<table><caption>"Technical" Start Date of Facility (This means, where the dates are not known, the earliest technically feasible start date is indicated - such that detector R&D readiness is not the delaying factor)</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Vertex Detector<sup>3)</sup></th><th rowspan="2">MAPS Planar/3D/Passive CMOS LGADs</th><th rowspan="2"></th><th colspan="10">&lt; 2030</th><th colspan="5">2030-2035</th><th colspan="3">2035 - 2040</th><th colspan="3">2040-2045</th><th colspan="3">&gt; 2045</th></tr><tr><th>X/X<sub>0</sub> (%/layer)</th><th>Power (mW/cm<sup>2</sup>)</th><th>Rates (GHz/cm<sup>2</sup>)</th><th>Wafers area (*)<sup>4)</sup></th><th>Timing precision σ<sub>h</sub> (ns)<sup>5)</sup></th><th>DROT 3.2</th><th>DROT 3.3</th><th>DROT 3.4</th><th>ALICE</th><th>ALICE 3</th><th>LHCb (≥L5A)<sup>1)</sup></th><th>ATLAS/CMS (≥ L5A)<sup>1)</sup></th><th>EIC</th><th>LHeC</th><th>ILC<sup>2)</sup></th><th>FCC-ee</th><th>CLIC<sup>2)</sup></th><th>FCC-th</th><th>FCC-eh</th><th>Muon Collider</th></tr><tr><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th></tr></thead><tbody><tr><td rowspan="2">Tracker<sup>6)</sup></td><td rowspan="2">MAPS Planar/3D/Passive CMOS LGADs</td><td rowspan="2"></td><td>X/X<sub>0</sub> (%/layer)</td><td>≤ 0.1</td><td>≤ 0.5</td><td>≤ 0.5</td><td>≤ 0.1</td><td>≤ 0.05</td><td>≤ 0.05</td><td>≤ 1</td><td></td><td>≤ 0.05</td><td>≤ 0.1</td><td>≤ 0.05</td><td>≤ 0.05</td><td>≤ 0.2</td><td>≤ 0.1</td><td>≤ 0.1</td><td>≤ 0.2</td><td></td><td></td><td></td></tr><tr><td>Power (mW/cm<sup>2</sup>)</td><td>≤ 0.1</td><td>≤ 0.1</td><td>≤ 0.1</td><td>≤ 0.1</td><td>≤ 0.1</td><td>≤ 0.6</td><td>≤ 0.1</td><td>≤ 0.1</td><td>≤ 0.1</td><td>≤ 0.05</td><td>≤ 0.05</td><td>≤ 0.5</td><td>≤ 30</td><td>≤ 0.1</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Calimeter<sup>7)</sup></td><td rowspan="2">MAPS Planar/3D/Passive CMOS LGADs</td><td rowspan="2"></td><td>Rates (GHz/cm<sup>2</sup>)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Wafers area (*)<sup>4)</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Time of Flight<sup>8)</sup></td><td rowspan="2">MAPS Planar/3D/Passive CMOS LGADs</td><td rowspan="2"></td><td>Radiation tolerance NIEL (x 10<sup>16</sup> neo/cm<sup>2</sup>)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Radiation tolerance TID (Grad)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">DROT 3.3</td><td rowspan="2">DROT 3.3</td><td rowspan="2">DROT 3.3</td><td>Timing precision σ<sub>h</sub> (ns)<sup>5)</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Radiation tolerance NIEL (x 10<sup>16</sup> neo/cm<sup>2</sup>)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">DROT 3.3</td><td rowspan="2">DROT 3.3</td><td rowspan="2">DROT 3.3</td><td>Radiation tolerance TID (Grad)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Timing precision σ<sub>h</sub> (ns)<sup>5)</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">DROT 3.3</td><td rowspan="2">DROT 3.3</td><td rowspan="2">DROT 3.3</td><td>Radiation tolerance NIEL (x 10<sup>16</sup> neo/cm<sup>2</sup>)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Radiation tolerance TID (Grad)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Values are indicative of performance targets and of operating conditions relevant to R&D. The latter are reported for the regions most exposed to radiation. Empty cells indicate either that projects are not concerned; or that specifications are already met or not yet fully established, for instance power consumption depends strongly on granularity and digital features that would be finally implemented.

1) LS3/LS4 are scheduled to start in 2025/2031 at the time of this document.

2) Reported rates are within the bunch trains.

3) LHCb/ATLAS/CMS consider Planar/3D sensors at the time of this document for rates and radiation tolerance. Pixelated LGADs could already be considered for NA62/Klever and for longer term Vertex Detectors for timing precision for high timing precision.

4) The size of wafers achievable can depend on technology (industrial process) with a general trend to benefits from larger areas.

5) Ultrafast timing ≤ 100 ps could be differently achieved by the various technologies.

6) In trackers, coarser longitudinal granularities could be considered for MAPSs. Thorough performance and cost comparison with Passive CMOS would be needed. Pixelated LGADs could be considered for potentially higher timing precision.

7) Two options exist for calorimetry: pads O[1] mm pitch with analog readout (applying to all technologies) and particle counting digital with MAPSs O[50] µm. LGADs could be considered for potentially higher timing precision. DRT 3.1 apply w/o the X/X<sub>0</sub> constraint. DRT 3.4 could achieve higher compactness and be needed for the digital options to integrate full readout within the sensor area.

8) TOF, as compared to 4D-tracking, concerns dedicated layers for very high pile-up, beam induced background or particle identification with highest possible precision. Timing performance of sensors w/o amplification (MAPS, planar/3D/CMOS passive CMOS) is subject to R&D, while LGADs w/ amplification are at this stage expected to potentially provide higher precision. DRT 3.1 and DRT 3.4 of Vertex Detector and Tracker apply with less stringent requirement.

Figure 3.2: This matrix complements Figure 3.1 by showing, in the same format, the required values of the quantities listed on the vertical axis as a function of time (and facility).

The choice to show the evolution of requirements with time reflects that the technologies discussed in the R&D themes are often valid alternatives for these (depending on which combination of properties is most needed) and in the longer term the distinctions between these different pixel approaches will become blurred as 3D-integration allows
more sophisticated monolithic solutions.

## 3.3 Key technologies

### 3.3.1 CMOS sensors: MAPS and passive CMOS

Silicon sensors manufactured using mainstream CMOS imaging technologies are referred to as MAPS (Monolithic Active Pixel Sensors) [Ch3-1], [Ch3-2], [Ch3-3]. The technology provides very small pitches yielding the best position resolution achieved so far. Furthermore, since the signal readout circuit is integrated into the sensitive element, they minimise multiple scattering, leading to further position and momentum resolution improvements. The radiation tolerance, timing, and efficiency of CMOS MAPS can be improved by applying the same principles that have guided standard sensors' designs in other technologies to achieve faster charge collection.

Recent developments have concentrated on two distinct process implementations for charged particle tracking: (i) MAPS designed with large collection electrodes, which provides the highest radiation tolerance and more uniform sensor timing but exhibits large input capacitance. (ii) MAPS with small collection electrodes yielding reduced capacitance at the amplifier inputs and, therefore, giving lower input noise and potentially faster signals. Traditionally, MAPS have been used in low radiation environments; however, recently, they have been further developed to cope with significant radiation levels through novel device engineering and processing methods. These improvements allow operating the sensors, regardless of large or small electrode designs, fully depleted and with optimised field configurations inside the sensing volume, which is key to reaching timing resolutions below 100 ps and radiation tolerance up to $1-2 \times 10^{15} \text{ neq cm}^{-2}$.

Smaller technology nodes (below the current 100-180 nm) in more advanced technologies or special imaging sensor features will reduce even further the pixel pitch and allow the implementation of digital functionalities necessary to sustain the high particle rates required by several applications. Moreover, with decreasing feature size, increased signal on small collection electrodes will enable precision timing. The lower power consumption will minimise the cooling needs and, therefore, the material in the tracking system. The radiation tolerance is also likely to improve substantially.

Stitching techniques must be developed to provide large area CMOS MAPS sensors, which are vital in minimising a system's material budget and building large area trackers. Large size sensors will require dedicated studies of the readout architecture for power distribution and to reduce consumption in the data transfer over substantially longer paths than achieved so far.

The thickness of the MAPS is the ultimate limit to the device's scattering material, and new designs must allow novel advances in post-processing techniques. Enabling technologies include post-processing techniques based on standard industrial processes adapted or optimised to specific needs. For example, thinning and dicing require optimisation, especially for very thin MAPS, to achieve the maximum sensitive area and implement stress relief to reduce damage.

In summary, to fulfil several and increasingly demanding constraints, the R&D on
MAPS will combine multiple strands that can build on common features, either gradually
in time or in parallel, linking also with the further developments in microelectronics
discussed in Chapter 7:

* MAPS, in sub-micron node(s) for smaller pixels pitch and stitching process for large area sensors to reach ultimate precision and radiation length in vertex detectors;

* MAPS for small-pixel trackers with radiation-hard cell designs and high hit-rate capability (sufficient charge collection after $1×10^{15}$ n$_{eq}$ cm$^{-2}$ to $1×10^{16}$ n$_{eq}$ cm$^{-2}$ Non-ionsing energy loss (NIEL), single event upsets (SEU) and single event effects (SEE) tolerant and power-optimised logic, concepts for high data volumes handling on a sensor);

* MAPS designs to reach ultimate timing precision in different processes;

* MAPS with reduced granularity and very low power consumption in very large area detectors for tracking and calorimetry applications.

Passive CMOS

The term passive CMOS sensors indicates pixel or strip sensors manufactured using a simplified CMOS process, e.g. with only two metal layers and without any active elements such as transistors. Passive CMOS [Ch3-4] sensors do not contain any active electronics, and they are read-out as standard analogue sensors via wire or bump bonding. Passive CMOS sensors can profit from using CMOS processing lines on large, high-resistivity wafers (kΩ cm), allowing for design improvements and production optimisations with n-well/p-well/metal layers for sensor implantation and biasing. The possibility of Multi-Project Wafers (MPWs) allows for design optimisations with small prototypes (guard rings, implant geometry, ...), limiting the costs. Several studies showed that in terms of breakdown voltage, radiation hardness, and particle detection efficiency, passive CMOS sensors are not of inferior quality with respect to standard silicon sensors. This was proven with pixel sensors up to $1×10^{16}$ n$_{eq}$ cm$^{-2}$ ($> 99\%$ efficiency at 400 V bias voltage) [Ch3-5]. The multiple metal layers present in CMOS processes allow new, advantageous variants in the sensor designs and the stitching technique (i.e. composing a larger pattern with more than one small image) allows exceeding reticle size to produce large sensor tiles, such as needed in silicon strip tracking systems.

Several aspects make passive CMOS sensors particularly interesting: (i) they might be cheaper to manufacture in large-scale production since they are manufactured in a CMOS chip technology line, which would be of particular importance for strip applications where the sensors are the cost driver. (ii) Their dependence upon a specific CMOS process is not as important as it is for active sensors or read-out electronics: there are many technology nodes to select from (AMS/TSI [Ch3-6], ESPROS [Ch3-7], IBM [Ch3-8], LFoundry [Ch3-9], TowerJazz [Ch3-10], Toshiba [Ch3-11], XFAB [Ch3-12], ...). (iii) Most design rules are not relevant for the large structures used in a sensor design, for example, the minimum rule set is usually not used. (iv) If one node is not available, porting a design to a new process is simple and can be achieved on a short timescale.
One should note that post-processing is usually required to add the implantation and
metallisation as back-plane contact.

The boundary between hybrid and monolithic sensors will become more vague in future developments as even MAPS might require multiple layers. Industrial 3D-integration techniques (see also Chapter 7) promise to integrate multiple specialised wafers with dedicated functions (sensing layer, analogue layer, digital circuit, photonics data transmission) into a single device. The product, a monolithic “sensor” from an assembly point of view, will allow coping with future demands from tracking detectors such as very high hit-rate capability, on-sensor signal processing, data compression, and high-speed data transmission. 3D-integrated imaging sensors currently commercially developed at Sony [Ch3-13] and Samsung [Ch3-14] also allow pitches in the µm range that would enable ultimate spatial resolution for particles entering sensors at different angles.

Feasibility studies on stitched devices will determine the size of the sensors for the
future, and whether and to what extent one can profit from wafer-scale integration.

3.3.2 Sensors for 4D-tracking

In the past ten years, silicon sensors moved from being considered sensors with a rel-
atively poor temporal resolution (usually linked to beam bunch spacing) to being the
detector of choice in high-precision timing systems. The level of timing accuracy needed
by future experiments depends on the targeted applications. Time of Flight (TOF) sys-
tems, normally formed by one or two sensor layers, require the best possible accuracy
as the time is typically measured in only a single (or at best a few) point(s). Large
4D-tracking systems requiring a good track timing identification might have a relatively
lower (50-100 ps) single point accuracy requirement, exploiting the capability of multiple
measurements to achieve the required precision on the track. MAPS and passive CMOS
sensors are also expected to meet this range of performance. On the other hand, 4D-
tracking systems that use the temporal information in their pattern recognition software
require a very high single hit timing accuracy, regardless of the number of layers. While
calorimeters (see Chapter 6) achieve excellent precision given their very large signals,
there is a growing interest in exploring calorimeter timing performances also for MIP
particles. They aim at deploying timing performance not only to eliminate collision
pile-up and beam induced background but also to be used for greater tracking precision
in the detailed reconstruction of particle showers.

Future 4D-trackers will advance from the present state-of-the-art design of timing lay-
ers [Ch3-15], [Ch3-16], investigated with IMB-CNM [Ch3-17], FBK [Ch3-18] and Hama-
matsu [Ch3-19], a silicon layer that provides precise temporal coordinate but only coarse
position, to tracker systems able to concurrently precisely measure the spatial and tem-
poral coordinates. The targeted temporal precision is about 30 ps for the detectors at
LHC, 20-30 ps at the EIC collider, and it decreases to about 10 ps for the TOF sys-
tems at FCC-ee, other Higgs-EW-Top factories, and FCC-hh. Similar ultimate precision
for FCC-hh would be highly desirable for 4D-track reconstruction at the foreseen 1000
multiple interactions per bunch crossing interval.

The primary specifications for these 4D-trackers are typically put in terms of spa-
tial and temporal precision. However, these two requirements do not adequately convey
the complexity of the design, as several other parameters are determining the overall
architecture: (i) material budget, (ii) power, (iii) rates, (iv) occupancy, (v) area, and
(vi) radiation hardness. In addition, the interplay between sensors and electronics in
timing applications is particularly important. In this view, a few essential parameters
that sensors for 4D-tracking should have, are ample (> 3 fC) and short (< 200-300 ps)
signal, uniform response, low capacitance<fnref n="2" />, very high fill factor, and 100% efficiency.
Presently, the two families of sensors that are delivering the best temporal performances
are (i) thin, planar Low-Gain Avalanche Diodes (LGADs) [Ch3-20] optimised for timing
(also known as Ultra-Fast Silicon Detectors [Ch3-21]) and (ii) 3D with columns [Ch3-22]
or trenches [Ch3-23]. The best performances in terms of temporal precision for these
sensors are similar, about 20-30 ps. A technology that might deliver excellent perfor-
mances is BiCMOS MAPS, exploiting the properties of SiGe transistors (low noise and
high gain) [Ch3-24]. In the following, a short description of the main characteristics of
sensors for 4D-tracking is provided.

* **Radiation levels:** LGADs are proven to work up to about $2 \times 10^{15} \text{ n}_{\text{eq}} \text{ cm}^{-2}$ [Ch3-25], while the 3D detector architecture is intrinsically more radiation-resistant;

* **Large area detectors,** covering areas above $1-2 \text{ m}^2$ can be obtained using LGADs (for example, the CMS timing layer requires $17 \text{ m}^2$ of sensors);

* **Low material budget** LGADs (critically dependent on substrate thickness) and MAPS will deliver the best performances; in 3D sensors the collected charge is proportional to the sensor thickness so they cannot be made very thin;

* **Ultimate temporal precision** might be achieved with thin LGADs, 3D sensors and BiCMOS MAPS;

* **Ultimate spatial precision** requires either small pixels (3D design), or it can be achieved with specialised evolutions of the LGAD concept: inverted LGAD (iLGAD) [Ch3-26], trench-isolated LGAD (TI-LGAD) [Ch3-27], and AC-coupled LGADs (AC-LGAD or RSD) [Ch3-28], [Ch3-29];

* **High fill factor** iLGAD, TI-LGAD, and 3D-column architectures.

The field of silicon sensors for 4D-tracking is very young; therefore, the rate of im-
provement is quite rapid, and the capabilities of present technologies have not yet been
fully exploited. For example, (i) MAPS are presently lagging in performances with
respect to hybrid systems. However, this situation might change in the near future.
(ii) The radiation hardness of the LGAD design is improving rapidly, and it has more
than doubled in the past five years. (iii) The full potentiality of 3D sensors for timing is
still to be reached. (iv) The production of BiCMOS MAPS for timing has just started.
Independently from each other, both the sensors' radiation resistance and temporal pre-
cision will improve in the coming decades.

<footnote n="2">²An important figure of merit is the ratio between generated charge and input capacitance. If the capacitance can be reduced significantly, the required minimum charge can be even lower than 3 fC.</footnote>
### 3.3.3 Silicon sensors for extreme fluences environments

Silicon is by far the most studied sensor bulk material at high fluences [Ch3-30], although studies of other materials such as SiC, GaN, and Diamond are also being performed due to the good potential shown so far. The expected behaviour of silicon sensors at high fluences obtained by predictions based on the damage parameters (introduction rate of space charge, trapping probabilities, generations current) measured at low fluences proved to be too pessimistic. Measurements above fluences of a few × 10¹⁵ $n_{eq}$ cm⁻² demonstrate that silicon sensors’ performance greatly surpass the predictions. The successful operations of thin silicon planar detectors at fluences above 2×10¹⁶ $n_{eq}$ cm⁻² and silicon 3D detectors above 3×10¹⁶ $n_{eq}$ cm⁻² have been reported [Ch3-31]. The few measurements available at fluences approaching those at FCC-hh, about 1-2×10¹⁷ $n_{eq}$ cm⁻² [Ch3-32], point to the possible operation of silicon 3D detector even above these radiation levels, maintaining signals around a few thousands of electrons. Although 3D detectors are the most promising technology for high-radiation environments, signals of around 1000 electrons were also observed in planar sensors of standard thickness (about 300 µm) [Ch3-33], with some indications of charge multiplication found in thin sensors [Ch3-34]. The signals (induced currents) at these fluences are very short, only a few 100 ps, and the losses are dominated by the charge trapping.

The changes of silicon properties at extreme fluences are currently poorly known. Reliable measurements of fundamental semiconductor properties such as carrier mobilities, impact ionisation coefficients, the introduction of charged defects, trapping, and generation centres are therefore prerequisites to any detector design. It is crucial that the properties of silicon sensors above fluences of 5-10×10¹⁶ $n_{eq}$ cm⁻² are measured and modelled.

The current limitations in exploring the semiconductor properties at extreme fluences are both in terms of the investigation techniques as well as in facilities that would allow the studies and exposure of the sensors to such extreme radiation levels. Any future progress in this field is very closely linked to improvement in both these aspects. The latter particularly depends on access to adequate resources.

The synergies with fusion reactor instrumentation are many and can be fruitfully exploited. The extension of current research lines of the RD50 [Ch3-35] research group and/or the creation of new R&D collaborations is needed to create the necessary resources to explore the extreme fluence frontier. This is particularly true given the associated issue of finding microelectronics solutions able to withstand such an environment, while coping with the heavily reduced signal size and the demands of ever faster timing capabilities (see Chapter 7).

### 3.3.4 Wide band-gap semiconductors

Wide band-gap (WBG) semiconductors have some attractive properties and also some associated problems. The balance between these benefits and drawbacks will decide how they could be used in future tracking detectors. Whilst a WBG reduces the leakage current, maintaining low noise levels even at high temperatures, it also increases the required electron-hole generation energy. This increase implies that the number of
electron-hole pairs generated for the same deposited energy is lower in WBG materials. For instance, the charge generation in SiC is approximately half of that in silicon. However, the substantial reduction of the noise level ensures that the overall signal-to-noise ratio (SNR) for WBG-based detectors is high enough, even after irradiation. In addition, the high breakdown field allows operation at high internal electric fields, minimising the carrier transit time and the trapping probability.

## Diamond

In the HEP community, the most studied material among WBG semiconductors is diamond. Diamond sensors have been studied intensively in the past decade [Ch3-36], and many of the initial hurdles have been passed [Ch3-37]. Detector-grade polycrystalline synthetic diamond (pCVDD) is available today with a charge-collection-distance (CCD) of about 400 µm. This level of quality has been achieved in close collaboration with manufacturers. The aim for the next 10 years is to increase the quality of as-grown pCVDD wafers from 400 µm to 500 µm, and at the same time improve the wafer uniformity in terms of CCD from currently 10% to 2% across the wafer. The radiation hardness of diamond has been verified with protons, neutrons, and pions; the damage constants for the different particle types and at different energies have been extracted. Based on these numbers, a rough estimate of the Schubweg<fnref n="3" /> at a fluence of $10^{17} \text{cm}^{-2}$ 24 GeV protons gives $\lambda \approx 16$ µm. This would lead to a significant reduction in signal efficiency for MIPs in planar detectors of approximately $\epsilon = \frac{\lambda}{d} = 3.4\%$ assuming a detector thickness of d=500 µm. A remedy to this problem is the use of 3D electrode geometries [Ch3-38]. In recent years, substantial progress has been achieved in the production of 3D diamond detectors, using a femtosecond laser process to convert diamond into graphite electrodes. The smallest cell size of 3D diamond detectors achieved so far has 50 µm in base length, which is equivalent to an electrode distance of d=35 µm. Smaller cell sizes should be possible, achieving an electrode distance similar to the expected Schubweg of $\lambda = 17$ µm for $10^{17} \text{cm}^{-2}$ 24 GeV protons, yielding good efficiency even at this fluence. The radiation damage of 3D devices has not been extensively studied yet. Still, preliminary results indicate that in a 50 µm cells size 3D detector, the collection efficiency decreases by $5 \pm 10\%$ after irradiation with $3.5 \times 10^{15} \text{cm}^{-2}$ 24 GeV protons, compared to a loss of $45 \pm 5\%$ for a planar diamond device at the same fluence [Ch3-39]. More studies are needed to assess the radiation tolerance of 3D diamond detectors comprehensively. The first 3D diamond detector devices in a collider experiment are planned as small beam condition monitors (BCMs) in the upgraded tracker as part of the ATLAS HL-LHC programme [Ch3-40]. This device will prove the readiness of the technology for small-scale applications on the timescale of the next ten years, and it represents a stepping stone towards larger area applications as needed for the FCC-hh to be developed over the next two decades.

## Silicon Carbide

In the past [Ch3-41], SiC radiation detectors were discarded for their use in large-area HEP experiments due to the low quality of the bulk materials produced by manufac-

<footnote n="3">³Schubweg, often identified with the letter λ is defined as the average distance a carrier traverses before being captured.</footnote>
turers. In addition, the possibility of buying only small-sized (50 mm) wafers was a limiting factor for large productions. Nowadays, the widespread use of SiC in power devices, together with the use of SiC as a substrate for GaN LEDs, has pushed the quality of this material to levels similar to those of silicon. In addition, 150 mm SiC wafers are now standard in the semiconductor industry. The high-quality material required for SiC sensors is typically epitaxially grown by Chemical Vapour Deposition (CVD). Epitaxy allows precise control of thickness, doping, and homogeneity of crystal films. High-purity detector-grade SiC epitaxial layers with a thickness of up to 150 µm have been recently obtained [Ch3-42]. The possibility of using thick substrates and fabricating cylindrical contacts with small electrode distances will also allow the fabrication of innovative SiC radiation detectors. Such a design would enable the sensors to be operated without cooling and to withstand high radiation fluences. The main technological challenges for SiC detectors in the following years are: (i) radiation hardness of high-quality materials, (ii) timing performances (LGAD option), (iii) reliable simulation models, (iv) production, in collaboration with industry, of large-area planar detectors with high yield and (v) explore the possibility of creating cylindrical electrodes (with different techniques such as laser, wet etching, Deep RIE) for extreme fluences.

### Innovative 2D-materials

Recently, photo-detectors based on novel materials such as graphene and meta-materials have received a lot of attention. As in other fields, graphene has been a game-changer and, as early as in 2009, its combined outstanding electronic and photonic properties achieved ultra-fast photo-detectors [Ch3-43]. In addition to graphene, other 2D-materials such as the family of transition metal dichalcogenides also exhibit interesting characteristic properties, including a set with (direct) bandgaps. As a related example, MoTe2 has been proven as efficient for radiation detectors [Ch3-44]. It would be of great interest to explore the use of 2D-materials intrinsic properties to test the limits of fast signal collection (ps), high spatial resolution (µm), ultra-thin active membranes (few nm), very high operation rates (higher than 10 Gcycles), and high radiation hardness (Grad or $1×10^{16} n_{eq} cm^{-2}$).

### 3.3.5 The future of interconnection technologies

Future 3D- and 4D-trackers will use hybrid and monolithic detectors. Contrary to the monolithic detectors, hybrid detector modules comprise a read-out ASIC and a solid-state sensor produced in two different technologies. The sensor can be of many different types, including passive CMOS, LGAD, planar and 3D silicon, planar and 3D diamond. The interconnection technologies used for the hybrid detector module production are state-of-the-art wafer-level post-processing technologies, including 2D-bumping, back end of line (BEOL) layer deposition, and 3D-integration technologies (see also Chapter 7). The future requirements on a highly flexible wafer-level processing line are:

* Process capabilities for different wafer sizes and sensor material types: 200 mm and 300 mm read-out chip (ROC) wafer produced in 65 nm technology node or below, 150/200/300 mm silicon sensor production wafers, single chip/small size diamond
wafers;

* Reduction of overall detector material budget requires cheap and flexible handling concepts or alternative bonding technologies for ultra-thin wafers;

* Reduction of interconnection pitches from standard 50 µm to around 25 µm or even below 20 µm.

In the construction of future tracking detectors, an increased hybridisation process-
ing at the vendor is foreseeable in combination with an associated cost reduction per
module. A chip-to-wafer and/or wafer-to-wafer bonding approach can address both as-
pects. However, both bonding technologies require an optimisation of the chip and the
wafer design. A chip-to-wafer assembly with a subsequent chip separation process is
only possible if the top chip is smaller than the substrate chip: this is not the case for
the current ROC on sensor chip design. The demands brought in by a wafer-to-wafer
bonding process are even more complex. ROC and sensor wafers have to match in wafer
size and require a matching reticle. If these requirements cannot be guaranteed, wafer
bonding using a chip-to-carrier re-configured wafer can be a fall-back.

Independently from the bonding technology used, electrically conductive intercon-
nections must be realised between every read-out cell and sensor pixel of the hybrid
module. The very flexible, state-of-the-art bump bonding technology will be available
also in the future, using a fast chip-to-wafer placement and parallel solder reflow process.
This technology has already been used successfully for interconnection pitches of about
10 µm. Alternative bonding processes using a reduced solder layer thickness, like Tran-
sient Liquid Phase Bonding (TLPB) / Solid Liquid Inter-face Diffusion (SLID) can be
applied for chip-to-wafer as well as wafer-to-wafer bonding. The metal-metal diffusion
bonding and metal-oxide-hybrid bonding processes are working without an additional
solder layer. Both technologies require highly planarised, residue- and particle-free bond-
ing surfaces. In addition, the metal-oxide-hybrid bonding processes are patent protected
(ZiBond®, DBI®) and are commercially available only under licence [Ch3-45].

New materials are under development to be used for an electrically conductive pixel-
to-read-out interconnection. Bonding with anisotropic conductive films (ACF), already
used in high volume production in the display manufacturing industry, can be a low-cost
interconnection technology option. Future research topics will focus on the availabil-
ity of ACFs with smaller and densly-packed filling particles, applicable for fine pitch
interconnects, and the reliability of this polymeric material for use in high luminosity
radiation environments [Ch3-46].

Advanced integration technologies will be required for the assembly of hybrid mod-
ules for inner tracking detector layers. 3D-integration technique with TSVs in the read-
out electronic chips will be one important approach. Wafers, produced in a via-first or
via-middle production, are required in order to reduce expensive post-processing steps.
Other integration technologies introduced by the semiconductor and manufacturing in-
dustry are using a chiplet-to-active interposer assembly for ultra-high bandwidth com-
munication [Ch3-47]. This concept can be adapted to an advanced hybrid module set-up.
In this approach, the interconnection to the thinned read-out electronics chip can be re-
alised either by TSVs to the chip backside or by a BEOL redistribution on the sensor wafer surface. Finally, photonic integration technologies allow even faster communication between chips and modules. Research on integration of photonic ICs, photonic wave-guides on chip and photonic interconnections will have to be carried out in the context of the radiation hardness requirements of future trackers and calorimeters.

### 3.3.6 Status and evolution of the simulation tools

A variety of commercial, open-source, and custom software is used within the HEP community, often in combination, to solve the various problems associated with the design, optimisation, and operation of radiation sensors. In the case of sensor design and optimisation, Synopsys' [Ch3-48] and Silvaco's [Ch3-49] commercial Technology Computer-Aided Design (TCAD) tools have played a decisive role for years, with the majority of users using Synopsys' tools, probably due to easy access to licenses via Europractice. TCAD was developed primarily for the semiconductor industry and consists mainly of process and device simulators. Process simulations are used to simulate the various fabrication steps of the sensors to obtain realistic doping profiles and structures. These doping profiles are essential for device simulators, allowing solution of the fundamental equations for semiconductor devices considering different physical models and boundary conditions. Since process details are usually confidential, close collaboration with vendors is required to optimise radiation sensors successfully. As tools for the semiconductor industry to develop new technologies, device simulators contain the necessary transport and physics models to design state-of-the-art devices. However, the challenge for radiation sensors lies more in the size of the structure to be simulated and in the implementation of satisfactory models for radiation damage effects resulting from bulk damage and surface damage. Bulk-damage models have been developed with various numbers of "effective trap levels", providing satisfactory results for fluences up to $2 \times 10^{16} n_{eq} \text{ cm}^{-2}$. The use of "effective trap levels" instead of microscopically measured point and cluster defects is because cluster defects cannot be correctly modelled. The main challenge for the coming years is the parametrisation of cluster defects. One possible way to do this might be based on occupation-dependent ionisation energy. Another important limitation of the simulation tools is that for many materials (SiC or GaN for example) the physical models are not as accurate as for silicon or, in other cases, the numerical modelling tools do not exist (such as for Diamond). Future R&D will require the investigation of radiation hardness and the development of reliable TCAD models for such materials.

A well known limitation of TCAD tools is that they are very demanding on computing time and often do not easily allow integration with other tools in order to facilitate a Monte Carlo approach, an essential method in high-energy physics given the stochastic nature of particle interactions. Allpix² framework [Ch3-50] and Garfield [Ch3-51] were developed to combine TCAD-simulated electric fields with a Geant4 simulation of the particle interaction with matter and can be used to investigate the behaviour of silicon sensors or MAPS to compare the predicted performance with measurements recorded in a particle beam.
## 3.4 Observations

### 3.4.1 Testing infrastructures

#### Irradiation facilities

In the past years, HEP has manage to have access to irradiation facilities for neutrons, charged hadrons, and photons [Ch3-52]. This has been accomplished by a combination of national funding, CERN, and European funding. Notably, European programmes such as AIDA [Ch3-53] had a twofold beneficial fallout: (i) they provided funding and (ii) demonstrated to the managers of the various facilities the relevance of the HEP irradiation programme. Moving forward, it is very important to keep the level of funding and awareness high so that the needs of the future facilities will be met (see Chapter 10 and Chapter 11). One item is particularly important: how to test materials to the irradiation levels foreseen at FCC-hh.

* **Neutron irradiation:** Irradiation at reactors can achieve the required fluence ($1×10^{17} n_{eq} cm^{-2}$ to $5×10^{17} n_{eq} cm^{-2}$) in a reasonable time interval (O(weeks)). The main issue is how to handle the irradiated material. Most sensors (and ASIC) carriers/cable/connectors will become activated. Remote handling might be required. Shipping of the material might become impossible;

* **Hadron irradiation:** Besides the problems connected with the activation of the irradiated material, hadron irradiation is much slower than neutron irradiation. Facilities able to irradiate at much higher rates are needed.

#### Test beam facilities

High-energy test beam facilities are critical infrastructures for R&D in detector technologies [Ch3-54]. Their relatively small number and high users' demand often make them the bottleneck for completing the many detector R&D programmes. A reduction in these facilities will pose a severe problem for the HEP detector R&D community. The delays in detector R&D for the HL-LHC introduced by the recent prolonged technical shutdown of the CERN accelerator complex and its associated test beam areas clearly illustrate how heavily the detector R&D community depends on these tests beam facilities. The maintenance and even increase of the test beam areas should be a priority for the laboratories hosting these infrastructures (again see Chapter 10 and recommendations in Chapter 11).

In addition to the beam facilities, current detector R&D programmes require additional ancillary instrumentation to achieve their characterisation goals. The beamlines need to have fine-pitch telescopes with fast read-out, precise time stamping of individual tracks, particle identification, magnetic fields, etc. In addition, and more importantly, dedicated user support in the operation of these auxiliary instruments is essential to optimise the use of the always scarce test beam time. Good models, illustrating the importance of such support, are the EUDET and AIDA beam telescopes that have been used very successfully in many detector R&D programmes. Maintenance and funding
support of these auxiliary instruments should be implemented by the accelerator and
detector R&D communities.

Finally, to facilitate more equitable user access to test-beam facilities, implementing transnational access programmes, such as those inherent in several EU-funded programmes, is highly desirable and should be promoted.

### 3.4.2 Industrialisation

Over the past years, several attempts to build a European (non-CMOS) large scale (order of several 100 m²) sensor production capability at microelectronics companies have failed. Several possible underlying reasons for these failures have been identified: (i) silicon sensors were considered a commodity bought from foundries without a sizeable monetary investment (of the order of 10 M€) to offset the R&D and production costs, (ii) the typical long R&D periods of the HEP experiments, and (iii) the uncertainty on the return of the initial investment (see Chapter 10). In the present scheme, R&D funding is provided via the experiments. There is no “oversight” body devoted to building the industrial capability of producing a large area of standard sensors in Europe. As an alternative R&D model, the magnets for LHC have been developed with solid financial investment from CERN. A similar investment in silicon sensors should focus on a partnership or collaboration with the foundries, including the post-processing companies, and the R&D efforts should be carried on jointly. The possibility of exploring this kind of approach would be highly beneficial in securing the survival of the manufacturing capabilities of standard silicon sensors.

The development of CMOS sensors (MAPS and passive) might provide an alternative solution for certain applications, for example in applications involving large areas of sensors. However, maintaining the production capability of standard sensors is fundamental to provide the required variety of detector design options and to offset the risk that the fast-evolving CMOS technology will not fully cover the needs of future experiments.

### 3.4.3 Related fields

The progress in solid state detectors and associated electronics in HEP feeds directly into many other fields, with a very fruitful interchange of technologies. One of the most valuable contributions is to the field of medical physics. Almost all countries have solid state R&D programmes dedicated to medical applications, where the technologies developed for HEP are transferred to the medical field. The importance of this transfer cannot be overstated, and it should be encouraged as an important aspect of what the particle physics community does for the wider community.
**3.5 Recommendations**

**3.5.1 Detector R&D Themes**

During the various ECFA Symposia (see Appendix C) and from the feedback through
the National Contacts, four main areas of research were identified.

The further evolution of active monolithic sensors is considered key to achieving
several of the goals at future facilities such as very small pixels, low material budget and
large area. MAPSs are also in a position to benefit greatly from the further evolution of
the main consumer electronics (DRDT 3.1).

The conditions at future facilities are making 4D-tracking a necessity, and not just
a tool to enhance the physics reach. For this reason, it is deemed necessary to continue
the research to identify the most appropriate sensors, with or without internal gain,
monolithic or hybrid (DRDT 3.2).

The understanding of the silicon properties at fluences above $1×10^{16}$ n$_{eq}$ cm$^{-2}$ is
a must for developing the sensors for experiments operating at or above that fluence
level. This research will be driven by experimental data. The first need is to collect a
large number of experimental measurements, and then it will be complemented by the
development of corresponding TCAD models (DRDT 3.3).

In many experiments, the required accuracy in terms of space and time represents a
true challenge for the electronics, as the space allowed for the circuitry is very limited,
even using the most advanced nodes. It is a key need to build a demonstrator to show
that 3D-vertical stacking is a possible option. In order for 3D-vertical stacking to happen,
the development of suitable sensors is necessary (DRDT 3.4).

Figure 3.3 shows the development timeline for the different technologies in terms of
years and facilities as presented in the ECFA Symposium of Task Force 3 (see Appendix
C). The solid-state detector R&D programmes of the next 20-30 years, summarised in
Figure 3.3, face the formidable challenge of providing the sensors needed to carry out
the various physics programmes at the many forthcoming future facilities.

**DRDT 3.1 - CMOS sensors.**

* Develop MAPS sensors with very high spatial resolution and low mass;

* Design and produce MAPS sensors for high fluence environments;

* Develop MAPS with very large areas and reduced granularity for tracking and calorimetry applications;

* Develop CMOS passive designs for pixel and strip sensors, as a complement to present standard silicon sensors;

* Explore the use of state-of-the-art CMOS imaging sensors technology for tracking and vertex detectors.
### DRDT 3.2 - Sensors for 4D-tracking.

* Understand the ultimate limit of precision timing in sensors with and without internal multiplication;

* Develop sensors with internal multiplication with 100% fill factors and pixel-like pitch;

* Investigate production of sensors with internal multiplication in a monolithic design;

* Increase radiation resistance, push the limit of 3D sensors and explore LGAD and MAPS capabilities;

* Investigate the use of BiCMOS MAPS, exploiting the properties of SiGe.

### DRDT 3.3 - Sensors for extreme fluences.

* Measure the properties of silicon sensors in the fluence range $1×10^{16} 
_{\text{eq}} 
_{\text{cm}^{-2}}$ to $1×10^{18} 
_{\text{eq}} 
_{\text{cm}^{-2}}$. Map the limit of 3D sensors and evolve their design to cope with the highest possible fluences;

* Optimise the simulation models with the measurements at high fluence;

* Develop simulation models based on microscopically measured point and cluster defects (instead of a model based on "effective trap levels");

* Explore the use of WBG semiconductors as radiation detectors at high fluences;

* Develop innovative 2D-materials that can offer high radiation hardness and operate at room temperature.

### DRDT 3.4 - A demonstrator of 3D-integration.

* Identify and produce silicon sensors designed to be used in multi-layer detectors;

* Develop flexible, cost effective and reliable chip-to-wafer and/or wafer-to-wafer bonding technologies;

* Build a multi-layer detector using vertically stacked layers of electronics interconnected by through-silicon vias;

* Include 3D-integration of a readout chip with a silicon photonics device.
<table><thead><tr><th rowspan="2">"Technical" Start Date</th><th colspan="3">&lt; 2030</th><th colspan="2">2030-2035</th><th colspan="2">2035-2040</th><th colspan="2">2040-2045</th><th colspan="2">&gt; 2045</th></tr><tr><th>ALICE LS3</th><th>Belle II<br>CBM</th><th>NA62</th><th>LHCb, ATLAS, CMS<br>(≥ LS4)<sup>1)</sup></th><th>ALICE 3 - EIC</th><th>ILC</th><th>FCC-ee</th><th>CLIC</th><th>FCC-hh</th><th>Muon Collider</th></tr></thead><tbody><tr><td rowspan="10">MAPS</td><td>technology node<sup>1)</sup></td><td>65 nm - stitching<br>10 - 20 µm</td><td>65 nm - stitching<br>10 - 20 µm</td><td></td><td></td><td>28 nm</td><td></td><td>≤ 28 nm</td><td></td><td>≈ 10 nm</td><td>≤ 28 nm</td></tr><tr><td>pitch</td><td></td><td></td><td></td><td></td><td></td><td></td><td>pitch ≤ 10 µm for q<sub>tr</sub> ≤ 3 µm in VD</td><td></td><td></td></tr><tr><td>wafer size<sup>2)</sup></td><td>12"</td><td>12"</td><td></td><td></td><td></td><td></td><td></td><td>12"</td><td></td></tr><tr><td>rate<sup>3)</sup></td><td></td><td></td><td></td><td>O(100) MHz/cm<sup>2</sup></td><td></td><td></td><td>5 GHz/cm<sup>2</sup></td><td>30 GHz/cm<sup>2</sup></td><td>α<sub>tr</sub> ≤ 20 ps</td></tr><tr><td>ultrafast timing<sup>4)</sup></td><td></td><td></td><td></td><td></td><td>α<sub>tr</sub> ≤ 100 ps</td><td></td><td></td><td></td><td></td></tr><tr><td>radiation tolerance</td><td></td><td></td><td></td><td>3 x 10<sup>15</sup> neq/cm<sup>2</sup></td><td></td><td></td><td></td><td>10<sup>18(16)</sup> neq/cm<sup>2</sup><br>VD/Cal.(Trk)</td><td></td></tr><tr><td>technology node<sup>1)</sup></td><td></td><td></td><td></td><td>ASIC 28 nm</td><td>ASIC 28 nm</td><td>ASIC ≤ 28 nm</td><td>ASIC ≤ 10 nm</td><td>ASIC ≤ 28 nm</td><td></td></tr><tr><td>pitch</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>wafer size<sup>2)</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>rate<sup>3)</sup></td><td></td><td></td><td></td><td>≤ 25 µm in VD</td><td></td><td></td><td>≤ 10 µm for α<sub>tr</sub> ≤ 3 µm in VD</td><td></td><td></td></tr><tr><td rowspan="5">Planar/3D/Passive CMOS</td><td>pitch</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>ultrafast timing<sup>4)</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>radiation tolerance</td><td></td><td></td><td></td><td>α<sub>tr</sub> ≤ 100 ps</td><td>α<sub>tr</sub> ≤ 100 ps</td><td></td><td></td><td></td><td></td></tr><tr><td>technology node<sup>1)</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>pitch</td><td></td><td></td><td>≥ 300 µm<br>(100% fill<br>facor)</td><td>≤ 50 µm<br>(100% fill facor)</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="5">LGADs</td><td>wafer size<sup>2)</sup></td><td></td><td></td><td></td><td>≥ 3"</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>rate<sup>3)</sup></td><td></td><td></td><td></td><td>6 GHz/cm<sup>2</sup></td><td></td><td></td><td></td><td></td><td>30 GHz/cm<sup>2</sup></td></tr><tr><td>ultrafast timing<sup>4)</sup></td><td></td><td></td><td></td><td>α<sub>tr</sub> ≤ 20 ps</td><td>α<sub>tr</sub> ≤ 20 ps<br>VD/Trk/Cal.</td><td>α<sub>tr</sub> ≤ 20 ps<br>VD/Trk/Cal.</td><td>α<sub>tr</sub> ≤ 20 ps<br>VD/Trk/Cal.</td><td></td><td></td></tr><tr><td>radiation tolerance</td><td></td><td></td><td></td><td>≥ 5 x 10<sup>3</sup> neq/cm<sup>2</sup></td><td></td><td></td><td></td><td>10<sup>18(16)</sup> neq/cm<sup>2</sup><br>VD/Cal.(Trk)</td><td></td></tr><tr><td>sensor thickness<sup>5)</sup></td><td>&lt; 50 µm MAPS</td><td>&lt; 50 µm MAPS</td><td></td><td>&lt; 150 µm<br>Planar/3D/Pas.<br>&lt; 50 µm LGADs</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">backend<br>processing</td><td>3D integration<sup>6)</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Only the projects requiring a new feature first are retained in this table. Values are indicative of performance targets and of operating conditions relevant to R&D. The latter are reported for the regions most exposed to radiation. The colors indicate when key progress (red) would be needed for a given technology, or when they would be desirable (yellow). Green indicates requirements are being met. The different technologies are alternatives for the various detectors, final choices will depend on their ability to achieve different performance parameters together. Heterogeneous layer designs can combine technologies to optimize the overall performance.

1) The evolution in technology node is progressive and indicative. It can depend on achievements in each node. It will also be driven by industrial standards.

2) The size of wafers achievable can depend on technology (industrial process, yield...) with a general trend of benefits from larger areas in all detectors. Either to bend sensors (depending on thickness and detector) or to house more than one sensor in a single substrate.

3) Reported rates are within bunch trains for ILC and CLIC.

4) The values reported are indicative of expected intrinsic performance, not excluding that it can be better with different achievements for sensors w/o amplification. Implementation of 4D-tracking at e-e colliders will depend on ability to maintain low X/XO for tracking precision.

5) Thin sensors is not a requirement for analog calorimetry energy resolution, while they could provide better timing precision.

6) 3D integration exist in commercial process (imagers) and could be beneficial in several performance aspects for future solid state devices. It may be needed to fulfill most stringent requirements and/or to enable desirable performance. Initial demonstrators could enter HL-LHC upgrades.

7) MAPS technology is only foreseen for use in the LHCb tracker. Planar/3D/passive CMOS are foreseen for the LHCb, ATLAS and CMS vertex detectors, rates and radiation tolerance are indicated for LHCb where values are the highest (conditions for ATLAS and CMS are already met).

Figure 3.3: Compilation of the technology R&D needs and timeline for future solid state detectors. The colour coding is linked not to the intensity of the required effort but indicates what key progress would be needed for a technology to enter a project (red), when it would be desirable (yellow), or when it is being met (green).

### 3.5.2 “Blue-sky” Research

Developments on detector technologies typically span timescales of 20 or more years from the first idea to a functional detector. The solid-state detector R&D programmes of the coming years will have to provide innovative sensors to carry out the physics programmes at the forthcoming future facilities. In this respect, the goals are very well defined, and the research plans should focus on achieving them. However, it should be stressed that the actual engine for innovation resides mostly in “blue-sky” research, where groups can explore innovative avenues focusing on mid-term R&D plans (about five years), without a short-term goal. As a recent notable example, the 4D-tracking innovation, started as a “blue-sky” R&D programme, is now embraced by many detectors at future facilities. National programmes that foster “blue-sky” innovations should be strengthened, acting
as incubators for new possibilities.

### 3.5.3 Further recommendations on industrialisation

Given the large demands of solid state sensors for future experiments, a major challenge relates to industrialisation where R&D funds are currently provided through experiments without concerted efforts devoted to building the industrial capability of producing the large areas of sensors in Europe. Although these are large orders by particle physics standards these are not by those of most semiconductor foundries. CERN should promote greater strategic coordination to achieve a stronger negotiating position with commercial partners and to provide them with relationships of greater continuity and depth.

## References Chapter 3

[Ch3-1] W. Snoeys, *CMOS monolithic active pixel sensors for high energy physics*, Nucl. Inst. Meth. A **765** (2014) 167-171.

[Ch3-2] R. Turchetta and et. al., *CMOS Monolithic Active Pixel Sensors (MAPS): Developments and future outlook*, Nucl. Inst. Meth. A **582** (2007) 866-870.

[Ch3-3] I. Perić and et al., *A novel monolithic pixelated particle detector implemented in high-voltage CMOS technology*, Nucl. Inst. Meth. A **582** (2007) 876-885.

[Ch3-4] D.-L. Pohl and et al., *Radiation hard pixel sensors using high-resistive wafers in a 150 nm CMOS processing line*, JINST **12** (2017) P06020.

[Ch3-5] Y. Dieter et al., *Radiation tolerant, thin, passive CMOS sensors read out with the RD53A chip*, Nucl. Inst. Meth. A **1015** (2021) 165771.

[Ch3-6] https://ams.com/full-service-foundry, https://www.tsisemi.com.

[Ch3-7] https://www.espros.com.

[Ch3-8] https://www.ibm.com.

[Ch3-9] http://www.lfoundry.com.

[Ch3-10] https://towersemi.com.

[Ch3-11] https://toshiba.semicon-storage.com/eu/top.html.

[Ch3-12] https://www.xfab.com.

[Ch3-13] https://www.sony-semicon.co.jp.

[Ch3-14] https://www.samsung.com/semiconductor.

[Ch3-15] CMS Collaboration, *A MIP Timing Detector for the CMS Phase-2 Upgrade*, CERN-LHCC-2019-003, CMS-TDR-020, 2019. https://cds.cern.ch/record/2667167.

[Ch3-16] ATLAS Collaboration, *Technical Proposal: A High-Granularity Timing Detector for the ATLAS Phase-II Upgrade*, CERN-LHCC-2018-023, LHCC-P-012, 2018. https://cds.cern.ch/record/2623663.
[Ch3-17] https://www.imb-cnm.csic.es/en.

[Ch3-18] https://www.fbk.eu/en.

[Ch3-19] https://www.hamamatsu.com.

[Ch3-20] G. Pellegrini and et al., Technology developments and first measurements of Low Gain Avalanche Detectors (LGAD) for high energy physics applications, Nucl. Inst. Meth. A **765** (2014) 12-16.

[Ch3-21] H.-W. Sadrozinski, A. Seiden, and N. Cartiglia, 4D tracking with ultra-fast silicon detectors, Rep. Prog. Phys. **81** (2017) 026101.

[Ch3-22] G. Kramberger and et. al., Timing performance of small cell 3D silicon detectors, Nucl. Inst. Meth. A **934** (2019) 26-32.

[Ch3-23] A. Lai and et al., First results of the TIMESPOT project on developments on fast sensors for future vertex detectors, Nucl. Inst. Meth. A **981** (2020) 164491.

[Ch3-24] G. Iacobucci and et al., A 50 ps resolution monolithic active pixel sensor without internal gain in SiGe BiCMOS technology, JINST **14** (2019) no. 11, P11008-P11008.

[Ch3-25] M. Ferrero and et al., Radiation resistant LGAD design, Nucl. Inst. Meth. A **919** (2019) 16.

[Ch3-26] G. Pellegrini and et al., Recent technological developments on LGAD and iLGAD detectors for tracking and timing applications, Nucl. Inst. Meth. A **831** (2016) 24-28. Proceedings of the 10th International “Hiroshima” Symposium on the Development and Application of Semiconductor Tracking Detectors.

[Ch3-27] G. Paternoster and et al., Trench-Isolated Low Gain Avalanche Diodes (TI-LGADs), IEEE Electr. Device L. **41** (2020) no. 6, 884.

[Ch3-28] M. Mandurrino and et al., Demonstration of 200-, 100-, and 50- µm Pitch Resistive AC-Coupled Silicon Detectors (RSD) With 100% Fill-Factor for 4D Particle Tracking, IEEE Electr. Device L. **40** (2019) no. 11, 1780.

[Ch3-29] M. Tornago and et. al., Resistive AC-Coupled Silicon Detectors: Principles of operation and first results from a combined analysis of beam test and laser data, Nucl. Inst. Meth. A **07** (2021) P11008.

[Ch3-30] I. Mandić and et al., Measurements with silicon detectors at extreme neutron fluences, JINST **15** (2020) P11018.

[Ch3-31] J. Lange and et al., Radiation hardness of small-pitch 3D pixel sensors up to a fluence of $3 \times 10^{16} n_{eq}/cm^2$, JINST **13** (2018) P09009.

[Ch3-32] M. Manna and et al., First characterisation of 3D pixel detectors irradiated at extreme fluences, Nucl. Inst. Meth. A **979** (2020) 164458.

[Ch3-33] G. Kramberger and et al., Charge collection studies on custom silicon detectors irradiated up to $1.6 \times 10^{17} n_{eq}/cm^2$, JINST **8** (2013) P08004.

[Ch3-34] I. Mandić and et al., Measurements with silicon detectors at extreme neutron fluences, JINST **15** (2021) P11018.

[Ch3-35] RD50 Collaboration. https://rd50.web.cern.ch.

[Ch3-36] RD42 Collaboration. https://rd42.web.cern.ch.
[Ch3-37] J. M. Holmes and et al., Neutralizing the polarization effect of diamond diode detectors using periodic forward bias pulses, Nucl. Inst. Meth. A **94** (2019) 162-165.

[Ch3-38] F. Bachmair and et al., A 3D diamond detector for particle tracking, Nucl. Inst. Meth. A **786** (2015) 97-104.

[Ch3-39] RD42 Collaboration, L. Bäni, *Radiation Tolerance of Diamond Detectors*, Presentation at International Conference on Technology and Instrumentation in Particle Physics (2021). https://indico.cern.ch/event/981823/contributions/4293567/attachments/2251572/3858302/Baeni-Radiation_Tolerance_of_Diamond_Detectors-TIPP2021-v2.pdf.

[Ch3-40] RD42 Collaboration, W. Trischuk, *Status Report*, 2020. https://rd42.web.cern.ch/rd42/lhcc-talks/RD42-LHCC-OpenSession2020posted.pdf.

[Ch3-41] M. Bruzzi and et al., Comparing radiation tolerant materials and devices for ultra rad-hard tracking detectors, Nucl. Inst. Meth. A **579** (2007) 754-761.

[Ch3-42] E.T.C. Epitaxial Technology Center S.r.l., Catania, Italy. http://www.sicepspa.it/en/works-sicep/e-t-c-epitaxial-technology-center-s-r-l.

[Ch3-43] F. Xia and et al., Ultrafast graphene photodetector, Nature Nanotechn **4** (2009) 839-843.

[Ch3-44] Y. Tao and et al., *A MoTe2-based light-emitting diode and photodetector for silicon photonic integrated circuits*, Nature Nanotech **12** (2017) 1124-1129.

[Ch3-45] G. Gao and et al., *Low Temperature Cu Interconnect with Chip to Wafer Hybrid Bonding*, 2019 IEEE 69th Electronic Components and Technology Conference (ECTC) (2019) 628-635.

[Ch3-46] M. Pinto and et. al., *Pixel detector hybridization and integration with Anisotropic Conductive Films*, TREDI2021, 16th Workshop on Advanced Silicon Radiation Detectors Trento, 16-18 February 2021 (2021). https://indico.cern.ch/event/983068/contributions/4223158/attachments/2191386/3711469/Trento_MVicente_ACF.pdf.

[Ch3-47] P. Vivet and et. al., *IntAct: A 96-Core Processor With Six Chiplets 3D-Stacked on an Active Interposer With Distributed Interconnects and Integrated Power Management*, IEEE Journal of Solid-State Circuits **56** (2021) no. 1, 79-97.

[Ch3-48] https://www.synopsys.com.

[Ch3-49] https://silvaco.com/tcad.

[Ch3-50] S. Spannagel and et. al., *Allpix2: A modular simulation framework for silicon detectors*, Nucl. Inst. Meth. A **901** (2018) 164-172.

[Ch3-51] https://garfield.web.cern.ch/garfield.

[Ch3-52] http://irradiation-facilities.web.cern.ch.

[Ch3-53] https://aida2020.web.cern.ch/aida2020/node/271.html.

[Ch3-54] http://test-beam-facilities.web.cern.ch.
# Chapter 4

# Particle Identification and Photon Detectors

## 4.1 Introduction

Photon detectors are at the heart of most experiments in particle physics. Moreover, they are also finding application in scientific fields as distant as chemistry and biology and are ubiquitous in society in general. As we encounter new environments where we need to collect the light, we require both advances in existing technology and transformative, novel ideas to meet the demanding requirements. Advancement in photon detector technology is therefore essential to address all the science drivers of future high energy physics experiments.

Reliable particle identification (PID) methods have become an indispensable experimental tool, in particular for the physics of heavy flavours, in studies of heavy-ion collisions and in electron-hadron experiments. PID has significantly contributed to our present understanding of elementary particles and their interactions, and will continue to be an essential ingredient in several of the planned experiments. The continuous advances in the development of pixelated single photosensors and fast and low-noise read-out electronics have pushed PID detectors, in particular Ring Imaging Čerenkov (CHerenkov) (RICH) counters, to unprecedented levels of performance. This has allowed a very efficient identification of charged particles and an outstanding background rejection in a vast momentum range from a few 100 MeV/c up to several 100 GeV/c. However the ever-growing demands of the future physics programme, from underground facilities to high luminosity colliders, require mastering a novel generation of PID detectors with high separation power over four to five orders of magnitude in momentum.

In what follows, particle identification and photon detectors are discussed. For technical details on several of the concepts used as identification tools, such as drift and time-projection chambers and transition-radiation detectors, the reader is referred to Chapter 1.
## 4.2 Main drivers from the facilities

There is a wide range of primary drivers for particle identification detectors in future particle physics experiments. Flavour physics experiments such as LHCb, NA62 and Belle II, and into the far future FCC-ee, are wholly reliant on PID to fulfil their physics goals to study heavy-quark charm and beauty decays, often rare decays with high multiplicity where backgrounds can otherwise be overwhelming. The future evolution of new PID techniques is similarly important for hadronic and heavy ion experiments such as ALICE, EIC, and the FAIR experiments. Moreover, photon detectors will be an essential requirement of all future particle physics experiments, whether it be to instrument PID detectors, calorimeters, tracking, neutrino and dark-matter experiments; from ultra-high rates to extreme low-noise requirements, and for all particle astrophysics applications.

The development over time of the major TF4 technology drivers is illustrated in Figure 4.1. Development of RICH and DIRC (Detectors for Internally Reflected Čerenkov light) technology is essential for LHCb, Belle II and the EIC, where hadron identification is paramount. Time of flight (TOF) for PID is also important for many of the physics aims, usually providing low-momentum coverage complementary to RICH information. Picosecond timing is an important theme running throughout all future particle physics applications, including the LHC GPDs and FCC-hh, however particle identification applications of timing detectors for the latter experiments will only be useful up to the few (3-4) GeV/c level.

The vital importance of silicon photomultiplier (SiPM) technology across most aspects of future particle physics applications is striking, where radiation hardness, lower noise and faster timing are important drivers. These devices are also applicable for scintillating detectors operating with liquid noble gases, in particular, underground low-noise experiments, where radio-purity, vacuum ultraviolet (VUV) and cryogenic operation will be important. Vacuum photon detectors also have an essential future role, where timing, large-area operation, rate capabilities and lifetime must be further developed.

Given the requirements discussed above, four main lines of R&D will be have to be pursued:

**DRDT 4.1 - Enhance timing resolution and spectral range of photon detectors.**

This is needed for fast timing in Čerenkov and time of flight detectors, for operation with high particle fluxes and pile-up, and in extending the wavelength coverage of scintillation photons from noble gases and Čerenkov photons.

**DRDT 4.2 - Develop photosensors for extreme environments.**

This being essential for operation in the high-radiation environments at the HL-LHC, Belle II upgrade, EIC and FCC-hh; and similarly for cryogenic operation.

**DRDT 4.3 - Develop RICH and imaging detectors with low mass and high resolution timing.**

As required for particle identification at HL-LHC, Belle II upgrade, EIC, and FCC-ee.
<figure><img src="image_16.png" /><figcaption>Figure 4.1: Schematic timeline of categories of experiments employing PID and photon detectors together with DRDTs and R&D tasks. The colour coding is linked not to the intensity of the required effort but to the potential impact on the physics programme of the experiment: Must happen or main physics goals cannot be met (red, largest dot); Important to meet several physics goals (orange, large dot); Desirable to enhance physics reach (yellow, medium dot); R&D needs being met (green, small dot); No further R&D required or not applicable (blank). [PDE stands for photon detector efficiency].</figcaption></figure>
**DRDT 4.4 - Develop compact high performance time-of-flight detectors.**

As a complementary approach for particle identification at HL-LHC, EIC and FCC-ee.

The R&D timelines for the above themes can be found at Figure 11.1 with attached explanation. R&D is needed for photon detectors in each of the half decade time blocks for facilities listed in Figure 3 and Figure 4 of the Introduction, all the way through to the FCC-hh/muon collider era (DRDT 4.1 and DRDT 4.2). For Particle Identification (DRDT 4.3 and DRDT 4.4), at the time of writing there were no definite plans for dedicated systems in experiments at the ILC, but several experiments requiring these techniques are expected in both the nearer term and looking forward to experiments during the FCC-ee/CLIC era. The future directions and advances of these key detector technologies and DRDTs are described below.

**4.3 Key technologies: particle identification**

Particle identification with a high separation power will continue playing an important role in future experiments. The growing demands of the physics programmes at forthcoming high luminosity facilities require a new generation of PID detectors, from RICH, DIRC and TOF detectors to gas-based $dE/dx$ and transition radiation detectors, as discussed below.

**4.3.1 RICH detectors**

The particle separation power of a RICH detector (DRDT 4.3) relies on a combination of the single-photon angular resolution and the number of detected photoelectrons per track [Ch4-1]. The best performance is achieved by designing a device with the broadest possible detection bandwidth coupled to a radiator characterised by a low dispersion refractive index, in order to reduce the contribution of chromatic aberration to the angular resolution. Since the chromatic aberration of the radiating medium is usually the dominant contribution to the detector precision, the design of a RICH detector that operates in the visible region will improve the angular resolution. However, owing to the falling power law of the number of Čerenkov photons as a function of wavelength ($dN/d\lambda \propto \lambda^{-2}$), working at longer wavelengths requires photon detectors with a high photon detection efficiency to compensate the narrower detector bandwidth. The capacity to detect a large number of photoelectrons not only improves the particle separation power but also increases the pattern recognition capability in presence of complex multi-particle events.

**RICH layout**

The choice of the detector layout will have to adapt not only to the exacting physics goals but also, in the case of particle identification at high momenta, to the often-critical integration constraints. The limited space of the interaction region for hermetic-coverage collider experiments (mandatory at the EIC and FCC-ee) requires designing performant
RICH detectors with a total length shorter than a metre. This is a very challenging task given that a standard focusing RICH counter encompasses a gas vessel, mirrors and respective support structures, and often a second radiator must be added to cover a wider momentum range. Despite the compact radiator regions at the FCC-ee resulting in low photon yields from necessarily low-n radiators, the R&D challenge will be to use achromatic gases with fast, high quantum efficiency (QE), high-granularity photon detectors to push the RICH discrimination power above 100 GeV/c.

To overcome the above issues, two alternatives are under study for the EIC. The first is to mimic the refractive index of fluorocarbon gases, which will likely in the future become unavailable (see below), by pressurising noble gases to several bars [Ch4-2]. This requires innovative engineering solutions for minimising the material budget of gas vessels able to resist the required pressures. The second solution is to design a very compact RICH detector working in the VUV region where the large number of generated Čerenkov photons compensates the short radiator length. In order to maximise the number of photoelectrons, the windowless approach pioneered by PHENIX [Ch4-3] is envisaged, implying the use of gaseous photon detectors operated with the radiator gas itself. This latter requirement poses strong limits on the choice of the radiator gas, and the high chromaticity in the VUV region could jeopardise the overall performance of the RICH detector. Moreover, the design of mirrors with a high reflectance for VUV photons and the control of gas purity at a level of parts per billion (ppb) are quite challenging issues (see below).

For many future RICH layouts, the levels of radiation doses, especially from the high-luminosity hadron facilities, require the use of radiation-tolerant detector components throughout.

### RICH detector timing

The capability to stand very high event rates in the harsh environmental conditions of future high-luminosity facilities requires the clean separation of signal from background hits. The spatial and temporal separation of events will be achieved by combining a precise Čerenkov angle measurement with accurate single photon timing (of order < 50 ps) and a high rate capability. A time-stamp of the Čerenkov photon arrival coupled to short gate intervals around the relevant bunch-crossings will enhance the selection of true Čerenkov photons over spurious hits caused by electronic noise, dark counts and out-of-time events. Moreover, in HL-LHC conditions, associating the photon hits via timing will be essential in associating track PID hypotheses to the associated primary vertex. Therefore timing will be essential for RICH detectors at high-luminosity machines to actually operate.

### Light collection systems

Usually a gas radiator is associated with a large gas-tight vessel of *O*(1-20 m), application dependent, to obtain the necessary light yield, and this necessitates an optical mirror system. The mirrors can often be in the detector acceptance (e.g. at LHCb), hence the development of spherical, parabolic and flat lightweight mirrors is essential. Lower-cost carbon-fibre technology is key to this. Moreover, the design of mirrors with a high reflectance for VUV operation is a necessary development. The mirror alignment is also
essential to achieve optimal performance.

With the aim to improve signal-to-noise ratios, further developments should be pur-
sued in designing dedicated light collectors (concentrators), either as quartz Winston
cone-like arrays, or dichroic reflectors. In this way, photons would propagate from a
larger entry window to a considerably smaller sensor, resulting in an improved signal
photon to dark count ratio. Methods have to be found to minimise the complication of
design of large RICH photon-detection planes.

Photon detectors

The development of large area single photon detectors capable of sustaining high count-
ing rates, and a total ionising dose up to a few Mrad and beyond 1×10¹² neq cm⁻² for
single photons (DRDT 4.2), is of paramount relevance for the next generation of RICH
counters at the high luminosity expected at HL-LHC (up to 1.5 · 10³⁴ cm⁻²s⁻¹ in the
LHCb interaction region). Moreover, the use of photon detectors featuring a high gran-
ularity (1 mm pixel size) and timing resolution of the order of a few tens of picoseconds
(DRDT 4.1) is mandatory to improve the background rejection capability, maintain a
manageable detector occupancy, and to allow a robust pattern recognition in view of
the anticipated high photon and charged particle fluxes exceeding several MHz/cm². In
addition, a good tolerance to magnetic fields is required in most applications in particle
physics.

SiPMs and micro-channel plates photomultiplier tubes (MCP-PMTs) seem the most
promising photon detector technologies that could suit the above features. The main
challenge for using SiPMs in a RICH detector is their high dark count rate with a pulse
height spectrum identical to that from individual Čerenkov photons; these rates can
be as high as tens of MHz/cm² at room temperature and increase with irradiation.
Commercial MCP-PMTs are presently limited in size and lifetime at high rates; their
high cost is a further major issue, as the total area to be instrumented will likely be in
the region of several square metres in any detector design.

In compact RICH counters for identification of high momentum particles at the EIC,
the implementation of solid photocathodes in micro-pattern gaseous detectors (MPGDs)
will continue playing a relevant role. These provide an affordable and low material-
budget approach to large surfaces in the presence of magnetic fields, in particular if
R&D efforts result in novel photo-sensitive materials that are more radiation-hard and
chemically more inert than CsI. These photocathodes should preferably be sensitive to
visual light, and where the positive-ion backflow and photon feedback that limit the
photocathode lifetime and operating speed are reduced. In the long term, an attrac-
tive choice for RICH photon detectors may also be superconducting sensors, but the
integration of these devices into accelerator-based experiments will be challenging.

Radiator materials

For the momentum region higher than 10 GeV/c, gas radiators are almost unique can-
didates, and many RICH devices employ fluorocarbon gases like C₄F₁₀ or CF₄ because
of their optical quality. Gas purification systems are indispensable to maintain high
transparency, and typically impurity quantities of H₂O and O₂ can be suppressed by a
level of several ppm. In the coming years, new developments, working with industry,
may reinforce gas purity performance to a level of ppb for VUV operation.

A further issue is the high greenhouse-gas Global Warming Potential (GWP) of fluorocarbons, for instance $C_4F_{10}$ where the GWP is 4800. $C_4F_{10}$ represents a prime choice for RICH radiators because of its good optical transparency and relatively low chromaticity. Such fluids will experience increasing use restrictions and future shortage of supply and, if used, leak-less systems will be imperative. New alternative gases to replace fluorocarbons must be investigated and, whilst there are candidates, they are flammable [Ch4-4]. New gas mixtures may be the way forward, e.g. by adding a large quantity of stable gas such as neon to satisfy the non-flammable conditions, implemented in association with photocathode development to longer wavelengths.

Regarding liquid radiators, and referring to Chapter 1. the biggest challenge is the water Čerenkov detector of Hyper-Kamiokande [Ch4-5] which stores a 280 kton volume, corresponding to a factor 10 larger fiducial mass than Super-Kamiokande. The most critical issue is the optical transparency of the water which must be retained with the larger volume, and this brings a major challenge for the re-circulation purification system.

Silica aerogel has been used successfully for many years in RICH detectors for PID below around 10 GeV/c, with the advantage that the refractive index can be adjusted by changing parameters in the synthesis process. The disadvantage is that the photon yield is low, which makes operation in a high occupancy environment problematic. Up to now, the refractive index of aerogel covers a wide range of 1.01 to 1.2 although tile dimensions may be limited, in particular, for higher refractive index [Ch4-6]. There is therefore scope for improvement of the process by producing larger aerogel tiles of high transparency, lower scattering, higher photon yield, and customised refractive index. Special skills to develop silica aerogels are available in only two production facilities in the world which could be a potential worry in the long term.

A long-term breakthrough in radiator materials could be the development of photonic crystals [Ch4-7] and meta-materials [Ch4-8] which match the refractive indices needed to identify particles at low and high momentum. The photonic crystal is a potential example of a material with a “tune-able” refractive index, consisting of two different materials with the thickness of sequential layers controlled to be the order of the photon wavelength. These technologies are still at a primitive stage, and there is vast scope for further study. Close cooperation with other disciplines and industry will be crucial to search for appropriate material characteristics, radiation hardness etc.

### 4.3.2 Detectors for Internally Reflected Čerenkov light

DIRCs were first pioneered by the BABAR collaboration [Ch4-9] and more recently by Belle II, extending the DIRC principle to measure Time of Propagation [Ch4-10]. These applications were for flavour physics experiments, where PID is essential. The DIRC techniques have now been further developed by, for example, PANDA [Ch4-11] and GlueX [Ch4-12].

The advantage of a DIRC detector is that it provides an extremely compact PID device in the barrel and endcap regions of an experiment, facilitating space for calorimeters and tracking detectors. DIRCs cover the low momentum region; current detectors pro-
vide $\pi/K$ separation up to around 4 GeV/c, hence are much more suited to lower energy machines. An advantage is they cover the relatively inaccessible region for $dE/dx$ from 1-2 GeV, which suffers from crossing $\pi/K/p$ bands. The main disadvantage is that the momentum range is not so suitable for high energy machines. Also multiple Coulomb scattering can be problematic.

Regarding future challenges (DRDT 4.3), R&D is required to make DIRC readout even more compact, to expand the momentum reach, and to facilitate more extensive use in the endcap region. It is also necessary to explore mitigation of RICH resolution terms, i.e. the chromatic dispersion and multiple scattering. Improved focusing design of the Čerenkov light is required, emphasising improved spatial resolution. Major advances, for example for the EIC, are necessary for a high performance DIRC [Ch4-13], with momentum reach up to about 7-8 GeV/c and above. A general requirement is to improve quartz technology, traditionally used as the DIRC radiator. Surface quality (sub $\sim$ nm surface roughness) is the current cost driver, plus uniform material quality; working with industry to drive down prices will be mandatory. State of the art timing will also be important ($O(10\ \text{ps})$ binning) to reduce backgrounds, for example, facilitated by the recent picoTDC development [Ch4-14]. To this end, photon detection with fast (few tens of ps) timing performance will be essential; MCP-PMTs are currently the detectors of choice, but SiPM readout needs to be developed. Fine-pitched photon detector granularity, radiation tolerance, low noise and photon sensitivity are important. There is recent common synergy with TOF applications, which can extend the PID performance to higher momenta (see below).

### 4.3.3 Time of flight detectors

TOF is a simple concept in principle and, like the DIRC technique, provides good PID separation in the low-momentum region. Here the RICH technique would have to rely on solid radiators where the use of aerogel, located in the acceptance region, has been problematic for high occupancy applications. Naturally for TOF, highly precise timing of order 10 ps is mandatory (DRDT 4.4).

The disadvantage of TOF is that the technique does not readily cover $\pi/K$ discrimination above 10 GeV/c for flight paths $\lesssim$10 m, and the separation power falls rapidly with momentum [Ch4-15]. Also the TOF technique provides only complimentary low-momentum PID coverage at high energy machines. The technique requires a large TOF distance, so multiple Coulomb scattering can become problematic. Since the detector area is often proportional to distance squared ($d^2$), there is also a trade-off between instrumenting a large area with the associated cost. In addition, a suitably accurate start time needs to be available. Despite these challenges, the scope for R&D in this area is very encouraging.

TOF is a very promising PID technique for flavour physics and hadronic physics experiments; examples are Panda/CBM, NA62/TauLV, ALICE, LHCb and FCC-ee etc. Timing is now compulsory for many future applications (for example, pile-up suppression for ATLAS/CMS and the FCC-hh). Hence there are several synergies with timing layers also providing PID using TOF, for example the addition of timing layers for calorimetry
and 4D-tracking silicon detectors.

There are several TOF techniques, all of which have their own specific R&D challenges, with the dream of pushing below the 10 ps level:

### Scintillators

Scintillator arrays have been developed for timing layers with fast photomultiplier readout, and the current state of the art is around ~100 ps [Ch4-16], combining readout from both scintillator ends. Similar performance can also be obtained with SiPMs [Ch4-17]. The CMS scintillator timing layers will bring a significant improvement using L(Y)SO:Ce crystals and SiPMs, where 35-60 ps is expected [Ch4-18]. An added benefit will be modest $\pi/K$ separation, however only up to ~3 GeV/c. Another technique with an area for development is in colloidal quantum dots for Dark Matter detectors (see Chapter 5). Scintillator TOF performance needs to be pushed with further R&D.

### Gaseous detectors

Referring to Chapter 1, multigap RPCs are a well established technique, with ALICE providing the state of the art resolution at 56 ps [Ch4-19]. Future R&D needs to improve performance by increasing the number of gaps and providing a higher rate capability. There should be a push to improve timing resolution and high flux performance further with micro pattern gas detectors (MPGDs). The RD51 Picosecond project [Ch4-20] detecting Čerenkov light with a CsI photocathode and Micromegas detector is an exciting development highly applicable for future applications.

### Silicon detectors

Silicon sensors are covered in detail in Chapter 3. Low-gain avalanche diodes (LGADs) have huge potential for TOF detectors and have common use for timing layers (e.g. ATLAS/CMS). The aim is to achieve a timing performance below the 10 ps level for the FCC-ee and FCC-hh. Further development of AC-, DJ-, TI-, iL-LGAD sensors should move forward, where 20 ps can be achieved [Ch4-21]. The radiation tolerance of LGAD technology also needs improvement (see Chapter 3).

### Large area MCP arrays

The principle is to tile large areas with MCP-PMTs, in which Čerenkov light can be detected from a charged particle passing through the PMT entrance window. At least a $O(15\ \text{ps})$ resolution per MIP needs to be achieved for useful TOF (DRDT 4.1). An example of an MCP-PMT with a large area coverage is the LAPPD [Ch4-22] (see Section 4.4.1). Challenges to overcome are the rate tolerance, radiation hardness and granularity, and operation is a high luminosity environment will require pixelated readout rather than strip. To make the technique feasible will also require an affordable cost per square metre.

### Čerenkov (DIRC)-based detectors

An operating example is the ATLAS Forward Proton TOF detector [Ch4-23], located 210 m from the interaction point, and utilising quartz fingers read out by commercial MCP-PMTs. A resolution of about 20 ps has been achieved. A more challenging project is the TORCH TOF detector [Ch4-24] based on the DIRC technology, developed for the LHCb Upgrade II, and which utilises large 10 mm thick quartz plates as Čerenkov
radiators. The quartz allows a large active area (30 m²) and is read out with customised MCP-PMTs. A 10–15 ps per-track resolution is achievable in providing π/K separation up to 10–12 GeV/c for a 10 m flight path. The technique requires 1 mrad precision for the detected photons, which dictates a detector measurement resolution down to the 100 μm level. The technology is also applicable to TauFV and FCC-ee. As for the DIRC technology, polished quartz surface quality (sub ~nm surface roughness) needs to be achieved at lower price.

In summary, there are several future R&D challenges common to all these technologies: a state of the art timing ASIC is compulsory (O(10 ps) binning), detectors with fast timing to the 10–20 ps level (e.g. MCP-PMTs, SiPMs, LGADs), radiation tolerance, low noise, and good photon sensitivity. Finally timing (clock) distribution R&D is essential, with very precise calibrations necessary.

TOF is also interesting in searches for long-lived Beyond-the-Standard Model particles where the signals would be delayed with respect to low-mass Standard Model hadrons. This places different requirements on the readout electronics which would need to provide timing resolution of O(1 ns) within a significantly larger time window.

### 4.3.4 Particle identification through dE/dx and TRD

The resolution of a dE/dx measurement can be parametrised as $\sigma(dE/dx) = 5.4\% \cdot (LP)^{-0.37}$, with length L in m and pressure P in bar [Ch4-25]. The P term is of interest when excellent PID is needed together with a large mass of the chamber gas (TPC, time-projection chamber, as a target). In this case, R&D topics include the search for suitable gas mixtures for high-pressure operation (as discussed in Chapter 1).

A sizeable improvement by a factor of about two in the dE/dx resolution could come from the determination of the energy loss via cluster counting rather than by measuring the charge [Ch4-26], [Ch4-27], [Ch4-28], [Ch4-29]. The two cluster counting methods are counting in time and counting in space. Cluster counting therefore requires fast electronics and sophisticated counting algorithms, or alternative readout methods. The main R&D topics are the development of readout electronics (e.g. wave-form sampling front-end electronics with FPGA processing) for the cluster counting in time, and a development of 2D-micropattern readout and cluster identification algorithms for cluster counting in space. In the latter case, studies are also needed to understand possible systematic uncertainties in cluster counting due to diffusion, and to optimise the gas composition. More details on gas chambers with dE/dx and cluster counting functionality are discussed in Chapter 1.

Transition radiation detectors (TRDs) work on the principle that transition-radiation X-rays are emitted when a highly relativistic charged particle with a Lorentz factor $\gamma$ exceeding $10^3$ crosses a boundary of two media with different refractive indices. TRDs with gas-based X-ray detectors are a mature instrument for identification at high energies. Due to the overlapping of the transition radiation signal with ionisation, a precise knowledge (and simulation) of the ionisation contribution is required. Another novel feature in TRDs is a GEM-based read-out of the chamber.

An attempt has been made in TRDs to improve cluster counting by means of a
GridPix readout. Some improvement was observed, although not sizeable with such a device [Ch4-30]. Another potential improvement may be reached by differentiating the response to X-ray photons and to particle ionisation; here extensive R&D is required. A completely new avenue could involve imaging the transition radiation X-rays with a high granularity 2D-detector like Timepix3 [Ch4-31], of potential relevance for hadron identification into the TeV range.

## 4.4 Key technologies: photon detection

For most of the last century, the photomultiplier tube has been the most important sensor for low light levels due to its excellent noise characteristics and scalability. Photon detection is crucial for a wide range of applications from high luminosity facilities to the ultra low-noise requirements of underground detectors. This requires an evolution of existing photon detectors, from traditional vacuum-based devices, silicon-based technologies and gas detectors, to novel superconducting devices (DRDTs 4.1 and DRDT 4.2). Detection of photons is a vital component also in neighbouring fields, e.g. in astro-particle physics, for neutrino observatories, or in applications such as medical imaging. For some technologies such as semiconductor photon detectors, a strong driver for R&D will be industrial applications, for example the automotive industry. For others, in particular in the detection of single photons, the R&D will remain in particle physics where advances in existing technologies are needed together with transformative, novel ideas. Photon detector technologies are discussed below.

### 4.4.1 Vacuum-based photon detectors

A number of photon detectors have evolved from the classic photomultiplier concept (PMT), based on the photoemission from a photocathode in vacuum and the subsequent electron multiplication by secondary emission. More recent developments focus on detectors which allow for high segmented anodes down to the millimetre scale [Ch4-32]. In vacuum detectors such as Hybrid Photon Detectors (HPDs) and Hybrid Avalanche Photo Detectors (HAPDs), the anode consists of a (segmented) silicon sensor leading to a single-step gain mechanism.

#### Micro-channel plate detectors (MCP-PMTs)

An MCP consists of a compact and close-packed set of miniature channel electron multipliers each acting as a continuous dynode. The geometry of the MCP and its surface have crucial impact on the detector performance. The geometry is characterised by the pore diameter (6-25 µm), the channel length (400–1000 µm), the length-to-diameter ratio (40-100) and the open-area ratio OAR (55-65%). For integration in an MCP-PMT, two MCPs are stacked in a “chevron” configuration that alternates their bias angle. This reduces ion and photon feed-back effects and optimises the overall amplification gain that reaches $10^6 – 10^7$. Segmentation of the readout anode allows to adapt the readout geometry to the application. Due to their compact size, MCP-PMTs are essentially immune to high magnetic fields (up to 1-2 T) as long as E and B fields are aligned, and also intrinsically feature high spatial and time resolutions (100 µm and 30 ps, respectively).
The technology has evolved significantly over the past ten years [Ch4-33]. A main breakthrough to increase lifetime (> 20 C/cm²) has been the introduction of atomic-layer deposition (ALD) coatings on the MCP to mitigate QE degradation from ion feedback. Modern tubes in flat-panel geometry show high area coverage (> 80%) and satisfactory QE although there is still a deficit of 10-15% absolute compared to high performance PMTs. Further developments are essential in the future in terms of QE, collection efficiency, lifetime (up to > 50 C/cm²) and rate limitation caused by saturation of the MCP (currently 10⁵ cm² which needs an improvement of at least a factor of 10). Operation at significantly lower gain ~ 10⁵ will relax the lifetime and rate issues, and this will require customised electronics development. An alternative to facilitate running at lower gain is the future development of hybrid MCP-HPD devices [Ch4-34], which would need to preserve the few 10's of picosecond timing capabilities. At present the suppliers of MCP-PMTs do not produce devices with massive parallelisation, which could bring down the current high cost of MCP-PMTs.

The Large Area Picosecond Photo-Detector (LAPPD) [Ch4-22] is produced on a pi-lot plant and commercialised by INCOM Inc. Close to ten years of development has led to an impressive MCP-PMT detector which combines flat 20 by 20 cm² tile geometry with MCP-typical timing performance (a transit time spread of 50-70 ps). The great advantage of the LAPPD development is the large area, which is well suited for a variety of applications, from RICH, TOF and dark matter detectors. The MCPs are ALD processed and deliver a gain of up to 10⁷. In the future, the QE (currently 25% at 365 nm) and other performance parameters will need to improve in routine production. The current GEN-I tubes are read out by strip line anodes (resolution 2.4 mm along and 0.76 mm across strips); further generation tubes will have capacitively coupled pad read-out, essential for high-rate high-granular applications, for example at the HL-LHC. For tiling large areas, the price per square metre will ultimately need to be more competitive.

## Photo-multipliers (including large areas)

Whilst the use of classic PMTs in accelerator-based HEP detectors is dropping, large-size PMTs are still the first choice in numerous applications such as water Čerenkov and liquid (noble and organic) scintillation detectors. In these experiments typically very large surfaces have to be covered (see Chapter 2). In recent years, new suppliers have adapted known concepts to large area phototubes with impressive performance [Ch4-35]. Parameters such as light collection efficiency, sensitivity (in the VUV) and operation at cryogenic temperatures have to be further optimised.

The radio-purity (radioactivity) of PMTs has improved by over a factor 100 in the last decade and is already adequate for existing dark matter experiments. However a further ~5-10 fold reduction, down to ~ 10⁻³γ s⁻¹ for a standard 3-inch PMT, is required for the new generation of 0νββ decay search experiments. A commensurate decrease in radioactivity from PMT bases is also required.

## Multianode PMTs (MaPMTs)

MaPMTs are based on the parallel (side-by-side) arrangement of several dynode channels and anodes in the same tube, requiring advanced micro-machining and processing techniques. Two decades of continuous evolution has led to a family of devices
of up to 5 · 5 cm² in size, with high QE (≥30%) and active area coverage, and mm-segmentation. MaPMTs are being deployed as photosensors in the current LHCb RICH upgrade [Ch4-36]. Further major improvements (QE, gain uniformity, timing) and significant cost reduction would be much welcomed, but not readily foreseeable.

## Hybrid photon detectors (HPDs)

Hybrid photon detector tubes combine vacuum photo-cathode technology with solid-state technology. In a “classic” HPD, the photoelectrons are accelerated to multi-keV energies (6-20 keV) and bombarded onto the backside of a standard silicon p-in-n sensor. In an HAPD, the Si sensor consists of an avalanche photodiode (APD) leading to an extra gain factor. The attractive features of H(A)PDs such as gain uniformity, low gain fluctuations, linearity, low noise, high speed and high-resolution photon counting capabilities have led to applications in high-energy physics for calorimetry and RICH detectors [Ch4-37], [Ch4-38]. Conversely, HPDs are in general highly sensitive to magnetic fields; an exception are proximity-focused H(A)PDs that can be operated in a B-field direction which is aligned with the sensor’s E-field. With the exception of the MCP-HPD hybrid, the need to encapsulate the readout electronics inside the vacuum envelope (in highly segmented devices) and their elevated cost mean H(A)PDs devices are less considered for future particle physics facilities.

## Other developments

There are relatively few groups developing novel types of vacuum photon detectors. Some groups work on the replacement of the glass MCP by amorphous silicon [Ch4-39], where the dynode channels are produced by dry ion etching techniques, or propose specially shaped and coated input geometries of the MCP in order to increase collection efficiencies and reduce late pulses.

The Vacuum Silicon PhotoMultiplier Tube (VSiPMT) concept [Ch4-40] integrates a SiPM as anode in the glass envelope of a PMT. Successful prototypes up to two-inch diameter have been built and tested. They work with a reasonably low HV (e.g. 2 kV) and achieve the expected high gain of the SiPM (10⁶). However they also feature the typical dark count rate of the SiPM of $O(100 \text{ kHz/mm}^2)$ at the single photon level. The VSiPMT can also be considered as a low-radioactive sensor; SiPM materials are intrinsically radio-clean, comparable to or better than PMT devices. However, this does not include interconnects or cold electronics which become the critical components, and for which R&D will be required.

The tynode concept [Ch4-41] places, in vacuum, a stack of transmission dynodes (tynodes) on top of a CMOS pixel chip, resulting in a single free electron detector. The assembly relies on Micro Electro Mechanical Systems (MEMS) fabrication techniques which could also serve for adding a photocathode and produce a very fast high-resolution photodetector.

### 4.4.2 Gas-based photon detectors

Gaseous Photon Detectors (GPDs) still represent the most effective solution for instrumenting large imaging surfaces (up to several square metres) which are embedded in high
magnetic fields, especially when low material budget and affordable costs are required, as discussed in Chapter 1. A further advantage offered by GPDs is the possibility to define a flat geometry for optimising detector acceptance and limiting dead regions. They also have sensitivity to single photoelectrons and the capability to perform photon counting due to the high gains achievable (up to $10^6$) via a virtually noise-free amplification mechanism [Ch4-42].

Over the last two decades, the revolutionary technology on which are based Micro-Pattern Gaseous Detectors (MPGDs) [Ch4-43], has allowed GPDs to achieve the localisation of photon conversion points to an accuracy of a few-tens of $\mu$m, good time resolutions (~1 ns) and a rate capability exceeding 1 MHz/mm$^2$. Furthermore, the implementation of elaborate combinations of GEMs [Ch4-44] and Micromegas [Ch4-45], the two basic MPGD architectures, had the great advantage of reducing the positive-ion backflow. Amongst the various phenomena leading to GPD performance degradation, this is the dominant mechanism for photocathode ageing. Such improvements have enabled GPDs to find wide applications in calorimetry, Čerenkov detectors, readout of scintillating fibre trackers and detection of electroluminescence photons. Future challenging applications require higher single photon detection efficiency, radiation hardness and improved time resolution. As an example, MPGD-based photon detectors are an option for equipping the windowless RICH detectors for the identification of particles at high momenta, under design at EIC [Ch4-2]. The devices envisaged must operate at a GHz or greater collision rate, with minimal material budget, and must be compact to fit within space requirements. The ongoing developments at the EIC, if successful, will pave the way for a wide use of GPDs in RICH detectors operated at upcoming fixed-target and collider experiments. Further applications are foreseen for the imaging of rare processes with large volume cryogenic detectors [Ch4-46].

Amongst the existing solid photocathodes for GPDs, CsI has been widely used for its easy implementation and storage in almost any moisture-free gas. However, high-rate applications of CsI photocathodes are limited by charging-up effects causing a severe decrease of the quantum efficiency after a collected charge of the order of around 1 mC/cm$^2$. In the quest for alternative photosensitive materials that are more radiation-hard and chemically inert than CsI, the ongoing investigation on the photo-emissive properties of layers of hydrogenated nano-diamond powders has provided promising results for applications in the VUV domain. However, further studies are required together with the exploration of novel materials.

### 4.4.3 Semiconductor photon detectors

Silicon Photomultipliers (SiPMs; also G-APD, SSPM, MPPC) are modern, nowadays well established, photon detection devices with a large number of applications in particle physics experiments and beyond [Ch4-47], [Ch4-48]. SiPMs consist of an array of typically 1000 single-photon avalanche diodes (SPADs, or pixels) per 1 mm$^2$, which in analogue devices are all connected in parallel. Each pixel is formed by a photodiode and a quenching element connected in series. The photodiode is operated at a few volts above the breakdown voltage such that an avalanche breakdown occurs if a photoelectron
is generated in the active volume; the avalanche multiplication is interrupted through the quenching circuit which can be either passive (analogue SiPMs) or active (digital SiPMs).

Important features of SiPMs are their compactness, a low operation voltage, insensitivity to magnetic fields and the low price-tag; these properties distinguish them, in particular, from PMTs. Today SiPM photon detection efficiencies (PDEs) commonly reach values of up to 60% in the visible range (350-600 nm). Disadvantages are their relatively high noise levels, with dark count rates (DCRs) at the present time typically 10 to 100 kHz/mm² at room temperature, and a somewhat limited dynamic range, depending on the pixel and SiPM array sizes used. Another disadvantage is only a moderate tolerance to radiation fields, particularly if detection of single photons is required. At fluences of a few times $1×10^{11}$ n$_{eq}$ cm$^{-2}$, SiPMs have no single photon sensitivity at room temperature.

Many novel detector concepts consider SiPMs for use in future particle physics experiments, however with the expectation of further-developed performance parameters. Required improvements include the following: reduced dark count rates well below 1 Hz/mm² especially for low rate, low light-level experiments in cryogenic conditions (e.g. DARWIN [Ch4-49], nEXO [Ch4-50] etc.); increased radiation tolerance for radiation levels of $\mathcal{O}(1×10^{14} n_{eq} cm^{-2})$ and above; reduced temperature parameter dependence; fast timing response at the 10 ps-level; highly efficient VUV and IR light detection; small cell sizes for larger dynamic range; large area coverage as well as low-noise; low-power dedicated multi-channel readout electronics. For ultra-cold applications, the cryogenic SiPM performance also needs to be better understood.

### Basic SiPM Technologies

Most SiPM devices presently make use of the analogue approach with a passive quenching circuit and all SPADs within the SiPM array connected in parallel. Advantages are a high yield and low-cost production process, the accomplishment of rather large PDE values and a comparably good control of DCR, cross talk and after-pulsing (the latter heavily rests on the customised SPAD production process). However, digital devices are a very viable alternative as they ultimately promise improved performance. Digital SiPMs (d-SiPM) utilize an active quenching circuit with a fast response time integrated for each SPAD; in the fully digital form each SPAD is in addition connected to its own readout circuit which discriminates and registers the SPAD signal [Ch4-51]. The production of d-SiPMs is still presently constrained to certain CMOS processes which are also used by industry for integrated circuit fabrication, with only minor modifications. This generally results in d-SiPMs being more noisy than their analogue counterparts, though recent developments seem promising. The individual SPAD readout of d-SiPMs offers several particular advantages, for example high single-photon time resolution and DCR control by disabling individual pixels. In addition, the use of industrial CMOS processes promises a high degree of scalability. However, the additional heating from the digital circuitry has to be dealt with.

More recent research activities attempt to combine the best of both worlds by 3D-integration or hybrid technologies with custom production of the SPADs connected to
CMOS-based readout electronics using, for example, through-silicon vias (TSVs) or other
interconnection methods. Whether such complex approaches lead to viable and afford-
able solutions for future particle physics experiments will need to be demonstrated.
Presently 3D-integration for single photon detection is driven by consumer electronics
and industry, with emphasis on infrared sensitivity.

**Recent Developments and Future Challenges**

Future SiPM applications in particle physics (and other research fields) require further developments and improvements, in particular, concerning PDEs in the infrared and deep UV, radiation hardness, fast timing, noise, cross talk and after pulsing, cell sizes and dynamic range, reduced cryogenic operation, improved temperature dependence, as well as dedicated fast, low-power readout electronics. There are many ongoing R&D activities, either generic, or in the context of a particular experiment. Several of the challenges and recent developments relevant for future particle physics applications are summarised below.

*Photon Detection Efficiency:* Modern SiPM devices reach PDEs as high as 50 to 60% depending on the wavelength and bias voltage; for blue and near-UV sensitivity n-on-p structures are used, whilst p-on-n SPADs are better for detecting green/red light. The main reasons for the increased PDE performance over the last decade are higher geometric fill factors and reduced noise levels, allowing SiPM operation at higher bias voltages. Present PDE research focuses mainly on improved efficiencies in the VUV and IR range. In particular, high VUV sensitivity is required for SiPM applications in dark-matter experiments and searches for $0\nu\beta\beta$ decays using large-scale liquid noble gas detectors. In addition, fast timing applications using Čerenkov radiation could profit from improved SiPM sensitivity in the UV range. A particular issue for extending SiPM PDEs into the deep UV is the fabrication of very thin, UV-transparent entrance windows. This requires a good understanding of the VUV optical properties of silicon. Alternative readout techniques for two-phase argon detectors have been proposed [Ch4-52], based on non-standard electroluminescence in the visible and near infrared (NIR). An increase of SiPM sensitivities in the red and IR range is also required outside HEP, for example for laser imaging, detection, and ranging (LiDAR) applications. This necessitates thicker depletion regions with possible limitations on breakdown uniformity and temperature stability.

*Timing:* SiPMs intrinsically provide very fast response, with single-photon time resolutions (SPTRs) reaching values of well below 100 ps. For a single SPAD, the timing spread is below 20 ps. SPTR limitations for multi-cell SiPMs include pixel non-uniformities (i.e. the timing spread between micro-cells), the overall SiPM capacitance, and wavelength-dependent tails. However, the main factor is the parasitic capacitance yielding a reduced signal slope and a degradation of the signal-to-noise ratio, particularly for large devices. The timing jitter due to the influence of electronic noise is the dominant factor degrading the SPTR for large area sensors [Ch4-53]. Possible ways to improve the SPTR degradation due to electronic noise are optimisations of the front-end electronics with respect to bandwidth, power and signal fluctuations, or improving the photon response at the device level. Operation at higher over-voltages can also yield a better timing perfor-
mance due to an increased signal slope. Hence, also lowering the DCR is a path to faster devices. Particularly good SPTR is promised by digital SiPMs with a single pixel read-out and active quenching elements, as these offer high uniformity and a much-reduced capacitance. In general, the ability of digital SiPMs to record and digitise the individual time stamps of all incoming photons, giving the full time information of a detected light pulse, might eventually lead to an ultimate timing performance of $O(10\ \text{ps})$ and below.

For the overall timing performance of a detector system with SiPM readout of light, the SPTR is only one performance parameter. Depending on the particular application, equally or even more important is the size and shape of the incoming light pulse, the optical coupling of the SiPM to the photon source (e.g. scintillator) or the SiPM PDE, which all have to be considered for an optimal timing response.

**Dark-count rate, cross talk and after-pulsing:** DCR, cross talk and after-pulsing are very important performance parameters of SiPMs. Dark counts are generated thermally at room temperature. Improved process engineering has led to a substantial reduction of the DCR component, at present yielding DCR values that can be as low as 10 kHz/mm². Operation at lower temperatures further reduces this value by about a factor of two every 10°C. However, at cryogenic temperatures (T < 200 K), the DCR is dominated by trap-assisted and band-to-band tunneling which primarily depends on the internal electric field strength. Very low noise applications at cryogenic temperatures thus require a very careful engineering of the high-field region within the SiPM SPADs. It has been observed that only a small number of SPADs are responsible for the majority of dark counts in a single SiPM array. Thus, SiPMs with single pixel readout provide the option to individually turn off single “screamer” SPADs for a substantial overall DCR reduction [Ch4-54]. However, whether this approach also helps to reduce the noise level for highly irradiated devices requires further research. One way to further improve the signal-to-noise ratio in SiPMs is to employ dedicated light collectors, either as Winston-cone-like arrays, or suitably designed meta-materials as discussed in Section 4.4.5.

After-pulsing occurs if charge carriers of the primary event are trapped during the avalanche process and released with a time delay, triggering a second discharge. As the trapping probability depends on the number of defects inside the silicon, after-pulsing can be reduced by careful process engineering. Optical cross talk comprises an additional SiPM noise component. This arises if photons generated within the signal avalanche induce a secondary event in a neighbouring pixel. The reduction of this cross talk is achieved by introducing trenches between pixels, with the challenge to retain high fill-factors for large PDEs.

**Radiation hardness:** For future applications in experiments with high radiation levels, radiation hardness of SiPMs is a key requirement. Many groups have studied the impact of radiation damage on the SiPM performance, the most evident being a substantial increase in DCR and a reduction of signal amplitudes. For multi-photon detection, available SiPM devices have been shown to work up to fluences of $1×10^{14} \text{ n}_{\text{eq}} \text{ cm}^{-2}$, although requiring operation at lower temperature and optimised bias voltage to maintain acceptable signal-to-noise ratios or timing performance. Conversely, single photon sensitivity is already lost at fluences of a few times $1×10^{11} \text{ n}_{\text{eq}} \text{ cm}^{-2}$. The increase of
SiPM radiation hardness is the first and foremost mechanism to reduce the DCR. This can be achieved for example by lowering the maximum electric field value (as above) or by reducing the active volume of the SPAD cells, which can be realised by thinning the depletion layer at the expense of increased tunnelling noise. To avoid a drop of the PDE caused by damage and charge trapping in the non-depleted SiPM entrance layer, optimisation of the dielectric-silicon $SiO_2/S_3N_4/Si$ interface needs to be investigated. Moreover, the requirement of operating irradiated SiPMs at low temperatures necessitates easy integration of cooling infrastructure, heat removal and temperature sensors. In addition, the possibility to heat for accelerated annealing also needs to be considered.

Radiation hardness of solid-state photo-multipliers could also be improved by exploring other semiconducting materials with wider band gaps and different material properties, like gallium indium phosphite (GaInP), gallium arsenid (GaAs), galium nitrate (GaN), or silicon carbide (SiC).

*Cryogenic application:* Application of SiPMs in large-scale cryogenic detectors such as nEXO, Darkside-20k, DARWIN or the DUNE Liquid Argon TPCs requires experiment-dependent very small DCR of less than 0.1 Hz/mm², reduced after-pulsing, NUV- and VUV-sensitivity, large area coverage with a high fill factor, good timing resolution and ultra-low intrinsic radioactivity. In addition, readout schemes suitable for large-scale, low temperature operation need to be developed. The first VUV-sensitive devices have been developed, e.g. for use by the MEG-II experiment [Ch4-55] or the nEXO experiment [Ch4-56] with PDEs in the 20% range. Particular challenges are the provision of UV-transparent entrance windows and a thin contact layer to accommodate the low penetration depth of VUV photons.

### Optimisation and integration

The performance of light detection with SiPMs must be seen in the context of a particular experiment, as the optimal performance parameters depend on the specific application. Hence many different aspects need to be considered when optimising SiPM parameters. Fast timing also requires fast input, hence depends on the radiator/scintillator used, its light yield and the spectral and temporal shape of the generated photons. Issues of potential wavelength-shifting, light collection efficiency or optical coupling including micro-lenses are also important. Readout electronics for fast timing must fulfil requirements of low power and low noise, and the need for cooling infrastructure and integration must be considered in both cryogenic and large-scale applications.

Finally, SiPMs are increasingly used in industry applications, also with very special performance requirements. Future particle physics experiments will benefit from the trends and possible performance development in this area, which requires a close exchange between industry and science.

### 4.4.4 Superconducting photon detectors

In this rapidly developing technology field, the three most established technologies are the superconducting nano-wire single photon detector (SNSPD), the transition edge sensor (TES), and the microwave kinetic inductance detector (MKID). The integration
of these cryogenic sensors into accelerator-based experiments would be challenging, but the sensor performance is impressive.

An SNSPD consists of a thin (4 nm) and narrow (100-250 nm) superconducting nanostrip that is current-biased just below its critical current. Absorption of a photon generates a resistive domain in the superconducting nanostrip, which leads to a transient voltage signal that can be detected. SNSPDs offer a unique combination of speed, both in terms of count rate (~ GHz) and low timing jitter (< 3 ps [Ch4-57]), a large range of wavelength sensitivity from VUV (120 nm) to mid-IR (10 µm), high detection efficiencies (approaching 100% for UV to near-IR), and low dark count rates (~ 5-10 Hz), making them appealing for a wide variety of demanding applications. These devices have been an enabling technology for quantum information science (QIS) applications. Outside of QIS, SNSPDs have been used for time-correlated single-photon counting (TCSPC), characterisation of single-photon emitters, molecular spectroscopy, space-to-ground communications, integrated circuit testing, fibre temperature sensing, and LiDAR systems.

Examples of SNSPDs in present use in particle physics are nanowire detectors for dark matter and dark photons. Work is in progress that could make these sensors relevant to HEP applications by lowering the energy threshold, increasing the area (using 300 mm wafers and larger) and pixel size, coupling via windows to cryogenic stages, and readout of arrays (superconducting electronics for data processing). Any application of these sensors with severe cryogenic requirements in large accelerator-based detectors would require an extensive R&D program.

### 4.4.5 Novel optical materials for fibre trackers and light collection

#### Scintillating fibres

Scintillating fibres offer a cost-effective way of instrumenting large areas for charged particle tracking at relatively low material budget. With the availability of small-pitch SiPM arrays, high resolutions are possible, as shown with the LHCb SciFi tracker upgrade [Ch4-58] presently being completed. To further advance the technology, e.g. for a second upgrade of the tracker envisaged for the HL-LHC, both the photosensor and the optical fibres need to be re-optimised to obtain higher light yield, allowing for smaller diameters, and thus higher precision and improved radiation tolerance.

Innovative materials such as Nanostructured-Organo-silicon-Luminophores (NOL) scintillators, exhibit stronger and faster light output than presently achieved; here the energy transfer from the primary excitation to the wavelength shifter is enhanced by silicon links, with respect to the radiative processes in standard materials [Ch4-59]. NOL fibres are almost a factor two (six) faster than the best blue (green) standard fibres, which makes them very interesting for timing-critical applications. Radiation-hardness tests by X-rays to a dose of 1 kGy have shown that the damage is at a level comparable to reference fibres [Ch4-60]. These are promising results but clearly more R&D is required.

#### Light collection methods

One way to improve the signal-to-noise ratio in photon detectors (of particular importance in SiPMs with high dark count rates) is to further develop dedicated light collectors,
either as quartz Winston-cone-like arrays [Ch4-61], [Ch4-62] or suitably designed meta-
materials [Ch4-63]. In this way, photons would propagate from a larger entry window
to a considerably smaller sensor, resulting in an improved signal-photon to dark-count
ratio. Another interesting idea uses light concentrators constructed from di-chroic re-
flectors. These sort photons by wavelength to aid Čerenkov-scintillation separation and
ultimately direction reconstruction in kiloton-scale neutrino detectors [Ch4-64].

Scintillation light emitted by LAr and LXe (128 nm and 178 nm, respectively) requires the use of wavelength shifting materials and di-chroic filters to bring the wavelength into the sensitive range of the photon detector (see Chapter 2).

4.5 Observations

The necessary development of PID and photon detectors for such a wide range of applica-
tions at future facilities presents a major challenge. “Blue-sky” R&D will be necessary,
and on the timescale of 20-30 years, totally new ideas and developments will surely
evolve, and even replace several of today’s preferred technologies, described above.

The development of innovative photon sensors requires cutting-edge technologies available only in very specialised industrial companies. Close collaboration with these industries will be essential. Industry is often driven by mass production outlets, and photon detector evolution should try to align itself to these drivers in order to ensure supply and lower cost. An example for SiPMs could be the automotive industry. A close synergy should also be developed with other research areas.

It is important to stress that the HEP community has very demanding requirements (speed, radiation tolerance, cost) which drive the R&D and ultimately benefit other communities and society in general. Teams with in-depth experience in HEP instrumentation are highly qualified for specifying, testing and characterising new devices and for suggesting technical improvements which feed through into other fields.

There will be benefit to establish a series of interdisciplinary workshops between experts of different research fields and representatives of the major companies developing innovative photon sensors. The goal would be to organise rather small workshops for experts meant to convey the needs of our community to industry, discuss technical challenges and trigger the interest of industry on novel ideas. There may be a benefit of R&D collaborations, e.g. on SiPMs, where issues of standardisation across research areas can be established. Device testing is an example.

The design and construction of PID detectors is in general a specialised area within
particle physics, although with clear synergies with other fields (e.g. in PET scanning
etc). Whilst participation of industry will be important, the detectors are driven by
physicists to fulfil the specific physics requirements of the experiments. The development
of PID devices requires highly skilled personnel, both physicists and engineers, and the
availability of specialised technical workshops in the institutes. It is important to define
a sustainable plan to recruit and train future HEP instrumentation experts for avoiding
generational gaps and maintaining dedicated technical facilities and infrastructures in
our European laboratories.
**4.6 Recommendations**

There are important recommendations to enable the implementation of the research directions discussed above.

**DRDT 4.1 - Enhance the timing resolution and spectral range of photon detectors.**

Sensors are required for fast timing in Čerenkov and time-of-flight detectors, and in extending the wavelength coverage of scintillation photons from noble gases and Čerenkov photons. In the shorter term (the next five years), advances in SiPMs for faster timing and UV sensitivity need to be made. In addition, light collection systems for SiPM arrays (quartz based, micro-lenses, meta-materials) must be developed. For MCP-PMTs, improvements of quantum and collection efficiencies and granularity are required, and extending to large area. Incremental improvements extending over the next 10 years need to be made in large-area gaseous photon detectors, in particular fine granularity, and incorporating fast timing.

**DRDT 4.2 - Develop photosensors for extreme environments.**

This is necessary for operation in the high particle fluxes and pile-up conditions of the HL-LHC, Belle II upgrade, EIC, and FCC-hh, and for experiments at fixed target facilities. In the shorter term (the next five years), improvement in the radiation hardness of SiPM technology is required. Also, for high-sensitivity experiments, radio-pure SiPM technology and operation in cryogenic systems must be realised. For MCP-PMTs, significant progress in detector ageing and high-rate performance needs to be made. Advances in gaseous photon detectors regarding photocathode ageing and rate capability are required, with evolving improvements over the next 10 years. NOL-based scintillation-fibre materials with higher light yield and shorter decay time must also be developed. Finally, on the timescale of 20 years, further advances in radiation hardness of SiPMs or other solid state sensors at and beyond $O(1×10^{14} n_{\text{eq}} \text{cm}^{-2})$ will be essential.

**DRDT 4.3 - Develop RICH and imaging detectors with low mass and high resolution timing.**

This will be needed for particle identification at the HL-LHC, Belle II upgrade, EIC, and FCC-ee. In the shorter term (the next five years), picosecond timing for RICH systems must be developed. Greenhouse-friendly radiator gases (including pressurised systems) will be necessary, together with highly-polished quartz and transparent aerogels. In the longer term (the next ten years), compact RICH systems with low $X_0$ will be required.

**DRDT 4.4 - Develop compact high performance time-of-flight detectors.**

This development is required for particle identification at the HL-LHC, EIC and FCC-ee. In the next 5-10 years, picosecond timing for TOF systems must be developed, together with high-granularity photosensors having long lifetime and high-rate capabilities.

**Further recommendations**

It is recommended that several “blue-sky” R&D activities be pursued. The development of solid state photon detectors from novel materials is an important future line of re-
search, as is the development of cryogenic superconducting photosensors for accelerator-
based experiments. Regarding advances in PID techniques, gaseous photon detectors
for visible light should be advanced. Meta-materials such as photonic crystals should be
developed, giving tune-able refractive indices for PID at high momentum. Finally, for
TRD imaging detectors, the detection of transition radiation with silicon sensors is an
important line of future research.

**References Chapter 4**

[Ch4-1] E. Nappi et al., *Ring imaging Cherenkov detectors: The state of the art and perspectives*, Riv. Nuovo Cim. **28** (2005) no. 8-9, 1-130.

[Ch4-2] R. A. Khalek et al., *Science Requirements and Detector Concepts for the Electron-Ion Collider: EIC Yellow Report*, arXiv:2103.05419 [physics.ins-det].

[Ch4-3] W. Anderson et al., *Design, construction, operation and performance of a Hadron Blind Detector for the PHENIX experiment*, Nucl. Instrum. Meth. A **646** (2011) no. 1, 35-58.

[Ch4-4] A. H. Harvey et al., *Candidates to replace R-12 as a radiator gas in Cherenkov detectors*, Nucl. Instrum. Meth. B **425** (2018) 38-42.

[Ch4-5] Hyper-Kamiokande Collaboration, K. Abe et al., *Hyper-Kamiokande Design Report*, arXiv:1805.04163 [physics.ins-det].

[Ch4-6] M. Tabata et al., *Development of transparent silica aerogel over a wide range of densities*, Nucl. Instrum. Meth. A **623** (2010) 339-341, arXiv:1112.5084 [physics.ins-det].

[Ch4-7] X. Lin et al., *Controlling Cherenkov angles with resonance transition radiation*, Nature Phys. **14** (2018) no. 8, 816-821, arXiv:1804.07118 [physics.app-ph].

[Ch4-8] *The EIC Tracking and PID Consortium (eRD6 Consortium)*, *EIC Detector R&D Progress Report, June 26, 2020*, https://wiki.bnl.gov/eic/upload/ERD6_ProgressReport_201907.pdf.

[Ch4-9] BaBar DIRC Collaboration, I. Adam et al., *The DIRC particle identification system for the BaBar experiment*, Nucl. Instrum. Meth. A **538** (2005) 281-357.

[Ch4-10] Belle-II PID Group Collaboration, K. Inami, *TOP counter for particle identification at the Belle II experiment*, Nucl. Instrum. Meth. A **766** (2014) 5-8.

[Ch4-11] PANDA Collaboration, B. Singh et al., *Technical design report for the PANDA Barrel DIRC detector*, J. Phys. G **46** (2019) no. 4, 045001.

[Ch4-12] F. Barbosa et al., *The GlueX DIRC detector*, Nucl. Instrum. Meth. A **876** (2017) 69-71.

[Ch4-13] L. Allison et al., *High-performance DIRC detector for use in an Electron-Ion Collider*, ICHEP2016 (2016) 300.
[Ch4-14] M. Horstmann et al., *PicoTDC: A 3ps bin size 64 Channel TDC for HEP experiments*, Presented at TWEPP 2019 Topical Workshop on Electronics for Particle Physics, Santiago de Compostela, Spain, September 3, 2019.
https://indico.cern.ch/event/799025/contributions/3486148/attachments/1902032/3140087/picoTDC_TWEPP_2019.pdf.

[Ch4-15] J. Va'vra, *Picosecond timing detectors and applications*, Journal of Physics: Conference Series **1498** (2020) 012013.

[Ch4-16] N. Abgrall et al., *NA61/SHINE facility at the CERN SPS: beams and detector system*, JINST **9** (2014) no. 06, P06005.

[Ch4-17] C. Betancourt et al., *Application of large area SiPMs for the readout of a plastic scintillator based timing detector*, JINST **12** (2017) no. 11, P11023.

[Ch4-18] F. Santanastasio, *Precision Timing in the CMS MTD Barrel Timing Layer With Crystal Bars and SiPMs*, IEEE Trans. Nucl. Sci. **67** (2020) no. 9, 2105-2110.

[Ch4-19] ALICE Collaboration, F. Carnesecchi, *Performance of the ALICE Time-Of-Flight detector at the LHC*, JINST **14** (2019) no. 06, C06023, arXiv:1806.03825 [physics.ins-det].

[Ch4-20] J. Bortfeldt et al., *PICOSEC: Charged particle timing at sub-25 picosecond precision with a Micromegas based detector*, Nucl. Instrum. Meth. A **903** (2018) 317-325.

[Ch4-21] Y. Zhao et al., *Comparison of 35 and 50 µm thin HPK UFSD after neutron irradiation up to 6×10¹⁵ nₐeq cm⁻²*, Nucl. Instrum. Meth. A **924** (2019) 387-393. 11th International Hiroshima Symposium on Development and Application of Semiconductor Tracking Detectors.

[Ch4-22] M. J. Minot et al., *Large area picosecond photodetector (LAPPD) offers fast timing for nuclear physics and medical imaging*, Nuovo Cim. C **43** (2020) no. 1, 11.

[Ch4-23] ATLAS Collaboration, T. Sykora, *ATLAS Forward Proton (AFP) time-of-flight (ToF) detector: construction existing experiences*, 2018.
https://cds.cern.ch/record/2320908.

[Ch4-24] N. Harnew et al., *Status of the TORCH Project*, JINST **15** (2020) no. 04, C04031.

[Ch4-25] M. Hauschild, *Progress in dE/dx techniques used for particle identification*, Nucl. Instrum. Meth. A **379** (1996) 436-441.

[Ch4-26] V. A. Davidenko et al., *Measurements of the relativistic increase of the specific primary ionization in a streamer chamber*, Nucl. Instrum. Meth. A **67** (1969) 325-330.

[Ch4-27] G. Malamud et al., *A Systematic study of primary ionization cluster counting at low gas pressures*, Nucl. Instrum. Meth. A **307** (1991) 83-96.

[Ch4-28] L. Cerrito et al., *Particle identification using cluster counting in a large drift chamber at normal conditions*, Nucl. Instrum. Meth. A **434** (1999) 261-270.

[Ch4-29] J.-F. Caron et al., *Improved Particle Identification Using Cluster Counting in a Full-Length Drift Chamber Prototype*, Nucl. Instrum. Meth. A **735** (2014) 169-183, arXiv:1307.8101 [physics.ins-det].

[Ch4-30] F. Hartjes et al., *Test beam studies of the GasPixel transition radiation detectorprototype*, Nucl. Instrum. Meth. A **706** (2013) 59-64.
[Ch4-31] J. Alozy et al., *Studies of the spectral and angular distributions of transition radiation using a silicon pixel sensor on a Timepix3 chip*, Nucl. Instrum. Meth. A **961** (2020) 163681.

[Ch4-32] T. Gys et al., *Position-sensitive vacuum photon detectors*, Nucl. Instrum. Meth. A **970** (2020) 163373.

[Ch4-33] M. Böhm et al., *Performance of the most recent MCP-PMTs*, JINST **15** (2020) no. 11, C11015.

[Ch4-34] M. Fiorini et al., *Single-photon imaging detector with O(10) ps timing and sub-10 µm position resolutions*, JINST **13** (2018) C12005.

[Ch4-35] Q. Wu et al., *Study of after-pulses in the 20-inch HQE-MCP-PMT for the JUNO experiment*, Nucl. Instrum. Meth. A **1003** (2021) 165351.

[Ch4-36] LHCb RICH Collaboration, S. Gambetta, *First results from quality assurance testing of MaPMTs for the LHCb RICH upgrade*, Nucl. Instrum. Meth. A **876** (2017) 206–208.

[Ch4-37] LHCb RICH Collaboration, M. Adinolfi et al., *Performance of the LHCb RICH detector at the LHC*, Eur. Phys. J. C **73** (2013) 2431, arXiv:1211.6759 [physics.ins-det].

[Ch4-38] S. Korpar et al., *A 144-channel HAPD for the Aerogel RICH at Belle II*, Nucl. Instrum. Meth. A **766** (2014) 145–147.

[Ch4-39] J. Löffler et al., *Amorphous silicon-based microchannel plate detectors with high multiplication gain*, Nucl. Instrum. Meth. A **912** (2018) 343, arXiv:1902.10447 [physics.ins-det].

[Ch4-40] F. C. T. Barbato et al., *The 2-inches VSiPMT industrial prototypes*, Nucl. Instrum. Meth. A **958** (2020) 162144.

[Ch4-41] H. van der Graaf et al., *The Tynode: A new vacuum electron multiplier*, Nucl. Instrum. Meth. A **847** (2017) 148–161.

[Ch4-42] F. Sauli, *Gaseous radiation detectors: Fundamentals and Applications*. Cambridge Monographs on Particle Physics, Nuclear Physics and Cosmology. Cambridge University Press, 2014.

[Ch4-43] A. Oed, *Position-sensitive detector with microstrip anode for electron multiplication with gases*, Nucl. Instrum. Meth. A **263** (1988) no. 2, 351–359.

[Ch4-44] F. Sauli, *GEM: A new concept for electron amplification in gas detectors*, Nucl. Instrum. Meth. A **386** (1997) no. 2, 531–534.

[Ch4-45] Y. Giomataris et al., *MICROMEGAS: a high-granularity position-sensitive gaseous detector for high particle-flux environments*, Nucl. Instrum. Meth. A **376** (1996) no. 1, 29–35.

[Ch4-46] D. Gonzalez-Diaz et al., *Gaseous and dual-phase time projection chambers for imaging rare processes*, Nucl. Instrum. Meth. A **878** (2018) 200–255, arXiv:1710.01018 [physics.ins-det].

[Ch4-47] F. Simon, *Silicon photomultipliers in particle and nuclear physics*, Nucl. Instrum. Meth. A **926** (2019) 85–100.
[Ch4-48] G. S. et al., *The silicon photomultiplier: fundamentals and applications of a modern solid-state photon detector*, Physics in Medicine & Biology **65** (2020) no. 17, 17TR01.

[Ch4-49] J. Aalbers et al., *DARWIN: towards the ultimate dark matter detector*, Journal of Cosmology and Astroparticle Physics **2016** (2016) no. 11, 017.

[Ch4-50] J. B. Albert et al., *Sensitivity and discovery potential of the proposed nEXO experiment to neutrinoless double-β decay*, Phys. Rev. C **97** (2018) 065503.

[Ch4-51] D. R. Schaart et al., *Advances in digital SiPMs and their application in biomedical imaging*, Nucl. Instrum. Meth. A **809** (2016) 31–52. Advances in detectors and applications for medicine.

[Ch4-52] A. Buzulutskov et al., *Infrared scintillation yield in gaseous and liquid argon*, EPL **94** (2011) no. 5, 52001, arXiv:1102.1825 [physics.ins-det].

[Ch4-53] *Analysis of single-photon time resolution of FBK silicon photomultipliers*, Nucl. Instrum. Meth. A **787** (2015) 34–37. New Developments in Photodetection NDIP14.

[Ch4-54] E. Charbon, *Single-photon imaging in complementary metal oxide semiconductor processes*, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences **372** (2014) no. 2012, 20130100.

[Ch4-55] W. Ootani et al., *Development of deep-UV sensitive MPPC for liquid xenon scintillation detector*, Nucl. Instrum. Meth. A **787** (2015) 220–223.

[Ch4-56] *FBK VUV-sensitive Silicon Photomultipliers for cryogenic temperatures*, Nucl. Instrum. Meth. A **982** (2020) 164478.

[Ch4-57] B. Korzh et al., *Demonstration of sub-3 ps temporal resolution with a superconducting nanowire single-photon detector*, Nature Photonics **14** (2020) 250–255.

[Ch4-58] T. Kirn, *SciFi – A large scintillating fibre tracker for LHCb*, Nucl. Instrum. Meth. A **845** (2017) 481–485. Proceedings of the Vienna Conference on Instrumentation 2016.

[Ch4-59] O. V. Borshchev et al., *Highly efficient wavelength-shifters: design, properties, and applications*, INEOS OPEN **4** (2019) 112–123.

[Ch4-60] LHCb SciFi Tracker Collaboration, L. Gruber, *LHCb SciFi – Upgrading LHCb with a scintillating fibre tracker*, Nucl. Instrum. Meth. A **958** (2020) 162025.

[Ch4-61] S. Korpar et al., *A module of silicon photo-multipliers for detection of Cherenkov radiation*, Nucl. Instrum. Meth. A **613** (2010) 195–199.

[Ch4-62] S. Korpar et al., *Test of the Hamamatsu MPPC module S11834 as a RICH photon detector*, Nucl. Instrum. Meth. A **766** (2014) 107–109.

[Ch4-63] R. H. Pots et al., *Improving light output and coincidence time resolution of scintillating crystals using nanoimprinted photonic crystal slabs*, Nucl. Instrum. Meth. A **940** (2019) 254–261.

[Ch4-64] T. Kaptanoglu et al., *Spectral Photon Sorting For Large-Scale Cherenkov and Scintillation Detectors*, Phys. Rev. D **101** (2020) no. 7, 072002, arXiv:1912.10333 [physics.ins-det].
# Chapter 5

# Quantum and Emerging
Technologies Detectors

## 5.1 Introduction

Devices exploiting the extreme sensitivity of quantum systems hold the promise to address, in a completely novel manner, the fundamental properties of space and time, and of particles and their interactions. The rapid progress being made in their development opens up new realms of exploration, complementary to traditional high energy physics approaches, with the prospect of significant advances on the time scale of a decade or less. A wide range of quantum technologies and methodologies is currently being actively explored, which have the potential that dedicated R&D in these areas could have profound impact in answering some of the most puzzling questions in fundamental physics. Several specific observations matched to specific time horizons can be articulated:

* With the current rapid growth of quantum technologies that may be relevant for particle physics, wide exploration of their potential and consolidation of promising approaches can open up the rapid exploration of new regions of parameter space, thus providing valuable and complementary windows on fundamental physics;

* On a time scale of ten years, the use of networks of quantum sensors holds the promise for significant further advances for fundamental physics beyond the reach of individual sensor systems;

* Similarly, the need to avoid environmental disturbances as well as rapidly falling costs encourages the development of devices that are suitable for operation on space-based science platforms, with impacts on the needs for e.g. miniaturisation, standardisation, cost reduction and scalability;

* On a longer time scale, major advances and improvements in quantum technologies will be required to address a breadth of physics topics of fundamental importance ranging from exploration of the dark universe to detection of relic neutrinos to probing the foundations of our understanding of the fabric of the universe;
* Next steps in developing and exploiting the wide range of quantum sensor tech-
nologies involve both exploration of emerging, or completely novel, technologies,
as well as scaling up existing successful technologies. With its decades-long experi-
ence of large high energy physics collaborations and concomitant quasi-industrial
detector fabrication, integration, characterisation and operation as well as project
management, the HEP community has an important role to play in facilitating
this via several routes: (a) developing effective distributed experimental collabora-
tions; (b) where relevant, the provision via national/international laboratories of
(i) ultra-low temperature test stands and other test facilities (ii) mechanical, cryo-
genic, vacuum, magnet or electronic engineering to design prototypes and build,
test and commission at large scales and (iii) where relevant the provision of civil
infrastructure such as accelerator tunnels and access shafts.

To reach such ambitious (but achievable) goals, support for both a dedicated expert workforce and for exploratory devices will be required. In particular, interdisciplinary training for both early-stage and experienced researchers will ensure the widest possible uptake of these rapidly growing technologies, while allowing developments to benefit from existing local, national and supranational research infrastructures and test facilities, for which it is thus equally important to sustain and provide stable support.

Major R&D themes for quantum sensing and other emerging technologies that have
been identified in the framework of this Task Force (common to the different detector
areas presented in Figure 5.1) include:

**DRDT 5.1 - Promote the development of advanced quantum sensing tech-**
**nologies.**

Given the demonstrated and potential further substantial impact of advanced quantum
sensing technologies on particle physics as well as in fundamental physics, their devel-
opment across the full range should be promoted.

**DRDT 5.2 - Investigate and adapt state-of-the-art developments in quantum technologies to particle physics.**

Developing new probes for the investigation of matter and fields will greatly benefit from exploring and adapting methodologies and technologies from quantum sensing, quantum materials, quantum computing and quantum communication. Investigation of their potential benefits to accelerator-based physics should be encouraged.

**DRDT 5.3 - Establish the necessary frameworks and mechanisms to allow**
**exploration of emerging technologies.**

Funding opportunities need to be put in place for rapid exploration of the potential of fundamentally new approaches as well as for consolidating longer term experimental efforts that build on and expand the initial proof-of-principle investigations; close ties to research in neighbouring fields and to industrial efforts, resulting in mutually beneficial cross-pollination, need to be established.
### DRDT 5.4 - Develop and provide advanced enabling capabilities and infrastructure.

Key advanced enabling capabilities such as cryogenic electronics, tailored coatings or dedicated developments in material science for quantum sensing devices need to be put in place; access to common infrastructures at the national and supranational levels for testing and evaluating the suitability of specific quantum technologies for their use in probing fundamental physics needs to be provided.

Many ambitious physics targets have been identified that could benefit from quantum sensing and other emerging technologies. In the following sections the physics motivations and quantum methodologies will be described in greater detail. To attempt to represent the rapidly evolving technological base, specific families of detector technologies have been identified that address various categories of physics targets. Figure 5.1 shows a range of technology areas and physics targets for various time frames and indicates the urgency of investment to enable these programmes. The colour coding for the detector R&D readiness is similar to that of the other Task Forces, with some slight adaptations as is described in the caption. Given the shifting landscape over time, specific physics targets are sometimes regrouped or merged under larger umbrellas (indicated by the same hues of grey), or new targets have been introduced.

Figure 5.1 covers the very wide range of physics targets and technologies discussed in Section 5.3, starting from current state-of-the-art small-sized, local setups to networked ensembles to space-based detector systems to address the rapidly expanding explorable parameter space in dark matter (DM) searches, in searches for new interactions or in probing fundamental symmetries or even foundational aspects of Quantum Mechanics (QM); longer term prospects (but still within the 2-decade framework) include transforming conceptual ideas for HEP detectors based on quantum sensing ideas into prototype or even functional HEP detectors, or even addressing completely new fields, such as searches for relic neutrinos. In Section 5.2 the physics targets that these approaches are particularly well suited to explore will be described. A representative summary of the expected impact on these physics targets of R&D in selected areas of quantum technologies is discussed in Section 5.4 and presented in Figure 5.3.

## 5.2 Theory Motivation

The unprecedented sensitivity and precision of quantum systems enables the investigation of questions of fundamental concern to particle physics. These include the nature of dark matter (DM), the existence of new forces, the earliest epochs of the universe at temperatures $T \gg 1$ TeV and the possible dynamics of dark energy (DE), the possible existence of dark radiation (DR) and the cosmic neutrino background, the violation of fundamental symmetries, and even the nature of interactions and space-time at scales as high as $M_{\text{planck}} \sim 10^{19}$ GeV.

The pseudo-scalar QCD axion is motivated as a leading solution to the strong CP problem [Ch5-1], [Ch5-2], [Ch5-3]. For the QCD axion there is a precise relation be-
<figure><img src="image_17.png" /><figcaption>Figure 5.1: Schematic timeline of categories of experiments employing detectors from the quantum sensing and emerging technology areas discussed in Section 5.3. A wide range of related topics are grouped under a common heading (e.g. tests of fundamental interactions also includes measurement of neutrino properties). The colour coding is linked not to the intensity of the required effort but to the potential impact on the intended physics programme and experiments. Must happen or main physics goals cannot be met (red, largest dot); Important or required to meet several physics goals (orange, large dot); Desirable to enhance physics reach (yellow, medium dot); R&D needs being met (green, small dot); Not applicable or fundamentally new approaches needed (blank).</figcaption></figure>

tween the large energy scale, *f*, whose inverse sets the overall size of the feeble couplings of the axion to the Standard Model (SM) and the particle mass $m_a \simeq 6$ meV ($10^9$ GeV/$f \simeq 1.5$ THz) [Ch5-4]. Axion-like-particles (ALPs), a generalisation of the QCD axion, have interactions again parametrically set by $1/f$, but now the ALP mass is a free parameter. The theoretical attractiveness of the QCD axion and ALPs is enhanced both by their natural, symmetry-protected light mass, and their ubiquitous presence in realistic completions of the SM and gravity, especially string theory [Ch5-5], [Ch5-6]. The details of their couplings and the relation between $1/f$ and their mass provides information on extremely high energy scales, potentially including Planck-scale physics. Importantly, both provide attractive DM candidates with natural early-universe production mechanisms [Ch5-7], [Ch5-8], [Ch5-9], [Ch5-10].

Massive spin-1 “dark photons” (ultra-light dark $Z'$), $A'^\mu$, are another attractive DM candidate with motivated production mechanisms [Ch5-11], [Ch5-12], [Ch5-13], as well as couplings to the SM, particularly kinetic mixing $\epsilon F_{\mu\nu} F^{\prime\mu\nu}$ with the photon [Ch5-14], [Ch5-15], [Ch5-16], [Ch5-17]. Here $\epsilon \ll 1$ is sensitive to physics even at the highest energy scales. Similarly to axions, vector bosons, either broken (massive) or
massless (so they could be dark radiation - see below), are ubiquitous in string theory and their light masses are symmetry-protected; such fields are again well-motivated by UV considerations. Another important target are ultra-light spin-0 CP-even particles such as dilatons or string moduli, which parameterise the size and shape of extra space dimensions which may exist, and whose variation generically leads to time- and space-varying SM couplings and masses. If light, they must be feebly-coupled to the SM to survive fifth-force and astrophysical constraints.

If such bosonic states comprise a substantial fraction of the DM density and their mass satisfies $m \lesssim 1$ eV, then they are well-described as a classical field oscillating at the Compton frequency $\nu_0 = 2.4 \times 10^{11}$ m/meV Hz with a $O(10^{-6})$ fractional line-width due to the virilised motion of DM in our galaxy [Ch5-18]. Thus, resonant detection of light QCD axion, ALP, dark photon and scalar moduli DM is possible via the limited number of leading couplings of the relevant classical field to the SM. As such, the QCD axion, ALPs, and the dark photons provide windows to both the earliest moments of the Big Bang via their production mechanisms and also the very highest energy scales (equivalently smallest lengths) via their feeble couplings and tiny masses.

Irrespective of whether such bosonic states comprise a component of the DM, they give rise to new “fifth” forces with a wide variety of possible types of couplings to the SM. At long-range compared to their Compton wavelength, $\lambda$, these new forces will be Yukawa-suppressed $\sim \exp(-r/\lambda)$, but at distances $r \lesssim \lambda$ the suppression is only power-law in $r$, the precise form depending on the spin and interactions of the exchanged particle [Ch5-19], [Ch5-20].

For almost all light ($m \lesssim 1$ MeV) feebly-interacting particles, whether constituting a component of the DM or not, astro-physical constraints are significant and must be taken into account when assessing the reach of proposed quantum-enhanced detection experiments. Although in principle these constraints can be avoided by more complicated constructions, these same complications can often invalidate the analysis or assumptions underlying the experimental search.

DM with $m \gtrsim 10$ eV is better described as individual particles scattering off a detector. Both light ($m < 1$ GeV) WIMP DM and feebly-interacting massive particle (FIMP) DM candidates with sub-weak-interaction strength populate this low-mass particle-like regime below 1 GeV [Ch5-21], [Ch5-22], [Ch5-23], [Ch5-24]. Non-WIMP heavy ($m \gtrsim 100$ TeV) DM is also a possibility, whether particle, soliton, or a composite state of asymmetric DM. Searches for DM with $m \gtrsim 10$ eV, involve ultra-sensitive quantum-enhanced detectors for photons, phonons, magnons, and sometimes exotic collective excitations of quantum materials or substances (see Chapter 2). Figure 5.2 highlights the potential mass ranges that quantum sensing approaches open up.

New CP-violating physics at high energy is motivated by the required generation of the observed matter-antimatter asymmetry (leptogenesis and/or baryogenesis). Searches for e⁻ and nucleon Electric Dipole Moments (EDMs) using high precision atom and molecular experiments can explore new CP-violating physics at $\sim 100$ TeV scales, with a reach that will only improve as the precision of these experiments improves with new quantum techniques.

Another well-motivated possibility is dark radiation (DR), a relativistic population,
<figure><img src="image_18.png" /><figcaption>Figure 5.2: Axion mass range accessible via novel advanced quantum sensing techniques compared to current experiments.</figcaption></figure>

not necessarily thermal, of new ultra-light or massless particles in the universe. While DM is known to exist, it is not known if there is DR. However DR arises frequently, especially in theories with new light particles, and it often arises in theories of DM. For example for any of the light DM candidates, e.g. axions, dark photons, etc., an abundance of relativistic particles, thus DR, would necessarily be produced alongside the DM. They could also be produced as a component of the dark energy (DE) density if the DE is dynamical [Ch5-25]. Such “DE radiation” can have significantly higher energy densities than other forms of DR, well above the usual Cosmic Microwave Background Radiation (CMBR) and Big Bang Nucleosynthesis (BBN) bounds on relativistic species, because they are produced at late times. DR of particles such as axions or dark photons would be an exciting signal to look for, quite distinct from cold DM. The cosmic neutrino background (CNB) is DR that is believed to exist. While challenging [Ch5-26], observation of this CNB would provide one of the only ways to probe the early pre-CMBR-formation universe. Further, a higher temperature population of cosmic neutrinos can also be produced by dynamical DE which would be significantly easier for experiments to detect [Ch5-25] and would shed light on the nature of DE.

The success of LIGO and VIRGO in detecting gravitational waves (GWs) in the 10 Hz $\lesssim$ ν $\lesssim$ 10 kHz band has vividly demonstrated the power of quantum detectors to advance fundamental physics [Ch5-27]. GWs enable investigations of general relativity, black holes and neutron stars, tests of a variety of Beyond-the-Standard Model (BSM) theories, and give a direct window on the earliest epochs of the Universe via primordial stochastic GWs. Near-future instruments will cover the 0.1-30 mHz (LISA) and 0.1 μHz
Pulsar Timing Array bands, while the Einstein Telescope (ET), a newly accepted European Large-Scale Next Generation Facility ESFRI<fnref n="1" /> project, will cover the frequency range from a few Hz to a few kHz. It is vital to extend this coverage to as wide a frequency range as possible. The ~0.03-3 Hz “mid-band” is a prime target for detectors based on atom interferometry, with MAGIS-100 [Ch5-28], AION [Ch5-29], MIGA [Ch5-30], already in early stages of construction. Future km-scale (such as ET) and satellite-based observatories (such as LISA) will have the capability to detect known astrophysical sources, as well as primordial GWs generated by physics at 1-100 TeV scales and above. Above VIRGO and LIGO, the band $\nu \gtrsim 30$ kHz is free of conventional astrophysical sources, and thus is a prime region to explore new physics [Ch5-31], e.g. via levitated sensors with sensitivity in this frequency range. Potential novel sources in all these bands include GWs from quantum super-radiance produced particle clouds around black holes, from axion inflation and oscillons, cosmic strings, and 1st-order phase transitions. Even smaller scale atom interferometers possess good reach for ultra-light “fuzzy” DM, some fifth force candidates, and equivalence principle and other tests of gravity.

Finally, improved macroscopic tests of QM, or of explicit violations of fundamental symmetries, such as CPT or high-energy Lorentz-invariance (low energy spontaneous breaking of either are better described as due to the presence of background “dark fields” such as DM, DR, or DE), are also a fascinating prospect. Modifications of QM are, of course, much less grounded in a reliable theoretical structure, so one must be careful in assessing the limits from any particular experiment, and also what limits may exist from other ultra-precise tests, such as those of CPT- or Lorentz-invariance.

## 5.3 Quantum Methodologies and Techniques

A wide range of technologies under development and being applied to ongoing or planned low-energy particle physics experiments build on advances in the fields of superconducting devices, atomic and molecular optics or in quantum technology-based materials science that have taken place in the past decades. A discussion of the domains of applicability of the individual technology families, together with an expectation for their further progress allows extrapolating which central questions of particle physics can reasonably be addressed in the coming two decades, and where targeted R&D would hold most promise. Several of these technologies appeared only very recently; flexibility in defining goals and means is thus central to unlocking the potential of quantum technologies.

### 5.3.1 Clocks

Optical atomic clocks now support frequency ratio measurements with fractional uncertainty below the level of $1 \cdot 10^{-17}$. Since atomic clock frequencies are defined by the laws of nature and the fundamental constants, these measurements are exceptional probes of fundamental physics. They test the symmetries of nature, such as Lorentz-invariance, and provide probes for BSM physics, such as the nature of dark matter.

<footnote n="1">¹The European Strategy Forum on Research Infrastructures</footnote>
In contrast to the situation with microwave atomic clocks, this level of clock perfor-
mance is accessible in a large array of optical clock species. Eight optical transitions are
currently listed as secondary representations of the second and dozens more have been
proposed as excellent clock candidates. Included in this list are atomic and molecular
species that exhibit high sensitivity to BSM effects and variations of fundamental con-
stants. The breadth of atomic systems amenable to high precision measurement thus
gives rise to numerous opportunities for fundamental physics. There also exists one low-
energy nuclear transition ($^{229}$Th) that is amenable to laser spectroscopy and could give
extremely high sensitivity to variations in fundamental constants.

Extending the reach of optical clock measurements to new species requires the devel-
opment of new protocols for laser cooling, trapping and state detection. One example
is the technique of quantum logic spectroscopy, which has enabled the highest accuracy
optical clocks to date, based on the $^1S_0 - ^3P_0$ transition in $^{27}$Al$^+$. Adaptation of this
technique has now allowed for precision spectroscopy on molecular and highly-charged
ions. Further developments are aimed at applying this technique to new systems and
scaling it to larger numbers of ions for improved measurement stability.

Quantum enhanced metrology also promises to improve measurement stability using spin-squeezed states of atoms or even maximally-entangled GHZ states. These techniques allow for frequency measurements beyond the standard quantum limit approaching the Heisenberg limit of measurement uncertainty. They have been demonstrated both for trapped ion systems and for atoms trapped in optical lattices. Future research will focus on improving this quantum advantage further and applying it to the stringent requirements of optical clock measurements.

Several grand challenges are evident in the field of frequency metrology. Building a
global network of high-stability and high-accuracy clocks is essential for advancing inter-
national time and frequency standards and would allow applications such as relativistic
goedesy and unprecedented sensitivity in the search for new physics such as ultralight
dark matter. Secondly, sending optical clocks to space would allow more stringent tests
of relativity and could enable the detection of low frequency gravitational waves. Lastly,
optical clocks based on entangled states of atoms would lead to measurements with even
higher stability, ultimately approaching the Heisenberg limit.

5.3.2 Spin-based sensors for axions and axion-like-particles

Spin-based sensors operating on the principles of Nuclear Magnetic Resonance (NMR) are a promising avenue for searching for the QCD axion in the mass ranges of 1 peV-10 neV with haloscopes, such as the CASPER experiment [Ch5-32], and 10 µeV-10 meV with fifth-force searches, such as ARIADNE [Ch5-20]. Ultimately limited by spin-projection noise, a figure of merit in these experiments is $p\sqrt{N\tau}$, so efforts to increase the relevant spin coherence time $\tau$, the number of interacting spins $N$, and the polarisation fraction $p$ are common to these approaches. Additional considerations regarding choice of material for the NMR medium may provide further enhancements in sensitivity. In addition, new quantum protocols may enable experiments to beat quantum projection noise with spin squeezing in nuclear magnetic resonance based detectors for QCD ax-
ion coupling to the strong force below ~ 1 µeV, and for the detection of short-range
spin-dependent interactions above ~ 1 µeV up to 10 meV.

Spin-based sensors are also able to perform sensitive searches for fifth-force axion-
induced couplings to electron spins, for example in the QUAX-gpgs experiment [Ch5-33],
or for the gradient of the cosmic axion field coupling to electrons as in the QUAX-ae
experiment [Ch5-34]. In the latter case, magnons, quanta of collective spin wave exci-
tations that arise in condensed matter systems, are coupled to photons of a microwave
cavity mode to realise a variant of the cavity haloscope. This scheme, that has excellent
prospects for model discrimination in the event of discovery, has been used for a prelim-
inary axion DM search based on a photon-magnon hybrid system consisting of ten small
spheres of yttrium iron garnet (YIG) coupled to a cylindrical cavity mode [Ch5-35].
To improve on the reported limit of the axion-electron coupling constant<fnref n="2" />, significant
R&D activity is required on magnetic materials and hybrid systems. Capability of single
magnon readout based on superconducting transmon qubits is also necessary.

Magnons are also promising channels for detection of DM with spin-dependent in-
teractions in the 10 keV to 10 MeV range. Their discovery potential has recently been
demonstrated for a YIG target with kilogram-year exposure [Ch5-36]. In this case the
DM scattering creates magnon excitations, to be detected with novel SC detectors
with less than 40 meV thresholds or single microwave photon counters.

An exciting possibility which combines quantum and traditional accelerator based
technologies is the use of storage rings as spin detectors. For example, using the
frozen spin method, storage rings could be used to search for a proton EDM (see
e.g. [Ch5-37], [Ch5-38]). The same technology would also allow one of the most sen-
sitive searches for axion (and other ultralight) dark matter in the lowest mass range, by
searching for the spin precession caused by the dark matter field [Ch5-39]. These appli-
cations require the use of both particles accelerated to relativistic speeds and held for
long periods, as well as high precision quantum sensors for measuring the spin precession
rate (and potentially also as beam position monitors).

**5.3.3 Superconducting approaches**

Over the last 30 years, the development and technological refinement of a broad class of
superconducting detectors has revolutionised major areas of experimental astrophysics.
The study of the large scale structure of the Universe through observations of the CMB,
the study of galaxy formation and evolution at high redshifts, the study of the chemistry
and dynamics of star formation in the local Universe, and the observation of black holes
and other highly energetic events have all benefited substantially from the development
of this technology.

The closely linked enabling technology of complex superconducting electronics has
been deployed in many ground-based, aircraft-based, high-altitude-balloon and space-
based observatories, indicating the degree to which this technology has been understood
and refined. Superconducting electronics is now playing a major role in quantum com-

<footnote n="2">²The obtained limit on axion-electron coupling constant $g_{ae} \sim 2 \cdot 10^{-11}$ at 43 µeV is four orders of magnitude above the QCD axion line.</footnote>
puting and communications, in the form of qubits, quantum state memory, and quantum noise limited detectors and amplifiers. The unique characteristics of these devices means that they will also play a pivotal role in a new generation of national and international particle and fundamental physics experiments, and any roadmap aimed at exploring the fundamental nature of space-time and low-energy excitations of quantum fields, must have the further development of superconducting electronics as one of its core priorities.

Crucially, over the past 30 years the vast majority of the development work has taken place in university and government laboratories, which brings into play the question of how best to ensure long-term continuity of supply and the guarantee of decades-long provision and support, and how best to provide science grade devices, and subsystems, which are compliant with high standards of reproducibility, durability and traceability.

A superconducting roadmap must simultaneously address four areas: (i) the innovation and exploration of new device physics; (ii) the processing of science-grade devices in a production environment; (iii) packaging, EMI and stray light mitigation, and general environmental control; (iv) readout and subsystem control electronics. These themes are closely related, and are all needed in order to realise operational experiments. Ultimately, the experimenter is not interested in what is in the box, but requires reliable performance, ease of operation, and above all artefact-free behaviour. The creation of robust, reliable, reproducible behaviour should be an intrinsic part of any development programme.

Fundamentally, superconducting devices work on the basis of either breaking Cooper pairs, or on exploiting the long-range order of the coherent superconducting state, and therefore not breaking pairs, and this determines the limits of applicability in each case. All, however, are capable of exceeding the operational performance of “classical” devices by orders of magnitude, and it is usually second-order effects, such as two-level-systems in deposited and surface oxides, impurity and defect scattering, magnetic flux trapping, proximity effects between dissimilar films, and quasiparticle heating that limit behaviour. Nevertheless a large repertoire of materials and processing techniques exist, which can be used to isolate and enhance the mechanisms of interest. All high-performance devices are based on either elemental materials, such as Nb, Al, Mo, Ti, Hf, Ir, Ta, or, in cases where short quasiparticle lifetimes are needed, the nitrides NbN, TiN, NbTiN; the silicides are also of interest, but have not been exploited much to date. These materials are combined with normal metals, and patterned dielectrics such as Si, SiO, SiO₂, and SiN, to create large-scale integrated electronics, often incorporating hundreds of devices on a single wafer. Because most devices are tens of microns in size, large-scale integration is straightforward where needed.

The devices of central importance are: (i) Superconductor Insulator Superconductor (SIS) mixers, which are based on the photon assisted tunneling of quasiparticles across a dielectric barrier to achieve frequency conversion. They allow conversion gain, which is a non-classical process, and quantum-noise limited sensitivity. (ii) Hot Electron Bolometer (HEB) mixers that operate at frequencies above the superconducting energy gap, and achieve frequency conversion through a power mediated non-linearity. (iii) Cold Electron Bolometers (CEB) achieve detection through a pair breaking process, but prevent the build up of a hot population of quasiparticles, which would otherwise generate
noise. They are extremely sensitive detectors, which have not yet been fully exploited. (iv) Transition Edge Sensors (TES) use the sharp normal-metal to superconducting transition in a thin film to detect radiation. Several modes of operation are possible. They can work from microwave to x-ray wavelengths, including gamma-ray spectroscopy, and are exceedingly popular in the most demanding astronomy applications. They are also being developed for massive particle detection, such as low-energy electron spectroscopy. (v) Kinetic Inductance Detectors (KID) are based on microwave thin-film distributed-or lumped-element resonators. When a photon is absorbed, quasiparticles are created, the inductance changes, and the resonant frequency shifts. A large array of KIDs can be fabricated, all connected to a single microwave readout line, and read out using digital electronics — essentially software-defined radio techniques. KIDs are also exceedingly well suited to a wide range of applications across the whole of the electromagnetic spectrum. (vi) In Superconducting Nanowire Single Photon Detectors (SNSPD) a long, very narrow (~100 nm) meandering superconducting wire forms a pixel. When a single optical or infrared photon is absorbed, it forms a “hot spot” where the line reverts to its normal state, and this is detected by a readout current. SNSPDs are exceedingly fast, and can be used for low-dark-count optical photon counting statistics. (vii) The Superconducting Quantum Interference Device (SQUID) is an ultra-low noise magnetic flux, or electrical current to voltage converter. A superconducting tunnel junction forms part of a small superconducting ring. The magnetic flux passing through the ring can only be changed in discrete quanta, and then the superconducting wave-function around the loop requires the voltage across the tunnel junction to change, which can be measured. SQUIDs are used for sensitive magnetic field measurement, and as ultra-low-noise current-to-voltage converters for reading out TESs. They are now also being used to realise quantum noise limited microwave parametric amplifiers. (viii) Josephson Junction Parametric Amplifiers (JJPA) are used routinely to achieve quantum noise limited microwave amplification. When the current through a superconducting tunnel junction changes, its inductance changes in a nonlinear way, and this can be used to achieve parametric amplification. (ix) Travelling Wave Parametric Amplifiers (TWPA) modulate the kinetic inductance of a long superconducting transmission line by applying a high-power ~0 dBm RF current. They have been developed to allow quantum noise limited amplification across wide instantaneous bandwidths and at millimetre wavelengths. They can also be used to generate a squeezed vacuum state, and so have numerous potential applications. Quantum noise limited amplification from ~1 GHz to 1 THz is entirely feasible. (x) Although the above list concentrates on the devices themselves, SQUID-based superconducting electronics has been developed to a high degree of sophistication to achieve Frequency Domain Multiplexing (FDM) and Time Domain Multiplexing (TDM), which is essential when reading out very large imaging arrays.

It should be appreciated that superconducting electronics is a complete technology, not merely a collection of individual device types. All of the above devices use the same thin-film processing techniques and so can be combined on wafers to create microcircuits having high degrees of functionality. The near-lossless nature of interconnecting traces ensures extremely wideband dispersionless behaviour of transmission lines, and in fact the kinetic inductance of the elemental superconductors is used extensively and routinely
to produce a variety of miniaturised passive components, such as RF filters, hybrid
couplers, and antennas to 1 THz.

Superconducting devices are a very promising approach for a direct determination of the neutrino mass scale via, for example, high resolution and high statistics measurements of low-energy electron capture and beta decay spectra. Although measuring the absolute neutrino mass is extremely challenging, having the aim to detect an extremely tiny spectral distortion in the end-point region of beta and electron capture spectra in an energy scale much less than 1 eV, R&D for quantum sensors is very well motivated given that we know that there is a lower bound for neutrino masses.

Overall the opportunities for creating a new generation of fundamental physics ex-
periments based on superconducting electronics is substantial. Areas such as combining
superconducting devices with micro-machined accelerometers and mechanical resonators
is largely untouched, but entirely realistic. Combining superconducting devices with sin-
gle and macroscopic spin systems is an area that is also starting to gain traction, and will
inevitably lead to major innovations. It appears that the application of superconducting
devices to massive particle detection has not been explored in-depth or indeed exploited,
but there is a steady trickle of disconnected papers in the open literature going back
for many years. We feel that this is also an area that needs assessing. Finally, to our
knowledge there have been no published quantitative studies exploring the application of
superconducting devices and electronics to traditional accelerator-based particle physics
experiments, and this is clearly a subject of substantial importance.

5.3.3.1 Dark matter searches with 3D microwave cavities

The most sensitive instruments to explore the parameter space of the QCD axion are the haloscopes, which rely on axion to photon conversion within microwave cavities under multi-Tesla fields [Ch5-40]. Since the axion mass is à priori unknown, all possible mass ranges need to be explored, and experimental efforts are made to enhance the speed at which haloscopes can scan through parameter space at some fixed axion to photon coupling $g_\gamma$ [Ch5-41]. The scan rate $R$ depends on experimental parameters according to $R \propto B^4 V_{\text{eff}}^2 Q_0 / N_{\text{sys}}^2$, with $B$ the magnetic field amplitude, $V_{\text{eff}}$ the effective cavity volume and $Q_0$ its intrinsic quality factor. $N_{\text{sys}}$ is the total system noise temperature. In spite of the promising fourth-power dependence, in most advanced haloscopes the magnetic field today does not exceed by more than 25-30% the value it had about 30 years ago. Cylindrical cavity volumes shrink for increasing frequencies<fnref n="3" />, and in addition, their quality factors are limited by the anomalous skin effect to less than $10^4$ at a few GHz frequency in normal conductors such as copper. Next generation experiments target the challenging 1-10 GHz frequency range by using SC technology to reach quality factors larger than copper by at least an order of magnitude, and by using Josephson junction parametric amplifiers (JJPAs) with total electronic noise very near its quantum limit [Ch5-42] to minimise total system noise, $N_{\text{sys}}$.

Accelerator cavities made of bulk niobium have been demonstrated with quality
factors exceeding $10^{10}$ [Ch5-43]. While this technology is of interest for dark photon

<footnote n="3">³The detection volume goes down with at least the second power of frequency increase.</footnote>
searches [Ch5-44] or so-called multi-mode axion searches [Ch5-45], it is not applicable to haloscopes as superconductivity in bulk Nb is lost at magnetic fields higher than its critical field (1 T). Metallic cavities coated with films of superconducting materials as YBCO, Nb₃Sn or Nb-Ti promise to largely exceed the performance of copper cavities. As for Nb₃Sn, experiments need to build expertise on different fabrication technologies including magnetron sputtering [Ch5-46] or vapour tin diffusion [Ch5-47] to obtain high quality films. Copper resonators coated with a Nb-Ti film produced by magnetron sputtering have already been obtained with a factor 10 improvement in $Q_0$ over copper cavities [Ch5-48]. A polygon-shaped cavity with inner walls covered by YBCO tape has been demonstrated with quality factors exceeding $3 \times 10^5$ at around 7 GHz, with no considerable degradation in the presence of magnetic fields up to 8 T [Ch5-49].

Even with cavity quality factors matching the axion linewidth, haloscopes relying on quantum limited amplifiers for readout of the cavity signal cannot probe the plausible parameter space within a reasonable amount of time. Squeezing, based on utilisation of two JJPAs, has proven effective in circumventing the standard quantum limit in linear amplification, and doubling the scan rate in the HAYSTAC receiver [Ch5-50], but a real breakthrough in this field could be achieved by switching to photon counting techniques. In the energy eigenbasis, a haloscope receiver would only be limited by dark counts and efficiency. Superconducting quantum circuits provide platforms for measuring microwave photons (E ~ $10^{-5}$ eV). Recently, following impressive progress made in the fabrication of these devices for circuit QED applications, different schemes for single microwave photon detectors (SMPD) based on transmon qubits have been demonstrated [Ch5-51], [Ch5-52], [Ch5-53], [Ch5-54]. SMPDs are available both for photons confined in resonators and for itinerant (travelling) photons. The first type of detector has already been applied to dark photon searches, where intense magnetic fields are not required, demonstrating acceleration of the search by a factor 1300 compared to quantum-limited amplification [Ch5-55]. Strategies based on itinerant photon detectors appear instead more suited to haloscope searches, even though they are more challenging to develop with the required low dark count, efficiency and bandwidth. Dissipation engineering [Ch5-56], a new paradigm introduced in circuit QED, has allowed unprecedented low dark count rates in this latter type of device [Ch5-51], and might significantly impact future haloscope searches in the 5-15 GHz range.

### 5.3.4 Optomechanical technologies

Optomechanical detectors have achieved remarkable sensitivity over the past several decades, for example, allowing the detection of the feeble strain on the earth caused by gravitational waves from distant astro-physical sources [Ch5-27]. Optomechanical sensors are also well suited for precision searches of the dark sector, including dark matter and “fifth-force” searches, as well as for fundamental tests of quantum mechanics and gravitation. In this section we highlight several optomechanical sensing technologies that are ripe for development for these purposes.

#### Ultra-light scalar DM (ULDM): Optical-cavity-based detectors

Scalar ULDM fields can produce a tiny atomic strain and oscillation in the atomic Bohr
radius, by coupling to the fine structure constant and/or the mass of the electron. This signal would cause the dimensions of solid objects to fluctuate at the DM Compton frequency, and can be searched for by tracking the resonance frequency of optical cavities [Ch5-57]. Monolithic reference cavities used for optical frequency standards are a mature technology offering exquisite strain sensitivity at frequencies between 0.1-100 kHz, little explored in the search for ULDM. A differential readout scheme, which is immune to parasitic coupling to the probe laser frequency [Ch5-57] is used. Here the length of a cavity with a rigid spacer can be compared with the length of a cavity with suspended mirrors. The rigid cavity undergoes a measurable strain due to the ULDM while the suspended cavity is unable to respond rapidly enough to see the signal due to the suspension. To achieve the best sensitivity, development of substrate materials with ultra-low mechanical loss and coefficient of thermal expansion (CTE), “supermirror” technology with ppm-absorption dielectric coatings, and mounting and spacer geometries that eliminate coupling to transverse vibrational modes can be employed. Operation at cryogenic temperatures (< 10 K) enables further reduction of mechanical loss, minimising thermal noise and giving access to shot noise limited readout over a wide range of acoustic frequencies [Ch5-57], limited by the cavity response time. Squeezed light techniques similar to those used in GW observatories [Ch5-58] may further improve sensitivity above the kHz range.

## Ultra-light DM: mechanical resonators

Bar detectors have also been proposed as detectors for ultra-light scalar DM in the audio-band [Ch5-59]. The resonant mechanical response of a material object can be harnessed to provide sensitivity to DM-induced strain at the resonance frequencies of a detector, and various readout mechanisms are possible, including optical and electronic. An analysis from data taken by the AURIGA bar detector [Ch5-60], [Ch5-59] provides the current best limit on scalar DM coupled to the electron mass at the frequency near 1 kHz. Vector “dark photon” DM may produce a weak force on atoms e.g. proportional to their neutron charge (B-L), oscillating at the DM Compton frequency. This force would deform an optical cavity or resonator made of two materials with differing (B-L), an effect which can be resonantly enhanced by utilising mechanical resonances at the DM Compton frequency [Ch5-61].

## Gravitational wave and axion detection: levitated microspheres

Optically levitated dielectric sensors have been identified as a promising technique for GW searches spanning a wide frequency band from a few kHz to ~300 kHz [Ch5-31]. A dielectric nano-particle suspended within an optical cavity will experience a force when a passing GW causes a time-varying strain of the physical length of the cavity. The particle will be displaced from the location of the trapping-light anti-node, resulting in a kick on the particle at the frequency of the GW. The sensitivity is limited by Brownian thermal noise and in ultra-high vacuum is quantum-limited by photon-recoil heating from discrete scattering events of individual trap laser photons [Ch5-62], rather than the displacement detection sensitivity. This results in improved sensitivity at higher frequency (unlike traditional interferometer style detectors, which decrease sensitivity at high-frequency due to laser shot noise) [Ch5-31]. The trapping frequency and mechanical
resonance line-width are widely tunable based on the laser intensity and laser cooling parameters chosen. Particularly motivated sources in this frequency range are gravitationally bound states of QCD axions with decay constant near the grand unified theory (GUT) scale that form through black hole superradiance and annihilate to GWs. A 1-m meter prototype Michelson-interferometer configuration detector called the Levitated Sensor Detector (LSD) is under construction in the U.S., with a target sensitivity of better than $h \sim 10^{-19}/\sqrt{\text{Hz}}$ at 10 kHz and $h \sim 10^{-21}/\sqrt{\text{Hz}}$ at 100 kHz [Ch5-63]. Fibre-based approaches are being investigated to permit longer cavities without the need for expensive optics [Ch5-64]. A detailed analysis of the search reach for GWs produced by axions via the black hole superradiance process is provided in [Ch5-63].

### Particle/extended-object DM: levitated microspheres

A recent search has been performed for composite DM particles scattering from an optically levitated nanogram mass, cooled to an effective temperature $\sim 200~\mu\text{K}$ [Ch5-65]. This search placed limits on the interaction strength between DM and neutrons, for DM masses in the range 1–10 TeV. These searches can probe models inaccessible to large WIMP detectors or to approaches aiming to detect single nuclear or electron recoils for DM particles that collectively scatter from multiple targets, and which transfer sufficiently small energy to normal matter that such recoils would not be visible in existing detectors [Ch5-65]. Since the scattering cross section scales as $\sim 1/q^4$, where $q$ is the momentum transferred to the sphere, future searches with lower detection thresholds (or consisting of arrays of many spheres) could further improve sensitivity to these models by many orders of magnitude [Ch5-66]. The LSD is also sensitive to GWs from binary coalescence of sub-solar-mass primordial black holes and as-yet unexplored new physics in the high-frequency GW window (see Figure 5.2).

### Particle/extended-object DM: Windchime experiment

The only interaction between SM particles and DM which has been observed is that due to gravity. If DM consists of heavy particles (at the GUT scale to the Planck scale or beyond), it is possible in principle to mechanically detect small signals induced by passing DM particles in a network of accelerometers [Ch5-67], [Ch5-66]. The Windchime collaboration [Ch5-67] aims to develop a proof-of-concept experiment. Apart from lower particle flux at higher masses, a challenge for these schemes is to achieve detection sensitivity below the standard quantum limit as well as low background noise.

### Tests of “Fifth-forces” and quantum aspects of gravitation

Mechanical sensors including torsion balances and micro-oscillators have been used for tests of the equivalence principle [Ch5-68] and tests of the Newton inverse square law [Ch5-69], [Ch5-70], [Ch5-71], [Ch5-72] where several BSM theories suggest deviations [Ch5-73], [Ch5-74]. Understanding and eliminating systematic effects and backgrounds in these experiments, for example from surface patch potentials, is a primary challenge [Ch5-75]. Optically levitated particles are also well suited for such investigations [Ch5-76], providing $10^{-21}$ N force sensitivity [Ch5-77], [Ch5-78] and a different set of systematic effects and backgrounds [Ch5-79]. While probing physics at the Planck scale directly may not be possible in the near future, examining the role that gravity plays in uniquely quantum phenomena such as entanglement can provide insight
into the quantum nature of gravity [Ch5-80], [Ch5-81], [Ch5-82]. Recent experimental progress has achieved quantum ground state cooling and quantum control in levitated optomechanical systems [Ch5-83], [Ch5-84]. Proposals have been presented for using macroscopic superpositions of massive nanoparticles to test whether the gravitational field can entangle the states of two masses [Ch5-85], [Ch5-86]. The state of embedded spins in the masses can be used as a witness to probe entanglement (e.g. [Ch5-86]). A significant consideration in these experiments is to avoid non-gravitational interactions due to surface forces, external fields, and other external environmental perturbations. Technologies such as low-vibration cryogenics and ultra-high vacuum are needed to realise the potential physics reach of these methods.

### Additional optomechanical techniques

Other mechanical sensors have been employed in fifth force or ultra-light DM searches, including torsion balances [Ch5-87], [Ch5-88], micro-cantilevers [Ch5-71], and superfluid-helium based detectors for gravitational waves and dark matter [Ch5-89]. Fifth-force and equivalence-principle tests using torsion balances for example have produced wide-sweeping limits extending over orders of magnitude of DM Compton frequencies [Ch5-59], [Ch5-88]. Further advances in these technologies may also result in substantial improvements in a number of domains of physics.

### 5.3.5 Atoms, molecules, ions and atom-interferometric probes

Precision control and manipulation of atomic, molecular and ionic systems lies at the heart of many of the most striking advances in the highly dynamic field of quantum optics and atomic and molecular physics, with a very large international university community having developed over the last decades the commensurate tools and expertise. Extremely well calculable and measurable transitions, correlations and interactions provide extraordinary sensitivity to even the weakest perturbations, and makes these systems particularly well suited to precision tests of fundamental constants, to searches for unknown fields, very weak interactions or deviations from standard interactions.

Atoms, molecules and (possibly highly charged) ions in traps offer extraordinary sensitivity to dark matter-induced shifts or temporal variations of internal energy levels, allow tests of the equivalence principle, or allow searching for violations of fundamental symmetries (e.g. Lorentz- or CPT-invariance). Further areas of application are highly sensitive searches for variations of fundamental constants, tests of QED or searches for non-SM interactions (fifth forces) [Ch5-90], which can also be carried via Ramsey spectroscopy of gravitationally bound quantum states of ultra-cold neutrons [Ch5-91].

Diatomic molecules are the focus of several attempts to improve the limits on the EDM of the electron (ThO, HfF$^+$, RaF), with first exploration of the potential of polyatomic molecules to improve sensitivity even beyond those systems [Ch5-92]; these systems also provide a window into searches for hadronic T-violation or CP-violation in the nucleus (RaF, RaOH$^+$). Similarly, searches for a neutron EDM via the Ramsey technique probe BSM CP-violating interactions at scales up to 1300 TeV [Ch5-93], with the potential of a further order of magnitude in sensitivity.

To benefit from the potential of these systems, close ties between the AMO and
the particle physics communities should be fostered, and technology R&D of mutual interest, e.g. development of VUV, XUV, and soft X-ray frequency combs and lasers to drive them for HCI spectroscopy, should be encouraged in an interdisciplinary manner.

Atom interferometry using macroscopic path lengths is a particularly dynamic and promising approach to search for low frequency (Hz) variations of interaction potentials that an ensemble of atoms is subject to. This opens a specific window (see Figure 5.2) for detection of slowly varying gravitational waves that several groups are working towards using clock atom interferometry based on single-photon transitions between long-lived atomic clock states (AION, MAGIS, MIGA, ZAIGA). The same devices are equally sensitive probes for the existence of a range of dark matter candidates: ultralight wave-like DM (mass $\le 10^{-14}$ eV) (Figure 5.2), scalar- and vector-coupled DM candidates, or the presence of new interactions (short range 5-th force-like interactions). The sensitivity of individual systems can furthermore be greatly enhanced – in the near future – by combining them in local or global networks of individual atom interferometers focused on high precision fundamental physics. These should operate in tandem (similarly to networked GW detectors) and in collaboration with already existing networks in metrology institutions.

Finally, because of their macroscopic nature (meter-scale wavepacket separation), these systems are excellent probes for studies of decoherence, nonlinear interactions and more generally, of fundamental aspects of quantum mechanics. Long-distance entanglement of individual atomic systems holds further great potential for even greater sensitivity.

The technological challenges to reach the aimed-for sensitivities are numerous, and range from scaling up from table-top devices to the 100 m (ongoing), 1 km (in the development phase) and beyond scale (requiring inter alia appropriate vertical shaft infrastructure and access), to magnetic shielding (passive and possibly active) and implementing photon squeezing (to improve sensitivity), among many others.

While current systems are ground based, reaching ultimate sensitivity will require operating them in space, as proposed in the context of e.g. ESA Voyage 2050; achieving this on time scales of 20 years requires, in addition to the above, sustained R&D focusing on a single atomic species and aiming at high technical readiness level versions of complex and fragile laboratory experiments. Miniaturisation, ruggedisation, and extreme reliability of experimental equipment, as well as redundancy of components, standardised protocols and platforms, and significant cost reductions are all equally essential.

### 5.3.6 Metamaterials, low-dimensional materials, quantum materials

Materials play a crucial role in quantum sensing through their potential role as sensor but also through their function in transduction and interrogation [Ch5-94]. Materials studies are often characterised by their dimensionality. Bulk properties of materials in three dimensions (3D-materials) are engineered to provide well-defined two-level systems with long coherence times, for example, as we have seen with superconducting devices. Topological materials are being studied broadly given their interesting quantum properties that are determined by topology. Topological insulators, insulating in bulk and
conducting at the surface, are quite persistent in the face of disruptions to their physical
structures and given their interesting quantum properties, can become an important
probe for testing fundamental quantum principles. Properties of topological materials
can indicate the existence of Majorana, Weyl and Dirac fermions.

With the advent of advanced technologies, such as molecular beam epitaxy, mate-
rials can be engineered as two-dimensional systems of atomically thin layers. Many
2D-materials are under study, such as ultra-thin layers of superconductors, but also
graphene, silicene, germanene, stanene, phosphorene, transition metal dichalcogenides
(TMDs), black phosphorus, or all-inorganic perovskites, with properties that could lend
themselves to act as sensor or detector. Graphene is characterised by high mobility and
high responsivity to a broad range of wavelengths, from from the visible (532 nm) up to
the mid-infrared (~10 µm), but also into the THz band, making this a promising material
for measuring low-level THz photons, albeit at low operation speeds (O(Hz)) [Ch5-95].
In Chapter 7 emphasis is placed on integration of front-end electronics in CMOS devices.
Recently there has been progress on the integration of photonics with CMOS electronic
circuits enabling on-chip optical interconnects. The integration of non-silicon electro-
optical materials with silicon integrated circuits has been challenging. Graphene and
related 2D-materials may provide for a pathway for opto-electronic integration combined
with photodetection with speeds up to 80 GHz and extreme broadband photodetection
for ultraviolet, visible, infrared and terahertz. In addition to detection of THz photons,
2D-materials may also play roles in more traditional environments, such as in the am-
plification stages of GEM detectors in HEP, or in photo-detection of UV photons, in the
form of graphene quantum dots [Ch5-96]. Of particular relevance to the physics targets
covered in this chapter are applications of graphene as a room-temperature bolome-
ter for photons [Ch5-97], specifically graphene nanoelectromechanical systems to detect
light via resonant sensing, whose resonant frequency can furthermore be tuned via tar-
geted membrane loading with individual atoms, nanodots (or nanocrystals) or additional
mono-layers.

One-dimensional materials are materials that have strong covalent bonds in one di-
rection and weaker bonds in cross-plane directions and are often prepared as crystalline
nanowires, that is, atomic chains. Within these chains, often quantum dots are em-
bedded, a 0D-material with special quantum properties. Quantum dots have a tunable
bandgap as a result of their size. When the size of a quantum dot approaches the size of
the material’s exciton Bohr radius, quantum confinement effects become prominent and
electron energy levels must be treated as discrete energy levels. Quantum dots thus have
an energy level spacing dependent on their size. Generally, the energy bandgap increases
with a decrease in size of the quantum dot. The adjustable bandgap of quantum dots,
nanodots or nanocrystals (e.g. perovskites) allows the construction of nanostructured
materials that can provide significant improvements in the performance of, for example,
ultra-rapid frequency-tuneable scintillator materials [Ch5-98].

While the sensor families discussed in this chapter often rely on properties that are
inherent to the material being used (superconductivity, energy transitions in bound sys-
tems), quantum materials represent a step forward in that systems with specific quantum
properties are engineered by fine-tuning these properties. Examples of such materials