[^1]

*Abstract*---The front-end electronics of the DØ Ur/liquid argon
calorimeter has been replaced to operate with the new shorter time
separation between particle bunches at the Run II of the Fermilab
Tevatron. The upgrades of the electronic readout system are described,
and the first results of the calibration and the performance of the
calorimeter are presented.

# INTRODUCTION

[The]{.smallcaps} Tevatron accelerator complex at Fermilab has been
upgraded after Run I (1992-1996). The center-of-mass energy of collision
has been increased from 1.8 TeV to 1.96 TeV. The major upgrades include
the construction of the Main Injector and the Antiproton Recycler,
within a common tunnel. The plan is to deliver an integrated luminosity
of 4.4 fb^-1^ as the base projection and 8.6 fb^-1^ as the design
projection by the end of FY2009 (Run I delivered luminosity is about 125
pb^-1^). At the time of the conference, the luminosity recorded at DØ is
about 200 pb^-1^. The higher instantaneous luminosity requires a
reduction in the time separation between particle bunches from the Run I
value of 3.56 μs to 396 ns in Run II.

The upgrade of the D∅ detector is designed to enhance its capabilities
from Run I and to accommodate the shorter bunch spacing time. Since the
start of the Tevatron Run II in March 2001, the upgraded D∅ detector is
in operation. In the following we will briefly overview major upgrades
of the D∅ detector and then discuss the upgrades for calorimeter in
details.

# D∅ Detector Upgrade

Fig. 1 depicts the overview of the upgraded D∅ detector. From inside to
outside are the three primary detector systems: inner tracking,
calorimeter and muon systems. The inner tracking system has been
completely replaced with a Silicon Microstrip Tracker (SMT) closest to
the beampipe, and a multi-layered scintillating Central Fiber Tracker
(CFT) outside of it. The new tracking system sits inside a 2 Tesla
magnetic field provided by a new 2.7 m long superconducting solenoid for
momentum measurement of charged particles.

The calorimeter, divided into a Central Calorimeter (CC) and two End-cap
Calorimeters (EC), has been kept in place, with a replacement of the
readout electronics and the online calibration system. A scintillator
based central preshower detector with wavelength shifting fiber readout
is added between the outer radius of the solenoid and the inner radius
of the central calorimeter cryostat to provide electron identification
and to compensate for energy loss in the solenoid. In the forward
region, a preshower detector similar to the central one is installed on
the faces of the endcap calorimeter cryostats. At a larger radius on the
inner surface of each EC cryostat, the photo multiplier tubes used for
the scintillator based Inter-cryostat Detector (ICD) have been relocated
to a region of lower magnetic field, to cover the gap between the CC and
EC calorimeters. New scintillator based muon trigger detectors covering
the full pseudorapidity range have been added to provide a fast trigger.
In the forward muon system, the proportional drift tubes have been
replaced with planes of plastic mini-drift tubes.

![](media/image1.png){width="3.515972222222222in"
height="2.509027777777778in"}

Fig. 1. Side view of the DØ upgraded detector in Run II.

A new 3-level trigger system and data acquisition system are used to
handle the high event rate and to provide pipelining of the front-end
signals from the tracking, calorimeter, and muon systems. The Level 1
(L1) trigger provides a fast decision on whether to keep or discard an
event based on fast detector pick-offs and reduces the input rate to 1.4
kHz. L1 trigger decision time is 4.2 μs and it is effectively
deadtimeless. The Level 2 trigger consists of dedicated processors that
can make the decision in 100 μs and further reduce the rate to 800 Hz.
The Level 3 trigger consists of a farm of commercial processors that
utilize fully digitized information to reduce the accept rate to 50 Hz.

# The Calorimeter

The D∅ uranium/liquid argon sampling calorimeter has been described in
details in \[1\]-\[2\], and is unchanged for Run II. In this section we
briefly summarize the important features of the calorimeter. As shown in
Fig. 2, the calorimeter consists of three liquid argon cryostats -- one
Central Calorimeter (CC) covering the region \|η\| \< 1.2, and two
End-cap Calorimeters (EC) extending the coverage to \|η\| ≈ 4. The
Inter-cryostat Detector (ICD) covers the overlapping region 1.1 \< \|η\|
\< 1.4 between CC and EC.

![](media/image2.png){width="3.5194444444444444in"
height="2.6395833333333334in"}

Fig. 2. The DØ calorimeter.

The calorimeter is highly modular and finely segmented in the transverse
and longitudinal shower directions. Three distinct types of modules are
used in the CC and EC: an electromagnetic section (EM) with relatively
thin and closely spaced uranium absorber plates, a fine hadronic section
(FH) with thicker and widely spaced uranium plates and a coarse hadronic
section (CH) with thick copper or stainless steel plates. Each module
consists of a row of interleaved absorber plates and signal readout
boards, as shown in Fig. 3. The 2.3 mm gap separating adjacent absorber
plates and signal boards is filled with liquid argon as the active
medium. The signal boards are constructed by laminating a copper pad
with two separate 0.5 mm thick G10 sheets at each end. The outer
surfaces of the boards are coated with a highly resistive epoxy. An
electric field is established by grounding the absorber plates while
applying a positive potential of 2.0 kV to the resistive surfaces of the
signal boards. The electron drift time in the liquid argon gap is about
430 ns, which sets the time scale for signal charge collection. Signals
from several signal boards at approximately the same η and φ are ganged
together in depth to form a readout cell. The readout layers are
arranged into pseudo-projective towers of size 0.1 × 0.1 in ∆η × ∆φ. A
projective tower consists of 8-12 layers. To capture the profile of
electromagnetic showers, the third layer of the EM section, which
corresponds to the shower maximum, is segmented more finely transversely
into 0.05 × 0.05 in ∆η × ∆φ.

![](media/image3.png){width="2.64375in" height="1.6479166666666667in"}

Fig. 3. Schematic view of a calorimeter cell.

## The Inter-cryostat Detector

In the intermediate region of the calorimeter (1.1 \< \|η\| \< 1.4), the
rapidly changing material profile and extra un-instrumented material
lead to reduced sampling of showers and hence a degradation of energy
measurements in this region. The Inter-cryostat Detector (ICD) \[3\]
restores energy resolution by providing additional sampling in this
region, which improves the jet energy and missing energy measurements.
The ICD consists of 16 supertiles per EC-region for a total of 378
scintillator tiles (one of the supertile is only half in size and has 6
tiles), as shown in Fig. 4. The tile segmentation of 0.1 × 0.1 in ∆η ×
∆φ matches that of the calorimeter projective towers. The scintillators
are read out by photomultiplier tubes (PMT) via wavelength shifting
fibers (WLS). The original Run I PMTs are reused but moved to a region
of reduced magnetic field, with 5-6 m long clear fiber waveguides to
transport the signals from the scintillator WLS fibers. The readout
electronics and electronic calibration scheme are the same as for the
liquid argon calorimeter but with adapted electronics and pulser shapes.
LED systems are used as an alternate approach to calibrate the PMTs.

![](media/image4.png){width="3.5194444444444444in"
height="2.1368055555555556in"}

Fig. 4. (a) Schematic of the ICD tile array hung on the inner surface of
each End-cap Calorimeter cryostat, with the placement of the clear fiber
piping to the low magnetic field region. (b) Schematic of the
scintillator within one ICD tile module. Straight lines indicate the
isolation grooves. Wavelength shifting fibers are placed in the curved
grooves.

## Upgraded Calorimeter Electronics

To accommodate the Run II bunch spacing time, and to maintain the Run I
noise performance, newly designed calorimeter electronics \[4\] is
necessary. The noise is the sum of electronic, uranium and pile-up
contributions. It is necessary to optimize the relative noise
contributions and to add analog storage device on which the calorimeter
signals can be stored until a L1 trigger decision be made. This also
requires a new strategy for the baseline subtraction.

A schematic diagram of the Run II calorimeter readout chain is shown in
Fig. 5. To minimize the sensitivity to reflections, new
impedance-matched 30 Ω signal cables replaced the former 110 Ω cables
between the detector and the low-noise preamplifiers for a total of
about 55,000 channels. The replacement cable lengths are also tuned for
faster charge collection and individual channel timing equalization.

![](media/image5.png){width="3.5194444444444444in"
height="0.8847222222222222in"}

Fig. 5. Readout chain of the calorimeter in Run II.

The new charge-integrating preamplifier (preamp) is a hybrid circuit on
ceramic, similar to the Run I version. Two input low-noise jFETs in
parallel are used to compensate for the higher electronic thermal noise
due to the shorter shaping time. Output driver stages are added for
terminated signal transmission and to provide current drive and
differential output capability. To make the preamplifier output waveform
as similar as possible for all channels, fourteen (and additional one
for ICD) different species of preamplifiers are used to compensate for
various capacitances of detector cells. Forty-eight preamplifier hybrids
are socket mounted on a single preamplifier motherboard that connects to
the backplane of a preamplifier box on the top of the calorimeter
cryostat. Each box houses 96 preamplifier motherboards and are powered
by low-noise commercial switching 2 kW power supplies, which are located
in steel boxes to shield the residual magnetic field of 300 gauss.
Calibration pulses are injected to preamplifiers through 0.1% precision
resistors.

Signals from the preamplifiers are driven through 115 Ω twist and flat
cables of about 25 m in length to the Baseline Subtractor (BLS) boards
which sit in crates on the platform underneath the detector. One BLS
board contains 4×12 signal shapers, trigger pick-off and summation
circuits, 4 BLS daughtercards, and output drivers to the ADCs. One BLS
daughtercard holds 5 Switch Capacitor Array (SCA) chips, each of which
contains a 12-channel by 48-depth capacitor array to pipeline the analog
signals. Four of the SCA chips (L1 SCAs) store the signals until a L1
trigger decision can be made 4.2 μs later. The SCA is not designed for
simultaneous reading and writing, so two SCAs are used for the same
channel so that one can be read out while the other is being written.
The SCA can store analog voltages from 0-5 V in level with a precision
greater than 12 digital bits. In order to achieve a 15-bit dynamic
range, the shaped signals are passed through two different gain paths
(×1 and ×8), where the signals are simultaneously sampled and stored
into two parallel SCAs. So a total of 4 L1 analog buffers are used for
every channel. This provides a deadtimeless operation for a L1 trigger
rate up to 10 kHz. If the L1 trigger system accepts the event, a
*baseline subtraction* is performed and the result is sent to a fifth
SCA (L2 SCA) at the same depth as the L1 pipeline, where it awaits a L2
trigger decision. After the L2 trigger acceptance is received, the
signal is read out from the appropriate depth in the L2 SCA and sent to
the ADCs (Analog to Digital Converters), which digitize and
zero-suppress the signals, and then send them to the Data Acquisition
System (DAQ). The ADCs have only a 12-bit dynamic range, but a low and
high gain path for each readout channel (×1 and ×8) maintain the 15-bit
dynamic range. This matches the measured accuracy of the SCA.

The shaped signals are sampled every 132 ns, and pipelined in L1 SCAs
for storage. The timing is tuned such that the shaped output can be
sampled at its peak at about 320 ns. Because of this earlier sampling
time compared to the liquid argon drift time of 430 ns, only 2/3 of the
charge in the detector is used to form the preamplifier signal. The
preamplifier output is an integral of the detector signal characterized
by a rise time of about 430 ns and a recovery time of 15 μs. In order to
remove slowly varying offsets and the pileup of events from neighboring
bunch crossings, the pulse height sampled 396 ns earlier is used as the
baseline, and then subtracted from the current pulse height. This
process is called the baseline subtraction.

A fast trigger pickoff signal for each channel is obtained by a hard RC
differentiation of the preamplifier output by the shaper hybrid. The
calorimeter L1 trigger uses the energy summed in trigger towers of size
0.2 × 0.2 in ∆η × ∆φ by trigger summers. Coordination between the
trigger framework and the calorimeter readout is done by the new timing
and control system. It receives trigger, accelerator and clock
information through the serial command link and provides information of
the appropriate locations in the pipelines to be readout at each trigger
level. The timing and control is performed by the FPGA (Field
Programmable Gate Array) chips.

## Calorimeter Calibration System

The calorimeter calibration system \[5\] consists of twelve identical
units used for the liquid argon calorimeter and one slightly modified
unit for the ICD. Each unit is composed of one pulser control board and
its power supply, located in the BLS racks on the detector platform, and
six active Fanout boards, housed inside of the preamplifier boxes on the
cryostats. The pulser control boards are programmed via a serial bus to
a VME I/O register, which can set the amplitude and the delay of the
calibration signal and select the channels to be pulsed. The pulser
control board delivers both a DC current corresponding to the chosen
pulse height for each selected channel and a differential ECL command
signal to the Fanout boards. The pulse heights are set through a 18-bit
DAC, with a 100 mA maximum current delivered, and the delays are set
through six programmable 8-bit delay lines with a step of size about 2
ns. There are sixteen switches located on each Fanout board. On the
reception of a command, the switch converts the DC current to
calibration pulse and delivers 2 calibration signals. Each calibration
signal is sent to 4 preamplifier boards, hitting 6 preamplifiers on each
board. Thus, one pulser talks simultaneously to 48 preamplifiers.

# Calorimeter Calibration

The installation and commissioning of the new calorimeter electronics
was complete. A variety of measurements and studies have been done and
some are still ongoing for various respects of the calorimeter
calibration. Here we only give an overview of some selected online and
offline calibrations.

## Online Calibration

In the calorimeter calibration system, both the pulser control board and
the Fanout board have been tested and shown to provide a pulser signal
with deviations from linearity within 0.1%. After the final installation
all the pulse shapes have been measured to estimate the systematic
effects on the signal amplitude, the timing and the charge injected. The
purity of the liquid argon is critical to the detector performance on
energy measurement. We have measured the liquid argon impurity to be
less than 0.15 ppm, which is less than the 0.5 ppm specification.

At the beginning of the commissioning we checked the continuity of the
cables from the calorimeter cells inside the cryostat to the
preamplifiers, by injecting a step pulse and measuring the reflected
pulse from the calorimeter cells. The reflected pulse shape can be
modeled using appropriate detector capacitances and cable impedances. We
have found less than 0.05% of the cables have problems that can not be
repaired. These bad cables are randomly distributed, thus have minimal
impact on the calorimeter data. We regularly debug the entire readout
chain by checking the mean values of pedestals and the noise for every
channel, as well as the response of every channel to the injected
calibration pulses. Efforts are made to fix bad channels and maintain
the percentage of bad channels within 0.1%, which are also randomly
distributed over the entire detector and are suppressed during data
taking. Other detailed debugging includes checking the variation of the
signals stored at all depths in the pipeline of SCAs, monitoring the
stability of the pedestals over time, etc. The correlated noises within
calorimeter readout channels, as well as those introduced by other
detector systems (e.g., tracking system) are being studied.

We tuned the timing to sample the shaped calorimeter signal at its peak.
The timing measurement was done by taking 3 samples of the same signal,
with the nominal peak sample taken at the point defined by the trigger,
an early sample taken 132 ns earlier and a late one 132 ns later. By
comparing the ratios of these three samples with a simulated model, we
can accurately determine the nominal sampling position with respect to
the shaped signal peak. We have measured the channel-to-channel
variation of the nominal sampling time is less than 10 ns, which implies
less than 0.1% variation in the peak sampled value due to timing
differences.

The linearity of the calorimeter electronics and the pulser system is
verified by a comparison of the electronics response versus the pulse
height for each gain path (×1 and ×8). We have measured excellent
linearity for calorimeter energy deposits greater than 1 GeV, where the
non-linearity is found to be \< 0.3%). For smaller energies the
non-linearity is non-negligible and has been traced back to the SCAs. We
use a parameterized function to correct the non-linearity, with the
parameters adjusted for all channels. We have further compared the
linearity from two calibration runs taken one year apart, and the
agreement between them shows the stability of the electronics.

## Offline Calibration

We keep online monitoring the calorimeter data quality (energy
distribution, channel occupancy, trigger rate, noisy cells, etc.) during
the data taking. Analyzing the offline data for physics studies also
gives feedbacks on the calorimeter performance, both at the cell energy
level and at the reconstructed particle level. Such feedbacks are
especially useful on subtle problems which may not be revealed by online
calibration. Some noisy calorimeter L1 trigger towers and trigger
cabling problems have been traced and fixed. A φ inter-calibration has
been performed to calibrate the uniformity of the calorimeter response
in φ from 0 to 2π. This takes into account the variations between the
different calorimeter modules due to the inhomogeneous distributions of
non-instrumented materials in front of the cryostats.

We calibrate the calorimeter EM energy scale (for electron and photon)
from the reconstructed events of Z^0^ resonance. The absolute energy
scale is obtained by reconstructing the mass of the Z^0^ resonance from
its decay product of two electrons, and adjusting the value to the LEP
measured Z^0^ mass value (about 91 GeV/c^2^). The comparison of E/p is
obtained using the reconstructed events of W decaying into an electron
and an electron neutrino, with the electron energy measured by
calorimeter and its momentum measured by tracking system. The EM energy
scale at low energy is calibrated using J/ψ (mass around 3.1 GeV/c^2^)
and Υ (mass around 9.5 GeV/c^2^) events. QCD events are used to
calibrate the calorimeter response to the hadronic jets. The measurement
of the missing transverse energy, obtained by summing up the transverse
energies of all cells as vectors, is also a good check on the
calorimeter performance.

The calorimeter respond to jets is crucial to determine the jet energy
scale. For most measurements on jets, the response of the jet should be
known at the particle level and be corrected for all detector effects.
To determine the calorimeter response of jets, γ-jet events are selected
and the transverse energy of the jet is compared to the calibrated
energy of the photon (the calorimeter response to photon is exactly the
same as to electron). The response function obtained for a
cone-algorithm jet is shown in Fig. 6. A new jet algorithm, the energy
flow algorithm, has been recently developed. Based on the
cell-nearest-neighbor clustering, this algorithm further combines the
information from both the calorimeter and the tracking system to improve
the jet energy resolution. The current performance of the energy flow
algorithm is comparable with the standard cone algorithm on Monte Carlo
jets. Studies are going on to improve this algorithm and to apply it on
data.

![](media/image6.png){width="1.9180555555555556in"
height="1.9930555555555556in"}

Fig. 6. Response function of jets compared to photons.

# Physics Studies

Physics analyses using Run II calorimeter data have been actively
performed at DØ. Preliminary results of QCD, electroweak, B, Higgs, top
and new phenomena physics have been shown on various conferences. For
example, Fig. 7 plots the inclusive jet cross section as a function of
the jet transverse energy, with the measured values compared to the
theoretical predictions of perturbative QCD. The jet energies are
corrected using the jet energy scale obtained from Run II calorimeter
data. As another example, the cross section of Z^0^ was measured in the
di-electron decay channel. The distribution of the invariant mass of the
two electrons for Z^0^ candidates is shown in Fig. 8 (QCD background has
been subtracted). The Z^0^ mass peaks at 91 GeV/c^2^ on the plot. The
cross section times branching ratio of Z^0^ is measured to be:

σ~Z~ × Br(Z→e^+^e^−^) = 275 ± 9 (stat.) ± 9 (syst.) ± 28 (lumi.) pb

![](media/image7.png){width="2.140277777777778in"
height="2.057638888888889in"}

Fig. 7. The inclusive jet cross section as a function of the jet
transverse energy. The data are compared to the theoretical predictions
of perturbative QCD in NLO (next-to-leading order).

![](media/image8.png){width="2.5194444444444444in"
height="1.8895833333333334in"}

Fig. 8. The invariant mass distribution for Z^0^ candidate events in the
di-electron decay channel. The points show the background-subtracted
data and the histogram shows the Monte Carlo.

# Conclusion

We have finished commissioning of the new calorimeter electronics, and
performed online and offline calibrations. The new calorimeter
electronics has been performing well in data taking. Physics analyses
using the calorimeter data are undergoing and preliminary results have
been shown. We will study the calibrations in more details to better
understand the data and to improve the physics results.

# Acknowledgment

Thanks to all the colleagues in the D∅ calorimeter group.

# References

1.  R. D. Schamberger, \"The D∅ calorimeter performance and
    calibration,\" in *Proc. fifth Calorimetry in High Energy
    Physics*, H. Gordon and D. Rueger, Upton, NY: World Scientific,
    1994, pp. 39-50.

2.  The D∅ Collaboration, S. Abachi et al., \"The D∅ detector,\" *Nucl.
    Instrum. Methods Phys. Res.*, A 338, p. 185, 1994.

3.  L. Sawyer et al., \"Technical design report for the upgrade of the
    ICD for DØ Run II,\" Internal DØ-note 2686, 1997.

4.  A. Kotwal et al., \"Calorimeter electronics upgrade for Run II:
    Technical design report,\" Internal DØ-note, 1998.

5.  P. Cornebise et al., \"Run II calorimeter calibration system,\"
    Internal DØ-note 3731, 2000.

[^1]: ^^Manuscript received October 18, 2003.

    S. Fu is with the Columbia University group at the DØ Experiment,
    Fermi National Accelerator Laboratory, Batavia, IL 60510 USA
    (telephone: 630-840-8757, e-mail: shfu@fnal.gov).