeg){width="5.000694444444444in"
height="3.410416666666667in"}

[]{#__RefHeading___Toc94556109 .anchor}Figure 9: Electron p~T~ spectra
from conversions reconstructed by requiring TPC tracking, a TPC hit and
2 SVT hits, or a TPC hit and 2 SVT hits and 2 HFT hits. The rejection
factor with an additional SVT hit (2 hits) is 3 and with an additional
pixel hit is 50.

The large reduction in electron background will enable us to observe
electromagnetic decays of short-lived vector meson, e.g. , *e*^+^*e* and
intermediate mass dileptons with a few hundred thousand central Au+Au
events in STAR. The rejection of π^0^ and  Dalitz decays and can be
achieved by measuring the pair simultaneously due to the large
acceptance of the STAR TPC. Table 2 shows our estimate of central AuAu
collisions needed in order to observe a signal with a statistical
significance of 3. Compared to a measurement incorporating only TPC and
TOF, the HFT will enhance the sensitivity of electro-magnetic vector
meson decay by a factor 20-40. We are in the process of simulating the
effect of the combinatorial background in more detail.

  ------------------------------------------- --------------- ------------
  Detectors                                                   

  TPC+TOF                                     8M              2M

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

[]{#__RefHeading___Toc94556146 .anchor}Table 2: Number of central Au+Au
events required to observe a 3- signal for and in their leptonic decay
channels under different detector configurations.

# 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 10). 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 , cover ±1.5 units in
rapidity, and track particles from 100 MeV/*c* to 20 GeV/*c*. The Heavy
Flavor Tracker will extend these unique capabilities even further by
providing particle identification for hadrons containing charm and
beauty.

![](media/image11.png){width="5.659055118110237in"
height="3.6271653543307085in"}

[]{#__RefHeading___Toc94556110 .anchor}Figure 10: 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.

In the current STAR detector configuration, the important tracking
elements at mid-rapidity are the Silicon Vertex Tracker (SVT), the
Silicon Strip Detector (SSD) and the Time Projection Chamber (TPC). The
SVT surrounds the interaction vertex with Silicon drift detectors at
radii of approximately 7, 11, and 15 cm. The SSD is installed at a
radius of 23 cm while the TPC tracking begins at a radius of
approximately 60 cm. The TPC tracking system extends out to a radius of
190 cm and yields a total of 45 position measurements along the track.
In the future, a complete Time of Flight detector will surround the TPC
barrel ( = 2).

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 of 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 particles 'whosthe 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 x 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 SVT in the analysis, the vertex resolution is
approximately 2 mm which substantially improves the efficiency for
finding topologically complex strange decays such as the baryon.

The HFT improves upon these capabilities even further because it will be
placed between the beam pipe and the SVT 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 m  30 m pixels and the
pointing accuracy of the system (including multiple coulomb scattering
in the beam pipe and Si layers) will then be 25 m 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 m. 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 3 and the detector is
shown schematically in Figure 11. The inner layer of the detector lies
at a mean radius of 1.5 cm radius and it is composed of 6 detector
ladders. The outer layer lies at a mean radius of 5.0 cm and it is
composed of 18 ladders. Each ladder contains a row of 10 monolithic CMOS
detector chips and each ladder has an active area of 19.2 cm by 1.92 cm.
Each CMOS detector chip contains a 640 by 640 array of 30 m square
pixels. It is our goal to thin down each chip to a total thickness of 50
m in order to minimize the multiple coulomb scattering (MCS) between
layers of the detector. The beam pipe must also be made thinner (0.5 mm)
to reduce scatterihe in front of the HFT. The HFT is designed to be as
close as possible to the interaction point. This will require a new,
smaller radius beam pipe. This smaller, thinner, beam pipe is an
essential part of this proposal (see section 6.6 ).

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

  Pixel dimension                          30 m 30 m

  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~               0.36 %

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

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

[]{#__RefHeading___Toc94556147 .anchor}Table 3: Selected HFT parameters.

![](media/image12.jpeg){width="3.0770833333333334in"
height="2.6506944444444445in"}
![](media/image13.jpeg){width="2.647222222222222in"
height="2.6006944444444446in"}

[]{#__RefHeading___Toc94556111 .anchor}Figure 11: 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.

## Simulated HFT Performance

The performance of the HFT has been studied 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 Vertex Tracker
(SVT) 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 generated Monte Carlo hits by a
Gaussian function with a width of =6m in the x-y-plane and in the
z-direction. This accounts for 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 Sec. 3.4. Tracks with at least 15 hits
in the TPC, 1 hit in the SVT and 2 hits in the HFT detector were
reconstructed with the STAR inner-tracker code.. Figure 12 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+SVT by triangles (green), and the TPC+SVT+HFT detector by
filled dots (blue). Note that the TPC tracking efficiency limits the
efficiency of the SVT 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 monoenergetic pions show that the efficiency stays approximately
constant up to 20 GeV/c.

![](media/image14.jpeg){width="4.94375in" height="3.4965277777777777in"}

[]{#__RefHeading___Toc94556112 .anchor}Figure 12: The efficiency for
finding tracks in central Au+Au collisions in the STAR TPC, SSD, SVT,
and the HFT. In this simulation, the beam luminosity is assumed to be
four times the RHIC design luminosity.

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 msec.
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.

![](media/image15.jpeg){width="5.695138888888889in"
height="1.9597222222222221in"}

[]{#__RefHeading___Toc94556113 .anchor}Figure 13: 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 13 shows the number of ghost tracks observed in the full HFT
tracking system. In this simulation, the SSD+SVTedetector was assumed to
be the intermediate tracker but we removed the first layer of the SVT to
test the performance of the system in this configuration. (This
configuration is called "SVT-1" and is pronounced SVT minus one). 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 12; the tracking
efficiency for the HFT is steeply dropping below 0.5 GeV/*c* and
substantially lower compared to TPC+SSD+SVT. 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 13).

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/image16.wmf){width="1.8472222222222223in"
height="1.1909722222222223in"}

where σ is the uncertainty in the track extrapolation from the SVT to
the pixel 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 caluclations 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.

![](media/image17.wmf){width="2.9319444444444445in"
height="3.9972222222222222in"}

[]{#__RefHeading___Toc94556114 .anchor}Figure 14: Probability of getting
the correct hit on the outer pixel layer, given two different SVT
geometries and a search window of 640 m

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+SVT+HFT tracking
system and the results are given in the next section.

### 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^+^, D~s~^+^  K π^+^ K^+^.
These decays represent a crucial sample of the physics capabilities that
can be achieved with the HFT detector.

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 Boltzman
distribution which reproduce the \<*p~T~*\> of D-mesons as measured by
STAR in d+Au collisions at √S~NN~ = 200 GeV 15\[\] and the rapidity
distribution suggested by perturbative QCD calculations applying the
program code Pythia. 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 collision \[48\]. Also,
the higher track reconstruction efficiency in single (D0 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, see Figure 15.

-   The decay length *l*, i.e. 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 between the two daughter tracks

-   The cos() with 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

-   The cos(\*) with \* the angle between the kaon in the D-meson Center
    of Mass frame and the D-meson momentum

Table 4 list the values of the cuts applied for D-meson reconstruction
in the SVT-1 configuration.

\<We miss a table here...\>

Table 4. Cuts applied for D-meson reconstruction in the SVT-1
configuration

Charged decay daughters were identified by their specific energy loss in
the TPC gas; i.e. to identify pions, we used a 3 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 700MeV/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 exisiting STAR data with a small set of the
full ToF detector\[ref\]. 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/image18.jpeg){width="4.566666666666666in" height="3.7in"}

[]{#__RefHeading___Toc94556115 .anchor}Figure 15: 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. The resulting yield of D^0^s
divided by the input D^0^ yield, as a function of transverse momentum is
shown in Figure 16. 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 m
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. The best results are achieved with the full
SVT 3 layer configuration; removing the innermost SVT layer leads to
somewhat smaller but still acceptable numbers. The main difference
between the SVT and SVT-1 configuration is the resulting smaller single
track efficiency of the HFT. Replacing the thinned Si wafers of the HFT
by ones similar to those used in the ALICE detector (3\* thicker), cause
the D0 reconstruction efficiency to drop by a factor 8, independent of
momentum. This would require an 8 times longer data taking period.
Therefore, we exclude the ALICE detector configuration.

![](media/image19.jpeg){width="4.998611111111111in" height="3.39375in"}

[]{#__RefHeading___Toc94556116 .anchor}Figure 16: The absolute yield of
D^0^s into the TPC+SVT+HFT divided by the input D^0^ yield. The
classical configuration includes all three layers of the SVT. 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, the Alice geometry
assumes thick Si detectors that are similar to the wafers used in the
ALICE vertex detector. For all three detector configurations shown the
D^0^ reconstruction cuts were optimized using the program MINUIT.

We 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 3. Figure 17 shows the number of central Au+Au
collisions needed as a function of the HFT hit resolution for different
Si ladder thicknesses. The curve is almost flat between 0 and 10 m 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 m and a total ladder thickness of 240 m
Si-equivalent (0.3% X~0~). We present projected statistical errors on
D-meson spectra and elliptic flow.

![](media/image20.png){width="3.0in" height="2.4006944444444445in"}

[]{#__RefHeading___Toc94556117 .anchor}Figure 17: The number of central
Au+Au events needed as a function of the Si ladder thickness and the
position resolution of the detector.

For the first year of data taking with the HFT, we assume a total of 50M
central Au-Au collisions at 200 GeV. (A similar number of events were
taken by STAR during the RHIC Run IV in 2004.) Figure 18 shows the
anticipated D^0^ spectrum from pQCD calculations (no flow) and a Blast
wave prediction (0.4 ~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. Usage of ALICE-type Si-ladders for
the HFT would require far more than 100M central Au-Au collisions which
is not feasible in one year of data taking.

![](media/image21.png){width="2.9694444444444446in"
height="2.0006944444444446in"}
![](media/image22.png){width="2.928472222222222in"
height="2.0006944444444446in"}

[]{#__RefHeading___Toc94556118 .anchor}Figure 18: Left: SVT-1
configuration without TOF. Right: SVT-1 configuration plus TOF. The
estimated statistical uncertainties are shown on the top line.

We performed a similar study for the D~s~ meson. Here the goal was to
measure the D~s~ yield and extrapolate over all p~T~ to extract the
dN/dy for hadro-chemistry studies. Figure 19 shows the estimated
statistical uncertainties of D~s~ measurement as a function of p~T~. Our
measurement will cover the momentum range 0-3 GeV/c which contains 95%
of the total D~s~ yield. Therefore, the extracted dN/dy will not rely
greatly on extrapolation and this minimizes systematic uncertainties.
The statistical errors in each p~T~ bin are larger than for the D~0~
decay since the D~s~ decays to 3 daughters with a much smaller decay
length of \~100 m. The estimated uncertainty in dN/dy (summed over all
p~T~ bins) is +-xx%. This is sufficient for probing the charmed quark
coalescence scenarios predicting an increase of the D~s~/D^0^ yield by a
factor of two.

[]{#__RefHeading___Toc94556119 .anchor}Figure 19: We need a D~s~ figure,
here.

Finally we study the performance of our D^0^ elliptic flow measurement.
This measurement is the clearest probe of D-meson (and therefore charmed
quark) flow. However, it requires a measurement of the D^0^ spectrum at
different angles with respect to the reaction plane and is therefore the
most challenging measurement of all. Figure 20 shows the estimated
statistical uncertainties in the D^0^ elliptic flow measurement on the
top line as a function of transverse momentum. We assume one year of
data taking. The uncertainties are largest at low momentum due to the
rather small D^0^ reconstruction efficiency, they reach a minimum around
2 GeV/c, and then increase due to the exponentially falling D^0^ yield
at larger momentum. The solid lines show the expected D^0^ azimuthal
anisotropy parameter v~2~ as a function of p~T~. The upper line is the
case of full charm quark flow (upper line) and for non charm-quark flow.
It is evident that after a year of data taking that the charmed quark
flow measurement can be fully addressed.

![](media/image23.jpeg){width="5.001388888888889in"
height="3.4791666666666665in"}

[]{#__RefHeading___Toc94556120 .anchor}Figure 20: Flow results

## 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 areal
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 4 and an estimate of the hit density is shown in Table 5.

  --------------------------------------------- -------------------------
  *Au+Au* Luminosity                            1x10^27^ / cm^2^ sec

  dN/dη (min bias)                              170

  Min bias cross section                        10 barns

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

[]{#__RefHeading___Toc94556148 .anchor}Table 4: Luminosity and other
parameters that determine the particle flux on the HFT.

As shown in Table 5, 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[^57]. 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 0.6%, 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
(110^27^/cm^2^/sec). This is 5 times the nominal "design luminosity".
(Design luminosity is defined to be 210^26^/cm^2^/sec averaged over a 10
hour fill. An instantaneous luminosity of 210^26^/cm^2^/sec 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 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%.

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

  Radius                                4.5 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___Toc94556149 .anchor}Table 5: Integrated hit loading
on the HFT and associated pileup.

**Table 6** 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                                 4.5 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___Toc94556150 .anchor}Table 6: 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/d = 700, so we see no evidence of additional
background beyond the pileup of normal Au+Au collisions[^58]. 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 STAR TPC that at larger
radii, 65 cm to 200 cm, the ionization density due to normal *Au+Au*
interactions is significantly larger than other contributions such as
beam gas[^59].

### Calculated background from Ultra Peripheral Collisions

The following addresses a question about a low momentum source of
background that is not visible in the SVT but might be an issue at a
smaller radius in the vicinity of the HFT. In ultra-peripheral *Au+Au*
collisions (UPCs), both nuclei interact only via the long range
electromagnetic force. Due to the large atomic number (Z = 79) the cross
section for these interactions is rather large. We examined the cross
section from UPCs generating *e*^+^*e* pairs which create hits in the
HFT. The total cross section of electron pairs created in UPC was
calculated in first order QED and determined to be 53 kbarn. These
electrons appear at low transverse momentum below 200 MeV/*c* with an
exponential spectral shape. The innermost ladder lies at a radius of 1.5
cm which requires electrons to have at least 22 MeV in the full STAR
magnetic field. Electron pairs were fed through GEANT with a realistic
vertex distribution of = 65 cm in width for UPC events. From these
simulations, the cross section for creating a single hit in the HFT is
estimated to be 17 barns. The spiraling of low momentum electrons in the
magnetic field may cause multiple hits and contributes about 12 % to the
estimated cross section. The cross section for hadronic interactions is
8 barns. Compared to the huge number of charged particles (dN/d = 700)
created in a central collision, we conclude that the contribution from
electrons created in ultra-peripheral *Au+Au* collisions is negligible.

# CMOS Sensors

## Introduction

The 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
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.

## Main Features and Established Performance

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[^60]. (See Figure
21.) 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 m thick.

[]{#__RefHeading___Toc94556121 .anchor}Figure 21: 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. 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 bulk silicon does not
participate in the detection process.

The ability of these sensors to provide charged particle tracking is now
well established[^61] and several prototypes, called MIMOSA[^62], have
been built by the IReS group in Strasbourg. Our 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 m can be achieved using a pixel
pitch of 20 m. 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 m) and the double hit resolution is
\~30 m. Our group has shown that results from 20 µm can easily be
extrapolated to 30 µm -- the proposed pitch of this detector.

The radiation tolerance of the sensors to bulk damage[^63] 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 kRads.

Most of the R&D at IReS was performed with small prototypes (a few
mm^2^) containing a few thousand pixels. A full size prototype (i.e.
\~3.5 cm^2^) called MIMOSA-5 was also built, see Figure 22. It is
composed of \~1 million of pixels per chip, and it was fabricated on a 6
inch wafer, as shown in Figure 22. The wafers were thinned down to 120 m
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 m
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.

![](media/image24.jpeg){width="2.717361111111111in"
height="1.9909722222222221in"}![](media/image25.jpeg){width="3.2152777777777777in"
height="1.9868055555555555in"}

[]{#__RefHeading___Toc94556122 .anchor}Figure 22: Wafer of reticle size
sensors (left) and zoomed-in view of individual chips (right).

Table 7 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 m                   \~2 m

  Granularity (pixel pitch) 30 m                   17 m

  Read-out time             4 -- 10 ms             24 ms ( 20 ms possible)

  Ionizing radiation        2.6 kRad/yr            100 kRad
  tolerance                                        

  Fluence tolerance         210^10^ n~eq~/cm^2^    10^12^ n~eq~/cm^2^

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

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

  Chip thickness            50 m                   120 m
  ------------------------- ---------------------- -----------------------

[]{#__RefHeading___Toc94556151 .anchor}Table 7: Comparison between
MIMOSA-5 characteristics and HFT specifications.

## MIMOSTAR-1 Sensor Design

Based on the results of MIMOSA-5, a new chip has been fabricated. It is
called MIMOSTAR-1. It's most significant design parameters are
summarized below:

-   Fabrication process: TSMC 0.25 m (digital option).

-   Pixel pitch: 30 m.

-   Pixel array: 640 x 640, i.e. active area of 19. 2x19.2 mm^2^.

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

-   Chip dimension: \~21x20 mm^2^.

-   Additional details can be found in Ref. \[[^64]\].

Since the read-out speed requirement is not very demanding, 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 64x640
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 2x10 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. Therefore, 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.

At the time of writing this proposal, the MIMOSTAR-1 chips have been
fabricated in Strasbourg. The specifications used to design the chip are
shown in Appendix I -- MIMOSTAR Specifications and a detailed guide to
using the chip has been completed and is included in Appendix II --
MIMOSTAR Users Guide. The testing of the chip's performance parameters
is not yet complete but so far the Strasbourg group has shown that the
chip responds well to a high radiation dose (0 to 24 kRad using a ^60^Co
source), that the conventional controls work (via the JTAG controller),
and that the bias circuits work and exhibit linear response via their
DACs. Finally, the analogue performance of the chips has been tested and
the gain on the pre-amplifiers is about 3.5 at the two 10 MHz outputs.
The next step will be to test each pixel on the sensor, but this process
has not yet begun.

We plan to learn from the MIMOSTAR-1 architecture and this will be the
final prototype run before trying to produce the actual chips to be used
in the HFT. These chips will be called MIMOSTAR-2 and a test production
run for these chips is scheduled for the end of 2005 or early 2006.

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
m[^65]. 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 or 14 microns in an epi layer. This
makes it well suited to applications where one has to cope with
substantial electronic noise generated in the pixel.

### Thinning

Up to now, MIMOSA-5 chips have been thinned down to 120 m thickness.
This operation is successful and we have not found any degradation of
the sensor parameters. The desired MIMOSTAR thickness is \~50 m.
Thinning a 6 or 8 inch wafer down to this thickness is not expected to
be an issue 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 plan to use MIMOSA-5 wafers to make these
investigations and several companies will be contacted in order to
demonstrate the required thinning.

## 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. \[[^66]\].

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 ^55^Fe, 1.5 GeV
electrons and in a scanning electron microscope.

Our measurements have confirmed the IReS results that APS sensors can
measure charged particles with excellent spatial
resolution[^67]^,^[^68]**.** They confirm much of the work that the IReS
group obtained. Figure 1A shows several 1.5 GeV electrons recorded at
LBNL's Advanced Light Source.

To study, the effects of radiation, we 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[^69].
These radiation measurements are complementary to the neutron exposures
measured by the IReS group.

We completed a study on the effect 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. 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 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 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 performances. 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 a
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[^70] 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 on 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[^71]. 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 make a better device. We have fabricated
a sensor with different diode sizes. We plan to study this device and
understand the effect of diode size on signal to noise. We will be soon
submitting an IC design without an epi-layer. 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 MimosaSTAR
design, the sensor is always sensitive to radiation. We will study a
sample and hold circuit to see if we can reduce the sensitivity.

These new APS designs are promising but 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.

# Data Acquisition

## Requirements

The data acquisition is designed to read out the very large body of data
from the individual sensors, to digitize the signals, to perform data
compression, and to deliver the sparsified data for event building and
taping. A summary of the requirements is provided in Table 8.

  ---------------------------------------------------- ------------------
  Total number of pixels                               98\*10^6^

  Number of pixels per chip                            640\*640

  Pixel Readout rate                                   100 ns (10MHz)

  Readout time per frame                               4 ms

  Dynamic range of ADC                                 10 bits

  Raw data from one sensor using a 10 bit ADC          1 Gbit/sec

  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___Toc94556152 .anchor}Table 8: Requirement Summary:
constraints for the APS, due to signal expectation, available design
process, and mechanics

Digitizing the analog signal on each pixel into 10 bit digital signal
yields 1 GBit/sec per sensor chip when reading out in 4 ms. This number
becomes 240 GBit/sec when multiplying by the number of chips equipping
the whole HFT. This volume of data must be reduced before being handled
by the STAR DAQ event builder and written to tape.

Data compression is achieved by performing correlated double sampling
(CDS) and then leakage current subtraction, i.e. subtraction of two
consecutive frames, then the subtraction of the pedestal, 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 MIMOSA5
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
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. The use of
forced air cooling limits the total power dissipation to about 100
mW/cm^2^.

## Architecture

A sketch of the readout architecture is shown in Figure 23. Reading all
sensors in parallel, the DAQ system must handle data rates of about 240
Gbits/. The *bottleneck* of the low-level DAQ, is the data path from the
sensors to the end of the ladder where Readout Transceiver PC Boards
(RTB) interface to the STARDAQ via high speed optical links.

Each sensor will be read out by a custom designed read-out chip that is
mounted together with the sensor on the ladder. This solution has the
disadvantage of doubling the thickness of the silicon and of increasing
the power dissipation on the ladder. These disadvantages are fully
compensated by the fact that we will not need long analog signal lines
to the end of the ladder. Multiple parallel analogue outputs from the
detector chips are wire bonded directly to the RDO chips. A serial
digital network (\~500 Mbit/sec) connects the read-out chips to the RTB
boards.

8 bits are necessary to store the signal after CDS is performed. Zero
suppression requires subtracting a pedestal from each pixel. This can be
directly followed by cluster finding. Applying this kind of algorithm
ensures the most efficient compression but at the risk of compromising
the data quality if the threshold setting and cluster finding are not
well optimized. On the other hand, a lossless Huffman encoding may be
applied to the data. Serial acquisition allows online computation of a
data entropy table. This allows assigning an optimal length prefix-free
symbol for each value before transmitting it over the readout network
significantly reducing the needed bandwidth.

![](media/image26.jpeg){width="5.997916666666667in"
height="4.544444444444444in"}

[]{#__RefHeading___Toc94556123 .anchor}Figure 23: DAQ Layout: schematic
of DAQ system. Depending on effective data rate, multiple ladders can be
combined to one optical link. Using PCI and gnu/Linux, allows flexible
and transparent access to complex test setups while minimizing changes
from prototype to the final detector.

The data processing steps are being developed in a modular fashion so
that they can later be configured to a complete readout-system of:
digitization, data (pre)processing, (de)compression, data transmission
and DAQ control. This modular approach permits DAQ modules to be
configured in hardware (custom ASIC or FPGA) and/or software located on
on-board PCs.

### Readout Chip

A schematic diagram of the readout chip is shown in Figure 24. The chip
contains 3 ADCs, which digitize in parallel and are located close to the
sensors to minimize noise and crosstalk pickup. It also includes logic
to control the acquisition and buffer data. Adding memory (RAM) to the
ROChip may allow data preprocessing (CDS), transcoding and compression
algorithms, before transmitting the data over the ladder-cable.

![](media/image27.jpeg){width="6.003472222222222in"
height="5.149305555555555in"}

[]{#__RefHeading___Toc94556124 .anchor}Figure 24: ROC block diagram:
preliminary schematic/floor-plan of readout chip. Internal design is
subject to recent development.

The readout chip is bonded to the sensor chip. Its task is to digitize
the signal and either transfer it directly to the end of the ladder, or
further apply compression schemes before transferring the data. Since
power dissipation depends directly on speed, the main logic will operate
as slow as appropriate, i.e. at 10MHz. ADCs need to get 12 times this
frequency (pipelined ADC). ADC design process and ASIC design flow
available to us through the university of Heidelberg lead us to using
the UMC L18 (.18 m) mixed mode process. Chip-geometry aspect-ratio
limitations, require using 54 ROChips per APS and 2 ADCs per ROC in a
smaller than 4x5 mm package as shown schematically in Figure 25.

![](media/image28.jpeg){width="5.997916666666667in"
height="5.548611111111111in"}

[]{#__RefHeading___Toc94556125 .anchor}Figure 25: Readout Layout:
schematic sketch of the readout-topology on a detector ladder. This
figure shows one of ten APS and the corresponding readout chip layout.
You can likewise add more APS chips. (Note: this plot does not aim to be
a complete view of the interfaces. The readout chips will be mounted
piggy-back on the APS.)

To perform CDS and pedestal subtraction, each readout chip needs to have
at least 160 kbyte of memory (4Mbit per APS, 54 ROC per APS). RAM blocks
in UMC technology: 4096 x 64 M16 (256kbit) - 2205x739 m -- 1107 W/MHz.
Static memory consumes no power when unused, which drives optimization
effort on RAM access algorithms.

## Data Synchronization

The readout of the HFT is continuous, i.e. it is not triggered. It will
enter the STAR DAQ as a separate sequential data stream. Synchronization
with the individually triggered events will be done by assigning a RHIC
clock time stamp to each HFT readout frame.

![](media/image29.jpeg){width="5.998611111111111in"
height="2.4631944444444445in"}

[]{#__RefHeading___Toc94556126 .anchor}Figure 26: Data flow and
processing stages in the readout chip. Each stage can be bypassed to
allow raw/unprocessed data to be routed through the DAQ.

## Alternate Read-out Strategy

As a fall-back solution for the custom designed read-out chip we are
developing a cable that brings the analog outputs of the sensors to an
area where the available volume is large enough to implement the
functionality of the read-out chip and the RTB boards in discrete
technology. A technology choice will be made in Spring of 2006.

# Integration with the Experiment

## Mechanical Introduction

Our simulations have shown that for *D* identification in the high track
density environment encountered in *Au+Au* collisions, it is imperative
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 the APS technology we can use thinned silicon
with modest connection requirements to minimize the support electronics
and cable requirements. 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. It is felt that 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 carbon composite work going on at
LBNL for the ATLAS pixel detector. In our development work with carbon
composites we are maintaining a close relationship with the LBNL
engineering and technician staff working on the ATLAS pixel project.
This should 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 have a limited base of users available to
us. The R&D effort to develop fabrication and fixturing methods is
allowing us to gain the required expertise with these new tools.

The design to be reported is a conceptual design which we have started
to evaluate with test structures, detector carrier support elements, and
digitizing and readout electronics boards.

## Support Carriage for Rapid Installation and Removal

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 27 as it will be positioned
for operation in the STAR detector system. The HFT is the small inner
detector in the center. It is shown positioned inside the SVT detector
(half of which is removed so that the HFT is visible). Alternative outer
tracking detectors could be installed in place of the SVT. 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 illustrated detector system permits
rapid removal and replacement while maintaining reproducible position
through the use of fully defined kinematic mounts.

![](media/image30.jpeg){width="6.0in" height="2.5027777777777778in"}

[]{#__RefHeading___Toc94556127 .anchor}Figure 27: The HFT is shown
integrated with the STAR inner detectors cone assembly.

For simplicity several existing STAR structures are not shown. One of
the cone supports that is integral to the support of the SVT and the SSD
is not shown and the SSD which provides an additional tracking layer
outside the SVT is not shown.

A close up view, Figure 28, shows the HFT ladder arrangement. There are
two tracking layers: one starting at 1.5 cm radius and the other
starting at 4.5 cm radius. The ladder design shown 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 also being developed which are simpler to construct but
pose additional challenges in mechanical integration.

![](media/image31.jpeg){width="6.0in" height="3.5902777777777777in"}

[]{#__RefHeading___Toc94556128 .anchor}Figure 28: Close up view of the
HFT ladders.

The detector support structure illustrated in Figure 29 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 outer structures that are part
of the SVT 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 30 showing the
three identical frames (brass colored) that are kinematically joined
around the beam pipe using ball on cylinder contacts.

![](media/image32.jpeg){width="6.0in" height="3.4027777777777777in"}

[]{#__RefHeading___Toc94556129 .anchor}Figure 29: 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 ball on cylinder pairs. Releasable capture mechanisms are
required to load the kinematic mounts. These devices are not shown in
the illustrations.

The common tie point joining the HFT detector system to the STAR support
cone is illustrated in Figure 31. 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/image33.jpeg){width="5.325in" height="3.8625in"}

[]{#__RefHeading___Toc94556130 .anchor}Figure 30: Kinematic mount
structure.

![](media/image34.jpeg){width="5.325in" height="3.99375in"}

[]{#__RefHeading___Toc94556131 .anchor}Figure 31: Precision tie points
coupling the HFT system to the STAR support cone

Note, this is an on going development. The illustrations represent the
current state of our 3D CAD model which is being used for evaluation and
refinement

### Position Alignment and Calibration

The required position resolution of 4 microns is a significant challenge
for calibration and alignment. The detector system being small permits a
design with a single kinematic support to define a reproducible position
centered in 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 30 can be separated and reassembled with out
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 (see Figure 33). 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
32. This structure must be stiff enough to be supported on one end only
and maintain reliable position stability to 4 microns. The ladder is
made up of a 150 micron 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. The thinned,
50 micron readout chip is bonded to the cable and the detector chip is
bonded on top with the active side. These components are staggered as
shown in the magnified view in Figure 33 to allow wire bonding between
the elements. Since the number of connections is modest it is believed
that wire bonding will suffice without need to resort to bump bonding.
It was found with test structures that it is relatively straight forward
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 but we
have tested fabrication methods that are promising using a uni-layer
silicon structure. We have used vacuum chuck fixturing to butt joint
chips side by side against a straight edge and have then bonded them to
kapton with Pyralux. This bonded structure was then bonded at room
temperature to the carbon beam using Hysol EA9396 aerospace epoxy. 75
micron 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. Parylene
coating is process 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
uniform without pin holes, so protection can be achieved without
compromising detector thickness. Application at room temperature avoids
introducing stresses that distort the ladder shape.

![](media/image35.jpeg){width="5.992361111111111in"
height="2.1319444444444446in"}

[]{#__RefHeading___Toc94556132 .anchor}Figure 32: Ladder assembly. The
current ladder is composed of a row of 10 thinned APS detector 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.

![](media/image36.jpeg){width="5.992361111111111in"
height="4.435416666666667in"}

[]{#__RefHeading___Toc94556133 .anchor}Figure 33: Close up view of the
ladder showing APS detector chips mounted on top of the readout chip.
The chips are mounted and wire bonded on a three layer aluminum kapton
cable.

### Detector Radiation Length

Multiple scattering in the beam pipe and the first inner layer of the
vertex detector sets the ultimate limit in precision of vertex
resolution. The radiation thickness for an inner ladder and the beam
pipe is given in Table 9. 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 the table.

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

  Beryllium beam pipe         35         500                      0.14

  Aluminum                    9.0        10                       0.007

  Silicon                     9.4        112                      0.12

  Kapton                      35         170                      0.05

  Adhesive                    35         85                       0.024

  Carbon Composite            28         174                      0.062

  Ladder Total                           550                      0.26
  --------------------------- ---------- ------------------------ ---------

[]{#__RefHeading___Toc94556153 .anchor}Table 9: Materials in the beam
pipe and the first detector layer with their radiation length fractions.
For details see Ref. \[[^72]\]

## Ladder Mechanical Tests

### Load Distortion Tests

Figure 34 and Figure 35 show a prototype of the ladder and detector
carrier that we have built. The mechanical tests have shown that the
stiffness and bending characteristics of the assembly are acceptable.
For these and similar tests[^73], 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 gm end load. The
deflection profile is shown in Figure 36 along with the calculated
deflection for a simple triangular closed beam. The stiffness is within
20% of expectations from the engineering model. The measured fundamental
frequency of the ladder is 140 Hz while the engineering model gives a
resonant frequency of 135 Hz[^74]. 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/image37.jpeg){width="5.992361111111111in" height="1.29375in"}

[]{#__RefHeading___Toc94556134 .anchor}Figure 34: A cross-section of the
prototype detector ladder showing its structure and materials
composition.

![](media/image38.jpeg){width="5.0in" height="3.479861111111111in"}

[]{#__RefHeading___Toc94556135 .anchor}Figure 35: The prototype ladder
on the testing apparatus.

![](media/image39.jpeg){width="4.694444444444445in"
height="3.703472222222222in"}

[]{#__RefHeading___Toc94556136 .anchor}Figure 36: 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 gm weight
placed on the other.

### Thermal Distortion Tests

We are in the process of measuring thermal generated deflections using
TV holography[^75]. 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 37). This
tool will also be used to test the stability of the final detector.

![](media/image40.jpeg){width="5.991666666666666in"
height="3.495833333333333in"}

[]{#__RefHeading___Toc94556137 .anchor}Figure 37: 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 37) 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[^76] 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 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[^77]. Additional information on the cooling studies is
available on the web[^78].

## Cabling

Connection to the detector readout chips will be done with an aluminum
kapton cable with three conduction layers. A connection topology that
does not require vias is shown in Figure 33 in which the bottom layer
carries analog and digital power. The mid layer carries dedicated data
lines, control lines and reference voltages. Common lines are daisy
chained through the silicon chips to avoid crossovers in the aluminum
flex PC cable. The top layer carries analog and digital grounds. The
power and ground layers are patterned to reduce conductor material. We
have recently found a commercial vendor that makes aluminum cables with
vias so the concept illustrated above will probably not be necessary.

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, but development work is required to
fabricate the staggered layer design illustrated in Figure 33. The issue
to be addressed is having clean bonding pads on the three layers after
cable fabrication.

Zero suppression or other data compression will be done locally, so that
a small number of traces should be sufficient for each readout chip to
carry the limited data load. This is key to minimizing the cable mass in
the tracking path while leaving some design flexibility for readout
speed and ladder length. The cable is part of the ladder and is located
underneath the two thinned silicon chips, APS detector and readout chip.

Connections leaving the detector structure will be limited to power,
fiber optic and possibly water cooling for data interface boards.

Figure 38 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 MIMOSA5 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[^79].

![](media/image41.jpeg){width="5.993055555555555in"
height="1.5659722222222223in"}

[]{#__RefHeading___Toc94556138 .anchor}Figure 38: A prototype readout
cable for the HFT.

Figure 39 shows a mechanical Prototype with 4 MIMOSA5 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 X
but has a "dish" shape in Y. 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[^80].

![](media/image42.jpeg){width="5.989583333333333in"
height="2.9784722222222224in"}

[]{#__RefHeading___Toc94556139 .anchor}Figure 39: Mechanical Prototype
with 4 MIMOSA5 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 40.

![](media/image43.jpeg){width="6.0in" height="3.73125in"}

[]{#__RefHeading___Toc94556140 .anchor}Figure 40: 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 41. The outer
shell has been made transparent in the figure to show the inner
structure.

![](media/image44.jpeg){width="6.0in" height="4.5in"}

[]{#__RefHeading___Toc94556141 .anchor}Figure 41: 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, have a minimum radius of 6
while 10 is very conservative[^81]. is the beam envelope at injection.
For STAR at injection the beam emittance is 15 π mradmm, ^\*^ is 10 m,
is 10.52 which gives = 1.5 mmError: Reference source not
found^,^[^82]^,^[^83]. For protons injection is smaller and therefore
less of a constraint. We have chosen an inner beam pipe radius of 14 mm
or \~9 . This should be tolerable because it is not the minimum aperture
in the STAR line. The abort kicker is the limiting aperture ( 7.3
horizontal plane and 6.0 vertical plane )[^84].

### 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[^85]^,^[^86]^,^[^87] 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 outgassing rate of 5.3 x 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 outgassing rate, but a factor of 10 greater value
is still tolerable.

### 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 42 shows how the
detector ladders mesh with the support spokes.

### 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[^88]. 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.

[]{#__RefHeading___Toc94556082 .anchor}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[^89].

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 1x10^9^
gold ions per bunch. This gives a wake field current of 110 mA (counting
both beams) which with a 1 micron 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 but
the scale from the simple view suggests that the wake field will not
compromise the detector signal.

![](media/image45.jpeg){width="6.0in" height="4.5in"}

[]{#__RefHeading___Toc94556142 .anchor}Figure 42: End view showing the
HFT ladders between spokes to the inner beam pipe support.

### 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
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 deg C
to 110 deg C or it could be a full activation of the NEG coating turning
it into a pumping surface[^90]. This later case requires baking at 150
deg C to 200 deg 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.

[]{#__RefHeading___Toc94556085 .anchor}Compatibility with the SVT and
FTPCs

If the SVT remains as the tracking detector outside of the HTF then the
inner layer of the SVT can be removed along with associated cabling to
make room for the HFT and it's supporting structure. The FTPC on one end
will have to be removed to accomedate the HFT and support electronics as
shown in Figure 27.

## 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 - 01/25/2005

A rough estimate has been made of cost and schedule. Figure 43 shows the
projected time line for completion of the major tasks associated with
the project. In this analysis the major tasks shown have been broken
down into at least design and fabrications phases and in many cases
include more detail such as prototype and testing tasks. The completion
date has been tied to the expected running time for the next long heavy
ion run of Au-Au in 2007. At this stage, consideration has been given to
the necessary completion times and expected effort requirements, but
analysis with more detailed task breakdown and resource loading is left
for the future.

![](media/image46.png){width="6.011111111111111in"
height="2.2256944444444446in"}

[]{#__RefHeading___Toc94556143 .anchor}Figure 43: Estimated time line
for major tasks in the building of the HFT detector.

In costing the project we have added a contingency of 75% for most
items. This has been reflected in the labor costs, but the time
contingency has not been included in the time line shown above. The
accumulated cost with time is shown in Figure 44 with a final completion
cost of 4.1 M\$. For purchases, costs are counted at the time of placing
the order.

![](media/image48.wmf){width="5.9944444444444445in"
height="4.344444444444444in"}

[]{#__RefHeading___Toc94556144 .anchor}Figure 44: Estimated cost
projection for the HFT project showing accumulated cost during the time
of design and construction. The final total is 4.1 M\$.

The cost estimate presented relies on a significant amount of
contributed labor that does not appear in the final sum. Table 10
provides a breakdown of the assumed contributed and project funded
effort. The cost of the contributed effort comes to 5.4 M\$ - this
includes the 75% contingency charge. At this stage effort has been
allocated to mechanical and electrical engineering at both BNL and LBNL,
technicians at both institutions and management. The significant
contributed effort already provided by IReS and on the detector chips as
well as their future effort has not been included. As shown in the
table, one of the biggest contributed items is 5.6 years of electronic
engineering from LBNL which over the life of the project exceeds the
expected resource at LBNL by \~1 person. This will have to be addressed
in some way, possibly through additional contributions from other
institutions.

+------------------------------------+----------------+----------------+
|                                    | contributed    | project        |
|                                    |                | supported      |
|                                    | (man years)    |                |
|                                    |                | (man years)    |
+------------------------------------+----------------+----------------+
| electronic engineer LBNL           | 5.1            | 0.0            |
+------------------------------------+----------------+----------------+
| electronic engineer BNL            | 0.4            | 0.0            |
+------------------------------------+----------------+----------------+
| mechanical engineer LBNL           | 3.5            | 0.3            |
+------------------------------------+----------------+----------------+
| mechanical engineer BNL            | 0.4            | 0.0            |
+------------------------------------+----------------+----------------+
| technician LBNL                    | 0.3            | 1.8            |
+------------------------------------+----------------+----------------+
| technician BNL                     | 2.7            | 0.5            |
+------------------------------------+----------------+----------------+
| management                         | 1.1            | 2.2            |
+------------------------------------+----------------+----------------+
| total                              | 13.4           | 4.8            |
+------------------------------------+----------------+----------------+

[]{#__RefHeading___Toc94556154 .anchor}Table 10: Estimated manpower
requirements for the HFT project

##  {#section-1 .unnumbered}

As mentioned above IReS/LEPSI is contributing significant manpower
resources developing the sensor chip which has not been included in the
above tabulation. This work involves several engineers and physicists,
most of who are part time only. A list of these contributors may be
found in Table 11r

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

                  

                  

                  

                  

                  

                  

                  

                  

                  

                  

                  

                  

                  

                  

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

### 

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

                                                        

                                                        

                                                        

                                                        

                                                        

                                                        

                                                        

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

-   -   
    -   
    -   
    -   
    -   

  ------------------ ----------------------------------------------------
  IReS               

  design engineers   F.Guilloux, A.Himmi, Ch.Hu, I.Valin

  test engineers     K.Jaaskelainen, Ch.Olivetto, M.Pelliciol

  physicists         A.Besson, M.Deveaux, G.Gaycken, D.Grandjean,
                     M.Winter

  LEPSI              

  design engineers   C.Colledani, G.Deptuch, W.Dulinski

  test engineers     G. Claus, M. Szelezniak
  ------------------ ----------------------------------------------------

Table 11 IReS/LEPSI manpower contributions developing the HFT readout
chip

## 

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

                   

                   

                   

                   

                   

                   

                   

                   

                   

                   

                   

                   

                   

                   

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

## 

## 

##  

# Summary

Assessing whether or not charm quarks pick up flow and thermalizes is
the next important step towards proving the formation of the Quark Gluon
Plasma at RHIC. Furthermore, measuring the energy lost by high
transverse momentum beauty 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 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 ( ≤
0.2 % radiation length) layers of Active Pixel Sensors (APS) having a
position resolution better than 10 m. APS technology is the only option
that fits these requirements while compromising neither the efficiency
nor the readout speed. Indeed, APS can be thinned down to 50 m 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 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 Specifications

La ti da

# Appendix II -- MIMOSTAR Users Guide

La ti da

# Appendix III -- Inner Tracker Interface Specification

La ti do

# Appendix IV -- SVT Interface Specification

La ti de

# References

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