Study of Open-Charm Production at RHIC

Patrick L McGaughey[^1] and David M. Lee

## Abstract

> This is the final report of a three-year, Laboratory-Directed Research
> and Development (LDRD) project at the Los Alamos National Laboratory
> (LANL). The Relativistic Heavy-Ion Collider (RHIC) at Brookhaven
> National Laboratory was constructed by DOE for research concerning the
> formation of the quark-gluon plasma. The charm quark provides for one
> of the most robust signals of plasma formation \-- suppression of the
> J/ψ resonance. Yet none of the existing experiments at RHIC have the
> ability to directly identify open charm via the standard technique of
> finding detached decay vertices. In this article we discuss the
> capabilities of, and conceptual design for, a new detector associated
> with the PHENIX muon spectrometers. This device would observe charm
> and beauty quarks through their decay vertices, using a silicon
> micro-vertex detector array.

**1. Background and Research Objectives**

Brookhaven\'s Relativistic Heavy-Ion Collider (RHIC) has successfully
collided gold ions at 200 GeV per nucleon center-of-mass energy (√S).
There is now little doubt that the extreme energy densities and
temperatures have been achieved and it appears likely that a new phase
of matter, named the quark-gluon-plasma (QGP), has been created. The
final confirmation of the existence of this new phase of matter awaits
measurements of the production of the J/ψ resonance, which is a bound
state consisting of a charm and anti-charm quark. The J/ψ particle
cannot be formed at temperatures much above the transition point at
which normal nuclear matter melts and forms a QGP. Thus the J/ψ acts as
a thermometer, since it disappears above a particular temperature.
Critical to the interpretation of the J/ψ signal is a robust measurement
of open charm (D mesons). The yield of open charm cannot be suppressed
by the presence of a QGP. In fact, a hot QGP is expected to radiate
additional thermal charm quark pairs.

The PHENIX experiment at RHIC was designed to measure the J/ψ production
rate, as a function of collision centrality. J/ψ decays to dielectrons
are measured with the central arms and decays to dimuons are detected
with the muon arms. During the last year, thousands of J/ψ decays were
recorded from *Au-Au* collisions. These data are currently being
analyzed. Unfortunately, none of the existing experiments at RHIC can
accurately measure the total open charm cross section in *Au-Au*
collisions. Until recently, the only data available were for single
electrons, which have large systematic uncertainties.

The conventional solution to the problem of measuring open charm is the
use of a vertex detector. Particles containing heavy quarks, such as D
mesons, have lifetimes in the picosecond range. For mesons moving at
velocities near the speed of light, these lifetimes result in a finite
displacement of the decay products from the point of production.
Specialized tracking detectors with very high position resolution,
called vertex detectors, are used to detect these displaced vertices.
None of the RHIC experiments currently have a detector with the required
resolution.

In this report we discuss the conceptual design of micro-vertex detector
intended for use with the PHENIX muon arms to directly identify heavy
quark decays. The design proceeded along the following steps : First,
using a detector simulation tool (GEANT) and a quark event generator
(PYTHIA), we determined the design requirements in terms of angular
coverage, placement of detectors and the required spatial resolution.
Next, we canvassed the available silicon detector technologies and
electronics to define what would satisfy these design requirements. Then
we developed up a conceptual mechanical design with Hytec, Inc. Finally,
we entered into a collaborative effort with Fermilab to develop and test
the readout electronics. The end goal of the project was to prepare a
DOE proposal for the construction of a silicon vertex detector for the
PHENIX muon arms.

**2. Importance to LANL's Science and Technolo****gy Base and National
R&D Needs**

Our project has strong ties to DOE's Nuclear and Particle Physics
programs and RHIC, as well as to previous work done at Fermilab and the
Superconducting Supercollider. This project resulted in a request for
capital construction funds from DOE to build a new micro-vertex detector
for PHENIX. This project also brings LANL to the forefront of silicon
detector research and development. Silicon pixel and strip detectors
have become the technology of choice for high-resolution particle
measurements, with myriad applications to LANL's basic and applied
research activities.

**3. Scientific Approach and Accomplishments**

There are three broad physics topics that are accessible using an Endcap
Si vertex detector.

-   A large increase in the range of Bjorken-*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 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 upgrade will operate well at 40× the RHIC design luminosity and many
of the measurements need the higher luminosity, e.g. upsilon and open
beauty.

The 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
almost the full azimuth angle with a resolution of \~150 μm. Each endcap
comprises four octagonal "lampshades" populated with Si mini-strip
detectors.

![](media/image1.jpeg){width="5.521527777777778in"
height="2.7506944444444446in"}

Figure 1. Cross-section view of the proposed vertex detector. The
central barrel layers are the four horizontal green lines at the center,
while the endcap layers are the four tilted lines on each end of the
barrel layers.

![](media/image2.png){width="4.4125in" height="3.297222222222222in"}

Figure 2. A schematic cut-away mechanical drawing of the proposed vertex
detector.

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.

## I. Scientific Goals of the Endcap Upgrade

### A. 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/image3.png){width="4.159027777777778in" height="3.4in"}

Figure 3. Expected Bjorken-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*~gluon~ = 0.001 using $D \rightarrow \mu X$.

-   $b\overline{b}$ production via gluon fusion. With the upgrade we can
    identify a displaced J/ψ from a $B \rightarrow \frac{J}{\Psi}$
    decay. This provides coverage in x between 0.005 - 0.3. The
    selection of semi-leptonic decays of
    $b\overline{b} \rightarrow e\mu X$ 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.

.

### B. 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/image4.png){width="4.034722222222222in"
height="3.3833333333333333in"}

Figure 4. Gluon shadowing predictions along with PHENIX coverage. The
red bars indicate the additional range provided by the Endcap vertex
upgrade, green bars are for the barrel upgrade, while the blue bars
cover the PHENIX baseline. The red and blue curves are theoretical
predictions of shadowing .

### 

### C. 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 Heavy-ion Physics Topics

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

####  {#section-2 .unnumbered}

#### 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/image5.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.

## II. 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
        100 μm, 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 50 μm by 2000 μm to 50 μm by
9000 μm as the radius increases. This keeps the occupancy at or below
1.5%.

### A. Open Charm Measurement

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

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 1 cm 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/image6.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/image7.jpeg){width="3.308333333333333in"
height="3.308333333333333in"}

Figure 7. The p~T~ distribution of muons that decay within 1 cm of the
collision vertex. The red histogram is for charm decays while the black
is for pion and kaon decays.

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^. A total
of about 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 pre-scale 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.

### 

### B. Open Beauty Measurement

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

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.

Applying a vertex cut on each reconstructed J/ψ has been used
successfully to identify B-production in experiments at lower
energies[^2]. 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/image8.jpeg){width="3.5145833333333334in"
height="3.3930555555555557in"}

Figure 8. The reconstructed Z-vertex distribution for J/ψ from B decays
(black line) and for prompt J/ψ (red line). Note that the J/ψ yield has
been scaled down by a factor of 100. The relative yield of J/ψ from B
decays versus prompt J/ψ is estimated to be about 1 %.

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 \~ 0.0036. The corresponding yield is \~320,000
reconstructed events. Thus, an excellent *B* measurement is possible.

### 

### C. 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[^3].

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.

###  

### D. 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 10) 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 10

  $$D \rightarrow \mu X$$                                        \~ 10^8^

  $$B \rightarrow \mu X$$                                        320000

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

### 

### E. 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.

## III. Technical Aspects of the Proposed Endcap Vertex Detector

### A. Silicon Readout Chip --PHX

The FNAL Electrical Engineering Department, ASIC development group
(Group leader, Ray Yarema) 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.

### B. 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.

![](media/image9.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.

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.

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/image10.jpeg){width="1.3493055555555555in"
height="3.435416666666667in"}![](media/image11.jpeg){width="1.8847222222222222in"
height="3.6840277777777777in"}![](media/image12.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/image13.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/image16.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/image17.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.

### C. 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 data collection modules (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.

### D. Mechanical Structure and Cooling

The mechanical structures and cooling will be designed with the
assistance of Hytec, inc. 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 below.

### 

### E. 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.

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/image18.jpeg){width="2.8055555555555554in"
height="2.502083333333333in"}
![](media/image19.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.

![](media/image20.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.

**F. Radiation Length**

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/image21.png){width="3.9972222222222222in"
height="2.1840277777777777in"}

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

**G. Readout and Level One Trigger**

Figure 17 shows the asynchronous readout concept on the sensor side. 11
readout chips will be mounted in one column to interface with one OASE
chip. This is achieved by having a group of six and a group of five PHX
chips one either side of the support wedge sharing one bus,
respectively. The OASE chip is a development by the University on
Heidelberg and will allow the translation from low voltage differential
signals (LVDS) to a 2.5 Gbit/s optical fiber. All components will be
integrated on the chip, including the driver for the fiber. One of the
interesting features of this chip is the bidirectional capability of the
communication protocol. While the rate of the sensor is 2.5 Gbit/s, in
order to allow for the fast racking discussed below, it is at the same
time able to receive slow control signals (on the order of 100 Hz) for
the readout chip control and communications. This will simplify the
system greatly.

In order to allow for a timely Level One decision, all the data has to
arrive on the receiver board within about 1 microsecond of the
collision. We think we can achieve this as follows: One sensor column
has 11 chips with 512 channels each. An occupancy of 1 percent
translates into about 60 hit pixels. Each pixel send a 24 bit
information onto the bus, 9 bits coding the position of the pixel on the
chips, 3 bits encoding the signal height, 4 bits of the chip id and then
an 8 bit time stamp. With an LVDS configuration of two busses, each with
6 lines of 160 Mbit/s and the fiber speed mentioned above, the data
transfer will be completed within 0.6 microseconds.

![](media/image22.jpeg){width="0.8338582677165355in"
height="3.02992125984252in"}![](media/image23.png){width="0.2688976377952756in"
height="0.9047244094488189in"}

**Figure 17.** Overview of the asynchronous readout of a wedge.

### 

Figure 18 shows the receiver board concept. We envision building boards
for LINUX based PCs which will host an OASE interface chip for the
sensor information input and front end slow control, an Arcnet interface
for the PHENIX slow control and GLink interface for the communication to
the PHENIX data acquisition system. The level one trigger output will
connect to the PHENIX level one processor.

The incoming sensor data will be processed by a FPGA to split the data
stream into two parts. Part one will emulate the existing PHENIX
acquisition standard, while the other stream will route ordered sensor
data into a fast tracking FPGA, which will perform hit matching and
displaced vertex tracking. The trigger decision will be derived from the
tracking information and be send to the PHENIX level one processor.

**Figure 18.** Overview of the receiver board.

### 

### 

### Publications

1\. H. Enyo, et.al., \"Silicon Vertex Upgrade Status Report,\" PHENIX
Technical note tn399.0,
http://www.phenix.bnl.gov/phenix/WWW/publish/cogilvie/techNote/SiTechnicalNote.ps,
(2002).

2\. M. Baker, et.al., \"Proposal for a Silicon Vertex Tracker (VTX) for
the PHENIX Experiment,\"
http://www.phenix.bnl.gov/WWW/publish/akiba/2003/SVTX/PHENIX-VTX-proposal.pdf
, (2003).

3\. D. M. Lee, \"A Proposed Silicon Vertex Tracker for the PHENIX
Detector at Forward Rapidity,\" IEEE Nuclear Science Symposium, Oct.
2004, Rome, Italy. , (2004).

4\. M. Baker, et.al., \"Proposal for a Silicon Vertex Tracker (VTX) for
the PHENIX Experiment,\"
https://www.phenix.bnl.gov/phenix/WWW/p/docs/proposals/VTX-PROPOSAL_jul2004.pdf
, (2004).

### References

[^1]: ^\*^<plm@lanl.gov>

[^2]: Jansen, D.M., et.al., (E789 Collaboration), "Measurement of the
    bottom quark cross-section in 800-GeV/c proton-gold collisions,"
    Phys. Rev. Lett. 74, 3118 (1995).

[^3]: Gagliardi, C.A., et al., (E789 Collaboration), \"Hardware Trigger
    System for Fermilab E866,\" Nucl. Inst. Meth. A 418, 322 (1998).