# The tracking detector of the FASER experiment

Henso Abreu  Claire Antel  Akitaka Ariga  Tomoko Ariga  Florian Bernlochner
Tobias Boeckh  Jamie Boyd  Lydia Brenner  Franck Cadoux  David W. Casper
Charlotte Cavanagh  Xin Chen  Andrea Coccaro  Olivier Crespo-Lopez  Sergey
Dmitrievsky  Monica D’Onofrio  Candan Dozen  Abdallah Ezzat  Yannick Favre
Deion Fellers  Jonathan L. Feng  Didier Ferrere  Stephen Gibson  Sergio
Gonzalez-Sevilla  Yuri Gornushkin  Carl Gwilliam  Shih-Chieh Hsu  Zhen Hu
Giuseppe Iacobucci  Tomohiro Inada  Sune Jakobsen  Enrique Kajomovitz  Felix
Kling  Umut Kose  Susanne Kuehn  Helena Lefebvre  Lorne Levinson  Ke Li
Jinfeng Liu  Chiara Magliocca  Josh McFayden  Matteo Milanesio  Sam Meehan
Dimitar Mladenov  Théo Moretti  Magdalena Munker  Mitsuhiro Nakamura
Toshiyuki Nakano  Marzio Nessi  Friedemann Neuhaus  Laurie Nevay  Hidetoshi
Otono<EMAIL_ADDRESS>Carlo Pandini  Hao Pang  Lorenzo Paolozzi
Brian Petersen  Francesco Pietropaolo  Markus Prim  Michaela Queitsch-Maitland
Filippo Resnati  Chiara Rizzi  Hiroki Rokujo  Elisa Ruiz-Choliz  Jakob
Salfeld-Nebgen  Osamu Sato  Paola Scampoli  Kristof Schmieden  Matthias Schott
Anna Sfyrla  Savannah Shively  John Spencer  Yosuke Takubo  Noshin Tarannum
Ondrej Theiner  Eric Torrence  Serhan Tufanli  Svetlana Vasina  Benedikt
Vormwald  Di Wang  Department of Physics and Astronomy, Technion—Israel
Institute of Technology, Haifa 32000, Israel  Département de Physique
Nucléaire et Corpusculaire, University of Geneva, CH-1211 Geneva 4,
Switzerland  Albert Einstein Center for Fundamental Physics, Laboratory for
High Energy Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern,
Switzerland  Kyushu University, Nishi-ku, 819-0395 Fukuoka, Japan  Universität
Bonn, Regina-Pacis-Weg 3, D-53113 Bonn, Germany  CERN, CH-1211 Geneva 23,
Switzerland  Department of Physics and Astronomy, University of California,
Irvine, CA 92697-4575, USA  University of Liverpool, Liverpool L69 3BX, United
Kingdom  Department of Physics, Tsinghua University, Beijing, China  INFN
Sezione di Genova, Via Dodecaneso, 33–16146, Genova, Italy  Joint Institute
for Nuclear Research, Dubna, Russia  Faculté de Physique et Ingénierie,
University of Strasbourg, 3-5 rue de l’Université, Strasbourg, France, 67000
University of Oregon, Eugene, OR 97403, USA  Royal Holloway, University of
London, Egham, TW20 0EX, UK  Department of Physics, University of Washington,
PO Box 351560, Seattle, WA 98195-1560, USA  Deutsches Elektronen-Synchrotron
DESY, Notkestrasse 85, 22607 Hamburg, Germany  Department of Particle Physics
and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
Department of Physics & Astronomy, University of Sussex, Sussex House, Falmer,
Brighton, BN1 9RH, United Kingdom  Nagoya University, Furo-cho, Chikusa-ku,
Nagoya 464-8602, Japan  Institut für Physik, Universität Mainz, Mainz, Germany
Institute of Particle and Nuclear Study, KEK, Oho 1-1, Tsukuba, Ibaraki
305-0801, Japan  Department of Physics, Chiba University, 1-33 Yayoi-cho
Inage-ku, Chiba, 263-8522, Japan  Dipartimento di Fisica “Ettore Pancini”,
Università di Napoli Federico II, Complesso Universitario di Monte S. Angelo,
I-80126 Napoli, Italy

###### Abstract

FASER is a new experiment designed to search for new light weakly-interacting
long-lived particles (LLPs) and study high-energy neutrino interactions in the
very forward region of the LHC collisions at CERN. The experimental apparatus
is situated 480 m downstream of the ATLAS interaction-point aligned with the
beam collision axis. The FASER detector includes four identical tracker
stations constructed from silicon microstrip detectors. Three of the tracker
stations form a tracking spectrometer, and enable FASER to detect the decay
products of LLPs decaying inside the apparatus, whereas the fourth station is
used for the neutrino analysis. The spectrometer has been installed in the LHC
complex since March 2021, while the fourth station is not yet installed. FASER
will start physics data taking when the LHC resumes operation in early 2022.
This paper describes the design, construction and testing of the tracking
spectrometer, including the associated components such as the mechanics,
readout electronics, power supplies and cooling system.

###### keywords:

Silicon microstrip detectors, Tracking detectors, FASER, LHC

††journal: Nucl. Instrum. Meth. Phys. Res. Sect. A

©2021 CERN for the benefit of the FASER Collaboration. Reproduction of this
article or parts of it is allowed as specified in the CC-BY-4.0 license.

##  1 Introduction

FASER (the ForwArd Seach ExpeRiment) was proposed in 2017 [1] to search for
new particles at the LHC at CERN. The experiment is placed in the unused
service tunnel, TI12, situated 480 m downstream from the ATLAS interaction
point on the beam collision axis. TI12 was formerly used for connecting the
Large Electron-Positron (LEP) collider to the Super Proton Synchrotron (SPS).
Excavation of the floor of TI12 provides a 5.5 m-long and up to 1.5 m-wide
trench centred on the beam collision axis, where the FASER detector is
installed. This location enables FASER to search for new weakly-interacting
long-lived particles (LLPs) with masses in the MeV to GeV range, produced from
the copious inelastic $pp$ scattering events at the LHC. The detector design
utilized the benchmark physics process of a dark photon $A^{\prime}$ decaying
into a pair of oppositely charged particles, e.g, $A^{\prime}\rightarrow
e^{+}e^{-}$. Along with this process, FASER is also sensitive to several other
new physics models presented in [2] which predict LLPs that travel to and
decay in the FASER detector into pairs of Standard Model particles including
photons. In addition to the LLP searches, the FASER location allows to study
neutrino interactions of all flavors in an uncharted energy region [3]
111Notably, a 29-kg pilot emulsion detector was temporarily installed in this
location during LHC running in 2018, which has been used to observe the first
neutrino interaction candidates at a collider [4].. The experiment location
greatly reduces background particles entering FASER from the LHC collisions,
since such particles would have to traverse the LHC magnets, absorbers and 90
m of rock.

The FASER detector for LLP searches was installed in Spring 2021 and is
currently being commissioned towards data taking during LHC operation in
2022-24 (Run 3). As a decay volume for LLP searches, the detector has a 1.5
m-long and 0.55 T dipole magnet with a 10 cm radius aperture to separate the
pair of charged particles arising from the LLP decay. Due to the limited space
and accessibility to TI12, a permanent magnet with a Halbach array design is
used. A veto scintillator station and a timing scintillator station are placed
upstream and downstream of this to ensure that the LLPs decay inside the decay
volume. Just behind the timing scintillator station, three tracker stations
and two 1 m long magnets are alternately installed to form a tracking
spectrometer allowing to measure the position and momentum of charged
particles. These 1 m-long magnets have the same design as the 1.5 m-long decay
volume magnet except for the length. Since the LLPs produced in the forward
direction of the LHC collisions have energy at the TeV-scale, the pair of
charged particles produced in their decay is highly collimated. In order to
resolve these particles the tracker stations must have a position resolution
of better than $\mathcal{O}$($100\text{\,}\mathrm{\SIUnitSymbolMicro m}$). For
example, for an LLP with mass $m=100~{}\textrm{MeV}$ and energy
$E=2~{}\textrm{TeV}$ decaying inside the decay volume, the separation between
the decay products at the first tracker station can be
$\mathcal{O}$($200\text{\,}\mathrm{\SIUnitSymbolMicro m}$). The tracker
stations consist of silicon microstrip detectors with a pitch of
$80\text{\,}\mathrm{\SIUnitSymbolMicro m}$, which fully cover the aperture of
the magnets; this paper will focus on details of these tracking detectors. The
most downstream detector is a pre-shower scintillator station and an
electromagnetic calorimeter with a depth of 2 and 25 radiation lengths
($\text{X}_{\text{0}}$), respectively, which will allow to discriminate
electrons from muons and to measure the electromagnetic energy. A trigger
signal for data taking is provided from the three types of scintillator
stations and the calorimeter. Based on FLUKA [5, 6] simulations and in situ
measurements [7, 8], a trigger rate of around 500 Hz is expected for a
luminosity of $2\times 10^{34}$cm-2s-1 222This is the expected peak luminosity
during LHC Run 3.. This rate is dominated by high-energy muons penetrating
through the 90 m of rock to get to FASER. Details of the trigger system are
described in [9].

To allow neutrino measurements at FASER, the detector will be augmented with
an additional veto scintillator station, a 1.1-ton tungsten/emulsion detector
and an interface tracker. These elements will be installed in front of the
FASER detector (described above) in late 2021. The interface tracker will
enable tracks from a neutrino interaction in the emulsion detector to be
matched to events in the tracker stations. This will enable FASER to measure
separately the muon neutrino and muon anti-neutrino interaction cross sections
by using the charge of the muon reconstructed in the FASER spectrometer. The
interface tracker will also improve the background rejection and energy
reconstruction for the neutrino analysis. The interface tracker has an
identical design to the tracker stations used in the FASER spectrometer. FASER
will therefore have four tracker stations in total, but this paper focuses on
the three tracker stations used in the FASER spectrometer and already
installed in TI12.

The layout of this paper is as follows. Section 2 presents the mechanical
design of the tracker stations including details of the silicon strip
detectors and on-detector electronics. The power supply, cooling, and data
acquisition systems are outlined in Section 3. The interlock and detector
control systems are summarized in Section 4. All components are integrated and
have been commissioned, as described in Section 5. Finally, conclusions and
outlook are given in Section 6.

##  2 Tracker stations

The FASER tracker consists of 72 double-sided silicon microstrip modules
arranged in three stations, each station being composed of three planes with
eight modules per plane. Given the short timescale to build the entire
tracker, spare modules of the ATLAS Semiconductor Tracker (SCT) have been
used. The SCT is one of the three sub-systems composing the Inner Detector
[10], the internal tracker of the ATLAS experiment at the LHC [11].

###  2.1 Silicon microstrip modules

The SCT barrel module [12] consists of four identical single-sided silicon
microstrip detectors glued by pairs on the two sides of a central baseboard,
and of a copper/polymide flex hybrid carrying the readout electronics. Each
sensor has $p^{+}$ strip implants on a high-resistivity
($>$4\text{\,}\mathrm{k\SIUnitSymbolOhm}\text{$\cdot$}\mathrm{cm}$$)
$285\text{\,}\mathrm{\SIUnitSymbolMicro m}$-thick $n$-type substrate. The
strip implants are AC-coupled to aluminum readout strips via a silicon dioxide
layer. The High-Voltage (HV) is applied via a $n$-type metalized implant
covering the back-plane and the strip implants are biased through polysilicon
resistors. Each sensor has a rectangular nominal geometry of $64\times
63.6~{}${\mathrm{mm}}^{2}$$, with 768 readout strips at a constant pitch of
$80\text{\,}\mathrm{\SIUnitSymbolMicro m}$. A guard-ring structure surrounding
the sensor active area is used to prevent electrical breakdown at large bias
voltages. The readout of 128 strip channels is done by means of the Atlas
Binary ABCD3TA ASIC [13], which comprises the front-end (FE) electronics, a
pipeline, buffer, readout and control logic. For each channel, the analog FE
consists of a preamplifier, shaper and discriminator. The signal delivered by
the preamplifier-shaper circuit has a peaking time of
$25\text{\,}\mathrm{ns}$, enough to ensure a discriminator timewalk of less
than $16\text{\,}\mathrm{ns}$ and a double-pulse resolution below
$50\text{\,}\mathrm{ns}$ as required for ATLAS operations. Since the
discriminator threshold remains common to all channels within the chip, the
effective threshold of a given channel corresponds to the offset of its
discriminator with respect to the common threshold. A per-channel threshold
correction 4-bit DAC allows to compensate for the spread of these threshold
offsets. The pipeline, clocked at 40 MHz rate and with a depth of 132 cells,
buffers the binary data of all channels for
$3.3\text{\,}\mathrm{\SIUnitSymbolMicro s}$. Upon reception of a trigger the
data from the last three cells are transferred to a second-level derandomizing
buffer where data compression is finally applied.

Figure 1: Photograph of a SCT barrel strip module.

Figure 1 shows a SCT barrel module held inside an aluminum test-box. On each
side of the module the sensors are bonded edge-to-edge to create $\sim
12.8\text{\,}\mathrm{cm}$-long readout strips. The total number of readout
channels in the module is 1536 (768 per side). A $40\text{\,}\mathrm{mrad}$
stereo angle between the front and back pairs of sensors allows the module to
provide a spatial resolution of $\sim 17\text{\,}\mathrm{\SIUnitSymbolMicro
m}$ in the precision coordinate (perpendicular to the strips) and $\sim
580\text{\,}\mathrm{\SIUnitSymbolMicro m}$ in the non-precision coordinate
(parallel to the strips) [12]. The flex hybrid, equipped with six ABCD3TA
chips per side, is bridged over the sensors via a carbon-carbon substrate. Two
NTC thermistors (one per hybrid side) allow to monitor the module temperature.
The baseboard, made of Thermal Pyrolytic Graphite (TPG) with an excellent in-
plane thermal conductivity and low radiation length, provides the mechanical
support to the sensors and allows to dissipate the heat generated by the FE
electronics. The hybrid is attached to beryllia (BeO) facing plates located on
the two ends of the TPG baseboard.

The strip modules used for the FASER tracker have been selected among the
existing spares of the SCT barrel module production that was completed in
2004. Since then, the modules were stored at CERN in individual sealed bags
within a controlled environment. Some of these spare modules failed the
initial quality assurance by the SCT collaboration, typically because there
were more than 1% defective channels, high leakage current (above
$4\text{\,}\mathrm{\SIUnitSymbolMicro A}$) or early breakdown before
$500\text{\,}\mathrm{V}$ (the maximum voltage expected after 10 years of
operation in ATLAS). For FASER, modules with the lowest number of defective
channels have been chosen. Given the much lower radiation levels expected in
FASER compared to ATLAS333with a total ionizing dose less than $5\times
10^{-3}$ Gy per year and a total fluence less than $5\times 10^{7}$ 1-MeV-
neutrons equivalent $\rm{n_{eq}/cm^{2}}$ per year as estimated from
simulations with FLUKA and as confirmed with in-situ measurements in the TI12
tunnel [8], the nominal operating voltage for the SCT modules is set to
$150\text{\,}\mathrm{V}$. This bias voltage is large enough to ensure full
depletion of the sensors, and it is not expected to be increased during the
lifetime of the experiment due to the lack of cumulated radiation damage. In
all cases, IV curves have been taken at different stages of the tracker
assembly and a good behaviour has been required at up to
$150\text{\,}\mathrm{V}$ (more details are given in Section 5).

###  2.2 Tracker plane

Figure 2: Photograph of a tracker plane with all eight SCT modules installed.
The beam axis is perpendicular to the plane.

A tracker plane consists of an aluminum frame (AW-5083) holding eight SCT
barrel modules as shown in Fig. 2. The modules are arranged by four on each
side (front and back) of the frame to accommodate the flex pigtails (see
Section 2.3). The distance between modules (closest sensors along the out-of-
plane direction) is $2.4\text{\,}\mathrm{mm}$, and the active area overlap
(in-plane, along the strip-length) is $2\text{\,}\mathrm{mm}$. The frame has a
size of
$$320\text{\,}\mathrm{mm}$\times$320\text{\,}\mathrm{mm}$\times$31.5\text{\,}\mathrm{mm}$$.
In order to minimize the material in front of the silicon detectors, the frame
is cut-out for most of the active area within the acceptance of the magnet
aperture, see Fig. 3. More details about the material distribution in the
active area of a tracker station is given in Section 2.4.

Figure 3: CAD view of a frame. The circle represents the
$200\text{\,}\mathrm{mm}$-diameter magnet aperture.

The aluminium frames are CNC (Computer Numerical Control) machined (Fig. 4a)
and then individually surveyed to check the quality and precision of the
production process. The SCT modules are positioned in the frame by using two
$1.8\text{\,}\mathrm{mm}$-diameter PEEK 1000 pins (Fig. 4b). One locating pin
provides the global precise positioning while a second pin (to be inserted
into the slotted washer of the SCT module) is used for the module orientation.
The PEEK material allows for a smooth and easy handling of the modules during
insertion-removal operations. The modules are then pressed down to the frame
by means of four small aluminum clamps fixed by four stainless steel M2 screws
(Fig. 4c). A $5\text{\,}\mathrm{mm}$-diameter inner cooling channel,
integrated by design into the frame (Fig. 4d), provides the thermal path to
extract by direct conduction through the water flow the heat generated by the
FE electronics. A good thermal contact between the SCT modules and the
aluminum frame is provided by the application of heat conducting thermal paste
(Electrolube HTCP-20S) at the contact surface between the frame and the bottom
side of the BeO facings. Finally, the frame also contains an inlet for dry air
to be flushed, to keep a low relative humidity inside.

(a)  (b)

(c)  (d)

Figure 4: (a) Bare aluminum frame, (b) exploded CAD view of a SCT module while
being positioned, (c) photograph of a module after its final fixation onto the
frame using clamps and (d) mid-plane cross section showing the inner cooling
channel. In (d), the arrows indicate the direction of the water flow. Figure
5: Metrology of a fully assembled tracker plane.

The metrology of each plane (Fig. 5) combines measurements performed with a
mechanical touch-probe and an optical camera. Each frame is equipped with four
stainless steel targets (one in each corner) used to define the plane
reference coordinate system. The targets are visible from both sides, allowing
to correlate measurements done on each side of the plane separately. The
silicon sensors of the SCT modules include a set of fiducial marks used for
the mechanical alignment during module assembly. Some of the fiducial marks
are measured in 3D with respect to the plane reference system. All assembled
planes were measured at the University of Geneva using a Mitutoyo CRYSTA-Apex
S CNC coordinate measuring machine with automatic probe changer. The precision
of the metrology machine for in-plane and out-of-plane measurements is
$5\text{\,}\mathrm{\SIUnitSymbolMicro m}$ and 10-15
$\mathrm{\SIUnitSymbolMicro m}$, respectively. It is found that all frames are
within the required tolerances ($\pm$20\text{\,}\mathrm{\SIUnitSymbolMicro
m}$$) with respect to the CAD manufacturing drawings. A maximum deviation of
$100\text{\,}\mathrm{\SIUnitSymbolMicro m}$ is found for the positioning of
the SCT modules with respect to the CAD model (corresponding to a perfect
alignment). This maximum deviation accounts for the combined effect of
machining tolerances, SCT modules positioning errors and assembly precisions.

###  2.3 On-detector electronics

(a)

(b)

Figure 6: The pigtails (a) and patch-panel (b).

A flexible printed circuit board (called the ”pigtail”) has been developed to
route the electric lines of the SCT module to the outside (Fig. 6a). Four
pigtails (one per module) at one side of the tracker plane are connected to a
single patch-panel (Fig. 6b). The connectors on the patch-panel are mounted on
the same side while the side of the module is arranged alternatively in the
tracker plane as shown in 7. For that reason, two types of the pigtail were
developed, arranging the lines in the opposite order. The size of the pigtail
is 6 cm $\times$ 15 cm with $150\text{\,}\mathrm{\SIUnitSymbolMicro m}$
thickness. The connector444SFMC-120-L3-S-D manufactured by SAMTEC, Inc on the
module side is oriented with 91.15 degrees on the pigtail to connect with the
hybrids on the module as shown in Fig. 7.

Figure 7: Schematic view of the tracker plane. One pigtail is connected to
each hybrid on the SCT module. Note that adjacent SCT modules are mounted on
opposite sides of the FASER module frame. The four pigtails in one side are
connected to one patch-panel.

The tracker is electrically connected to the Data Acquisition (DAQ) and
powering systems. The patch-panel (Fig. 6b) was developed as the interface
between them. A patch-panel is placed at each side of the tracker plane and
fixed on the aluminum frame. The size of the patch-panel is 10 cm $\times$ 30
cm with 1.6 mm thickness. The Tracker Readout Board (TRB, see Section 3.3) in
the DAQ system is connected to two patch-panels via eight twinax Firefly
cables555ECUE-08-300-T2-FF-01-1 manufactured by SAMTEC, Inc, i.e. one cable
per SCT module. The cable provides all LVDS lines necessary for operation and
readout of the ABCD3TA chips on the module. LVDS repeaters are mounted for all
LVDS lines on the patch-panel to transmit signals along 3 m of the cable. In
addition, LVDS receivers are placed to receive the operation signals for the
ABCD3TA chips from the TRB. HV and Low-Voltage (LV) as well as their return
lines are provided from a HV splitter board and a LV protection board to bias
the sensors and power the ABCD3TA chips, respectively (see Section 3). Every
patch-panel is connected with the HV splitter board, biasing the sensors on
the four modules with one HV channel. LV for the analog and digital circuits
in the ABCD3TA chips is provided to each module separately. The patch-panel
works as the interface for the NTCs on the modules and temperature and
humidity sensors on the plane frame with the Tracker Interlock and Monitoring
Board (TIM, see Section 4.2) which is used for the environmental monitoring.
In addition, there are 5 V power lines on the patch-panel for the LVDS
repeaters and receivers as well as a line to transfer an interlock signal
generated by the TIM to the LV power supply.

###  2.4 Tracker stations

(a)  (b)

Figure 8: (a) Exploded CAD view of a tracker station and (b) photograph of a
fully assembled station. The black cover on top of the station is a carbon-
fibre plate. The cooling loops of each plane are connected together, so that
per station there is only one inlet and one outlet for the cooling fluid.

A tracker station is an assembly of three planes (including the patch-panels)
as shown in Fig. 8. Within the station the three planes are staggered along
the vertical direction, with a relative shift of the middle (last) plane of
$+$5\text{\,}\mathrm{mm}$$ ($-$5\text{\,}\mathrm{mm}$$) with respect to the
first plane. This configuration ensures at least two 3D reconstructed space-
points for a track crossing the station. Each station is flushed with dry air
to reduce the relative humidity ensuring safe operating conditions against dew
point. The patch-panels fit within the $32\text{\,}\mathrm{mm}$ thickness of
one plane. All aluminium parts are post-treated with a trivalent chromium
passivation666SURTEC-650 coating to prevent any corrosion by the corona effect
that may occur after putting the frames in contact during the station
assembly. In addition, an O-ring sealing joint between frames provides a good
tightness to keep the humidity inside the station as low as possible
(typically about 5%). The station is connected only after the metrology and
full commissioning of each individual plane is completed. The inter-plane
alignment is done via two high-precision pins located in
$5\text{\,}\mathrm{mm}$-diameter H7 holes while the fixation between two
adjacent planes is done via four M5 screws. The two end covers that close the
station volume are made of $400\text{\,}\mathrm{\SIUnitSymbolMicro m}$-thick
carbon-fibre plates (standard T300 fibers). Without cables, the total weight
of one station is about $15\text{\,}\mathrm{kg}$.

(a)  (b)

Figure 9: (a) FEA simulations on a complete station and (b) close up view on a
single module. The maximum temperature estimated on the module is
$28\text{\,}\mathrm{\SIUnitSymbolCelsius}$ (on the readout electronics). See
text for additional details on the different simulation parameters.

Various Finite Element Analyses (FEA) simulations have been carried out to
check the thermal performance of the system throughout the evolution of the
frame design. Over-heating of the SCT modules can cause problems in the
mechanical integrity and alignment of the SCT modules due to the glass-
transition of glues. The glue 777ARALDITE 2011 manufactured by Huntsman
Corporation: https://samaro.fr/pdf/FT/Araldite_FT_2011_EN.pdf used for the SCT
module assembly should have the lowest glass-transition temperature around
$35\text{\,}\mathrm{\SIUnitSymbolCelsius}$, which should be the absolute
maximum temperature for the SCT modules. The safety scheme to ensure this is
described in Section 4.

Figure 9 shows the FEA of a complete station. The different simulation
parameters have been set to match the testing conditions during the plane and
station commissioning at CERN. In particular, the coolant temperature has been
fixed at $15\text{\,}\mathrm{\SIUnitSymbolCelsius}$, the water flow at
$3\text{\,}\mathrm{l}\text{$\cdot$}{\mathrm{min}}^{-1}$ (for a heat transfer
coefficient of water of
$500\text{\,}\mathrm{W}\text{$\cdot$}{\mathrm{m}}^{-2}$), and the outside air
convection at $23\text{\,}\mathrm{\SIUnitSymbolCelsius}$. The FEA gives a
maximum temperature on the FE chips of
$\sim$28\text{\,}\mathrm{\SIUnitSymbolCelsius}$$, neglecting the temperature
rise within the water channel due to the heat load (estimated to be
+$0.6\text{\,}\mathrm{\SIUnitSymbolCelsius}$ for
$3\text{\,}\mathrm{l}\text{$\cdot$}{\mathrm{min}}^{-1}$). The FEA results are
in good agreement (within 2-$3\text{\,}\mathrm{\SIUnitSymbolCelsius}$) with
the measurements taken during the commissioning at CERN. Given the good
modeling of data measurements by the FEA, the temperatures on the silicon
sensors are estimated to be in a comfortable range between 21 and
$23\text{\,}\mathrm{\SIUnitSymbolCelsius}$.

Figure 10: Side (left) and front (right) views of a tracker station. The
circle represents the $200\text{\,}\mathrm{mm}$-diameter magnet aperture. The
dot and star markers respectively correspond to the reference positions for
material calculation in the central and edge regions.

Figure 10 shows the front and side CAD views of a station. Two different
reference positions have been considered to estimate the material distribution
along the station depth ($Z$-axis, perpendicular to the station front-plane):
a central region, close to the geometrical center of the plane, and an edge
region. Table 1 summarizes the material budget in each case. The central
region corresponds to a particle traversing the least amount of material
inside the station, i.e. six silicon sensors and two carbon-fibre covers that
account for a total of 2.1% $\text{X}_{\text{0}}$. The edge region is a worst-
case position that corresponds to a particle traversing the six SCT modules
(including sensors, TPG baseboard, flex hybrid with carbon-carbon bridge and
readout ASICS), aluminum frames and station covers, accounting for a total of
21.5% $\text{X}_{\text{0}}$. For a benchmark dark photon model
(m${}_{A^{\prime}}$=100 MeV, $\epsilon$ = 10-5) for dark photons that decay in
the FASER magnet apperture, 70% will be in the low material central region of
the tracker. Given the range of particle momentum expected in the experiment
the contribution of multiple scattering from the traversed material is
expected to be negligible.

Component | Material | Number |  $X_{0}$ (%)
---|---|---|---
|  | / station | Central region | Edge region
Silicon sensor | Si | 6 | 1.8% | 1.8%
Station Covers | CFRP | 2 | 0.3% | 0.3%
SCT module support | TPG | 3 | - | 0.6%
C-C Hybrid | C (based) | 3 | - | 2.2%
ABCD3TA chips | Si | 3 | - | 6.5%
Layer frame | Al | 3 | - | 10.1%
Total / station | - | - | 2.1% | 21.5%
Table 1: Amount of material in $X_{0}$ in the active area of a tracker station
for two regions: i) the central region with only the silicon sensor material
and ii) the edge region. Details of the material in the SCT module are given
in Table 8 of Ref. [12]. The numbers are calculated directly from the CAD
description of the tracker station.

The three tracker stations are mounted into the FASER detector with an
aluminum structure (AW-5083) called the backbone as shown in Fig. 11a. Each
station is fixed to the backbone via an aluminum support interface (Fig. 11b)
with six M6 screws and two locating pins. The backbone has been CNC machined
in one go for what concerns its key parts (stations interface areas, pin
holes) to optimize the final precision. It has been assembled at the
University of Geneva and surveyed by the CERN group to check its overall
flatness, parallelism and straightness. The survey results show a
$0.13\text{\,}\mathrm{mm}$ flatness over the three station interfaces, each
one being $0.04\text{\,}\mathrm{mm}$ without load and
$0.10\text{\,}\mathrm{mm}$ when loaded with the station weight.

(a)  (b)

Figure 11: (a) CAD model of the backbone with three stations attached and (b)
detail of the station interface.

The backbone is the primary global mechanical structure of the tracker as it
links together the three stations. It allowed for an easy handling and
transportation (Fig. 12a), and serves as a reference structure for the tracker
alignment. The backbone is then supported by the first and second magnet
cylinders via clamps (Figs. 12b and 12c).

(a)  (b)

(c)

Figure 12: (a) Transport of the backbone above the LHC, (b) CAD view of the
FASER tracker and (c) photograph of the tracker after installation onto the
detector in March 2021.

##  3 Powering, cooling and readout system

Since access to the FASER location is guaranteed only every two or three
months during technical stops of the LHC, it is important that the system is
robust. The FASER tracker powering and cooling systems are based on commercial
products, and the readout system is based on a custom general purpose I/O
(GPIO) board with a dedicated interface card. Peripheral equipment is designed
exclusively for the FASER tracking detector.

###  3.1 Power supply system

In order to provide HV and LV to the SCT modules described in Section 2.1,
three 19-inch rack mountable crates called the MPOD LV/HV Power Supply
System888MRAAH2500A2H manufactured by W-IE-NE-R Power Electronics GmbH. are
installed in TI12, which host 3 HV modules999EHS 84 05p manufactured by Iseg
Spezialelektronik GmbH. and 18 LV modules101010MPV 8008I manufactured by W-IE-
NE-R Power Electronics GmbH. in total. For other electronics, one 19-inch rack
mountable box (PSbox) is also installed, which holds fifteen 24V power
supplies 111111TXL 035-24S manufactured by Traco Power.

Three types of printed circuit boards, the HV splitter board, the LV
protection board and the 24V/6V board, were developed. These are mounted on
the detector, directly above the tracker stations. The HV splitter board
divides one HV channel into the four to supply the SCT modules connected to
one patch-panel, corresponding to half a tracker plane. The LV protection
board equips an integrated circuit121212LTC4365 manufactured by ANALOGUE
DEVICE which protects against over-voltage possibly induced by radiation in
TI12. The 24V/6V board is powered by the PSbox, and it is used to distribute
24V to the TIM, described in Section 4.2 and 6V to the patch-panel.

###  3.2 Cooling system

The cooling system was designed and built by the CERN cooling and ventilation
group (EN-CV). It consists of two air-cooled water
chillers131313HRS030-AF-20MT manufactured by SMC Corporation: one circulating
chilled water through the tracker stations and the second acting as a hot
spare.

As shown in Fig. 13, the cooling system is mounted on a single frame together
with all instrumentation. An additional water reservoir is also installed,
which makes it possible to refill the water tank inside the chiller in an
automatic manner. The cooling capability of each chiller is about 1.8 kW at a
$None$C water outlet temperature with $\Delta T=$$\textrm{C}$ between inlet
and outlet temperature. Since one SCT module consumes 6 W, corresponding to an
overall power consumption of 450 W for the full tracker, the cooling system is
therefore sufficient to regulate the temperatures of the tracker stations.

In case of a failure of the chiller in use for the tracker stations, the
cooling circuit is re-routed by controlling valves to the spare chiller to
take over the cooling. Since in normal conditions both chillers are running in
the normal state, one connected to the detector and the other in bypass mode,
this swap can seamlessly take place without any impact on the operation of the
tracker stations. If both chillers are not operating correctly, the power
supply system is forced to be turned off without software intervention
(Section 4.3).

A dry air system with a dew point of $None$C coming from compressed air
supplied from LHC point 8 is used to flush the tracker stations to avoid
condensation on the electronic due to the cooling. In case of lack of dry air
an alarm is triggered and the cooling and detector will be stopped.

Figure 13: Photograph of the cooling system.

###  3.3 Data acquisition system

The Tracker Readout Board (TRB) is used for operation and readout of the SCT
modules in the FASER tracker. It consists of a GPIO board and an adapter card
as shown in Fig. 14. The GPIO board was developed as a general readout board
centered around a CYCLONE V A7 FPGA (Field Programmable Gate Array). The FPGA
is driven either by a 40 MHz oscillator on the GPIO board or by an external
clock via LVDS input through dedicated two-pins LEMO connectors. The GPIO
board is operated with an input voltage of 24 V and provides 5.0, 3.3 and 2.5
V to various active devices either on the GPIO board or to the adapter board.
The adapter card is directly attached to the GPIO board and acts as an
interface to the patch-panel and the Trigger Logic Board (TLB) [9]. The TLB,
also a GPIO board, is the central trigger board of the FASER trigger and data
acquisition system, which subsequently provides a global trigger accept signal
(L1A).

Figure 14: TRB consisting of GPIO board and adapter card.

For standalone operation during qualification of the tracker planes, the TRB
can work independently without any external control and clock signals. On the
other hand, in combined operation in the FASER detector, the TRB is controlled
by the TLB which provides the 40 MHz clock used by the SCT modules, Level-1
trigger and bunch counter reset signals. A busy signal is also sent by the TRB
to the TLB to hold the trigger during SCT data readout. The TRBs are housed on
the detector in a custom-made minicrate, and the minicrate backplane provides
24 V power as well as the TLB signals for the GPIO boards. The TRB is
connected to the TLB via RJ45 connector located on the rear side of the
minicrate backplane.

Figure 15: TRBs installed in the custom-made minicrate.

One TRB reads out eight SCT modules corresponding to one tracker plane.
Therefore, a total of nine TRBs (Fig. 15) are used for the three tracker
stations in the FASER spectrometer. In addition, three further TRBs are used
in a separate minicrate for the interface tracker. The TRB adapter card is
connected to the tracker plane patch-panels via eight Samtec twinax Firefly
cables, one for every module. Each Firefly cable provides eight LVDS lines
used for two clock, two command and two data signals as well as one reset and
one select signal. The GPIO board communicates with a DAQ PC, situated at the
surface, via a 1 Gbps Ethernet link.

The TRB FPGA logic is defined by the TRB firmware, and controlled by the DAQ
software running on the surface PC. The software is written in C++ code and
manages all operational procedures such as the calibration sequence as
described Section 5.4 [9]. The firmware is designed to send low level protocol
commands to the ABCD3TA chips on the SCT modules and read out data from them.
The high level protocol commands are provided from the PC to the GPIO board,
and then the corresponding operation signals are sent to the chips. The data
from the chips are stored into First-In First-Out (FIFO) memories in the FPGA
and sent to the PC. The software decodes the bit-stream of data from the SCT
modules. The event and BCID (Beam Crossing Identifier) counters are
implemented in the firmware to ensure synchnorization between all readout
elements (the chips, different TRBs and TLB). The software in the central DAQ
system can send a command to reset the chips as well as the counters and FIFOs
in case desynchronization is detected. In addition, if an error is detected in
the firmware, it provides error information to the DAQ PC, for example, in the
case that the L1 trigger is received while hit data remain in the FIFOs.

The firmware contains four circuit blocks to set the readout timing. Two
blocks are used to adjust the clock phase of the 40 MHz input clock with 390
ps steps and 25 ns range, with each four modules on the patch-panel connected
to one of these blocks. The other two blocks are used for the circuits in the
firmware to latch correctly the output data signals from the modules and
compensate for the propagation delay of the cables and the latency in the
modules.

The typical data size per plane is expected to be $\sim 212$ Bytes/event. At
500 Hz trigger rate in the FASER tracker, that corresponds to a required
readout bandwidth of at least 106 kBytes/s (848 kbps), or an occupancy of just
0.04% of the 1 Gbps Ethernet link. On the other hand, the calibration scans
lead to an occupancy of typically 25% of a module, corresponding to 8.5 kBytes
of data per plane per L1A trigger.

##  4 Interlock and Detector control system

The aim of the safety system of the FASER tracker is to protect the delicate
silicon tracker modules from damage under all circumstances. The FASER tracker
follows hereby the common approach of a multi-level protections system
consisting of high-level software-based detector monitoring (DCS: Detector
Control System) and a low-level hardware-based interlock system. The software
system is capable of triggering automatic actions that can turn off individual
detector components in a controlled way while the hardware interlock system
turns off power supplies immediately and acts therefore as the last level of
safety.

###  4.1 Safety scheme

Generally, there are two sources of danger against which the detector has to
be protected. One is electrical failures which can cause persistent damage on
the detector electronics and silicon sensor due to over-voltage or over-
current. Over-voltage and over-current are defined as
$V_{\mathrm{nominal}}+200$ mV [13] and $I_{\mathrm{nominal,max}}+200$ mA,
respectively, where $V_{\mathrm{nominal}}$ is the nominal operation voltage
and $I_{\mathrm{nominal,max}}$ is the maximum current with
$V_{\mathrm{nominal}}$. The other is over-heating caused by a failure of the
cooling system. As discussed in Section 2.4, the temperature of the SCT module
is required to be less than $35\text{\,}\mathrm{\SIUnitSymbolCelsius}$.

Figure 16: Schematic overview of the FASER safety and interlock system.

Figure 16 gives an overview of the protection system of the FASER tracker. In
the upper left corner, one of the two patch-panels of a tracker plane is
sketched. Each patch-panel serves four silicon strip modules and each module
is equipped with two temperature sensors (NTC-10k thermistor) that are used
for temperature monitoring close to the readout part of the tracker module. In
addition, each patch-panel also connects to one temperature sensor (NTC-10k
thermistor) that is thermally attached to the mechanical frame inside of the
plane as the frame makes the thermal contact to the silicon strip modules
themselves. Finally, there is one sensor (HIH-4000) per plane which measures
the humidity inside of the tracker plane.

All these sensors are read out by the TIM (Tracker Interlock and Monitoring
Board). One TIM can receive the signals from a complete tracker station (three
planes, six patch-panels). All sensor values are digitized and provided to the
software DCS system via an Ethernet connection. Additionally, the frame
temperatures of each plane are used as input to an analog comparator circuit
which generates an interlock signal for the relevant LV and HV power supplies.
The granularity of the temperature interlock can be changed from only a single
tracker plane up to the full station by configuring six hardware switches on
the TIM.

The interlock signal for HV is provided to the HV power supplies (one per
station) in a separate cable, while the interlock signal for the LV power
supplies (one per patch-panel) is propagated to the LV power supply through a
dedicated signal line in the LV cable via the patch-panel.

In order to prevent the detector modules from electrical damage, each power
supply has an independent supervision module that can turn off a channel as
soon as it leaves the defined operation range. As an extra level of
protection, an additional ASIC for over-voltage protection is placed in the LV
power path.

The final component in the interlock system is the MPOD Interlock (MPOD-I)
which receives the Detector Safety Signal (DSS) from the cooling plant and
provides a crate-wide interlock to the MPOD crates in case of a cooling plant
failure.

In the following sections the specific hardware components of the tracker
safety system are described in more detail.

###  4.2 Tracker Interlock and Monitoring

Figure 17 shows a picture of TIM as well as its corresponding block diagram.
The core of the TIM is the AM335X micro-controller as well as three
independent comparator based interlock circuits.

Figure 17: TIM board and corresponding block diagram (signal path shown for
two patch-panels).

The TIM can connect to up to six tracker patch-panels via DB25 connectors each
connector carrying the signal from one frame temperature sensor, $4\times 2$
module temperature sensors, and one humidity sensor. There is one additional
DB9 input that allows to connect up to 2 additional environmental temperature
sensors. The latter is currently not used in the FASER trackers setup, but
leaves flexibility for further monitoring in the future.

All the mentioned sensors are routed to the microprocessor for digitization
and monitoring. In addition, the frame temperature sensors of patch-panels
that belong to the same tracker plane enter into one of the three comparator-
based circuits in which a hardware interlock signal is generated if the analog
signal of those sensors lies outside of the valid range. The range itself can
be adjusted with potentiometers. Eventually, two interlock signals are
generated for the relevant power supply channels per comparator circuit: One
for the LV power supplies and the other for the HV power supplies. The
interlock is released at the power supply side in both cases when a logical
high level is applied. In case of the LV interlock signal the logical high
level amounts to $9\,\mathrm{V}$ while for the HV interlock signal
$5\,\mathrm{V}$ is used. Additional hardware switches on the TIM allow to
combine (or exclude) individual outputs of the three comparator circuits into
a single interlock signal in case of the HV interlock signal.

Finally, the TIM features several interfaces for higher level communication:
an Ethernet port for the communication with the DCS as well as several serial
interfaces for debugging and optional connection to other devices.

###  4.3 MPOD Interlock Board

The MPOD Interlock Board (MPOD-I) represents the interface between the DSS
signal of the cooling plant and the FASER interlock system. The board and its
corresponding block diagram are depicted in Fig. 18. The cooling plant PLC
controls a loop which is physically interrupted in case of a cooling plant
failure (DSS signal). The MPOD-I checks this loop constantly and translates
the DSS signal into a voltage interlock signal which is distributed to up to
four MPOD crates simultaneously. It is important to note that this interlock
signal, in contrast to the interlock signals described earlier, is a crate-
wide signal causing the shutdown of all the power supply modules which are
part of the crate directly via the crate controller. A high logical signal
($5\,\mathrm{V}$) needs to be actively supplied in order to release the
interlock at the controller side.

Figure 18: MPOD-I board and corresponding block diagram.

###  4.4 The DCS system

The DCS system is the central location where all monitoring data from the TIM
boards as well as from the power supplies come together and is stored
persistently into a database. Additionally, it provides high level controls of
all the power supplies. The software is, therefore, capable of executing
automatic actions in case the detector is leaving the operation parameter
space indicated by one of the many available sensors. Due to the significantly
larger number of sensor readings in the DCS system compared to the interlock
system, a more sophisticated warning and protection scheme can be implemented
at the cost of the dependence on a software process. Details of the DCS system
is described in [9].

###  4.5 Limits for DCS automatic actions and hardware interlock

Table 2 shows the thresholds values of temperature and humidity used for the
operation of the tracker safety system. The thresholds for the automatic
actions as well as the hardware interlock need to be chosen carefully such
that the software-based automatic actions will step in before the actual
hardware interlock in case of any abnormal temperature increase.

sensor | DCS warning | DCS automatic action | hardware interlock
---|---|---|---
module temperature | $>30.0\,^{\circ}\mathrm{C}$ | $>31.0\,^{\circ}\mathrm{C}$ | -
plane humidity | $>10\%$ | - | -
frame temperature | $>23.0\,^{\circ}\mathrm{C}$ | - | $>25.0\,^{\circ}\mathrm{C}$
Table 2: Thresholds of temperature and humidity to activate the different
protection mechanism in the FASER tracker safety system.

In order to verify these limits, a potentially destructive test was performed
on a prototype tracker plane. All protection mechanisms were disabled, the
cooling was stopped, and the plane remained fully powered. Figure 19 shows the
thermal evolution measured by a representative frame and module temperature
sensor. The condition for the hardware interlock
($T_{\mathrm{frame}}>25.0\,^{\circ}\mathrm{C}$) was reached after about 9
minutes. The corresponding module temperature at that moment was measured to
be $T_{\mathrm{module}}>32.0\,^{\circ}\mathrm{C}$ and was therefore well below
the required $35.0\,^{\circ}\mathrm{C}$. The threshold for the automatic DCS
actions are adjusted in such a way that they are sufficiently far away from
the normal operation point, but low enough in order still trigger before the
hardware interlock under normal circumstances.

Figure 19: Thermal behaviour of frame and module in case of a cooling stop. A
special single plane was used for this test and all protection mechanisms were
deactivated.

##  5 Tests during construction and commissioning of the FASER tracker

The FASER tracker elements were tested at each stage of the construction. The
electrical performance and behavior of the silicon sensors as a function of
the applied bias voltage were investigated for the single SCT modules,
individual tracker planes and full tracker stations on the surface before
installation of the tracker stations into the FASER experimental site. In
addition, metrology was performed for the layers and stations. After the
installation, the performance was also tested using cosmic rays and random
triggers deploying the central DAQ system of the FASER experiment. In this
section, the test setup, procedure and results of the thermal and electrical
tests are described.

###  5.1 Test setups during testing on surface

Three test setups were installed to perform the required measurements at the
different stages, namely tests of a single module, plane and station.

The single module test was done with the readout system developed at Cambridge
University [14]. It was used to evaluate the electrical performance of a
single module. A chiller was used to cool down an environmental enclosed box
to keep the module below 30∘C. The number of functional strips, the noise
value and dependency on applied bias voltage (high-voltage behaviour) of the
silicon sensors were verified and compared to the results in the module
production of the ATLAS SCT detector [12].

The individual planes and stations were qualified on surface in test-stands
which used equipment later used in the FASER experiment. Both the interlock
and monitoring of the temperature, voltage and current were handled by the TIM
unit (see Section 4.2). This allowed the DCS information to be monitored live
and be archived into a database for evaluating the detector performance. The
same powering and DAQ systems, cables and calibration software as used for the
experiment were used during the testing on surface. This allows to compare the
electrical performance at different stages of the assembly and installation as
well as from different data taking periods in the tunnel. The surface
commissioning with the components used in the real experiment made it possible
to test their operation and investigate the long term behaviour early on. The
operation of the different tracker planes and stations was conducted with
specific finite state machines which took the modified detector mapping on the
surface compared to the one in the tunnel into account. Figure 20 shows a
picture of the test setup with its main components for station commissioning.

Figure 20: The test setup for the station commissioning on surface.

###  5.2 Test procedure

The procedures of the plane and station testing were consistent to allow for
comparisons of the collected data. After installation of the plane or station
under test into the test setup, the thermal and electrical measurements were
conducted. The thermal measurements comprised of measurements of the frame NTC
and SCT module NTCs without powering the SCT module, after powering and after
configuring the ABCD3TA chips.

During the electrical characterization, the leakage current of the sensors was
measured. The LV and its current consumed were also investigated after
powering up and configuring the chips. Further important parameters measured
in the electrical calibration sequence are the number of non-functional
channels, so-called masked channels, the noise, the gain and the noise
occupancy of each module after trimming the threshold offset of the ABCD3TA
chips as explained in Section 5.4. The planes and stations were powered for at
least 24 hours to investigate their long-term stability and behaviour of the
leakage current in the silicon sensors.

The results were compared between those in the test of the individual modules,
plane and station commissioning. The plane and station were signed-off,
considering a list of the required parameters such as thermal stability, less
than 0.5% masked strips, noise and gain values comparable to the single module
testing. After installing all three stations into the FASER experiment in
TI12, the same thermal and electrical tests were conducted and the test
results were compared to the surface commissioning data. A general agreement
was found as shown in more detail below.

###  5.3 Results of thermal tests

Figure 21 shows the temperature of the two NTC sensors on each module in one
plane of one station, which was measured during surface commissioning of the
first station. The temperature values are listed for power-off (green), after
powering the modules (orange) and after powering and configuring the chips
(blue). The values without powering correspond to the coolant temperature of
15∘C. The maximum values of around 27∘C are reached after powering and
configuring the modules. The module temperatures vary slightly in a pattern
along the flow of the cooling loop. It leads to the behaviour that typically
the modules in the corners of the plane (modules 0, 3, 4, and 7) are the
coldest ones.

Figure 21: Thermal test results of one plane in the first station during
surface commissioning. Temperature values of the SCT module NTCs for power-off
(green), after powering the modules (orange) and after powering and
configuring (blue).

All the temperature values are well below 35∘C which is the required maximum
temperature of the module. The difference was within 1∘C, compared to the
results in the plane commissioning. A similar thermal performance was obtained
for the other two stations and after installation into TI12.

Figure 22: Temperature values of NTCs on SCT modules of plane 5 during a scan
sequence for 24 hours in plane commissioning.

As an example of the temperature stability of the tracker planes, Figure 22
shows the temperature measured with one of two NTC sensors on each module
(NTC1 and NTC2) in plane 5 during a period of 24 hours covering one
calibration sequence. The analogue and digital currents consumed by the
ABCD3TA chips fluctuate during the calibration, causing a slight variation of
the module temperature, however this never exceeds 35∘C. When the module is
powered off, the temperature decreases to the coolant temperature of 15∘C. It
was confirmed that the temperature was stable during the nominal operation. In
addition, the dew point inside the planes was kept well below the coolant
temperature during the commissioning.

###  5.4 Calibration procedure

The readout of the SCT barrel modules is binary, meaning there is only a hit
or no-hit information depending on whether the current pulse generated by a
charged particle passing through the silicon sensor is above or below the
threshold set. Since the analog information of the signal pulse is not
recorded, a good calibration of the ABCD3TA chips on the SCT module is
essential. For this purpose, the chip contains an internal calibration circuit
that enables to simulate a hit in a strip. Each channel of the readout ASIC
has a $100\text{\,}\mathrm{fC}$ calibration capacitor connected to its input.
An internal 8-bit DAC and chopper circuit are used to generate a voltage pulse
that is sent to the calibration capacitor, thus by-passing the sensor strips.
The amplitude of the calibration pulse can be set in the range from 0 to
$160\text{\,}\mathrm{mV}$ (corresponding to input charges up to
$16\text{\,}\mathrm{fC}$), and its delay with respect to the clock phase can
be adjusted in 64 steps within 50 ns. Every fourth channel can be tested
simultaneously.

Figure 23: Illustration of a threshold scan, in which a certain number of
calibration charges are injected at each threshold point. The input signal is
convoluted with electronics noise, described by a Gaussian distribution
function. For a given threshold $\tau$, the hit occupancy is the fraction of
signals above threshold (dashed area). The resulting s-curve, essentially a
complementary error function, corresponds to the probability $p(\tau)$ that
the signal is above the threshold $\tau$.

The calibration of the chips mostly relies on so-called threshold scans (see
Fig. 23), in which the discriminator threshold is varied in discrete steps and
a set of well defined fixed-amplitude calibration charges is sent at each
step. The hit occupancy, defined as the fraction of injected signals above
threshold, is computed for each readout channel at each threshold point. Since
the signal amplitude is convoluted with Gaussian electronics noise, the hit
occupancy does not follow an ideal step function but is smeared out to give
rise to the so-called s-curve. The threshold at which the occupancy is 50% is
called the $vt_{\text{50}}$ point and corresponds to the input charge
amplitude. The width of the distribution is the noise at the discriminator
output. The calibration procedure implemented for the FASER tracker largely
follows that established by the ATLAS SCT collaboration [15], which has been
extensively used during past years to perform the electrical characterization
of the SCT modules. Typically, several tests / scans are run in sequence, and
the chip parameters are updated along the sequence. A typical calibration
sequence is listed below.

  1. 1.

Mask-scan. This test aims at determining two sorts of main defects, i) dead /
non-responsive channels, and ii) very noisy channels. This is achieved by
setting a very low (high) threshold, then sending a number of trigger signals
(without charge injection) to read the data stored in the digital pipelines,
and finally checking how many channels are below (above) threshold. The
identified defective channels are disabled by tagging them in a dedicated in-
chip mask register.

  2. 2.

StrobeDelay scan. This scan is used to determine for each chip the optimum
delay between the calibration charge and the clock. The calibration delay is
varied in its full range and at each step the occupancy per channel is
computed after charge injection. Then, the occupancies are projected to the
calibration delay axis and a fit is carried out. The optimum delay is
calculated to be in the plateau of maximum occupancy.

  3. 3.

Three-Point-Gain. This test allows to estimate the gain and noise of the
preamplification stage. Threshold scans are performed for three different
input charges. A linear fit of the $vt_{\text{50}}$ thresholds versus injected
charge is carried out to obtain the gain (slope) and threshold offset
(intercept). The input noise is calculated as the standard deviation of the
s-curve fits (corresponding to the noise at the discriminator output) divided
by the gain. This scan is also used to obtain a conversion for the measured
thresholds in mV to fC or electrons, respectively.

  4. 4.

Trimming. This scan aims at correcting for the threshold dispersion inside the
chip. A 4-bit resolution DAC (TrimDac) allows to set individual channel
threshold corrections. The TrimDac has four possible ranges, as the ABCD3TA
chip was conceived to operate in the LHC environment in which the threshold
spread increases with time due to accumulated radiation damage to the sensors
and electronics. In the trimming procedure threshold scans are performed for
various settings of the TrimDac (ranges and step values). Given the excellent
linearity of the DAC, four out of the sixteen possible steps per range are
used. Using a linear fit to the $vt_{\text{50}}$ as a function of trim value,
a channel is flagged as “trimmable” if its offset can be corrected with
respect to a given threshold target. The trimming settings are those that
correspond to the minimum TrimDac range and the minimum threshold target
(within that range) for which a maximum number of channels are trimmable. This
is done on a module-by-module basis to achieve a good threshold uniformity
across all channels of a given module.

  5. 5.

ResponseCurve. With this test an accurate threshold-to-charge relation is
obtained. Threshold scans are performed for ten input charges, from
$0.5\text{\,}\mathrm{fC}$ to $8\text{\,}\mathrm{fC}$. After the corresponding
s-curve fits, the $vt_{\text{50}}$ as a function of the input charge is fit to
a first-degree polynomial with an exponential correction term to account for
small non-linearities at very low and high injected charges,
$\tau=p_{2}+p_{0}/(1+e^{-q/p_{1}})$, where $p_{0}$, $p_{1}$ and $p_{2}$ are
the fit parameters, and $\tau$ and $q$ are the threshold in mV and fC,
respectively. The average parameters for each individual ASIC are obtained to
determine the mV to fC conversion.

  6. 6.

NoiseOccupancy. Although this test is formally not part of the chip
calibration (as no calibration parameters are derived from it), it is
typically performed to assess the goodness of the above-mentioned procedure.
The noise occupancy (NO) is defined as the probability for a strip to give
rise to a hit only due to noise. This typically occurs when fluctuations at
the discriminator input exceed its threshold. For the ATLAS SCT modules the
noise occupancy per strip is specified to be less than $5\times 10^{-4}$ at
$1\text{\,}\mathrm{fC}$ threshold and the nominal operating temperature. The
NO is determined by performing a threshold scan without any input charge. The
number of triggers sent is increased progressively as the threshold is raised.

###  5.5 Results of electrical and calibration tests

The calibration scans described in Section 5.4 were repeated during individual
plane commissioning, station commissioning, and commissioning _in situ_ after
installation in TI12 as described in Section 5.1. The aim of the calibration
is to achieve uniform threshold distribution, high hit efficiency ($>99\%$)
and low noise occupancy ($<5\times 10^{-4}$) at the nominal operating
threshold of 1 fC. In this sub-section, electrical calibration results are
shown and compared between single module testing, plane testing and station
testing - unless otherwise stated the station test results shown were obtained
after the installation in TI12 during March/April 2021, however they are
consistent with the station results obtained on the surface.

The leakage current of the silicon sensors was measured as a function of the
applied bias voltage in the tests of a single module, plane and station. The
curves are generally in fair agreement between the different stages. Figure 24
shows a comparison of the leakage current measured during single module
commissioning and the plane commissioning. The individual module leakage
currents are typically in the range $100$–$800$ nA at $15^{\circ}$C. Since in
the plane four modules are connected to one HV channel, the comparison is made
by summing the individual module leakage current measurements for the four
modules in a half a plane (either Module 0-3 or 4-7) (scaled to 15 ∘C) and
comparing to the measurement in the plane itself (channel 0 or channel 1).
Similar agreement was also found when comparing results of plane and station
commissioning.

Figure 24: Comparison of leakage current measurements during single module
testing (Sum of Modules 0-3/4-7 in green/red) and plane commissioning (Plane
Channel 0/1 in blue/orange).

In order to achieve a high hit efficiency, the number of ‘bad’ strips – those
identified as either dead or very noisy during the mask scan – in each module
was carefully monitored during the tests for a single module and the plane and
station commissioning. The modules with lower numbers of bad strips were
selected to be mounted at one of the four inner module positions which are
inside the central region of the magnet acceptance, as demonstrated in Figure
10. In addition, the station with the highest quality – that with lowest
number of the bad strips in this region – was chosen to be placed at the front
(upstream) of the spectrometer while the station with the lowest quality is
located at the back (downstream). A pair of an electron and positron from the
decay of a dark photon is most collimated at the upstream station, therefore,
the station with the best performance was selected for that position. Table 3
shows the number of masked channels observed during the _in situ_
commissioning after the installation. In the inner region of the magnet
acceptance the number of masked channels is below 0.1%. Even in the region
mostly outside the magnet acceptance, this is less than 0.5%.

Station | Inner region | Outer region
---|---|---
Station 1 (upstream) | 0.02% | 0.06%
Station 2 (middle) | 0.03% | 0.04%
Station 3 (downstream) | 0.08% | 0.31%
Table 3: Fraction of the masked channels in each tracker layer for the four
modules in the inner (outer) region of the magnet acceptance.

The gain measured in the three-point-gain measurements are shown in Fig. 25a.
The average over all the chips in all stations is 54 mV/fC, which is in good
agreement with the $\sim 55$ mV/fC expected from the module specification
[12]. The dependence of the gain over a larger range of injected charges is
tested in the response curve scan. Figure 25b shows the typical result of the
response curve scan for a single strip.

After the trim scan, an even response between different channels in a module
is obtained, as shown in Fig. 26. The majority ($99.9\%$) of the channels can
be trimmed using the two lowest trim range settings, as expected for
unirradiated modules, and an additional $0.05\%$ of channels can be trimmed
using one of the larger trim ranges. The remaining channels cannot be trimmed
even at the largest trim range setting.

(a)  (b)

Figure 25: (a) Distributions of the gain per chip in each station. (b) Example
response curve fit for a single strip.

(a)  (b)

(c)  (d)

Figure 26: Threshold scan for an example module with 1.5 fC injected charge
before (a), and after (b) optimal trim settings from the trim scan are
applied. The distributions of the $vt_{\text{50}}$ points for all strips are
also shown before (c), and after (d) the trim scan.

It is important to keep the level of readout noise as low as possible to
maintain low thresholds and accordingly realize high tracking efficiency. An
estimate of the input Equivalent Noise Charge (ENC) is obtained, using the
three-point-gain measurement during the routine calibration scans described in
Section 5.4. This provides a measurement of the output noise value with an
injection charge of 2 fC. The distribution of ENC for all the chips in the
three stations are shown in Fig. 27a, whose mean value is 1515 electrons. This
is in good agreement with the $\sim 1500$ electrons expected for unirradiated
modules from the specifications [12].

The additional dedicated noise occupancy scans were also performed for a more
direct measurement of the noise, where no charge is injected, as shown in Fig.
27b. Due to the large number of triggers required at high thresholds these
take a considerably longer time than the three-point-gain measurement. The
noise occupancy scans are, however, more sensitive to the tails of the noise
distribution and to external sources such as a commonly increased noise on
several channels (common-mode noise). The ENC from the noise occupancy scans
is extracted from a linear fit to log (noise occupancy) vs threshold2 in fC2.

The ENC evaluated from the noise occupancy scan and the three-point-gain
measurement correlate well as shown in Fig. 28a, and are in good agreement
with measurements of the ATLAS SCT modules [12]. Fig. 28b shows the noise
occupancy at the nominal 1 fC thresholds using randomly triggered events
during combined system runs _in situ_ , which provides a further cross-check
of the measured noise. Over 99.7% of the the strips in the tracker satisfy the
performance criteria that the noise occupancy per channel is less than
$5\times 10^{-4}$ at the nominal 1 fC threshold. The main reason of noisy
strips is a non-linear behavior in the response curve illustrated in Fig. 25b.

(a)  (b)

Figure 27: (a) ENC per chip in each tracker station, and (b) noise occupancy
scan for a single chip.

(a)  (b)

Figure 28: Correlation between (a) ENC measured in the three-point-gain and in
the noise occupancy scans, and (b) strip-by-strip noise occupancy measured
from random triggers during combined runs.

There is the trend of a slight reduction in noise (for the same gain) in
station commissioning compared to the plane commissioning step. This could be
caused by an improved grounding scheme in the station.

###  5.6 Magnetic field test

One of the concerns for the installation of the FASER tracker in between the
gaps of the permanent FASER magnets was the presence of the stray magnetic
field during the lowering of the tracker planes. In order to exclude any
negative impact on the performance or damage due to electromagnetic induction
a test stand was set up that allowed to lower the FASER prototype tracker
plane vertically in a controlled way at different distances to the aperture of
one of the FASER magnets for testing purposes.

In a first step, the tracker plane was positioned $1\,\mathrm{m}$ away from
the aperture (reference position) and a standard calibration and
characterization sequence was run, i.e. the identification of noisy and non-
responsive strips as well as the measurement of gain, threshold, electronics
noise, and the sensor IV response. At the reference position the magnetic
stray field is negligible. In a second stage, the mechanical mounting
structure was placed in a distance of 65 mm from the magnet aperture which
corresponds roughly to the distance of the first tracker plane in the final
FASER assembly. At this location, the magnetic field strength of the stray
field amounts to up to $60\,\mathrm{mT}$. The plane was lowered at a very low
speed of about $0.03\,\mathrm{m}/\mathrm{s}$ until it was finally centered in
front of the magnet aperture. During the lowering process all cables (data,
power, DCS) remained attached to the patch-panel of the plane, but were
disconnected at the off-detector end. A second set of performance measurements
was taken at this position. In a final step, a further reference measurement
was taken at the reference position after the plane has been extracted in the
reverse order with respect to the installation.

The number of noisy and non-responsive strips was found to be very consistent
between the different measurement positions. The same conclusion applies to
the measured gain as well as thresholds. Only for the noise, changes in the
order of a few percent could be observed between the different measurement
positions, but no correlation with respect to the presence of the magnetic
field was observed. It is more likely that the observed changes can be
attributed to small differences in the grounding after the mechanical movement
of the setup.

In conclusion, no sign for any damage or performance degradation could be
found in this installation tests.

###  5.7 Cosmic ray test

In order to test the FASER tracker station as a full detector system including
trigger and DAQ, a cosmic ray test stand has was set up in the FASER surface
laboratory. Figure 20 shows the setup in which the tracker station is placed
horizontally in between two large trigger scintillators covering the full
detector acceptance.

After successfully timing in the tracker with respect to the trigger,
correlated hits could be reconstructed across all tracker layers. Figure 29
depicts an example event display of a cosmic ray candidate traversing all
three tracker layers leaving a clear signature. The reconstructed space points
from the crossing of two strips per layer are well compatible with a straight
track through the detector.

Beyond the bare proof of the tracker functionality and its interplay with the
larger FASER trigger/DAQ system, the collected cosmics data set is very
valuable for the intra-station alignment due to the absence of any stray
magnetic field which will not be the case once integrated in between the FASER
permanent magnets. The successful cosmic ray test concluded the tracker
station commissioning on the surface.

Figure 29: Event display of a cosmic ray candidate traversing all three layers
of one FASER tracker station.

###  5.8 Summary of commissioning

Three stations consisting of nine planes were successfully assembled and
commissioned on the surface and subsequently _in situ_ after installation.
Overall a very good performance was found, which is comparable between surface
plane commissioning and station commissioning and in the tunnel. The long term
stability of full stations were additionally confirmed in long runs which
lasted several weeks and was partially accompanied with data taking of cosmic
ray events.

##  6 Conclusion and outlook

The FASER tracker was designed to fulfill the requirements needed to search
for light new long-lived particles, produced in LHC collisions at the ATLAS
interaction point, and decaying to two oppositely charged particles inside the
FASER decay volume. The decay products would be high energy and extremely
collimated, and the tracker must be able to resolve the two closely spaced
charged particles down to $\mathcal{O}$(100 $\mu$m). Other constraints driving
the tracker design were the need to minimize the required services, the need
for a robust design, and to be able to construct and install the tracker in a
short timescale (during LHC long shutdown 2) and at a low cost.

The FASER spectrometer was constructed with three tracker stations, comprising
of nine planes of the silicon strip modules that were originally the spares
for the ATLAS SCT barrel detector. All other parts of the tracker, including
dedicated support frames, DCS and cooling system, readout electronics and
services were newly developed for FASER. After construction and assembly,
metrology of the tracker, to ensure a precise alignment within the mechanical
tolerances, was performed for each plane and station. The design of the system
was carried out during 2018, prototype components were produced and tested
during 2019, and the final system fabricated and commissioned on the surface
during 2020. The installation of the tracker into the TI12 tunnel was carried
out in March 2021.

The installed detector is fully operational and shows excellent performance,
well within the specifications. The number of dead channels is less than 0.5%
(including the region outside the main magnet acceptance). The electrical
performance of the detector was tested after calibration of the ABCD3TA
readout chips at each stage of the construction. During the _in situ_
commissioning of the final FASER setup, the average noise (ENC) was evaluated
to be 1515 electrons, and the noise hit occupancy at 1 fC threshold was
$2.3\times 10^{-5}$ which is within the requirement of $<5\times 10^{-4}$.
Long-term operation as part of the commissioning showed that all the SCT
modules can be kept below the required maximum temperature of 35 ∘C with
coolant temperature of 15 ∘C. In addition to standalone tests, the tracking
detector was tested as part of the full FASER detector commissioning using the
final TDAQ system, for example for combined cosmic ray data taking.

A new tracker station, identical to the three already installed in FASER, and
called the “Interface Tracker”, will be installed between the FASER$\nu$
emulsion detector and the main FASER detector. This Interface Tracker will
allow matching tracks between the emulsion detector and the FASER tracker, and
enable to distinguish $\nu_{\mu}$ and $\overline{\nu}_{\mu}$ interactions by
measuring the charge of the produced muon in the FASER spectrometer. The
Interface Tracker construction and surface commissioning started in June in
2021, and the installation is planned for late-2021.

The FASER experiment will start physics data-taking in proton-proton
collisions from the start of LHC Run 3 operations in 2022. The tracker will
act as one of the crucial detectors to allow to search for new light long-
lived neutral particles in FASER.

## Acknowledgement

We thank the technical and administrative staff members at all FASER
institutions, including CERN, for their contributions to the success of the
FASER effort. We are grateful to the ATLAS SCT Collaboration for donating
spare SCT modules to FASER, and to the LHCb Collaboration for the loan of the
calorimeter modules. We thank Steve Wotton and Floris Keizer for providing
their single module readout system, which enabled us to evaluate the spare SCT
modules. We gratefully acknowledge input from Steve McMahon for his careful
reading of the FASER ”Letter Of Intent” and his input on the initial design of
the FASER tracking detector, and from Dave Robinson, Bruce Gallop and Nobu
Unno for their very useful advice when we faced unexpected behavior of the SCT
modules. In addition, we thank the CERN cooling and ventilation group (EN-CV)
for their work related to the FASER cooling system. This work is supported in
part by Heising-Simons Foundation Grant Nos. 2018-1135, 2019-1179, 2020-1840,
Simons Foundation Grant No. 623683, U.S. National Science Foundation Grant
Nos. PHY-2111427, PHY-2110929, and PHY-2110648, JSPS KAKENHI Grants Nos.
JP19H01909, JP20K23373, JP20H01919, JP20K04004, and JP21H00082, and by the
Swiss National Science Foundation.

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