## R&D Proposal for High Resolution Inner Vertex Detector Development for STAR

Principle Investigator: H. Wieman

Lead Electronic Engineer: Fred Bieser

Main Investigators:

S. Kleinfelder

H. Ritter

H. Matis

F. Retiere

E. Yamamoto

M. Oldenburg

We propose development of a new high resolution inner vertex detector
for STAR which will open new dimensions in the study of heavy ion
collisions at RHIC.

In elementary collisions, heavy quark production provides a benchmark
process to study perturbative QCD. The heavy quark mass, m~Q~, defines
the scale at which the strong coupling constant, α~s~, is evaluated.
Since m~Q~ » Λ~QCD~, the inclusive production properties should be
calculable within perturbation theory. Furthermore, measuring the charm
and the bottom quark yield would provide a better understanding of some
non-perturbative processes. Indeed, due to their different masses (m~c~
≈ 1.0 -- 1.6 GeV/c^2^, m~b~ ≈ 4.1 -- 4.5 GeV/c^2^), pQCD is probed at
different Q^2^, where the contributions of non-perturbative effects are
different.

For this reason, heavy quark production (c,b) has drawn tremendous
interest in the heavy ion community. Due to their large mass, c and b
quarks are predominantly produced from the (pQCD-scale) interactions of
initial state partons. Lighter quarks, however, may be produced
throughout the evolution of a heavy ion collision. By this virtue, the
absolute c and b yields provide a direct connection to the initial
state. The process of hadronization of heavy quarks (manifestly
non-perturbative), however, will depend on the dynamical evolution of
the system.

With the High Resolution Inner Vertex Detector, we will have the
capability of making high precision measurements of charmed and bottom
particles at STAR. Since the majority of charm quarks produced will end
up in open charm (D^0^, D^0^bar, D^+^, D^-,^ D~s~^+^, Table 1),
measurement of D mesons is necessary to obtain an absolute yield for
c-cbar production (N~ccbar~). The relative yields of charmed mesons from
previous experiments and theory are non-negligible, thus requiring a
measurement to minimize the uncertainty in N~ccbar~ (Table 2).

The final distribution of heavy quarks in hadrons, whether in quarkonium
(QQbar bound states), or in open states has been suggested as a probe of
the matter created in relativistic heavy ion collisions. If, for
example, charm quarks are able to escape the system undisturbed, the
relative yields of the charmed hadrons should be the same in p-p and
Au-Au collisions. In heavy ion collisions, however, the initially
produced charm quarks may go through a strongly interacting system where
they become thermalized before freezing out. In this case, the relative
charm hadron yield would be significantly modified as shown in the
rightmost column of Table 2. Since charm hadrons have presumably low
cross sections with other hadrons, such thermalization would only be
possible if charm quarks go through a thermalized medium of quarks and
gluons. Thus, measuring the yield of the various charmed hadrons in p-p
and heavy ion collisions may provide a signature that a new kind of
matter is created in heavy ion collisions.

The J/Ψ yield measured by the NA50 collaboration at the CERN SPS may
also be explained along the same line. As the collision become more
central, initially produced J/Ψs go through a hotter and hotter system,
which is eventually able to dissociate the c and cbar quarks. The charm
quarks may then be included in a thermal bath, where hadronization
occurs in a thermal fashion, i.e. the relative yield of J/Ψ is defined
by thermal parameters. Thus, the ratio of the yield of J/Ψ over open
charm (e.g. D0) also tests whether or not charm quarks merged into a
thermalize system. In general, it is more precise to study the yield
with respect to the yield of open charm rather than with respect to the
yield of related hard processes such as the Drell Yan that was used by
the NA50 collaboration. In depth understanding of the J/Ψ yield requires
the knowledge of the open charm yield.

![](media/image1.png){width="4.727777777777778in"
height="3.727777777777778in"}

Figure 1: Last 3 points are D^0^/D^+^, D~s~ and Λ~c~

If charm quarks merge within a bath of partons, they may be affected by
the system tranverse expansion. Evidence of the system collective
transverse expansion have been established from transverse mass spectra,
elliptic flow analysis and two-particle correlation analysis. Such
transverse flow is due to the multiple collisions between particles, but
it is unclear whether flow arises from hadronic or partonic
interactions. To answer this question, hadrons, such as the Ω baryons,
which presumably interact weakly with other hadrons, are studied.
Presumably, charm hadrons fall into the same category; the cross
sections between charm hadrons and pions are small. Thus, charm hadrons
transverse mass spectra probe the system expansion at the partonic
level. Figure 1 show the mean transverse momentum of various particle
species as a function of their mass. The blast wave calculation is used
as the transverse flow baseline because it reproduces pion, kaon and
proton transverse mass spectra as well as the pion source size and the
elliptic flow. Pythia provides the no-flow baseline where the Au-Au
collisions are a superposition of p-p collisions. Transverse flow
significantly increases the mean transverse momentum of charm hadrons.
Measuring such an increase from p-p to Au-Au collisions would provide
evidence of the system collective expansion at the partonic level.

![](media/image2.png){width="5.0625in" height="4.447916666666667in"}

Figure 2: lines are various pQCD calculations.

The current available p-p collisions data poorly constrain the charm
production cross section, as shown in Figure 2. High quality
measurements of the charm quark yield in p-p collisions at 50 GeV \< √s
\< 500 GeV, would help to constrain these calculations.

In the Standard Model, pQCD calculations predict an A dependence of
c-cbar production for central A-A collisions that goes as:

〈N~ccbar~〉~AA~ = 〈N~ccbar~〉~NN~ \* A 4/3

A measurement of open charm will provide a unique opportunity to set
limits on the applicability of pQCD at these collision energies.

While the large mass of the charm quark makes it an attractive probe of
heavy ion collisions, this property makes it difficult to produce charm.
The yield of c-cbar pairs is small (\~1 per event), when compared to the
yield of lighter quarks. Extracting a high precision measurement of
charm will require excellent background rejection. It is possible to
reject background if one can utilize the lifetime of the charmed
particles. With lifetimes measured in 100's of microns, it is necessary
to have a detector that can resolve a vertex displacement on the order
of the lifetime.

The High Resolution Inner Vertex Detector will provide STAR with this
capability. The detector will provide, for each charged particle that
traverses its fiducial area, two high precision space points within 4 cm
radially of the collision vertex. The proximity of the detector to the
collision point is necessary to achieve the resolution required to
reject background.

The short-lived particles will be measured by calculating the invariant
mass from the decay particles. The number of events required to achieve
a given level of statistics scales as N ∝ B/S^2^, where B is the
background under the invariant mass peak and S is the signal in the
peak. By aiding in the selection of tracks that do not point back to the
primary vertex, the vertex detector limits the number of tracks
contributing to the combinatorial background, B and has a dramatic
effect on the amount of beam time required to achieve a given physics
result. Our GEANT simulations (see Fig. 3) show that the combinatorial
background, B, has a very strong dependence on the thickness of the
first detector layer. We are proposing a 50 μm thick inner silicon layer
plus aluminum Kapton cable located at a 2 cm radius from the beam plus a
1.8 cm radius^1^ 600 μm thick Be beam pipe. The GEANT simulation shows
that the combinatorial background for our design of 50 μm is 19 times
smaller than using 300 μm of silicon, a thickness believed possible with
the more conventional pixel designs using a hybrid approach. A factor of
19 reduction in required beam time is strong justification for
developing the thinnest technology.

![](media/image3.wmf)

**Figure 3:** Combinatorial background from a GEANT calculation as a
function specified detector silicon thickness. The simulation is for a
760 μm beam pipe with silicon layers at 2.8 cm and 3.8 cm. The curve
guides the eye.

We have focused on an Active Pixel Sensor in CMOS approach for this
vertex detector development, because like CCDs a detector can be built
that utilizes a single layer of thinned silicon. The current APS
technology is close to a realizable detector system, if all of the
pixels are read out like a CCD. It has two main advantages over CCDs
which are important for STAR. One it is more radiation hard than CCDs,
and two it can be readout faster and with less power than CCDs. Another
distinct advantage is the promising future of combining sophisticated
electronics on the same chip as the detector. This has the potential for
much faster readout speeds and low power if one can accomplish on chip
zero suppression. Such a technology could be satisfactory at the highest
predicted RHIC luminosities. Our initial approach, however, is to
develop a simpler readout scheme with off chip data correction and zero
suppression which can be used for a first generation APS detector in
STAR. This will work with the anticipated RHIC luminosity of the next 10
years.

There are several items, both electronic and mechanical, to be addressed
by this R&D program which are important preparation for the construction
and use of the proposed high resolution vertex detector in STAR.

Already, several APS chips have been demonstrated to work with minimum
ionizing signals. It is believed that further design and testing will
result in better signal to noise performance which will improve the
normal tradeoff of particle detection efficiency vs accidental rate. R&D
work will be done to demonstrate that this technology can be
successfully scaled from our small test chips to a full large area
ladder design. A design constraint for the large system design is full
detector readout in 20 ms. This speed is sufficient for the expected
RHIC luminosity. Two approaches are under consideration for the large
scale readout: analogue vs digital; either way requires development
work. A differential analogue driver or an ADC will have to be developed
as part of the APS chip design. An initial constraint for either design
is that it adhere to a 100 mw/cm^2^ power budget which is known to be
within the cooling capabilities of forced air. We believe this can be
done but it must be demonstrated in the R&D program.

The 20 ms readout time complicates the DAQ interface since a single APS
detector read potentially contains information for several trigger
events. Developing an approach for merging vertex data with the main
STAR data stream is one of the projects for this R&D program.

Additional R&D effort will be devoted to on board data processing and
sparsification. This is particularly important for a detector of this
type where the total number of pixels, in excess of 90 million, is more
than the TPC pixel count. The fraction of pixels with useful hit
information is less than 1%, so the benefits of sparsification are
significant.

The APS detector breaks new ground mechanically and requires R&D
accordingly. To achieve the ultimate in thinness, we will explore
construction methods based on thinned silicon and associated cables
supported under tension with no additional backing or cooling material.
We also will develop a mechanical design that allows rapid insertion and
removal of the detector. As far as we know rapid insertion capability in
the center of a large coaxial detector has not been attempted before,
but we believe there are two reasons why this is a necessary requirement
for operating at RHIC at such a small radius. The main reason is
potential destruction by beam excursions. Since the area of silicon is
relatively small for a vertex detector, detector destruction is not a
major catastrophe if it can be rapidly replaced without significant loss
of beam time. A second advantage of rapid retraction is the solution to
the beam pipe bake out problem. R&D on the support structures will
include controlling and measuring detector position. Extending forced
air cooling capability while limiting structure vibration will also be
explored. Progress in this direction will expand electronic options.

Mechanical R&D will be invested in the development of a small radius
beam pipe which is integral to this detector. Currently STAR uses a 4 cm
radius beryllium beam pipe which is a uniform cylinder construction that
must support both vacuum and the gravitational load of an extended pipe.
By exploring a design which separates these two mechanical functions we
should be able to reduce the wall thickness in the region of the vertex
detector and improve detector performance.

Our R&D program will require significant software effort for detector
diagnostics, APS data reduction, integrated tracking for simulation
within the STAR environment. To the extent possible this development
will be done with goal of having components that can be used in a final
installation of the detector into STAR.

The R&D of this program will focus on developing a detector for early
implementation in STAR. While it is hoped that APS breakthroughs are
made such as photo-gate development which allows rapid on chip zero
suppression, the emphasis of the effort will be development of a
detector that utilizes as much as possible the APS technology as it
stands today.

The following summarizes deliverables for the 3 year R&D program. This
is followed by a partial list of development items.

**[Year 1:]{.underline}**

**Deliverables**

1.  

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1.  Physics driven requirements for the APS vertex detector.

2.  Develop and evaluate APS designs appropriate for a first generation
    detector system.

3.  Develop and evaluate mechanical concepts for support and cabling of
    thinned silicon ladders

4.  Simulate performance in the STAR environment

**Personnel**

  -------------------------- ------------ -------------------------------
  Senior Electrical Engineer 25 % FTE     provided from LBNL base

  IC design Engineer         20 % FTE     supported by UCI

  Senior Physicists          100 % FTE    from LBNL base + LDRD

  Post-doc                   100 % FTE    from LBNL base

  Post-doc                   50 % FTE     supported with R&D funds

  Grad. Student              125 % FTE    supported Univ. Heidelberg +
                                          LBNL LDRD

  Summer Under Grad          25 % FTE     LBNL LDRD

  Mechanical Engineer        25 % FTE     from BNL Instrument Div.

  Total personnel            4.7 FTE      
  -------------------------- ------------ -------------------------------

# Travel, purchases and subcontracts

  -------------------------------------------------------- --------------
  Travel                                                   \$10k

  IC foundry                                               \$30k

  Subcontracts- for board assembly                         \$10k

  Machine shop- for test structures                        \$30k

  Purchases- misc. parts; test equip.                      \$10k

  total non-personnel costs                                \$90k
  -------------------------------------------------------- --------------

**[Year 2:]{.underline}**

**Deliverables**

2.  1\. Single APS with demonstrated required readout speed

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2.  Ladder module mechanical conceptual design with stability tests

3.  Detector assembly conceptual design with STAR interfacing

4.  Readout chip data reduction conceptual design

5.  Beam pipe conceptual design with initial prototype testing

**Personnel**

  -------------------------- ------------ -------------------------------
  Senior Electrical Engineer 80 % FTE     provided from LBNL base

  IC design Engineer         20 % FTE     supported by UCI

  Senior Physicists          100 % FTE    from LBNL base

  Post-doc                   50 % FTE     from LBNL base

  Post-doc                   100 % FTE    supported with R&D funds

  Grad. Student              125 % FTE    supported Univ. Heidelberg +
                                          R&D funds

  Summer Under Grad          25 % FTE     R&D funds

  Mechanical Engineer        150 % FTE    R&D funds

  Total personnel            5 FTE        
  -------------------------- ------------ -------------------------------

# Travel, purchases and subcontracts

  -------------------------------------------------------- --------------
  Travel                                                   \$10k

  IC foundry                                               \$60k

  Subcontracts- for board assembly                         \$10k

  Machine shop- support test structures                    \$20k

  Purchases- misc. parts; test equip.                      \$10k

  total non-personnel costs                                \$80k
  -------------------------------------------------------- --------------

**[Year 3:]{.underline}**

**Deliverables**

4.  1\. Prototype APS ladder with readout and data reduction module

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2.  Prototype mechanical ladder module

3.  Mechanical conceptual design for detector support and insertion

4.  Conceptual design for alignment determination

5.  Design document to support system proposal

**Personnel**

  -------------------------- ------------ -------------------------------
  Senior Electrical Engineer 30 % FTE     provided from LBNL base

  Electrical Engineer        50 % FTE     R&D funds

  IC design Engineer         20 % FTE     supported by UCI

  Senior Physicists          100 % FTE    from LBNL base

  Post-doc                   50 % FTE     from LBNL base

  Post-doc                   100 % FTE    supported with R&D funds

  Grad. Student              125 % FTE    supported Univ. Heidelberg +
                                          R&D funds

  Summer Under Grad          25 % FTE     R&D funds

  Mechanical Engineer        50 % FTE     R&D funds

  Total personnel            5.5 FTE      
  -------------------------- ------------ -------------------------------

# Travel, purchases and subcontracts

  -------------------------------------------------------- --------------
  Travel                                                   \$10k

  IC foundry                                               \$50k

  Subcontracts- for board assembly                         \$10k

  Machine shop- proto ladder module                        \$40k

  Purchases- misc. parts; test equip.                      \$10k

  total non-personnel costs                                \$120k
  -------------------------------------------------------- --------------

The Table below summarizes the anticipated costs for this R&D task:

+---------------+-------------+-----+-------+-------------------------+
| # Category    | **[De       | **  | **[FY | **[FY                   |
|               | scription/c | [FY | 2004  | 2005]{.underline}**     |
|               | omments]{.u | 2   | K\$   |                         |
|               | nderline}** | 003 | ]{.un | **[K\$]{.underline}**   |
|               |             | K\$ | derli |                         |
|               |             | ]{. | ne}** |                         |
|               |             | und |       |                         |
|               |             | erl |       |                         |
|               |             | ine |       |                         |
|               |             | }** |       |                         |
+---------------+-------------+-----+-------+-------------------------+
|               |             |     |       |                         |
+---------------+-------------+-----+-------+-------------------------+
| Salaries      | Only for    | \   | \$1   | 223.2k                  |
| (Includes     | personnel   | $32 | 53.7k |                         |
| fringe)       | supported   | .5k |       |                         |
|               | from R&D    |     |       |                         |
|               | funds       |     |       |                         |
+---------------+-------------+-----+-------+-------------------------+
| Travel        | Pixel       | \$  | \$10k | \$10k                   |
|               | conferences | 10k |       |                         |
|               | and         |     |       |                         |
|               | LEPSI/IReS  |     |       |                         |
|               | visits      |     |       |                         |
+---------------+-------------+-----+-------+-------------------------+
| Purchases     | See text    | \$  | \$10k | \$10k                   |
|               | above       | 10k |       |                         |
+---------------+-------------+-----+-------+-------------------------+
| Subcontracts  | See text    | \$  | \$60k | \$100k                  |
|               | above       | 60k |       |                         |
+---------------+-------------+-----+-------+-------------------------+
| Overhead      | Only for    | \   | \$1   | \$151.8k                |
| costs         | personnel   | $20 | 02.8k |                         |
|               | supported   | .8k |       |                         |
|               | from R&D    |     |       |                         |
|               | funds       |     |       |                         |
+---------------+-------------+-----+-------+-------------------------+
| Total         |             | \$  | \$3   | \$495k                  |
|               |             | 133 | 36.5k |                         |
|               |             | .3k |       |                         |
+---------------+-------------+-----+-------+-------------------------+

The following is a partial list of projects and tasks to be addressed in
the R&D program. Some have been started, others have not been addressed
or planned. These items give some scope to the exercises outlined above.

**[Simulation:]{.underline}**

Abbreviated simulations, quantify performance expectations.

Integrate vertex detector into STAR tracking

Simulate vertex detector performance in the STAR software environment

**[Electronic:]{.underline}**

APS 0

1^st^ copy of LEPSI/IReS CMOS APS. Single point readout -- pixel
connected to output buffer and non-buffered output with row and column
selection via shift registers. 0.25 micron TSMC process through MOSIS. 4
variations -- two with TX isolation. (DONE)

APS 1

2^nd^ copy same as APS but with more FET size variations and each column
has own current source. (DONE)

APS 2

A small part of a larger UCI chip. 0.5 micron technology

APS 3

An APS with focus on fast readout. Requires on chip ADC or analogue
differential driver.

APS ladder 1

This will require a full wafer run. The ladder will consist of
electrically isolated chips arranged in a row and cut from a single
wafer.

Read out board 0

Small test board for reading out APS 0 and 1. Connects to LabVIEW on
MAC. Used for first characterization of APS 0 and 1. Used for ALS Beam
tests and radiation damage characterization. (DONE)

Read out board 1A

Next generation readout board with modern ADC, FPGA and memory. Designed
for speed and development of readout approaches that could be used in
STAR. Improvements for better multi-board integration to help in
multi-chip telescope testing in the ALS electron beam.

Read out board 1B

Modified from 1A to provide a development platform for on board data
filtering and data reduction involving cluster finding and reduction to
hit locations.

Read out board 2

To readout a multi-APS ladder. This will serve as a prototype for the
final STAR vertex readout system and will include capabilities developed
in Read out board 1B.

DAQ Interface

Development work is required on determining how to interface the APS
Vertex detector to the STAR DAQ system. The vertex system presents a
special interface issue because this vertex detector will read out
slower than other detectors and a vertex detector event will contain
useful hit information associated with several trigger events. A
development plan for this activity remains to be done.

**[Mechanical:]{.underline}**

Support Structure 0

This device tests ability to mechanical couple to thin silicon and load
it under tension and provides quantitative working load limits. (DONE)

Support Structure 1

Develops concepts for supporting thinned silicon under tension. Explores
issues of mechanical stability, limits of air cooling velocity and
induced silicon surface vibration.

Support Structure 2

Develops concepts for support of thinned silicon plus aluminum flex PC
cable and investigates ability to use wire bond connections from the
silicon to the cable.

Support Structure 3

Develops a concept for supporting a group of 3 ladders. 12 of these 3
ladder groups will form the two layer silicon structure.

Cooling test

Determine limits of ladder cooling by air flow either with mechanical
test module or calculation.

Readout Support

Develops a concept for supporting the ladder groups and carries the off
ladder readout electronics with water cooling. Includes insertion and
retraction mechanics used for routine maintenance and removal for beam
line bake out.

Aluminum Cable

Development of manufacturing methods for producing aluminum flex PC
cable. Explores issues of pattern control and methods for making vias
(electrical connections between layers)

Calibration and Alignment

Develop methods for high precision determination of detector silicon
location.

Beam Pipe Design

A new small diameter beam pipe must be developed to accommodate the APS
detector design. A concept is under consideration, but design work and
prototyping remains to be done.

**[Software Development:]{.underline}**

DAQ 0

A LabVIEW program for reading out and testing APS 0 and 1 has been
developed. (DONE)

DAQ 1

Modified LabVIEW program to interface to Read Out Board 1A and 1B.
Additional setup options and designed data output formatting.

DAQ 2

Program to readout a full multi APS ladder. Will operate with Read Out
Board 2

Tracking Code 1

Diagnostic code for setting up ALS electron beam testing with multiple
APS telescope. Capable of online track recognition to be used for
telescope alignment.

Tracking Code 2

Integration of the APS vertex detector to STAR tracking. Used initially
for simulation, but later will serve as a diagnostic tool in the STAR
environment.

Pixel Monitor

Rapid specialized measure of hardware function. To be used initially
with Read out board 2 with later use in the final STAR installation.

Footnote:

1\. A smaller beam pipe of around 2 cm radius should not pose a problem
for the RHIC beam. Private Communication Steve Peggs.