PROPOSAL

for a Silicon Vertex Tracker (VTX)

for the PHENIX Experiment

**Proposal for a Silicon Vertex Tracker (VTX) for the PHENIX
Experiment**

M. Baker, R. Nouicer, R. Pak, P. Steinberg

Brookhaven National Laboratory, Chemistry Department, Upton, NY
11973-5000, USA

Z. Li

Brookhaven National Laboratory, Instrumentation Division, Upton, NY
11973-5000, USA

J.S. Haggerty, J.T. Mitchel, C.L. Woody

Brookhaven National Laboratory, Physics Department, Upton, NY
11973-5000, USA

A.D. Frawley

Florida State University, Tallahassee, FL 32306, USA

J. Crandall, J.C. Hill, J.G. Lajoie, C.A. Ogilvie, H. Pei, J. Rak,
G.Skank, S. Skutnik,

G. Sleege, G. Tuttle

Iowa State University, Ames, IA 56011, USA

M. Tanaka

High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki
305-0801, Japan.

N. Saito, M. Togawa

Kyoto University, Kyoto 606, Japan

H.W. van Hecke, G.J. Kunde, D.M. Lee, M. J. Leitch, P.L. McGaughey, W.E.
Sondheim

Los Alamos National Laboratory, Los Alamos, NM 87545, USA

T. Kawasaki, K. Fujiwara

Niigata University, Niigata 950-2181, Japan

T.C. Awes, M. Bobrek, C.L. Britton, W.L. Bryan, K.N. Castleberry, V.
Cianciolo,

Y.V. Efremenko, K.F. Read, D.O. Silvermyr, P.W. Stankus, A.L.
Wintenberg, G.R. Young

Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

Y. Akiba, H. En'yo, Y. Goto, J.M. Heuser, H. Kano, H. Ohnishi, V. Rykov,
T. Tabaru, K.Tanida, J. Tojo

RIKEN (The Institute of Physical and Chemical Research,) Wako, Saitama
351-0198, Japan

A. Deshpande

RIKEN BNL Research Center, Brookhaven National Laboratory, Upton, NY
11973-5000, USA

S. Abeytunge, R. Averbeck, A. Dion, A. Drees, T.K. Hemmick, B.V. Jacak,
C. Pancake,

V.S. Pantuev, D. Walker

Stony Brook University, Department of Physics and Astronomy, Stony
Brook, NY 11794, USA

B. Bassalleck, D.E. Fields, M. Malik

University of New Mexico, Albuquerque, NM, USA

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

[2. Physics Overview 4](#physics-overview)

[2.1 Probes of Early, Highest Energy-Density Stage of Heavy-ion
Reactions
4](#probes-of-early-highest-energy-density-stage-of-heavy-ion-reactions)

[Potential enhancement of open charm production
5](#potential-enhancement-of-open-charm-production)

[Open Beauty Production 6](#open-beauty-production)

[Ratio of charm and beauty production and its centrality dependence
7](#ratio-of-charm-and-beauty-production-and-its-centrality-dependence)

[Energy-loss of heavy-quarks 7](#energy-loss-of-heavy-quarks)

[Open charm reference to J/ψ suppression and enhancement
9](#open-charm-reference-to-jψ-suppression-and-enhancement)

[Open beauty and J/ψ suppression 10](#open-beauty-and-jψ-suppression)

[Thermal di-lepton and open charm 11](#thermal-di-lepton-and-open-charm)

[High *p*~T~ phenomena with light flavor in 10 -- 15 GeV/c in *p*~T~
12](#high-pt-phenomena-with-light-flavor-in-10-15-gevc-in-pt)

[Measurement of Upsilon states 12](#measurement-of-upsilon-states)

[2.2 Determination of spin structure of nucleon.
12](#determination-of-spin-structure-of-nucleon.)

[Exploring the spin structure of the nucleon: The past
12](#exploring-the-spin-structure-of-the-nucleon-the-past)

[Gluon polarization measurement at RHIC:
13](#gluon-polarization-measurement-at-rhic)

[The role of Silicon Vertex Detector:
14](#the-role-of-silicon-vertex-detector)

[The heavy quark physics (open charm and beauty production)
15](#the-heavy-quark-physics-open-charm-and-beauty-production)

[Direct photon + Jet measurement: 17](#direct-photon-jet-measurement)

[Other advantages of the Silicon Vertex Detector:
17](#other-advantages-of-the-silicon-vertex-detector)

[2.3 Exploration of the nucleon structure in nuclei
18](#exploration-of-the-nucleon-structure-in-nuclei)

[3. Physics measurements with the VTX detector
23](#physics-measurements-with-the-vtx-detector)

[3.1 Performance Requirements and the VTX detector geometry
23](#performance-requirements-and-the-vtx-detector-geometry)

[Performance Requirements 23](#performance-requirements)

[VTX detector geometry 24](#vtx-detector-geometry)

[Central Track -- VTX matching 25](#central-track-vtx-matching)

[3.2 Open Charm and Beauty Measurement
26](#open-charm-and-beauty-measurement)

[Open Charm measurement from semi-leptonic decay
26](#open-charm-measurement-from-semi-leptonic-decay)

[Direct Measurement of *D^0^K^-^π^+^* at high *p~T~*
29](#direct-measurement-of-d0k-π-at-high-pt)

[Open Beauty Measurement 32](#open-beauty-measurement)

[3.3 Photon and jets measurement in polarized p+p
34](#photon-and-jets-measurement-in-polarized-pp)

[3.4 Improved momentum resolution and pt resolution
37](#improved-momentum-resolution-and-pt-resolution)

[3.5 Event rate estimates 38](#event-rate-estimates)

[4. VTX Detector system 44](#vtx-detector-system)

[4.1 Overview 44](#overview)

[4.2 Hybrid pixels 46](#hybrid-pixels)

[Sensor 47](#sensor)

[Readout chip 48](#readout-chip)

[Interconnection of Sensor and Readout Chip ("Bump Bonding")
49](#interconnection-of-sensor-and-readout-chip-bump-bonding)

[Readout Bus 51](#readout-bus)

[Pilot module 51](#pilot-module)

[Front End Modules 52](#front-end-modules)

[Pixel detectors operating in the NA60 experiment:
53](#pixel-detectors-operating-in-the-na60-experiment)

[4.3 Silicon Strip Detector 54](#silicon-strip-detector)

[Strip Sensors 55](#strip-sensors)

[Laboratory tests of the strip sensors
56](#laboratory-tests-of-the-strip-sensors)

[Sensor Prototypes in Test Beam 58](#sensor-prototypes-in-test-beam)

[SVX4 readout chip 61](#svx4-readout-chip)

[Sensor Readout Card (ROC) / Readout Bus
63](#sensor-readout-card-roc-readout-bus)

[Pilot Module 64](#pilot-module-1)

[Zero Suppression 64](#zero-suppression)

[4.4 Mechanical Structure and Cooling
65](#mechanical-structure-and-cooling)

[Design Criteria 66](#design-criteria)

[Structural Support 67](#structural-support)

[Detector Ladders and Cooling 68](#detector-ladders-and-cooling)

[Radiation Length 71](#radiation-length)

[4.5 Detector integration into PHENIX
71](#detector-integration-into-phenix)

[VTX detector assembly 71](#vtx-detector-assembly)

[Integration into PHENIX 72](#integration-into-phenix)

[DAQ 73](#daq)

[5. R&D 75](#rd)

[6. Project management and responsibilities
78](#project-management-and-responsibilities)

[6.1 Project background 78](#project-background)

[6.2 The management plan for the VTX
79](#the-management-plan-for-the-vtx)

[PHENIX management structure 79](#phenix-management-structure)

[PHENIX subsystem leadership 79](#phenix-subsystem-leadership)

[Role of BNL 80](#role-of-bnl)

[Specification of deliverables 80](#specification-of-deliverables)

[6.3 Institutional involvement 81](#institutional-involvement)

[Brookhaven National Laboratory, Chemistry Department (BNL CHEM)
82](#brookhaven-national-laboratory-chemistry-department-bnl-chem)

[Brookhaven National Laboratory, Instrumentation Division (BNL ID)
82](#brookhaven-national-laboratory-instrumentation-division-bnl-id)

[Brookhaven National Laboratory, Physics Department (BNL PHY)
83](#brookhaven-national-laboratory-physics-department-bnl-phy)

[Iowa State University (ISU) 83](#iowa-state-university-isu)

[Kyoto University (Kyoto) 84](#kyoto-university-kyoto)

[Los Alamos National Laboratory (LANL)
84](#los-alamos-national-laboratory-lanl)

[University of New Mexico, Albuquerque (UNM)
84](#university-of-new-mexico-albuquerque-unm)

[Oak Ridge National Laboratory (ORNL)
85](#oak-ridge-national-laboratory-ornl)

[RIKEN Institute (RIKEN) 85](#riken-institute-riken)

[Stony Brook University, Physics Department (SBU)
86](#stony-brook-university-physics-department-sbu)

[6.4 Foreign contributions 86](#foreign-contributions)

[7 Budget and schedule 88](#budget-and-schedule)

[7.1 Total estimated cost (TEC) 88](#total-estimated-cost-tec)

[Fiscal Responsibilities 88](#fiscal-responsibilities)

[Contingency Analysis 88](#contingency-analysis)

[Overhead Estimate 88](#overhead-estimate)

[Budget 89](#budget)

[7.2 Schedule 92](#schedule)

[Appendix A ENDCAP Extension 97](#appendix-a-endcap-extension)

[Introduction 97](#introduction)

[A.1 Goals of the Endcap Upgrade 98](#a.1-goals-of-the-endcap-upgrade)

[A.1.1 Spin Structure of the Nucleon
98](#a.1.1-spin-structure-of-the-nucleon)

[A.1.2 Exploration of Gluon Structure in Nuclei
100](#a.1.2-exploration-of-gluon-structure-in-nuclei)

[A.1.3 Probes of Early, Highest Energy-Density Stage of Heavy-ion
Reactions
100](#a.1.3-probes-of-early-highest-energy-density-stage-of-heavy-ion-reactions)

[Energy Loss of Heavy Quarks 101](#energy-loss-of-heavy-quarks-1)

[Open Charm and Beauty Enhancement
101](#open-charm-and-beauty-enhancement)

[J/ψ Suppression 101](#jψ-suppression)

[Other Physics Topics 101](#other-physics-topics)

[A.2 Simulations and Required Performance for the Si Endcap Upgrade
102](#a.2-simulations-and-required-performance-for-the-si-endcap-upgrade)

[A.2.1 Open Charm Measurement 103](#a.2.1-open-charm-measurement)

[A.2.2 Open Beauty Measurement 105](#a.2.2-open-beauty-measurement)

[A.2.3 Trigger Plans 106](#a.2.3-trigger-plans)

[A.2.4 Si Endcap Event Rates 107](#a.2.4-si-endcap-event-rates)

[A.2.5 Matching to Muon Spectrometers
107](#a.2.5-matching-to-muon-spectrometers)

[A.2.6 Integration with PHENIX 107](#a.2.6-integration-with-phenix)

[A.3 Technical Aspects of the Proposed Endcap Vertex Detector
108](#a.3-technical-aspects-of-the-proposed-endcap-vertex-detector)

[A.3.1 Silicon Readout Chip --PHX 108](#a.3.1-silicon-readout-chip-phx)

[A.3.2 Silicon Ministrip Sensors 108](#a.3.2-silicon-ministrip-sensors)

[A.3.3 Silicon Ministrip Control Chip
111](#a.3.3-silicon-ministrip-control-chip)

[A.3.4 Mechanical Structure and Cooling
112](#a.3.4-mechanical-structure-and-cooling)

[A.3.5 Endcap Ladder Structure 112](#a.3.5-endcap-ladder-structure)

[6.3.7 Endcap Analysis Summary 114](#endcap-analysis-summary)

[A.4 R+D Schedule, Responsibilities and Budget
114](#a.4-rd-schedule-responsibilities-and-budget)

[A.4.1 Schedule 114](#a.4.1-schedule)

[A.4.2 Responsibilities 115](#a.4.2-responsibilities)

**List of Figures**

[Figure 1 Charm enhancement expected in RHIC energy in ref. 3. The both
panels, contribution from the initial gluon fusion (solid), pre-thermal
production (dot-dashed), and thermal production (dashed, lowest) are
shown. The left panel is the calculation with energy density of 3.2
GeV/fm^3^, while the right panel shows the case with energy density 4
times higher. 6](#__RefHeading___Toc55021889)

[Figure 2 Ratio of Jet Quenching factor QH/QL of heavy quark (QH) and
light quark (QL) in high density QCD medium as function of pT of the
quark, from ref. 8. The solid line is with no energy cut-off for gluon
and the dashed line is with cut off of 0.5 GeV.
8](#__RefHeading___Toc55021890)

[Figure 3 Single electron data of PHENIX compared with two extreme
models of charm *p*~T~ distribution. From ref. 12.
9](#__RefHeading___Toc55021891)

[Figure 4 The ratio of J/Ψ yield and open charm yield predicted in ref.
10](#__RefHeading___Toc55021892)

[Figure 5 The di-electron effective mass distribution in PHENIX central
arm acceptance in central Au+Au collision at $\sqrt{s_{\text{NN}}}$= 200
GeV predicted by Ralf Rapp^19^. In the intermediate mass region (1 \<
Mee \< 2.5 GeV), the dominant sources of electron pairs are open charm
and thermal radiation from the QGP and hot hadronic gas.
11](#__RefHeading___Toc55021893)

[Figure 6 Expected x-ranges for polarized and un-polarized gluon
distribution measurements in PHENIX using different channels. The blue
bars indicate PHENIX detector's existing capability while the red bars
indicate the enhanced coverage provided by the proposed silicon vertex
detector upgraded PHENIX. 15](#__RefHeading___Toc55021894)

[Figure 7 - Gluon shadowing from Eskola as a function of x for different
Q^2^ values: 2.25 GeV^2^ (solid), 5.39 GeV^2^ (dotted), 14.7 GeV^2^
(dashed), 39.9 GeV^2^ (dotted-dashed), 108 GeV^2^ (double-dashed) and
10000 GeV^2^ (dashed). The regions between the vertical dashed lines
show the dominant values of x~2~ probed by muon pair production from
DDbar at SPS, RHIC and LHC energies. 19](#__RefHeading___Toc55021895)

[Figure 8 - Gluon shadowing predictions along with PHENIX coverage. The
red bars indicate the additional range provided by the vertex upgrade,
while the blue bars cover the PHENIX baseline. The three theoretical
predictions are for different Q transferred, blue, green and red lines
are Q = 10, 5 and 2 GeV/c respectively, from Frankfurt and Strikman.
20](#__RefHeading___Toc55021896)

[Figure 9 - Dimuon mass spectrum from E866/NuSea showing the mass region
used in their analysis which excludes masses below 4 GeV. Lower masses
were excluded because of the large backgrounds from open charm in that
region. 21](#__RefHeading___Toc55021897)

[Figure 10 (a) Cross section of the silicon vertex tracker (VTX) along
the beam axis. The inner pixel hybrid layer is located at a radial
distance of 2.5 cm from the beam pipe and extends over \~22cm in beam
direction. The silicon strip outer layers are located at 6, 8 and 10 cm.
All three extend over \~26 cm in beam direction. The Be beam pipe with 2
cm radius is also shown. (b) Cut through the silicon vertex detector in
the xy-plane transverse to the beam axis. The VTX is assembled in two
half shells with small acceptance gaps at top and bottom. The different
layers of each half shell have 5, 7, or 9 rows of silicon detectors,
depending on the radial location of the layer.
24](#__RefHeading___Toc55021898)

[Figure 11 DCA distribution for electrons from Dalitz, charm and beauty
decays simulated through four 1% Si layers on the left and four 2%
layers on the right. 27](#__RefHeading___Toc55021899)

[Figure 12 Signal to Background ratios as a function of minimum electron
p~T~ cut. The signal corresponds to detached electrons from charm decays
using a DCA cut of 200μm (circles) or no DCA cut (diamonds). The
background corresponds to electrons from Dalitz decays and photon
conversions which pass the corresponding DCA cuts, assuming four layers
of Silicon with 1 or 2% of a radiation length per layer.
28](#__RefHeading___Toc55021900)

[Figure 13 Correlation between the transverse momentum of the D mesons
and the minimum p~T~ cut applied to the electrons (using a DCA cut of
120μm). The points represent the most probable value of the D meson p~T~
while the spread represents the (asymmetric) full width at half maximum.
29](#__RefHeading___Toc55021901)

[Figure 14 The DCA distributions in cm for pions with the inner pixel
having 1%X~0~ thickness. On the left is the DCA for direct pions with pt
\>1 GeV/c and on the right is the DCA for pions from *D*^0^ decay.
30](#__RefHeading___Toc55021902)

[Figure 15 The invariant mass distribution for background pairs from
central Au+Au events. Each pion+kaon pair has a pair p~T~ \> 2 GeV/c.
30](#__RefHeading___Toc55021903)

[Figure 16 The *S/B* for D^0^Kπ with a pt \>2 GeV/c for central Au+Au
events into the west-arm of PHENIX. On the left is the simulation for
1%X~0~ thickness per layer. On the right is simulation for 2%X~0~
thickness per layer. 31](#__RefHeading___Toc55021904)

[Figure 17 The S/B for D^0^+D^0^-\>Kπ with a pt \>2 GeV/c for central
Au+Au events into the west-arm of PHENIX. On the left is the simulation
for 1%X~0~ thickness per layer. On the right is simulation for 2%X~0~
thickness per layer. 31](#__RefHeading___Toc55021905)

[Figure 18 DCA distribution for electrons from Dalitz, charm and beauty
decays simulated through four 1% or 2% Si layers
33](#__RefHeading___Toc55021906)

[Figure 19 Signal to Background ratios as a function of the minimum
electron p~T~ cut. The signal corresponds to detached electrons from
beauty decays using a DCA cut of 200μm (circles) or no DCA cut
(diamonds). The background corresponds to electrons from Dalitz decays
and photon conversions which pass the corresponding DCA cuts, assuming
four layers of Silicon with 1 or 2% of a radiation length per layer
33](#__RefHeading___Toc55021907)

[Figure 20 In each panel, the green histogram shows the pseudo-rapidity,
η~q~-distribution of the final hard scattered partons, which initiated
the recoil jet; the blue histogram shows the η~q~-distribution of recoil
jets within the barrel VTX acceptance; and the red histogram show the
(η^jet^ -η~q~)-distribution, where η^jet^ is for the pseudo-rapidity
reconstructed for the recoil jets. Different panels are for the event
samples with direct photon of different transverse momenta, starting
from 4-5 GeV/c in the upper left to 9-10 GeV/c in the lower right panel.
35](#__RefHeading___Toc55021908)

[Figure 21 Correlation between x reconstructed and true *x*-value from
PYTHIA. In the plot on the left, ![](media/image1.wmf) and no jet
information has been used. The plot in the right panel is obtained,
using the reconstructed jet axes in the barrel VTX.
36](#__RefHeading___Toc55021909)

[Figure 22 The relative widths (RMS) of the (x(true) --
x(reconstruct))/x(true) distributions, using the reconstructed jet axes
in the barrel 37](#__RefHeading___Toc55021910)

[Figure 23 - Separation of Upsilon states in the di-electron spectrum
with a vertex detector (yellow) and without (black). The number of ϒs in
this plot represents our expectation for a Au-Au run with a recorded
effective luminosity of \~1 nb^-1^ (see chapter 3.5).
38](#__RefHeading___Toc55021911)

[Figure 24 GEANT model of the VTX detector. It consisted of the
inner-most pixel layer and three outer strip layers.
44](#__RefHeading___Toc55021912)

[Figure 25 (a) Cross section of a pixel detector half ladder designed
for the ALICE experiment. The hybrid pixel detector itself consists of a
readout chip that is connected via solder bump-bonds to a sensor chip.
Every sensor pixel has a corresponding individual signal processing
electronic in the readout chip. They are interconnected with small
solder balls (\`\`bump-bonds\'\') in a flip-chip process. Eight pixel
detector assemblies are wire-bonded to a readout bus structure that runs
along the detector on top of the sensors. The half ladder is mounted
onto a mechanical support with includes embedded cooling lines to remove
about one Watt of power dissipated by the readout chip. (b) Arrangement
of two sensor assemblies with four chips each to form a PHENIX pixel
detector half ladder. A bus connects all readout chips. A pilot module
outside of the acceptance of the sensors interfaces the readout of the
half-ladder to the data acquisition system.
46](#__RefHeading___Toc55021913)

[Figure 26 Photograph of a corner of a pixel detector sensor chip, seen
through a microscope. A guard electrode surrounds the array of pixel
implants. The scribe line defines the outer dimensions of the die.
48](#__RefHeading___Toc55021914)

[Figure 27 Test result of a typical high-quality ALICE1LHCb assembly for
the NA60 experiment: (a) Test pulse injection into readout chip: 8 out
of 8192 pixels are dead, the rest of the pixel array responds. (b)
Source measurement with Sr^90^ to test the bump bonding quality: 3 out
of 8192 bonds are open (or pixels do not respond electrically). (c)
Image of a beta source with shadow of the depletion voltage contact
needle on the silicon sensor. 50](#__RefHeading___Toc55021915)

[Figure 28 Map of working pixels from a source measurement of a thin
ALICE pixel sensor assembly. The sensor assembly consists of five
thinned readout chips of 150 μm thickness that are bump-bonded to a 200
μm thick silicon sensor substrate. The fraction of working pixels is
indicated for every chip 50](#__RefHeading___Toc55021916)

[Figure 29 The vertex spectrometer of the NA60 experiment comprises a
16-plane pixel detector telescope mounted in a 2.5 T dipole magnetic
field in 7 cm to 32 cm distance downstream of the targets. Every plane
is built from four or eight ALICE1LHCb single-chip pixel detector
assemblies, which are mounted on ceramic printed circuit boards
54](#__RefHeading___Toc55021917)

[Figure 30 Average-multiplicity event in collisions of a 30\~GeV/c Pb
beam onto three Pb targets, reconstructed with three pixel detector
planes during a test run of NA60 in Fall 2002
54](#__RefHeading___Toc55021918)

[Figure 31 A schematic view of p+ cathode structure of the pixels.
55](#__RefHeading___Toc55021919)

[Figure 32 A schematic view of the prototype silicon strip sensor.
56](#__RefHeading___Toc55021920)

[Figure 33 Current and capacity characteristics of a prototype sensor.
57](#_Ref54530969)

[Figure 34 Schematic layout of the laser test setup.
58](#__RefHeading___Toc55021922)

[Figure 35 The laser test setup for the strip sensor. 58](#_Ref54530872)

[Figure 36 The prototype detector 59](#_Ref54543695)

[Figure 37 Charge correlations in between x-strips and u-strips found in
tests with a radioactive source and with beams of charged particles.
60](#__RefHeading___Toc55021925)

[Figure 38 Hit residuals from tracks found using the silicon strip
sensors in a test beam experiment. 60](#__RefHeading___Toc55021926)

[Figure 39 Photograph of an SVX4 chip. 62](#_Ref54760321)

[Figure 40 Schematic Diagram of SVX4 test board.
62](#__RefHeading___Toc55021928)

[Figure 41 Three views of Si Strip ladder. Panel a) shows a top view of
a single sensor and its two ROCs and associated chips. The sensor
appears as blue-green. The four rows of bonding pads for each sensor are
shown as red and dark blue lines -- two per orientation per sensor. The
corresponding SVX4s are also shown in red and dark blue. They are
wire-bonded to the sensor through the holes shown in the ROCs (light
green). The digital ASICs are shown in black and arrows indicate the
direction of the signal bus for each orientation. The signal buses (and
power and ground) are carried across ROC and sensor boundaries (shown by
the solid black line bisecting the sensor) by wire-bonding necessary
pads of adjacent ROCs. Panel b) shows the short edge-on view of a
ladder. The vertical development is shown -- the support/cooling
structures are gray; the sensor is blue-green, the ROC is light green
and the SVX4s are red. Panel c) shows the long edge-on view of a pair of
sensors (separated at the heavy black vertical line) using the same
color scheme. The hatched green regions show where the holes are in the
ROC to allow wire-bonded connection between the sensor and the SVX4.
63](#__RefHeading___Toc55021929)

[Figure 42 Design concepts studied for the vertex detector support
structures. The center most concept with the constant outer diameter
shell had the highest fundamental frequency.
67](#__RefHeading___Toc55021930)

[Figure 43 First mode shape that dominated the dynamic structural
stiffness analysis 67](#__RefHeading___Toc55021931)

[Figure 44 Displacement and principle stress from a 1.0g gravity load on
a full mass loaded structure 68](#__RefHeading___Toc55021932)

[Figure 45 3D model of the barrel region on the left and the ladder
structure on the right showing a cooling tube mounted on a C-C thermal
plane and the sensor and electronics on the underside.
69](#__RefHeading___Toc55021933)

[Figure 46 Left panel shows the out of plane distortions and the right
panel shows the bowing for the 0 deg solution.
70](#__RefHeading___Toc55021934)

[Figure 47 Management chart of the VTX project. The fiscal
responsibilities for the individual tasks are specified in bold letters.
The intitutions participating in each task are given in italic. In
PHENIX the DAQ is a separate subsytem and therefore not connected to the
VTX management. 80](#__RefHeading___Toc55021935)

[Figure 48 The overall schedule for the VTX Project.
93](#__RefHeading___Toc55021936)

[Figure 49 The schedule for the strip layers.
93](#__RefHeading___Toc55021937)

[Figure 50 The schedule for the pixel layers
94](#__RefHeading___Toc55021938)

[Figure 51 The schedule for the auxiliary systems and infrastructure
95](#__RefHeading___Toc55021939)

[Figure 52 Budget profile for the VTX project
96](#__RefHeading___Toc55021940)

**List of Tables**

[Table 1 Table of efficiency factors that must be applied to delivered
pp, dAu and AuAu luminosities to calculate expected signal yields. The
single and two track reconstruction efficiencies are for electrons in
the central arm. 40](#__RefHeading___Toc55013006)

[Table 2 Table of effective luminosities from a 19 week production run,
after reality factors are taken into account. The delivered luminosities
use the average of the most pessimistic and most optimistic C-AD
estimates of how the luminosity will evolve by 2008-2009. The signal
yield for a given process is found by multiplying the cross section for
the process by the effective luminosity and by the detector acceptance.
For d-Au and Au-Au collisions and the effective Ldt columns, the
nucleon-nucleon luminosities are shown in the parenthesis).
40](#_Ref54638032)

[Table 3 Event rate calculated for selected physics processes. The
effective integrated luminosity used in the calculation is shown in
Table 2. For the meaning of "no VTX" column, see the text. In both of
Au+Au and p+p, the collision energy $\sqrt{s_{\text{NN}}}$ is 200 GeV
per nucleon pair. The yields include the anti-particle channels. The DCA
cut value for the single electron measurement is DCA\>200 μ. For the
lowest pT bin, the number with DCA\>400μ is shown in parenthesis.
42](#__RefHeading___Toc55013008)

[Table 4 Summary of physics measurement gained by the VTX detector. The
column "without VTX" shows the present capability of PHENIX, while the
measurement range with the VTX detector is shown in the column "with
VTX". If the process is not measurable, it is marked as "No".
43](#_Ref54757390)

[Table 5 Summary of main parameters of the 4 VTX layers.
45](#__RefHeading___Toc55013010)

[Table 6 Map of construction tasks and WBS numbers onto the proposed
fiscal responsibilities. 90](#__RefHeading___Toc55013011)

[Table 7 Overview of the total estimated cost for the VTX project
90](#__RefHeading___Toc55013012)

[Table 8 Cost breakdown for tasks to be funded through the DOE. Tasks
which do not show a cost correspond to deliverables for which the RIKEN
Institute will take fiscal responsibility.
91](#__RefHeading___Toc55013013)

# 1. Executive Summary

We propose the construction of a Silicon Vertex Tracker (VTX) for the
PHENIX experiment at RHIC. The VTX will substantially enhance the
physics capabilities of the PHENIX central arm spectrometers. Our prime
motivation is to provide precision measurements of heavy-quark
production (charm and beauty) in *A+A*, *p(d)+A*, and polarized *p+p*
collisions. These are key measurements for the future RHIC program, both
for the heavy ion program as it moves from the discovery phase towards
detailed investigation of the properties of the dense nuclear medium
created in heavy ion collisions, and for the exploration of the nucleon
spin-structure functions. In addition, the VTX will also considerably
improve other measurements with PHENIX. The main physics topics
addressed by the VTX are:

-   Hot and dense strongly interacting matter

    -   Potential enhancement of charm production

    -   Open beauty production

    -   Flavor dependence of jet quenching and QCD energy loss

    -   Accurate charm reference for quarkonium

    -   Thermal dilepton radiation

    -   High *p*~T~ phenomena with light flavors above 10-15 GeV/c in
        *p*~T~

    -   Upsilon spectroscopy in the e^+^e^− ^ decay channel

-   Gluon spin structure of the nucleon

    -   ∆G/G with charm

    -   ∆G/G with beauty

    -   x dependence of ∆G/G with γ-jet correlations

-   Nucleon structure in nuclei

    -   Gluon shadowing over broad x-range

With the present PHENIX detector, heavy-quark production has been
measured indirectly through the observation of single electrons. These
measurements are inherently limited in accuracy by systematic
uncertainties resulting from the large electron background from Dalitz
decays and photon conversions. Also we are currently unable to separate
charm from beauty production. The VTX detector will provide vertex
tracking with a resolution of \<50 μm over a large coverage both in
rapidity (\| η\| \< 1.2) and in azimuthal angle (∆φ \~ 2π). With this
device, significantly enhanced and qualitatively new data can be
obtained. A more robust and accurate measurement of heavy-quark
production over a wide kinematics range will be possible.

The main benefits are in three areas. Firstly, by selecting electrons
with a distance of closest approach (DCA) to the primary vertex larger
than \~100 μm, the background will be suppressed by several orders of
magnitude and thereby a clean and robust measurement of heavy flavor
production in the single electron channel will become available.
Secondly, because the lifetime of mesons with beauty is significantly
larger than that of mesons with charm, the VTX information will allow us
to disentangle charm from beauty production over a broad *p*~T~ range.
Thirdly, a DCA cut on hadrons will reduce the combinatorial background
of *Kπ* to an extent that a direct measurement of *D* mesons through
this decay channel will become possible. In addition, the VTX detector
will substantially extend our *p*~T~ coverage in high *p*~T~ charged
particles, and it also will enable us to measure γ+jet correlations.

The proposed VTX detector has four tracking layers. To avoid cost
intensive and time consuming R&D, we have investigated to what extent
existing technology can meet our needs. For the inner most layer we
propose to use a silicon pixel device with 50×425 μm channels that was
developed for the ALICE experiment at the CERN LHC. Our preferred
technology choice for the outer layers is a silicon strip detector
developed by the Instrumentation Division at BNL. With stereoscopic
strips of 80 μm × 3 cm, these devices achieve an effective pixel size of
80 × 1000 μm. We plan to use the SVX4 readout chip developed at FNAL to
readout the strip detectors.

With the help of institutional contributions PHENIX was able to maintain
a small but well focused effort over the past two years to gain
experience with these technologies and to launch the necessary R&D to
adapt them to the PHENIX requirements. We are confident that the
remaining issues can be solved within the next year and that the
detector construction could be started by beginning of FY05.

A collaboration of 65 members from 14 institutions has formed to carry
out the project. The collaboration brings in expertise in all phase of
the construction of a silicon vertex detector, design and commissioning
of modern readout electronics, mechanical and integration issues,
detailed knowledge of all aspects of the PHENIX experiment as well as
expertise in data analysis and a broad interest in different physics
aspects addressed by the VTX.

We anticipate that the project will be funded by two agencies, the DOE
Office of Nuclear Physics and the RIKEN Institute of Japan. For a
successful completion of the project we propose clear responsibilities
and scope of deliverables for both agencies. A preliminary management
plan of the VTX detector project, which also discusses the role and
expected responsibilities of the participating institutions, is included
in this document.

We propose to construct the VTX detector over a period of three years,
US FY05, FY06 and FY07. Parts of the detector will be ready and
installed in time for the expected RHIC run in (RUN7). The project will
be completed before RUN8. To carry out this project we seek funding of a
total of \$5.6M through DOE. These funds would be supplemented with
deliverables equivalent to about \$3M US dollar provided by the RIKEN
Institute during calendar years 2004 to 2006.

The proposal has the following structure. The physics motivation for the
upgrade and the proposed measurements are documented in section 2. The
feasibility of these measurements and the required detector performance
are discussed in section 3. Section 4 gives a detailed description of
the vertex tracker and the technical aspects of the proposed project. A
draft of our management plan, section 6, specifies deliverables and
institutional responsibilities. Section 7 lays out the budget request
and the proposed schedule. Finally, in appendix A we present our future
plan to also upgrade the PHENIX muon arms with vertex tracking by
augmenting the silicon barrel detector proposed here by endcap
detectors.

# 2. Physics Overview

Heavy-flavor production provides a wide-ranging pallet of key
information in three broad areas of physics addressed by the
relativistic heavy ion collider RHIC at Brookhaven National Laboratory.
The current available experiments at RHIC are inadequately equipped to
fully exploit the opportunities heavy-flavor production provides. Many
of the necessary measurements are either not possible or can be
performed only with very limited accuracy. Precise vertex tracking is
imperative for a robust measurement of heavy-flavor production. The
proposed VTX detector adds tracking capabilities to the central arms of
the PHENIX experiment. With this detector charged particles detected in
the central arms can be identified as decay products from charm- or
beauty-carrying particles by the displacement of their trajectories to
the collision vertex. A broad *p~T~* range for charm and beauty
measurements is achieved by using different decay channels to reach
different parts of phase space.

The addition of the VTX to PHENIX will significantly extend the physics
program of PHENIX. In heavy ion collisions open charm and beauty
production will help to probe the high-density matter created early
during the reaction. Specifically accurate measurements will help to
establish:

-   if heavy-quarks are produced only in the initial parton-parton
    collisions or also during the later phases of the collision.

-   the flavor dependence of the energy-loss, which has already been
    observed for light partons.

-   a firm baseline to quantify the suppression or possible enhancement
    of J/ ψ.

-   quantitatively the rate of thermal dilepton emission.

-   quark confinement forces at larger binding energies via the yield of
    upsilon states.

Measurements of open beauty in polarized *p+p* reactions add new
channels in which the gluon spin structure function of protons can be
measured. Robust charm measurement and jet reconstruction over large
acceptance significantly extend the *x*-range of the currently possible
measurements. In *p+A* reactions shadowing of the gluon structure
function in nuclei can be addressed both with open charm and beauty
measurements.

## 2.1 Probes of Early, Highest Energy-Density Stage of Heavy-ion Reactions

As RHIC moves to the second half of this decade the research focus will
shift from the discovery phase to a detailed exploration of quark
matter. Charm and beauty production, measured as yield and spectra of
heavy flavor mesons, provide information about the earliest stages of
heavy ion collision. Several key measurements discussed in these
sub-sections can be made with the addition of the proposed VTX detector
to PHENIX. Of particular importance is the broad reach in transverse
momentum, which extends PHENIX's existing capability to measure
low-*p~T~* open charm.

PHENIX has extracted the cross-section for open charm in the momentum
range *p~T\ ~*\< 2 GeV/c via inclusive electron spectra[^1]. This method
relies on the fact that a fraction of the electrons originates from
decays of heavy-flavor mesons (charm or beauty) and on the ability to
subtract the large background from light-meson decays. This procedure
suffers from uncertainties due to the limited knowledge of the
background sources that are subtracted. The addition of a silicon vertex
detector to PHENIX will allow a much more convincing and accurate
determination of the heavy-quark component in these spectra. Requiring
the leptons to be displaced from the collision will substantially reduce
the background and thus extend the range of the charm measurement to
smaller *p~T~*. At moderate and high *p~T~* decays of beauty-flavor
mesons also contribute to the single-electron spectrum. The present
PHENIX detector cannot distinguish the charm from the beauty
contribution and thus our ability to measure charm is limited to
*p^e^~T~* \< 2.5 GeV/c, i.e. the range where charm is the dominant
source of single electrons after background subtraction. The proposed
upgrade adds the capability to detect charm and beauty production
separately with high accuracy, which will enable us to measure not only
the yield of open beauty production but also to extend the charm
measurement to higher *p~T~*. Complementary to the measurement of
inclusive electrons with displaced vertex, at high p~T~ we can also
measure exclusive decays such as $D \rightarrow K\pi$.

With the extended capability of heavy quark measurement with the VTX
detector, we can address the following critical questions.

### Potential enhancement of open charm production

It has been predicted that open charm could be enhancement in
high-energy nucleus-nucleus collisions relative to the expectation from
elementary collisions[^2]^,^[^3]^,^[^4]. Heavy quarks are produced in
different stages of a heavy ion reaction. In the early stage charm and
beauty are formed in collisions of the incoming partons. The yield of
this component is proportional to the product of parton density
distribution in the incoming nuclei (binary scaling). If the gluon
density is high enough a considerable amount of charm can be produced
via fusion of energetic gluons in the pre-equilibrium stage before they
are thermalized. Finally, if the initial temperature is above 500 MeV,
thermal production of charm can be significant. The last two mechanisms
(pre-equilibrium and thermal production) can enhance charm production
relative to binary scaling of the initial parton-parton collisions.
These are the same mechanisms originally proposed for strangeness
enhancement, but in the case of charm may reveal more about the
critical, early partonic-matter stage of the reaction since the rate of
heavy-quark production is expected to be negligible later in the
reaction when the energy density has decreased. In comparison,
strangeness production is expected to continue even in the later
hadronic stages of the reaction.

![](media/image2.jpeg){width="2.7222222222222223in"
height="3.0375in"}![](media/image3.jpeg){width="2.428472222222222in"
height="2.9763888888888888in"}

[]{#__RefHeading___Toc55021889 .anchor}Figure 1 Charm enhancement
expected in RHIC energy in ref. Error: Reference source not found. The
both panels, contribution from the initial gluon fusion (solid),
pre-thermal production (dot-dashed), and thermal production (dashed,
lowest) are shown. The left panel is the calculation with energy density
of 3.2 GeV/fm^3^, while the right panel shows the case with energy
density 4 times higher.

At RHIC energies the anticipated enhancement is small effectError:
Reference source not found^,Error:\ Reference\ source\ not\ found^. The
contributions to charm production from various stages of an Au+Au
collision are shown in Figure 1 (taken from reference Error: Reference
source not found). From the left panel of the figure it is evident that
for an initial energy density of 3.2 GeV/fm^3^ the pre-thermal or
pre-equilibrium production contributes about 10% of total charm
production, while the thermal contribution is negligible. However, the
yield is very sensitive to the initial density, and with 4 times the
energy density the pre-equilibrium contribution can be as large as the
initial fusion. This is illustrated in the right panel of the figure.
Present single electron measurements of PHENIX indicate that within
\~40% systematic uncertainty charm production approximately scales with
the number of binary collisions. Thus, charm enhancement, if it exists,
cannot be a large effect. A measurement of the charm yield with
substantially higher accuracy and precision is therefore required to
establish a potential charm enhancement.

The VTX detector will improve the accuracy of charm measurement through
single electrons by significantly reducing the background from Dalitz
and photon conversions. This will extend the single electron measurement
to the *p*~T~ region below 0.5 GeV/c, which is essential for an accurate
determination of the total charm yield since more than half of the
single electron yield from charm decays is in this *p*~T~ region.

### Open Beauty Production

Beauty quarks are predominantly produced by the initial parton-parton
collision. Because of the large mass almost no additional production is
expected from the pre-equilibrium stage or thermalized phase. As a
consequence, the measurement of open beauty is ideally suited to probe
the parton density in the coming nucleus and thus the initial parton
luminosity.

The open beauty yield can be measured via inclusive electron production,
or more directly through its decay *B  J/ψ +X* (B.R. 1.14%). The VTX
detector is essential for the detection of both channels. In the single
electron measurement, we cannot distinguish single electrons from open
charm and open beauty with the present PHENIX detector. Below *p*~T~ \~
2.5 GeV/c the open charm contribution to the non-photonic electron
spectrum, which is the inclusive electron spectrum after subtraction of
the light meson decay background, is much larger than that of beauty.
Thus, it is not possible to determine the open beauty component in this
low *p*~T~ range. This is *p*~T~ range that contains about 90% of the
electrons from beauty decays. Even in the high *p*~T~ region (*p*~T~\>3
GeV/c), where beauty is expected to be the leading source of
non-photonic electrons, there is a large uncertainty due to the unknown
charm contribution. Since beauty has a larger *c*τ (*B*^0^: 462 μm,
*B^+^*: 502 μm) than charm (*D*^0^: 123 μm, *D^+^*: 317 μm), we can
accurately split the beauty component of single electron from the charm
component using a precise displaced vertex measurement from the VTX.

The VTX also enables us to measure the *B**J/*ψ+*X* decay by tagging
*J*/ψ\'s with a vertex detached from the collision point. Although this
mode has a small cross section, it gives a clean signal of *B* in wide
momentum range, down to *p*~T~ = 0.

### Ratio of charm and beauty production and its centrality dependence

One of the interesting opportunities opened by a beauty measurement
using the VTX is the extraction of the (*c**e*)/(*b**e*) ratio as
function of the collision centrality. In this ratio, most of the
systematic uncertainties including acceptance, reconstruction
efficiency, luminosity, and number of collisions per event cancel. In
addition, since little or no enhancement of beauty relative to binary
scaling is expected at RHIC energy, the denominator (*b*  *e*) may
serve as a precise monitor of the initial parton luminosity, a role
similar to that of Drell-Yan production of muon pairs for J/ψ
suppression measurement by NA50. This ratio could provide a very
sensitive method to observe a small charm enhancement like it was
discussed in the previous section. As discussed in section 3.5, we could
obtain an accuracy of the centrality dependence of this ratio close to
\~1 % in statistical precision.

### Energy-loss of heavy-quarks

Colored high-pt partons are predicted to lose energy as they propagate
through the dense nuclear medium[^5]. The dominant mechanism is likely
medium-induced gluon radiation[^6],[^7] with a smaller contribution from
elastic collisions with lower-energy partons. Gluon radiation and
energy-loss are exquisitely sensitive to interference effects, since the
gluon formation time is comparable to the time between successive
collisions. Hence before we can quantitatively use the measured
energy-loss as a probe of the dense medium, we need to be confident that
the interference effects in the model calculations are well tested by
data. One powerful strategy is to change the amount of
gluon-interference by using heavy-quarks instead of light quarks.

![](media/image4.jpeg){width="3.3541666666666665in"
height="2.236111111111111in"}

[]{#__RefHeading___Toc55021890 .anchor}Figure 2 Ratio of Jet Quenching
factor QH/QL of heavy quark (QH) and light quark (QL) in high density
QCD medium as function of pT of the quark, from ref. Error: Reference
source not found. The solid line is with no energy cut-off for gluon and
the dashed line is with cut off of 0.5 GeV.

Heavy-quarks are predicted[^8] to lose less energy in the plasma because
of the "dead-cone effect". Qualitatively the large quark mass eliminates
the favored collinear gluon Bremsstrahlung. It also shortens the gluon
formation time and leads to a distinctly different destructive
interference around the heavy-quark's trajectory. Figure 2 shows the
ratio of jet quenching factor Q~H~/Q~L~ for heavy quarks (Q~H~) and
light quarks (Q~L~) as function of the *p*~T~ of the quark calculated in
reference Error: Reference source not found. The smaller energy loss due
to the "dead cone" effect leads to a factor of 2 less suppression of
high *p*~T~ charm quarks compared to light quarks.

Recent studies suggest that the magnitude of the
dead-cone[^9]^,^[^10]^,^[^11] may be smaller than anticipated in
reference Error: Reference source not found, which would lead to an
energy-loss for heavy quarks closer to that for light quarks. Djordjevic
and GyulassyError: Reference source not
found^,Error:\ Reference\ source\ not\ found^ have proposed that the
energy-loss for heavy-quarks is further reduced due to a plasmon
frequency cut-off effect in a thermalized medium. As a result precise
measurement of heavy-quark energy loss through open charm may enable a
measurement of partonic effective thermal masses in the medium.

As the opposite extreme, Batsouli *et al*[^12] have suggested that the
first electron measurements at RHIC can be reproduced by assuming that
charm particles flow hydrodynamically, i.e. the charm particles interact
with the medium with a large cross-section. To distinguish between these
effects and to explore this physics will require measuring the *p*~T~
spectra for open charm at high transverse momentum, out to several
GeV/c. This point is illustrated in Figure 3. The figure, taken from
reference Error: Reference source not found, illustrates that the *p*~T~
distribution of *D* mesons and single electrons from charm have little
difference in the two extreme scenario of no medium effect (shown in
dashed curves) and hydrodynamic model (shown in solid curves) within the
*p*~T~ range accessible by the current PHENIX setup. Obviously, a
measurement at much higher *p*~T~ range is required to distinguish the
models. Such a measurement is not feasible without the VTX upgrade.

![](media/image5.jpeg){width="4.054166666666666in"
height="3.438888888888889in"}

[]{#__RefHeading___Toc55021891 .anchor}Figure 3 Single electron data of
PHENIX compared with two extreme models of charm *p*~T~ distribution.
From ref. Error: Reference source not found.

Using the displaced vertices of kaons and pions we will be able to
measure the high-*p*~T~ spectra of charm directly via the hadronic decay
channels, e.g. *D**K*+π. In addition, it will also be possible to
separate single electron from beauty and charm decays. This extends the
momentum range of charm measurement in the inclusive electron channel
from *p*~T~^e^ \< 2.5 GeV/c to *p*~T~^e^ \~ 6 GeV/c into the range where
the effect of finite energy loss of charm quark is expected.

### Open charm reference to J/ψ suppression and enhancement

In the J/ψ studies done at CERN by NA38/50[^13] the J/ψ yields were
usually determined relative to the Drell-Yan di-muon yields with the
argument that the latter should have little final-state nuclear
dependence. But it is not clear how reliable this comparison really is
since the Drell-Yan process involves quarks ($q\overline{q}$
annihilation) while J/ψ production involves gluons (gluon fusion). It is
likely that the nuclear effects on the initial parton distributions for
quarks and gluons as well as their energy loss in the initial state
before the hard interaction are different. Additionally, the yield of
Drell-Yan dimuon pairs is quite small and thus limits the statistical
accuracy of the measurement. It seems much more natural to compare J/ψ
production to open-charm production, where the initial-state effects are
probably the same. Therefore a robust measurement of open-charm is quite
important for the physics of the J/ψ. At CERN this is now provided by
the NA60 experiment. It has also been suggested by some theoretical
groups[^14] that the effective gluon distributions are process
dependent, and different for e.g. open- and closed-charm production.
These models suggest that comparisons of open and closed charm are
important to establish the extent of higher-twist contributions to
closed charm production.

![](media/image6.jpeg){width="3.734027777777778in"
height="3.0368055555555555in"}

[]{#__RefHeading___Toc55021892 .anchor}Figure 4 The ratio of J/Ψ yield
and open charm yield predicted in ref. [^15]

Recently, a new mechanism for charmonium production in high-energy
nucleus-nucleus collisions has been proposedError: Reference source not
found^,^[^16]^,^[^17]. The basic idea is that charmonium can be formed
by re-combination of *c* and $\overline{c}$quarks when the bulk of the
hadrons are formed. Since about 10 to 20 $c\overline{c}$ pairs are
produced in a single event in central Au+Au collisions at RHIC, this
contribution can be very significant. It has been predicted that the
charmonium yield increases with the square of the open charm yield.
Figure 4 shows a prediction of reference Error: Reference source not
found, one of the recombination models. In this model, the ratio of J/ψ
yield over open charm yield has a minimum at $\sqrt{s}$\~ 40 GeV due to
interplay between J/ψ suppression in QGP and J/ψ formation via
recombination mechanism. An accurate measurement of charmonium to open
charm ratio over a broad range of impact parameters and collision
energies is essential to test these models.

### Open beauty and J/ψ suppression

Another important area, especially for J/ψ measurements, is the
production of beauty quarks. The decay of *B* mesons will produce J/ψ's
(BR \~ 1.14%) that tend to have somewhat higher *p*~T~ than prompt J/ψ
production. In a scenario where color-screening in a QGP destroys most
of the primary J/ψ's, it is conceivable that a large fraction of the
observed J/ψ's comes from *B* decays. An estimate by Lourenco[^18]
several years ago indicated that for central collisions the fraction of
J/ψ's from *B* decays might be as large as 20% overall, with even larger
fractions at high *p*~T~. Clearly one would like to measure the *B*
cross sections at RHIC energies so that a more reliable estimate of
their contribution to the J/ψ production can be made, an issue which
would be particularly important should a large suppression of J/ψ's be
seen in central Au-Au collisions at RHIC. How strong the suppression
actually is will be difficult to quantify without establishing how many
of the remaining J/ψ's do come from *B* decays.

### Thermal di-lepton and open charm

The hot and dense system that is created in the heavy ion collision
should emit electro-magnetic radiation during its time evolution, either
in the form of real photons, or in the form of virtual photons, which
materialize as lepton pairs. This thermal electro-magnetic radiation
directly probes the dense system. The production rate of the thermal
di-leptons is a steep function of temperature, and thus an accurate
measurement may enable us to determine the initial temperature of the
system.

![](media/image7.png){width="3.5131944444444443in" height="2.975in"}

[]{#__RefHeading___Toc55021893 .anchor}Figure 5 The di-electron
effective mass distribution in PHENIX central arm acceptance in central
Au+Au collision at $\sqrt{s_{\text{NN}}}$= 200 GeV predicted by Ralf
RappError: Reference source not found. In the intermediate mass region
(1 \< Mee \< 2.5 GeV), the dominant sources of electron pairs are open
charm and thermal radiation from the QGP and hot hadronic gas.

There are several processes that contribute to the di-lepton continuum.
Qualitatively, the Drell-Yan process dominates the high-mass region,
while thermal pairs from the hadron gas dominate the low-mass region. At
RHIC energies, thermal radiation from the quark-gluon plasma is
predicted to be the major source of di-leptons in the intermediate mass
region of 1\<M~ee~\<3 GeV. In this mass range thermal radiation competes
with a large background from semi-leptonic decays of correlated
$D\overline{D}$pairs. Figure 5[^19] compares predictions for the thermal
di-electron continuum above the φ resonance to di-leptons from open
charm. The di-electron yield from open charm is comparable to the
thermal di-lepton signal at M~ee~=1 GeV, and is two to three times as
large in the mass range of 1.5 to 2.5 GeV. It is clear that one cannot
extract the thermal dilepton yield without knowledge of open charm
contribution. Knowledge of single electron production from open charm is
insufficient to subtract this contributions, since the mass of the
di-lepton also depends on the correlation between the $D$ and
$\overline{D}$ mesons. Thus it is imperative to directly measure the
di-lepton spectrum from correlated charm pairs in order to observe and
to accurately determine the thermal dileptons from the quark-gluon
plasma. This measurement will only be possible with the VTX upgrade.

### High *p*~T~ phenomena with light flavor in 10 -- 15 GeV/c in *p*~T~

The suppression of the high *p*~T~ particle production is probably the
most direct evidence of formation of very dense matter in high-energy
nucleus-nucleus collisions at RHIC so far. The creation of dense matter
is now firmly established from the high *p*~T~ data in Au+Au collision
and the comparison data in d+Au collisions. The natural next step is to
extend the data, now in *p*~T~ range of up to 10 GeV/c, to reach much
higher *p*~T~ to study the nature of the high *p*~T~ suppression.

In the present PHENIX detector, the *p*~T~ range of the charged particle
measurement is limited to 10 GeV/c in *p*~T~ due to a large background
from photon conversion and decay in flight of light mesons. The present
central arm spectrometer suffers from these backgrounds since it
measures particle tracks only outside of the magnetic field. Thus, it
cannot distinguish a real high *p*~T~ track that originates from the
event vertex from a background track that is produced far from the
vertex either by photon conversion or by decay-in-flight. The VTX
detector will eliminate these backgrounds by providing additional
tracking near the event vertex. In addition, the VTX measurement will
improve the *p*~T~ resolution by about a factor of three (see 3.4) by
measuring the initial emission angle of the track in a slightly
increased magnetic field. Combined, the *p*~T~ range of the charged
particle measurement in PHENIX will be extended to beyond 15 GeV/c or
more, and will be limited only by the statistics.

### Measurement of Upsilon states

Given sufficient RHIC luminosity, we will be able to measure the
ϒ-states ($b\overline{b}$bound states), and to compare closed and
open-beauty production. It is particularly interesting to measure the
relative yield of the three ϒ states, as we can study the suppression of
heavy quarkonia as function of the binding energy in a region of large
binding energy that is not accessible by charmonium production. In
addition, unlike charmonium, the contribution to ϒ production due to
quark recombination must be negligible since the number of
$b\overline{b}$ pairs produced in an event is very small. Thus in the ϒ
production we can directly access the de-confinement effect in dense
matter. As mentioned previously with the VTX detector, the momentum
resolution will be improved by about factor three, which reduces the
mass resolution to \~ 60 MeV so that a clean separation of the 1S, 2S
and 3S ϒ states becomes possible. However, this measurement will only be
possible if luminosities significantly above the RHIC design value of
2×10^26^ cm^-2^ are reached.

## 2.2 Determination of spin structure of nucleon.

### Exploring the spin structure of the nucleon: The past

Most of what we know about the origin of the nucleon spin comes from
Deep Inelastic Scattering (DIS) experiments performed over the last
three and half decades. They used polarized electron or muon beams in
the momentum range 20-200 GeV/c to impinge on stationary polarized
gaseous or solid-state targets. The partonic interaction that occurs in
such experiments is between the virtual photons (coming from the
polarized lepton beams) and the quarks inside the nucleons of the
stationary targets. Naturally, DIS is an excellent probe of the quark
polarization in the nucleons. In the late 1980s, measurements were made
for the first time at higher energies and a significant deficiency in
the quark contribution to the nucleon spin was discovered. Often called
in the literature "Spin Crisis", the quest to understand this deficiency
has driven the experimental and theoretical work in the field of nucleon
spin since then. Where is the rest of the nucleon spin? The obvious
place to look is the gluons and to measure their contribution. The
virtual photons in the DIS only interact weakly with the gluons, as
such, one can access the gluon spin dynamics in DIS only through scaling
violations of spin structure functions which requires their measurement
over a large range of x and Q^2^. As of today, such an experimental
facility is unavailable and so one has to consider other techniques and
tools to access the gluon spin.

### Gluon polarization measurement at RHIC:

The new tool that we have been waiting for is the Relativistic Heavy Ion
Collider (RHIC). It enables collisions between polarized proton beams at
high energy (up to 250 GeV/c). The expected luminosities at these top
beam energies are 2x10^32^ cm^-2^ sec^-1^. As of now, 100 GeV/c
polarized protons have been collided with few x 10^30^ cm^-2^ sec^-1^
luminosity. Since protons are abundant sources of gluons, polarized
proton-proton collisions allows a direct exploration of the gluon spin
dynamics at the partonic level. The differences in the hadronic final
states originating from gluon-gluon and quark-gluon interactions in the
polarized proton collisions measured by the detectors when the proton
spins in the two colliding beams are aligned vs. anti-aligned gives us
access to the gluon spin contribution to the proton. For a partonic
interaction of the kind (a+b c+d) occurring in polarized pp collisions,
assuming factorization one can write:

$A_{\text{LL}} = \frac{\Delta a}{a}\frac{\Delta b}{b}a_{\text{LL}}(a + b \rightarrow c + d)$
(1)

Here ∆*a/a* and ∆*b/b* are the ratios of polarized to unpolarized
distributions for parton distributions of *a* and *b* respectively, and
*a~LL~* is partonic analyzing power calculable in pQCD. *A~LL~* is the
double spin asymmetry measured in the experiment as a result of the
polarized proton proton scattering for the final state in which *c* and
*d* are created and measured in the detector. In this particular
example, either *a* or *b* or both could be gluon distributions in the
colliding protons.

In the PHENIX experiment we will measure ∆G/G using many different
processes. A partial list includes gg, gq in the partonic initial state
resulting in different final states:

1)  inclusive neutral and charged pions

> $$a_{\text{LL}}(\text{gg},\text{gq} \rightarrow \pi^{0, \pm},X)$$

2)  inclusive photon production (direct or prompt photon production)

> $$a_{\text{LL}}(\text{gg},\text{gq} \rightarrow \gamma + X)$$

3)  charm & anti-charm and beauty-anti-beauty pair production

> $$a_{\text{LL}}(\text{gg},\text{gq} \rightarrow \overline{c}c,\overline{b}b + X)$$

4)  direct photon production along with jet

> $$a_{\text{LL}}(\text{gg},\text{gq} \rightarrow \gamma + \text{jet} + X)$$

For different final states, experimentally we measure the following
double spin asymmetry (a counting rate asymmetry):

$A_{\text{LL}} = \frac{1}{P_{B}P_{Y}} \ast \frac{N_{\text{++}} - R \ast N_{+ -}}{N_{\text{++}} + R \ast N_{+ -}}$
(2)

Where P~B/Y~ are the blue and yellow beam polarizations, *N*~++/+-~ the
counting rates measured with the ++(parallel) and +-(anti-parallel)
orientations of the proton beam spin vectors and R is the ratio of
luminosities for ++ and +- spin orientation collisions. (Ideally, R=1).

### The role of Silicon Vertex Detector:

The different channels with which PHENIX can make measurements of the
gluon polarization cover different kinematic regions in x and Q^2^.
Figure 6 shows the x coverage possible with RHIC p-p running at 200 GeV
center of mass, \~70% beam polarization and \~300 pb^-1^ luminosity
(delivered) with the PHENIX detector for the above mentioned physical
processes under two different scenarios. Here x is the gluon momentum
fraction of the proton momentum, and "coverage" implies we measure the
ratio ∆G/G with \~20% relative uncertainty of its expected value at that
x. The baseline PHENIX detector is capable of covering a range: 0.02 \<
x \< 0.3 (shown in blue). We note that although the coverage extends
over one decade in x, between the different channels there is little
overlap. The coverage extended by the VTX silicon is shown in the same
figure (in red).

The proposed silicon vertex detector will be crucial in the
determination of gluon distribution in two significant ways:

1)  Different measurements will cover the same kinematic regions: this
    would enable the much-needed cross-checks within PHENIX for
    accessing the polarized gluon distribution. The vertex detector
    extends the reach in *x* for many of the measurements and hence adds
    a significant amount of overlap in *x*-range coverage.

2)  By being able to observe displaced vertices at low-pt for
    semi-leptonic decays of charm and beauty, the VTX detector enables a
    larger x-range over which we will make gluon polarization
    measurements. It is estimated that the x reach of the silicon-vertex
    upgraded PHENIX will be 0.01 \< x \< 0.3.

![](media/image8.png){width="4.506944444444445in"
height="3.6618055555555555in"}

[]{#__RefHeading___Toc55021894 .anchor}Figure 6 Expected x-ranges for
polarized and un-polarized gluon distribution measurements in PHENIX
using different channels. The blue bars indicate PHENIX detector's
existing capability while the red bars indicate the enhanced coverage
provided by the proposed silicon vertex detector upgraded PHENIX.

Since the two measurements of open charm and beauty and of gamma+jet
crucially depend on the silicon vertex detector more details are
provided on these two channels below.

### The heavy quark physics (open charm and beauty production)

By requiring an additional cut on displaced vertex information coming
from the vertex detector, we gain significantly in the robustness of the
heavy-quark results by improving the purity of the event sample.

We plan to observe charm production through its semi-leptonic decay to
*e*^±^. We will need a good vertex resolution to identify the displaced
vertices in such events. The main backgrounds expected for this physics
include Dalitz decays and photon conversions. This has been studied
(Section 3.2) using a GEANT detector simulation. We estimate that the
SVTX could achieve \~50μm DCA resoluiton. Using a DCA cut value \~200 μm
for tracks with *p*~T~ \> 1 GeV/c, we should be able to achieve a
significant background reduction. As a result of the DCA cut the purity
of the event sample increases from \~50% to \~90% (see Figure 12 in
section 3.2).

Another possible channel to access gluon distributions is open beauty
production. Beauty production measured at the Tevatron at 1.8 TeV, and
the next-to-leading order pQCD calculation missed the data by factor of
2 or greater. The discrepancy between the experimental data and the
theory has sparked much debate and excitement recently. New data on
beauty production would be crucial, especially at RHIC, since they would
be obtained at different values of  s (200 and 500 GeV).

Measurements of beauty production can be performed in the present PHENIX
detector using electron-muon coincidence using the central and forward
muon arms. With the limited acceptance for the detector subsystems, this
results in a narrow kinematics coverage and small detection cross
section. With the VTX detector, we have two additional channels to
measure beauty production at RHIC: the single electron in the central
arm and *B*J/ψ+X. The single electron channel provides us much higher
statistics compared with the μ-*e* channel.

The main background in the *b* physics measurements is expected from the
charm semi-leptonic decay, Dalitz decays, and photon conversions.
Information provided by the VTX detector will enable a cut on the DCA to
produce a highly pure sample of events involving beauty quarks with less
than \~10% impurity from charm quark events in the low *p*~T~ range (\<
3 GeV) and even purer *b*-sample at higher *p*~T~. Without the SVTX this
impurity is expected to be more than 75% (see Figure 19, section 3.2).
The VTX and the DCA analysis of data it will thus produce a reliable
data set highly devoid of charm events and other impurities for the
comparison with theory for beauty production cross section.

The displaced vertex resolution possible with the VTX detector enables
additionally one more measurement: *B*J/ψ+X. *B* mesons could be
identified with J/Ψ decays detected as displaced electron-pair vertices.
This process identifies open beauty production with no charm
contribution and will be a clean probe of the polarized and the
un-polarized gluon distributions.

Finally, a recent theoretical study (I. Bojak, Ph.D. Thesis, April 2000,
Univ. Dartmund) of the expected values of the open charm and open beauty
asymmetries at high energy concluded that they would be of the order of
a few times 10^-3^ at RHIC energies. The open beauty asymmetries are
expected to be slightly larger (private discussions with W. Vogelsang).
False asymmetries related to bunch-to-bunch variation of luminosity in a
collider are potentially a show-stopper for any spin measurement if they
are comparable in magnitude to the asymmetry one is interested in.
However, from the ongoing RUN 3 analysis we already know at RHIC these
false asymmetries can be controlled to be smaller than a few times
10^-4^. Although this situation could potentially get worse with the
RHIC luminosity increase (due to difficulties associated with handling
higher beam currents), additional tools are being discussed at RHIC that
are expected to reduce uncertainties due to such effects by a factor of
\~10 using techniques such as simultaneous spin flips in both RHIC beams
using a spin flipper magnet and beam re-cogging. With such anticipated
developments we will be able to pursue the open charm and open beauty
*spin physics* measurements at PHENIX with the proposed Silicon Vertex
detector.

### Direct photon + Jet measurement:

One of the limitations of the direct photon measurement that is possible
with existing PHENIX detectors, is an imperfect determination of the
partonic kinematics in the event. The uncertainty in the determination
of the x of the gluon exists because we observe only a single photon in
the final state. Event-by-event reconstruction of the event kinematics
is impossible, and one has to rely on the Monte Carlo simulations to
understand the event kinematics coupled to the detector acceptance. This
has been studied (see section 3.3) using a PYTHIA simulation. The
proposed VTX detector enables the tagging of the hadronic activity
(originating from a single quark/jet), hence determination of the jet
axis (Figure 20), and will make the uncertainties related to the
reconstruction of the event kinematics significantly better (Figure 21).
Our dependence on Monte Carlos is factored out. Additional uncertainties
related to the determination the total jet energy, remain, however one
does better by tagging the jet with the proposed VTX.

PHENIX's limited acceptance in rapidity as well as azimuth has been a
significant hurdle in our measurement of any jet related physics. The
silicon vertex detector with its good hit resolution and large
acceptance will serve as a high-resolution tracker and provide the much
needed jet axis measurement in co-incidence with the direct photon
measurement. Monte Carlo studies indicate a significantly improved
determination of x-gluon (20% relative compared to \~40% without the
VTX). The VTX detector can also be used to detect charged tracks around
the direct photon candidate. This may allow an improved isolation
selection for the direct photon in the event.

The silicon vertex detector is hence crucial in determining the
polarized gluon distribution using the direct photon channel. For this
particular measurement it converts PHENIX detector in to a high
resolution - large acceptance detector.

### Other advantages of the Silicon Vertex Detector:

There are other advantages of the silicon vertex detector, which we
mention briefly in this section.

#### Background suppression for W physics event sample 

*W* physics at PHENIX allows a unique possibility to distinguish the
flavor (*u* and *d*) dependence of quark structure function and its
polarization: *W*^+^ is produced by collision of $u + \overline{d}$ ,
while *W*^-^ is produced by $d + \overline{u}$. However, if one wants to
explore *W* physics with electron final states in the central arm,
backgrounds from hadrons can be a significant problem. Improved momentum
resolution and (hence) background suppression is the way to reduce the
background. Using information from the silicon vertex detector in the
momentum reconstruction, the moment resolution is improved by a factor 2
or 3. In addition, the large solid angle coverage of SVTX will allow us
to apply an isolation cuts for the single electron candidate and thereby
to improve S/B ratio of the W decay electron. In general, electron from
W decay is isolated from a jet activity, while the backgrounds (very
high *p*~T~ charged particle decays and high *p*~T~ electrons from heavy
quark decay) have associated jet activity around it.

#### Improved correlation measurements between particles

It is expected that the large acceptance of the silicon vertex detector
in rapidity and azimuthal coverage, will enable us understand
correlations between particles produced in the hadron-hadron collisions.
One important measurement is related to the transversity distribution:
Transversity structure function is as fundamental as any other
(un-polarized and polarized structure function of the nucleon), but it
is yet to be measured. It is a helicity odd object, and it needs to be
measured in experiments as a product of another helicity odd object so
that the product is helicity even. Measurements of this kind involve
measuring many particles and their angular correlations in the final
state in addition to possible hadronic jet activity in the primary
interaction. One example of this is the Collins fragmentation function,
which refers to a correlation between hadron distributions around the
jet axis. The orientation of π+ π- (hadron-) pair is also expected to
show correlation with the transverse fragmentation function in single
transverse spin *pp* scattering at RHIC. The Silicon vertex detector is
expected to improve determination of this orientation in spite of the
fact that lack of particle-ID associated with such an event will dilute
the correlation. Through these correlation functions we plan to measure
the transversity distribution. Needless to say, enhanced acceptance,
resolution provided by the silicon vertex detector would be crucial for
such a measurement.

## 2.3 Exploration of the nucleon structure in nuclei

Proton-nucleus collisions not only provide important baseline
information for the study of QCD at high temperatures, they also address
the fundamental issues of the parton structure of nuclei. Since the
discovery of the EMC effect in the 1980\'s, it is clear that the
parton-level processes and structure of a nucleon are modified when
embedded in nuclear matter[^20]. These modifications reflect fundamental
issues in the QCD description of the parton distributions, their
modifications by the crowded nuclear environment of nucleons, gluons and
quarks, and the effect of these constituents of the nucleus on the
propagation and reactions of energetic partons that pass through them.

Of particular interest is the depletion of low momentum partons (gluons
or quarks), called shadowing, which results from the large density of
very low momentum partons. For gluons at very low momentum fraction, *x*
\< 10^-2^, one can associate with them, following the uncertainty
principle, a large distance scale. These high-density gluons then will
interact strongly with many of their neighbors and by gluon
recombination or fusion are thought to promote themselves to larger
momentum fraction, thus depleting small values of *x*. In most pictures
the overall momentum is conserved in this process and so the small *x*
region gluon density is depleted while the moderate *x* region above
that is enhanced. In recent years a specific model for these processes,
called gluon saturation, has been discussed extensively by McLerran and
collaborators[^21]. Gluon saturation affects both the asymptotic
behavior of the nucleon gluon distributions as *x* approaches zero and
the modification of this behavior in nuclei, i.e. shadowing.

At RHIC energies many of the observables are affected by parton
distributions at small x where nuclear shadowing is thought to be quite
strong. However, theoretical predictions of the amount of shadowing
differ by factors as large as three. For example, in the production of
J/ψ in the large rapidity region covered by the PHENIX muon arms, models
from Eskola et al (Figure 7) predict only a 30% reduction due to gluon
shadowing, while those of Frankfurt & Strikman[^22] (Figure 8) or
Kopeliovich[^23] predict up to a factor of three reduction. Results from
the measurements of the just-completed d-Au run should help to clarify
how much shadowing we have, but increased statistics from higher
luminosity runs and more definitive measurements via observables that
are sensitive to gluon structure functions over several channels will be
necessary to test the theory with sufficient power to constrain the
underlying QCD processes.

![](media/image9.png){width="4.079861111111111in"
height="2.6381944444444443in"}

[]{#__RefHeading___Toc55021895 .anchor}Figure 7 - Gluon shadowing from
Eskola[^24] as a function of x for different Q^2^ values: 2.25 GeV^2^
(solid), 5.39 GeV^2^ (dotted), 14.7 GeV^2^ (dashed), 39.9 GeV^2^
(dotted-dashed), 108 GeV^2^ (double-dashed) and 10000 GeV^2^ (dashed).
The regions between the vertical dashed lines show the dominant values
of x~2~ probed by muon pair production from DDbar at SPS, RHIC and LHC
energies.

In particular, it is clear that a precise knowledge of the shadowed
gluon structure functions in nuclei is essential towards understanding
several of the important signatures for QGP in heavy-ion collisions at
RHIC, including open and closed heavy-quark production. Recombination
models for J/ψ production, which might cause an enhancement of that
production in heavy-ion collisions due to the large density of charm
quarks created in a collision, must be constrained by an accurate
measurement of the amount of charm produced given the shadowing of the
gluon densities in the colliding nuclei.

A number of other physics issues besides shadowing also need to be
understood. Energy loss of partons in the initial state is thought to
have a small effect at RHIC since the energy loss per fm, in most
models, is thought to be approximately constant and small compared to
the initial-state parton energies at RHIC. On the other hand, partons in
the final state could show some effects of energy loss since their
momentum is lower, while heavy-quarks are expected to lose less energy
than light partons due to the dead-cone effect[^25]. These issues are
very important in the high-density regions created in heavy-ion
collisions, but need a baseline for normal nuclear densities from
proton-nucleus collisions. Another general feature of most produced
particles comes from the multiple scattering of initial-state partons,
which causes a broadening of the transverse momentum (Cronin effect) of
the produced particles.

In general, all processes suitable for the measurement of gluon spin
structure in nucleons are also ideal for probing the gluon distributions
in nuclei. The reach in Bjorken *x* is indicated in Figure 8,
superimposed on calculations of the ratio of nuclear to nucleon gluon
structure functions.

![](media/image10.png){width="4.034722222222222in" height="3.15625in"}

[]{#__RefHeading___Toc55021896 .anchor}Figure 8 - Gluon shadowing
predictions along with PHENIX coverage. The red bars indicate the
additional range provided by the vertex upgrade, while the blue bars
cover the PHENIX baseline. The three theoretical predictions are for
different Q transferred, blue, green and red lines are Q = 10, 5 and 2
GeV/c respectively, from Frankfurt and Strikman[^26].

The red bars indicate the additional coverage provided by the vertex
upgrade compared to the baseline of PHENIX. The vertex upgrade extends
the *x*-range from the anti-shadowing region into the shadowing domain
and therefore will provide a measurement of shadowing and establish the
shape of the shadowed structure functions versus *x*.

Drell-Yan measurements, which provide a direct measure of the anti-quark
distributions in nucleons or nuclei, have always been limited in the
past in their reach to low *x* by the inability to separate the
Drell-Yan muon pairs below the J/Ψ in mass from copious pairs from
open-charm decays in that mass region. For example, in FNAL E866/NuSea,
information extracted from the Drell-Yan process was limited to masses
above 4 GeV.

![](media/image11.png){width="4.843055555555556in"
height="2.3361111111111112in"}

[]{#__RefHeading___Toc55021897 .anchor}Figure 9 - Dimuon mass spectrum
from E866/NuSea[^27] showing the mass region used in their analysis
which excludes masses below 4 GeV. Lower masses were excluded because of
the large backgrounds from open charm in that region.

On the other hand, PHENIX, with the addition of a vertex detector,
should be able to identify and quantify the portion of the lower mass
dimuon continuum from charm decays and therefore isolate the Drell-Yan
process at these lower mass and lower *x* values. In the
central-rapidity barrel region values as low as *x~2~* \~ 0.7x10^-2^
could be accessed. This will still be a challenge because of the small
cross sections and yields for Drell-Yan at RHIC, but has the potential
of providing information on the anti-quark distributions at much smaller
values of *x*. At the same time one would also learn more about charm
production and the correlation of the charm pairs through the charm
pairs found in the continuum.

In summary, the silicon vertex barrel, which covers the PHENIX central
arm mid-rapidity range ( \|*y*\| \< 0.35 ), addresses the following
physics in dA reactions :

-   Charm and beauty at high p~T~ and mid-rapidity via high-*p~T~*
    electrons and also exclusive decays such as $D \rightarrow K\pi$ and
    $D \rightarrow K\text{ππ}$.

-   A gluon structure measurement in the anti-shadowing region as a
    baseline for shadowing measurements at small *x*.

-   Charm measurements at mid-rapidity as a baseline for J/ψ production,
    i.e. for comparisons of open and closed charm which should share the
    same initial-state effects in nuclei.

-   Accurate measurement of nuclear dependence of charm cross section

-   Beauty cross sections at mid-rapidity as a constraint of the
    contributions of $B \rightarrow \frac{J}{\psi}$to J/ψ production.

-   Comparison of light and heavy-quark *p*~T~ distribution to determine
    differences in energy loss and Cronin effects.

-   Better separation in high-luminosity measurements of ϒ measurements
    of the three ϒ states.

-   Low-mass electron pairs and anti-quark shadowing at small *x*
    values.

For many of these topics the physics picture that can be obtained is
significantly strengthened with the planned addition of an endcap
silicon vertex detector as described in Appendix A.

# 3. Physics measurements with the VTX detector

## 

The proposed VTX detector provides us the tool to measure new physics
observables that are to date not accessible at RHIC or only with very
limited accuracy. These include a precise determination of the charm
production cross section and transverse momentum spectra - particularly
at high *p*~T~, a measurement of beauty, and the detection of recoil
jets in direct photon production. In this section, we discuss how the
proposed VTX detector makes these measurements possible, or
significantly improves our capability to address these observables.

Before discussing the simulation results and the expected performance of
the VTX for each of the observables, we briefly explain the design of
the proposed VTX detector and discuss the required performance. More
technical details of the implementation of the VTX detector are
presented in the section 4. In the last sub-section, we summarize the
expected rate of physics signals and the physics reach that will become
available with the VTX detector.

## 3.1 Performance Requirements and the VTX detector geometry

### Performance Requirements

The performance requirements for the detector are

-   ability to match tracks reconstructed in the central arms to hits or
    track segments from the silicon vertex detector.

    -   sufficient position accuracy so that the displacement resolution
        of the track with respect to the collision point is less than
        the cτ of charm and beauty decays, i.e. a resolution less than
        100μm, preferably at the level of 30-50 μm.

    -   high resolution predominantly in rφ direction, i.e. the main
        bend plane of the magnetic field, matched to the central arm
        resolution.

A variety of simulations and first principle calculations have shown
that the displacement resolution is dominated by the position accuracy
of the two inner most detector layers and by the amount of
multiple-scattering between the collision point and the two position
measurements. Assuming that the multiple scattering occurs at the
location of the first layer, the Distance to the Closest Approach (DCA)
of a trajectory to the beam axis in the main bend plane can be measured
with a resolution given approximated by:

$$\sigma^{2_{\text{DCA}}} \approx \frac{\left( \sigma_{1^{2}}r_{2}^{2} + \sigma_{2}^{2}r_{1}^{2} \right)}{(r_{2} - r_{1})^{2}} + \theta_{\text{ms}^{2}}\frac{r_{1}^{2}}{\text{sin}^{2}\theta}$$

Here σ~1~, σ~2 ~ and *r*~1~, *r*~2~ are the *rφ* resolution and radial
position of the first and second layer, respectively. The average
multiple-scattering angle, denoted by *θ*~ms~, is given by the beam pipe
thickness and the first detector layer. The first term quantifies the
contribution due to the finite position resolutions, while the second
term is the effect of the multiple scattering. Given standard silicon
detector segmentation of 50 to 100 μm in *rφ* and a typical thickness of
1 to 2 % of a radiation length, both terms contribute to the final
resolution. For our \"strawman\" layout of the VTX, which is discussed
below, typical DCA resolution from this estimate is \~ 40 μm: a value
confirmed by detailed simulations.

It is evident from the equation for the DCA resolution that in order to
minimize the DCA resolution, the first layer should be as close to the
collision point as is practical, which at RHIC is about 2 cm, and the
first layer plus beam-pipe should be as thin as possible.

![](media/image12.png){width="2.901388888888889in"
height="2.825in"}![](media/image13.png){width="2.825in"
height="2.825in"}

[]{#__RefHeading___Toc55021898 .anchor}Figure 10 (a) Cross section of
the silicon vertex tracker (VTX) along the beam axis. The inner pixel
hybrid layer is located at a radial distance of 2.5 cm from the beam
pipe and extends over \~22cm in beam direction. The silicon strip outer
layers are located at 6, 8 and 10 cm. All three extend over \~26 cm in
beam direction. The Be beam pipe with 2 cm radius is also shown. (b) Cut
through the silicon vertex detector in the xy-plane transverse to the
beam axis. The VTX is assembled in two half shells with small acceptance
gaps at top and bottom. The different layers of each half shell have 5,
7, or 9 rows of silicon detectors, depending on the radial location of
the layer.

### VTX detector geometry

After exploring different configurations, we decided to perform all
feasibility studies with the detector layout depicted in Figure 10. The
layout features four concentric barrels of silicon detectors with a
length of approximately 30 cm along the beam axis. The outer three
barrel layers are silicon strip detectors placed at radial positions of
*r~4~*=10 cm (barrel 4), *r~3~*=8 cm (barrel 3), and *r~2~*=6 cm (barrel
2). The inner barrel is composed of a silicon pixel device and is
located at *r~1~*=2.5 cm (barrel 1). The beam-pipe has a design radius
of 2.0 cm and is made of 500 μm thick Be.

The pixel detectors for the inner barrel layer have a segmentation of 50
μm by 425 μm. The outer layers are assumed to have 80 μm × 3cm strips.
This choice together with the radial location guarantees that the
occupancy of the innermost strip layer (barrel 2) remains below 10% even
in central Au-Au collisions. In the low occupancy environment of pp
collisions a stereoscopic readout of the strip layers reduces the
effective channel size to 80μm × 1000μm.

These detectors have been implemented as our \"strawman design\" into
the GEANT simulation of the PHENIX detector. At this point no details of
cooling, mechanical support, cables etc. are put into the simulation,
however these materials are approximated by an effective thickness of
the detector layers. From a survey of existing silicon detectors we
conclude that a thickness of 2% of a radiation length per layer should
be easily achievable while 1% of a radiation length will be challenging.
We have chosen these two values for this effective thickness, 1% and 2%
of a radiation length, to bracket the potential range of thickness and
performed simulations with both values.

### Central Track -- VTX matching

In order to make full use of the VTX capabilities, tracks reconstructed
in the central arms have to be matched to hits or track segments in the
VTX. In the following we estimate the fraction of ambiguous track
matches for the most difficult case, namely for central Au-Au collisions
at 200 GeV. For *pp* or more peripheral Au-Au collisions track matching
should not be a problem.

In central Au-Au collisions the charged particle density has been
measured and is *dN/dη* \~ 600 at η=0. In the following we double this
number to account for hit splitting, noise, and background hits. In the
pixel detector the occupancy then is about 0.7% per pixel or 0.2 hits
per milli-steradian. This is a conservative estimate. We expect that
merging hits to clusters can eliminate the effect of hit splitting and
that most of the noise and background hits can be removed by correlating
information from several planes. Thus, more optimistically the hit
density may be closer to 0.1 per mili-steradian and consequently the
following occupancy estimates may be reduced by a factor 2.

The pointing resolution of reconstructed tracks in the central arm
spectrometers is well known. It is given by the performance of the drift
chamber, which has a single point spatial resolution of 150 μm in
azimuth and 2 mm in inclination. Providing a reference point, *P*, at a
reference radius, *R*, in the drift chamber active volume, an angle in
the bend plane, α, can be defined as the angle between a straight line
projection from the collision vertex to *P*, and the bend plane vector
of the reconstructed track at *P*. The momentum (in GeV/c) of a track is
related to α by *p* = 87 mrad GeV/c / α. Here, 87 mrad GeV/c is the
effective field integral, *K*~1~, of the trajectory to point *P*.
Utilizing this relationship, the momentum resolution can be related to
the intrinsic angular resolution of the drift chamber, σ~α~ and the
angular resolution due to multiple scattering from the collision vertex
to point *P*, σ~ms~, as:

$\left( \frac{\delta p}{p} \right)^{2} = \left( \frac{\sigma_{\alpha}}{K_{1}}p \right)^{2} + \left( \frac{\sigma_{\text{ms}}}{K_{1}\beta} \right)^{2}$

Using the measured widths of distributions of the square of the
reconstructed mass for identified protons, pions, and kaons, as a
function of momentum, the values of σ~α~ and σ~ms~ have been extracted
from a simultaneous fit of all three of these particle species from the
RHIC 200 GeV run. The fit result yields σ~α~ = 0.86 mrad and σ~ms~ =
0.84 mrad, averaged over positive and negative particles. The results
are independent of centrality. Added in quadrature, the bend-plane
angular resolution is 1.20 mrad. The angular resolution of the drift
chamber in the non-bend plane is 2.70 mrad if the information from the
event vertex is used; here the *z* resolution of the PHENIX beam-beam
counter, which is 6 mm, was assumed for the vertex.

We assume that in central Au-Au collisions the VTX information will
allow to determine the interaction point to better than 10×10 μm^2^ in
the bend-plane and to better than 50 μm along the beam direction. With
this vertex accuracy the VTX determines the original direction of the
track with an uncertainty of 0.7 mrad in azimuthal and 5 mrad
inclination direction. To match the central tracks in angular space we
need to account for the central track resolution and multiple scattering
in the 4 silicon layers (assume 2% X~0~ per layer and 1 GeV/c momentum).
If we assume ±2σ matching window the search area is ±8 mrad in azimuth
and ± 12 mrad in inclination. The solid angle of this search area is
0.384 milli-steradian and thus the expected random match probability is
7.8%. Requiring hits from the different VTX layers will reduce the
random match.

This estimate is based on the vertex position and the hit information
from the pixel detector. If tracks or track segments can be
reconstructed in the VTX the number of random matches can be further
reduced. The reduction will depend on the level of track reconstruction
in central collisions that will be possible with the VTX alone. Any
tracking will reduce the number of hits in the pixel detector to a
smaller number of tracks. Basic track reconstruction will allow charge
matching and more sophisticated analysis may even allow momentum
matching to the central arms. Quantification of the potential reduction
requires detailed Monte Carlo studies which are under way.

## 3.2 Open Charm and Beauty Measurement

### Open Charm measurement from semi-leptonic decay

Open charm and beauty spectra and yields are a sensitive probe of the
early stages of heavy-ion collisions, and are keys for the physics goals
of gluon spin structure and for structure function studies in *pA*
reactions. For heavy-ion collisions, the goal is to improve the accuracy
and precision of the charm measurement, and to extend the reach to
higher *p*~T~ to measure the energy-loss of open charm. The yield of
beauty in heavy-ion collisions should be dominated by the initial hard
collisions and hence will serve as a critical benchmark for the first
stage of reaction.

For open charm our strategy is to use semi-leptonic decays to electrons
in a wide *p*~T~ range and to complement this at high *p*~T~ with
hadronic decay channels. The four layers of the central silicon barrel
provide an accurate measurement of the trajectory and impact parameter
of tracks near mid-rapidity. Single electrons at different momenta were
simulated and tracked through the GEANT implementation of PHENIX
including the straw-man vertex detector. The simulation was run assuming
zero magnetic field. The hits from the electrons were tracked back to
calculate the transverse distance-of-closest approach (DCA) to the known
point-of-origin. For all cases the DCA resolution is better than or
comparable to the cτ of charm and beauty decays.

![](media/image14.jpeg){width="4.024305555555555in"
height="3.8090277777777777in"}

[]{#__RefHeading___Toc55021899 .anchor}Figure 11 DCA distribution for
electrons from Dalitz, charm and beauty decays simulated through four 1%
Si layers on the left and four 2% layers on the right.

The power of this resolution is seen by comparing the distribution of
DCA from charm, beauty and Dalitz decays of π^0^ in Figure 11. The
spectra were generated from p+p events (PYTHIA) passed through GEANT.
The design thickness for the inner pixel layer is 1% of radiation length
(X~0~) and for the outer barrel layers is 1.7% X~0~. Hence for these
simulations we bracket the range of possibilities by running simulations
with either 1% or 2% X~0~ per layer. In the bottom panels of Figure 11
are the DCA distributions for electrons above 1 GeV/c. A DCA cut of 200
μm removes the majority of Dalitz contribution from the electron yield.
Note that this is the momentum of the electron and given the large
Q-value of the *D*-decay, these electrons predominantly come from
low-momentum *D*'s. Since beauty decays have longer lifetimes, the
electrons from *B* decays dominate at large DCA values. By fitting the
full DCA distribution with the expected shapes from the different cτ we
should be able to simultaneously extract integrated charm and beauty
yields for electron momenta above 1 GeV/c.

Below 1 GeV/c the extraction is more difficult, but even down to 500
MeV/c charm dominates the DCA distribution above 200 μm for a thickness
between 1 and 1.5% of a radiation length. In particular, the vertex
detector will provide a dramatic improvement over the previous
measurement^15^, which was limited to 40% systematic error resulting
from the uncertainties in the background subtraction. Figure 12 shows
the "signal to noise" for the optimistic (1% X~0~) and pessimistic (2%
X~0~) cases compared to the measurement without the vertex detector
available. This should allow a much cleaner extraction of the
background-subtracted electron spectra for *p*~T~\>0.4 GeV/c than was
previously available, including the charm-specific *d^2^N~e~/dydp~T~*
for electrons up to about 2.5 GeV/c.

![](media/image15.wmf){width="4.0in" height="2.8826388888888888in"}

[]{#__RefHeading___Toc55021900 .anchor}Figure 12 Signal to Background
ratios as a function of minimum electron p~T~ cut. The signal
corresponds to detached electrons from charm decays using a DCA cut of
200μm (circles) or no DCA cut (diamonds). The background corresponds to
electrons from Dalitz decays and photon conversions which pass the
corresponding DCA cuts, assuming four layers of Silicon with 1 or 2% of
a radiation length per layer.

Figure 13 shows that there is a useful correlation between the *p*~T~
cut applied to the electrons and the *p*~T~ of the parent *D* meson. The
points represent the most probable value of the parent *p*~T~ while the
error bar represents the FWHM spread. Using the correlation, one can,
for example, determine the *p*~T~ distribution of the parent *D*-meson
from the decay electron spectrum.

.

![](media/image16.wmf){width="4.472916666666666in"
height="3.2104166666666667in"}

[]{#__RefHeading___Toc55021901 .anchor}Figure 13 Correlation between the
transverse momentum of the D mesons and the minimum p~T~ cut applied to
the electrons (using a DCA cut of 120μm). The points represent the most
probable value of the D meson p~T~ while the spread represents the
(asymmetric) full width at half maximum.

### Direct Measurement of *D^0^K^-^π^+^* at high *p~T~*

For higher values of the electron *p*~T~, electrons from *B*-decays
dominate those from *D*-decay and the extraction of the charm
contribution becomes difficult. This can be overcome by a direct
measurement via hadronic decay channels, which only becomes possible
only at high *p*~T~ because of the small solid angle of the central arm
spectrometers of PHENIX.

![](media/image17.png){width="2.977777777777778in"
height="2.0027777777777778in"}![](media/image18.png){width="2.977777777777778in"
height="2.0027777777777778in"}

[]{#__RefHeading___Toc55021902 .anchor}Figure 14 The DCA distributions
in cm for pions with the inner pixel having 1%X~0~ thickness. On the
left is the DCA for direct pions with pt \>1 GeV/c and on the right is
the DCA for pions from *D*^0^ decay.

We have simulated *D*^0^ production using *p+p* PYTHIA events and
tracked them through GEANT as described above. The decay pions and kaons
where then compared to those directly produced in a central Au+Au
background event. Figure 14 shows the DCA distribution for directly
produced pions with a *p*~T~ cut of 1 GeV/c compared to the DCA
distribution for pion daughter particles from *D*^0^ where the parent
*D*^0^ has *p*~T~ above 2 GeV/*c*. The pions from *D*^0^ have a broader
DCA distribution than directly produced pions. Clearly a DCA cut will
remove a larger fraction of the direct pions than pions from *D*^0^
decay.

We have estimated the signal/background (S/B) for a *D*^0^ analysis for
central Au-Au collisions where the S/B is the smallest. In this study
only *D*^0^s with *p*~T~ above 2 GeV/c are were used to better match the
opening angle of the daughters to the PHENIX acceptance. We require that
both the pion and kaon from the decay fall into the acceptance and that
they have not decayed before reaching the outer tracking detectors. We
also assume that kaons are identified by one of the PHENIX PID
detectors, an aerogel plus TOF detector covering the full acceptance of
the west arm, and the existing TOF detector in the east arm. Lifting
this requirement will deteriorate the signal/background by roughly a
factor of 5, depending on the p~T~ of the kaon and the centrality of the
event.

![](media/image19.png){width="2.977777777777778in"
height="2.0027777777777778in"}

[]{#__RefHeading___Toc55021903 .anchor}Figure 15 The invariant mass
distribution for background pairs from central Au+Au events. Each
pion+kaon pair has a pair p~T~ \> 2 GeV/c.

Figure 15 shows the invariant mass for combinatorial background from
pions kaons pairs that have a pair *p*~T~ \> 2 GeV/*c*. Only a few
percent of the combinatorial pairs have an invariant mass near the
*D*^0^ mass of 1.86 GeV. We count the background in a 40 MeV window
around 1.86 GeV. The window corresponds to 2 times the rms mass
resolution, which is 1% calculated from the known momentum resolution of
the PHENIX tracking.

To count the signal we scale the number of reconstructed *D*^0^\'s in
the PYTHIA events by the number of binary collisions for a central Au+Au
events. The signal/background (*S/B)* is then studied as a function of a
simultaneous DCA cut on the pions and kaons. Figure 16 shows the *S/B*
for different DCA cuts on pairs detected in the west-arm. The left- and
right-hand panels are for simulations with 1% X~0~ and 2% X~0~ per
layer. With no DCA cut the *S/B* is less than 0.1%, placing a DCA cut of
100μm increases the *S/B* to a level of 2-3%. This *S/B* should be
further improved by requiring that the parent particle point back to the
collision vertex.

The statistical significance of the signal can be estimated by
$\frac{S}{\sqrt{B}}$. This ratio can be interpreted as the number of
sigma of the extracted counts in the signal over the fluctuating
background. It increases with the square root of the number of events.
Using the run assumptions described in section 3.5, we will
approximately collect 200M central Au+Au events within a 10 cm
collision window in a running period. Under these assumptions the
combined (*D*^0^+*D*^0^) S/B for different DCA cut strategies for
pairs detected in the west-arm of PHENIX is shown in Figure 17.

![](media/image20.png){width="2.977777777777778in"
height="2.0027777777777778in"}![](media/image21.png){width="2.977777777777778in"
height="2.0027777777777778in"}

[]{#__RefHeading___Toc55021904 .anchor}Figure 16 The *S/B* for D^0^Kπ
with a pt \>2 GeV/c for central Au+Au events into the west-arm of
PHENIX. On the left is the simulation for 1%X~0~ thickness per layer. On
the right is simulation for 2%X~0~ thickness per layer.

![](media/image22.png){width="2.977777777777778in"
height="2.0027777777777778in"}![](media/image23.png){width="2.977777777777778in"
height="2.0027777777777778in"}

[]{#__RefHeading___Toc55021905 .anchor}Figure 17 The S/B for
D^0^+D^0^-\>Kπ with a pt \>2 GeV/c for central Au+Au events into the
west-arm of PHENIX. On the left is the simulation for 1%X~0~ thickness
per layer. On the right is simulation for 2%X~0~ thickness per layer.

From Figure 17, we can conclude that the D^0^ peak for particles above
p~T~ \> 2 GeV/c can be extracted robustly. This is the worst case for
the most central collisions where the combinatorial background is
largest. Requiring the parent to point to the collision vertex can
further reduce the background.

Simulations indicate that at higher-pt the acceptance for D^0^ increases
since the opening angle between the daughters is smaller. The background
due to combinatorics decreases. For D^0^s above p~T~ \> 3 GeV/c, even
though the open charm yield is lower, the significance of the
(*D*^0^+*D*^0^) peak maintains a level of 7sigma in central Au+Au
collisions.

### Open Beauty Measurement

*B* meson production, while more rare than *D* production, is somewhat
simpler to measure with the VTX detector because of the larger *cτ*. The
main challenge is the relatively low rate. We have at least two methods
to measure *B* with the VTX detector:

-   Semi-leptonic decays: Since beauty mesons have a larger lifetime
    than charm mesons, it is possible to extract the beauty yield at low
    transverse momentum from the distribution of decay distances. At
    large transverse momentum beauty decays dominate the DCA
    distribution.

-   The decay channel *B*  J/ψ produces J/ψ that are displaced from the
    collision.

For momenta greater than 3 to 4 GeV/*c* electrons with displaced
vertexes are dominated by beauty decays. This is clearly seen in Figure
18. By placing a DCA cut on the order of 150 μm we should be able to
cleanly separate electrons from beauty from all other sources. Note
however that this clean separation is only possible with the VTX
detector even in this high *p*~T~ region. Although the high *p*~T~
region is dominated by beauty, there is a significant contribution from
charm component, and the separation of these two components is possible
only with the DCA measurement. With an accurate determination of the *b*
component, the charm component will also accessible up to 6 GeV/*c*
using a simultaneous fit of the DCA distribution. As discussed in the
previous section, the high *p*~T~ charm component will also be measured
directly in *D**K*π decay.

The signal to background ratio for beauty decays at high momentum is
shown in Figure 19. The effect of the DCA cut is even more favorable
than for charm.

![](media/image24.jpeg){width="4.152777777777778in"
height="4.245833333333334in"}

[]{#__RefHeading___Toc55021906 .anchor}Figure 18 DCA distribution for
electrons from Dalitz, charm and beauty decays simulated through four 1%
or 2% Si layers

![](media/image25.wmf){width="4.188888888888889in"
height="3.0180555555555557in"}

[]{#__RefHeading___Toc55021907 .anchor}Figure 19 Signal to Background
ratios as a function of the minimum electron p~T~ cut. The signal
corresponds to detached electrons from beauty decays using a DCA cut of
200μm (circles) or no DCA cut (diamonds). The background corresponds to
electrons from Dalitz decays and photon conversions which pass the
corresponding DCA cuts, assuming four layers of Silicon with 1 or 2% of
a radiation length per layer

## 3.3 Photon and jets measurement in polarized p+p

Direct photon emission a key process to measure the gluon distribution
of the nucleon and the polarization of the gluons. At RHIC energies,
quark-gluon Compton scattering, *q*+*g*  *q*+*γ* is the dominant
parton-level process in the production of high p~T~ direct photons. In
*pp* collisions the cross section of this process is directly
proportional to the quark and gluon densities multiplied by the pQCD
cross section. Thus measuring the direct photon together with the recoil
jet is a direct and clean way to measure the gluon density and
polarization as function of the momentum fraction *x*. With the present
PHENIX setup, direct photons are measured with the finely segmented
electro magnetic calorimeters (EMCal) in the central arms. However, due
to the limited coverage of the central arm tracker, most of the recoil
jet cannot be measured. Therefore we can only measure the direct photon
averaged over the recoil jet kinematics.

In *p+p*, *p+A*, and light ion collisions, the VTX detector works as a
stand-alone, large solid angle charged particle tracker. The expected
momentum resolution for tracks reconstructed solely by the VTX detector
is about 10% for a 1 GeV/*c* track. This resolution is sufficient to
reconstruct the recoil jets in wide rapidity range (\|*η*\|\<1). With
the knowledge of the recoil jet we can constrain the initial kinematics
(*x*~1~ and *x*~2~) of incoming partons and thus determine the gluon
density and polarization as function of *x*.

We have studied the potential improvements due to the VTX detector in a
Monte Carlo simulation. In the simulation, the direct photon events have
been generated using the PYTHIA event generator. The recoil jet is then
reconstructed from the charged tracks within the VTX detector acceptance
(\|*η*\|\<1.2) but in opposite azimuthal direction of the direct photon.
In the first step of the algorithm, tracks with momentum greater than 1
GeV/*c* at an azimuth angle opposite to the direct photon *(\|φ --
φ~γ~\| \>* π*/2*) are selected. Then, the direction of the jet is
estimated as the momentum weighted average of the selected tracks, as

$$\eta^{\text{jet}} = \frac{\sum_{}^{}{\eta^{i}p_{T}^{i}}}{\sum_{}^{}p_{T}^{i}},\varphi^{\text{jet}} = \frac{\sum_{}^{}{\varphi^{i}p_{T}^{i}}}{\sum_{}^{}p_{T}^{i}}$$

Next, the tracks with momenta above 1.0 GeV/c and within the cone of
radius
$R = \sqrt{(\eta^{\text{jet}} - \eta^{i})^{2} + (\varphi^{\text{jet}} - \varphi^{i})^{2}} < 0\text{.}5$are
selected. In a second iteration the recoil jet axis is corrected with
the same algorithm, using the tracks within the cone. This procedure is
iterated until the direction of the axis no longer changes.

![](media/image26.png){width="5.897916666666666in"
height="4.351388888888889in"}

[]{#__RefHeading___Toc55021908 .anchor}Figure 20 In each panel, the
green histogram shows the pseudo-rapidity, η~q~-distribution of the
final hard scattered partons, which initiated the recoil jet; the blue
histogram shows the η~q~-distribution of recoil jets within the barrel
VTX acceptance; and the red histogram show the (η^jet^
-η~q~)-distribution, where η^jet^ is for the pseudo-rapidity
reconstructed for the recoil jets. Different panels are for the event
samples with direct photon of different transverse momenta, starting
from 4-5 GeV/c in the upper left to 9-10 GeV/c in the lower right panel.

Figure 20 illustrates how well the direction of the recoil jet is
determined by this simple algorithm. In the figure, the pseudo-rapidity
*η~q~* of scattered quark (obtained from the event generator) are
plotted as green histograms. The blue histograms in the figure show the
distributions of *η~q~* for events in which the recoil jet is
reconstructed within the VTX acceptance. The red histograms show the
difference *η~q~ - η^jet^* between the true pseudo-rapidity of the
recoil quark and that of reconstructed jet. The large uncertainty of the
recoil jet kinematics, as seen from the wide distribution of *η~q~*, is
much reduced by the reconstruction of the recoil jet direction.

From the measurement of *η^jet^* and the transverse momentum *p~T~* of
the direct photon, the kinematics of the initial partons can be
determined. Under the assumption that the *p~T~* of the direct photon
and the recoil jet is the same, the fractional momenta of initial
partons *x*~1~ and *x*~2~ are determined from the following relations:

$$\begin{array}{r}
x_{1} + x_{2} = \frac{2}{\sqrt{s}}(E^{\text{jet}} + E^{\gamma}) = \frac{2p_{T}}{\sqrt{s}}(\text{cosh}(\eta^{\text{jet}}) + \text{cosh}(\eta^{\gamma})), \\
x_{1} - x_{2} = \frac{2}{\sqrt{s}}(p_{Z}^{\text{jet}} + p_{Z}^{\gamma}) = \frac{2p_{T}}{\sqrt{s}}(\text{sinh}(\eta^{\text{jet}}) + \text{sinh}(\eta^{\gamma}))
\end{array}$$

Thus *x~1~* and *x~2~* are calculated as

$$\begin{array}{r}
x_{1} = \frac{p_{T}}{\sqrt{s}}(\text{exp}(\eta^{\text{jet}}) + \text{exp}(\eta^{\gamma})), \\
x_{2} = \frac{p_{T}}{\sqrt{s}}(\text{exp}( - \eta^{\text{jet}}) + \text{exp}( - \eta^{\gamma}))
\end{array}$$

![](media/image27.wmf){width="2.595833333333333in"
height="2.5833333333333335in"}![](media/image28.wmf){width="2.5680555555555555in"
height="2.5680555555555555in"}

[]{#__RefHeading___Toc55021909 .anchor}Figure 21 Correlation between x
reconstructed and true *x*-value from PYTHIA. In the plot on the left,
![](media/image29.wmf) and no jet information has been used. The plot in
the right panel is obtained, using the reconstructed jet axes in the
barrel VTX.

The two panels of Figure 21 shows the correlation between the
reconstructed *x* and its true value known from the event generator. If
the recoil jet direction is not known, left panel, the best estimate for
*x* based on the photon p~T~ is only loosely correlated to the true *x.*
However, with the recoil jet reconstruction, right panel, there is a
narrow correlation between the reconstructed *x* and its true value.

How well initial parton kinematics, *x~1~* and *x~2~*, can be extracted
is summarized by Figure 22. Here, the widths (RMS) of (*x*(true) *--
x*(reconstruct))/*x*(true) are shown as function of *x~1~*. In the range
of *x*\>0.04, the x-values are determined with an accuracy of \~20%. The
respective accuracy for the measurements without knowledge of the jet
axis deteriorates by a factor \~2 to 3.

![](media/image30.png){width="4.145833333333333in" height="3.875in"}

[]{#__RefHeading___Toc55021910 .anchor}Figure 22 The relative widths
(RMS) of the (x(true) -- x(reconstruct))/x(true) distributions, using
the reconstructed jet axes in the barrel

## 

## 3.4 Improved momentum resolution and pt resolution

In the present PHENIX detector, drift chambers that are located outside
of the central magnet measure the momentum of the charged particles.
Since there is little magnetic field at the location of the drift
chambers, charged particles traverse them on almost straight
trajectories. As discussed in 3.1 the momentum *p* of a particle is
related to the bend angle *α * measured at the drift chamber
approximately as 87 mrad/*p* for *p* in GeV/c.

The momentum resolution of the central detector will be much improved
with the VTX detector. This is because in the present PHENIX central arm
spectrometers the effective field kick of 87 mrad GeV/c is only about
40% of the total angular deflection *∆φ* in the magnetic field. The
field integral at the location of the drift chamber is about 0.7 Tm,
which gives *∆φ  =  *210 mrad */p.* Since the VTX measures the initial
direction of the particles the full value of *∆φ * is measured rather
than the angle α at the edge of the magnetic field. In addition, a
second field coil, which has been installed in 2003, allows increasing
the field integral to roughly 1 Tm. With this field integral the total
field kick increases to 300 mrad GeV/c. The improvement in momentum
resolution is directly given by the ratio of the field kicks *∆φ*/ α ∼
300 mrad / 87 mrad \~ 3.

The higher momentum resolution with the VTX detector will improve the
high *p*~T~ measurements. At present, a momentum resolution of about
1%/*p* has been achieved. With the VTX detector, the resolution of a 30
GeV/c track can be reduced from 30% to about 10%. In addition, the track
confirmation close to the vertex provided by the VTX will eliminate the
decay and conversion background, which currently limits the *p*~T~ reach
of the PHENIX charged particle tracking to *p*~T~ \< 10 GeV/*c*.

The improvement of the momentum resolution may have a significant impact
on the data quality in the higher e^+^e^-^ mass region around the ϒ
states. With the better momentum resolution also the mass resolution
decreases to a level that the members of the ϒ family, the resonances
ϒ~1S~ (9.46 GeV), ϒ~2S~ (10.02 GeV) and ϒ~3S~ (10.36 GeV) can be clearly
separated. This is shown schematically in Figure 23. The VTX together
with the increased magnetic field reduces the resolution at the ϒ states
from \~170 MeV to \~60 MeV for the e^+^e^-^ decay channel, thus allowing
to separate the individual states. We note that this measurement will be
a significant challenge and only possible if sufficiently high
luminosities expected for RHIC II are available for extended running
periods.

![](media/image31.png){width="4.4944444444444445in"
height="3.0430555555555556in"}

[]{#__RefHeading___Toc55021911 .anchor}Figure 23 - Separation of Upsilon
states in the di-electron spectrum with a vertex detector (yellow) and
without (black). The number of ϒs in this plot represents our
expectation for a Au-Au run with a recorded effective luminosity of \~1
nb^-1^ (see chapter 3.5).

## 

## 3.5 Event rate estimates

In this section we estimate the event rates of selected physics
processes corresponding to the major goals of the VTX detector. First,
we summarize the common assumptions that go into estimating yields for
the physics signals that the VTX detector will make available. We start
with integrated luminosity estimates, based on the luminosity evolution
at RHIC expected by the BNL Collider Accelerator Division. Then we fold
in estimates of all of the practical efficiency factors that reduce the
yields measured by PHENIX. The obtained "effective luminosity" is then
used to estimate the signal yields.

The efficiency factors are summarized in Table 1 for the p-p, d-Au and
AuAu cases. The first three factors, (1) to (3), are the factors that
reduce the CA-D delivered luminosity to the recorded luminosity that is
written on data tape by PHENIX, and therefore they are common for all
three beam species. These factors are based on the actual performance of
RHIC and PHENIX during RUN2 and RUN3, and to be conservative we assume
no improvements of these factors in the future. The largest loss of the
recorded luminosity comes from the vertex cuts, factors (1) and (2) in
the table. Here we used longitudinal length of the collision diamond of
σ=20cm and a vertex cut of \|z\|\<10 cm since the inner-most layer of
the VTX detector covers \|z \|\< 11 cm. The combined reduction of these
two factors is gives about 28%. With 60% PHENIX uptime, which was
achieved in RUN3, the recorded luminosity is 17% of CA-D delivered
luminosity.

The effective luminosity is further reduced by the fraction of "good
data" that can be used for offline analysis (the factor (a) ) and the
offline reconstruction efficiency (the factors (b) and (c)). Again,
these numbers are based on actual PHENIX performance and reconstruction
efficiency (tracking plus electron identification) for single electrons
in central arms in the RUN2 Au+Au data analysis. The reconstruction
efficiency includes the loss due to the dead channels in the central arm
detectors. The efficiency depends on the beam species, and it decreases
from *pp* to dAu to AuAu. This reduction is caused by occupancy
dependent efficiency losses, which are also based on the actual
performance in RUN2 data analysis. The reconstruction efficiency factors
are shown for both single electron (factor (b)) and electron pairs
(factor (c)).

We should emphasize that the reality factors given in Table 1 are all
based on achieved performance of RHIC and PHENIX and we assume no
improvement in the future. Therefore the effective luminosity presented
here is a very conservative estimate and potential improvements will
increase the effective luminosity usable by the VTX detector. In
particular, improvement in diamond size of the beam and the efficiency
of the storage RF would greatly increase the effective luminosity.

In Table 2 we list the delivered integrated luminosity, the recorded
integrated luminosity with the VTX (delivered integrated luminosity
times factor (4) in Table 1), and the effective integrated luminosity.
For the delivered integrated luminosity, we used the CA-D projection of
the RHIC luminosity in year 2008 RUN and assume constant effort, which
means 19 weeks of physics data taking per year. Since there is a very
large variation of the CA-D luminosity projection, we use the average of
the most pessimistic estimate and the most optimistic estimate. This
luminosity estimate is consistent with the five year extended beam use
proposal presented by PHENIX to the Physics Advisory Committee (PAC) in
the fall 2003. The most optimistic luminosity figure is about a factor
of 2 larger than shown in the table.

[]{#__RefHeading___Toc55013006 .anchor}Table 1 Table of efficiency
factors that must be applied to delivered pp, dAu and AuAu luminosities
to calculate expected signal yields. The single and two track
reconstruction efficiencies are for electrons in the central arm.

  ----------------------------- ------------ -------------- --------------
  Quantity                      p-p factor   D+Au factor    Au-Au factor

  \(1\) Storage RF efficiency   75 %         75 %           75 %

  \(2\) Event vertex cut (±10   38 %         38 %           38 %
  cm)                                                       

  \(3\) PHENIX uptime           60 %         60 %           60 %

  \(4\)                         17%          17%            17%
  L^recorded^/L^delivered^                                  
  =(1)×(2)×(3)                                              

  \(a\) Good run fraction       80 %         80 %           80 %

  (b)Single track efficiency    85 %         80 %           50 %

  \(c\) Two track efficiency    72 %         64 %           25 %

  Total (single                 12 %         11 %           6.8 %
  track)=(4)×(a)×(b)                                        

  Total (two track)=(4)×(a)×(c) 10 %         8.8 %          3.4 %
  ----------------------------- ------------ -------------- --------------

[]{#_Ref54638032 .anchor}

Table 2 Table of effective luminosities from a 19 week production run,
after reality factors are taken into account. The delivered luminosities
use the average of the most pessimistic and most optimistic C-AD
estimates of how the luminosity will evolve by 2008-2009. The signal
yield for a given process is found by multiplying the cross section for
the process by the effective luminosity and by the detector acceptance.
For d-Au and Au-Au collisions and the effective Ldt columns, the
nucleon-nucleon luminosities are shown in the parenthesis).

+-------+---------+---------+------------+------------+--------------+
| beam  | $$\     | de      | recorded   | Effective  |              |
|       | sqrt{s_ | livered | Ldt        | Ldt        |              |
| sp    | {\text{ | Ldt     |            |            |              |
| ecies | NN}}}$$ |         |            |            |              |
+-------+---------+---------+------------+------------+--------------+
|       |         |         |            | Single     | Double track |
|       |         |         |            | track      |              |
+-------+---------+---------+------------+------------+--------------+
| p-p   | 200     | 160/pb  | 27/pb      | 18/pb      | 15/pb        |
+-------+---------+---------+------------+------------+--------------+
| d-Au  | 200     | 40/nb   | 6.8/nb     | 4.4/nb     | 3.5/nb       |
|       |         |         |            | (1.7/pb)   | (1.4/pb)     |
+-------+---------+---------+------------+------------+--------------+
| Au-Au | 200     | 2.2/nb  | 370/μb     | 150/ub     | 74/μb        |
|       |         |         |            | (5.8/pb)   | (2.9/pb)     |
+-------+---------+---------+------------+------------+--------------+
| p-p   | 500     | 540/pb  | 93/pb      | 63/pb      | 54/pb        |
+-------+---------+---------+------------+------------+--------------+

In Table 3, the estimated signal yields of selected physics processes
are summarized. The "Yield" column of the table shows the raw signal
yield calculated as the product of the cross section, the geometrical
acceptance of the central arms, and the effective luminosity given in
Table 2. However, most of these raw signal yields cannot be measured or
cannot be separated from other competing process without the VTX
detector. The column "no VTX" indicates which of the physics signals can
be measured without the VTX detector (marked as "Yes") or not (marked as
"No"). If the signal can be measured with limitations or with a large
systematic uncertainty, the column is marked as "Limited". For example,
in charm decay electron measurement (*c**e*), the first row (1\< *p*~T~
\<2 GeV/c) is marked as "Yes" since charm is the dominant source of
non-photonic electron in this *p*~T~ bin. The next row (2\< *p*~T~ \<3
GeV/c) is marked as "Limited" since there is a large uncertainty due to
the beauty contribution. All other rows are marked as "No" since it is
not possible to separate charm signal from the larger beauty signal for
*p*~T~\>3 GeV/c without the VTX detector. In these *p*~T~ bins, the
measurement of beauty decay electron (*b**e*) are marked as "Limited"
since we cannot separate *b* and *c* signal in a model independent way,
but the beauty contribution is larger than the charm contribution.

For the yield estimate of single electron from open charm (*c**e*), the
momentum distribution of the charm decay electron is calculated using
PYTHIA event generator. The generator is tuned to re-produce the low
energy charm data from fixed target experiments and single electron data
at the ISR. The electron spectrum predicted by the tuned PYTHIA
simulation agrees well with PHENIX data at $\sqrt{s_{\text{NN}}}$=130
GeV and 200 GeV. The corresponding total charm cross at
$\sqrt{s_{\text{NN}}}$=200 GeV is 650 μb per nucleon-nucleon collisions.
The single electron spectrum is then scaled assuming the binary scaling,
and then multiplied by the geometrical acceptance and the effective
luminosity to give the expected yield (sum of *e*^+^ and *e*^-^) shown
in the "Yield" column in the table. The expected yield in low *p*~T~
region (*p*~T~ ≤ 3 GeV/c) is consistent with the observed charm decay
electron yield in RUN2 Au+Au data. Since this estimate comes from a
scaled *p+p* PYTHIA simulation, it corresponds to a scenario with no
charm enhancement nor any energy loss. If there is a substantial energy
loss of charm in high *p*~T~, the actual yield of charm decay electron
could be smaller than the value in the table.

The yield of single electron from open beauty is estimated in a similar
way using PYTIA generator, assuming that total beauty cross section
is$\sigma_{b\overline{b}}$ = 3.8 μb per N-N collisions. This cross
section is estimated by tuning the PYTHIA calculation of *b* production
with *b* production data from the Tevatron (1.8 TeV) and
$\text{Sp \{}\overline{p}S$collider (630 GeV) and then extrapolating
down to lower energies at RHIC (200 GeV). Binary scaling of the cross
section is assumed for the Au-Au estimates.

For both the charm and beauty decay electron measurements, the signal
yields with DCA \> 200*μ* are shown in the column labeled "with DCA
cut". With the DCA cut, beauty is dominant over charm for *p*~T~ \>2
GeV/c, and can be separated from the charm signal. In the lowest *p*~T~
bin (1.0\<*p*~T~\<2.0 GeV/c), charm is still dominant over beauty by
about factor 5 with this DCA cut. The *b/c* ratio is improved to \~1/2
with tighter DCA cuts (DCA\>400*μ*), as shown in the table, and *b/c*
ratio becomes about 1 with DCA\>800μ (see Figure 18). Using the DCA
distribution, we can statistically separate the *b*-decay signal from
charm over the range 1\<*p*~T~\<6 GeV/c. The table shows that even with
the pessimistic assumptions used in the estimate, we will have
sufficient statistics for a *b/c* signal separation in this *p*~T~
range. Although the e/π separation power of the RICH detector is reduced
above its Cereknov threshold for pions (*p*~T~\>4.7 GeV/c), the
RICH+EMCAL combination has a sufficient *e*/π separation up to this
*p*~T~ range.

As discussed earlier, one of interesting measurement with the VTX is the
ratio of (*c**e*)/(*b**e*) and its centrality dependence. Since most
of the systematic uncertainties cancelled in this ratio, the measurement
is primarily limited by the *b/c* separation from the DCA measurement
and the statistics of *b**e*. The expected yield of *b* and *c* signal
with and without the DCA cuts in Table 3 shows that we can reach \~1%
statistical precision in the ratio measurement.

A summary the physics program addressed with the VTX detector and how it
compares to the capabilities without the VTX is given in Table 4. For
many of these physics topics, a measurement is not possible without the
VTX detector or very marginal. For the processes that PHENIX can measure
without the VTX detector, the VTX will substantially extend the
kinematical range of the measurement. In addition, the accuracy and the
precision of the measurements are improved.

[]{#__RefHeading___Toc55013008 .anchor}Table 3 Event rate calculated for
selected physics processes. The effective integrated luminosity used in
the calculation is shown in Table 2. For the meaning of "no VTX" column,
see the text. In both of Au+Au and p+p, the collision energy
$\sqrt{s_{\text{NN}}}$ is 200 GeV per nucleon pair. The yields include
the anti-particle channels. The DCA cut value for the single electron
measurement is DCA\>200 μ. For the lowest pT bin, the number with
DCA\>400μ is shown in parenthesis.

+--------------------------+-------+------------------+---------------+
| Process                  | no    | Yield            | Yield with    |
|                          | VTX   |                  | DCA cuts      |
+--------------------------+-------+------------------+---------------+
| AuAu*c**e*             |       |                  |               |
+--------------------------+-------+------------------+---------------+
| 1.0\<*p*~T~\<2.0 GeV/c   | Yes   | 3M               | 150K (40K)    |
+--------------------------+-------+------------------+---------------+
| 2.0\<*p*~T~\<3.0 GeV/c   | Li    | 130K             | 6K            |
|                          | mited |                  |               |
+--------------------------+-------+------------------+---------------+
| 3.0\<*p*~T~\<4.0 GeV/c   | No    | 5K               | 0.3K          |
+--------------------------+-------+------------------+---------------+
| 4.0\<*p*~T~\<5.0 GeV/c   | No    | 1K               | 50            |
+--------------------------+-------+------------------+---------------+
| 5.0\<*p*~T~\<6.0 GeV/c   | No    | 0.2K             | 10            |
+--------------------------+-------+------------------+---------------+
| AuAu*b**e*             |       |                  |               |
+--------------------------+-------+------------------+---------------+
| 1.0\<*p*~T~\<2.0 GeV/c   | No    | 200K             | 50K (20K)     |
+--------------------------+-------+------------------+---------------+
| 2.0\<*p*~T~\<3.0 GeV/c   | No    | 70K              | 15K           |
+--------------------------+-------+------------------+---------------+
| 3.0\<*p*~T~\<4.0 GeV/c   | Li    | 17K              | 3K            |
|                          | mited |                  |               |
+--------------------------+-------+------------------+---------------+
| 4.0\<*p*~T~\<5.0 GeV/c   | Li    | 4K               | 0.7K          |
|                          | mited |                  |               |
+--------------------------+-------+------------------+---------------+
| 5.0\<*p*~T~\<6.0 GeV/c   | Li    | 1K               | 0.2K          |
|                          | mited |                  |               |
+--------------------------+-------+------------------+---------------+
| Au+Au*D**K*π           |       |                  |               |
+--------------------------+-------+------------------+---------------+
| *p*~T~ \>2 GeV/c         | No    | 4                | 1000          |
|                          |       | 900. (S/B\~0.1%) | (S/B\~3%)     |
| *p*~T~ \>3 GeV/c         | No    |                  |               |
|                          |       | > 2900 (S/B\~1%) | 600 (S/B      |
|                          |       |                  | \~5%)         |
+--------------------------+-------+------------------+---------------+
| Au+Au*B**J*/ψ*ee*     | No    | 100              | 50            |
+--------------------------+-------+------------------+---------------+
| *pp**c**e*             |       |                  |               |
+--------------------------+-------+------------------+---------------+
| 1\<*p*~T~\<3 GeV/c       | Yes   | 10M              | 0.5M          |
+--------------------------+-------+------------------+---------------+
| *p*~T~\>3 GeV/c          | No    | 20 K             | 1K            |
+--------------------------+-------+------------------+---------------+
| *pp**b**e*             |       |                  |               |
+--------------------------+-------+------------------+---------------+
| *p*~T~\>1 GeV/c          | No    | 0.9M             | 0.2M          |
+--------------------------+-------+------------------+---------------+
| *pp*γ+jet               |       |                  |               |
+--------------------------+-------+------------------+---------------+
| 4\<*p*~T~\<5 GeV/c       | No    | 300K             | N.A.          |
+--------------------------+-------+------------------+---------------+
| 5\<*p*~T~\<6 GeV/c       | No    | 150K             | N.A.          |
+--------------------------+-------+------------------+---------------+
| 6\<*p*~T~\<7 GeV/c       | No    | 70K              | N.A.          |
+--------------------------+-------+------------------+---------------+
| 7\<*p*~T~\<8 GeV/c       | No    | 40K              | N.A.          |
+--------------------------+-------+------------------+---------------+
| 8\<*p*~T~\<9 GeV/c       | No    | 20K              | N.A.          |
+--------------------------+-------+------------------+---------------+
| 9\<*p*~T~\<10 GeV/c      | No    | 12K              | N.A.          |
+--------------------------+-------+------------------+---------------+
| *pp**B**J*/ψ*ee*      | No    | 560              | 280           |
+--------------------------+-------+------------------+---------------+

[]{#_Ref54757390 .anchor}

Table 4 Summary of physics measurement gained by the VTX detector. The
column "without VTX" shows the present capability of PHENIX, while the
measurement range with the VTX detector is shown in the column "with
VTX". If the process is not measurable, it is marked as "No".

  ----------------------- ----------------------- -----------------------
  Process                 Without VTX             With VTX

  *c**e*                 0.5 \< *p*~T~ \< 2.5    0.3 \< *p*~T~ \< 6
                          GeV/c                   GeV/c

  *D* *K*π ( *p*~T~\>2   No (2σ significance in  \> 7 σ significance in
  GeV/c)                  central Au+Au)          central Au+Au

  Total charm yield       \~ 20 %                 \~ 10 %

  (*c**e*)/(*b**e*)     No                      \~ 1 %
  ratio                                           

  *b**e*                 *p*~T~\>3 GeV/c with    1 \< *p*~T~ \< 6 GeV/c
                          model dependence        

  *B**J*/ψ               No                      ∆σ/σ \~ 10 - 15 %

  Total beauty yield      No                      \~ 10 %

  High *p*~T~ charged     *p*~T~ $$ 10 GeV/c      *p*~T~ \< 15 -20 GeV/c

  ∆G(x) from *c**e*      0.03 \< *x* \< 0.08     0.01 \< *x* \< 0.15

  ∆G(x) from *b**e*      No                      0.02\< *x* \< 0.15

  ∆G(x) from g+jets       No                      0.04\< *x* \< 0.3

  Nuclear shadowing of    0.03 \< *x* \< 0.3      0.01 \< *x* \< 0.3
  G(x)                                            
  ----------------------- ----------------------- -----------------------

# 4. VTX Detector system

## 4.1 Overview

The VTX detector system is composed of 4 layers of silicon detectors, an
inner layer with silicon pixel hybrid detectors and three outer layers
with silicon strip detectors. A 3-D view of the detectors is shown in
Figure 24 and its cross sectional views are shown in Figure 10 in the
previous chapter. The geometrical dimensions are summarized in Table 5.

![](media/image32.png){width="4.4375in" height="4.4375in"}

[]{#__RefHeading___Toc55021912 .anchor}Figure 24 GEANT model of the VTX
detector. It consisted of the inner-most pixel layer and three outer
strip layers.

For the inner layer we will use silicon pixel detectors. The technology
is the ALICE1LHCb sensor-readout hybrid, which was developed at CERN for
the ALICE and LHCb experiment. The 200 μm thick silicon sensor holds
32×256×4 cells, or pixels, each with an active area of 50×425 μm^2^. The
sensor is bump bonded to four matching readout chips of 150 μm
thickness, and each of the read-out chips has 32×256 individual
amplifier discriminator channels. The readout chip also holds the
electronics to pipeline the data flow. We call a sensor bump-bonded to 4
readout chips a sensor assembly. Two sensor assemblies are bonded on to
a kapton aluminum readout bus, which itself is supported by a carbon
fiber structure. The bus is connected to a unit called pilot module,
which provides all service voltages, control and timing signals, and
reads out the pixel data. The pilot module transmits the data via
optical fiber from the detector for further processing.

A unit of two sensor assemblies and one pilot module form a half ladder,
the basic building block of the detector. Two half ladders form a full
ladder and span approximately 22 cm in beam direction. Five such ladders
on each side of the beam pipe result in almost full azimuthal coverage.
A total of 10 ladders or 20 half ladders complete the inner layer. The
combined materials of silicon sensor, readout chip, readout bus and
mechanical structure including cooling add up to about 1.2% of a
radiation length.

The outer layers employ silicon strip sensors. The sensor, developed by
the BNL Instrumentation Division, allows stereoscopic readout on a
single sided sensor. Each sensor is about 3.43×6.46 cm^2^, with 2×384
active strips of 80 μm width and 3.1 cm length in beam direction
(x-projection) and a similar number of strips at an angle of 4.6^0^ to
the beam direction (u-projection). Due to the stereoscopic readout the
effective pixel size is 80×1000 μm. Four sensors are mounted in a ladder
that subtends the full 25.8 cm longitudinal extent of the barrel. A
total of 42 ladders are required to cover the azimuth acceptance as
shown in Figure 24.

[]{#__RefHeading___Toc55013010 .anchor}Table 5 Summary of main
parameters of the 4 VTX layers.

+-------------+----------------+-----------+--------+-------+--------+
| VTX         | Layer          | R1        | R2     | R3    | R4     |
+-------------+----------------+-----------+--------+-------+--------+
| Geometrical | R (cm)         | 2.5       | 6      | 8     | 10     |
| dimensions  |                |           |        |       |        |
+-------------+----------------+-----------+--------+-------+--------+
|             | ∆z (cm)        | 21.8      | 25.8   |       |        |
+-------------+----------------+-----------+--------+-------+--------+
|             | Area (cm^2^)   | 280       | 890    | 1240  | 1600   |
+-------------+----------------+-----------+--------+-------+--------+
| Channel     | Sensor size    | 1.28 ×    | 3.43 × |       |        |
| count       |                | 1.36      | 6.46   |       |        |
|             | R × z (cm^2^)  |           |        |       |        |
+-------------+----------------+-----------+--------+-------+--------+
|             | Channel size   | 50 × 425  | 80 μm  |       |        |
|             |                | μm^2^     | × 3 cm |       |        |
|             |                |           |        |       |        |
|             |                |           | (eff   |       |        |
|             |                |           | ective |       |        |
|             |                |           | 80 ×   |       |        |
|             |                |           | 1000   |       |        |
|             |                |           | μm^2^) |       |        |
+-------------+----------------+-----------+--------+-------+--------+
|             | Sensors/ladder | 2 × 8     | 4      |       |        |
+-------------+----------------+-----------+--------+-------+--------+
|             | Ladders        | 10        | 10     | 14    | 18     |
+-------------+----------------+-----------+--------+-------+--------+
|             | Sensors        | 160       | 40     | 56    | 72     |
+-------------+----------------+-----------+--------+-------+--------+
|             | Readout chips  | 160       | 480    | 672   | 864    |
+-------------+----------------+-----------+--------+-------+--------+
|             | Readout        | 1,310,720 | 61,440 | 8     | 1      |
|             | channels       |           |        | 6,016 | 10,592 |
+-------------+----------------+-----------+--------+-------+--------+
| Radiation   | Sensor         | 0.2%      | 0.4%   |       |        |
| length      |                |           |        |       |        |
|             |                |           |        |       |        |
| (X/X0)      |                |           |        |       |        |
+-------------+----------------+-----------+--------+-------+--------+
|             | Readout        | 0.16%     | 0.7%   |       |        |
+-------------+----------------+-----------+--------+-------+--------+
|             | Bus            | 0.14%     |        |       |        |
+-------------+----------------+-----------+--------+-------+--------+
|             | Ladder &       | 0.7%      |        |       |        |
|             | cooling        |           |        |       |        |
+-------------+----------------+-----------+--------+-------+--------+
|             | Total          | 1.2%      | 1.8%   |       |        |
+-------------+----------------+-----------+--------+-------+--------+

Each strip sensor is wire-bonded to and read-out by twelve read-out SVX4
ASICs, six per orientation, and there are 128 channels on each SVX4
chip. Fermilab and LBNL have developed these chips for other silicon
vertex detectors. The twelve SVX4s servicing each sensor are mounted on
two beryllium oxide readout cards (ROCs). The six SVX4s per ROC are
readout by two identical, custom, digital ASICs, one per orientation.
These chips compress and parallelize the data sufficiently to meet
PHENIX readout speed requirements. Power, serial control, timing and
readout for a ladder are all carried on the ROCs, which are bussed
together via wire-bonding at their edges (one bus per orientation). This
bus runs the length of a ladder and out of the acceptance to a Pilot
Module, which transmits the data via an optical fiber for further
processing. The present estimate is that the mechanical support, ROC\'s
and sensor add up to about 1.8% of a radiation length.

Table 5 summarizes the main parameter of the VTX layers. The detector
system is discussed in more detail in the following sub sections.

## 4.2 Hybrid pixels {#hybrid-pixels .unnumbered}

The inner layer of the vertex tracker will be build based on silicon
pixel devices designed for the ALICE experiment at CERN. The ALICE
collaboration has developed and is now constructing a silicon pixel
detector for its inner tracker. This detector system has to fulfill
similar requirements as the inner layer of the vertex detector proposed
for PHENIX. We therefore plan to develop and built the inner layer for
PHENIX in close collaboration with ALICE.

  ----------------------------------------------------- ----------------------------------------------------
  ![](media/image33.png){width="2.8027777777777776in"   ![](media/image34.png){width="2.857638888888889in"
  height="1.6277777777777778in"}                        height="1.0708333333333333in"}

  \(a\)                                                 \(b\)
  ----------------------------------------------------- ----------------------------------------------------

[]{#__RefHeading___Toc55021913 .anchor}Figure 25 (a) Cross section of a
pixel detector half ladder designed for the ALICE experiment. The hybrid
pixel detector itself consists of a readout chip that is connected via
solder bump-bonds to a sensor chip. Every sensor pixel has a
corresponding individual signal processing electronic in the readout
chip. They are interconnected with small solder balls
(\`\`bump-bonds\'\') in a flip-chip process. Eight pixel detector
assemblies are wire-bonded to a readout bus structure that runs along
the detector on top of the sensors. The half ladder is mounted onto a
mechanical support with includes embedded cooling lines to remove about
one Watt of power dissipated by the readout chip. (b) Arrangement of two
sensor assemblies with four chips each to form a PHENIX pixel detector
half ladder. A bus connects all readout chips. A pilot module outside of
the acceptance of the sensors interfaces the readout of the half-ladder
to the data acquisition system.

For PHENIX the pixel devices will be arranged in ladders, each ladder
carries 4 sensor assemblies, each with 4 readout chips bump-bonded to
one sensor chip. Two of these assemblies, comprising a half ladder, are
wire-bonded to a Kapton-aluminum bus on which power, control and data
lines run. This is illustrated in Figure 25. A pilot module that is part
of the half ladder but outside of the sensors\' geometrical acceptance
processes the incoming control signals and outgoing data of a half
ladder. It carries an analog pilot chip for the power and reference
voltage supplies of the pixel readout chips, a digital pilot chip for
their controls and readout, and an optical link chip and transmitter for
the data transfer to the acquisition system.

### Sensor

The pixel detector sensors are designed in a planar technology of
CANBERRA and produced as p-in-n structures on 5" silicon wafers of 200
μm thickness. A pixel cell is defined by p^+^ implants in one side of
the n-type silicon. The pixel dimensions are 50 μm by 425 μm. Every
sensor pixel has a contact pad for bump-bonding to the matching
electronics pixel on the readout chip.

The array of 32 by 256 pixels is read-out by a single readout chip. The
array is surrounded by a guard ring electrode that protects the
detection area from leakage currents deteriorating the signals generated
by charged particles in the depleted silicon. The guard ring also
encircles a "snake" structure for quality tests of the bump bonding
during the mass production of the assemblies. Those "snakes" have
matching structures and contact pads for probing access on the readout
chip. A scribe line defines the outer dimensions of the device for the
wafer dicing and also contains alignment marks. Figure 26 shows a
photograph of a corner of such sensor.

A sensor chip as produced for ALICE pixel detector prototypes and the
NA60 pixel vertex spectrometer, is of 12.8 by 13.6 mm^2^ size and has
array of 32 by 256 pixels. On a PHENIX pixel sensor chip, four such
sensor pixel arrays are implemented in a linear arrangement on a single
substrate[^28]. The long side of the pixels is parallel to the long
direction of the chip. The 200 μm thick sensors deplete typically
already at 12V applied to the aluminized n^+^ implanted continuous back
plane. During operation in the experiment, the sensors are over-biased.
The leakage currents are as low as a few nA in well diced material but
can go up to a few μA without any danger to the sensors' functionality.

Several sensor chips are arranged on the production mask of a 5" wafer.
Dedicated p-in-n diodes between the sensor chips are production-specific
to the CANBERRA technology and serve the quality survey in the foundry.

  -----------------------------------------------------------------------
  ![](media/image35.wmf)

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

[]{#__RefHeading___Toc55021914 .anchor}Figure 26 Photograph of a corner
of a pixel detector sensor chip, seen through a microscope. A guard
electrode surrounds the array of pixel implants. The scribe line defines
the outer dimensions of the die.

### Readout chip

The pixel detector front-end electronics ("readout chip") results from a
research and development effort at CERN that started more than a decade
ago[^29]. The recent chip "ALICE1LHCb" is a mixed analog-digital ASIC
designed in CERN\'s EP-MIC group and the ALICE and LHCb teams for an
application in the Silicon Pixel Detector of the ALICE experiment and
the RICH photo detection of the LHCb experiment at the LHC[^30]. It is
also being applied in the vertex spectrometer of the NA60 experiment at
the SPS[^31]. The chip is designed in a 0.25 μm process with radiation
tolerant design layout techniques. Each chip has 32 by 256 pixels of 50
μm by 425 μm size. It is designed to be bump-bonded with solder balls of
about 20 μm diameter to a silicon sensor that contains an array of
sensor pixels with matching size and pitch. The chip provides a binary
output signal for every pixel that indicates a charge release by a
traversing particle. The threshold of the pixels\' discriminators and
various other parameters of the front-end electronics can be adjusted
and programmed individually for every pixel. An analog pilot chip
provides the reference levels for the readout chip. Typical thresholds
for particle detection are below 2000 e^-^, while the electronic noise
is below 200 e^-^ equivalent noise charge. For single pixel hits the
mean sensor signal from minimum ionizing particles in 200 μm Si is about
16000 e^-^ so that a superb signal-to-noise ration is achieved. Each
ALICE1LHCb chip is readout by clocking out 256 32-bit words at a
frequency of 10 MHz.

### Interconnection of Sensor and Readout Chip ("Bump Bonding")

The term "bump-bonding" denotes the micro-interconnection of electrical
structures with small "balls" of metals or alloys. It is essential for
the construction of modern hybrid structures with two-dimensional arrays
of high-density designs. Bump-bonding and flip-chip interconnection
techniques were already introduced in the late 1960s. They became
commercially available only about 15 years ago. Bonding with dimensions
smaller than approximately 50 μm is still offered by only a rather small
number of vendors or specialized laboratories, and becomes challenging
at below 20 μm for specialized applications.

The Technical Research Centre of Finland (VTT), Microelectronics
Department of the Information Technology Division, is the vendor of
choice at CERN for the production of flip-chip assembled pixel detectors
for the ALICE and LHCb experiments. VTT offers an integrated package of
bump-bond deposition, wafer thinning and flip-chip bonding[^32]. Key
features of the process are:

-   200-mm (8\'\') wafer capability.

```{=html}
<!-- -->
```
-   Tin-lead solder alloy bumps are used for mechanical strength of the
    bonded assemblies.

-   Bump deposition by electroplating.

-   Optional thinning (back grinding) of bumped readout wafers from
    native thickness of 750 μm down to 150 μm.

-   Clean dicing with front side protection using either photo resist or
    tape.

-   Fluxless flip chip bonding. Alignment accuracy better than 3μm

-   Bump size down to smaller than 20μm diameter.

-   Throughput 3-4 assembly bondings per hour.

Statistics from the recently finished production of bump-bonded
assemblies for the NA60 pixel vertex spectrometer, with chips of 750 μm
and sensors of 300 μm thickness, illustrates that the industrial process
of VTT is able to produce good yield for 20 μm bump bonds.

In 2003, a grand total of 132 single-chip pixel detector assemblies were
produced in multiple batches for NA60 at VTT. Out of those, 115
assemblies passed the acceptance test, i.e. had less than 1 % dead or
noisy pixels. An example is shown in Figure 27. The average yield of
accepted assemblies was 87 %. Several large batches, especially towards
the end of the production, had a significantly higher yield than this
average and reached even 100 % of accepted assemblies. In the same
process, first pixel sensor assemblies were successfully produced for
the ALICE experiment. Each of the assemblies contains five readout chips
placed on a silicon sensor substrate of about 14 mm by 70 mm size that
has five matching sensor pixel arrays implemented. The processing
included the thinning of the readout chips to 150 μm before they were
bump-bonded to the sensor.

  ---------------------------------------------------- ----------------------- -----------------------
  ![](media/image36.png){width="5.999305555555556in"                           
  height="1.8305555555555555in"}                                               

  \(a\)                                                \(b\)                   \(c\)
  ---------------------------------------------------- ----------------------- -----------------------

[]{#__RefHeading___Toc55021915 .anchor}Figure 27 Test result of a
typical high-quality ALICE1LHCb assembly for the NA60 experiment: (a)
Test pulse injection into readout chip: 8 out of 8192 pixels are dead,
the rest of the pixel array responds. (b) Source measurement with Sr^90^
to test the bump bonding quality: 3 out of 8192 bonds are open (or
pixels do not respond electrically). (c) Image of a beta source with
shadow of the depletion voltage contact needle on the silicon sensor.

The requirement for thinnest possible detectors especially in the
internal layers of the vertex detector, to minimize multiple scattering
and photon conversion, demands the thinning of the readout chips. The
native thickness of electronics wafers is 750 μm when they leave the
foundry. Since they are processed only from one side, in a layer of only
a few micron depths, a large fraction of the bulk can be removed without
affecting the electrical properties of the chip. Sensor wafers are
processed from both sides to establish the p-in-n structures, and have
to be directly produced in their final thickness. At VTT, a procedure
has been developed to thin readout chips in a way that conforms to the
mechanical stability requirements for bump-bonding and flip-chip
assembly. After the electrolytic deposition of solder bumps on a full
readout wafer, and before the flip-chip process is performed, the
wafer\'s surface is first protected with a removable film. The backside
is then grinded mechanically in several steps. Finally, wet chemical
etching or chemical mechanical polishing removes the defect layer. The
minimum thickness achieved with 8\'\' wafers is 150 μm, with thickness
variations of less than 5 μm.

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[]{#__RefHeading___Toc55021916 .anchor}Figure 28 Map of working pixels
from a source measurement of a thin ALICE pixel sensor assembly. The
sensor assembly consists of five thinned readout chips of 150 μm
thickness that are bump-bonded to a 200 μm thick silicon sensor
substrate. The fraction of working pixels is indicated for every chip

A small number of thin sensor assemblies have been produced so far for
the ALICE experiment, prior to the start of the mass production. The
test production yielded several well working sensor assemblies. Figure
28 illustrates a test result from one thinned sensor assembly with five
readout chips.

### Readout Bus

The bus is a printed circuit sandwich made from Kapton and aluminum
layers. It contains the power, data and control lines for the operation
and parallel readout of eight pixel chips (see next section on Pilot
module) per half ladder. The supply, control and data lines are between
50 μm and 8 μm thick and insulated with 8 μm Kapton layers. Micro-vias
interconnect readout and control lines across several metal layers in
order to manage the routing towards the side of the multi-chip interface
module. The metal layers are arranged in vertical steps along the long
side of the bus in order to be able to wire-bond the readout chips to
the bus. Several passive components (decoupling capacitors, pull-up
resistors etc.) will be placed on the top layer. The total thickness of
the bus is 280 μm. Prototypes of the bus and first specimens of the
final Kapton-aluminum bus have been produced at CERN for the ALICE Pixel
Detector.

### Pilot module

The pilot multi-chip module carries four chips that support the readout
and control of the pixel readout chips of every half ladder: Analog and
digital pilot chip, link driver chip with optical link,
flash-analog-digital converter that reads several temperature sensors on
the half ladder. The components are arranged on a multi-layer Kapton
printed circuit with copper lines and interconnections. It is as wide as
the pixel bus and less than 70 mm long.

The role of the digital pilot is to read the data from the eight
ALICE1LHCb chips. Upon pixel chip readout being initialized, each
ALICE1LHCb presents 256 sequential words of pixel data on a 32-bit bus
synchronously at a 10MHz clock. Thus, 25.6μs are required to read the
data from a single chip. In PHENIX data is read-out in 80μs to achieve a
maximum throughput of 12.5 kHz. A sequential, readout of the 8 chips on
two ladders will require 204μs to empty the pixel chip data into the
pilot and hence does not meet our timing constraints. To increase the
total readout speed several pixel chips from one half ladder must be
readout in parallel.

We are currently investigating several options to overcome the readout
speed limitation. Key to all of them is to introduce a modified pixel
bus with parallel branches of data readout lines. One solution could be
to readout out each of the parallel branches with one of the existing
ALICE pilot ASIC\'s. This solution is not very attractive since it
requires a lot of space on the ladder and implies large power
consumption. Other solutions are based on a new pilot chip. This may be
implemented as a new version of the ALICE pilot ASIC adapted to PHENIX,
or as a radiation hard FPGA. Both options are being investigated in
collaboration with the ALICE pixel group and a decision will be made
between the two by spring 2004.

The ASIC option uses the same design rules and radiation tolerant
technology as the pilot ASIC that is presently already functional. It
can be submitted as part of a multi project wafer at CERN.

The operating environment of the pilot module is such that the expected
total ionization dose of the order of 3.8 kRad per 27-week run (50% duty
cycle) is sufficiently low to employ radiation-hard FPGA technology.
These FPGAs have been developed for the satellite industry and are
guaranteed to work up to 200kRad. Between 200 and 500 kRad there is some
degradation in timing observed at far higher frequencies than we would
operate the device. An available package size of 12 mm by 12 mm is
sufficiently small to fit onto the pilot module. A FPGA solution is more
flexible than an ASIC especially it may allow on-ladder zero-suppression
at the 32-bit word level, which reduces the amount of data that needs to
be transmitted. It also may be applicable with little modification in
the strip detector readout chain. A first prototype design with parallel
readout and zero suppression has been finished and is being tested on an
FPGA. A rad-hard FPGA will be tested near the intersection region in the
run-4 RHIC run.

A bus with several parallel branches of data lines is essential to all
of the technical approaches mentioned above. It requires a design with
higher density of readout lines than presently produced in prototypes,
in order to keep the overall bus thickness at the present level. The
capabilities of industrial partners are being explored for a production
that can yield high line densities.

### Front End Modules 

Power, slow control, and data readout of a pixel ladder are provided
through the pilot module. For the ALICE experiment this module has been
developed and we expect that with few modifications most components can
be reused. The pilot module transmits its data via optical fiber. A
front-end module (FEM) will communicate with the pilot module on one
side, and the PHENIX DAQ and slow control system on the other side.
Since the pilot module transmits the data via optical fiber, the FEM can
be located in electronic racks away from the vertex region. Each FEM may
receive data from several pixel half ladders and thus reduce the number
of Data Collection Modules (DCM) needed to interface to the PHENIX DAQ.
In order to allow simple manipulations of the data, the FEM will pipe
the data through an FPGA. This FPGA will add data headers and trails to
for standard PHENIX data packages. Optionally the data may be further
zero suppressed at the bit-level at this stage to reduce the data volume
that need to be transferred. The design of the FEM is very similar to
FEM\'s that are currently employed in the PHENIX readout system.

We are investigating to what extent the FEM\'s may also be used to
extract information from the vertex tracker for the second level
trigger. The basic idea is to let the FPGA group the data into super
pixels, which provide rough hit information. The hit information from
all FEM\'s could then be transmitted to a local trigger processor to
calculate trigger primitives in real time. The hit information could be
correlated with other information, like electron identification in the
PHENIX RICH and EMCal, by more elaborate algorithms, while the event is
analyzed by the PHENIX second level trigger processors farm. In
principle, if the data of the silicon strip detectors is also passed
through similar modules at the same location the information of all
layers of the vertex detector could be correlated and a rough secondary
vertex tracking may be feasible.

### Pixel detectors operating in the NA60 experiment:

Hybrid pixel detectors as described above are already being applied in
the NA60 experiment at the SPS at CERN. NA60 studies the production of
open charm and prompt dimuons in collisions induced by proton and heavy
ion beams on nuclear targets. For this task, several novel detector
systems were added to the dimuon spectrometer and zero-degree
calorimeter, which were previously used in the NA50 experiment. The main
upgrade is a new silicon pixel vertex spectrometer, which contains a
tracking telescope, shown in Figure 29 that is built from ALICE1LHCb
pixel detectors. Ninety-six single-chip pixel assemblies are arranged on
16 tracking planes with 4 or 8 chips that are operated on pixel busses
on ceramic substrates. The readout electronics utilizes the pilot
prototype chip that was developed for the ALICE Pixel Detector, and the
driver chip developed at CERN for the optical link.

In the year 2002, during the construction phase of the NA60 pixel
detector modules, a sub-set of three tracking planes was operated in the
experiment and used to track particles in low-energy Pb-Pb collisions
Figure 30. The run confirmed the feasibility of the detector concept in
both technical aspects of the module construction and the vertex
reconstruction performance[^33]. One half of the telescope was then
characterized in a test beam run in September 2003. The full telescope
with 16 detector planes is now ready and installed in the NA60
experiment for the physics run with high-energy Indium beams in October
2003. The full readout chain is working well in the NA60 data
acquisition system.

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  height="1.4659722222222222in"}

  ![](media/image39.png){width="6.020833333333333in"
  height="2.4791666666666665in"}
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[]{#__RefHeading___Toc55021917 .anchor}Figure 29 The vertex spectrometer
of the NA60 experiment comprises a 16-plane pixel detector telescope
mounted in a 2.5 T dipole magnetic field in 7 cm to 32 cm distance
downstream of the targets. Every plane is built from four or eight
ALICE1LHCb single-chip pixel detector assemblies, which are mounted on
ceramic printed circuit boards

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  ![](media/image40.png){width="5.997916666666667in"
  height="1.3597222222222223in"}

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[]{#__RefHeading___Toc55021918 .anchor}Figure 30 Average-multiplicity
event in collisions of a 30\~GeV/c Pb beam onto three Pb targets,
reconstructed with three pixel detector planes during a test run of NA60
in Fall 2002

##  {#section-3 .Style-Heading-2-+-Times-New-Roman-Not-Italic}

## 4.3 Silicon Strip Detector {#silicon-strip-detector .Style-Heading-2-+-Times-New-Roman-Not-Italic}

We plan to instrument the outer three layers of the VTX with silicon
strip detectors of a novel design. The sensors developed by the BNL
Instrumentation Division provide two-dimensional position sensitivity
with single-sided processing. A first set of prototypes has been
produced and tested using the VA2 readout chip, which was not well
matched to the sensor. The next generation of prototypes is under
development. They will be readout by SVX4 chips, which were developed
for silicon strip detectors by FNAL and LBL. If the test results are
satisfactory the SVX4 will be our choice for the readout of the sensors.
In the final readout system the SVX4 chips would be arranged on readout
cards (ROC\'s) and mounted directly on the sensors. The ROC\'s would
also provide the data bus, power distribution, and all necessary control
signals.

### Strip Sensors

Principle Design

The sensor is a finely grained detector with 80μm×1000μm pixels. Each
pixel region has two serpentine-shaped metal strips (Figure 31) that
collect charge that diffuses after an ionizing particle transverses the
silicon. Any one hit will deposit charge on two serpentines. A metal
strip connects those serpentines that are in a straight line
(X-direction), while a second strip connects serpentines that are at a
4.6 degree angle (U-direction). This provides a stereoscopic X-U readout
and thus two-dimensional information from one side of the sensor.

![](media/image41.jpeg){width="5.992361111111111in"
height="2.7576388888888888in"}

[]{#__RefHeading___Toc55021919 .anchor}Figure 31 A schematic view of p+
cathode structure of the pixels.

Figure 32 shows a schematic view of the prototype strip sensors and
their geometrical dimensions. The total size of the sensor is
34.3mm×64.6mm. Each sensor is divided into two sides of separate
sensitive areas with a dimension of 30.7mm×30.0mm. The active areas are
segmented into pixels of 80μm×1000μm, which are connected to X- and
U-strips as discussed above. Each strip sensor has a total of 1536
channels, 2×384 channels per side, half of them in X- and the other half
in U-direction. First prototypes where produced with 250 and 400 μm
thickness. These detectors have been tested in the laboratory and in
beam. The next generation of prototypes is being developed and we expect
to test their performance in the first half of 2004.

![](media/image42.png){width="5.956944444444445in"
height="4.958333333333333in"}

[]{#__RefHeading___Toc55021920 .anchor}Figure 32 A schematic view of the
prototype silicon strip sensor.

### Laboratory tests of the strip sensors 

The initial tests of the prototype sensors have shown good results.
Figure 33 shows the measured current and capacity as a function of the
bias voltage. Full depletion is achieved at a bias voltage of \~80 V. At
this voltage the capacitance of each strip is \~10 pF and the leakage
current is less than 10 nA.

To test the sensors in more detail, the Instrumentation Division in BNL
has constructed a laser test setup. During the R&D phase the laser setup
allows to studies of the properties of the sensors and to find the
optimum operation condition, which includes the depletion voltage, the
charge collection efficiency, the response to the carrier drift process,
electric field profile inside the sensor and so on. The transient
current technique (TCT)[^34] \[1\] is applied to obtain the electrical
properties of the sensors. The technique is based on the analysis of the
current and/or charge pulse shapes, which arise from electron-hole pairs
created inside the detector by injecting the fast laser light. During
the mass production of sensors, the existing setup or a newly developed
setup based on our experience will provide the basic tool for quality
control of the strip sensors and later of the assembled detector
ladders.

![](media/image43.wmf){width="3.12992125984252in"
height="2.5515748031496064in"}![](media/image44.wmf){width="2.765354330708661in"
height="2.351574803149606in"}

[]{#_Ref54530969 .anchor}

Figure 33 Current and capacity characteristics of a prototype sensor.

shows the schematic layout of the laser test setup. The picture of the
laser test setup is shown in Figure 35. A nano-second pulsed laser
coupled into a focuser through an optical fiber is operated with a pulse
generator. Two types of laser are used in the laser tests. One is a red
laser with the wavelength of 635 nm and the penetration depth of 3 μm in
silicon and the other is an infrared laser with the wavelength of 1060
nm and the penetration depth of 800 μm in silicon. The red laser is used
to study the separate collection of electrons and holes. The infrared
laser is used to study the response of the detector to the charge
deposition of a minimum ionizing particle (MIP). The focuser was fixed
to a XYZ motorized stage. The precision of the laser spot positioning
with the motorized stage is 0.5 μm. The laser spot size achieved after
focusing is about 10μm in diameter. The current laser test setup is
controlled with manual controllers and will be upgraded to have an
automatic control with a LabVIEW compatible multi-axis motor drive.

![](media/image45.png){width="4.639583333333333in"
height="2.7756944444444445in"}

[]{#__RefHeading___Toc55021922 .anchor}Figure 34 Schematic layout of the
laser test setup.

![](media/image46.jpeg){width="4.444444444444445in"
height="3.3465277777777778in"}

[]{#_Ref54530872 .anchor}

Figure 35 The laser test setup for the strip sensor.

### Sensor Prototypes in Test Beam

To test the prototypes in beam, the sensors were mounted on readout
cards designed to read out signals from one side of the sensors (Figure
36). Each plane consists of a silicon strip sensor, six readout chips, a
base-board, two fan out boards (pitch adaptors) and two SMT boards. A
bias voltage was applied in the sensor backplane (ohmic side) through
the base-board. An analog multiplexer readout chip with 128-channel
charge sensitive preamplifier-shaper circuits, VA2 chip of Ideas
ASA[^35], was used to read out signals from the sensor. The
preamplifier-shaper has a 1-3 μsec peaking time and a nominal gain of 30
mV/pC. The SMT board was designed to operate the VA2 chips and read out
multiplexed analog signals through an interface with a VME data
acquisition system. Three detectors planes with 400 μm thick sensors and
four with 250 μm thick sensors were constructed.

![](media/image47.png){width="3.561111111111111in"
height="2.676388888888889in"}

[]{#_Ref54543695 .anchor}

Figure 36 The prototype detector

The sensor performance in terms of charge sharing properties, detection
efficiency and position resolution has been evaluated with a ^90^Sr
β-source and in the test beam at KEK. The beam test was performed at T1
beam line in KEK-PS, which delivered a positively charged particle beam
with momentum of 0.5 - 2.0 GeV/*c*.

Charge sharing property in x-strip and u-strip can be characterized by
the asymmetry, A~Q~ = ( Q~x~ - Q~u~ ) / ( Q~x~ + Q~u~ ), where Q~x~ and
Q~u~ represent collected charges in x-strip and in u-strip,
respectively. For an optimum two-dimensional position sensitivity charge
sharing one expects that A~Q~ on average is zero with a narrow width.
The results are plotted in Figure 37. The source test shows the A~Q~
distribution peaking at zero with a width of about 0.18. This
demonstrates that the principle of two-dimensional position sensitivity
by charge sharing works. In the beam tests A~Q~ exhibited a dependence
on the incident angle of the beam normal to the sensor surface, θ~inc~.
For θ~inc~ = 15 degrees and 30 degrees narrow distribution with a peak
at zero were observed. However, at θ~inc~ = 0 degrees broad
distributions which did not peak at zero were found. This deficiency was
traced back to the line width (8μm) and gap spacing (5μm) of the p+
electrode structure in a single pixel. For the second-generation
prototype sensors a narrower line width (5μm) and gap spacing (3μm) was
chosen to overcome this problem.

![](media/image48.png){width="6.0in" height="1.6888888888888889in"}

![](media/image49.png){width="6.0in" height="1.8486111111111112in"}

[]{#__RefHeading___Toc55021925 .anchor}Figure 37 Charge correlations in
between x-strips and u-strips found in tests with a radioactive source
and with beams of charged particles.

The position resolution was measured from the residuals of hits on
reconstructed particle trajectories. As is shown in Figure 38 the rms
width of the residual distribution for a single strip is about 40 μm.
After deconvoluting the fit bias and the contribution of multiple
scattering, the intrinsic position resolution of the detector is 23 - 25
μm. This is comparable to the resolution expected for a strip detector
of 80 μm pitch (80/$\sqrt{\text{12}}$=23.1).

![](media/image50.jpeg){width="6.0in" height="2.272222222222222in"}

[]{#__RefHeading___Toc55021926 .anchor}Figure 38 Hit residuals from
tracks found using the silicon strip sensors in a test beam experiment.

A reliable measurement of the efficiency of the sensors was not
possible. In part this is due to the imperfections in the charge
sharing, but more prominently the readout chain suffered from severe
common mode noise, which lead to a small signal-to-noise ratio and
prohibited an efficient hit detection. At present only lower limits for
the efficiency can be quoted, they are 88% for the 400 μm thick sensor
and about 60% for the 250 μm sensor. The detection efficiencies and the
signal-to-noise ratios are under study and are expected to be improved
by optimizing the operational condition of the detector and by
introducing a more appropriate readout chain based on the SVX4 chip.

### SVX4 readout chip

The strips will be read out with the SVX4 chip, which was developed by a
FNAL/Berkeley collaboration[^36]. The chip is shown in and is shown in .
The SVX4 is implemented in the 0.25 μm TSMC process and is inherently
rad-hard. It is a 128-channel chip with a 46-deep pipeline that is
cycled by the beam-crossing clock, thus providing the LVL1-latency
required by the PHENIX DAQ. LVL1-accepted events are stored for future
pipelined readout. The SVX4 allows up to 8 bits of analog information,
although the number of bits are programmable. Several
pedestal-subtraction steps offer robust protection against common-mode
noise. On-board zero suppression is provided for, but can be turned off
(see discussion below). The SVX4 also provides for four-deep multi-event
buffering required by the PHENIX DAQ.

A test board has been developed that is a simplified version of the
PHENIX DAQ. A schematic diagram of the test board operation is shown in
Figure 40. With this test board we can download serial strings, pulse
test patterns, send in trigger signals and read out the data. This will
allow us to completely verify compatibility of the SVX4 with the PHENIX
DAQ bandwidth, latency and buffering requirements. In addition, we will
use this board to perform an integrated SVX4/sensor test in the next few
months. Initial system tests have started at ORNL.

In fall 2003 FNAL cancelled the Tevatron Run-II silicon upgrade
projects. There are already sufficient SVX4 chips that have been
produced to meet our needs. We are in negotiations with FNAL for the
purchase of these chips. In addition, FNAL has a complete wafer-testing
station and has indicated a willingness to take on the required wafer
testing of our chips.

![](media/image51.jpeg){width="4.779861111111111in"
height="3.584722222222222in"}

[]{#_Ref54760321 .anchor}

Figure 39 Photograph of an SVX4 chip.

![](media/image52.wmf){width="5.8375in" height="3.4145833333333333in"}

[]{#__RefHeading___Toc55021928 .anchor}Figure 40 Schematic Diagram of
SVX4 test board.

### Sensor Readout Card (ROC) / Readout Bus

The readout geometry is illustrated in Figure 41, which shows three
different views of a portion of a strip ladder. Each sensor is
wire-bonded to and read out by twelve SVX4s, six per orientation. The
twelve SVX4s servicing each sensor are mounted on two beryllium oxide
readout cards (ROCs). There are 2 ROCs per sensor, 4 sensors per ladder
and 42 ladders for a total of 336 ROCs (2016 SVX4s).

![](media/image53.wmf){width="5.844444444444444in"
height="3.988888888888889in"}

[]{#__RefHeading___Toc55021929 .anchor}Figure 41 Three views of Si Strip
ladder. Panel a) shows a top view of a single sensor and its two ROCs
and associated chips. The sensor appears as blue-green. The four rows of
bonding pads for each sensor are shown as red and dark blue lines -- two
per orientation per sensor. The corresponding SVX4s are also shown in
red and dark blue. They are wire-bonded to the sensor through the holes
shown in the ROCs (light green). The digital ASICs are shown in black
and arrows indicate the direction of the signal bus for each
orientation. The signal buses (and power and ground) are carried across
ROC and sensor boundaries (shown by the solid black line bisecting the
sensor) by wire-bonding necessary pads of adjacent ROCs. Panel b) shows
the short edge-on view of a ladder. The vertical development is shown --
the support/cooling structures are gray; the sensor is blue-green, the
ROC is light green and the SVX4s are red. Panel c) shows the long
edge-on view of a pair of sensors (separated at the heavy black vertical
line) using the same color scheme. The hatched green regions show where
the holes are in the ROC to allow wire-bonded connection between the
sensor and the SVX4.

In addition to six SVX4s, each ROC has two identical, custom, digital
ASICs (or perhaps radiation hardened FPGAs), one per orientation. These
chips read the data out of their corresponding SVX4s. Reading out SVX4s
on different ROCs in parallel is required in order to meet the PHENIX 40
μs readout criteria since without zero suppression 129 clock ticks are
required to read the data out of each SVX4. This defines a maximum of
three SVX4 chips that can be read out serially if the SVX4 readout clock
is cycled at the beam clock frequency (\~10 MHz). The readout clock can,
in principle, run at up to roughly four times the beam clock frequency,
reducing the required number of these chips by the same factor of four.
However, faster readout results in more SVX4 power consumption, and the
reduction would require development of two different flavors of ROC. In
addition to parallelizing the data, these chips also serve to remove
strip address information and compress the 8-bit data to 4-bits.
Finally, they also serve to minimize the effects of single-point
failures (one broken chip will take out all of the chips in its serial
path). The decision to use an ASIC or a rad-hard FPGA will be made based
on radiation dose measurements. Since these chips are purely digital the
development process can be done inside an FPGA (greatly simplifying any
necessary revisions) and a technology choice can be postponed.

Power, serial control, timing and readout for a ladder are all carried
directly on the ROCs. All ROCs on a ladder are bussed together with
wire-bonds at the edges of each ROC pair. There is one signal bus per
orientation. These carry the digitized data from each orientation to
Pilot Modules at opposite ends of the ladder.

### Pilot Module

The two pilot modules per ladder are wire-bonded to the ROCs at the far
ends of each ladder and sit outside the detector acceptance. These
modules serve as the interface to the PHENIX timing, data collection and
serial control systems. These modules consist of ARCNet connectors and
corresponding chipset, optical fiber links for timing and data
communication, and an FPGA that handles data formatting and direction of
serial control traffic. Space issues may require this module to be
implemented in two boards, one local to the ladder and one remote with
optical connection between the two. The bulk of the data formatting
tasks will be handled by the ROC-mounted FPGAs (or digital ASICs).
Aspects of this module are extremely similar to other PHENIX DAQ
interface modules previously developed. With four-bit data read out by a
clock at twice the beam frequency on 16 bit words (specifications for
the existing Data Collection Modules) each pilot module would supply
data to one DCM and fit within the PHENIX bandwidth requirements.

### Zero Suppression

The architecture above assumes that we do not take advantage of the
capability of the SVX4 to zero-suppress its data. The advantage of this
approach is that we can use existing DCM technology. This simplifies
design of the pilot module and decouples the development cycle from the
development cycle of a new DCM. In addition, the current PHENIX DAQ is
not designed to handle zero-suppression prior to the DCM. The pipelined
architecture assumes a fixed length data packet. Any specified maximum
data volume that is smaller then the data volume when every channel is
struck can be exceeded. A mechanism to either throw out anomalously
large events or a method to incorporate a BUSY signal into the DAQ would
need to be developed. Note: an active R&D program in the PHENIX DAQ is
investigating these options since the inner-most barrel (pixels) data is
zero-suppressed on the detector.

Allowing on-board zero-suppression by the SVX4 would have several
advantages however. First, the ROC-mounted FPGA (or digital ASIC) would
not strictly be needed. With a generous safety factor on the expected
occupancy all SVX4s on a ladder could be read out serially. This would
save on FPGA/ASIC development costs, on precious ROC real estate and on
material in the aperture. (However, note: even with zero suppression
implemented there is motivation to retain this chip since it does limit
the effects of single-point failures.) Second, the data reduction
provided by on-board zero-suppression would allow us to combine several
ladders worth of data into a single DCM. This would likely make any
future hardware level-2 trigger easier to implement. Finally, it is
reasonable to expect some modest reduction in power consumption due to
the reduced amount of data to read out. These points will be studied
further prior to the development of a final design.

## 4.4 Mechanical Structure and Cooling {#mechanical-structure-and-cooling .unnumbered}

A conceptual design of the silicon vertex detector was commissioned by
the LANL group to HYTEC, Inc. HYTEC is the mechanical designers for the
ATLAS silicon group and has 15 years of design experience with silicon
vertex detectors. For PHENIX they have also designed the station 1 muon
detectors and the station 2 spider and also did the finite element
analysis of the station 3 octants. The VTX mechanical conceptual design
was finished and the report is summarized in this section.

\[<http://p25ext.lanl.gov/~hubert/phenix/silicon/HTN-111003-0001.pdf>\].

The next steps following this conceptual design are to fully specify the
requirements for the VTX mechanical support and cooling. These
requirements will incorporate the integration needed for all the upgrade
PHENIX detectors in the inner region, including the VTX barrel described
in this proposal, the endcap described in Appendix A, and the HBD/TPC to
proposed later. Hence there are two major mechanical engineering tasks
contained in this proposal: 1) the internal support and cooling of the
VTX detector (described in this section) and 2) the integration of the
VTX detector into the inner region of PHENIX (described in the next
section).

For the internal support and cooling of the VTX detector, the major
results of the conceptual design are:

-   The use of sandwich composites will satisfy the radiation length
    requirements and provide the required stiffness.

-   The outer frame structure should be a single diameter encompassing
    both the barrel and endcaps.

-   The modular clamshell design can satisfy the stability requirements
    provided the connection issues are studied further.

-   An octagon arrangement is suggested to facilitate utility routing
    and fabrication.

-   Structural end disks at either end of the structure are recommended
    to prevent deformation

-   The ladders should have a simple support at one end and floating
    support at the other end to minimize thermal strains

The R&D issues identified are:

-   Building prototypes of ladder assemblies to verify calculations.

-   Building full-scale prototype to test static and dynamic stiffness.

-   Develop connections of modules.

-   Develop support design.

-   Refine calculations and develop full concept for 0 deg operation if
    necessary.

### Design Criteria

The goal of the study is to establish a feasible design and to identify
outstanding design issues. The study is based on a preliminary list of
design requirements and a strawman layout of the detector structure. To
adequately address all structural and mounting issues a fully integrated
design, which includes the barrel detectors and future endcaps
extension, is needed. This design needs to address all integration
issues not only for the barrel and the endcap vertex trackers, but also
with other potential PHENIX upgrades.

The design requirements of the conceptual study were,

-   Modular Design

    -   Endcaps detectors can be mounted independently at a later time

    -   Support structure separated vertically into two half shells

-   Detector Coverage

    -   Hermetic design

    -   Four barrel layers

    -   Four endcap layers layers in each forward section

    -   Fiducial volume \< 20 cm radius, z \< 40cm

-   Radiation length goal \< 1% per layer

-   Room temperature operation desirable, 0 deg Celsius if needed

-   Dimensional stability \< 25 microns

In the conceptual study, the ALICE1LHCb pixel hybrids for the inner
layer and silicon strip detectors with SVX4 readout for the three outer
layers provide the basis for cooling and radiation length analysis of
the barrel detector. At the time, for the endcaps a modified ALICE type
chip that has 1/5 to 1/10 of the number of pixels was assumed[^37].
These choices resulted in a estimated heat load of typically 0.7 W/cm^2^
for the barrel and 0.1 W/cm^2^ for the endcaps.

### Structural Support

The selection of materials for the support structure is based upon the
above criteria where the most important material properties are high
radiation length, low density, high stiffness, and availability. Out of
three candidates (i) beryllium, (ii) graphite fiber reinforced plastic
(GFRP), and (iii) Carbon-Carbon, the GFRP was chosen for the study
because of its wide availability, works well in sandwich composites, and
has good radiation length and strength properties.

#### Structural Analysis

The structural analysis includes two studies, a first study using finite
element analysis models and the resulting modal frequencies to look at
dynamic stiffness of tracker concepts and a second study to look at the
static stiffness with mass loaded structures. The lower modal frequency
limit is set at 70 Hz on a fully loaded structure so that the natural
frequencies due to environmental conditions such as pumps, traffic, etc.
do not couple into the structure and cause instabilities greater than 25
microns.

![](media/image54.png){width="5.997916666666667in"
height="1.5319444444444446in"}

[]{#__RefHeading___Toc55021930 .anchor}Figure 42 Design concepts studied
for the vertex detector support structures. The center most concept with
the constant outer diameter shell had the highest fundamental frequency.

![](media/image55.jpeg){width="2.875in"
height="2.0069444444444446in"}![](media/image56.jpeg){width="2.873611111111111in"
height="1.975in"}

[]{#__RefHeading___Toc55021931 .anchor}Figure 43 First mode shape that
dominated the dynamic structural stiffness analysis

Various support structures shown in Figure 42 were studied. The center
most structure has the highest frequency limit.

The dumbbell shaped structure has the lowest mode frequencies below 53
Hz while the concept with the uniform shell with constant outside
diameter has the lowest fundamental mode at 132 Hz, well above 70 Hz. In
Figure 43 the associated first mode shape of the concept that has the
highest fundamental frequency is shown

The static analysis under gravitational load is shown in Figure 44 for
the concept with the uniform shell. A 1.0 G load is applied vertically
to the fully loaded structure. The maximum displacement is 14.5 microns
and the maximum stress is 130 psi. These satisfy the design criteria so
the uniform shell with constant diameter has been chosen as the concept
to be pursued.

![](media/image57.jpeg){width="2.748611111111111in"
height="1.8930555555555555in"}![](media/image58.jpeg){width="2.748611111111111in"
height="1.8930555555555555in"}

[]{#__RefHeading___Toc55021932 .anchor}Figure 44 Displacement and
principle stress from a 1.0g gravity load on a full mass loaded
structure

### 

### Detector Ladders and Cooling 

The mechanical design draws on a ladder concept for supporting both the
inner layer of pixel and the outer strip detector layers. The pixel
detectors and strip detector array are arranged in a longitudinal
fashion, and at a slight cant angle, which provides a small amount of
overlap for hermeticity. The pixel and silicon strip detector both
contain on-board electronics, thus necessitating cooling along the
Z-axis of the detectors, thus the ladders simultaneously must provide
mechanical support and cooling.

The barrel region is about 30cm in length and thus the ladders need
supported only at their ends. Open ring like structures at the two ends
of the ladders, Figure 45, provide attachment points for the ladders and
serve to combine the staves into two halves of a clamshell. Figure 45
also illustrates the cross section of a ladder structure, it is composed
of a thermal plane (Carbon-Carbon) onto which the pixel modules or strip
detectors are mounted. The thermal plane collects the distributed
electronic heat, as well as providing a conductive path to the cooling
tube. All elements are bonded with thermally conductive, rigid setting
adhesives. The omega shaped piece holding the cooling tube on to the C-C
thermal plane provides significant stiffness to the ladder.

![](media/image59.jpeg){width="2.748611111111111in"
height="1.8854166666666667in"}![](media/image60.jpeg){width="2.126388888888889in"
height="0.7576388888888889in"}

[]{#__RefHeading___Toc55021933 .anchor}Figure 45 3D model of the barrel
region on the left and the ladder structure on the right showing a
cooling tube mounted on a C-C thermal plane and the sensor and
electronics on the underside.

#### Cooling Analysis

The total heat load that must be removed from the vertex detector is
approximately 2.2 kW, which is dominated by power dissipation in the
barrel detectors. There are generally two choices for the coolant
system, single-phase and two-phase. In a single-phase system the coolant
is circulated through pipes in a closed system. In a two-phase system
the coolant arrives as a liquid and then evaporates into the detector
volume where it is collected and removed. The two-phase system has the
advantage that it is a more efficient coolant and a lower total mass is
required. However, the two-phase system is more difficult to implement
because of issues concerning more sophisticated temperature and pressure
control. The single-phase system is simpler in implementation and design
and therefore chosen to be the baseline design.

An important issue in the design of the mechanical structure is the
operating temperature. In the initial design requirements we stressed
the importance of room temperature operation and the design presented
here is based on this assumption. However, the design engineers have
developed the concepts that will allow operation at 0 deg by using
suitable coolants that can operate at these lower temperatures. A more
detailed study of the enclosure and thermal stresses would need to be
done to confirm the concepts at reduced operating temperature. After
this study was concluded, it was noted that the outer 3 layers of the
barrel might perform better at 0 deg because of the mismatch between the
SVX4 chip which is an AC coupled device and the strip sensor which is a
DC coupled device. The concern centered on leakage currents from the
sensor that could saturate the SVX4 chip. The other option is to reset
the chip regularly during the empty beam-crossing at RHIC.

The selection of coolants was based on previous experience in the design
of the ATLAS detector and the choice was to use one of several
perfluocarbon candidates, commonly called fluorinerts. These fluids can
be used in either a single or a two-phase system and are environmentally
acceptable. Based on a number of considerations the fluorinert C5F12 was
chosen as the baseline.

In comparison to the ATLAS detector the heat load is very modest and a
single-phase system can be used. Since the barrel is only 30 cm long the
design assumes that the ladders need only be supported at the ends and
the cooling structure can serve as the means of joining the two half
ladders of the pixel layers.

The concerns with this approach center on,

-   Out of plane distortions from thermal strains due to different CTE's

-   Gravity sag

-   Mass of the structure exceeding radiation length guidelines

A series of calculations were done on the thermal aspects of this
concept by using the outer barrel ladders since the greatest total heat
load, 27 W, exists there. Using a temperature rise of 2 deg C as the
maximum allowed temperature rise in the ladder the results indicate a
good solution exists with an Al tube diameter of 3mm and a wall
thickness of 0.2 mm.

The result for the out-of-plane distortions for room temperature
operation is quite acceptable, 0.18 microns. For the possible design
requirement of 0 deg operation the out-of-plane distortions increase
because of the difference in CTE's of the C-C and silicon but is still
acceptable at 3.5 micron. Bowing along the length of the ladder due to
thermal strain because of the temperature gradient at room temperature
is an acceptable 6.8 microns. However, if the detector is cooled, the
bowing increases to over 80 microns. This is more than desirable so
addition R&D is necessary.

![](media/image61.jpeg){width="2.8722222222222222in"
height="1.6118055555555555in"}![](media/image62.jpeg){width="2.997916666666667in"
height="1.53125in"}

[]{#__RefHeading___Toc55021934 .anchor}Figure 46 Left panel shows the
out of plane distortions and the right panel shows the bowing for the 0
deg solution.

Figure 46 illustrates the 0 deg solution. While the temperature drop for
the coolant is still the same as at room temperature the reference
temperature is always 25 deg since that is the temperature of assembly.
The increased bowing is due to reducing the operating temperature to 0
deg.

For the gravity sag the general bowing of the ladder is acceptable at
18-20 microns.

### Radiation Length

The radiation length budget for the ladder describe above exclusive of
the sensor and electronics consists of:

-   Composite thermal backplane

-   The cooling tube and tube support

-   The omega piece

-   The coolant

The combined radiation length is 0.7% with the single largest
contributor being the tube support (0.28%). The liquid coolant
contributes 0.074%, cooling tube \~0.1%, and the omega piece and C-C
facings \~ 0.2%.

## 4.5 Detector integration into PHENIX  {#detector-integration-into-phenix .unnumbered}

Once the individual detector components and the mechanical support
structures become available, the VTX system needs to be assembled,
integrated into the PHENIX setup, and connected to the support and
ancillary systems as well as to the DAQ. These steps are critical for
the successful completion of the project and need to be addressed well
in advance and in close contact with the developments of the detectors
and support structures.

### VTX detector assembly 

Each step from delivery of sensors and readout chips, to the assembly of
detector ladders, to their final mounting in the mechanical support
structure, will require intensive quality control and testing of the
components. Special equipment, infrastructure and expertise are
necessary to complete these tasks. We have started to plan the necessary
steps and to optimize the use of expertise within our collaboration and
the institutions that we cooperate with.

For the pixel detectors we will greatly profit from the close
cooperation with the ALICE pixel detector group and from the RIKEN staff
that now has several years of experience with the ALICE pixel detectors.
All necessary steps will be carried out either at CERN or at facilities
of the RIKEN institute, except for the final installing of the pixel
ladders into the VTX detector, which will be done at BNL.

For the silicon pixel detectors the major steps that need to be taken
are

-   Test sensor ladders which include one sensor and 4 readout chips

-   Readout bus to read data from sensor ladders manufactured and tested

-   Sensor ladders mounted on readout bus and tested

-   Pilot card to readout pixel hybrids from one readout bus produced
    and tested

-   Half ladders, combining readout bus, two sensor ladders and pilot
    module assembled and tested

-   Assembly and commissioning of half ladders to full ladders

For the silicon strip detectors the major steps that need to be taken
are:

-   Test sensors

-   Test SVX4s

-   Test bare readout card

-   Mount SVX4s on ROCs

-   Test ROC assembly

-   Attach ROCs to sensor

-   Test ROC/sensor combo

-   Assemble ROC/sensors into ladders

-   Attach pilot modules to ladder

-   Test complete ladder

-   Assemble ladders to full detector

-   Full detector test

For the testing of the strip sensors and the SVX4 chips, we will benefit
from the knowledge of the BNL Instrumentation Division and the
cooperation with the FNAL silicon laboratory. The ORNL group, which is
developing the ROC, will work together with other groups, including
University of New Mexico, to test and assemble the ROCs. The University
of New Mexico group draws on knowledge and experience gained during
similar work for the ATLAS and CDF vertex detectors. The BNL Chemistry
Department will participate in all assembly steps, but will take
responsibility for the final assembly and commissioning of the strips.
This group will draw from the experience gained while testing,
assembling and commissioning the silicon detectors for PHOBOS. Stony
Brook has started to build up a group, which will support this effort in
all stages and expects to play a major role in the assembly,
installation in PHENIX and commissioning of the detector. The final
assembly will either make use of facilities available at Stony Brook or
at the BNL Chemistry Department.

### Integration into PHENIX

#### Mechanical mounting

The mechanical mounting of the detector will be done by BNL PHENIX
Operations, in close collaboration with HYTEC. The current concept would
be similar to the existing PHENIX MVD mounting. This will support the
VTX detector directly on the copper nose cones with attachments to the
ends of the detector. The two halves of the split barrel will be either
hinged together at the top or the bottom, or be mounted independently.

The development of this concept will require careful consideration of a
number of issues, including precision and stability requirements,
accommodation of future detectors (e.g. silicon vertex detector endcaps
to cover the muon detector acceptance, and a hadron blind detector (HBD)
in the acceptance of the central arms), the attachment and routing of
cables, fibers and hoses.

#### Support and Ancillary Systems

Additional platforms installed on top of the PHENIX central magnet yoke
will be required to support electronics racks and cooling system
equipment. These racks will include space for low voltage and bias
supplies, and whatever readout electronics for the silicon detectors.
The platforms must also provide area for cooling system pumps, and
compressors or chillers. In addition, these platforms will be used for
ancillary systems in future detector upgrades in the central magnet such
as the HBD.

#### Beam pipe

A new beryllium beam pipe with a smaller 4 cm inner diameter and 500 μm
nominal thickness will be critical to perform the anticipated
measurements. The inner surface of this pipe may need to be NEG
(non-evaporative getter) coated to help reduce beam induced vacuum
instabilities. This technology is currently being implemented in the
upcoming RHIC run. The new beam pipe will need to taper out to match the
standard 5 in diameter lines of the RHIC rings.

### DAQ 

The VTX readout system will be matched to the requirements of the PHENIX
high rate data acquisition (DAQ) system. The current DAQ system has a
bandwidth corresponding to approximately 12.5 kHz level 1 (L1) trigger
rate with a planned upgrade to 25 kHZ. Currently each front-end module
(FEM) sends uncompressed events of fixed length to Data Collection
Modules (DCMs) through a gigabit optical link. Each DCM has four sets of
optical receivers, FPGAs and digital signal processors (DSP), which
zero-suppress and buffer the events from four FEMs. An additional DSP is
used to merge the data from the four sets. The DCMs and FEMs pipeline
events with the capability to buffer at least 4 or 5 L1 triggers. Since
the FEM data is of known length no data flow control between FEMs and
DCMs is necessary in this design and only the DCM participates in the
busy logic.

Because the VTX will deliver substantially larger data volumes than the
currently operated detectors and it will be necessary to zero suppress
the data at the front-end before it is transmitted to DCM\'s. As a
consequence some of the current DCM-functionality must be taken over by
the pilot modules on the detector and the FEMs off the detector. In
particular, after the zero suppression, the FEM data will no longer be
of fixed length and therefore the FEMs must participate in the busy
logic.

Since the new requirements are common to most of the planned PHENIX
upgrades detectors we have launched an R&D effort to develop the new
DCM\'s independent of this proposal. We anticipate that the development
will be completed within 2 years, in time to produce new DCM\'s for the
VTX detector. Once the new DCM\'s are completed the PHENIX Electronics
Facilities and Infrastructure (EF&I) and Online Computing Systems (ONCS)
groups will guide the electronic integration and readout of the VTX.

# 5. R&D

With the help of institutional contributions it was possible to maintain
a small, but well focused, effort over the past two years to explore
technologies for silicon detectors suitable for PHENIX. The results of
these investigations are the basis of the present proposal. The outcome
and corresponding technology choices, which are the ALICE silicon pixel
hybrid detectors and a BNL silicon strip sensor with SVX4 readout, have
been discussed in section 4. More recently the R&D effort has shifted
towards adapting the technologies to the PHENIX requirements. In part
this is possible due to the support PHENIX received following a request
to BNL management for generic R&D funds. Here two key issues remain open
and need to be address. The first one concerns the readout speed of the
pixel ladder and the second one the readout scheme for the strip
detectors.

The readout speed of the ALICE1 pixel chip is limited to 10 MHz, which
means that it takes 25.6 μs to read one chip. A sequential readout of
all chips on a ladder, like it is done in ALICE, is out of question
since it does not meet the PHENIX design specification to read each
detector in 40 μs. Several options to overcome this limitation are under
study. They all involve parallel readout on the ladders, which requires
us to replace the existing ALICE digital pilot chip. We have focused on
two options

-   Use of rad-hard FPGA

-   Modification of the ALICE pilot custom ASIC

Feasibility studies for the ASIC option are underway. The modified ASIC
would be developed at CERN together with the designers of the original
pilot chips. A promising alternative which would avoid expensive ASIC
development is the use of a radiation hard FPGA. An FPGA is more
flexible and would allow us to implement zero-suppression on the ladder.
This potentially may reduce the amount of data to be transferred by a
factor of 10, which may increase the bandwidth and simultaneously reduce
the number of necessary FEM\'s and DCM\'s. A first design of the FPGA
code is completed and is currently being evaluated. We plan to make the
final technology choice in the first half of 2004.

The second issue concerns the readout of the BNL silicon strip sensors.
First prototypes sensors have been tested in the laboratory and in beam.
The performance is very encouraging, however, some limitations have been
found which we believe result from poor signal-to-noise ratio of the
readout electronics used. The next generation of improved prototypes
will be tested with the SVX4, our preferred choice for the readout
chips, in early 2004. Based on the information at hand and multiple
discussions with the experts at FNAL, ORNL, and BNL instrumentation this
combination should provide the necessary performance.

If the present limitations are not overcome an alternative solution
could be provided by the PHX chip + mini-strip sensors. We are currently
investigating this technology, which is a spin off of the FNAL
development for BTeV, for the endcap extension of the VTX. This
technology should also work for the outer barrel layers, with
potentially additional advantages of reduced occupancy since the sensors
are more finely segmented and a reduced heat-load, hence potentially
thinner support. The R&D for these detectors has started and will
continue for the endcaps independent of the technology choice for the
VTX.

For many parts of the VTX, the technology choices have been made, and
R&D is focusing on prototyping the designs. We need to pursue and
complete these topics over the next year to start EDIA and construction
of the VTX in FY05. Generic R&D funding through both RIKEN and from the
DOE (at a level of \$286k) will be required to complete these tasks. In
detail the projects are:

*Inner Pixel layer*

-   Pixel Bus, multiple layers, fine lines on Kapton

    -   Uses ALICE bus as starting point

    -   Draws on rapid advances in signal interconnect technology

    -   Design and prototyping completed by Sep 2004

    -   RIKEN manpower

-   Pilot Module

    -   Uses ALICE pilot card as starting point, containing existing
        ALICE analog PILOT and optical chips, and either the new FPGA or
        ASIC digital pilots.

    -   Technology decision by April 2004

    -   RIKEN, ISU manpower

-   Pixel FEM

    -   First prototype complete by Sep 2004

    -   SBU manpower

*Outer Strip Layers*

-   Readout card for SVX4

    -   First prototype cycle completed by July 2004

    -   RIKEN, ORNL manpower

-   Serpentine strip sensor

    -   Second prototype tested with SVX4 by April 2004

    -   RIKEN manpower

-   Bus

    -   First prototype cycle completed by Sep 2004

    -   RIKEN, ORNL manpower

*Auxiliary System and Integration*

-   Specifications of mechanical design

    -   List of design parameters by July 2004

    -   BNL/LANL/HYTEC manpower

-   Design study

    -   LANL/HYTEC manpower

# 6. Project management and responsibilities 

The organization and management of the proposed effort is embedded in
the management structure of the PHENIX experiment, which is part of the
BNL RHIC project. The new organization must satisfy a number of
requirements including a clear interface to the existing RHIC and PHENIX
management structure, clear roles and responsibilities within the
existing PHENIX subsystem structure. Particular attention has to be paid
to the fact that a significant portion of the project is supported by
foreign contributions. Clear deliverables, responsibilities for
deliverables and the accountabilities of the participating funding
Agencies have to be defined. These responsibilities will be formalized
in memoranda of understanding (MOU\'s) between PHENIX and the
participating institutions. In this section, we outline our proposed
management organization and delineate responsibilities within the
project.

## 6.1 Project background {#project-background .unnumbered}

The proposed project is part of a detailed upgrades program to enhance
the physics capabilities of PHENIX over the next 8 years. Realizing this
plan will enable PHENIX to remain competitive well beyond the turn on of
LHC expected for 2008, as well as advance our understanding of QCD by
fully exploiting the unique spin physics capabilities of RHIC. The plan
covers a broad range of measurements in AA, pA, and pp and its goal is
to provide key measurements which currently can either not be addressed
at RHIC or only with limited accuracy.

The development of the PHENIX upgrade program started in response to the
recent NSAC long-range plan, which was developed in 2000. The strategy
was consolidated at a workshop in Montauk, NY March 2001 and at BNL
August 2002. Since then individual upgrades have been followed up in
detail within four PHENIX study groups.

The PHENIX upgrades plan was first presented to BNL management in a
proposal seeking funds to initiate a broad-based R&D effort to develop
the detector technology necessary to realize our goals. BNL charged a
Detector Advisory Committee (DAC) to review the R&D plan. After the
review December 19-20, 2002, at BNL the committee named the PHENIX
silicon vertex tracker a high priority in the future development of
PHENIX. Some quotes from this report:

\"Physics Motivation: ... Charm has emerged as valuable probe of QCD
dynamics ... The measurement of hadrons with open charm requires
excellent vertex identification, which can only be achieved with state
of the art silicon vertex detectors ...\"

\"PHENIX Microvertex Tracking Detector: The extension of the
capabilities of the PHENIX detector to provide a direct measurement of
charm and open beauty will have major impact on a number of physics
questions both in AA and pp collisions. This upgrade is therefore a very
high priority in the future development of PHENIX."

"It is likely that a detector system with adequate performance can be
constructed using existing technology and it is therefore possible and
desirable that a detailed proposal for this upgrade be in place in time
for inclusion in the planning process for FY05 construction funds.\"

Responding to the DAC review, the silicon study group developed a PHENIX
internal letter of intend proposing a two component silicon vertex
tracker system, one component - a barrel detector - extending the
capabilities of the PHENX central arm detectors and the other - a endcap
detectors - extending the physics reach of the muon detector system.
This LOI was reviewed in March 2003 at a joint meeting of the PHENIX
detector council (DC) and executive council (DC). Following their
recommendations PHENIX management (PM) endorsed both projects and
charged the silicon study group to develop a proposal for a silicon
barrel detector for immediate presentation to DOE through BNL.

## 6.2 The management plan for the VTX {#the-management-plan-for-the-vtx .unnumbered}

### PHENIX management structure

The VXT project is part of the PHENIX project and as such integrated
into the PHENIX management structure as described by the PHENIX bylaws.
The PHENIX Detector Council (DC) will advise PHENIX management on the
design, construction, and integration of the VXT. The DC is co-chaired
by the operations manager (E.O\'Brien) and the upgrades manager
(A.Drees). The VTX subsystem manager will serve as a member of the DC.

### PHENIX subsystem leadership

We expect that the proposed VTX project will be funded through two
agencies, the DOE Office of Nuclear Physics (DOE-NP) and the RIKEN
Institute. A successful completion of the VTX will require close
collaboration and well-defined responsibilities and scope of the
contributions of both agencies in terms of deliverables. The
deliverables and foreign contributions are outlined below. Within
PHENIX, the responsibility for the VTX subsystem will be shared by the
subsystem leader, Yasuyuki Akiba (RIKEN) and his deputy, Craig Ogilvie
(ISU). The subsystem leader reports to PHENIX PM and will represent the
VTX in the PHENIX DC.

Simultaneously, Ogilvie will serve as the DOE contract project manager
(CPM), and will have the fiscal and construction responsibility for the
DOE funded deliverables as outlined in this proposal. This involves
appropriate planning, budgeting, and reporting. Akiba will serve as
RIKEN counterpart and have similar responsibilities for the RIKEN funded
deliverables.

### Role of BNL

Because we expect that all DOE funding for this effort will be directed
through the BNL Physics Department, BNL line management will have
ultimate fiscal and management responsibility for the construction of
the VTX and for its subsequent operation.

![](media/image1.wmf)

[]{#__RefHeading___Toc55021935 .anchor}Figure 47 Management chart of the
VTX project. The fiscal responsibilities for the individual tasks are
specified in bold letters. The intitutions participating in each task
are given in italic. In PHENIX the DAQ is a separate subsytem and
therefore not connected to the VTX management.

### Specification of deliverables

Figure 47 shows the construction and organization chart for the VTX
project. The VTX is divided into subprojects, which themselves are
divided into tasks. The tasks closely relate to deliverables, which need
to be completed before the VTX construction project can be considered
complete. The proposed fiscal responsibility for each deliverable,
either RIKEN or DOE, is indicated in brackets. Expected institutional
involvement is listed together with the fiscal responsibilities in
Figure 47. The present and future involvement of individual institutions
is discussed in more detail in the next section.

Outer layer silicon strip detectors:

-   Silicon strip sensors designed and tested to specifications (RIKEN)

-   SVX4 readout cards (ROC\'s) developed and manufactured (DOE)

-   Strip sensors integrated with ROC\'s (RIKEN)

-   Readout bus designed and produced (DOE)

-   Pilot modules to readout ROC\'s from detector assembly (DOE)

-   Front-end modules (FEM\'s) to collect data from pilot modules (DOE)

-   Detector ladder, sensors-ROC\'s assemblies with pilot module mounted
    on readout bus (DOE)

-   Commissioning of strip detector ladders (DOE)

Inner layer silicon pixel detector:

-   Tested sensor ladders which include one sensor and 4 readout chips
    (RIKEN)

-   Readout bus to read data from sensor ladders manufactured and tested
    (RIKEN)

-   Two sensor ladders mounted on readout bus and tested (RIKEN)

-   Pilot card to readout pixel hybrids from one readout bus produced
    (RIKEN)

-   Half ladders, combining readout bus, two sensor ladders and pilot
    module assembled and tested (RIKEN)

-   Assembly and commissioning of half ladders to full ladders (RIKEN)

-   Front-end modules (FEM\'s) ready to collect data from pilot modules
    (DOE)

DAQ system

-   Data collection modules for pixel and strip detector manufactured
    and tested (DOE)

Auxiliary Systems and Integration

-   System support requirements specified, including heat loads, power
    distribution, mechanical tolerances and ground scheme (DOE)

-   Mechanical support structure designed and manufactured (DOE)

-   Ancillary systems operational, including power distribution and
    cooling system (DOE)

-   Strip and pixel detectors ladders mounted in mechanical support
    (DOE)

-   Full system tested in assembly laboratory (DOE)

-   Commissioning of full detector system (DOE)

-   VTX installed at final location in PHENIX experimental (DOE)

-   Full system test in PHENIX (DOE)

-   Integration into PHENIX DAQ system (DOE)

## 6.3 Institutional involvement {#institutional-involvement .unnumbered}

Currently 14 institutions with a total of 65 collaborators are involved
in the VTX project. The different institutions bring in diverse research
background, physics interests and expertise, which form a broad base to
carry out the proposed project. All groups have taken on specific
responsibilities and will actively participate in the VTX project.
Matching their expertise these group will be involved in construction,
installation, commissioning, operation, and data analysis. Formal
commitments of the involved institutions will be specified in MoU\'s. At
this early stage of the project naturally some groups have been actively
involved and launched R&D through institutional contributions while
others have joined the project only recently. In the following we will
briefly discuss the different groups, their expertise and their
potential involvement. Florida State University and the RIKEN BNL
Research Center have made major contributions to this proposal and will
specify their specific responsibilities in the future. The BNL
Instrumentation Division, KEK, and Niigata join the VTX project because
of their interest in the technological development, but are not members
of the PHENIX collaboration.

### Brookhaven National Laboratory, Chemistry Department (BNL CHEM)

Mark Baker and his group bring in broad experience with silicon detector
technology. In addition to management and physics experience, each group
member brings specific technical skills that will be important for the
successful construction, installation, commissioning and operation of a
silicon detector in the RHIC environment.\
Rachid Nouicer is one of the two primary silicon experts active on
PHOBOS, having participated in the construction, installation, and
commissioning of the detector. He is currently responsible for the
multiplicity barrel, rings, and vertex silicon detectors within PHOBOS
and he led the successful effort to replace damaged ring modules by
building new ones from scratch. Robert Pak is responsible for the
silicon cooling and dry air systems, for the radiation-monitoring
chipmunks, and also the beryllium beam pipe. Andrei Sukhanov is an
expert in data acquisition (real-time programming, firmware programming,
triggering, electronics etc.). Finally, Peter Steinberg played a major
role in the design, development, and implementation in the silicon
detector signal processing algorithms and software chain.

At present the focus of the group is on operations and analysis of the
PHOBOS experiment, but over the next three years, their hardware effort
will increasingly focus on the VTX project. The group will actively
participate in assembly, integration and testing of the strip detector
assemblies and of pixel and strip detector ladders mounted in the
mechanical support. In the longer run the group foresees playing a
leading role in the commissioning and operation of the VTX system in
PHENIX.

### Brookhaven National Laboratory, Instrumentation Division (BNL ID)

BNL\'s Instrumentation Division\'s Silicon Detector Development and
Processing Laboratory (SDDPL) will be involved in the development of
silicon strip detectors for PHENIX Upgrades. The activity will include
detector simulation, design, and processing of prototype detectors.
SDDPL will also be involved in laser scan tests of those prototypes,
these tests will be carried out in close collaboration with the RIKEN
Institute and RIKEN-BNL Research Center. Concerning the mass production
of detectors, SDDPL will develop the detector processing receipt,
produce the final mask set, and help transfer the processing technology
and mask set to a industrial manufacturer. Zheng Li, the group leader of
SDDL, Don Elliott, the Processing Engineer, and Rolf Beuttenmuller, the
Senior Processing Technician will be involved in these activities.

### Brookhaven National Laboratory, Physics Department (BNL PHY)

The PHENIX Group from the BNL Physics Department provides infrastructure
and technical support as part of Systems Engineering and Integration
(SE&I) for the entire PHENIX experiment. It has a staff of mechanical
and electrical engineers and a group of experienced technicians who are
intimately familiar with the detector, and work closely with the BNL
Collider-Accelerator Department for operations and any modifications to
its present design. They designed much of the infrastructure for the
baseline detector, including racks, cable trays, electrical power,
cooling, access, safety systems and numerous other services, and carried
out the installation of all of the present subsystem detectors. This
group will now be closely involved with the design of the infrastructure
and support for the silicon vertex detector, and with its installation
into PHENIX.

The BNL PHENIX group also has primary responsibility for a number of
major subsystems. These include Electronics Facilities and
Infrastructure (EF&I), Online Computing Systems (ONCS) and Offline
Computing. These groups will participate in the electronic integration
and readout of the silicon detector into the PHENIX data acquisition
system, and will be involved with track reconstruction and offline data
analysis.

### Iowa State University (ISU)

As DOE project manager, Craig Ogilvie plays a leading role in the
management of the VTX project. Over the past two years Ogilvie\'s
leadership role in the PHENIX Silicon Vertex Tracker working group was
essential to bring the project to the proposal stage. In addition, the
Iowa State group has been designing a parallel readout for the pixel
detectors that will work within PHENIX's readout time and replaces the
custom designed sequential readout ASIC\'s on the ALICE pixel detector
pilot module. The new parallel readout is implemented in radiation hard
FPGA\'s The first design of the pixel pilot with parallel readout and
zero-suppression is completed and testing is underway. A MOU is being
negotiated with CERN to be able to include the control code into the new
Pilot. The FPGA design effort is led by Prof Garry Tuttle of the
Electrical Engineering Department and Professors John Lajoie and John
Hill. Gary has extensive experience with the design of digital and
analog systems and Prof. John Lajoie has considerable expertise in fast
digital logic from his prior work on the LVL1 detector in PHENIX. This
work will form the master's theses of two students, Jonathon Crandall in
Electrical Engineering and Steve Skutnik of the Physics Department.
Senior technician Gary Sleege and Harold Skank will help in the final
stages of the design and testing of the pixel pilots. The ISU group will
also work with ORNL on the design of the pilot for the strip layers. In
addition ISU graduate students will be available to help in various
assembly tasks for the VTX project as construction progresses.

### Kyoto University (Kyoto)

Naohito Saito together with Manabu Togawa is involved in the R&D effort
focused on the silicon strip sensors. Especially the beam test at KEK-PS
together with the RIKEN group provided crucial information on the
further development of the of the sensor prototype. The group will
participate further for the evaluation of the sensors. In addition the
group actively participates in the development of analysis techniques
and physics simulations to establish the design of the silicon strip
detectors. The advantage of VTX detector especially in photon-jet
production has been clearly demonstrated in the PYTHIA based
simulations. Such efforts will be continued to further optimize the
physics analysis strategy.

### Los Alamos National Laboratory (LANL)

The Los Alamos group has established three major initiatives for the VTX
upgrade in conjunction with our interest in pursuing new physics with
the muon arms. (A) We have worked with the mechanical design group of
the ATLAS silicon tracker, HYTEC, Inc, to develop a concept for the
mechanical structures for the integrated (barrel and endcap) design of
the fully completed vertex tracker. Dave Lee, Walt Sondheim, and other
LANL members will continue to lead the effort to bring the concept to a
fully engineered mechanical design and to work closely with other
members of the VTX upgrade group and BNL to ensure effective integration
of the detector into PHENIX. (B) LANL group has also established
collaboration with the FNAL electronics group headed by Ray Yarema to
modify an existing chip design to read out mini-strips of the endcap
extension. Gerd Kunde will continue that coordination responsibility
since it is vital to the endcap effort. (C) An ongoing simulation effort
will be very important to these efforts and Pat McGaughey, Hubert van
Hecke, and other members of the LANL team will pursue these activities.

The LANL group has extensive experience with silicon detectors
including, 1) a measurement of the of the *B* production cross section
and charm production nuclear dependence with a silicon vertex detector,
E789, at FNAL using electronics developed by Ray Yarema, 2) implementing
the SVXH readout chip for the L3 micro-vertex detector at CERN, 3)
extensive prototyping and testing in conjunction with the mechanical and
electronic designs for the GEM/SSC silicon micro-vertex detector, and
management responsibility for GEM; and 4) similar responsibilities for
the MVD. Over the next 3 years the LANL group will gradually reach a
level of effort equivalent to 3 FTE's per year to be devoted to the VTX
upgrade.

### University of New Mexico, Albuquerque (UNM)

The University of New Mexico medium energy physics group can contribute
in several areas of development for the silicon tracker including
simulations, sensor testing, hardware design, and fabrication. The
Nuclear and Particle physics groups at UNM have a long history of
silicon development, including a testing facility currently being used
for the ATLAS sensors and managed by technical staff member Martin
Hoeferkamp, and an extensive simulation and hardware development program
for the CDFII silicon vertex detector by the group of Prof. Michael
Gold. Profs. Douglas Fields and Bernd Bassalleck have experience with
smaller scale silicon projects at BNL and elsewhere, and have been
deeply involved in the design of the PHENIX Muon Tracking System in
collaboration with LANL and HYTEC, Inc. Our desire is to build upon this
local expertise and to help with the development of a quality assurance
program for the strip sensors, local testing of sensors, R&D for
hardware (experience in regulated power supplies, kapton cable design,
etc.), and simulation efforts. We have already become involved in the
simulation geometry development through the efforts of one of our
graduate students Michael Malik. In addition, we plan to hire two
additional postdocs, one of which would be resident at BNL.

### Oak Ridge National Laboratory (ORNL)

The Oak Ridge group consists of physicists and electrical engineers. The
group (Vince Cianciolo, Ken Read, Paul Stankus, Terry Awes, Glenn Young,
Chuck Britton, Bill Bryan, Alan Wintenberg and Miljko Bobrek) recently
secured internal funding dedicated to developing the system-level
architecture for the electronic readout of the outer barrel (strip
layers) using the SVX4 chip, and to test the compatibility of the SVX4
chip with the PHENIX architecture and with the proposed strip-pixel
sensors.

The ORNL group has extensive experience with design, construction and
commissioning of PHENIX front-end electronics systems and associated
ASICs. We anticipate leading the design, prototyping and fabrication
efforts for the various pieces of the readout electronics for the outer
barrels and collaborating with RIKEN and BNL groups in testing the
production components and integrating the full detector into PHENIX.

### RIKEN Institute (RIKEN)

The RIKEN group, lead by Hideto Enyo, has a key role in the VTX project.
Yasuyuki Akiba leads the VTX project and represents it in the PHENIX
detector council. Akiba previously lead design and construction of
PHENIX RICH detector, and he brings in ample experience from the RICH
construction. As leader of the RICH project he has gained experience in
all aspects of large scale construction projects and in particular also
in coordinating a close collaboration of US and Japanese institutions.

Two years ago RIKEN has taken the lead in a broad R&D effort to
establish silicon vertex detector technologies suitable for PHENIX.
RIKEN has started collaboration with the ALICE pixel group. At present
three members of the RIKEN group are at CERN working with ALICE to help
finalize the detector development and test the first generation of
detectors in the NA60 experiment, which is currently taken physics data
with a multi-layer silicon pixel hybrid telescope. Johann Heuser, who
has worked for many years developing pixel detectors at CERN and
commissioning a pixel detector system for the DELPHI experiment before
he joined the RIKEN Institute, and Hiroaki Ohnishi, a software expert
working with RIKEN, are members of the NA60 pixel team since two years
and also working with ALICE. Hiroyuki Kano, who is now also at CERN, is
working with ALICE to adapt the ALICE pixel pilot to the PHENIX
requirements.

RIKEN has also initiated R&D with the BNL Instrumentation Division to
develop novel silicon strip detectors. The effort includes R&D together
with ORNL to develop suitable readout electronics based on the SVX4
readout chip developed at FANL. Yuji Goto, Junji Tojo, and Atsushi
Taketani are participating in this endeavor.

In addition, the RIKEN group has engaged in Monte Carlo simulations to
establish basic performance of the VTX detector and in the development
of analysis strategies and software.

### Stony Brook University, Physics Department (SBU)

Axel Drees together with a group of students has launched an effort to
adapt an FEM, originally built for the PHENIX drift chamber, to readout
the ALICE silicon pixel hybrids. In collaboration with NA60 the group is
gaining the necessary experience with the silicon pixel detectors. The
setup will serve as test sight for the PHENIX silicon pixel detectors.
Eventually, the effort will lead towards a new FEM design customized to
collect data from both the he pixel and strip assemblies. The design,
construction and commissioning of the FEM\'s will be carried out with
the help of the electronics workshop of the Stony Brook Physics
Department, lead by C. Pancake, the engineer who spearheaded the
development of the PHENIX drift chamber FEM\'s.

Stony Brook group expects to strengthen its effort in the future and to
take on major responsibilities for testing of individual components,
assembly, commissioning of the VTX detector. The group is presently
recruiting a new Assistant Professor. Though the position is not filled
yet, it seems likely that the new faculty member, together with a new
post-doc and students, will participate in this effort. The clean room
facilities at Stony Brook used for the PHENIX drift chamber assembly can
be upgraded with moderate effort to serve as basis for the VTX assembly.

## 6.4 Foreign contributions {#foreign-contributions .unnumbered}

The VTX project is funded in part by the RIKEN Institute. The tasks for
which the RIKEN Institute has taken fiscal responsibilities are
identified in the project chart (Figure 47) and are given in more detail
below:

Outer layer silicon strip detectors:

-   Silicon strip sensors designed and tested to specifications (RIKEN)

-   Strip sensors integrated with ROC\'s (RIKEN)

Inner layer silicon pixel detector:

-   Tested sensor ladders which include one sensor and 4 readout chips
    (RIKEN)

-   Readout bus to read data from sensor ladders manufactured and tested
    (RIKEN)

-   Two sensor ladders mounted on readout bus and tested (RIKEN)

-   Pilot card to readout pixel hybrids from one readout bus produced
    (RIKEN)

-   Half ladders, combining readout bus, two sensor ladders and pilot
    module assembled and tested (RIKEN)

-   Assembly and commissioning of half ladders to full ladders (RIKEN)

To enhance the expertise to carry out the work RIKEN has initiated
several cooperations with the BNL Instrumentation Division on silicon
strip sensors and with the ALICE/CERN pixel group on the silicon pixel
detectors. For the pixel hybrid detectors a memorandum of understanding
between RIKEN and CERN is in place. The objective of the memorandum is
the delivery of silicon pixel hybrids developed for the ALICE experiment
at CERN to RIKEN.

# Budget and schedule

## 7.1 Total estimated cost (TEC) {#total-estimated-cost-tec .unnumbered}

The costs and schedule for the VTX project have been developed using
engineering estimates, vendor quotes and experience from the
construction of the silicon pixel detector for NA60 and ALICE. The
proposed project is based on a cost sharing between the DOE Office of
Nuclear Physics and the RIKEN Institute of Japan. Responsibilities for
specific deliverables have been discussed in the previous chapter. RIKEN
funding started in April 2003 and we expect that it will continue at a
level of approximately \$1M per year for the next three years. In
addition, RIKEN provides substantial manpower (equivalent to \~ \$1M)
through in house contributions. The total estimated costs for the
proposed DOE construction project is \$5.6M including an average
contingency of 39%. We have implemented a work breakdown structure
(WBS), which is the basis for the following is a more details on the
cost estimate and schedule.

### Fiscal Responsibilities

Table 6 summarizes the mapping of the major construction tasks onto the
proposed fiscal responsibility as discussed in the previous section.
Completion of some of these tasks will require collaboration and
expertise from institutions funded through the DOE as well as from
foreign institutions. In order to proceed with the project in a timely
manner and to bring technologies ready as soon as possible, R&D and
prototype cycles for some of the major tasks have been developed based
on both DOE and RIKEN funding. For example, the first round prototyping
of the Strip Pilot Card SVX4 (1.1.1.1) and the Strip Bus (1.1.1.4) will
be funded by RIKEN to advance with the project, while the fiscal
responsibility will reside with the DOE.

### Contingency Analysis

In order to estimate the necessary contingency we have taken the
following approach:

-   For all tasks that require design of components and the production
    of prototypes, contingency is taken to be 50% of the cost of one
    extra design plus prototype cycle.

-   For all testing, assembly, and installation tasks, we assume that
    the contingency is 50% of the costs.

-   For purchases based on vendor information, 25% of the cost is
    included as contingency.

-   For all other purchases, the contingency is 50% of the costs.

### Overhead Estimate

For the overhead we assume a 17.5% rate for MST at BNL as is the current
practice for capital construction project at BNL, e.g. US-ATLAS at BNL.
This 17.5% overhead is listed separately in our budget tables. Also
listed as overhead are 9% for funds that are transferred from BNL to
other institutions. The additional overhead charged by local institution
is not listed separately. However, these costs are included in the
budget tables. In particular, all manpower costs are fully burdened
costs, including all overheads.

### Budget

Table 7 summarizes the estimated costs for the VTX project and shows the
split between the two funding agencies. For those items for which we
seek funding through the DOE, a detailed cost-breakdown is given in
Table 8.

[]{#__RefHeading___Toc55013011 .anchor}Table 6 Map of construction tasks
and WBS numbers onto the proposed fiscal responsibilities.

  --------------- ---------------------------------- ---------- ----------
  WBS             Construction Task                  DOE        RIKEN

  ***1.1***       ***VTX***                                     

  **1.1.1**       **Strip Layers**                              

  1.1.1.1.        Strip ROC Card SVX4                X          

  1.1.1.2         Strip Sensor                                  X

  1.1.1.3         Integration Pilot Card/Sensor                 X

  1.1.1.4         Strip Bus                          X          

  1.1.1.5         Strip Pilot                        X          

  1.1.1.6         Strip Assembly and test            X          

  1.1.1.7         Services, LV, Racks                X          

  **1.1.2**       **Pixel Layer**                               

  1.1.2.1         Pixel sensor                                  X

  1.1.2.2         Pixel Readout-chip                            X

  1.1.2.3         Pixel Hybrid                                  X

  1.1.2.4         Pixel Bus                                     X

  1.1.2.5         Pixel Pilot Card                              X

  1.1.2.6         Pixel System test                             X

  1.1.2.7         Pixel FEM                          X          

  1.1.2.8         Pixel Assembly and test                       X

  ***1.2***       *DAQ*                                         

  **1.2.1**       Strip DCM                          X          

  **1.2.2**       **Pixel DCM**                      X          

  ***1.3***       ***Auxiliary Systems +                        
                  Integration***                                

  **1.3.1**       Specifications                     X          

  **1.3.2**       **Mechanical**                                

  1.3.2.1         Support                            X          

  1.3.2.2         Ladders                            X          

  **1.3.3**       **Infrastructure**                 X          

  **1.3.4**       **Safety**                         X          

  **1.3.5**       **Installation**                   X          
  --------------- ---------------------------------- ---------- ----------

[]{#__RefHeading___Toc55013012 .anchor}Table 7 Overview of the total
estimated cost for the VTX project

  --------- ------------------------------- ------------ ----------- -----------
  **WBS**   **Name**                        **DOE**      **RIKEN**   **Total**

                                                                      

  1.1.1     STRIP                           2,138,761    1,726,500   3,865,261

  1.1.2     PIXEL                           500,914      1,163,750   1,664,664

  1.2       DAQ                             460,125      0           460,125

  1.3       AUXCILARY SYSTEMS & INTEGRATION 2,483,225    67,500      2,550,725

  1         Total                           5,583,025    2,957,750   8,540,775
  --------- ------------------------------- ------------ ----------- -----------

[]{#__RefHeading___Toc55013013 .anchor}Table 8 Cost breakdown for tasks
to be funded through the DOE. Tasks which do not show a cost correspond
to deliverables for which the RIKEN Institute will take fiscal
responsibility.

  ------------ --------------------- --------------- --------------- --------------- ------------- --------------- --------------- ---------------
  **WBS**      **Name**              **Fixed Costs   **Labor**       **DOE Const**   **DOE         **Contingency   **DOE           **Total DOE**
                                     (Parts)**                                       Overhead**    Percentage**    Contingency**   

  5.11.1.1     **STRIP**                                                                                                            

  5.11.1.1.1   Strip ROC Card SVX4   235,000         307,000         542,000         48,780        40.6            240,073         830,853

  5.11.1.1.2   Strip Sensor                                                                                                         

  5.11.1.1.3   Integration of ROC    0               23,000          23,000          0             50.0            11,500          34,500
               Card & Sensor                                                                                                       

  5.11.1.1.4   Strip bus             20,000          80,000          100,000         9,000         27.8            30,248          139,248

  5.11.1.1.5   Strip pilot           30,000          313,000         343,000         26,190        22.6            83,521          452,711

  5.11.1.1.6   Assembly and Testing  220,000         120,000         340,000         38,500        50.0            189,250         567,750
               of Strip                                                                                                            

  5.11.1.1.7   Services, LV, racks   56,000          10,000          66,000          9,800         50.0            37,900          113,700
               etc                                                                                                                 

               Totals:               561,000         853,000         1,414,000       132,270       38.3            592,491         2,138,761

                                                                                                                                    

  5.11.1.2     **PIXEL**                                                                                                            

  5.11.1.2.1   Pixel Sensors                                                                                                        

  5.11.1.2.2   Pixel ROC                                                                                                            

  5.11.1.2.3   Pixel Hybrid                                                                                                         

  5.11.1.2.4   Pixel Bus                                                                                                            

               Pixel Pilot FPGA      96,000          52,250          148,250         13,343        34.8            56,271          217,864

  5.11.1.2.6   Pixel Systems Test                                                                                                   
               (Hybrid+Pilot)                                                                                                      

  5.11.1.2.7   Pixel FEM             63,000          77,000          140,000         12,600        39.3            59,950          212,550

  5.11.1.2.8   Pixel Assembly and                                                                                                   
               Test                                                                                                                

  5.11.1.2.9   Services, LV, racks   40,000          0               40,000          7,000         50.0            23,500          70,500
               etc                                                                                                                 

               Totals:               199,000         129,250         328,250         32,943        38.7            139,721         500,914

                                                                                                                                    

  5.11.2       **DAQ**                                                                                                              

  5.11.2.1     Strip DCM             165,000         30,000          195,000         28,875        50.0            111,938         335,813

  5.11.2.2     Pixel DCM             45,000          30,000          75,000          7,875         50.0            41,438          124,313

               Totals:               210,000         60,000          270,000         36,750        50.0            153,375         460,125

                                                                                                                                    

  5.11.3       **AUXCILARY SYSTEMS &                                                                                                
               INTEGRATION**                                                                                                       

  5.11.3.1     Specifications        0               67,500          67,500          0             50.0            33,750          101,250

  5.11.3.2     Mechanical Structure  367,000         333,000         700,000         90,030        26.7            211,133         1,001,163

  5.11.3.3     Infrastructure        90,000          550,000         640,000         15,750        50.0            327,875         983,625

  5.11.3.4     Environmental Safety  0               50,000          50,000          0             50.0            25,000          75,000
               & Compliance                                                                                                        

  5.11.3.5     Installation          10,000          220,000         230,000         1,750         39.0            90,438          322,188

               Totals:               467,000         1,220,500       1,687,500       107,530       38.3            688,195         2,483,225

                                                                                                                                    

  **5.11**     **Totals:**           **1,437,000**   **2,262,750**   **3,699,750**   **309,493**   39.3            **1,573,783**   **5,583,025**
  ------------ --------------------- --------------- --------------- --------------- ------------- --------------- --------------- ---------------

## 7.2 Schedule

The following plots summarize the construction schedule to complete the
VTX detector upgrade for PHENIX. The overall schedule is shown in Figure
48. It assumes start of the DOE construction project in FY05. The
subsequent plots, Figure 49 to Figure 51, show the detailed schedule for
each major WBS element. The schedule implies that construction dollars
from RIKEN are available 2003 through 2005 and that the DOE construction
funds become available in FY05.

Figure 52 gives the funding profile for the DOE project. In this
schedule the inner pixel layer and part of the strip layers will be
installed in summer 2006 and the full VTX detector will be completed in
summer 2007. The proposed schedule assumes that \$286k of R&D funds will
be available from the DOE for the VTX project for FY04.

![](media/image2.jpeg){width="6.736111111111111in"
height="2.6055555555555556in"}

[]{#__RefHeading___Toc55021936 .anchor}Figure 48 The overall schedule
for the VTX Project.

![](media/image3.png){width="6.152083333333334in"
height="4.254861111111111in"}

[]{#__RefHeading___Toc55021937 .anchor}Figure 49 The schedule for the
strip layers.

![](media/image4.png){width="6.4631944444444445in"
height="4.002777777777778in"}

[]{#__RefHeading___Toc55021938 .anchor}Figure 50 The schedule for the
pixel layers

![](media/image5.png){width="6.95625in" height="7.143055555555556in"}

[]{#__RefHeading___Toc55021939 .anchor}Figure 51 The schedule for the
auxiliary systems and infrastructure

![](media/image6.wmf)

[]{#__RefHeading___Toc55021940 .anchor}Figure 52 Budget profile for the
VTX project

# 

# Appendix A ENDCAP Extension

## Introduction

This section outlines our plans to propose and construct a Silicon
Endcap Vertex Detector for PHENIX. The Endcap vertex detector
complements the barrel vertex detector by providing larger Bjorken-*x*
coverage, higher total rates and greater reach in transverse momentum.
In the following sections, we outline the physics case, specify the
requirements, list possible technical options, and define the needed
R&D. There are three broad physics topics that are accessible using an
Endcap Si vertex detector.

-   A large increase in the range of *x* over which we can extract the
    gluon spin structure function in protons with measurements of open
    charm and beauty in polarized *p+p* reactions.

-   Robust measurement of the shadowing of the gluon structure function
    in nuclei with measurements of open charm and beauty in *p+A*
    reactions.

-   Probing the early, highest energy-density phase of the matter formed
    in a heavy-ion reaction using the production of heavy flavor. There
    are several opportunities:

    -   Measuring the high-*p*~T~ spectra of open charm and beauty above
        4 GeV/c. The energy-loss of high-pt heavy-quarks is predicted to
        be less than for lighter-quarks.

    -   Measuring the yields of both open-charm and beauty in multiple
        channels to firmly establish whether heavy-quarks are enhanced
        in the pre-equilibrium phase.

    -   Using the open charm yield to form the ratio J/ψ/(open charm)
        and hence to quantify the suppression of J/ψ.

    -   Identification of upsilon states at rapidity near zero.

Our physics goals require that we measure charm and beauty mesons over a
broad range of rapidity and transverse momentum. The proposed Endcap
vertex detector achieves this by measuring displaced tracks that are
matched to the muon arms of PHENIX. A broad *p*~T~ and rapidity range is
achieved by using different decay channels to reach different parts of
phase space. The planned upgrade will operate well at 40× design
luminosity and many of the measurements need the higher luminosity, e.g.
upsilon and open beauty.

The proposed detector (VTX) is shown schematically in Figure 1, where
there are three distinct parts, a central barrel and two endcaps. The
Endcap silicon detectors are designed to provide coverage in the angular
acceptance of the forward Muon Arms. They cover 1.2 \< \|η\| \<2.7 and
the almost full azimuth angle with a resolution of \~150 μm. Each endcap
comprises four octagonal "lampshades" populated with Si mini-strip
detectors.

![](media/image7.jpeg){width="4.331944444444445in" height="2.26875in"}

Figure 1. Cross-section view of the proposed vertex detector.

![](media/image8.png){width="3.3694444444444445in"
height="2.5208333333333335in"}

Figure 2. A schematic cut-away mechanical drawing of the proposed vertex
detector (from Hytec).

A schematic mechanical drawing developed by HYTEC engineering is shown
in Figure 2. The proposed detector complements the existing muon arm
detectors and significantly enhances the muon physics program.

## A.1 Goals of the Endcap Upgrade

### A.1.1 Spin Structure of the Nucleon

PHENIX has the existing capability shown in Figure 3 as the blue lines.
However there are significant gaps in this *x*-range that will make it
difficult to fully address the spin-crisis. The proposed Si Endcap
detector extends the coverage to the lowest and highest *x*-values,
0.001 \< *x* \< 0.3, as well as providing significant regions where
multiple channels overlap. This overlap will provide vital cross-checks
that will improve the reliability of global fits to the spin structure
function.

![](media/image9.png){width="4.159027777777778in" height="3.4in"}

Figure 3. Expected x-range for different channels used to extract the
gluon spin structure function. The blue bars indicate PHENIX's existing
capability, green bars are for the Barrel upgrade, while the red bars
indicate the additional coverage provided by the proposed Endcap vertex
upgrade. The curves show various estimates of the expected gluon
polarization.

The Endcap vertex detector provides the following improvements in
x-range over a Barrel only detector. These have been estimated by
simulating *p+p* collisions with PYTHIA and requiring sufficient counts
in each exit channel to be able to make a reasonable measurement.

-   $c\overline{c}$production via gluon fusion. The x-range is extended
    considerably down to *x* = 0.001 using $D \rightarrow \mu X$.

-   $b\overline{b}$ production via gluon fusion. With the upgrade we can
    identify displaced J/ψ from $B \rightarrow \frac{J}{\Psi}$ decay.
    This provides coverage in x between 0.005 - 0.3. The selection of
    semi-leptonic decays $b\overline{b} \rightarrow e\mu X$ at high
    momentum is improved using displaced vertices. This extends the
    *x*~gluon~ coverage for these semi-leptonic decays to 0.01 - 0.3.
    Measurement of $B \rightarrow \mu X$ is also possible by placing a
    cut on the *p*~T~ of the muon.

.

### A.1.2 Exploration of Gluon Structure in Nuclei

The reach in *x*-range for the Si Endcap is indicated in Figure 4,
superimposed on calculations of the ratio of nuclear to nucleon gluon
structure function. The red bars indicate the additional coverage
provided by the Endcap vertex upgrade compared to the baseline of
PHENIX. The Endcap vertex upgrade provides extends the *x*-range from
the anti-shadowing region into the shadowing domain, which means we will
be able to establish the shape of the gluon structure function in
nuclei. The shadowing region is not accessible with the Barrel-only
upgrade. While the *x*-range for J/ ψ production also extends into the
shadowing range, final state effects, such as dissociation, complicate
the extraction of the gluon structure function. Open charm and beauty
measurements are unaffected by these final state effects.

![](media/image10.png){width="4.034722222222222in"
height="3.3833333333333333in"}

### A.1.3 Probes of Early, Highest Energy-Density Stage of Heavy-ion Reactions

Information from the Si Endcap concerning the yield and spectra of
heavy-flavor mesons is discussed in the following sub-sections. Of
critical importance, is the broad reach in rapidity and transverse
momentum made possible by the proposed Endcap upgrade.

### 

### Energy Loss of Heavy Quarks

Measuring the high-p~T~ (above 4 GeV/c) spectra of open charm and beauty
will provide a test of the various theoretical models of energy loss.
Since the cross section for these events is low, the large acceptance of
the Endcap vertex upgrade versus the Barrel is a distinct advantage.

### Open Charm and Beauty Enhancement

The Endcap vertex detector, combined with the muon spectrometers,
provides excellent rates of open charm and beauty detection. The Endcap
upgrade identifies and separates charm and beauty production with high
accuracy, even at low *p*~T.~

### J/ψ Suppression

To quantitatively understand suppression requires knowledge of the
initial production of $c\overline{c}$pairs. The effectiveness of a
deconfined medium in preventing the formation of J/ψ can be quantified
using the ratio J/ψ/(open charm) in the same acceptance as PHENIX
measures J/ψ. The Endcap upgrade provides for the detection of open
charm over about the same rapidity interval as for J/ψ decays to
dimuons.

### Other Physics Topics

The Si Endcap vertex upgrade will help other physics programs in PHENIX
:

#### 

#### The J/ψ resolution in the muon arms will be improved using a vertex detector, from \~150 MeV down to \~100 MeV. This is important for separating the ψ' from the J/ψ. The physics interpretation is cleaner for the  ψ' than the J/ψ, since it does not have such a large contribution from feeddown of the χ~c~ states. Figure 5 demonstrates the improved separation possible with the Si Endcap. 

#### The signal to background ratio for muon-pairs will be greatly improved by the removal of muons from long-lived pion and kaon decay. This is especially important for pairs with mass below the J/ψ, where the backgrounds are large in nucleus-nucleus collisions. 

#### 

![](media/image11.png){width="4.117361111111111in"
height="2.7729166666666667in"}

Figure 5 -- Mass spectra for the J/ψ and ψ\', showing the substantial
improvement in separation expected with a vertex detector (yellow, 100
MeV resolution) compared to that without a vertex detector (black, 150
MeV resolution). The number of J/ψ and ψ' in this plot represents our
expectation for a \~25 pb^-1^ p-p run.

## A.2 Simulations and Required Performance for the Si Endcap Upgrade

The performance requirements for the Si Endcap detector are :

-   Ability to match tracks from a muon arm to hits in multiple layers
    of the Si detector.

    a.  Sufficient position accuracy so that the displacement resolution
        of the track with respect to the collision point is less than
        the cτ of charm and beauty decays, i.e. a resolution less than
        100μm, preferably at the level of 30 - 50 μm.

    b.  Good resolution in both rφ and z are required.

    c.  Sufficient segmentation to operate well in Au-Au and high
        luminosity p-p collisions.

For the simulations we have used two nominal thickness for each layer:
1% and 2% radiation length. This includes detector, readout and cooling
in a simplified one-volume effective layer. Our current concept will
have a radiation length close to 1% because we are implementing a design
that has incorporated a readout bus in the silicon chips and sensors and
we are able to thin the chips. We are striving to minimize this
thickness, in particular for the critical first disk.

The endcaps mini-strips vary in size from 50μm by 2000μm to 50μm by
9000μm as the radius increases. This keeps the occupancy at or below
1.5%.

### A.2.1 Open Charm Measurement

> **Si Endcaps:** $D \rightarrow \mu X$**,**
> $D\overline{D} \rightarrow \mu\text{eX}$**,**
> $D\overline{D} \rightarrow \mu^{+}\mu^{-}X$

Each silicon endcap detector has four layers of pixel detectors, which
measure the trajectory of particles within the nominal rapidity
acceptance of the muon arms. The impact parameter of each track is
determined accurately along the Z (beam) direction. For each detected
muon, the impact parameter is used to eliminate muons that come from
pion and kaon decays. These long-lived decays are the primary source of
background muons.

Contrasted with these background muons are \"prompt\" single muons,
which come from more short-lived decays, e.g. open charm and beauty. For
transverse momenta below \~5 GeV/c the prompt muons are primarily from
semi-leptonic charm decay. Other processes that produce prompt muons,
such as J/ψ or Drell-Yan decays to muon pairs, have much smaller
cross-sections times branching ratios. Muons from *B* decays become
important only at larger transverse momenta.

The PYTHIA event generator was used to simulate semi-leptonic charm
decays to muons. The total charm pair cross-section was set at 350 µb,
which is consistent with recent NLO theoretical calculations and with
the published PHENIX measurement at a somewhat lower energy. The decay
muons were tracked through the proposed silicon vertex detector and then
through the muon spectrometer using PISA.

The mean vertex of the detected muons from charm decay is 785 μm from
the interaction vertex. This is \~2.5 times larger than the proper decay
length of semi-leptonic charm decays (318 μm), due to the Lorentz boost.
The impact parameter resolution for these muons ranges from 92 to 115
μm, depending on how many layers of silicon are transversed. By
requiring that the muon vertex is within 1cm of the collision point we
remove many of the muons from pion and kaon decay while retaining prompt
muons from charm and beauty.

Figure 6 shows a simulated muon *p*~T~ spectrum, including charm, beauty
and light quark decays, before the application of a vertex cut. The
background from light quark decays dominates the spectrum below 4 GeV/c.
The *p*~T~ distribution of muons that survive a 1 cm vertex cut is shown
in Figure 7. This vertex cut reduces the muon background from light
mesons by about an order of magnitude over what the muon arm alone can
achieve, making a charm measurement possible even at low *p*~T~. Note
that the removal of the muon background from pion and kaon decays could
be achieved with a detector with less spatial resolution. The resolution
requirement is driven by the physics program of measuring open beauty
(see next section).

![](media/image12.png){width="3.8916666666666666in" height="2.79375in"}

Figure 6. Single muon p~T~ distributions for charm, beauty and
backgrounds from low-mass meson decays, as expected for the 2003 d-Au
run. Note that the light-meson decays are above charm up to near 4
GeV/c. The black curve is for pion and kaon decays, green is charm and
red is beauty.

![](media/image13.jpeg){width="3.682638888888889in"
height="3.682638888888889in"}

To calculate the yield of charm, we assume a 650 µb D pair
cross-section, integrated *p-p* luminosity on tape of 50 pb^-1^,
acceptance time branching ratio of 0.0091 and two Si Endcaps. A total of
about 3.2 x 10^8^ semi-leptonic charm decays would be reconstructed.
This rate is before application of a vertex or impact parameter cut.
Even if a large prescale is required for single muon triggers, the yield
is still very large.

Since charm is produced in pairs, coincidence measurements of
opposite-sign lepton pairs may serve to further enhance the signal to
noise in *p-p* and *p-A* reactions. One could use vertex identified
muon-electron coincidences to obtain a clean charm pair signal in the
rapidity interval midway between the PHENIX central and muon arms.

### A.2.2 Open Beauty Measurement

B meson production, while much more rare than *D* production, is
somewhat simpler to measure. The challenge is the relatively low rate.
There seem to be at least two possibilities :

-   Since beauty mesons have a larger lifetime than charm mesons, it is
    possible to extract the beauty yield from the distribution of decay
    distances of single muons from semi-leptonic decays. At large
    transverse momentum beauty decays dominate the DCA distribution.

-   The decay channel *B*  J/ψ+X produces J/ψs that are displaced from
    the collision point.

**Si Endcaps:**
$B \rightarrow \frac{J}{\psi} \rightarrow \mu^{+}\mu^{-}$**,**
$B \rightarrow \mu X$

Applying a vertex cut on each reconstructed J/ψ has been used
successfully to identify B-production in experiments at lower
energies[^38]. Since the B cross-section is larger at RHIC energies, the
measurement should be easier. As the average p~T~ of J/ψ from beauty
decays is larger than for prompt J/ψ, a p~T~ cut could also be used to
enrich the beauty sample.

Pythia was used to simulate
$B \rightarrow \frac{J}{\psi} \rightarrow \mu^{+}\mu^{-}$  decays. The
resulting muons are tracked through the silicon and muon spectrometers
using PISA. The muons have an impact resolution of \~55 μm,
significantly better than muons from D decays, due to their larger
average momentum. The pair z-vertex resolution is \~133 μm, while the
mean decay length is \~1.1mm. With a downstream pair z-vertex cut of 1
mm, 39% of the B decays are retained, while the prompt J/Ψ are
attenuated by a factor of 2x10^-4^. Figure 8 shows the reconstructed
Z-vertex distribution for the J/ψ from B decays as well as prompt J/ψ.

![](media/image14.jpeg){width="3.651388888888889in"
height="3.5340277777777778in"}

We have assumed a total B cross-section of 2 microbarns and 4 microbarns
for J/ψ production. The branching ratio of 1.2% for
$B \rightarrow \frac{J}{\Psi}$ has been previously measured. The total
acceptance \* BR for these events using two Si Endcaps is \~
4.0\*10^-5^. Assuming an integrated luminosity to tape of 50 pb^-1^,
about 1900 $B \rightarrow \frac{J}{\Psi}$ events would be reconstructed
after the application of a 1 mm vertex cut. For $B \rightarrow \mu X$,
the acceptance \* BR is \~ 0.0087. The corresponding yield is \~780,000
reconstructed events. Thus, an excellent *B* measurement is possible.

### A.2.3 Trigger Plans

We plan to use the level 1 single and di-muon triggers as the main
physics trigger for the Si Endcaps. Higher level triggers could be a
level 2 displaced track trigger, possibly similar to the trigger used by
CDF and E789. Both CDF and E789 implemented the trigger in hardware to
optimize it for speed. For PHENIX this could be ported to level 2. The
algorithm for the Si Endcaps / muon spectrometers would be very similar
to that done previously by E789.

The E789 algorithm functions in three steps (translated into PHENIX
language); 1) Muon tracks are found from stubs in the Muon ID and Muon
Tracker. The momentum and angle of each track are determined. 2) These
tracks are then matched to hits in the Si Endcap using a pre-computed
lookup table. 3) Si hits within the matching window are formed into a Si
track stub. The stub is then fitted with a straight line to determine
the momentum, angle and impact parameter of the track.

These events would then be passed to the level 2 triggers of displaced
vertexes and/or high-momentum tracks. For pair triggers, tracks could be
combined and fitted to determine a pair vertex.

###  A.2.4 Si Endcap Event Rates

The event yields in the previous sections are summarized below. They
assume an integrated p-p luminosity on tape (for Run 8) of 50 pb^-1^.
Yields from a comparable Au-Au run would be about a factor of 3X lower.
The yields for semileptonic heavy quark decays are about an order of
magnitude larger than for the Si Barrel, due to the larger acceptance of
the Si Endcap. The B decay rates could benefit from the increased
luminosity in the RHIC II proposal.

  -------------------------------------------------------------- -----------------------------------
  Observable                                                     Counts per RHIC p-p Run 8

  $$D \rightarrow \mu X$$                                        3.2 x 10^8^

  $$B \rightarrow \mu X$$                                        780k

  $B \rightarrow \frac{J}{\Psi X} \rightarrow \mu^{+}\mu^{-}$    1900
  -------------------------------------------------------------- -----------------------------------

### A.2.5 Matching to Muon Spectrometers 

Track matching between the Si Endcaps and the Muon Spectrometers was
studied by using hijing Au-Au central collisions in a PISA simulation. A
muon track was embedded in a hijing event. The muon track was found in
station 1 from the muon tracker by demanding that the muon reached the
middle of the MUID, i.e. the muon energy was \> 2.5 GeV. The
distribution of the muon hits in station 1 was found to be +- 2 cm from
the projection of the Si Endcap track, due to multiple scattering in the
central magnet steel. No other track in the tracker was found to be in a
+- 2cm cut around the muon hit in station 1. We then looked for all
tracks in the Si Endcaps that had their projection fall into the 2 cm
cut about the muon track. In addition to the muon, typically 3 other
tracks fell into this cut. Of these candidate tracks all except the muon
came from the primary interaction vertex. The background would be the
fraction of primary tracks that fall beyond a 1 mm cut.

### A.2.6 Integration with PHENIX

The proposed Endcap vertex detector matches and extends the capability
of the existing muon spectrometer arms. In addition a joint HBD/TPC is
being proposed to sit outside the vertex detector. The Detector Advisory
Committee recommended studies exploring the impact of the VTX on the
HBD/TPC with the possibility of standalone running for either detector.
Because the Endcaps are outside of the acceptance of the HBD/TPC we
believe that both detectors can operate simultaneously.

## A.3 Technical Aspects of the Proposed Endcap Vertex Detector

### A.3.1 Silicon Readout Chip --PHX

Ray Yarema's group at FNAL has designed the FPIX2 chip, a low-noise
programmable Si pixel readout chip for the planned BTeV experiment. The
chip is an advanced mixed analog/digital DC-coupled design optimized for
a detector with 50 μm by 400 μm pixels. The device has very low noise
(60 electrons RMS!) and high-speed readout, including the ability to
interface to a level 1 type trigger. Approximately 3000 FPIX2 chips have
been produced in an engineering run, with a high yield of fully
functional devices. Test results are very encouraging, with the
prototypes demonstrating excellent performance and minimal cross-talk.
Only minor design tweaks are needed before a production run of the
devices could be made.

The electrical design of the FPIX2 chip is similar to that needed for
the Si Endcap pixels. The main change required is to adapt the physical
chip geometry to accommodate the Endcap sensors larger mini-strips.
These modifications are not very difficult, with an estimate of 2
man-years. Yarema has offered the services of his engineers and
facilities to perform this work. They have already completed a
conceptual layout of the modified PHX readout chip, which is shown in
Figure 9. This elegant design has the readout bus structure integrated
into the chip itself, simplifying the sensor-readout assembly process.
The PHX chip will be bump-bonded to the sensor, with 200 μm bump
spacing. This relatively large spacing was chosen to ensure high yields
during the assembly process. Yarema's team have also simulated the FPIX2
response with input capacitances corresponding to our larger mini-strips
and found it to be acceptable.

The schedule and R&D budget for the PHX chip development are given in
section 6.4 below. These were provided to us by Ray Yarema.
Approximately two years are need to complete all of R&D related to the
PHX chip.

### A.3.2 Silicon Ministrip Sensors

We plan on using existing technology for the silicon sensor. Pixel
Sensor technology from the either the ALICE, ATLAS or CMS efforts will
have the pixel layout (masks) modified to match the longer mini-strips
that we need. Developing the masks for this effort will be done in
concert with the vendors of the sensors. Lengthy and costly R&D for the
sensors is not necessary.

Three different silicon sensors of trapezoidal shape are used to tile
the active areas of the Si Endcap, as shown in Figure 10. Also shown is
the arrangement of the readout chips on each of the sensors. The largest
sensor is 79 mm high and 27 mm wide at its large end. Six PHX chips are
used to readout the 3072 mini-strips. The smaller sensors contain 2560
and 1536 strips.

![](media/image15.png){width="1.15625in" height="3.51875in"}

Figure 9: Conceptual layout of the PHX pixel readout chip. The left side
graphic depicts the general layout of the chip. Green is the area for
bonding, blue the programming interface, red the discriminator, orange
the pipeline and yellow the digital interface. The right side graphic
shows the bonding layout, the spacing is 200 micron. The signal and
power bus will be routed on the surface on the chip and bonded via the
bump bonds on the ends of the chip.

The Si Endcap detector layers are assembled as shown in Figures 11
through 13. First, the sensors are tiled on carbon panels that serve as
the support and cooling structure for each of the sector assemblies
(Figure 11). Next, 24 sectors are joined to form each of the four
z-Stations (Figure 12). Finally, the four z-Stations are assembled in
each Si Endcap detector (Figure 13). Each Endcap contains approximately
two million strips.

![](media/image16.jpeg){width="1.3493055555555555in"
height="3.435416666666667in"}![](media/image17.jpeg){width="1.8847222222222222in"
height="3.6840277777777777in"}![](media/image18.jpeg){width="1.7708333333333333in"
height="3.348611111111111in"}

Figure 10: Three silicon detectors will be used. The largest will have 6
chips reading out 3072 strips, the intermediate silicon will have 5
chips reading out 2560 strips and the smallest silicon is half the size
of the largest with 3 chips reading out 1536 strips. (All dimensions are
in millimeter)

![](media/image19.png){width="1.645138888888889in"
height="3.5541666666666667in"}

Figure 11: A sector assembly will have 24 carbon panels (one shown here
in brown) in azimuth, each of them carrying 4 silicon detectors (blue),
two in the front and two in the back. They overlap on the edges by a few
millimeters to avoid dead areas. The bus on a silicon assembly is routed
on the chips as described earlier, the connection of the inner silicon
detectors is realized via a kapton bus (golden).

![](media/image22.png){width="6.0in" height="3.3666666666666667in"}

Figure 12: Each station carries 96 silicon detectors. The stations are
placed at \~20, 26, 32 and 38 cm from the interaction point.

![](media/image23.png){width="4.189583333333333in"
height="3.3305555555555557in"}

Figure 13: Each endcap will have 4 stations of silicon detectors. The
inner station has a reduced size in order to not interfere with the HBD
acceptance.

### A.3.3 Silicon Ministrip Control Chip

The Control Chip serves as a receiver for the PHX chip digital output
and is located at the outer edge of each sector. It formats the data to
be compatible with the PHENIX DCMs, converts the data into serial form
and drives a 2.5 gigabit/sec fiber. The other end of the fiber connects
to the DCM in the counting house. It is simpler than the PILOT chip used
to interface the Si barrel, as it does not need to combine separate data
streams to meet PHENIX's DAQ speed requirements. (The PHX chip is
already fast enough for PHENIX). We envision that the Control Chip would
consist of an FPGA plus a laser diode and driver. The University of
Heidelberg has expressed interest in developing the Control Chip with
us.

### A.3.4 Mechanical Structure and Cooling

The mechanical structures and cooling are part of the integrated design
discussed in Chapter 4. The majority of the support structure will be
designed as part of the barrel effort and remaining issues concerning
ladders and cooling specific to the endcaps will be discussed here.

### A.3.5 Endcap Ladder Structure

The forward regions consist of 4 conical arrays of ladder modules tilted
from the normal to the beam pipe by 22 deg. Conceptually, we have chosen
a flat octagonal panel structure with sensors and electronics mounted on
either side of the panel so that we can achieve hermetic coverage.
Figure 14 shows this arrangement on the left and an octagon panel
structure on the right.

![](media/image24.jpeg){width="2.8055555555555554in"
height="2.502083333333333in"}
![](media/image25.jpeg){width="2.2118055555555554in"
height="2.7527777777777778in"}

##  

Figure 14. 3D model of octagonal disk like structures for the endcap
ministrips. Cooling tubes are shown to demonstrate both the number and
routing. The octagon panel structure is on the right with the cooling
channel shown. A heat load of 0.1 W/cm\*\*2 is assumed.

Each endcap has a total heat load of approximately 450W, or about 15W
per octant panel. In comparison to the barrel this is a very small heat
load and greatly simplifies the removal of the waste heat. The octant
panel structure consists of a composite sandwich of C_C facings on
either side of a carbon foam in which is embedded an aluminum cooling
tube( Figure 15). Thermal and gravity sag calculations were performed in
a manner similar to those discussed in chapter 4 and no serious
distortions were observed. For the case of 0 deg operation, more work is
necessary.

![](media/image26.png){width="3.9319444444444445in"
height="2.2104166666666667in"}

Figure 15. Illustration of an embedded cooling passage arrangement in
the composite sandwich used in the endcap thermal and static
calculations. The upper panel depicts a circular tube with supports and
the bottom panel shows a flattened tube which enhances heat transfer and
provides a thinner sandwich.

[A.3.6 Radiation Length]{.underline}

The thermal and static design studies produced a range of solutions for
the endcaps Figure 16 shows the radiation length estimate for different
cooling tube dimensions. The parameters used in the calculations are:

-   Al tube, 200 micron.

-   4 mm carbon foam separator.

-   Tube support 2 mm wider than tube diameter.

-   Sandwich facings of 400 micron.

![](media/image27.png){width="4.179861111111111in" height="2.28125in"}

Figure 16. Estimated normal radiation length for the endcap octant panel
for different tube diameters.

### 6.3.7 Endcap Analysis Summary

######## 

######## The conceptual design studies revealed the following:

######## Single phase cooling is well suited to the endcaps.

-   Two adjacent octant panels can be cooled in series thus reducing
    service connections.

-   2mm cooling tubes and panel thickness are adequate.

-   The radiation length of the octant panel exclusive of sensor and
    electronics is \~ 0.6 %.

The R&D issues consist of refining the calculations, designing
attachment points to the main support structure, and prototyping the
octant panels.

## A.4 R+D Schedule, Responsibilities and Budget

### A.4.1 Schedule

The R+D schedule for project 2 is shown below. Also shown is a
construction and installation schedule assuming funds are available now.
This will slide with funds availablity. The R+D is expected to begin in
early FY04 and be completed by late FY05.

![](media/image28.wmf)

### 

### A.4.2 Responsibilities

The LANL Group will work together with HYTEC inc. to develop the design
for the Endcap mechnical ladder and cooling. LANL has formed a
collaboration with FNAL to design, prototype and test the PHX readout
chip. LANL is forming collaboration with Ecole Poly-Technique and
Saclay, who will contribute to both the hardware and software efforts.

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[^35]: Ideas ASA (Norway), <http://www.ideas.no/>

[^36]: http://www-cdf.lbl.gov/users/mweber/svx4/

[^37]: After the completion of this study a more promising technology
    for the endcaps has been identified (see appendix A), but the heat
    load is expected to be similar.

[^38]: E789 Phys. Rev. Lett. 74, 3118 (1995).