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

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936227.doc

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**A Stand-alone Heavy Flavor Tracker**

**for STAR**

Z. Xu

Brookhaven National Laboratory, Upton, New York 11973

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

University of California, Irvine, California

H. Huang, A. Tai

University of California, Los Angeles, California 90095

V. Kushpil, M. Sumbera

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

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

Institut de Recherches Subatomique, Strasbourg, France

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

Massachusetts Institute of Technology, Cambridge, MA 02139

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

Lawrence Berkeley National Laboratory, Berkeley, California 94720

I. Kotov

Ohio State University, Columbus, Ohio 43210

Contents

[1 Executive Summary 10](#executive-summary)

[1 Executive Summary 10](#__RefHeading___Toc119662821)

[1.1 Scientific Motivation 10](#scientific-motivation)

[1.2 Detector Concept 11](#detector-concept)

[1.3 Cost and Schedule 12](#cost-and-schedule)

[2 The Physics of the HFT 13](#the-physics-of-the-hft)

[2.1 Introduction 13](#introduction)

[2.2 Initial Heavy Quark Production 14](#initial-heavy-quark-production)

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

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

[2.3.1 Elliptic Flow 21](#elliptic-flow)

[2.3.2 Charm Hadro-Chemistry 24](#charm-hadro-chemistry)

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

[2.4.1 R~AA~ and Energy Loss 27](#raa-and-energy-loss)

[2.4.2 Charm Angular Correlations 28](#charm-angular-correlations)

[2.4.3 Baryon -- Meson Ratios 30](#baryon-meson-ratios)

[2.4.4 B Jet Tagging 31](#b-jet-tagging)

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

[2.6 Spin Measurements 36](#spin-measurements)

[3 HFT Conceptual Design and Performance
38](#__RefHeading___Toc119662839)

[3.1 Overview of the Experiment 38](#overview-of-the-experiment)

[3.2 Design of the HFT 39](#design-of-the-hft)

[3.3 Simulated HFT Performance 40](#simulated-hft-performance)

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

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

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

[4 CMOS Sensors 52](#cmos-sensors)

[4.1 Introduction 52](#introduction-1)

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

[4.3 MIMOSTAR-1 Sensor Design 55](#mimostar-1-sensor-design)

[4.3.1 Thinning 56](#thinning)

[4.4 Additional R&D 56](#additional-rd)

[5 Data Acquisition 61](#data-acquisition)

[5.1 Requirements 61](#requirements)

[5.2 Architecture 62](#architecture)

[5.3 Data Synchronization 65](#data-synchronization)

[5.4 Data Rates 65](#data-rates)

[6 Integration with the Experiment 66](#integration-with-the-experiment)

[6.1 Mechanical Introduction 66](#mechanical-introduction)

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

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

[6.3 Ladder Design and Fabrication 71](#ladder-design-and-fabrication)

[6.3.1 Detector Radiation Length 73](#__RefHeading___Toc119662862)

[6.3.2 Expected Radiation Exposure 73](#expected-radiation-exposure)

[6.4 Ladder Mechanical Tests 73](#ladder-mechanical-tests)

[6.4.1 Load Distortion Tests 73](#load-distortion-tests)

[6.4.2 Thermal Distortion Tests 75](#thermal-distortion-tests)

[6.4.3 Cooling Measurements 76](#cooling-measurements)

[6.5 Cabling 76](#cabling)

[6.6 Beam Pipe 79](#beam-pipe)

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

[6.6.2 Beam Pipe Radius - Vacuum Considerations
80](#__RefHeading___Toc119662871)

[6.6.3 Supporting Section 81](#supporting-section)

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

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

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

[6.6.7 Bake Out 82](#bake-out)

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

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

[7 Cost, Schedule and Manpower 84](#cost-schedule-and-manpower)

[7.1 Cost and Schedule - 01/25/2005 84](#cost-and-schedule---01252005)

[8 Summary 87](#summary)

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

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

[11 References 89](#references)

List of Figures

[Figure 1: Masses of the six quark flavors. The current and constituent
masses are shown by blue- and yellow-boxes, respectively. The figure is
adapted from Ref. \[11\]. 15](#__RefHeading___Toc119665993)

[Figure 2: 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. \[14\].
17](#__RefHeading___Toc119665994)

[Figure 3: Total cc production cross-sections per nucleon-nucleon
collision vs. 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 adapted from Ref. \[16,
18\]. 17](#__RefHeading___Toc119665995)

[Figure 4: Solid- and dashed-curves represent the charm- (red) and
beauty-hadron (blue) spectra from Blast-Wave and Pythia model
calculations, respectively. The corresponding heavy flavor decayed
electron spectra are shown as black curves. The data are the measured
single electron distributions measured in 10% central Au+Au collisions
at 130 GeV by the PHENIX collaboration. The figure is adapted from Ref.
\[19\]. 18](#__RefHeading___Toc119665996)

[Figure 5: Nuclear modification factor R~AA~ of D-mesons assuming the
measured spectrum from d+Au collisions scales as the light charged
hadrons in central Au+Au collisions (blue line, I) and from a
hydro-dynamically inspired parameterization assuming a collective flow
velocity of \<~r~\> = 0.4*c* for D-mesons (red line, II); the
corresponding electron decay-spectra are shown by the blue dashed (I)
and red dashed (II) lines. 19](#__RefHeading___Toc119665997)

[Figure 6: Elliptic flow of strange hadrons at RHIC as measured by the
STAR detector. The top panel demonstrates typical hydro-dynamical mass
ordering up to a momentum p~T~ \< 2GeV/c and saturation at larger
momentum. The bottom panel shows the scaling of elliptic flow with the
number n (baryons, n=3; mesons, n= 2) of constituent quarks in the
saturation region. 23](#__RefHeading___Toc119665998)

[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. The dashed-dotted-line
represents the input D-meson v~2~ distributions, assuming the scaling of
constituent quarks for meson v~2~. The corresponding decayed electron
v~2~ are shown as the hatched band. The dashed-line represents the
results from Greco et al. In this coalescence calculation, similar v~2~
distributions for c- and u-quarks are assumed. The figure is adopted
from Ref. \[34\]. 24](#__RefHeading___Toc119665999)

[Figure 8: The ratio of suppression factors in hot matter for charm (H)
and light (L) quarks. The solid line represents results from
calculations with unrestricted gluon radiation, while the dashed line is
based on calculations with a cut on gluon energies \> 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. \[54\]. 27](#__RefHeading___Toc119666000)

[Figure 9: Non-photonic electron R~AA~ in central Au+Au collisions from
STAR (open-symbols, top 5%) and PHENIX (filled-symbols, top 10%) from
Refs. \[61,62,63\]. Theoretical predictions for charmed-hadron and charm
and beauty-hadron decayed electrons are shown as blue- and yellow-bands,
respectively. In these calculations, the initial gluon density was
assumed to be 3500, Ref \[66\] . 28](#__RefHeading___Toc119666001)

[Figure 10: D-meson correlation functions for 200 GeV p+p collisions.
Default parameters in the Pythia model were used in these calculations.
A clear back-to-back correlation in the angular distribution of charmed
mesons is observed (shown by the open circles). The Solid-line and the
diamonds represent the results with angular smearing for and,
respectively. 29](#__RefHeading___Toc119666002)

[Figure 11: Distance of the B^+^ decay position from the primary vertex.
Full circles are from high-momentum simulated B^+^ e^+^ + X decays, open
circles show the distribution of combinatorial background. The arrow
indicates a cut at 250 m optimizing the signal to background ratio.
32](#__RefHeading___Toc119666003)

[Figure 12: Left top: TPC dE/dx vs p in d+Au collisions. Left bottom:
TPC dE/dx vs p after TOFr PID selection of \| 1- \| \< 0.03. Clean
electron identification is achieved. Right: dE/dx from the TPC after
TOFr PID selection for 1.0 \< p \< 1.5 GeV/c.
34](#__RefHeading___Toc119666004)

[Figure 13: 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 HFT hit is 50.
35](#__RefHeading___Toc119666005)

[Figure 14: Di-electron invariant mass distribution. The solid black
curve is a prediction from R. Ralf Ref. \[81 using the full STAR
acceptance. The red curve at the top is the total di-electron invariant
mass seen with the 2004 configuration of STAR but assuming electron PID
from Full TOF coverage. The pink dashed line is the e+e- pair invariant
mass from charm semi-leptonic decays. The gray curve is the charm e+e-
pair invariant mass after cut on the HFT DCA \< 80um. The dot dashed
line is the Dalitz decays from 0 and after rejection from TPC.
36](#__RefHeading___Toc119666006)

[Figure 15: The STAR detector at RHIC. The primary detector elements at
mid-rapidity are shown in this cut-away view of the detector. For scale,
the TPC is 4.2 meters long and 4 meters in diameter. Heavy Ion beams
enter from the left and right while collisions take place at the
centerline under the Silicon trackers. 38](#__RefHeading___Toc119666007)

[Figure 16: Left - Schematic view of the HFT detector ladder arrangement
with 6 ladders forming the inner layer and 18 ladders forming the outer
layer. The width of the ladders (the dimension seen in this view) is 19
mm. The shingled layers provide cross calibration of the ladders and
provide an open geometry for ease of installation. Right - The HFT is
shown embedded inside the SVT+SSD. 40](#__RefHeading___Toc119666008)

[Figure 17: 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. 41](#__RefHeading___Toc119666009)

[Figure 18: 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. 42](#__RefHeading___Toc119666010)

[Figure 19: Probability of getting the correct hit on the outer HFT
layer, given two different SVT geometries and a search window of 640 m
43](#__RefHeading___Toc119666011)

[Figure 20: 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. 45](#__RefHeading___Toc119666012)

[Figure 21: The absolute yield of D^0^ into the TPC+SSD+HFT divided by
the input D^0^ yield. In the SVT-1 configuration, the innermost layer of
the SVT has been removed in order to allow more room for the HFT. And
finally, Tthe Alice geometry assumes thick Si detectors that are similar
to the wafers used in the ALICE vertex detector. The D^0^ reconstruction
cuts were optimized using the program MINUIT.
46](#__RefHeading___Toc119666013)

[Figure 22: The number of central Au+Au events needed as a function of
the Si ladder thickness and the position resolution of the detector
(left). Results from Monte Carlo Simulations and full tracking in all
sub-detectors including TOF and the proposed HFT (right).
47](#__RefHeading___Toc119666014)

[Figure 23: Left: SVT-1 configuration without TOF. Right: SVT-1
configuration plus TOF. The estimated statistical uncertainties are
shown on the top line. 47](#__RefHeading___Toc119666015)

[Figure 24: Ds reconstruction efficiency as a function of transverse
momentum 48](#__RefHeading___Toc119666016)

[Figure 25: Flow results if charmed quarks flow, or do not flow.
49](#__RefHeading___Toc119666017)

[Figure 26: 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.
53](#__RefHeading___Toc119666018)

[Figure 27: Wafer of reticle size sensors (left) and zoomed-in view of
individual chips (right). 54](#__RefHeading___Toc119666019)

[Figure 28: An APS Sensors developed by the LBNL/UIC group. The picture
shows 16 separate test structures. Each structure has a 36 x 36 array of
20 µm pixels. 57](#__RefHeading___Toc119666020)

[Figure 29: This graph shows the results of one event taken with 1.5 GeV
electrons. Each bin represents one pixel and the height is proportional
to the measured charge. Several electrons can be identified in the plot.
58](#__RefHeading___Toc119666021)

[Figure 30: DAQ Layout: schematic of DAQ system for a single ladder.
Analog data is carried as differential current on the low mass cable at
50 MHz. The signals are driven in parallel over short (\~1m) twisted
pair cables to the motherboard. Analog to digital conversion, CDS and
data reduction are performed in the Motherboard / Daughter cards. The
reduced hit data is transferred digitally to the SIU and carried to
linux based readout PCs via an optical fiber. Control, synchronization
and event ID tagging are accomplished in the Synch/Trigger board.
63](#__RefHeading___Toc119666022)

[Figure 31: Ladder Layout - sketch of the readout-topology on a detector
ladder. This figure shows the ten APS and the corresponding current to
voltage conversion and driver electronics. The drivers will be located
out of the low mass region of the detector and may require additional
cooling. 64](#__RefHeading___Toc119666023)

[Figure 32: HFT Motherboard functional layout. This figure shows the
functional layout of the HFT Motherboard and Daughter cards. There are 5
daughter cards per motherboard to process the data from one ladder.
Correlated double sampling, pedestal subtraction and hot pixel removal
happen at the first stage of processing on the daughter cards. Hit
finding and reduction of the data to addressed clusters happens on the
motherboard. Data is then transferred via optical fiber to the RORC
cards where it is picked up by the STAR DAQ.
64](#__RefHeading___Toc119666024)

[Figure 33: Data rates at the various stages of the HFT readout chain.
65](#__RefHeading___Toc119666025)

[Figure 34: The HFT is shown integrated with the STAR inner detectors
cone assembly. 67](#__RefHeading___Toc119666026)

[Figure 35: Close up view of the HFT ladders.
68](#__RefHeading___Toc119666027)

[Figure 36: Detector support structure with kinematic mounts to insure
repeatable detector positioning. 68](#__RefHeading___Toc119666028)

[Figure 37: Kinematic mount structure. 69](#__RefHeading___Toc119666029)

[Figure 38: The HFT is shown integrated with the STAR inner detectors
cone assembly. 70](#__RefHeading___Toc119666030)

[Figure 39: Ladder assembly. The current ladder is composed of a row of
10 thinned APS detectors on top of a matching row of thinned readout
chips and a three-layer aluminum Kapton cable. The silicon cable
structure is bonded to a carbon composite v, closing the beam to make a
rigid structure. 72](#__RefHeading___Toc119666031)

[Figure 40: 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.
72](#__RefHeading___Toc119666032)

[Figure 41: A cross-section of the prototype detector ladder showing its
structure and materials composition. 74](#__RefHeading___Toc119666033)

[Figure 42: The prototype ladder on the testing apparatus.
74](#__RefHeading___Toc119666034)

[Figure 43: Measured bend of a silicon/carbon composite ladder test
structure and the calculated bend shape. The ladder was rigidly
supported at one end with a 10 gram weight placed on the other.
75](#__RefHeading___Toc119666035)

[Figure 44: TV Holography system viewing test ladder in a small
transparent wind tunnel. Inset shows diffraction pattern with color map
of the surface displacement. 76](#__RefHeading___Toc119666036)

[Figure 45: A prototype readout cable for the HFT.
77](#__RefHeading___Toc119666037)

[Figure 46: Mechanical Prototype with 4 MIMOSA-5 detectors glued to the
Kapton cable assembly. 78](#__RefHeading___Toc119666038)

[Figure 47: Beam pipe structure with outer support barrel in the center.
79](#__RefHeading___Toc119666039)

[Figure 48: View of the beampipe mid-section.
80](#__RefHeading___Toc119666040)

[Figure 49: End view showing the HFT ladders between spokes to the inner
beam pipe support. 81](#__RefHeading___Toc119666041)

[Figure 50: The technically-driven time line for major tasks in the
building of the HFT detector. 84](#__RefHeading___Toc119666042)

[Figure 51: Estimated cost of the HFT project versus the technically
driven schedule. The accumulated costs are shown in millions of dollars.
Design and construction costs are shown in blue, and the value of
contributed labor is shown in purple. 85](#__RefHeading___Toc119666043)

List of Tables

[Table 1: Charm quark fragmentation functions. The left column is from
Ref. \[47,48\]. The right column is from Ref. \[46\]. The D^+^ and D^0^
yields include feed-down from D^\*+^ and D^\*0^ decays.
25](#__RefHeading___Toc119662936)

[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. 35](#__RefHeading___Toc119662937)

[Table 3: Selected HFT parameters. 40](#__RefHeading___Toc119662938)

[Table 4: Cuts applied for D-meson reconstruction
44](#__RefHeading___Toc119662939)

[Table 5: Cuts applied for D~s~^+^ reconstruction
48](#__RefHeading___Toc119662940)

[Table 6: Luminosity and other parameters that determine the particle
flux on the HFT. 50](#__RefHeading___Toc119662941)

[Table 7: Integrated hit loading on the HFT and associated pileup.
50](#__RefHeading___Toc119662942)

[Table 8: Hit loading on the HFT from Au+Au central collisions and
associated pileup. 51](#__RefHeading___Toc119662943)

[Table 9: Comparison between MIMOSA-5 characteristics and HFT
specifications. 54](#__RefHeading___Toc119662944)

[Table 10: Requirement Summary - constraints for the APS, due to signal
expectation, available design process, and mechanics
61](#__RefHeading___Toc119662945)

[Table 11: Materials in the beam pipe and the first detector layer with
their radiation length fractions. For details see Ref. \[\]
73](#__RefHeading___Toc119662946)

[Table 12: Estimated manpower requirements for the HFT project
85](#__RefHeading___Toc119662947)

[Table 13: IReS manpower contributions developing the HFT readout chip
86](#__RefHeading___Toc119662948)

# Executive Summary

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

## Scientific Motivation

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

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

The most exciting prospective measurement with the pixel detector is to
perform a measurement of the elliptic flow of D mesons down to very low
p~T~ values. It is a generally accepted fact that elliptic flow is
established in the partonic phase. If charm quarks, with a mass much
larger than the temperature of the system, elliptic flow then it has to
arise from many collisions with the abundant light quarks. Thus, flow of
charm quarks can be taken as a probe for frequent rescatterings of light
quarks and is an indication of thermalization that may be reached in the
early stages of heavy ion collisions at RHIC. Proof of thermalization
requires additional measurements such as thermal photons and low mass
dilepton spectra. We believe that proof of thermalization constitutes
the last step towards the establishment of the QGP at RHIC and this
measurement requires a very thin detector.

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

The HFT detector will allow us to measure the ratio of different D meson
states with high precision. This measurement can be used to test if the
different states are produced in the same ratio as in elementary
particle collisions or if the ratios are established according to
chemical equilibrium. The heavy quark can also be used to probe the
properties of the medium created in heavy ion collisions. The production
of gluons is kinematically suppressed for heavy flavors (due to the dead
cone effect) and as a consequence heavy flavors should lose less energy
in the dense medium.

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

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

## Detector Concept

The proposed HFT detector will sit inside the STAR TPC and it will
surround the interaction vertex. It will exploit all of STAR's unique
features including full azimuthal coverage and tracking from the lowest
to the highest p~T~. The HFT has two tracking layers composed of
monolithic CMOS pixel detectors using 30 m 30 m square pixels. These
critical innermost tracking layers lie at radii of 1.5 cm and 5 cm,
respectively, and these layers are active over 20 cm in Z and have
\~ 100 million pixels. The HFT will provide tracking information for
decaying particles that are displaced by only a few tens of microns from
the primary vertex. The silicon chips for the detector will be thinned
to 50 m and will be mounted on low mass carbon fiber structures to
minimize pointing errors generated by multiple Coulomb scattering. In
this respect, the STAR HFT is unique. No other Si detector at RHIC
combines this kind of extreme pointing accuracy and is effective for all
particles down to 150 MeV/c.

## Cost and Schedule

The total financial resources required to complete the Heavy Flavor
Tracker falls in the range from 8 to 10 million dollars, including the
costs of the contributed labor which will come from the base support of
the participating institutions. A more refined estimate is presented
later in this document. The estimate includes a 75% contingency on most
items, including the labor.

We propose to build and install the detector over a three year period
and we plan to have it ready to take data in time for the next full
energy Au+Au run at RHIC.

# The Physics of the HFT

## Introduction

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

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

Rare processes provide 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) can
generate significant energy loss effects[^6]^,^[^7]. For example, hard
partons are indirectly observable in elementary particle collisions (
*e^+^+ e*, *p* +*p* ) due to their fragmentation into collimated sprays
of energetic hadrons at large transverse angles with respect to the
beam, these processes are known as jets. Jets can also be identified in
high energy nuclear collisions on a statistical basis; the modification
of their properties may signal novel flavor dependent energy loss
mechanisms in a dense medium by the parton that initiated the jet. Full
jet reconstruction in heavy ion collisions is exceedingly difficult but
leading hadrons (i.e. high *p*~T~ hadrons, which typically carry a large
fraction of the jet energy) and their correlations 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
was produced.

Heavy quark (*c,b*) production provides some of the most important
observables. Due to their large masses, *c* and *b* quarks are produced
dominantly by the interactions of the initial incoming partons whereas
lighter quarks are produced throughout the 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 interaction 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, an additional way to study
partonic energy loss. If charmed quarks participate sufficiently in
re-scattering processes, they will develop flow (i.e. transverse radial
and elliptic flow) which can be observed in charmed hadron momentum
distributions. Finally, charmed quarks might achieve equilibrium in the
surrounding medium leading to statistical hadronization. In this case,
the relative abundances of charmed hadrons are significantly modified.

The study of the structure of the nucleon is an important frontier in
strong interaction physics. Despite considerable experimental and
theoretical progress over the past several decades, many open questions
remain. A striking example is the spin structure of the proton which has
been of key interest ever since the European Muon Collaboration
published their data on the spin structure function
*g~1~(x,Q^2^)*[^8]^,^[^9]. The EMC data and subsequent data indicate
that the quark helicity contribution to the proton spin is remarkably
small. The natural question regarding the fraction of the nucleon spin
carried by gluon helicities remains basically unanswered. Unlike
previous nucleon spin measurements, STAR will observe processes where
the polarized gluon distribution function *G(x,Q^2^)* contributes at
leading order via a hadronic, not electromagnetic, probe. Heavy quark
production is expected to provide direct access to gluons in the proton
with a large analyzing power for longitudinal spins.

## Initial Heavy Quark Production 

Quarks are elementary particles and, depending on the energy scale,
there are two mechanisms that generate their masses with different
degrees of importance: current quark masses are generated by the
electroweak symmetry breaking mechanism (Higgs mass) and spontaneous
Chiral symmetry breaking leads to the constituent quark masses in QCD
(QCD mass). The QCD interaction affects only the light quarks (u, d, s)
while the heavy quark masses(c, b, t) are determined by the Higgs
mechanism, see Figure 1. In high-energy nuclear collisions at RHIC,
heavy quarks are produced through gluon fusion and *qq*
annihilation[^10]. Heavy quark production is also sensitive to the
parton distribution function. Unlike the light quarks, heavy quark
masses are not modified by the surrounding QCD medium[^11] and the value
of their masses are much higher than the initial excitation of the
system. It is these differences between light and heavy quarks in a
medium that make heavy quarks an ideal probe to study the properties of
the hot-dense medium created in high-energy nuclear collisions.

![](media/image2.png){width="5.006944444444445in"
height="3.0631944444444446in"}

[]{#__RefHeading___Toc119665993 .anchor}Figure 1: Masses of the six
quark flavors. The current and constituent masses are shown by blue- and
yellow-boxes, respectively. The figure is adapted from Ref. \[Error:
Reference source not found\].

Understanding the yield of charmed hadrons in hadron-hadron collisions
requires a knowledge of the projectile and target parton distribution
functions, the cross 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 charmed (bottom)
hadrons. The parton distributions within the proton can be extracted by
parameterizing the data from electron-proton collisions while the
cross-sections for gluon fusion and *qq* annihilation are calculated in
a perturbative QCD framework up to next-to leading-order (NLO)[^12].
However, the parton and gluon distribution functions within the nucleus,
relevant to the charm and bottom quark production at RHIC energies, are
poorly understood[^13] and thus leave room for precise measurements of
charm cross sections in p+p, d+Au and Au+Au collisions.

Perturbative QCD predictions for the cross section (*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 in these cross-sections arise from a reasonable
variation of quark masses (*m~c~* = 1.2-1.4 GeV, *m~b~* = 4.5-5.0 GeV),
factorization and renormalization scales (~R~ and ~F~), and parton
distribution functions (MRST, CTEQ, GRV). The cross section values are
normalized per participating nucleon-pair so, in the calculation of the
yield for central Au+Au collisions, the number of pairs produced per
central Au+Au collision were obtained by multiplication with the nuclear
overlap integral T~AA~ (29.3 mb^-1^), corresponding to \~ 1150 binary
nucleon-nucleon scatterings per Au+Au collision.

In heavy ion collisions, final state interactions may also enhance charm
production relative to the binary scaling of initial parton collisions
and may also lead to additional production of charm via "thermal"
processes. An analogous mechanism dominates strangeness production. The
heavy quark mass (*m~c~* ≈ 1.2 to 1.8 GeV, *m~b~* ≈ 4.5 to 5.0 GeV),
however, should greatly suppress this channel[^14], making heavy quark
production rates primarily sensitive to the dynamics of the early
partonic stage of the collision. This is especially true for the b
quark. Figure 2 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.

STAR and PHENIX have made preliminary measurements of charm production
in Au+Au, d+Au, and p+p collisions at RHIC. The PHENIX data for Au+Au
collisions suggests that the cross-section for open charm production is
consistent with the expectations of pQCD. These data were measured using
single electron spectra[^15], see Figure 3. The STAR results show that
the cross-section for open charm production in d+Au .vs. Au+Au is
consistent with binary collision scaling but the total yield may be
enhanced with respect to simple pQCD models. Both the p~T~ integrated
yield of dN/dy and the value of \<p~T~\> of the measured D-meson
spectrum are larger than the pQCD model predictions. The STAR data were
measured by doing a direct reconstruction of the open charm hadrons[^16]
as well as through single electron spectra.

![](media/image3.jpeg){width="3.5277777777777777in"
height="3.9930555555555554in"}

[]{#__RefHeading___Toc119665994 .anchor}Figure 2: 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. \[Error: Reference source not found\].

![](media/image4.png){width="4.330555555555556in"
height="3.1006944444444446in"}

[]{#__RefHeading___Toc119665995 .anchor}Figure 3: Total cc production
cross-sections per nucleon-nucleon collision vs. collision energy. The
dashed line depicts a PYTHIA calculation[^17]. 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[^18]. The
figure is adapted from Ref. \[Error: Reference source not found, Error:
Reference source not found\].

### The Need for Direct Topological Reconstruction of Open Charm

The first successful measurements of D-meson spectra at RHIC were seen
in d+Au collisionsError: Reference source not found by measuring single
electron spectra.  When direct measurements of heavy flavor hadrons are
not possible, non-photonic electrons from heavy flavor decays can be
used to study charm production but there are serious limitations in such
situations. As shown Batsouli, Kelly, Gyulassy and Nagle in Ref.
\[[^19]\], the decayed electron distributions are insensitive to the
intrinsic shape of the D-meson transverse momentum distribution. Due to
the decay kinematics and the light mass of the electrons and positrons,
the dynamical information in the primary spectrum is lost. This is shown
in Figure 4, where there is a clear difference between the zero mean
free path hydrodynamic flow prediction (solid) and the infinite mean
free path pQCD Pythia calculations (dashed-line) for D-mesons; but the
resulting electron spectra are indistinguishable. So two extremely
different models, one soft and the other hard, are indistinguishable
when observed with single electron spectra. In order to extract useful
information about heavy flavor production in heavy ion collisions, we
must measure the charm and beauty-hadrons by direct topological
reconstruction. Single electron spectra are not sufficient. The same
conclusion is reached when doing the analysis for R~AA~ of heavy flavors
because R~AA~ is a ratio of these data to the corresponding spectrum in
p-p collisions.

![](media/image5.jpeg){width="4.198611111111111in"
height="3.609027777777778in"}

[]{#__RefHeading___Toc119665996 .anchor}Figure 4: Solid- and
dashed-curves represent the charm- (red) and beauty-hadron (blue)
spectra from Blast-Wave and Pythia model calculations, respectively. The
corresponding heavy flavor decayed electron spectra are shown as black
curves. The data are the measured single electron distributions measured
in 10% central Au+Au collisions at 130 GeV by the PHENIX collaboration.
The figure is adapted from Ref. \[Error: Reference source not found\].

The nuclear modification factor, R~AA,~ for D-mesons is shown in Figure
5. Two vastly different models are shown. The figure shows that there
are large differences in  the two D-meson R~AA~ curves but the
corresponding decayed-electron R~AA~ curves  are essentially identical
the for all p~T~. These are theoretical curves without error bars;
additionally, an electron measurement comprises large systematic
uncertainties at lower momentum due to the large experimental background
from photonic electron productionError: Reference source not
found^,Error:\ Reference\ source\ not\ found^~.~ For a precise
measurement of charm quark flow, a direct reconstruction of D  π + K is
required. At even higher p~T~, electrons from B-meson decays will become
more abundant making the electron measurement for charm physics even
less realistic.  This clearly calls for a  direct measurement of heavy
flavor hadrons, i.e. D- and B-mesons.

In general, single electron spectra only provide an indirect measure of
the charm and bottom hadron yields because additional, secondary, decays
of charm and bottom 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 the bottom hadrons). Large systematic errors are associated
with the extraction of the charm and bottom quark yields because of the
large background subtraction required at low momentum, and the
assumptions that need to be made regarding the charm and bottom hadron
composition and spectra.

The proposed Heavy Flavor Tracker (HFT) will measure these spectra by
the direct toplogical reconstruction of the various charmed hadrons,
including the D^+^, D, D^0^, D~s~^+^, and possibly ~c~^+^. Thus the HFT
will enable us to overcome the systematic problems that are inherent in
single electron spectra.

![](media/image6.png){width="4.0in" height="2.8152777777777778in"}

[]{#__RefHeading___Toc119665997 .anchor}Figure 5: Nuclear modification
factor R~AA~ of D-mesons assuming the measured spectrum from d+Au
collisions scales as the light charged hadrons in central Au+Au
collisions (blue line, I) and from a hydro-dynamically inspired
parameterization assuming a collective flow velocity of \<~r~\> = 0.4*c*
for D-mesons (red line, II); the corresponding electron decay-spectra
are shown by the blue dashed (I) and red dashed (II) lines[^20].

## Probing Medium Thermalization: Charm Quark Re-interactions

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

Collectivity in heavy ion reactions is addressed by studying flow. Many
measurements of transverse radial and elliptic flow of identified
hadrons, containing light quark flavors (*q = u, d, s*), have been
performed at RHIC. The main conclusions from these studies are that the
systems created in ultra-relativistic Au+Au collisions exhibit strong
collective expansion. Compared to measurements at lower SPS energies,
the degree of collectivity is stronger. In addition, measurements of the
spectra of multi-strange hadrons ( and ) at RHIC suggest that they
freeze-out at a higher temperature and lower collective velocity than
the lighter hadrons; however a significant amount of elliptic flow is
also observed for multi-strange baryons. The elliptic flow of the
multi-strange baryons is comparable to the amount of flow observed for
the non-strange baryons and is in good accord with the number of
constituent quarks (ncq -- scaling) hypothesis that describes the
non-strange quark bearing mesons and baryons so well[^21]. These results
have been interpreted as an indication that sizeable partonic
collectivity develops at RHIC. Details of these studies can be found in
the literatureError: Reference source not
found^,^[^22]^,^[^23]^,^[^24]^,^[^25].

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

Theoretical calculations indicate that thermalization may be reached at
RHIC at temperatures T~eq~ 0.3-0.5 GeV[^26] and that the duration of the
equilibrium period is on the order of 5-10 fm/c[^27]^,^[^28]. Thermal
production of *cc* pairs is suppressed due to their large masses
(m~cc~ = 2.2 -- 2.3 GeV/*c*^2^), however, charmed hadrons may still be
produced in a thermalized fashion by gluon fusion and *qq* annihilation
if the *c* and *c* quarks become embedded in a thermalized bath of light
quarks. The relative probability of creating different charmed hadrons
will be driven by the properties of the medium which is providing the
additional quarks necessary to form the hadrons. The relative yield of
various charmed hadrons is thus sensitive to the collective properties
(temperature and chemical potentials) of the light quark medium and so
charmed hadron yields are something we want to measure.

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

The spectra of low mass particles, such as pions, are dominated by a
temperature parameter, while the spectra of heavier mass particles are
dominated by a 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, which is more
pronounced for heavier particles. Hence, the measurement of particles to
the lowest possible momentum is essential for spectral analysis in the
charm sector. Also, extended momentum coverage is necessary to reliably
extract integrated yields because you want to minimize the extrapolation
of the measured data points when doing the integration and because the
yields at low momentum are not well known theoretically.

Thermal motion and collective flow can only be distinguished in the
soft-hadron spectra by a thermal model analysis. Measured transverse-
mass spectra can be successfully fit with a hydrodynamically motivated
model such as the Blast Wave model with parameters for 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 parameters for kinetic thermal
equilibrium and is not sensitive to chemical equilibrium.

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

### Elliptic Flow

In non-central heavy ion collisions, the overlap of the colliding nuclei
is spatially deformed and has an ellipsoidal shape. The build-up of
pressure and pressure gradients cause collective motion of the
constituents of the medium if the matter produced in the reaction zone
re-scatters sufficiently, and in this way the initial state spatial
anisotropy can be transformed into a final state momentum space
anisotropy. For example, the largest momentum anisotropies are obtained
in the hydrodynamic limit[^29] where there is a zero length mean free
path and infinitely fast re-scatterings; this leads to instantaneous
local thermal equilibrium. Note that re-scattering is a sufficient
condition for the development of these anisotropies and thermalization
is not required.

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
a Fourier decomposition of the distribution in azimuth.

The largest contribution comes from the second Fourier coefficient
*v*~2~(*p*~T~, *y*), the *elliptic flow coefficient*. In the evolution
of elliptic flow, the strong and early spatial deformation decreases
because the matter begins to expand more rapidly in the direction of the
shorter axis of the ellipsoid[^30]. As the spatial deformation
disappears, the build-up of flow due to pressure gradients ceases and
the elliptic flow saturates. For this reason, elliptic flow is thought
to be a signal that develops in the early stages of a collision. RHIC
dataError: Reference source not found^,^[^31]^,^[^32] show that in
semi-central Au+Au collisions, the elliptic flow reaches the
hydrodynamic limit for transverse momenta up to 2 GeV/c and this
suggests very early thermalizationError: Reference source not found.
Thus, information about the equation of state[^33] can be determined. If
all hadron species experience the same anisotropic flow, their *v*~2~
coefficients should obey simple hydrodynamic relationsError: Reference
source not found and exhibit a characteristic mass dependence. As an
example, Figure 6 (top) shows the measured elliptic flow of strange
hadrons up to, and including, the multiply-strange cascade baryon. At
low momentum, all particles exhibit a linear rise in *v*~2~ and a clear
mass ordering appears; from the lower mass kaon to the heavier cascade.
The bottom plot in Figure 6 shows the measured elliptic flow versus
transverse momentum; where both axes are scaled with the number of
constituent quarks. Quark coalescence models predict a universal scaling
of *v*~2~/n versus *p*~T~/n at intermediate momentum where quark
recombination is (supposedly) the dominant hadron production mechanism.
In these models, collectivity -- the elliptic flow of constituent quarks
-- is intrinsically built in and supports the idea of partonic
collectivity. These analyses demonstrate that collectivity is
established at the partonic level.

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

[]{#__RefHeading___Toc119665998 .anchor}Figure 6: Elliptic flow of
strange hadrons at RHIC as measured by the STAR detector. The top panel
demonstrates typical hydro-dynamical mass ordering up to a momentum p~T~
\< 2GeV/c and saturation at larger momentum. The bottom panel shows the
scaling of elliptic flow with the number n (baryons, n=3; mesons, n= 2)
of constituent quarks in the saturation region.

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

Figure 7 shows the first indication of charm particle elliptic flow at
RHIC measured in the inclusive electron channelError: Reference source
not found^,Error:\ Reference\ source\ not\ found^. The predictions
assume elliptic flow for the light quarks as determined by fits to
experimental data. Presently, the data support the idea that the heavy
charm quarks flow. 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 charm quarks.

![](media/image8.jpeg){width="6.002777777777778in"
height="3.748611111111111in"}

[]{#__RefHeading___Toc119665999 .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[^34] and STAR[^35]. Only statistical errors
are shown for both results. The dashed-dotted-line represents the input
D-meson v~2~ distributions, assuming the scaling of constituent quarks
for meson v~2~. The corresponding decayed electron v~2~ are shown as the
hatched band[^36]. The dashed-line represents the results from
Greco[^37] et al. In this coalescence calculation, similar v~2~
distributions for c- and u-quarks are assumed. The figure is adopted
from Ref. \[Error: Reference source not found\].

### Charm Hadro-Chemistry

Hadronic yields and their ratios have been measured from AGS to RHIC
energies and have been successfully described by statistical
models[^38]^,^[^39]. 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[^40]. The extracted chemical freeze-out temperature T~ch~
is higher than the kinetic freeze-out temperature T~fo~ extracted from
momentum spectra[^41] and this suggests that chemical freeze-out occurs
before kinetic freeze-out, as expected.

It has been claimed, at the SPS and RHIC[^42], that for most particles
the chemical freeze-out temperature coincides with the critical
temperature T~c~ predicted from lattice QCD. Meson resonances with a
very wide width might be an exception[^43]. Obviously, the critical
temperature T~c~ serves as an upper limit for hadronic system but even
higher temperatures might be observed when measuring thermal spectra of
penetrating electro-magnetic probes such as dileptons and photons
because these probes emerge early, during the partonic phase of the
collision sequence.

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

These arguments lead to the idea of statistical hadronization of charm
quarks[^44]^,^[^45]. Statistical coalescence scenarios predict large
changes in open (regarding relative abundances, e.g. the D^0^ /
D~s~*^+^* ratio) and hidden charm production with a strong centrality
dependence[^46]. Statistical coalescence implies that charm quarks
travel over significant distances, e.g. in a QGP. Therefore, we believe
that a consistent description of precision data by these models would be
an indication of deconfinementError: Reference source not found.

Measuring the total charm and bottom yields requires measuring the
yields of several different hadrons. Indeed, charm quarks may fragment
into a variety of hadrons as shown in the first column of Table 1. These
fragmentation ratios have recently been compiled[^47]^,^[^48] using
*e-p* and *e^+^-e^-^* collision data. The ratios are found to be
independent of the collision energy and the collision system (e-p or
e^+^-e^-^). They are likely to be the same in p-p collisions at
mid-rapidity where the HFT can be used to detect charmed particles at
RHIC. Note that the ratios calculated using PYTHIA[^49], shown in the
second column of Table 1, differ significantly from the measured ratios.
To avoid any uncertainties in the charge yield measurement, these ratios
will have to be measured in *p+p* collisions at RHIC energies.

+-----------------+------------------+----------------+---------------+
|                 | *e*-*p* and      | Pythia         | Statistical   |
|                 | *e*^+^-*e*       |                | coalescence   |
|                 |                  |                |               |
|                 | average          |                |               |
+-----------------+------------------+----------------+---------------+
| (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___Toc119662936 .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 (see Ref. Error: Reference source not found). The calculation
assumes that charm quarks statistically coalesce with the lighter
quarks, (i.e. according to the temperature and chemical potential of the
light-quark system) so the temperature and chemical potential are set so
that the light hadron yields measured at RHIC are reproduced. In
addition, the number of charm quarks present in the system is set by
pQCD calculations and this is reflected by a charm-chemical potential in
the statistical coalescence model. The results of the model show that
statistical coalescence increases the yield of the D~s~*^+^* meson by
80% and the J/ 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 very sensitive probes of thermal
charm hadron production.

In terms of being able to measure these ratios, we are fortunate that
measuring the ratio of D~s~*^+^* to D*^+^* yields will cause most of the
systematic errors in the individual spectra to cancel out because both
are reconstructed in very similar decay channels: D*^+^* K π^+^ π^+^ and
D*~s~^+^* K π^+^ K^+^.

A precise measurement of the total charm production (mostly carried by
open charm mesons) will also serve as a baseline for J/
enhancement/suppression measurements suggested as a possible signature
for QGP formation.

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

If heavy quarks travel a significant distance through partonic matter,
then they become a tool to study the properties of that matter. Any
change in the charmed and bottom hadron properties in proton-proton or
d+Au as compared to heavy-ion collisions will be interesting. Possible
phenomena to look for include thermal and kinetic equilibration and the
subsequent statistical coalescence of the charm quarks. If this occurs,
it would be strong evidence for partonic matter formation since the
pion-charm hadron cross-section is small[^50]. In addition, exotic
phenomena such as the formation of charmed penta-quarks[^51] or an
enhancement of the B~c~ meson yield[^52] may also take place. But
perhaps one of the most fundamental measurements is the rate of energy
loss for a heavy quark in dense partonic matter.

Results reported at the recent QM05 conference on the nuclear
modification factor R~AA~ indicate that the rate of energy loss for
heavy-quarks in central Au+Au collisions is surprisingly similar to that
for the light-quarks (u, d, s). This experimental observation
contradicts our early understanding of the pQCD interactions of
energetic partons in a hot and dense medium where much less energy loss
was expected for heavy-quarks compared to the light
ones[^53]^,^[^54]^,^[^55]. Since there have been no directly
reconstructed heavy-quark hadron distributions from RHIC experiments so
far, non-photonic electrons in the transverse momentum range 4 \< p~T~
\< 10 GeV/c were used for these heavy-quark studies. When analyzing the
data with only no-photonic electron p~T~ distributions, the data are
complicated by an unknown mixture of charm and beauty contributions.
Therefore, the only way to disentangle these effects is by a direct
topological reconstruction of charmed-hadron distributions and a
measurement of charmed hadron angular correlations at RHIC.

### R~AA~ and Energy Loss 

The discovery of a factor of 4 to 5 suppression of high *p~T~* hadrons
(5 \< *p~T~* \< 10 GeV/c) produced in Au+Au collisions at RHIC and the
disappearance of the away-side jet has been interpreted as evidence for
jet quenchingError: Reference source not found^,^[^56]^,^[^57]^,^[^58].
This effect was predicted to occur due to radiative energy loss of high
energy partons that propagated through a dense and strongly interacting
medium[^59]. Heavy quarks are also predicted to lose energy as they
traverse the medium, however, their energy loss is predicted to be
significantly less because of a suppression of gluon radiation at angles
\< *M~Q~ /E* ; where *M~Q~* is the heavy quark mass and *E* is the heavy
quark energy[^60]. This kinematic effect is known as the "dead cone"
effect. The suppression of small angle radiation has the advantage that
the heavy quark fragmentation function and the spectrum of light
particles produced in association with the heavy quarks can be
calculated perturbatively.

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

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

[]{#__RefHeading___Toc119666000 .anchor}Figure 8: The ratio of
suppression factors in hot matter for charm (H) and light (L) quarks.
The solid line represents results from calculations with unrestricted
gluon radiation, while the dashed line is based on calculations with a
cut on gluon energies \> 0.5 GeV. The size of the static medium
traversed by the fast quark is assumed to be 5 fm. The figure is from
Ref. \[Error: Reference source not found\].

Figure 9 shows R~AA~ for non-photonic electrons[^61]^,^[^62]^,^[^63]
from STAR and PHENIX. The data extend up to p~T~ \~ 10 GeV/c. The figure
shows that the suppression factor for single electron spectra is in the
range of 0.2-0.3 which is almost exactly the same as the suppression
factor observed for charged hadrons and pions[^64]^,^[^65] (i.e. light
quarks). Using an unrealistically large initial gluon density of
dN~g~/dy = 3500, Djordjevic, Gyulassy, Vogt and Wicks have done a pQCD
calculation[^66], including gluon radiative energy-loss, which can
barely reproduce the electron R~AA~ (see the blue-line in Figure 9). The
model ignores the contributions from beauty-hadrons, and when beauty is
added to the model, the resulting prediction for R^AA^ does not fit the
data at all (see the yellow band in Figure 9 ); the model with beauty
over-predicts the data by a factor of 2-3. Note that for light-quark
hadrons, like pions, a gluon density of \~1000 has been used to
reproduce the observed R~AA~. These results raise a serious challenge to
our understanding of both the mechanism for heavy-quark production and
the mechanism for energy-loss in a hot and dense medium. In order to
resolve these important issues, we have to do direct topological
reconstruction of open charm. It is simply too risky to use single
electron spectra because the electrons from D-mesons are obscured by
electrons from bottom hadron decay and its decay kinematics.

![](media/image10.png){width="5.35625in" height="3.8159722222222223in"}

[]{#__RefHeading___Toc119666001 .anchor}Figure 9: Non-photonic electron
R~AA~ in central Au+Au collisions from STAR (open-symbols, top 5%) and
PHENIX (filled-symbols, top 10%) from Refs. \[Error: Reference source
not found,Error: Reference source not found,Error: Reference source not
found\]. Theoretical predictions for charmed-hadron and charm and
beauty-hadron decayed electrons are shown as blue- and yellow-bands,
respectively. In these calculations, the initial gluon density was
assumed to be 3500, Ref \[Error: Reference source not found\] .

### Charm Angular Correlations

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

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

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

[]{#__RefHeading___Toc119666002 .anchor}Figure 10: D-meson correlation
functions for 200 GeV p+p collisions. Default parameters in the Pythia
model were used in these calculations. A clear back-to-back correlation
in the angular distribution of charmed mesons is observed (shown by the
open circles). The Solid-line and the diamonds represent the results
with angular smearing for and, respectively.

Now lets consider a few potential observables. For example, when a
charm-quark interacts with the medium, it will lose energy and the
original angular correlation pattern, as shown in the figure, will be
modified. We have tried angular smearings of and. In addition, the
changes in the angular correlation depends on the nature of the
interaction. Most of the (semi)elastic scatterings are directed in a
narrow cone in the forward direction[^67]^,^[^68] while the inelastic
scatterings, such as the gluon radiative energy lossError: Reference
source not found will lead to a much wider smearing in the final
correlation. In the inelastic scattering scenarioError: Reference source
not found^,Error:\ Reference\ source\ not\ found^, the energy-loss
occurs deep inside the plasma and the final correlation function
reflects the hot/dense properties of the medium. On the other hand, the
resonant scattering happens near
T~cError:\ Reference\ source\ not\ found~. Although both scenarios lead
to sizable energy-loss, the angular correlations may allow us to
distinguish these two different mechanisms in high-energy nuclear
collisions. In order to perform the measurement, a large acceptance with
the reconstructed charmed hadrons is essential. The proposed HFT, plus
STAR TPC and TOF, will be necessary for this study.

Recently, the measurements on charm production by the Belle
Collaboration[^69] show a surprisingly large cross-section for J/
production in s = 10.6 GeV e^+^+e^-^ collisions. Even more surprising is
the fact that more than half of the observed J/'s are accompanied by cc
pairs. This result contradicts our current understanding for J/
production in the pQCD framework, such as those discussed in Refs.
\[Error: Reference source not found,[^70],[^71],[^72]\], and implies a
different production mechanism for heavy-quarks in elementary
collisions[^73]. As proposed in Ref. \[Error: Reference source not
found\], gluon fragmentation is increasingly important for collisions at
higher bombarding energies and so RHIC energies are very interesting. In
elementary collisions, the main difference between the new and
conventional processes lies in the angular correlation of the produced
charmed hadrons. With the proposed HFT and STAR EMC (Electro-Magnetic
Calorimeter), we will be able to study the correlation of D mesons to
further understand pQCD in p+p collisions. We will also study the
correlation of D mesons in high-energy nucleus-nucleus collisions, were
the gluon density is high[^74]^,^[^75], and this might enhance the
effects observed in elementary p+p collisions. These studies will
certainly shed light on the production mechanisms for charm and
charmonium at RHIC.

As mentioned earlier, heavy-quark production leads to a back to back
correlation between particle and anti-particles. This correlation is
also reflected in their decay products, such as the electron pairs. In
this case, it causes the background in the intermediate mass region[^76]
1 \< m~ee~ \< 3 GeV/c^2^ to have a correlation, too. (Here m~ee~ is the
invariant mass of the electron pair.) This creates a significant
background for a low mass vector meson analysis. Using the HFT to
measure the correlated electron pairs will greatly reduce the background
for vector meson and charm measurements via non-photonic electron
spectra.

### Baryon -- Meson Ratios

In the intermediate p~T~ region 2 \< p~T~ \< 6 GeV/c, light-quark baryon
yields are known to be enhanced compared to meson yields in Au+Au
collisions at RHIC. The enhancement is found to be proportional to the
collision centrality. These observations have been confirmed by both
R~AA~ and v~2~ measurements. The results are usually explained by a
hadronization mechanism involving collective multi-parton coalescence
rather than by independent vacuum fragmentation . The success of the
coalescence approach implies deconfinement and possibly thermalization
of the light quarks prior to hadronization. Since ~c~ is the lightest
charmed-baryon and its mass is not far from the other D-mesons, it will
be very interesting to measure the R~AA~ of ~c~ and compare it with the
R~AA~ of the other charmed mesons in order to see if there is a
meson-baryon difference. Theories about heavy quark deconfinement and
collectivity can be tested with these comparisons. In addition, due to
the different branching ratios for the semi-leptonic decays, the
measurement of ~c~ spectra will help us to understand the surprising
suppression observed in the non-photonic electrons. In this case, even
if charmed quark production scales with the number of binary collisions,
an increase in the ratio ~c~/D-mesons similar to that seen for the /Kaon
ratio will lead to a \~20% suppression in non-photonic electrons ( 1 \<
p~T~ \< 5 GeV/c ) for central Au+Au collisions[^77].

### B Jet Tagging

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

If an electromagnetic shower can be matched to a track that is
reconstructed in the TPC, it will be tagged as an electron or positron.
A significant background that lies under the reconstructed electron
signal comes from charged pions undergoing charge-exchange, i.e.
becoming neutral pions, in the calorimeter. The neutral pions decay
immediately into 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
these processes. The only measurable difference between an electron or
positron from D and B decay and a high transverse momentum pion,
producing an electromagnetic shower, is their spatial origin near the
vertex. High p~T~ charged pions will come from the primary vertex while
electrons and positrons from heavy meson decay will come from the B and
D decay vertex which are displaced from the primary vertex by a few
hundred m. Thus, these different decay locations can be selected by the
HFT.

The CDF collaboration has shown that B jet tagging can be achieved by
associating the electron or positron with one of the other charged
hadron decay products[^78]. Figure 11 shows a feasibility study where we
applied a topological technique to RHIC data, similar to what can be
done with the HFT, that imitates the CDF technique to remove background.
The channels they studied include
$B^{+} \rightarrow {\overline{D}}^{0} + e^{+} + \nu$ and
${\overline{D}}^{0} \rightarrow K^{+} + \pi^{-}$. The positron is
matched with either a kaon or a pion from the D meson decay, but only if
the distance of closest approach is less than 150 microns. The B decay
vertex is then located at the mid-point between both points of closest
approach on each helix. The variable shown in Figure 11 is the distance
between the B and the primary vertex as found by the HFT. The sign is
given by the scalar product of the electron momentum with the vector
constructed from the primary vertex to the reconstructed decay vertex.
The background is simulated by embedding high transverse momentum tracks
into events that match the momentum distribution measured in 200 GeV
central AuAu collisions below 3 GeV/c. The relative normalization
between the background and B decay distribution is arbitrary, since the
charge-exchange cross-section is not well known. By applying a
signed-decay-length cut of more than 250 m, the background is suppressed
by a factor 200 while keeping 60% of the signal.

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

![](media/image12.jpeg){width="3.9868055555555557in"
height="3.4972222222222222in"}

[]{#__RefHeading___Toc119666003 .anchor}Figure 11: Distance of the B^+^
decay position from the primary vertex. Full circles are from
high-momentum simulated B^+^ e^+^ + X decays, open circles show the
distribution of combinatorial background. The arrow indicates a cut at
250 m optimizing the signal to background ratio.

## Measuring Vector Mesons with Dileptons 

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

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

On the other hand, processes which produce photons and leptons are rare
and are overwhelmed by photons and leptons from electromagnetic decays
of hadrons and subsequent conversions to leptons. In addition, the final
state electrons and positrons are the integrated yield of radiation over
the whole evolution of the colliding system. Despite all the
experimental difficulties, the low and intermediate mass dileptons have
been measured at the CERN-SPS and an excess of radiation above the
hadron cocktail has been observed in the invariant mass region of 0.2 \<
M~ee~ \< 0.6 GeV/c^2^ in semi-central Pb+Au collisions[^79]^,^[^80].
Also, new and exciting results from the first three RHIC runs indicate
that jets have lost a large fraction of their energy in dense matter and
the hadronic phase is relatively short. We believe that these
observations at RHIC favor experimental measurements of photons and
dileptons due to thermal radiation and will result in a clearer
signature than from lower energy heavy ion collisions.

At STAR, electron identification is made possible by a combination of
two measurements: a measurement of the energy lost by charged particles
due to ionization in the TPC gas (dE/dx), and a velocity measurement
with the time-of-flight system. The relativistic rise of the electron
dE/dx separates the electron dE/dx from those of hadrons except at the
crossovers with pions at a momentum of \~ 0.2 GeV/c, kaons at \~ 0.6
GeV/c, protons at \~ 1.1 GeV/c and deuterons at \~ 1.5 GeV/c. A
time-of-flight measurement, using a requirement that \|1-\| \< 0.03,
eliminates slow hadrons and cleans up the crossovers. This results in
clean electron identification, as shown in Figure 12. The top plot on
the left shows the conventional dE/dx measurement in the TPC gas as a
function of particle momentum. The bottom plot on the left shows the
same dE/dx measurement but now requiring \|1-\| \< 0.03 from the TOF
detector. The upper electron band is clearly separated from the lower
pion band. Heavier particles (e.g. kaons and protons) are completely
removed by the TOF requirement. The right hand plot of Figure 12 shows a
dE/dx projection in the transverse momentum range p~T~ = 1.0-1.5 GeV/c.
Even at this rather large momentum, the distribution of electrons is
clearly separated from the distribution of charged hadrons.

![](media/image13.jpeg){width="2.7944444444444443in"
height="2.7180555555555554in"}
![](media/image14.jpeg){width="2.9298611111111112in"
height="2.7055555555555557in"}

[]{#__RefHeading___Toc119666004 .anchor}Figure 12: Left top: TPC dE/dx
vs p in d+Au collisions. Left bottom: TPC dE/dx vs p after TOFr PID
selection of \| 1- \| \< 0.03. Clean electron identification is
achieved. Right: dE/dx from the TPC after TOFr PID selection for 1.0 \<
p \< 1.5 GeV/c.

These electrons are, to a large extent, from photons converting into
electron-positron pairs e^+ ^e^-^ in the detector material. The HFT
detector will reduce the background of electrons and positrons from
these conversions. By requiring hits in the HFT, electrons from photon
conversion outside the HFT, i.e. the SSD, and TPC inner field cage, are
rejected. Figure 13 shows the resulting yield from pure Monte Carlo
simulations (no tracking involved) of conversion electrons when
requiring hits in the TPC only (open circles), hits in the TPC and hits
in the TPC+SSD+HFT (closed circles). The requirement of hits in the HFT
reduces the detected yield of conversion electrons by a factor of 50.
Full simulations with appropriate pile-up events show that the rate of
random ghost tracks is 5%. This means that we can achieve a maximum
rejection by a factor of 20. To estimate the signal-to-background ratio
in the vector meson measurements, we take a reasonably conservative
approach and assume that the HFT can reject conversions by a factor of
10. Another source of electron background comes from the semi-leptonic
decays of heavy quark hadrons. The dominant source at intermediate
mass[^81]^,^[^82] is from the semi-leptonic decay of open charm. Due to
the large charm yield at RHIC energies, the number of e^+^e^-^ pairs
from charm-anti-charm decays is comparable to that from conversion and
and Dalitz decays after the HFT rejection factor, described above, is
applied. Due to the lifetime of \~ 100 m of heavy-flavor hadrons, this
charm semi-leptonic decay background can be greatly reduced by measuring
the displaced decay vertex with high resolution as can be provided by
the HFT detector. Detailed simulations show that HFT is capable of
rejecting 75% of e^+^e^-^ pairs from the D^0^ decay while preserving 50%
of the primordial e^+^e^-^ pairs. This increases the
signal-to-background ratio by a factor of 2.

![](media/image15.jpeg){width="5.000694444444444in"
height="3.410416666666667in"}

[]{#__RefHeading___Toc119666005 .anchor}Figure 13: 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
HFT hit is 50.

The large reduction in photonic-electron background will enable us to
observe electromagnetic decays of short-lived vector meson, e.g. ,
*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
by a factor of 3 (single track) can be achieved by measuring both
electrons, of a pair, which is possible because of the large acceptance
of the STAR TPC. Table 2 shows our estimate of central Au+Au collisions
needed in order to observe a signal with a statistical significance of
3. Compared to measurements incorporating only the TPC and TOF, the HFT
will enhance the sensitivity to electro-magnetic vector meson decay by a
factor 20-40.

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

  TPC+TOF                                     8M              2M

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

[]{#__RefHeading___Toc119662937 .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.

Figure 14 shows the di-electron invariant mass distribution for various
signals plus the background. The signals for vector mesons and thermal
radiation, are shown as a black curveError: Reference source not found
and have been calculated using the full acceptance of the STAR detector.
The red curve, at the top, is the total di-electron invariant mass yield
using the STAR configuration in 2004. This is obtained from single
inclusive electron spectrum measurements in 200 Au+Au collisions with
the assumption of electron PID from Full TOF coverage. The pink dashed
line is the e^+^e^-^ pair invariant mass spectrum for charm
semi-leptonic decays derived from the non-photonic single electron
spectra measured in Au+Au collisions. The gray curve is the e^+^e^-^
pair invariant mass distribution for charm after a cut on the HFT DCA \<
80m. The dot dashed line show the Dalitz decays from and after rejection
from the TPC. The standard method of dealing with the residual
background is by a mixed-event method. This method has been used in
CERES and NA52/NA60 at the SPS, and will be used in both PHENIX and
STAR.

![](media/image16.jpeg){width="5.686111111111111in"
height="4.0784722222222225in"}

[]{#__RefHeading___Toc119666006 .anchor}Figure 14: Di-electron invariant
mass distribution. The solid black curve is a prediction\
from R. Ralf Ref. \[Error: Reference source not found using the full
STAR acceptance. The red curve at the top is the total\
di-electron invariant mass seen with the 2004 configuration of STAR but
assuming electron PID from Full TOF coverage. The pink dashed line is
the e+e- pair invariant mass from charm semi-leptonic decays. The gray
curve is the charm e+e- pair invariant mass after cut on the HFT DCA \<
80um. The dot dashed line is the Dalitz decays from 0 and after
rejection from TPC.

## Spin Measurements

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

The main goal of the RHIC spin program[^83] is to precisely determine G
by measuring double spin asymmetries in longitudinally polarized
proton-proton collisions for a series of complementary reaction channels
and over a broad range of the gluon momentum fraction x~g~ and
transverse momenta p~T~. One of the promising reaction channels in STAR
is the production of prompt photons and jets with high transverse
momenta.

The production of charm and bottom quarks proceeds, in leading order in
the strong coupling constant ~s~, through gluon-gluon fusion and quark
anti-quark annihilation. The gluon-gluon fusion process is known to be
dominant in the spin averaged case for all experimentally relevant
kinematic regions. In the spin dependent case both processes have large
analyzing powers, as measured by the parton-parton spin asymmetry a~LL~,
which approaches -1 for asymptotically large transverse momenta. The
masses of the heavy quarks lead to large modifications of the
parton-parton spin asymmetry in the kinematic region accessible with
STAR[^84]. Next-to-leading order corrections are available and predict
relatively sizable changes in the observed asymmetry as compared to
leading order expectations[^85].

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

As is the case for the study of heavy quark energy loss described in
this proposal, the relative smallness of the production cross section at
large transverse momenta requires triggering with the STAR
electromagnetic calorimeters. In spin measurements, the trigger serves
to relate the event to the RHIC beam bunch crossing, which determines
the spin orientations of the colliding protons. Commonly used
parameterizations of the deep-inelastic scattering data[^86] lead us to
expect measurable, and negative, double longitudinal spin asymmetries at
the level of --5\*10^-3^ in heavy flavor production in STAR. These
asymmetries originate from gluon polarization at intermediate momentum
fractions x~g~ and are expected to grow in size with increasing p~T~ of
the decay electron or positron.

In order to make these measurements, fast external pointing with
high-rate capability is required to distinguish collisions from
different bunch crossings and, hence, different spin orientations.

[]{#__RefHeading___Toc119662839 .anchor}HFT Conceptual Design and
Performance

## Overview of the Experiment

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

![](media/image17.jpeg){width="6.0in" height="3.772222222222222in"}

[]{#__RefHeading___Toc119666007 .anchor}Figure 15: The STAR detector at
RHIC. The primary detector elements at mid-rapidity are shown in this
cut-away view of the detector. For scale, the TPC is 4.2 meters long and
4 meters in diameter. Heavy Ion beams enter from the left and right
while collisions take place at the centerline under the Silicon
trackers.

The important tracking elements at mid-rapidity will be the Silicon
Strip Detector (SSD) and the Time Projection Chamber (TPC). The SSD is
installed at a radius of 23 cm around the center of the beam pipe while
the TPC tracking begins at a radius of approximately 60 cm. The TPC
tracking system extends out to a radius of 190 cm and yields a total of
45 position measurements along the track. At the time of HFT
installation, a complete Time of Flight detector will surround the TPC
barrel ( = 2). This detector will replace the Central Trigger Barrel
(CTB) which is an array of scintillators. The TOF surrounds the outer
field cage of the TPC. By the time the HFT is installed in STAR, the
existing Silicon Vertex Detector (SVT), will be decommissioned.

The goal of the STAR detector is to start measuring a particle's
trajectory as close as possible to the interaction vertex and then to
follow it continuously to a large radius as it traverses a uniform
magnetic field up to 0.5 T. The STAR tracking philosophy, on the other
hand, is to start from the outside where the hit density is lowest and
to follow the track inwards. This design allows us to measure the
collision vertex with great precision. Using this information we can
identify a short-lived particle whose decay is displaced from the vertex
by a significant distance. The *K~S~*^0^ is one example. It decays into
two pions with a mean lifetime of 0.896 10^-10^ seconds and it has a
characteristic decay distance of 2.68 cm. It is relatively easy to
identify this non-vertex decay using the TPC, alone, because the TPC has
a vertex pointing accuracy of approximately 5 mm.

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

The HFT improves upon these capabilities even further because it will be
placed between the beam pipe and the SSD detector. It will have two
layers of silicon pixel detectors; one at 1.5 cm radius and the other at
5 cm radius. Each layer will be composed of 30 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 16. The innermost layer of the detector
lies at a mean radius of 1.5 cm radius and it is composed of 6 detector
ladders. The outer layer lies at a mean radius of 5 cm and it is
composed of 18 ladders. Each ladder contains a row of 10 monolithic CMOS
detector chips and each ladder has an active area of 19.2 cm 1.92 cm.
Each CMOS detector chip contains a 640 640 array of 30 m square pixels.
We will thin down each chip to a total thickness of 50 m to minimize the
multiple coulomb scattering (MCS) between layers of the detector. The
HFT is designed to be as close as possible to the interaction point.
This will require a new, smaller radius, beam pipe. The beam pipe must
also be made thinner (0.5 mm) to reduce scattering in front of the HFT.
This smaller, thinner, beam pipe is an essential part of the proposal
and is discussed in section 5.6.

  ---------------------------------------- ------------------------------
  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___Toc119662938 .anchor}Table 3: Selected HFT
parameters.

![](media/image18.jpeg){width="3.0770833333333334in"
height="2.6506944444444445in"} ![](media/image19.jpeg){width="2.65625in"
height="2.6013888888888888in"}

[]{#__RefHeading___Toc119666008 .anchor}Figure 16: Left - Schematic view
of the HFT detector ladder arrangement with 6 ladders forming the inner
layer and 18 ladders forming the outer layer. The width of the ladders
(the dimension seen in this view) is 19 mm. The shingled layers provide
cross calibration of the ladders and provide an open geometry for ease
of installation. Right - The HFT is shown embedded inside the SVT+SSD.

## Simulated HFT Performance

The performance of the HFT has been studied using tracks from √sS~NN~ =
200 GeV central Au+Au collisions generated by the "MEVSIM" code. We have
also studied the performance of the detector by using single particle
tracks. The simulated tracks and simulated events were processed through
GEANT and, finally, a detailed response function for the Time Projection
chamber (TPC) and a response function for the Silicon Drift Detector
(SSD) were applied to the data. These response functions carefully take
into account physical effects in the detector and the subsequent
electronics. For the HFT, we smeared the Monte Carlo hits by a Gaussian
function with a width of =6 m in the x-y-plane and in the z-direction.
This simulates the finite hit resolution we expect for the HFT. The
limited read-out speed of the 1^st^ generation HFT will lead to pile-up
of up to 120 Au+Au collisions in the HFT at four times the RHIC design
luminosity. We account for that by randomly uploading hits on top of the
Monte Carlo generated hits in the HFT. The topic of pile-up is discussed
in detail in section 2.10. Tracks with at least 15 hits in the TPC, 1
hit in the SSD and 2 hits in the HFT detector were reconstructed with
the STAR inner-tracker code. Figure 17 shows the efficiency for tracking
pions with \|*y*\| \< 0.5 in various detector configurations. The TPC
results are designated by the open circles (red), TPC+SSD by triangles
(green), and the TPC+SSD+HFT detector by filled dots (blue). Note that
the TPC tracking efficiency limits the efficiency of the SSD at low
transverse momentum and, in addition, the efficiency of the HFT at low
momentum is affected by ghost tracks and event pileup. But at high
momentum, the acceptance of the detectors is quite uniform and the
overall efficiency for the HFT tracking system is 50% or above at a
transverse momentum larger than 500 MeV/c. Simulations using
mono-energetic pions show that the efficiency stays approximately
constant up to 20 GeV/c.

![](media/image20.jpeg){width="4.94375in" height="3.4965277777777777in"}

[]{#__RefHeading___Toc119666009 .anchor}Figure 17: 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 ms. So
we have studied the influence of the ghost tracks in the HFT detector by
using simulated central Au+Au collisions in the TPC+SSD+SVT+HFT. At low
momentum, ghost tracks dominate due to multiple Coulomb scattering.

![](media/image21.jpeg){width="5.695138888888889in"
height="1.9597222222222221in"}

[]{#__RefHeading___Toc119666010 .anchor}Figure 18: 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 18 shows the number of ghost tracks observed in the full HFT
tracking system. The bulk of the ghost tracks appear below 0.5 GeV/*c*
and the effect of these miss-identified tracks can be seen in Figure 17;
the tracking efficiency for the HFT is steeply dropping below 0.5
GeV/*c*. The ratio of ghost tracks to good tracks is 10% at 0.5 GeV/*c*
and saturates at 4% above 2 GeV/*c* (see the right hand panel of Figure
18).

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

![](media/image22.wmf)

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

![](media/image23.wmf){width="2.9319444444444445in"
height="3.9972222222222222in"}

[]{#__RefHeading___Toc119666011 .anchor}Figure 19: Probability of
getting the correct hit on the outer HFT 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+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^+^ and
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 and the rapidity
distribution suggested by perturbative QCD calculations applying the
program code PythiaError: Reference source not found. The background is
simulated using the MevSim event generator parameterized to reproduce
the experimentally measured particle multiplicities in Au+Au collisions
at √s~NN~ = 200 GeV. Our parameterization is accurate for particles
below 3 GeV/c. It may underestimate the background above this momentum.
The distributions of reconstructed D-meson signal and background were
scaled to match the expected D-meson production per central Au+Au
collisionError: Reference source not found. Also, the higher track
reconstruction efficiency in single (D^0^ signal) events compared to
central Au +Au collisions (background) was taken into account.

The HFT detector allows us to identify a D^0^ decay-vertex by
reconstructing the trajectory of its two daughters. The following
selection criteria were used to separate the D^0^ signal from
background, also see Figure 20.

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

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

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

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

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

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

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

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

  DCA~K~                                 \< 50 m

  cos()                                  \< 5.0 degrees

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

[]{#__RefHeading___Toc119662939 .anchor}Table 4: Cuts applied for
D-meson reconstruction

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

![](media/image1.jpeg){width="4.933333333333334in" height="4.00625in"}

[]{#__RefHeading___Toc119666012 .anchor}Figure 20: 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 21. 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; however, 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 sensor similar to those used in the ALICE detector
(3 times thicker) results in the D^0^ reconstruction efficiency dropping
by a factor 8, independent of momentum. This would require an 8 times
longer data taking period.. As a result, we have exclude the ALICE
detector technology from consideration.

![](media/image24.jpeg){width="5.001388888888889in"
height="3.3895833333333334in"}

[]{#__RefHeading___Toc119666013 .anchor}Figure 21: The absolute yield of
D^0^ into the TPC+SSD+HFT divided by the input D^0^ yield. In the SVT-1
configuration, the innermost layer of the SVT has been removed in order
to allow more room for the HFT. And finally, Tthe Alice geometry assumes
thick Si detectors that are similar to the wafers used in the ALICE
vertex detector. The D^0^ reconstruction cuts were optimized using the
program MINUIT.

We have studied the influence of different HFT hit resolutions and Si
ladder thicknesses. As a figure of merit we use the number of central
Au+Au collisions required to measure a D^0^ signal with a statistical
significance of 3. Figure 22 shows the number of central Au+Au
collisions needed to make the D^0^ measurement as a function of the HFT
hit resolution for different Si ladder thicknesses. The curve is almost
flat between 0 and 10 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 micron Si-equivalent (0.3% X~0~) and with these
assumptions we present projected statistical errors on D-meson spectra
and elliptic flow.

![](media/image25.jpeg){width="2.953472222222222in"
height="2.152083333333333in"}![](media/image26.jpeg){width="2.953472222222222in"
height="2.152083333333333in"}

[]{#__RefHeading___Toc119666014 .anchor}Figure 22: The number of central
Au+Au events needed as a function of the Si ladder thickness and the
position resolution of the detector (left). Results from Monte Carlo
Simulations and full tracking in all sub-detectors including TOF and the
proposed HFT (right).

For the first year of data taking with the HFT, we assume a total of 50
M central Au+Au collisions at 200 GeV. (A similar number of events were
taken by STAR during the RHIC Run IV in 2004.) Figure 23 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.

![](media/image27.jpeg){width="2.9611111111111112in"
height="2.0708333333333333in"}![](media/image28.jpeg){width="2.9611111111111112in"
height="2.0708333333333333in"}

[]{#__RefHeading___Toc119666015 .anchor}Figure 23: 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~^+^ + and K^+^ + K^-^. Here the
goal was to measure the D~s~ yield and integrate over all p~T~ to
extract the dN/dy for hadro-chemistry studies. The D~s~^+^ has a
lifetime of \~ 100 m while the -meson decays immediately at the
secondary decay vertex. Table 5 lists the cuts used in our studies.
These cuts have been optimized using the program MINUIT analogous to the
D^0^ studies.

  ------------------------------ ----------------------------------------
  *l*                            \> 200 m

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

  *m*,                           20 MeV/*c*^2^

  *DCA*                          \< 60 m

  cos()                          \< 5.7 degrees

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

[]{#__RefHeading___Toc119662940 .anchor}Table 5: **Cuts applied for
D~s~^+^ reconstruction**

The acceptance for D~s~ reconstruction is shown in Figure 24. Due to the
large background in a 3-body decay reconstruction, Strange D-mesons can
be detected only above 1.0 GeV/c with a saturation in efficiency of 2%
starting at 4 GeV/c. The rather low efficiency compared to D^0^ is also
due to the larger acceptance losses of the three daughter particles.

![](media/image29.jpeg){width="4.236805555555556in"
height="2.877083333333333in"}

[]{#__RefHeading___Toc119666016 .anchor}Figure 24: Ds reconstruction
efficiency as a function of transverse momentum

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

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 25 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 for
full charm quark flow and the lower line is for non charm-quark flow.
The differences for the two scenarios are on the order of a factor two,
while the projected statistical uncertainties are expected to be smaller
than 10% in the momentum region 1-3GeV/c. It is evident that after a
year of data taking that the charmed quark flow measurement can be fully
addressed.

![](media/image30.jpeg){width="5.781944444444444in"
height="3.5854166666666667in"}

[]{#__RefHeading___Toc119666017 .anchor}Figure 25: Flow results if
charmed quarks flow, or do not flow.

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

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

To set the scale of the problem, consider the extraneous hit density
from normal interactions. The parameters used in the calculation are
shown in Table 6 and an estimate of the hit density is shown in Table 7.

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

  dN/d (min bias)                               170

  Min bias cross section                        10 barns

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

[]{#__RefHeading___Toc119662941 .anchor}Table 6: Luminosity and other
parameters that determine the particle flux on the HFT.

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

  Radius                                5 cm              1.5 cm

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

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

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

  Probability of Tracking Window Pileup 10 %              10 %

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

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

[]{#__RefHeading___Toc119662942 .anchor}Table 7: Integrated hit loading
on the HFT and associated pileup.

As shown in the table, the accumulated hit density from the integrated
Au+Au collisions is 17/cm^2^ in the outer layer and 72/cm^2^ in the
inner layer of the HFT[^87]. This produces a pileup probability in the
track search window of 10% at both the inner and outer HFT barrels.. The
area of the track search window in this case is based on the projection
resolution for 1 GeV/c pions as determined from GEANT simulations. For
comparison the unresolved hit pileup intrinsic to the HFT detector is
much less, 0.14% and 0.58%,%, for the outer and inner barrel
respectively. This meets our specifications.

The luminosity applied in this case is the maximum instantaneous
luminosity that was recorded during the Au+Au run in 2004 (110^27^
cm^-2^s^-1^). This is 5 times the nominal "design luminosity". (Design
luminosity is defined to be 2 10^26^ cm^-2^s^-1^ averaged over a 10 hour
fill. And for future reference, an instantaneous luminosity of 210^26^
cm^-2^s^-1^ is equivalent to an instantaneous ZDC coincidence rate of
2000 Hz.).) It should be noted that the RHIC beam luminosity starts high
and drops with time over the course of a fill. During the RHIC II era,
the most important improvements in RHIC luminosity will be longer decay
times during the fill, but we also expect an increase in peak luminosity
by about a factor of 4. This is important, and so under RHIC II
conditions we will have to improve the readout time of the HFT in order
to limit pileup in the tracking search window to about 10%.

Table 8 gives the same loading information resulting from a single
central Au+Au collisionError: Reference source not found. The numbers
for a single collision is a factor of 10 less then the integrated load,
which demonstrates that an isolated collision is easily tracked by the
HFT.

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

  Radius                                 5 cm            1.5 cm

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

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

  Probability of Tracking Window Pileup  1%              1.1%

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

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

[]{#__RefHeading___Toc119662943 .anchor}Table 8: 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 events[^88]. This measurement was done at B =
0.25 T, half the normal STAR magnet field, to open the acceptance to
lower momentum particles. It would be desirable to use SVT events with a
zero bias trigger to have a measure of other background sources such as
beam gas showers, but so far our noise suppression filters for the SVT
are not sufficient for this task.

Although we do not have a direct measure of beam showering, we do know
from studies of space charge distortions in the STAR TPC that at larger
radii, 65 cm to 200 cm, the ionization density due to normal Au+Au
interactions is larger than other contributions such as beam gas[^89].

# CMOS Sensors

## Introduction

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

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

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

## 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[^90] (See Figure
26). 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.

![](media/image31.jpeg){width="4.5in" height="3.4347222222222222in"}

[]{#__RefHeading___Toc119666018 .anchor}Figure 26: 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 ability of these sensors to provide charged particle tracking is now
well established[^91] and several prototypes, called MIMOSA[^92], have
been built by the IReS group in Strasbourg. The LBL/UCI group has also
built and successfully tested these sensors. The prototype detectors
have explored different fabrication processes and key parameters of the
charge sensing system, and the results demonstrated that a detection
efficiency of \~ 99% and a single point resolution of \~2 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. We have also shown that
results from 20 µm can be extrapolated to 30 µm -- the proposed pitch of
the HFT.

The radiation tolerance of the sensors to bulk damage[^93] was also
investigated. No significant performance loss was observed up to
fluences close to 10^12^ n~eq~ cm^-2^. As far as ionizing radiation
damage is concerned, the real potential of this technology is still
being explored, but it is already established that it stands up to more
than 100 kRad.

Most of the R&D at IReS was performed with small prototypes (a few
mm^2^) containing a few thousand pixels. A full size prototype (i.e.
\~3.5 cm^2^) called MIMOSA-5 was also built, see Figure 27. It is
composed of \~ 1 million pixels per chip, and it was fabricated on a 6
inch wafer, as shown in Figure 27. 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/image32.jpeg){width="2.717361111111111in"
height="1.9909722222222221in"}![](media/image33.jpeg){width="3.2152777777777777in"
height="1.9868055555555555in"}

[]{#__RefHeading___Toc119666019 .anchor}Figure 27: Wafer of reticle size
sensors (left) and zoomed-in view of individual chips (right).

Table 9 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 2 cm^2^            1.9 1.7 cm^2^

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

[]{#__RefHeading___Toc119662944 .anchor}Table 9: 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: 640 640, i.e. active area of 19.2 19.2 mm^2^.

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

-   Chip dimension: \~ 21 20 mm^2^.

-   Additional details can be found in Ref. \[[^94]\].

Since the read-out speed requirement is modest, the chip's architecture
is based on relatively slow signal processing at the pixel level. This
feature favors moderately low noise and modest power consumption. On the
other hand, high speed signal processing is needed at the chip level
(i.e. after amplification and multiplexing) due to the need for short
integration times and limited shot noise. In order to achieve these
goals, the chip is subdivided into 10 groups of 64 640 pixels. Each
group is split into two equal size sub-groups, and each of these
sub-groups is connected to a common amplifier, which is placed on the
outer edge of the chip. The 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. As a result, the chip read-out time is approximately 8 ms.

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

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

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 tests of the chip's performance parameters are
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 there will be at
least one more prototype run before trying to produce the final chips to
be used in the HFT.

Meanwhile, several fabrication processes will be explored, aiming to
find the process providing the smallest leakage current. In general,
several parameters underlying the sensor performance parameters depend
on features specific to each fabrication process and so the process
specific characteristics of the chips need to be explored in parallel
with the development of the chip architecture. For instance, a new
fabrication technology, relying on a lightly doped substrate but
exhibiting no epitaxial layer was investigated with two different
prototypes chips. The IReS tests show that a detection efficiency of
99.9% can be achieved, as well as a single point resolution of about 2.5
m[^95]. A major advantage of this technology is that a large signal can
be generated because the charges are collected from several tens of
microns of Si instead of from \~ 10 µm in an epitaxial layer. This makes
it well suited to applications where one experiences substantial
electronic noise.

### Thinning

Up to now, MIMOSA-5 chips have been thinned down to 120 m thickness.
This operation was successful and we have not found any degradation of
the sensor parameters. In the HFT design, the MIMOSTAR thickness will be
50 m. Thinning a 6 or 8 inch wafer down to this thickness is not
expected to be a problem since the process is an industry standard
technique. We have demonstrated that 50 µm sensors are mechanically
stable and can be assembled into ladders. We have used low-yield
MIMOSA-5 wafers to make these investigations and have successfully
thinned detectors to 50 µm[^96]. Testing is in progress.

## 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. \[[^97]\].

Starting with a IReS design, the LBNL/UCI group has built several
generations of APS sensors. These devices were built in the 0.25 µm
process at TSMC. These ICs have been tested using different sources,
i.e. ^55^Fe, 1.5 GeV accelerator electrons and a scanning electron
microscope. Figure 28 shows one of the sensors that has 16 different
test structures.

![](media/image34.jpeg){width="3.011111111111111in"
height="2.765277777777778in"}

[]{#__RefHeading___Toc119666020 .anchor}Figure 28: An APS Sensors
developed by the LBNL/UIC group. The picture shows 16 separate test
structures. Each structure has a 36 x 36 array of 20 µm pixels.

Our measurements have confirmed the IReS results that APS sensors can
measure charged particles with excellent spatial
resolution[^98]^,^[^99]**.** Figure 29 shows several 1.5 GeV electrons
recorded at LBNL's Advanced Light Source.

![](media/image35.jpeg){width="4.013194444444444in"
height="3.8444444444444446in"}

[]{#__RefHeading___Toc119666021 .anchor}Figure 29: This graph shows the
results of one event taken with 1.5 GeV electrons. Each bin represents
one pixel and the height is proportional to the measured charge. Several
electrons can be identified in the plot.

To study the effects of radiation, we have exposed the chips to protons
at LBNL's 88" Cyclotron. Afterwards, we measured the change in leakage
current and pulse height. These results show that there was a modest
change with an irradiation of 300 krad. What was most interesting is
that after 6 months, the device self annealed. Therefore, the effect of
gradual radiation is much less than that of an acute exposure[^100].
These radiation measurements are complementary to the neutron exposures
measured by the IReS group.

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

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

We can increase the fraction of charge collected by a single pixel using
the photo-gate technology. In principle, this technology allows us to
use a large area photo-gate for charge collection, while not increasing
the capacitance, because the charge is collected through a small size
gate. Even though we have demonstrated the sensitivity of the photo-gate
structure to X-rays, we have not achieved the expected performance of
the device. The transfer time required for moving electrons from the
photo-gate to the drain appears to be very long (several ms). We believe
this undesirable signal delay is probably consistent with surface traps
at the Si0~2~ silicon boundary. Studies to find a way to avoid this
delay are in an SBIR proposal that is currently being considered.

Correlated double sampling (CDS) is a standard technique that is used to
remove the fixed pattern noise introduced by the reset transistor. Its
main drawback is to require reading out at least two frames, hence
doubling the power consumption. To avoid doing CDS, we have produced a
clamp circuit[^101] that reduces the reset noise by a factor of 3**.**

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

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

We have also been collaborating with colleagues at UC San Diego on the
suitability of using an APS sensor in an electron microscope[^102]. We
have been studying the response of electrons from energies of 100 keV to
300 keV. Results show that single electrons can be detected with a good
single to noise ratio. This technique is well suited for this
application overcoming problems with traditional CCDs that suffer
radiation damage. Using APS devices are very appealing for this
different type of application.

We have several ideas on how to further improve our device. We have
fabricated a sensor with different diode sizes. We plan to study this
device and understand the effect of diode size on signal to noise. We
will soon be 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 MIMOSTAR-2 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 still speculative. These R&D
studies will be done concurrently with the construction of the HFT
detector. If any of these ideas leads to significant improvements, then
they can be added to a second-generation vertex detector.

# Data Acquisition

## Requirements

The data acquisition system is designed to read out the large body of
data from the individual sensors, to digitize the signals, to perform
data compression, and to deliver the sparsified data to an event
building and storage device. A summary of the requirements is provided
in Table 10.

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

  Number of pixels per chip                            640 640

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

  Readout time per frame                               4 ms

  Dynamic range of the ADC                             10 bits

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

  Fixed pattern noise                                  2000 *e*

  Noise after Correlated Double Sampling               10 *e*

  Maximum signal                                       900 *e*

  Dynamic range after Correlated Double Sampling       8 bits

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

[]{#__RefHeading___Toc119662945 .anchor}Table 10: Requirement Summary -
constraints for the APS, due to signal expectation, available design
process, and mechanics

Digitizing the analog signal on each pixel into a 10 bit digital signal
yields approximately 1 Gb/s per sensor chip when read out in 4 ms. Thus,
the total front end data rate is \~240 Gb/s. Clearly the volume of data
must be reduced before being handled DAQ event builder and written to
storage.

Data compression is achieved by performing correlated double sampling
(CDS) and then leakage current subtraction, i.e. subtraction of two
consecutive frames, 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 MIMOSA-5
chip to be 2000 electrons. The noise remaining after CDS must be on the
order of 10 *e* to guarantee an efficiency of greater than 99%. The
maximum signal is estimated from dE/dx calculation and by measuring how
the charge spreads over pixels. The signal can be truncated above 900
*e* without compromising either the efficiency or the position
resolution, so 8 bits is a sufficient dynamic range for signal storage.
Hit finding and the reduction of the data to addressed clusters then
reduces the data to a manageable rate. The use of forced air-cooling on
the low mass region of the ladder limits the total power dissipation to
about 100 - 150 mW/cm^2^.

## Architecture

A sketch of the readout architecture and a description of the data flow
is shown in Figure 30. Figures 30 and 31 show a schematic representation
of the ladder assembly and the HFT motherboard / daughter card
assemblies. The basic flow of a ladder data path starts with the APS
sensors. Analog data is carried from the two 50 MHz outputs in each
sensor in parallel on the low mass cable to discrete electronics at the
end of the ladder and out of the low mass detector region. This
electronics performs current to voltage conversion and drives the
signals in parallel over a short (1 m) twisted pair cable to the
motherboard. The daughter cards on the motherboard perform analog to
data conversion of the data, correlated double sampling, and pedestal
subtraction and hot pixel removal.. The data is then transferred to the
motherboard for hit finding and data reduction. The reduced data is then
buffered and transferred to the STAR DAQ system over a high speed
bidirectional fiber link. We intend to use the Source Interface Unit
(SIU) and Readout Receiver Cards (RORC) developed for ALICE as our
optical link hardware to transfer data to and from the STAR DAQ system.
These links have been chosen as the primary readout connections for the
new STAR TPC FEE. Leveraging existing hardware and expertise in STAR
allows for a faster and more reliable design than developing our own
custom solution. The complete system consists of a parallel set of
ladder readouts consisting of 24 separate chains. The data may be
combined at the optical transfer stage depending on the final rates,
which will be determined by our final data reduction efficiency.

![](media/image36.wmf){width="5.991666666666666in" height="4.5in"}

[]{#__RefHeading___Toc119666022 .anchor}Figure 30: DAQ Layout: schematic
of DAQ system for a single ladder. Analog data is carried as
differential current on the low mass cable at 50 MHz. The signals are
driven in parallel over short (\~1m) twisted pair cables to the
motherboard. Analog to digital conversion, CDS and data reduction are
performed in the Motherboard / Daughter cards. The reduced hit data is
transferred digitally to the SIU and carried to linux based readout PCs
via an optical fiber. Control, synchronization and event ID tagging are
accomplished in the Synch/Trigger board.

![](media/image37.wmf){width="5.427083333333333in"
height="1.6041666666666667in"}

[]{#__RefHeading___Toc119666023 .anchor}Figure 31: Ladder Layout -
sketch of the readout-topology on a detector ladder. This figure shows
the ten APS and the corresponding current to voltage conversion and
driver electronics. The drivers will be located out of the low mass
region of the detector and may require additional cooling.

![](media/image38.wmf){width="5.996527777777778in" height="3.25625in"}

[]{#__RefHeading___Toc119666024 .anchor}Figure 32: HFT Motherboard
functional layout. This figure shows the functional layout of the HFT
Motherboard and Daughter cards. There are 5 daughter cards per
motherboard to process the data from one ladder. Correlated double
sampling, pedestal subtraction and hot pixel removal happen at the first
stage of processing on the daughter cards. Hit finding and reduction of
the data to addressed clusters happens on the motherboard. Data is then
transferred via optical fiber to the RORC cards where it is picked up by
the STAR DAQ.

Data from the ADCs is 10 bit. Performing CDS and pedestal subtraction
requires 2 data samples to be stored along with the pedestals. This
drives the need for external RAM on the daughter cards. Pedestal
subtraction and hot pixel removal can be done in the same lookup
operation and data is then passed to the motherboard. After CDS and
pedestal subtraction, the data can be represented by 8 bits.

We are investigating methods of hit finding and data reduction for use
on the motherboard. A simple high threshold hit read out with the
surrounding 8 pixels is our default approach. This can be implemented in
an FPGA and run as a pipeline filling the output buffer with ADC and
address values. A simple example of an FPGA logic diagram that
accomplishes this can be found on the web[^103]. Additional method under
investigation include summing algorithms around different thresholds .

## 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. Assigning a
RHIC clock time stamp to each HFT readout frame will provide a method to
synchronize with the triggered events. In addition, synchronization
between the ladders/boards must be maintained. We will provide
functionality to allow the motherboards to be synchronized at startup
and any point thereafter.

## Data Rates

We have determined that 10 bits of dynamic range is required to give us
sufficient signal to noise given the expected signal sizes, noise.[^104]
The data rate from each 640 x 640 MIMOSTAR2 detector is thus
approximately 1 Gb / sec. The total rate of raw data entering the
processing chain in the detector is thus approximately 240 Gb / sec.
After CDS, the data can be represented by 8 bits. 10 bits of column
address and 10 bits of row address are required for each detector and an
additional 4 bits of detector-in-ladder address and 5 bits of ladder
address are required to map the detector pixel space. Combining this
with the occupancy per layer and the 4 ms frame read time gives a data
rate from the detector of 2.46 Gb/sec. This is shown graphically in
figure 30. After the data with complete addresses for each pixel are
read into the DAQ structure, further reductions in the data rate are
possible. The encoding of just the seed address and a fixed pattern of
surrounding ADC values can significantly reduce the rate at which data
is saved to RCF.

![](media/image39.wmf){width="6.002777777777778in"
height="1.2520833333333334in"}

[]{#__RefHeading___Toc119666025 .anchor}Figure 33: Data rates at the
various stages of the HFT readout chain.

# Integration with the Experiment

## Mechanical Introduction

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

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

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

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

## Support Carriage for Rapid Installation and Removal

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

A conceptual design has been developed for the support carriage that
will permit rapid insertion and removal without moving other detector
components. This system is shown in Figure 34 as it will be positioned
for operation in the STAR detector system. The HFT is a small detector
at the center of the STAR system. An additional layer of tracking (not
shown), the SSD, lies out side of the HFT. Room is left for another
detector between the SSD and the HFT that can be added at a later date
for improved performance). As shown, the mechanical support and
electronic service for the HFT is located on one end only. The light
weight guide track and support bridge are permanent elements of the HFT
design. The detector system permits shown in the figure allows for rapid
removal and replacement while maintaining reproducible position through
the use of fully defined kinematic mounts.

![](media/image41.jpeg){width="6.0in" height="4.5in"}

[]{#__RefHeading___Toc119666026 .anchor}Figure 34: The HFT is shown
integrated with the STAR inner detectors cone assembly.

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

A close up view in Figure 35 shows the HFT ladder arrangement. There are
two tracking layers: one at 1.5 cm radius and the other at 5 cm radius.
The ladder design shown in the figure consists of a single thin gull
shaped carbon fiber beam with a flex PC cable plus multiple thinned
silicon APS detector chips. Other two layer carbon fiber laminates are
being developed which are simpler to construct but pose additional
challenges in terms of mechanical integration.

![](media/image42.jpeg){width="5.0in" height="3.0in"}

[]{#__RefHeading___Toc119666027 .anchor}Figure 35: Close up view of the
HFT ladders.

The detector support structure illustrated in Figure 35 shows elements
of the kinematic position control assembly. The detector ladders are
arranged in 6 modules with 3 outer layer ladders and 1 inner ladder on
each module. The module is supported on a long structure, shown in
green. The long green structure allows stable support of the ladder
while avoiding contact with cabling and other structures that are part
of the intermediate tracking system.. The brass colored structures
provide common tie points for coupling the detector system in a
reproducible manner. A more detailed view of this support system appears
in Figure 37 showing the three identical frames (brass colored) that are
kinematically joined around the beam pipe using ball on cylinder
contacts.

![](media/image43.jpeg){width="5.2875in" height="2.7090277777777776in"}

[]{#__RefHeading___Toc119666028 .anchor}Figure 36: Detector support
structure with kinematic mounts to insure repeatable detector
positioning.

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

![](media/image44.jpeg){width="5.325in" height="3.8625in"}

[]{#__RefHeading___Toc119666029 .anchor}Figure 37: Kinematic mount
structure.

The common tie point joining the HFT detector system to the STAR support
cone is illustrated in Figure 38. 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/image45.jpeg){width="5.325in" height="3.99375in"}

[]{#__RefHeading___Toc119666030 .anchor}Figure 38: The HFT is shown
integrated with the STAR inner detectors cone assembly.

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

### Position Alignment and Calibration

The required position resolution of better than 10 microns is a
significant challenge for calibration and alignment. The detector system
is small and so it permits a design with a single kinematic support to
define a reproducible position centered on the STAR system. With the
proposed mechanical design the detector system can be moved as a unit
between the visual coordinate measuring system and the docking support
in the STAR experiment without disturbing the relative HFT ladder
positions. The brass colored structures shown in Figure 37 can be
separated and reassembled without changing their relative alignment.
This opening of the structure is necessary both for installation around
the beam pipe and for visually accessing the inner layer detector
surfaces with the visual coordinate measuring machine. The detector
silicon will be patterned with a fiducial grid in the top aluminum
connection layer (see Figure 40). 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
39. This structure must be stiff enough to be supported at one end,
only, and maintain reliable position stability to better than 10 µm. The
ladder is made up of a 150 µm two ply carbon composite open beam, which
is closed with the detector sandwich to make a rigid structure. The
bottom layer of the detector sandwich is an aluminum Kapton cable. Wire
bonds will provide connections between the APS chip and the cable.

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

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

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

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

![](media/image46.jpeg){width="5.992361111111111in"
height="2.1319444444444446in"}

[]{#__RefHeading___Toc119666031 .anchor}Figure 39: Ladder assembly. The
current ladder is composed of a row of 10 thinned APS detectors on top
of a matching row of thinned readout chips and a three-layer aluminum
Kapton cable. The silicon cable structure is bonded to a carbon
composite v, closing the beam to make a rigid structure.

[]{#__RefHeading___Toc119662862 .anchor}Detector Radiation Length

Multiple scattering in the beam pipe and the first inner layer of the
vertex detector sets the intrinsic limit of precision for vertex
resolution. The radiation thickness for an inner ladder and the beam
pipe is given in Table 11. The detector ladder is placed at an angle and
is quite close to the interaction point so particles pass through the
material at varying angles. This results in path length through the
materials that is greater than the thickness. The average effective
thickness is represented in Table 11.

  ---------------------------- ---------- ----------------------- ---------
  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        55                      0.06

  Kapton                       35         170                     0.05

  Adhesive                     35         85                      0.024

  Carbon Composite             28         174                     0.062

  Total for the ladder                    490                     0.20
  components                                                      
  ---------------------------- ---------- ----------------------- ---------

[]{#__RefHeading___Toc119662946 .anchor}Table 11: Materials in the beam
pipe and the first detector layer with their radiation length fractions.
For details see Ref. \[[^105]\]

### Expected Radiation Exposure

RHIC luminosity projected to 2008 with the assumptions of a 21 week
running period and a 40% operating efficiency give a radiation dose of
approximately 2 kRad/year. This calculation may be found on the
web[^106]. The materials that we anticipate using in the HFT detector
are the same as used in much higher radiation environments like the
Atlas pixel detector. One exception is the acrylic adhesive, which
exhibits excellent radiation resistance (Radiation Index \~5)[^107]. Our
structure should not exhibit any mechanical degradation from the
absorbed dose.

## Ladder Mechanical Tests

### Load Distortion Tests

Figure 39, Figure 40, and Figure 42 show a prototype of the ladder and
detector carrier that we have built. Figure 41 shows a cross-sectional
view of the ladder. The mechanical tests have shown that the stiffness
and bending characteristics of the assembly are acceptable. For these
and similar tests[^108], the ladder's mechanical structure was supported
on one end and the surface contours were measured using the vision
measuring machine at LBNL with and without a 10 gram end load. The
deflection profile is shown in Figure 43 along with the calculated
deflection for a simple triangular closed beam. The stiffness is within
20% of expectations from an engineering model. The measured fundamental
frequency of the ladder is 140 Hz while the engineering model gives a
resonant frequency of 135 Hz[^109]. 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/image48.jpeg){width="5.992361111111111in" height="1.29375in"}

[]{#__RefHeading___Toc119666033 .anchor}Figure 41: A cross-section of
the prototype detector ladder showing its structure and materials
composition.

![](media/image49.jpeg){width="6.0in" height="4.329861111111111in"}

[]{#__RefHeading___Toc119666034 .anchor}Figure 42: The prototype ladder
on the testing apparatus.

![](media/image50.jpeg){width="6.002083333333333in" height="4.725in"}

[]{#__RefHeading___Toc119666035 .anchor}Figure 43: Measured bend of a
silicon/carbon composite ladder test structure and the calculated bend
shape. The ladder was rigidly supported at one end with a 10 gram weight
placed on the other.

### Thermal Distortion Tests

We are in the process of measuring thermally generated deflections using
TV holography[^110]. 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 44). This
tool will also be used to test the stability of the final detector.

![](media/image51.jpeg){width="5.991666666666666in"
height="3.495833333333333in"}

[]{#__RefHeading___Toc119666036 .anchor}Figure 44: 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 44) 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[^111] 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[^112]. Additional information on the cooling studies is
available on the web[^113].

## Cabling

Connection to the detector readout chips will be done with an aluminum
Kapton cable. We have recently found a commercial vendor that makes
aluminum cables with vias.

Fabrication tests have been done with 0.35 mil aluminum with 1 mil
Kapton plus two 1 mil acrylic adhesive layers. Wire bonding tests with
this material have been successful .

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

Figure 45 shows the prototype for a HFT readout cable. The prototype is
an active cable with a buffer and differential amplifier for each sector
of 10 MIMOSA detectors for use in testing the MIMOSA-5 detectors. The
final detector readout cable will not have the components shown but will
narrower and will be sized to be slightly wider than the detectors
themselves to allow wire bonding. The cable in the figure uses a copper
conductor on a 25 µm Kapton insulator. The final cable will be a 4-layer
low-mass aluminum conductor cable with a radiation length equivalent of
84 µm of Si. The prototype has been successfully tested and is working
in our prototype ladder readout system. Additional and more recent
information on cable and ladder developments may be found on the
web[^114].

![](media/image52.jpeg){width="5.993055555555555in"
height="1.5659722222222223in"}

[]{#__RefHeading___Toc119666037 .anchor}Figure 45: A prototype readout
cable for the HFT.

Figure 46 shows a mechanical Prototype with 4 MIMOSA-5 detectors glued
to the Kapton cable assembly with a thin acrylic film adhesive. The
adhesive has a bond line of 50 µm. The MIMOSA is relatively flat in
horizontal direction but has a "dish" shape in vertical (y) direction.
The magnitude of these distortions is, however, quite small. The largest
deviation from a linear fit in y is only 6 µm. This low rate of
deformation will allow us to make a minimum number of measurements of
the detector positions on the ladders in order to locate the individual
pixel position to the required accuracy by a parameterized location
function[^115].

![](media/image53.jpeg){width="5.989583333333333in"
height="2.9784722222222224in"}

[]{#__RefHeading___Toc119666038 .anchor}Figure 46: Mechanical Prototype
with 4 MIMOSA-5 detectors glued to the Kapton cable assembly.

## Beam Pipe

The vertex detector extends the measurement radius to 15 mm from the
interaction axis and will require a new small radius beam pipe for STAR.
The design concept for this pipe is shown in Figure 47.

![](media/image54.jpeg){width="6.0in" height="3.73125in"}

[]{#__RefHeading___Toc119666039 .anchor}Figure 47: 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 48. The outer
shell has been made transparent in the figure to show the inner
structure.

![](media/image55.jpeg){width="6.0in" height="4.5in"}

[]{#__RefHeading___Toc119666040 .anchor}Figure 48: View of the beampipe
mid-section.

### Minimum Beam Pipe Radius Consistent with Injected Beam

We have chosen a minimum radius for the beam pipe which, now that the
RHIC optics have stabilized, is reasonable conservative. From the
standpoint of beam pipe interaction with beam optics, the beam pipe wall
should, according to accelerator physicists at BNL, have a minimum
radius of 6 while 10 is very conservative[^116]. 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 mm plus a 5 mm beam offset at
injectionError: Reference source not found^,^[^117]^,^[^118][^119]. For
protons injection is smaller and therefore less of a constraint. We have
chosen an inner beam pipe radius of 14.5 mm or \~ 7 . This is
satisfactory for RHIC operation, however, depending on focusing details
this could be the limiting aperture, leaving the HFT vulnerable to
uncontrolled beam dumps118.

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)[^120].

[]{#__RefHeading___Toc119662871 .anchor}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[^121]^,^[^122]^,^[^123] which gives a pressure
increase at the center in the interaction region of \~ 10^-11^ torr.
This is the pressure increase above the pressure at the pumping station
and it is based on a surface out-gassing rate of 5.3 10^-11^ W/m^2^ and
a conservative estimate of the pipe conductanceError: Reference source
not found. This is well within the requirements. The greatest
uncertainty in this estimate is the out-gassing rate, but a factor of 10
greater value is still tolerable. If an Active NEG coating is used the
inner section will be a pumping surface rather than an out gassing
surface

### Supporting Section

The central supporting section will be attached to the existing carbon
cone in STAR and will provide the support for the center portion of the
beam pipe. The outer cylinder and spoke section, made of either carbon
composite or beryllium, is designed to support the pipe sections
extending from either end and constrain them sufficiently that a very
thin beam pipe structure can be used in the center region. This outer
cylinder with spokes plus flex sections will isolate the thin central
pipe from outside forces so that the central pipe only has to support
the vacuum load. The spokes that couple the extended beam pipe to the
outer cylinder are shaped to allow free insertion of the vertex detector
into the enclosed center region. The end view in Figure 49 shows how the
detector ladders mesh with the support spokes.

![](media/image56.jpeg){width="5.875in" height="4.40625in"}

[]{#__RefHeading___Toc119666041 .anchor}Figure 49: End view showing the
HFT ladders between spokes to the inner beam pipe support.

### Central Beam Pipe Thin Window Section

The current plan for the thin central region is to use a beryllium pipe
with a 0.5 mm wall thickness. It is believed that this will provide the
minimum coulomb scattering while maintaining a reasonable structural
safety margin. Preliminary analysis indicates a 50:1 safety margin for
buckling[^124]. There is a structure failure limit imposed by the degree
of roundness of the cylinder. A stress analysis indicates that a 1 mm
deviation from a cylinder (1 mm difference between major and minor axis
of an ellipse) sets a 20:1 safety margin for material failure.

### RF Background from the Wake Field

It is believed that the beam pipe in STAR will be sufficient to suppress
wake field signals in the detector to well below the expected signal.
The evidence for this belief comes from experience with the gas jet
polarimeter and carbon strip polarimeter located at the 12 o'clock
intersection region of RHIC. These polarimeters have silicon detectors
located inside the beam pipe close to the beam and it has been found
that a few microns of conductor are sufficient to shield these detectors
from the wake field[^125].

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

### Beam Pipe Insertion and Removal

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

### Bake Out

Beam pipe bake out is still under discussion. It is expected that the
same matter NEG coating will be used and there are two possibilities in
how this will be treated. It may either be a minimum bake out to remove
primarily water as it is done now with a hot dry nitrogen scrub gas at
150 C to 110 C or it could be a full activation of the NEG coating
turning it into a pumping surface[^126]. This later case requires baking
at 150 C to 200 C with vacuum. This could impose additional constraints
on beam pipe materials and would require additional thermal isolation
from the other detectors. If it is decided that the less aggressive bake
out procedure is adequate then carbon composites can be used in the
construction of the beam pipe exoskeleton. In either case the HFT will
be withdrawn during bake out from the operating location to isolate it
from the heated beam pipe.

## Compatibility with the SSD and other cone mounted detectors

The HFT will be supported and installed from one end only. There will be
no other detectors requiring support or access at this end. Other
detectors requiring cone support will be outside of the HFT space. The
FTPC on one end will have to be removed to accommodate the HFT and
support electronics as shown in Figure 34.

## 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 for the cost and the technically-driven
schedule for the HFT. Figure 50 shows the projected time line for the
major tasks associated with the project. In this analysis the major
tasks have been broken down into design and fabrications phases and in
most cases include more detail such as prototype and testing tasks.
Figure 50 is a high level summary of a detailed set of MS Project files
which are available upon request. When it comes time to develop a
resource constrained schedule, it is important to set the completion
date so that it is tied to the expected running time for the next long
heavy ion run of Au+Au at RHIC. At this stage, consideration has been
given to the technically-driven durations and expected effort
requirements, but a determination of the actual start date and analysis
with more detailed task breakdowns and resource loading is left for the
future.

![](media/image57.png){width="6.011111111111111in"
height="2.2256944444444446in"}

[]{#__RefHeading___Toc119666042 .anchor}Figure 50: The
technically-driven time line for major tasks in the building of the HFT
detector.

The total resources required to complete the Heavy Flavor Tracker is 8
to 10 million dollars, including the costs of the contributed labor
which will come from the base support of the participating institutions.
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. For
purchases, costs are counted at the time of placing the order.
Contingency on the contributed labor has been included in the
construction costs and we do not assume that the contributing
institutions can assume the contingency risk on their contributed labor.
This is an assumption that can be re-evaluated at a later time.

The total resources required to complete the project as a function of
time are shown in Figure 51. The figure shows the contributed labor and
materials.

![](media/image58.wmf)

[]{#__RefHeading___Toc119666043 .anchor}Figure 51: Estimated cost of the
HFT project versus the technically driven schedule. The accumulated
costs are shown in millions of dollars. Design and construction costs
are shown in blue, and the value of contributed labor is shown in
purple.

**The construction costs shown in Figure 51 include the contingency on
the contributed labor; so for example, the construction costs are
estimated to be 5.8M\$, the contributed labor is valued at 2.0M\$ and
the contingency on the contributed labor is 1.5M\$. The resources
required from all sources to complete the project is therefore 9.3M\$ in
this estimate.**

+----------------------------+--------------------+-------------------+
|                            | contributed labor  | project supported |
|                            | (FTE)              |                   |
|                            |                    | (FTE)             |
+----------------------------+--------------------+-------------------+
| electronic engineer LBNL   | 3.0                | 2.1               |
+----------------------------+--------------------+-------------------+
| electronic engineer BNL    | 0.4                | 0.0               |
+----------------------------+--------------------+-------------------+
| mechanical engineer LBNL   | 3.0                | 0.8               |
+----------------------------+--------------------+-------------------+
| mechanical engineer BNL    | 0.8                | 0.0               |
+----------------------------+--------------------+-------------------+
| technician LBNL            | 0.3                | 1.8               |
+----------------------------+--------------------+-------------------+
| technician BNL             | 2.7                | 0.5               |
+----------------------------+--------------------+-------------------+
| management                 | 1.1                | 2.2               |
+----------------------------+--------------------+-------------------+
| total                      | 11.3               | 7.4               |
+----------------------------+--------------------+-------------------+

[]{#__RefHeading___Toc119662947 .anchor}Table 12: Estimated manpower
requirements for the HFT project

Table 12 provides a breakdown of the contributed and project funded
effort. 7.4 man years of effort (FTE) is funded by the project. 11.3 man
years is contributed by LBNL and BNL. A significant amount of
contributed effort has already been provided by IReS for the detector
chips. This effort, as well as their future effort, is not included in
the totals presented in the table.

Table 13 lists the names of the participants at IReS. This work involves
several engineers and physicists, most of whom are part time only.

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

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

[]{#__RefHeading___Toc119662948 .anchor}Table 13: IReS manpower
contributions developing the HFT readout chip

# Summary

Probing charm quark flow and thermal equilibration at RHIC may prove to
be the final step towards the discovery of a Quark Gluon Plasma.
Furthermore, measuring the energy lost by high transverse momentum heavy
(c,b) quarks while traversing the medium will help disentangle between
energy loss scenarios in cold nuclear matter and in partonic matter. The
HFT is designed to tackle both tasks by precisely measuring open charm
hadron yields, spectra and elliptic flow (v~2~) as well as tagging the
electrons produced by high transverse momentum beauty hadrons. The
design requirements are fulfilled by having two thin ( ≤ 0.36% radiation
length) layers of Active Pixel Sensors (APS) with a position resolution
better than 10 m. APS technology is the only option that fits these
requirements while compromising neither the efficiency nor the readout
speed. Indeed, an APS can be thinned down to 50 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. This feature allows the detector to be
externally aligned, repaired and upgraded. By combining cutting edge
sensor and readout technologies with a flexible and robust mechanical
design, the HFT will provide the high precision data on heavy flavor
hadrons that are crucial to understand the nature of the medium formed
in Au+Au collisions at RHIC.

# Appendix I -- MIMOSTAR Specifications

Attached.

# Appendix II -- MIMOSTAR Users Guide

Attached.

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[^88]: Technical report,
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[^96]: http://www.lbnl.leog.org/thinned_mimosa_testing.htm

[^97]: Technical report,
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[^98]: H. S. Matis, F. Bieser, S. Kleinfelder, G. Rai, F. Retiere, H.G.
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[^103]: http://www.lbnl.leog.org/data_reduction.pdf

[^104]: http://www-rnc.lbl.gov/\~wieman/ADCrange.htm

[^105]: Technical report,
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[^106]: [[http://www-rnc.lbl.gov/\~wieman/radiationdoseinapsDulinskiValue.htm]{.underline}](http://www-rnc.lbl.gov/~wieman/radiationdoseinapsDulinskiValue.htm)

[^107]: http://doc.cern.ch/yellowrep/2001/2001-006/p1.pdf

[^108]: <http://www.lbnl.leog.org/writeup.htm>

[^109]: Technical report,
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[^110]: TV Holography System at LBNL, courtesy of Eric Andersen and the
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[^111]: Technical report,
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[^112]: Technical report, <http://www.lbnl.leog.org/writeup.htm>

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[^115]: Technical report,
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[^116]: Steve Peggs and Steve Tepikian, Private Communication.

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[^119]: Technical report,
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[^120]: Wolfram Fischer, private communication, 28-Nov-2004.

[^121]: Technical report,
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[^122]: John O'Hanlon, *A User's Guide to Vacuum Technology*. John Wiley
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[^123]: Seth Wieman, Private Communication.

[^124]: Technical report,
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[^125]: Sandro Bravar, Private Communication.

[^126]: Dan Weiss, Private Communication 30-Nov-2004.