**Technical Advisory Committee Review**

**STAR Tracking Upgrade Plan**

**January 30-31, 2007**

[Committee members:]{.underline} M. Cooper (LANL), H. Fenker (JLab), C.
Haber (LBNL), V. Radeka (BNL), R. Van Berg (Univ. Pennsylvania)

[Also participating:]{.underline} T. Ludlam (BNL, convener), G. Rai
(DOE)

**[Summary]{.underline}**

The STAR Collaboration is proposing an extensive upgrade of its tracking
capability, providing precision vertex tracking at mid-rapidity for
direct measurement of displaced vertices from charm and beauty decays,
and tracking at forward angles, subtending the End Cap Calorimeter, to
determine the charge sign of high-energy electrons from W decays.
Following a review in March 2006 of development efforts then underway
for possible detector technologies, the collaboration has focused on
designing an optimized scheme that would form a viable basis for a
construction effort. This work has emphasized the development of
simulation tools to study the physics capability of possible detector
configurations for an integrated system, including the present Time
Projection Chamber, and the future capabilities of the upgraded data
acquisition system for STAR and the planned RHIC II luminosity.

The specific proposal that has emerged calls for construction of a
next-generation micro-vertex detector using a combination of monolithic
active pixel sensors (MAPS) and conventional silicon strip technology,
and a forward tracking detector based on the emerging gas electron
multiplier (GEM) technology. STAR is aiming for a major R&D investment
in these upgrades in FY 2007-2008, and a start of construction in FY
2009. The overall estimates of cost and schedule presented to the
Committee for these upgrades do not differ significantly from those
given in BNL's "Mid-Term Strategic Plan for RHIC", submitted to DOE in
February 2006.

The purpose the present review was to assess whether the detector
configuration presented by STAR is indeed optimal for the proposed
physics goals, and to provide BNL and DOE with advice on the readiness
of the R&D plans, the technical designs, and the cost and schedule
estimates for this effort to proceed on the time scale proposed by STAR.
The review agenda is attached as an appendix to this report.

The Committee's detailed findings, comments and recommendations are
presented in the following sections. We summarize the key outcomes of
the review here:

-   The Committee finds that the scientific case presented for the
    tracking upgrades is well established, and that the proposed
    detector configurations are well motivated technologically. The
    collaboration has made a convincing case that compelling physics
    goals can be achieved with the resources requested.

-   Because each is required for a common set of scientific goals, the
    Heavy Flavor Tracker (HFT) and Intermediate Silicon Tracker (IST)
    should be combined as a single project. It appears that the cost of
    the Forward GEM Tracker (FGT) can be kept below the \$2M threshold
    for designation as a DOE Major Item of Equipment, allowing it to be
    pursued as a capital equipment project within the RHIC operations
    budget. If a more careful analysis bears this out, it may be
    possible to proceed on a technically-driven schedule that can lead
    to earlier physics results.

-   STAR identified a number of management challenges that resonated in
    the committee. The collaboration needs to identify project leaders
    for three crucial roles. (a) It would be good if one physics group
    could take responsibility for the existing Silicon Strip Detector
    (SSD). They may draw on dispersed resources from around the
    collaboration, but it is important for one person to take the
    coordinating role for this pivotal element of the planned
    upgrades. (b) A single project leader needs to be identified for the
    IST + HFT detectors, and subsystem managers are needed for the two
    detectors. (c) A project leader must be identified for the FGT. Both
    mechanical and electronic integration engineers should be identified
    for the overall project.

-   The combined IST + HFT project will require the DOE critical
    decision process. The first step is CD-0, the establishment of
    mission need. STAR should make every effort to prepare a proposal by
    late spring of this year for submission to DOE in order to obtain
    first construction funds in FY'09. This proposal will need to make
    the physics case in a compelling way. It will need a reasonable but
    preliminary management plan, and should establish a reliable cost
    range. It will need a schedule that provides the basis for a
    proposed funding profile.

-   The MAPS chip design for the HFT relies critically on the on-going
    R&D program at IRES, Strasbourg. This is a broad-based effort that
    will continue past the point when the design for STAR will have to
    be frozen to meet the HFT schedule. The plan for when and how this
    "freeze point" will be captured must be spelled out as part of the
    project plan.

**[Overview of the proposed upgrades]{.underline}**

The STAR collaboration presented a coherent plan for determining
detached vertices and the interaction vertex for low multiplicity
events. Four systems were discussed: the silicon strip detector (SSD),
the heavy flavor tracker (HFT), the intermediate silicon tracker (IST),
and the forward GEM tracker (FGT). The presentations were well
constructed and provided the committee with the vast majority of the
information needed. Each detector covered the physics goals, the
mechanical and electronics challenges, responsibilities, cost estimates,
and schedule proposals. The committee appreciates the effort put forth
to present their plans.

The primary motivation for the mid-rapidity tracker is to study heavy
quark production in p+p, d+A, and A+A collisions at RHIC. If the flow
measured for heavy quarks is found to be the same as for the light
quarks, strong evidence would exist that the strongly interacting quark
matter produced is in thermodynamic equilibrium. The key to measuring
the flow of the heavy quarks is observation of a large ensemble of D^0^
 K^-^ + π^+^ and charge conjugate events, which are identified by full
topological reconstruction that measures the displaced decay vertex as
being clearly distinguished from the interaction vertex. The
presentations were convincing that the proposed detector would allow for
success in understanding this physics. The committee finds this
principal physics goal to be compelling; it justifies the resources
requested.

The FGT is designed to probe the flavor dependence of the sea quark
polarization from polarized p+p collisions. The technique utilizes the
parity-violating W production and decay to establish the helicity of the
antiquark involved in formation of the charged W. This works because the
anti-quark polarization is preserved in the leptonic observables. The
crucial step in separating the sea quark properties is distinguishing
electrons and positrons at high rapidity. The FGT allows for the proper
identification, which should lead to a better determination of the sea
quark role in the spin of the nucleon. The committee also finds the
physics case for the FGT to be very strong.

The committee sees the refurbishment of the SSD as a relatively small
effort that should be started as soon as funding can be made available
from the RHIC operations grant. Its performance goals should be
compatible with the full tracking system. The mid-rapidity tracker has a
common set of scientific goals that warrant both the IST and HFT to be
merged into a single project. The scientific objectives of the FGT are
different from the IST and HFT. The committee recommends that it be made
a separate project. The breakdown into separate projects still requires
good integration practices because all these detectors are in very close
proximity.

As presented at this review, the plan for the HFT involves an extended
R&D phase, followed by implementation of a full detector with MimoSTAR 4
detectors, and finally the implementation of UltraSTAR chips. The
Committee is not convinced that this is the most effective way to
complete this project. As noted in the following section, much depends
on the role of the IRES group in this project, and the degree to which
the MAPS development in France is driven by the STAR requirements. These
issues will need to be clarified before the scope of this project can be
fully defined.

The mid-rapidity tracker has had substantial simulation, but a few
important questions remain to be answered. For example, at this review
the overall tracking efficiency was unknown when only two of the three
IST + SSD layers are present. Since the time to accumulate the required
statistics is quadratic in the tracking efficiency, it is important to
know how the expected detector imperfections will affect the data
collection time. The committee recommends that the simulation of the mid
rapidity tracker continue to a higher level of sophistication.

The individual detector discussions cover the need for prototypes in a
number of areas. A vigorous prototyping plan should be put forward. The
committee sees the need to demonstrate the positioning reproducibility
of the three cantilevered supports for the HFT and the production of
full quadrant sized GEM foils as the most critical ones.

The project cost estimate appears to have reasonable number of spares as
well as acceptable contingency methodology. Escalation should not be
forgotten. As the project proceeds, more rigorous estimates will be
expected.

In most cases, the committee viewed the proposed schedules as very
success oriented. There did not seem to be sufficient schedule
contingency or time allotted for the DOE funding process. The schedule
is more complicated than shown due to important outside events. Two
examples are startup of a RHIC running cycle and the availability of
funds during the fiscal year, which is usually in January. Thus the
tight HFT schedule that assumes funds arrive on October 1 of 2009 for
the first phase of the HFT to be ready for beam a year later is probably
not achievable. STAR is urged to put more effort into their estimates of
the schedules.

The committee believes these detectors can be built on a reasonable
schedule and produce exciting physics results.

**[The Heavy Flavor Tracker]{.underline}**

The proposed MAPS technology is at this time the most promising choice
for high granularity, low radiation length and low power dissipation
vertex tracking in an environment such as RHIC. The proposed design
configuration maximizes the solid angle coverage while keeping the
number of detector layers and the area of silicon at a minimum. This is
a cutting edge technology and it will be used for the first time on a
fairly large scale in a large physics experiment.

The basic sensor cell (pixel), while based on "standard" CMOS process,
depends critically on some properties of the process usually not
specified by the foundry and not subject to the suggestions/requests of
small volume customers. For this device the thickness of the epitaxial
layer from which the signal charge is collected is of primary
importance.

While the broad based and outstanding R&D program at IRES is
commendable, their developments will be continuing past the point when
the design for STAR will have to be frozen to meet the schedule. The
plan for when and how this "freeze point" will be captured has not been
spelled out.

The mechanical design concept makes use of advanced composite materials.
The proposed design is primarily motivated by the desire to make
possible installation and removal of the detector in a short time.
Design considerations which could provide some quantitative measure of
the position repeatability after reinsertion and stability of the
detector components with respect to each other have not been presented.

The project schedule planning is greatly influenced by the limitations
in the available manpower and funding, particularly with respect to
mechanical engineering. One consequence of this has been that the tests
of functionality and performance of the technologically most critical
components, the ULTRA-STAR detector/readout and the nesting cantilevered
mechanical structure, are planned too close to the desired completion
date in 2011. The projected total time for the project might be adequate
if some of the design, modeling and demonstration activities take place
earlier in the project.

However, given the cutting edge technology, the proposed schedule does
not contain sufficient float.

The cost estimate has been made with realistic contingency figures.Given
the risks of radiation damage near the beam pipe, allowing for four
detector copies of the "final" design is justified.

[Recommendations for the HFT]{.underline}

-   The project should move rapidly on the mechanical system, by
    performing modeling of the nesting concept and then building a
    prototype to demonstrate the repeatability and position stability.
    Realistic alignment tolerances need to be derived from the track
    reconstruction requirements.

-   STAR ought to make certain that the IRES schedule matches the STAR
    schedule. By CD1 there will have to be a formal agreement on this.
    It will be advisable at that point to verify with the IRES' funding
    agency, that IRES will be funded for that work throughout the
    duration of the project.

-   It will be advisable to have a closer connection with IRES in the
    design of the readout architecture (a physicist/engineer from one of
    the STAR institutions participating in the Design?).

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-   Power dissipation of the electronics is an important input to the
    mechanical design both for cooling and thermal deformation. Detailed
    power estimates (with upper bounds if possible) for both MimoSTAR4
    and MimoSTAR Ultra should be provided to the mechanical team as soon
    as possible. The schedule for this should be defined.

[Some questions for consideration by the HFT team:]{.underline}

Most, if not all, of the design elements required for the MimoSTAR Ultra
chip seem to have been tested or at least simulated at Strasbourg - is
there a fundamental reason, other than uncertainties in particular
process suitability for creating good sensor diodes, for not assembling
and prototyping a MimoSTAR Ultra 0 chip (full Ultra architecture but
small pixel array) within the next year? If there is a fundamental
problem, what is the plan to address that problem?

What are the expected detailed differences between MimoSTAR4 readout and
MimoSTAR Ultra readout and how do those two different systems couple to
DAQ1000?

Tests of MimoSTAR2 have encountered some problems at the single and two
chip level, how soon can one understand whether these problems are
inherent in the present chip design or are merely artifacts of the test
system?

Is the JTAG control interface for MimoSTAR4 (or MimoSTAR Ultra) planned
to be single ended as in the MimoSTAR2 device or is there a plan to go
to an LVDS (or similar) differential signaling standard?

A complete ladder test of MimoSTAR4 or MimoSTAR Ultra will provide
important verification of cooling, mechanical stability, and readout
performance. When is such a test or tests (of the separate issues)
planned?

The planned use of Aluminum on Kapton cables reduces the radiation
length of the detector. If Aluminum were to prove very difficult to work
with what are the physics implications of going to Copper/Kapton cables
or other types of laminates?

Air cooling of the detector system(s) (HFT, IST, SSD, and FGT) is a way
to keep the radiation length of the detectors as low as possible.
However, the power dissipation, especially in the intermediate rapidity
region, will be much greater after the proposed upgrade than presently.
How soon can a reasonably detailed mechanical analysis be completed to
give confidence that there will be adequate space to bring in and
exhaust sufficient cooling air (gas?) at reasonable velocities to
satisfy the cooling needs of all the proposed detectors and the already
existing SSD without introducing any undesirable effects? Is there a
temperature control system needed, particularly for the HFT?

**Intermediate Silicon Tracker (IST) and Silicon Strip Detector (SSD)**

The Intermediate Silicon Tracker (IST) and Silicon Strip Detector (SSD)
layers provide crucial tracking functions to link the STAR TPC with the
proposed Heavy Flavor Tracker (HFT). The function, performance
specifications, and in the case of IST, the conceptual design and
layout, of these devices was presented by the Collaboration. While a
rational case was made for these components, certain questions and
concerns remain which should be addressed by the Collaboration. The
Committee does not see any fundamental problem with the ideas and
approach proposed but does recognize that certain adjustments may have
to be made with not insignificant cost or effort implications.

The SSD is an existing system which lacks a focused effort for
operational and scientific support within STAR. Its perceived value is
growing and in fact it is viewed as critical for the HFT era. A clear
and careful assessment of the state of the SSD needs to be made and
presented. Whatever prospects there are for repair, replacement, or
resurrection, of bad or otherwise out of specification components needs
to be assessed and confronted. The acceptable performance level
(efficiency etc) for the SSD needs to be clearly stated and justified.
After this, if the Collaboration still believes the SSD is needed then
Management has to put in place the proper resources to repair and
maintain it.

The SSD may not be recoverable or maintainable. In this case, the
Collaboration has to consider and present a back-up plan. They may
choose to remove the SSD and rely only on the two IST layers or they may
choose to replace the SSD with a third IST layer around 25 cm in radius.
Track finding and linking performance for these two scenarios should be
studied and presented. If there is a replacement, the case for a
particular strip and/or pad geometry should be made. Obviously a project
to build a third IST layer should be closely coupled, in all respects,
to IST1 and 2 in order to benefit from the same non-recurrent
engineering, component designs, and procurements. IST3 should not become
a separate project.

The technical challenges of designing and building the IST layers are
mostly similar to those of other recent projects. A few points raise
concern and the Committee wants to emphasize those to the Collaboration.
The time for development and iteration is rather short so the
Collaboration needs to move quickly to assess these technical choices
and adopt or replace them so that a proper cost estimate, schedule, and
technical (rather than conceptual) design can be made.

-   The mechanical support of the ladders is of some concern. Carbon
    foam alone may not be nearly rigid enough. One would expect to clad
    it with carbon fiber (or other) facings with low elasticity. This
    needs to be considered. The idea of blowing air through channels in
    the foam is novel. This may lead to microphonic vibration and
    excessive thermal gradients. There may also be substantial thermal
    resistance from the foam core up to the readout chips which are the
    main heat source.

-   As a general approach, air cooling was shown to work on SSD. For IST
    the density of ladders is higher in that radial region and that
    raises additional concern.

-   The IST ladders consist of 10-12 sensors in a row. There are many
    different element orientations (strips, z, phi, pads). The readout
    hybrids may be mounted differently for each and the access by the
    long readout cables may be complicated. One alternative is to
    provide clocks and commands to the APV25 in a multi-drop
    configuration to all chips on a ladder. Another issue is power
    distribution. Additional material reduction may result with serial
    powering of all the hybrids on a ladder. This is a technical
    approach with certain risks but also the focus of much R&D in the
    LHC community. How is all this going to be routed? If the readout
    cables are placed beneath the sensors (between the sensor and the
    carbon) they will add to the thermal resistance of the system. This
    needs to be considered.

-   The choice of Hamamatsu as the sensor vendor is good considering
    their excellent track record. The AC coupled pad detector is more
    complicated than the more common strip design. The biasing scheme
    for this has to be worked out and presented.

-   The use of kapton hybrids and cables is not without risk. This
    technology is attractive for mass reduction and integration.
    However, essentially all past projects have had major problems
    getting kapton circuits to work, although they eventually succeeded.
    The major implication is for cost and schedule. How many iterations
    are acceptable? The Collaboration should reduce its exposure by
    working with vendors having a proven track record for physics
    instrumentation projects. The Collaboration should also develop a
    back-up plan in case the kapton development does not converge soon
    enough. A large contingency would be appropriate here.

**The Forward GEM Detector**

The proposed Forward GEM Tracker system appears to be well matched to
the need to determine the charge sign of electrons from W decays in the
rapidity range from 1 to 2 units. The demonstrated 70 micron position
resolution obtained in existing GEM detectors is more than sufficient to
achieve this measurement, making the proposed detector very likely to
succeed. The readout uses an existing front-end which has already been
proven to work, and the collaboration has an existing healthy
relationship with a GEM production facility (Tech-Etch, Inc.) which
already produces working GEMs and which is actively improving their
production process. The collaborators have demonstrated a thorough
understanding of the current limitations of GEM technology and have the
tools in place to provide the necessary quality control of parts that go
into the final detector.

Because of the availability of components and the relative simplicity of
the FGT, it seems quite realistic to achieve its development and
construction well before the HFT and IST systems. Its physics reach is
independent of these systems. Organizational approaches that would allow
the FGT to proceed more quickly than possible for a formal DoE Project
would provide valuable physics returns from the planned 500 Gev running
in 2009. The aggressive but achievable schedule submitted to the
Committee would reach this goal.

While good quality prototype GEM foils have been regularly provided by
Tech-Etch, it is likely that the yield of acceptable foils will go down
as their size is increased. Therefore the Committee recommends that
large size GEM foils be procured and tested at an early date so that a
realistic estimate of the yield can be made, and so that unanticipated
technical problems related to the size of the GEMs can be identified and
dealt with.

Associated upgrades of the GEM measuring and testing hardware may be
required to accommodate this increase in size.

A draft of the final charge-collection strip layout should be developed
promptly. Details of the strip length and the r-phi geometry were not
provided. These will be affected particularly by the choice of radial
subdivisions within a quarter-disc module, and will impact the strip
capacitance, signal routing, and readout chip placement. The width of
phi-measuring strips and the distance between them will presumably vary
with radius and could lead to a radial dependence of the resolution. It
would be wise to identify critical areas in the design at an early
stage. It would be especially interesting to understand the signal
routing for those strips that do not terminate on a quadrant boundary.

This layout may also have an impact on the possibility that signals will
be degraded by coupling to the nearby control, clock, and readout lines
servicing the APV-25 chips. Particularly if the chips and digital lines
are to reside on the same (multilayer) PC board that carries the readout
strip material, this board may be difficult to engineer and fabricate.

The Committee suggests that a prototype detector which incorporates a
readout board representing a realistic sub-section of the final board be
fabricated and tested as soon as practical. APV-25 chips, logic, power,
and readout lines should appear on the board similar to the way they
would appear on the final board. Consideration should be given to the
way in which high voltages are supplied to the drift electrodes and
GEMs, if these have any impact on the board layout.

As indicated in the presentations, no conceptual design yet exists for
the routing of cables and gas to the FGT. Cooling air for the FGT
electronics and for the mid-rapidity tracking systems must also be
routed through the FGT support cone. High-voltage isolation from the TPC
inner field cage must be assured. These considerations may well lead to
rearrangement of the FGT disk layout and result in changes in
acceptance. An early engineering study of how to satisfy these
requirements should proceed at least to the level required to assess the
possible impact on detector system performance.

**Appendix**

**Technical Advisory Committee Review**

**STAR Tracking Upgrade Plan**

**January 30-31, 2007**

Room 2-160, BNL Physics Dept.

*[Agenda]{.underline}*

[Tuesday, Jan. 30]{.underline}

+-----------+-------------------------------------+--------------------+
| *8:30 --  | *Executive Session*                 |                    |
| 9:00*     |                                     |                    |
+-----------+-------------------------------------+--------------------+
| 9:00 --   | Overview:                           | T. Hallman (20+10) |
| 9:45      |                                     |                    |
|           | Physics motivation                  | H. Matis (10+5)    |
|           |                                     |                    |
|           | Optimization process; proposed      |                    |
|           | configuration                       |                    |
|           |                                     |                    |
|           | SSD status and perspective          |                    |
+-----------+-------------------------------------+--------------------+
| 9         | Mid-Rapidity Tracking               | J. Thomas (30+10)  |
| :45-10:25 |                                     |                    |
+-----------+-------------------------------------+--------------------+
| 10        | Mechanical progress on the Heavy    | H. Wieman (15+5)   |
| :25-10:45 | Flavor Tracker (HFT)                |                    |
+-----------+-------------------------------------+--------------------+
| 10        | Progress on Monolithic APS &        | M. Szelesniak      |
| :45-11:05 | associated electronics              | (15+5)             |
+-----------+-------------------------------------+--------------------+
| *11:      | *Coffee Break*                      |                    |
| 05-11:20* |                                     |                    |
+-----------+-------------------------------------+--------------------+
| 11        | Intermediate Silicon Tracker design | M. Miller (20+10)  |
| :20-11:50 |                                     |                    |
+-----------+-------------------------------------+--------------------+
| 11        | Technical implementation, cost,     | B. Surrow (15+10)  |
| :50-12:50 | schedule, and R&D plan for          |                    |
|           | Mid-Rapidity Tracking upgrade       | J. Thomas (25+10)  |
+-----------+-------------------------------------+--------------------+
|           |                                     |                    |
+-----------+-------------------------------------+--------------------+
| *12       | *Committee Working Lunch*           |                    |
| :50-2:00* |                                     |                    |
+-----------+-------------------------------------+--------------------+
|           |                                     |                    |
+-----------+-------------------------------------+--------------------+
| 2:00 --   | Forward Tracking Upgrade:           | E. Sichtermann     |
| 2:30      |                                     |                    |
|           | Physics Motivation                  | (20+10)            |
+-----------+-------------------------------------+--------------------+
| 2:30 --   | GEM progress; optimized             | F. Simon (20+10)   |
| 3:00      | configuration                       |                    |
+-----------+-------------------------------------+--------------------+
| 3:00 --   | Forward Tracking cost, schedule,    | B. Surrow (20+10)  |
| 3:30      | R&D plan                            |                    |
+-----------+-------------------------------------+--------------------+
| *3:30 --  | *Coffee Break*                      |                    |
| 3:40*     |                                     |                    |
+-----------+-------------------------------------+--------------------+
| 3:40 --   | Mechanical integration of tracking  | J. Kelsey (30+10)  |
| 4:20      | upgrades                            |                    |
+-----------+-------------------------------------+--------------------+
| 4:20 --   | Project summary                     | T. Hallman         |
| 4:30      |                                     |                    |
+-----------+-------------------------------------+--------------------+
| *4:30 --  | *Executive Session*                 |                    |
| 6:00*     |                                     |                    |
+-----------+-------------------------------------+--------------------+
| *7:00*    | *Dinner at local restaurant*        |                    |
+-----------+-------------------------------------+--------------------+

[Wednesday, Jan. 31]{.underline}

  -------------- ------------------------------------- --------------------
  *8:30 -- 9:00* *Executive Session*                   

  9:00 -10:00    Follow-up meetings with STAR          

  10:00-12:00    Committee: report preparation         

  *12:00-1:00*   *Committee Working Lunch*             

  1:00 -- 2:00   Committee: report preparation         

  2:00 -- 2:30   Close-out                             

  2:30           Adjourn                               
  -------------- ------------------------------------- --------------------