that rely on fabrication control at the atomic scale (and thus go beyond metamaterials) encompass atomic-layer thin surface coatings (control of the optical properties of a crystal), formation of artificial atoms in two-dimensional materials, or manipulations of trapped neutral systems to form macroscopic, extremely weakly bound Rydberg molecules, but equally topological quantum materials, van der Waals heterostructures based on atomically-thin 2D-crystals, spin torque materials, Moiré materials and many others [Ch5-99].

Materials with quantum mechanically tuneable electronic, magnetic or structural properties (which can furthermore be combined into heterostructures) hold great promise as their emergent properties are highly sensitive to environmental perturbations or transient phenomena, and can greatly amplify, via collective transitions, any interactions related to spin, charge or orbital states in response to external fields.

## 5.4 Observations and Prospects

Developments in the field of quantum sensing and other emerging technologies are taking place at a very rapid pace. Consequently, extrapolation beyond the next 15 years is fraught with risk. Nonetheless, a number of increasingly ambitious physics targets would clearly benefit from increased, dedicated R&D efforts over the next two decades. In Figure 5.1 the R&D needs for the presented families of quantum sensing approaches was summarised to achieve the science goals in the different categories of evolving physics targets. That representation maps the technologies onto physics targets for different time windows, with the colour code indicating the R&D needs. In Figure 5.3 a different representation is used where the impact of the proposed R&D programme is presented. The quantum and emerging technology categories are indicated again, but under each category the reach of the directed R&D for the different physics targets is quantified, indicating the timeframe within which this could be achieved. Increases in sensitivity or accessible range are highlighted in red.

This representation covers a very wide range of physics targets and technologies, 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 DM searches, in searches for new interactions, probing fundamental symmetries, measuring the relic neutrino mass or even foundational aspects of quantum mechanics. Long-term prospects include transforming conceptual ideas for HEP detectors based on quantum sensing ideas into prototype or even functional HEP detectors, or even addressing completely novel fields, such as searches for relic neutrinos.

The promise of quantum-enhanced and fundamentally new sensors will have transformational impact with a dedicated R&D programme that is fully complementary, but equally impactful, to the traditional methods of exploration in particle physics.
<figure><img src="image_19.png" /><figcaption>Figure 5.3: Prospective time line for selected developments with a range of quantum and emerging technologies; increases in sensitivity or accessible range are highlighted in red.</figcaption></figure>

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# Chapter 6

# Calorimetry

## 6.1 Introduction

During the last decades, the role of calorimeters in High-Energy Physics (HEP) has become more and more important, and complex. Today’s detectors are required to:

* Measure energy deposits with high accuracy and precision. Thanks to the statistical nature of showering process(es) and the high number of particles involved, the response is typically linear while the relative precision scales with the inverse square root of the energy;

* Provide highly selective trigger information, in particular in high-rate high-background environments. Calorimeters are in general based on fast processes and can easily provide triggering signals at high rate in correspondence to (high) local energy depositions;

* Provide information for particle identification and final-state identification (through the shower topology and through timing).

Past and present implementations are normally addressing two complementary sets of measurements: the identification and measurement of all energy deposits generated by primary electromagnetic (EM) particles ($e^{\pm}$ and $\gamma$-initiated showers), done with EM calorimeters, and the measurement of hadron-initiated showers which are observed in combined systems of an EM calorimeter in the front and a hadronic calorimeter behind. In addition EM calorimeters are separated into:

* Sampling calorimeters, that allow the realisation of fast and highly granular devices at affordable costs. They can provide an energy resolution of $\mathcal{O}(10\%)/\sqrt{E}$;

* Homogeneous calorimeters, that often have higher costs and a lower granularity (and often a slower response) but a much better energy resolution, $\mathcal{O}(1-3\%)/\sqrt{E}$.

On the other hand, hadronic showers develop both a hadronic component (due to nuclear interacting particles) and an EM one (due to the EM decay of neutral mesons such as $\pi^0$ and $\eta$) which produce, in almost all detector implementations, a different signal
response ($e/h \neq 1$), due to "invisible" contributions to the hadronic energy deposition, such as nuclear excitation. In this case, the calorimeter is said to be "non-compensating", and the hadronic energy measurements are compromised by the fluctuations in the EM fraction ($f_{EM}$) and ultimately limited by the fluctuations of the invisible component. Nonetheless, strategies for response equalisation (i.e. compensation) exist and, indeed, hadron calorimeters can be divided into:

* Non-compensating calorimeters, offering higher degrees of freedom to optimise cost or 3D-segmentation;

* Compensating calorimeters, that require a fixed sampling fraction and specific choices of absorber and active medium, but have a much better energy resolution ($\sim 30\%/\sqrt{E}$ compared to $\sim 45-60\%/\sqrt{E}$ or worse for non-compensating calorimeters).

For many years, there have been two main lines of research to tackle the problem of optimising the energy resolution for hadronic jets and achieving the best possible overall energy measurement in collision events. The first exploits Particle Flow (PF) Algorithms, employing highly granular systems to disentangle neutral- and (dominating) charged-particle contributions and using the main tracker to precisely measure the latter ones. The highly granular calorimeter information can be used in addition to improve the hadronic energy resolution by equalising the response to EM and hadronic shower components in the reconstruction ("software compensation"). The second approach (Dual Readout, DR) is based on the measurement of all the energy deposits through two different processes, usually scintillation and Čerenkov light emission, the former produced by all ionising particles, the latter only by relativistic charged particles (i.e. mostly $e^{\pm}$ from the EM shower component). After a calibration with electrons, the combination of the two signals allows an event-by-event estimate of $f_{EM}$, strongly improving the hadron-shower energy measurements. In all cases, new developments target highly granular 5D-(3D-space, $E, t$)-detectors where timing plays an important role as well.

## 6.2 Main drivers from the facilities

Broadly speaking the near- and mid-term R&D programme has to meet the needs of the HL-LHC experiments, of the calorimeters for future $e^{+}e^{-}$ Higgs-EW-top factories and for an electron-ion collider. For the longer term, strategic R&D to address requirements of muon and hadron colliders, e.g. radiation tolerance significantly beyond today's standards, must be pursued. This leads to the target definitions and associated required progress in technology that are summarised in Figure 6.1 and described further below.

### DRDT 6.1 - Develop radiation-hard calorimeters with enhanced electromagnetic energy and timing resolution.

Priority developments are geared towards the successful completion of R&D programmes for the variety of HL-LHC detector upgrades aiming at much finer spatial granularity
and high accuracy timing while still delivering excellent energy resolution in high radia-
tion environments. LHCb plans an upgrade of the inner part of their EM calorimeter for
LHC LS4. The upgrade aims at a time resolution of 50 ps and an EM energy resolution
of around 10%/$\sqrt{E}$[GeV] $\oplus$ 1%. LHCb shares R&D goals with the fixed target experi-
ment KLEVER [Ch6-1] in terms of sustainable rates and time resolution. An excellent
EM energy resolution of a few % over $\sqrt{E}$ is targeted for an EIC Detector, while the
ALICE FOCAL (discussed below) primarily targets ultra-high granularity.

<figure><img src="image_20.png" /><figcaption>Figure 6.1: Schematic timeline of categories of experiments employing calorimetry 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).</figcaption></figure>
**DRDT 6.2 - Develop high-granular calorimeters with multi-dimensional read-out for optimised use of particle flow methods.**

This theme principally addresses calorimeters at future Higgs-EW-Top factories, but a granular EM calorimeter is also an interesting option for the near detector of DUNE. During LS3 ALICE envisages the installation of the FOCAL detector, which introduces CMOS sensors to calorimetry. Typically for a given Higgs-EW-Top factory one aims at a jet-energy resolution of 3%-4% over the entire centre-of-mass range, through PF methods. Developments are focused on combining information from the calorimeter with other detector systems. Capitalising on the experience at the HL-LHC, the objective is to achieve 3D-pixelated calorimetry to measure each particle individually and use of precise tracking information for charged particles. This goal can be achieved using silicon sensors (Chapter 3) or SiPMs (Chapter 4) coupled to scintillator tiles or strips as active medium, as well as gaseous detectors (Chapter 1) in the case of hadronic calorimeters. Liquified noble gas (Chapter 2) may often represent a performant and cost-effective alternative.

Further developments are to be pursued to exploit the different signals produced by the electromagnetic and non-electromagnetic components of hadron showers in a Dual Readout system. Eventually, combining both approaches might lead to an optimised jet energy resolution; for example, using finely-segmented crystals with excellent EM energy resolution to complement a hadronic DR section. In all cases the inclusion of timing at the 10 ps level can significantly improve the quality of the event reconstruction.

A notable difference between linear and circular colliders is the beam structure, with the former allowing for a pulsed operation of the front-end electronics, leading to reduced services (e.g. cooling, powering) and allowing a more compact detector design when compared to that possible at circular machines with continuous operation.

**DRDT 6.3 - Develop calorimeters for extreme radiation, rate and pile-up environments.**

Long-term R&D is to start for calorimetry systems at a future hadron collider to address up to two orders of magnitude more severe requirements than at the HL-LHC, related to unprecedented radiation hardness, pattern recognition in the presence of severe pile-up and the associated data handling. Radiation levels at a Muon Collider are between that of HL-LHC and the FCC-hh. The Muon Collider also has challenges in pattern recognition and the need for techniques to reduce beam induced backgrounds.

The R&D timelines for the above themes can be found at Figure 11.1 with attached explanation. R&D for calorimetry has a particularly long lead-time due to the duration of the stage for experiment specific final prototyping, procurement, production, assembly, commissioning and installation. For the facilities listed in Figure 3 and Figure 4 in the Introduction, the most demanding requirements associated with DRDT 6.1 include those for next decade further upgrades of ALICE and LHCb. Particle Flow based on high granularity calorimetry (DRDT 6.2) is particularly important for future e$^+$e$^-$ Higgs-EW-top factories and an option to be seriously considered for the electron-ion-colliders. Beyond the HL-LHC upgrades this decade, the requirement of extreme
radiation-hardness and pile-up rejection (DRDT 6.3) becomes critical for the FCC-hh in particular.

# 6.3 Key technologies

## 6.3.1 Silicon-based calorimeters

Silicon offers several attractive features for calorimeters in HEP experiments, in particular for highly-granular EM sampling calorimeters. Silicon sensors are highly segmentable, show a robust and stable performance over time and over a wide range of environmental parameters, and are tolerant to high levels of radiation. Silicon has a relatively high density, resulting in suitable sampling fractions for analog energy-based calorimetric measurements also when using relatively thin sensor layers, and enables very compact systems with good shower separation in combination with high-density absorbers such as tungsten. There are two general classes of applications of silicon-based calorimeters in HEP:

### Smaller systems with specific applications

Examples include: beam/luminosity calorimeters; dedicated far-forward calorimeters for photon tagging and reconstruction [Ch6-2], [Ch6-3]; and compact calorimeters in satellite borne experiments [Ch6-4].

### Main calorimeter systems in collider detectors

Developments of highly-granular calorimeters using Si/W as the main EM calorimeters for collider experiments were pioneered by the CALICE collaboration [Ch6-5], achieving excellent two-particle separation in combination with a highly granular hadron calorimeter [Ch6-6], and an EM energy resolution of approximately $16.5\%/\sqrt{E[GeV]} \oplus 1\%$ [Ch6-7]. A first large-scale highly granular silicon-based calorimeter system, the High Granularity Calorimeter (HGCAL) [Ch6-8] will be implemented for the Phase II upgrade of the CMS endcap calorimeters, with a total silicon area of approximately 600 m². ALICE-FOCAL and the ALICE pre-shower detector would be first applications of CMOS based sensors for calorimetry. Systems with sensor areas of up to 2500 m² and $10^8$ channels and more are being planned for the main calorimeters (barrel and endcap) of detectors at Higgs-EW-Top factories. Spin-off applications of the technology developments for towards these large systems, such as the planned LUXE experiment [Ch6-9] at DESY with a scale of typical prototypes (i.e. silicon area of approximately 1 m²), will also directly benefit from the further R&D described in this section.

### 6.3.1.1 Challenges and requirements for future projects

The applications introduced above have overlapping, but also specific technological requirements and challenges, which fall into two broad categories: one related to the system aspects; the other to sensor technology.

On the system side, large Si-based calorimeter systems at colliders require fully embedded electronics with high dynamic range, low noise, low power and maximum compactness, and suitable interconnect technologies between sensors and electronics PCBs,
which also address edge effects that arise in particular in circular endcap geometries. An integrated approach for the mechanical and electronic design, as well as for cooling and services is crucial, as are scalability, automation in assembly, and documentation. The large number of channels ($O(10^8)$ or more), and the large sensor surfaces impose new challenges in operation and calibration, and result in the need for redundancy and complex monitoring.

Smaller Si-based calorimeters typically do not have the same system-level requirements, but target specific applications that require maximum compactness with the smallest possible Molière radius, sensors and front-end systems capable of handling occupancies close to 100%, and extreme mechanical precision. This latter can be a few 10 µm for luminosity monitoring at linear $e^+e^-$ colliders, and as low as 1 µm for circular $e^+e^-$ colliders targeting high-precision Z-pole programmes with $10^{12}$ recorded Z-bosons.

On the sensor side, small dead zones, in particular at the sensor edges, are important for the overall calorimetric performance. Precision timing will gain in importance as requirements increase and the performance in this area improves, as discussed in more detail in Section 6.3.5. Large signals, either through thicker sensors, or via physical gain built-in to the sensor (like e.g. LGADs), can improve the signal-to-noise ratio (SNR) and, consequently, energy and time resolution. Small pads or pixels improve the spatial granularity of the calorimeter systems and can also improve SNR at the expense of more electronic channels. Radiation tolerance significantly beyond the current state of the art becomes relevant for future hadron colliders. Here, sensitivity to minimum-ionising particles also at the end of life of the system is important for a robust calibration scheme. Calorimeter applications are in general significantly more sensitive to sensor cost than tracking detectors, since the required sensor area is typically 30 times larger, up to 2500 m² or beyond, for full systems at future electron-positron colliders.

### 6.3.1.2 Main R&D Directions

#### Silicon based calorimeters

<table><caption>Table 6.1: Overview of main R&D needs and corresponding directions of development for silicon-based calorimeters connected to facilities and DRDTs.</caption><thead><tr><td>R&D Need</td><td>Main direction</td><td>Target facilities</td><td>Related DRDT</td></tr></thead><tbody><tr><td>Reduction of passive space</td><td>Larger wafers, smaller guardrings, suited mechanical structures</td><td>ILC, FCC-ee, CLIC, FCC-hh, Muon Collider</td><td>6.2, 6.3</td></tr><tr><td>Increase of signal</td><td>Thicker Si sensors, active gain</td><td>ILC, FCC-ee, CLIC, Muon Collider</td><td>6.2, 6.3</td></tr><tr><td>New Technologies</td><td>CMOS based sensors, digital SiPMs</td><td>HL-LHC, ILC, FCC-ee, CLIC, FCC-hh, Muon Collider</td><td>6.1, 6.2, 6.3</td></tr><tr><td>New Materials</td><td>GaAs for beam calorimeters, improved radiation hardness</td><td>ILC, FCC-ee, CLIC, FCC-hh, Muon Collider</td><td>6.2, 6.3</td></tr></tbody></table>

From the requirements outlined above, a few main R&D directions arise for silicon-based calorimeters.

* The reduction of dead space, through larger wafers and novel guard-ring designs. For full exploitation novel precise mechanical structures play a key role;
* *Increase of signal*, via increased sensor thickness or physical gain;

* *Control of power consumption*: The high channel density of silicon based calorimeters calls for power economical solutions for the front-end electronics. Power pulsed operation is one approach. For continuous powering a power consumption well below 1 mW/cm² should be targeted. Therefore electronics for silicon based calorimeters are particularly challenging use cases for the goals formulated in DRDT 7.1 (see Chapter 7);

* The use of *new technologies*, such as CMOS-based monolithic active pixels sensors. Calorimeters have to cover large areas. This requires large Si wafers and/or reliable stitching techniques. Both are among R&D topics described in Chapter 3 but would need to be adapted to calorimeter needs (coarser granularity than for trackers, miniaturisation of services, signal transport over larger distances). 3D-integration of sensors may lead to reduction of dead space in addition to the means described above. New technologies may have increased intelligence for digital EM calorimeters and possible fully-digital approaches with internal gain for faster response inspired by d-SiPMs, see Section 4.4.3. For low-occupancy applications, additional position sensitivity within pads may yield lower channel counts and reduced power in the electronics;

* *New materials*, which offer higher density and suitable band-gaps may offer performance benefits for specialised applications, as already seen for GaAs in luminosity detectors. Further improved radiation tolerance will become relevant in the more distant future.

These R&D directions are put into the context of future facilities and overarching DRDTs in Table 6.1.

### 6.3.2 Calorimetry Based on Liquified Noble Gases

Calorimetry based on liquified noble gases was successfully used in many high-energy experiments (e.g. NA48, SLD, H1, D0, ATLAS) due to its energy resolution (a stochastic term of $10\%/\sqrt{E}$ can be easily achieved), linearity, stability, uniformity, timing resolution ($O(100 \text{ ps})$ for highly-energetic showers has been achieved) and radiation tolerance. While the latter is only a concern for future hadron colliders, all other properties are necessary for all future colliders. The granularity, can be adjusted to the needs, with readout cell sizes of $x_\Theta \times x_\phi \times x_{\text{depth}}$ of $5 \times 10 \times 20 \text{ mm}^3$ achievable by carefully designing the readout electrodes. Liquid argon is the most common medium (e.g. ATLAS LAr [Ch6-10]) and a similar design, using either argon or krypton, has been proposed for the FCC-hh baseline detector [Ch6-11] and a design is underway for FCC-ee. The R&D programme will combine the excellent EM energy resolution of noble-liquid based calorimeters with the benefits of high readout granularity, facilitating particle-flow with 4D- and 5D- shower imaging.
### 6.3.2.1 Challenges and requirements for future projects

The challenges and requirements for calorimeters differ substantially between $e^{+}e^{-}$ Higgs-EW-top factories and future hadron colliders. However, there are some commonalities:

* High readout granularity: For pile-up mitigation and particle-flow reconstruction, including techniques using 4D- or 5D-shower images;

* Timing information: Timing precision of order $O(1 \text{ ns})$ will be crucial for triggering and attributing the energy deposits to the correct bunch crossing. A timing resolution of $O(30 \text{ ps})$ will be essential for pile-up rejection in future hadron colliders;

* Minimisation of passive material in front of the calorimeter: Cryostats, mechanical support structures and services need to be minimised in terms of material. Heavy calorimeters (100's of tonnes) need to be supported by these "low-material" cryostats.

Specific challenges for the different major collider projects are summarised here:

* Future $e^{+}e^{-}$ Higgs-EW-Top factories: The required EM energy resolution (sampling term of 5 to 8%/$\sqrt{E}$) and photon-energy measurement down to $\sim 300$ MeV call for low-material-budget cryostats and extremely low-noise readout electronics. Excellent jet-energy resolution is only achievable by particle-flow algorithms relying on high-granularity measurements. Extremely low noise levels, to enable detection of single MIPs in each of the cells, require a careful optimisation of the readout electrodes and the readout electronics. For timing precision the situation is much less clear, and a final requirement will have to take into account the price of better timing information in terms of power and readout noise; but some performance improvement could be expected by completing the 4D-imaging of the particle showers with its time development;

* Future 100 TeV Hadron Colliders: An expected 1 MeV neutron equivalent fluence and total integrated dose in the central calorimeters (forward calorimeters) of $4 \times 10^{15} \text{n}_{\text{eq}} \text{cm}^{-2}$ ($5 \times 10^{18} \text{n}_{\text{eq}} \text{cm}^{-2}$) and 4 MGy (5 GGy), respectively (for $30 \text{ ab}^{-1}$), will require a careful selection and radiation testing of all materials used. The huge number of up to 1000 simultaneous collisions per bunch crossing will require an efficient pile-up rejection, which will have to rely on precision timing $O(30 \text{ ps})$, highly-granular calorimeter information and an efficient combination with the tracker measurements.

### 6.3.2.2 Main R&D Directions

The above listed challenges and requirements lead to the following R&D directions:

* R&D to achieve high granularity readout, comprising R&D on low-noise and low cross-talk, finely segmented readout electrodes and dense signal feedthroughs. This requires multi-layer PCB readout electrodes with embedded traces sandwiched
between ground layers to prevent cross-talk. These ground layers increase the capacitance of the readout cells and therefore impact the readout noise;

* R&D on warm (i.e. outside the cryostat) and cold (i.e. inside the cryostat) readout electronics minimising noise. Cold electronics would improve considerably the signal-over-noise ratio. It faces however technological challenges such as more involved maintenance and potential heat dissipation inside the noble liquid. The R&D carried out for DUNE (see Chapter 2) may give valuable input on this matter. In case of warm electronics, the high granularity of future calorimeters implies an increased signal density at the feedthroughs of up to 20-50 signals/cm². This is a factor about five to ten more than in the ATLAS experiment;

* R&D on low material-budget cryostats, see also Section 8.3.5. For composite-material cryostats, vacuum-tight junctions between the cryostat and metallic flanges (e.g. for feedthroughs) for warm and cryogenic temperatures will be required;

* Performance optimisation addressing the sampling frequency, the sampling fraction, the choice of the active medium (LAr or LKr), the absorber material and the overall geometry;

* R&D on the limits of timing precision aiming at $\mathcal{O}(30 \text{ ps})$ for larger energy deposits, as required for the application in future hadron colliders. The precision for timing of MIPs must also be optimised.

In addition it is important to study whether using the scintillation light produced in the noble liquid, following the R&D described in Chapter 2, could be used to improve on any of the above parameters.

These R&D needs are put into the context of future facilities and overarching DRDTs in Table 6.2.

<table><caption>Calorimeters based on Liquified Noble Gases</caption><thead><tr><td>R&D Need</td><td>Main direction</td><td>Target facilities</td><td>Related DRDT</td></tr></thead><tbody><tr><td>High granularity for PFA and 4D-imaging</td><td>Finely segmented readout electrodes, dense signal feedthroughs</td><td>ILC, FCC-ee, CLIC, FCC-hh, Muon Collider</td><td>6.2, 6.3</td></tr><tr><td>Low noise and low power electronics</td><td>Electronics at room temperature or in cryogenic environment minimising the read-out noise</td><td>DUNE, ILC, FCC-ee, CLIC, Muon Collider</td><td>6.1, 6.2, 6.3</td></tr><tr><td>Low material budget</td><td>Thin cryostats to maximise calorimeter performance using composite materials or Al honeycomb</td><td>ILC, FCC-ee, CLIC, FCC-hh, Muon Collider</td><td>6.2, 6.3</td></tr><tr><td>Optimisation of calorimeter performance</td><td>Choice of sensitive material (e.g. LAr or LKr), absorber material, geometry optimisation, detection and exploitation of scintillation light</td><td>DUNE, ILC, FCC-ee, CLIC, FCC-hh, Muon Collider</td><td>6.1, 6.2, 6.3</td></tr><tr><td>Timing precision</td><td>Timing precision of $\mathcal{O}(30 \text{ ps})$ will be extremely beneficial for pileup rejection</td><td>FCC-hh, Muon Collider</td><td>6.3</td></tr></tbody></table>

<caption>Table 6.2: Overview of main R&D needs and corresponding directions of development for calorimeters based on Liquified Noble Gases connected to facilities and DRDTs.</caption>
### 6.3.3 Calorimetry Based on Gaseous Detectors

Gas detectors as active elements have been proposed mainly for future hadronic calorimeters but could also be options for EM calorimeters (see Chapter 1). While the intrinsically-low sampling fraction results in rather modest energy resolution, gas detectors offer some attractive features: adaptability of readout pad sizes (from 1 cm² to 10 cm²) and sampling frequency; high efficiency for single MIPs; excellent spatial resolution of 50 to 100 μm; and applicability to large cost-effective calorimeter systems. This is especially true for $e^{+}e^{-}$ Higgs-EW-top factories with benign radiation environments, modest particle rates and relatively-low particle multiplicity, but where excellent two-jet invariant-mass resolution is needed to classify all relevant final states. Gas detectors are also an interesting option for pre-samplers and tail catchers. Timing on the $\mathcal{O}(5 \text{ ns})$ level can be easily achieved and some configurations can be optimised for < 100 ps. Still, their integration in a calorimeter would require specific studies. While at $e^{+}e^{-}$ Higgs-EW-top factories timing resolution of $\mathcal{O}(10 \text{ ps})$ might be used to improve shower reconstruction through 5D imaging, it is mandatory for pile-up rejection in future 100 TeV hadron colliders.

Calorimeters based on Resistive Plate Chambers (RPCs) or MicroPattern Gas Detectors (MPGDs) have recently been studied by the CALICE collaboration [Ch6-12]. Two full hadronic prototypes using single-gap RPCs have been built and tested in particle beams: one digital HCAL with Fe and W absorber (38 +14 layers 1 m²) and a semi-digital HCAL with Fe absorber (48 layers 1 m²). Several layers of Micromegas and prototypes of GEM and of resistive plate WELL detectors have been built and tested. The SCREAM project [Ch6-13] is studying sampling calorimeters with resistive anode MPGDs.

#### 6.3.3.1 R&D needs for Gaseous Calorimeters

While R&D on gas detectors is generally covered in Chapter 1, specific challenges are listed here related to their use in calorimeters for future collider projects.

* Scalability of the technology and the production of large-area detectors with sufficient uniformity and response stability. PCBs of several m² with per-cent level flatness are beyond today's industry standards, calling for a close collaboration with industry;

* For future hadron colliders the rate capability has to be improved. R&D on semi-conductive glass RPCs is one way to improve on this parameter;

* Clever solutions to avoid double counting on cell edges have to be developed;

* Dedicated R&D targeted to achieving the required timing precision for particular applications ($e^{+}e^{-}$ or hadron collider).

These R&D needs are put into the context of future facilities and overarching DRDTs in Table 6.3.
Calorimeters based on Gaseous Readout

<table><thead><tr><th>R&D Need</th><th>Main direction</th><th>Target facilities</th><th>Related DRDT</th></tr></thead><tbody><tr><td>Scalability of technology</td><td>Large area PCBs with robust interconnection, large scale precise absorber structures</td><td>ILC, FCC-ee, CLIC, FCC-hh</td><td>6.2, 6.3</td></tr><tr><td>Rate capability</td><td>Semi-conductive Glass RPC</td><td>FCC-hh</td><td>6.3</td></tr><tr><td>Control of pad multiplicity</td><td>Avoid/reduce double counting on cell edges</td><td>ILC, FCC-ee, CLIC, FCC-hh</td><td>6.2, 6.3</td></tr></tbody></table>

Table 6.3: Overview of main R&D needs and corresponding directions of development for calorimeters based on gaseous readout connected to facilities and DRDTs.

### 6.3.4 Calorimeters with light-based readout

#### 6.3.4.1 State-of-the-art

Light-based-readout calorimeters exploit the properties of light-transparent media to emit light either when excited by ionising radiation (scintillators) and/or when crossed by fast charged particles (Čerenkov radiators). The amount of emitted light correlates with the energy absorbed by the medium. The readout is light-based: a photosensitive device is either directly coupled to the medium or through wavelength-shifting (WLS) fibres. Classical readout options exploit vacuum photomultiplier tubes, while advanced configurations – permitting compact designs and high segmentation – include avalanche photodiodes (APDs) and silicon photomultipliers (SiPMs). Light-based-readout calorimeters are well established, with a long record of successful applications in many experiments. Crystals, plastic scintillators and light-emitting fibres are excellent candidates for future calorimeters. These active media can be adopted in compact designs for HEP collider experiments, but also for specific needs of low-energy and fixed target experiments.

High-resolution measurements of photons and electrons down to small particle energies call for crystal-based calorimeters (e.g. L3, Babar, Belle, CMS ECAL, etc.). When the requirements on the EM energy resolution are less stringent, sampling EM calorimeters, consisting of an active medium and a passive absorber (e.g. Pb, Cu, W), are often chosen for cost considerations. Popular solutions include lead with scintillating fibres (“SPACAL”), sandwiches with WLS fibres crossing through the active (and passive) material (“Shashlik”), or tiles with local readout of the light. The actual sampling fraction depends on performance needs, integration and radiation constraints. Both, crystal-based (e.g. LHCb R&D) and plastic-tile options (e.g. DUNE Near Detector R&D, CALICE ScECAL R&D) are under study.

Plastic scintillators (tiles or strips), allowing a cost-effective coverage over large areas, are particularly well suited for hadron sampling calorimeters (e.g., ATLAS TileCal and CMS HCAL). For ultimate precision in PF reconstruction of the jet energy, fine segmentation is paramount, in both, the EM and the hadronic calorimeter compartments (e.g. CMS HGCAL, CALICE AHCAL and ScECAL R&D). Dual readout (DR) of scintillation and Čerenkov light provides a complementary approach for jet-energy precision measurements. Fibre-based calorimeters are the baseline candidate for DR energy reconstruction (DREAM/RD52) in highly granular 4D-detectors [Ch6-14]. An integrally active DR calorimeter is currently tested for the REDTOP/ADRIANO pro-
posal [Ch6-15]. Compact systems combining PF (segmentation in space and potentially in time) and DR (multiple readout) may compound the benefits of either reconstruction method and have a great potential interest for future applications [Ch6-16], [Ch6-17].

### 6.3.4.2 Challenges and requirements for future projects

Precision EW measurements at future $e^{+}e^{-}$ factories require two-jet invariant-mass resolutions of about 3% at the Z-, W-, and Higgs boson mass values for efficient identification of the final state. An EM compartment with enhanced performance ($< 3\%/\sqrt{E}$) would provide access to the reconstruction of exclusive states for precision tests in flavour physics. To match these requirements with light-based-readout calorimeters, several challenges arise related to system aspects, to the validation of the conceptual designs, and to the sensor technology, with significant differences between pure PF and pure DR calorimeter systems. At $\mu^{+}\mu^{-}$ and hadron colliders, the performance goals are similar, while beam background and the radiation levels impose further requirements on the time response and radiation tolerance of the system.

At system level, PF calorimeters with fine 3D-segmentation require a significant integration effort with embedded photon detectors and readout electronics with high dynamic range, low noise, and low power. The routing of signals, services and cooling should be compliant with a hermetic design. Complex thermal management, possibly including options for local cooling and in-situ annealing of the photon detectors (e.g. with thermoelectric coolers/heaters), is relevant for future hadron colliders. In DR calorimeters, the complexity of the integration is moved to the back of the calorimeter where the optical fibres exit. Options for longitudinal segmentation are presently focused on timing measurements. Both systems share the need for precision machining of absorbers. The optimisation of a high-performance EM calorimeter compliant with PF and/or DR reconstruction is another challenge. In PF systems, a (crystal-based) EM calorimeter with a high-sampling fraction should be finely longitudinally segmented. In DR systems, crystals with an appropriate spectral difference of the Čerenkov and scintillation light emission are required. Typical EM calorimeters consisting of homogeneous crystal blocks cannot detect the event-by-event fluctuations between the EM and the strong interaction components of hadron showers and have e/h ratios that spoil the hadron energy resolution.

Photodetectors with high photoelectron efficiency and a broad range of spectral responses are needed, to match the emission spectra of a variety of scintillators and Čerenkov materials. Response linearity, B-field immunity, compact devices, and excellent timing are crucial. Radiation hardness significantly beyond the current state of the art is particularly relevant for future hadron colliders. These requirements are common across the different light-based readout calorimeters and are typically similar, if not less stringent, to those for application in particle identification systems.

Challenges in the choice and development of the active media are different for crystals, plastic scintillators, and fibres, but fall into three main categories: the search for novel materials; the optimisation of time measurements; the identification of radiation-tolerant materials or configurations for application at future hadron colliders.
### 6.3.4.3 Main R&D directions

Development and optimisation of the *photon detectors* is a field of intensive R&D. Close collaboration with industrial partners yielded SiPMs with spectral responses covering a wide range of wavelengths with high quantum efficiency. Further evolution, such as digital SiPMs, appears desirable. Novel SiPM materials, such as high band-gaps semiconductor, hold the promise of higher radiation tolerance. More details about the main R&D directions for photon detectors are covered in the Section 4.4.3.

#### New material technologies

Novel techniques for crystal growth have broadened the range of potential configurations for crystal-based calorimeters, including crystal-fibre EM calorimeters and multiple-readout calorimeters [Ch6-18], [Ch6-19], [Ch6-20]. A SPACAL calorimeter, using co-doped garnet crystal fibres (GAGG, YAG, GYAGG), is proposed for the upgrade of the LHCb ECAL [Ch6-21], for improved energy resolution, shower timing with ten ps precision, and appropriate radiation hardness. Further improvements in radiation hardness will become relevant for future hadron colliders. Heavy scintillating glasses such as DSB : Ce³⁺ are investigated as a cost effective alternative to e.g. the common PbWO₄ crystals [Ch6-22], [Ch6-23]. Beyond, new plastic scintillators will be needed to improve radiation hardness and/or for use in multiple-readout options with Čerenkov and scintillation emission. The exploration of 3D-printing technologies in the production of scintillators [Ch6-24], as well as for mass production of precision absorbers in collaboration with industrial partners are promising R&D lines.

#### Timing in event or shower reconstruction

The use of timing in event or shower reconstruction is emerging as an interesting new field, with promising potential benefits for the jet-energy resolution. While some precision timing with crystal-based calorimeters has been demonstrated, specific R&D is needed with both, plastic and crystal scintillators for further appraisal of the critical design parameters (e.g. light output, time constants, discrimination threshold, granularity of the time information) for a calorimeter offering intrinsic fast-timing capabilities. Promising new technologies in this area include Nanostructured-Organo-silicon-Luminophores (NOLs) (see Section 4.4.5) due to the innovative transmission of the scintillation light to the wavelength shifting fibre. Chapter 5 also introduces quantum dots. This approach yields a higher transparency since the embedded nanomaterial has a different band structure to that of its surroundings.

These R&D directions are put into the context of future facilities and overarching DRDTs in Table 6.4.

### 6.3.5 Precision timing in calorimetry

The main use of sub-ns timing precision in calorimeters has been largely focused on in-time pile-up suppression at hadron colliders such as the LHC and HL-LHC. For the latter in particular, with a possible pile-up of 200 simultaneous primary collisions, the new wave of calorimeters is being designed with ~30 ps precision to allow the many vertices that overlap spatially to be separated in the time domain. Achieving this level of precision
<table><caption>Calorimeters based on Optical Readout</caption><thead><tr><td>R&D Need</td><td>Main Directions</td><td>Target facilities</td><td>Related DRDT</td></tr></thead><tbody><tr><td>Optimisation of Photon detectors</td><td>Novel SiPMs with large spectral sensitivity and high-band semiconductors for higher radiation tolerance, Digital SiPMs</td><td>ILC, FCC-ee, CLIC, FCC-hh, Muon Collider</td><td>6.2, 6.3</td></tr><tr><td>Novel crystal technologies</td><td>Co-doped garnet crystal fibres</td><td>HL-LHC, ILC, FCC-ee, CLIC, FCC-hh, Muon Collider</td><td>6.1, 6.2, 6.3</td></tr><tr><td>Longitudinal information</td><td>Longitudinal segmentation of crystals, z-position from timing</td><td>HL-LHC, ILC, FCC-ee, CLIC, FCC-hh, Muon Collider</td><td>6.1, 6.2, 6.3</td></tr><tr><td>Novel plastic scintillators</td><td>Radiation hardness, implementation of dual readout</td><td>ILC, FCC-ee, CLIC, FCC-hh, Muon Collider</td><td>6.2, 6.3</td></tr></tbody></table>

Table 6.4: Overview of main R&D needs and corresponding directions of development for calorimeters based on Optical Readout connected to facilities and DRDTs.

requires fast signal production with a good signal-to-noise ratio, with high-precision TDCs and clock distribution to match. Existing R&D has targeted all these three critical components with excellent results. The ongoing R&D for the LHCb SPACAL has already been introduced in Section 6.3.4.3. R&D on crystals with different dopants is targeting faster decay times. For example, $BaF_2$ has a decay time of ~0.5 ns when doped with yttrium, making it suitable for ultrafast calorimetry for fast-repetition-rate colliders. $PbF_2$ is investigated for KLEVER and the Muon Collider. Fast sampling can be used for the removal of out-of-time pile-up, as done, for example, for the present CMS ECAL using *template fits* to crystal time samples.

### 6.3.5.1 Use of timing information for enhanced calorimetric performance

An emerging field is the use of high-precision timing to trace the evolution of EM and hadronic showers and extract information that can be used to enhance the calorimeter performance, see Figure 6.2. It is easily apparent that measurements of the spatial evolution of showers at different times can provide information about the particle type and support particle separation in calorimeters. The inclusion of timing information into convolutional and graph neural networks shows an improvement in energy resolution when compared to simple cluster sums, particularly when shorter time slices are included, see also Ref. [Ch6-25]. Similar studies have shown that the use of time information can improve calorimeter linearity, particularly important for ultra-high-energy colliders such as FCC-hh. Note finally that time stamping of calorimeters hits can be used to search for new long-lived particles.

### 6.3.5.2 R&D Needs

As described before and sketched in Table 6.1 trends in calorimetry and the challenges of future colliders require timing precision down ~10 ps by 2030 and maybe better in the longer term. There are promising developments in fast crystals (including crystal fibres) that can be suitable for $e^+e^-$ and muon colliders (low dose/fluence) but there
<figure><img src="image_21.png" /><figcaption>Figure 6.2: Left: Time evolution (0-1.3 ns, shown with a colour scale) of a 250 GeV positron shower (including bremsstrahlung photon showers) as measured with a CMS-HGCAL prototype; Centre: R-Z evolution of electron (top) and charged-pion (bottom) showers for two different time periods; Right: energy resolution of a simulated high-granularity Cu-Si calorimeter, showing the improvement in performance for 30 and 100 GeV pions when using neural networks (CNN), and when including time-slice information (GNN) [Ch6-25].</figcaption></figure>

remains significant R&D to optimise the rise/fall times and scale these systems up to large-volume detectors. Developments of ultra-fast integrated photon detectors, such as digital SiPMs, may provide timing performance to match these new crystals, at the same time negating the need for separate ASICs. The most challenging future collider, on the timescale considered here, will be the FCC-hh, due to its expected pile-up of 1000 events, complex collision topologies and possible bunch-crossing frequency of 200 MHz. A timing precision of ~5 ps will be required to help mitigate these challenging conditions. This will require significant R&D in the domains of active materials, light detectors, front-end ASICs and clock distribution, taking into account signal propagation times over large areas, e.g. in light guides or on PCBs and through cables/connectors. On-ASIC fast digitisation and advanced processing may be beneficial but, as with high-precision TDCs/PLLs, it comes at the price of increased power consumption, so that R&D into low-power ASIC blocks is mandatory.

High-precision timing may be necessary in every layer of a calorimeter, but R&D for its use in dedicated layers, either for preshowers (using e.g. LGADs) or at specific depths in the shower (e.g. at shower-maximum, as proposed by RADiCAL) should be carried out in order to optimise the complete detector performance whilst taking into account cost and integration.

Finally, R&D into advanced software able to exploit the capabilities of the calorimeters needs to be performed. In this context “software” may refer to programmable features on ASICs as well as in back-end processors, and should include the use of neural networks and machine learning.
### 6.3.6 Readout Systems for Calorimetry

Over the past 40 years calorimeters have mainly grown in size, but not significantly in channel density (with the exception of the silicon-based *Preshower* of CMS). The next-generation calorimeters being developed for the HL-LHC will push the boundaries in terms of both spatial density and timing resolution, to facilitate 5D-reconstruction for particle flow. This requires some major developments of the accompanying electronics. For future colliders, the challenges for calorimeter electronics will be even more extreme, but also heavily collider-dependent. The general prospects for electronics R&D are given in Chapter 7. Aspects specific to calorimeters are outlined in the following.

#### 6.3.6.1 Breadth of challenges for calorimeter readout systems

Linear $e^+e^-$ colliders have a low duty cycle coupled with low occupancy, resulting in rather modest levels of radiation. This means that the available readout bandwidth can be used to stream all data from high-granularity calorimeters and techniques such as power-pulsing can be exploited. The CALICE collaboration has made significant R&D along these lines, with great success so far. At the other end of the spectrum lies the FCC-hh, with a high duty-cycle coupled with extremely high levels of pile-up (in-time and out-of-time). Calorimeters at FCC-hh therefore need to survive extreme levels of radiation, must include high-precision timing information in order to mitigate pile-up (see Section 6.3.5) and, due to bandwidth limitations, will most likely require triggered data flow. There is consequently no “one size fits all” recipe for calorimeter readout systems: they must be tailored to the application.

#### 6.3.6.2 Necessary ASIC developments

Possibly, the most challenging aspect of front-end ASICs specific to calorimeters is the large dynamic range coupled to low noise for small signals (necessary for calibration of most sampling calorimeters). A related aspect is the response linearity over the full dynamic range. For smaller technology nodes, with lower operating voltage than the present generations, this will certainly require dedicated R&D.

The increased granularity of all future calorimeters requires a move to *processing* at all stages, from front-end to back-end, including data compression, encoding/decoding and embedded neural networks. Some recent developments for HL-LHC have included neural-network-based encoders for front-end triggering inside a 130 nm ASIC - i.e. putting “software” into the front-end. Future colliders will need to go much further, with more programmability and functionality built into the ASICs.

#### 6.3.6.3 Discrete components for compact calorimeters

Despite increased integration on ASICs, and notwithstanding MAPS-like developments, there will remain the need for discrete, and sometimes bulky, on-detector components. For example, the power-pulsing methodology for CALICE-like detectors requires large
capacitors as close as possible to the front-end, to reduce transient effects; and switched-mode DC-DC converters require high-inductance coils. These components are most challenging for high-density EM calorimeters, but the needs are similar in hadronic calorimeters and quite different from trackers or muon detectors, where space is more readily available. Although industry drives much of the development of passive components, specific R&D into in-PCB coils, small-volume capacitors (including for bias voltages) and high-reliability connectors (a common weak point in all HEP detectors) should be carried out.

### 6.3.6.4 Connection technologies for large-area solid-state sensors

The move to higher-granularity solid-state calorimeters for present and future collider detectors brings challenges in terms of interconnections between the sensors and the read-out boards. This is particularly true for large-area pad sensors, where direct soldering to FR4-based PCBs is not practical due to mismatches of the CTEs<fnref n="1" />, even when the detectors are operated at room temperature. For colder and even cryogenic operation, this issue becomes even more serious. Through-hole wire bonding can work for low-density sensors. But R&D into different interconnect technologies should be explored, including such things as Anisotropic Conductive Films/Pastes and PCBs based on materials with the same CTE as silicon. These would allow a move to larger-scale industrialisation necessary, in particular, for the next generation of hadron-collider experiments.

### 6.3.6.5 Readout integration and the new wave of FPGAs

Pattern recognition through artificial intelligence (AI) could clearly be beneficial, especially in the back-end readout systems, with the complex topologies of showers, for particle identification, energy measurements etc. Indeed it is becoming apparent that the new wave of FPGAs is focusing less on large arrays of programmable logic and more on having many dedicated programmable AI cores. The HEP community must exploit synergies with AI developments in the commercial world, possibly through industrial R&D partnerships, as the manner in which FPGAs can be incorporated into our readout systems will necessarily significantly change.

## 6.4 Observations

Calorimetry is going through a change of paradigm from integral to differential (5D-) detectors. Therefore the performance metric has to be somewhat rethought. The new metric has to take into account the changing role of calorimeters as a key element in the global event reconstruction of detectors, rather than devices purely for the measurement of single- (or multi-) particle energies.

From the beginning, individual components such as sensitive materials, ASICs and mechanical structures, will have to be considered holistically, as a single system of several

<footnote n="1">¹Coefficient of Thermal Expansion</footnote>
layers, deep enough to absorb highly-energetic particle showers. Large-scale demonstra-
tors are an essential ingredient of the R&D programme. These demonstrators are early
applications of novel technologies at system level, including novel materials for precise
mechanical structures outlined in Section 8.3.5. Data recorded with these demonstrators
validate and constrain the Monte Carlo (e.g. Geant4 [Ch6-26]) predictions, in particular
for assessing the relative contributions of the different nuclear models. The validation in
the time domain will become important in the coming years. The validation of Monte
Carlo predictions has a general interest, even outside the HEP field.

The complexity of calorimeters already at the prototype level calls for the formation
of R&D collaborations that include research centres and laboratories with the necessary
engineering competences. An important point to take into account is the time needed for
the construction of calorimeters. The R&D for calorimeters should finish approximately
8-10 years before the start of data taking. This requires well-conceived demonstrators
at an early stage that allow for a smooth transition from the development phase to the
construction phase.

Robustness and reproducibility are important factors that have to be developed in
collaboration with industry, due to the large number of modules involved in modern
calorimeters. It is, however, observed that: (i) in many fields particle physics is not
the main technology driver anymore, and (ii) the quantities required for particle-physics
detectors are still small compared to today's industrial standards.

Thus, future projects and national and international funding agencies should develop
strategies for collaboration with industry, valuing the training opportunities that flow
from the construction of calorimeters and their prototypes as well as, of course, the gain
in knowledge through the research itself.

Traditionally, calorimeters have strong overlap with other fields such as medicine or
earth science. The advent of complex granular devices makes calorimeters also an ideal
“playground” for the application of modern pattern recognition technologies.

**6.5 Recommendations**

In order to implement the research directions the following set of recommendations is
formulated.

* Implementation of DRDT 6.1. Support of R&D on novel optical materials and corresponding readout technologies to optimally prepare for the LHCb Upgrade II (in $\ge$ LS4). Experiments such as KLEVER could provide an early use case of developments for LHCb. The development of heavy glasses for the Electron-Ion-Collider should be followed closely and European groups are encouraged to join this effort;

* Implementation of DRDT 6.2. In order to meet a start of data taking for a Higgs factory around 2035, the planning should make sure that advanced options can reach maturity in the coming years but should enable also a judgement on alternatives. Therefore, where demonstrators (e.g. CALICE & Dual-Readout prototypes)
are planned or existing, the programme should be fully supported. Where possible
one should capitalise on HL-LHC experience to accelerate the process;

- Where R&D is still at a smaller level (e.g. for MAPS or Noble-liquid devices) or at the beginning, the necessary steps should be supported and the construction of larger-size demonstrators should be assessed in two to three years from now. MAPS based sensors will however already benefit from the planned construction of the ALICE FOCAL detector;

- Experimental setups in beam tests should be able to validate simulation models of EM and hadronic cascades;

* Implementation of DRDT 6.3. Develop a strategy for calorimeter R&D at a future hadron machine;

- Test beam and irradiation facilities have to approach the conditions at a future hadron collider as closely as possible. System tests should be combined with suitable computer simulations of the conditions at a high energy hadron collider. The HL-LHC will deliver valuable real-world experience;

* Understand how calorimeters can benefit from progress in terms of precision timing for single particles and showers, including within showers;

- Section 6.3.5 distinguishes between timing used for pile-up mitigation and for improvement in measurable parameters, such as resolution and linearity. In particular the latter requires systematic simulation studies before a dedicated hardware development for calorimeters. These simulation studies will have to be completed on a time scale of around two years from now;

* Changes in readout architectures need to be followed up and integrated into our planning;

- The R&D programme must be able to keep up with the rapid development in industry. This concerns in particular the introduction of Artificial Intelligence at the FPGA level;

* The uncertain landscape and the size of demonstrators emphasise the role of R&D Collaborations (or at least open collaborative efforts without a label). These collaborations should take into account that calorimeter R&D is a worldwide effort;

- R&D collaborations and demonstrators should also be fora/infrastructures to discuss/test new developments at an early stage beyond test-bench level. It is, for example, recommended that the Muon Collider Community integrates into existing R&D structures for calorimeters. The current R&D Collaborations may also be the best route to coordinating contributions to the DUNE Near Detector;
* Present and future calorimetry projects may have a lifetime of about 30 years or even longer. It is therefore important that calorimeters can be upgraded with state-of-the-art technology during the lifetime of a project. This will ensure, on one hand, an optimal scientific output and, on the other hand, is a premise that the field will remain attractive for coming generations of scientists and engineers. Therefore, upgradeability must become a design criterion.

## References Chapter 6

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[Ch6-3] OPAL Collaboration, G. Abbiendi et al., *Precision luminosity for Z0 line shape measurements with a silicon tungsten calorimeter*, Eur. Phys. J. C **14** (2000) 373-425, arXiv:hep-ex/9910066.

[Ch6-4] P. Picozza et al., *PAMELA: A Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics*, Astropart. Phys. **27** (2007) 296-315, arXiv:astro-ph/0608697.

[Ch6-5] CALICE Collaboration, J. Repond et al., *Design and Electronics Commissioning of the Physics Prototype of a Si-W Electromagnetic Calorimeter for the International Linear Collider*, JINST **3** (2008) P08001, arXiv:0805.4833 [physics.ins-det].

[Ch6-6] CALICE Collaboration, C. Adloff et al., *Tests of a particle flow algorithm with CALICE test beam data*, JINST **6** (2011) P07005, arXiv:1105.3417 [physics.ins-det].

[Ch6-7] CALICE Collaboration, C. Adloff et al., *Response of the CALICE Si-W electromagnetic calorimeter physics prototype to electrons*, Nucl. Instrum. Meth. A **608** (2009) 372-383, arXiv:0811.2354 [physics.ins-det].

[Ch6-8] CMS Collaboration, *The Phase-2 Upgrade of the CMS Endcap Calorimeter*, CERN-LHCC-2017-023, CMS-TDR-019, 2017. https://cds.cern.ch/record/2293646.

[Ch6-9] H. Abramowicz et al., *Conceptual Design Report for the LUXE Experiment*, arXiv:2102.02032 [hep-ex].

[Ch6-10] ATLAS Collaboration, *ATLAS liquid-argon calorimeter: Technical Design Report*, 1996. https://cds.cern.ch/record/331061.

[Ch6-11] M. Aleksa et al., *Calorimeters for the FCC-hh*, 2019. arXiv:1912.09962 [physics.ins-det].

[Ch6-12] *First results of the CALICE SDHCAL technological prototype*, JINST **11** (2016) no. 04, P04001-P04001.
[Ch6-13] D. Shaked Renous et al., *Towards MPGD-based (S)DHCAL*, J. Phys. Conf. Ser. **1498** (2020) 012040.

[Ch6-14] RD-52, INFN RD-FA Collaboration, M. Antonello et al., *Development of a Silicon Photomultiplier based dual readout calorimeter: The pathway beyond the proof-of-concept*, Nucl. Instrum. Meth. A **936** (2019) 127-129.

[Ch6-15] REDTOP Collaboration, C. Gatto, *The REDTOP experiment*, arXiv:1910.08505 [physics.ins-det].

[Ch6-16] B. Bilki et al., *Total Absorption Dual Readout Calorimetry R&D*, Phys. Procedia **37** (2012) 309-316.

[Ch6-17] CEPC calorimetry working group Collaboration, Y. Liu, J. Jiang, and Y. Wang, *High-granularity crystal calorimetry: conceptual designs and first studies*, JINST **15** (2020) no. 04, C04056.

[Ch6-18] V. Kononets et al., *Growth of long undoped and Ce-doped LuAG single crystal fibers for dual readout calorimetry*, Journal of Crystal Growth **435** (2016) 31-36.

[Ch6-19] K. Kamada et al., *Growth and scintillation properties of Li and Ce co-doped Lu₃Al₅O₁₂ scintillator*, Journal of Crystal Growth **452** (2016) 85-88.
ACCGE-20-OMVPE-17 (The 20th American Conference on Crystal Growth and Epitaxy in conjunction with the 17th US Biennial Workshop on Organometallic Vapor Phase Epitaxy, August 2-7, 2015, Big Sky, MT, USA).

[Ch6-20] K. Kamada et al., *Large Size Czochralski Growth and Scintillation Properties of Mg²⁺ Co-doped Ce: Gd₃Ga₃Al₂O₁₂*, IEEE Transactions on Nuclear Science **63** (2016) no. 2, 443-447.

[Ch6-21] L. Martinazzoli, *Crystal Fibers for the LHCb Calorimeter Upgrade*, IEEE Trans. Nucl. Sci. **67** (2020) no. 6, 1003-1008.

[Ch6-22] E. Auffray et al., *DSB:Ce³⁺ scintillation glass for future*, J. Phys. Conf. Ser. **587** (2015) no. 1, 012062.

[Ch6-23] T. Horn et al., *Scintillating crystals for the Neutral Particle Spectrometer in Hall C at JLab*, Nucl. Instrum. Meth. A **956** (2020) 163-175, arXiv:1911.11577 [physics.ins-det].

[Ch6-24] G. A. Dosovitskiy et al., *First 3D-printed complex inorganic polycrystalline scintillator*, CrystEngComm **19** (2017) 4260-4264.

[Ch6-25] N. Akchurin et al., *On the Use of Neural Networks for Energy Reconstruction in High-granularity Calorimeters*, arXiv:2107.10207 [physics.ins-det].

[Ch6-26] J. Allison et al., *Recent developments in Geant4*, Nucl. Instrum. Meth. A **835** (2016) 186-225.
# Chapter 7

# Electronics and Data Processing

## 7.1 Introduction

For fifty years, electronics developments have supported and enabled all aspects of experimental particle physics. Current and near-future detector electronics compares in complexity with the most challenging developments in industry, and is the means by which data and control for many elements come together to constitute an experiment. “Electronic” systems use a wide range of technologies, from nanoscale semiconductors, to high current and voltage power supplies, to optoelectronics. They rely on complex software and firmware. A breadth of skills in engineering, modelling, simulation, software and systems integration is needed to design, deliver and operate them.

The performance of the next generations of electronic systems will be a limiting factor in the scientific reach of future experiments, unless significant R&D, new organisational structures, and a new relationship with industry are put in place. Predicted needs and timescales indicate that this must be done now, building on the lessons from LHC and other current projects. Near-future experiments needing low-power, exceptional granularity, and in some cases very low mass, require focused R&D in the coming years. For further-future collider experiments, today’s technologies lack the necessary combination of performance, power efficiency and radiation hardness, and fundamental R&D is needed. Non-collider experiments with extreme conditions pose a range of challenges, again requiring the use of technologies currently in their infancy.

## 7.2 Main drivers from the facilities

### 7.2.1 Technical requirements

The trend in future detectors is for finer granularity, more channels and more data, presenting a challenge in power dissipation. The close coupling of readout ASICs to sensors, improving performance and hermeticity, makes this all the more important. Some applications require ultra-high radiation hardness; all require exceptional reliability for both on- and off-detector electronics. Precise timing information for triggering
and reconstruction is a common requirement, and will also increase the amount of data, requiring more and faster links using the smallest possible amount of material. Data selection and reduction will increasingly be addressed by intelligent processors close to the front end, reducing data movement. Here, R&D must identify affordable compromises between performance, complexity, power consumption and flexibility. A summary of requirements is shown in Figure 7.1.

The requirements of future lepton and heavy-ion colliders seem to be mostly within reach of technologies from HL-LHC, but this is not true for FCC-hh or a muon collider, even assuming reasonable progress over the next two decades. Large-scale neutrino and dark matter experiments pose a different set of technical challenges, but have overlaps in areas such as controls, reliability techniques and software. Smaller specialised experiments will continue to pose unique and individual challenges, which may form valuable stepping stones and test-beds for advanced technologies of wider applicability. For the first time, electronics has become an enabling, but potentially also limiting, aspect. The effort required to maintain access to some of the required technologies and tools is so large that a completely new collaborative model must be implemented to focus on a limited set of affordable and targeted R&D lines. The R&D strategy will be dictated by what is feasible and affordable, and not solely by requirements of the experiments.

### 7.2.2 The inheritance from HL-LHC

The HL-LHC detector projects are currently the largest concentration of advanced electronics developments in HEP. Over 40 front-end ASICs have been developed for readout of silicon pixel and strip sensors, calorimeters, and muon or timing detectors, in 130 and 65 nm CMOS technology. Schemes have been put in place to improve the efficiency of design teams: access to foundry and design tools, run centrally from CERN; common projects for the specific developments (data links [Ch7-1], power conversion [Ch7-2], etc.); and collaborative developments across experiments (RD53 [Ch7-3]) and beyond HEP (Medipix [Ch7-4]). At the back end, HL-LHC will use large, cost-effective data acquisition and processing systems. These systems typically use commercial FPGAs to interface between custom front-end links and online computing, but carry out the majority of data processing using modern computing platforms, including increasing use of co-processors.

### 7.2.3 Industrial developments

For ASICs, the microelectronics industry has reached nanoscale CMOS nodes, based on new transistor structures (FinFETs, gate-all-around devices, etc.) to improve the speed and power performance of digital systems. EDA tools have simultaneously reached unprecedented performance levels and are essential in the design process. In contrast to most (purely digital) complex commercial ASICs, future readout ASICs will be mixed signal systems, with analogue interfaces to sensors. Achieving the required analogue performance with advanced nodes will be a crucial target of the R&D programme. High-performance commercial CMOS image sensors are based on stacked active layers to
<figure><img src="image_22.png" /><figcaption>Figure 7.1: Schematic timeline of categories of electronics 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).</figcaption></figure>

reduce pixel pitch and increase functionality. These industrial advances can provide the means to enhance or even revolutionise the performance of future detector readout ASICs and their interconnection to high granularity detectors.

Commercial communications and data processing technologies continue to evolve rapidly. Optical links are now ubiquitous in homes, offices and data centres and enable massive data transfers across the globe. Data-processing technologies are using increasingly specialised co-processing, stream-processing, and multi-processing architectures, as opposed to more powerful single devices. The difficulty in HEP will be in keeping pace with these extremely complex new developments, which unlike today's FPGAs are largely proprietary and not typically available for use in custom developments.

These considerations feed into a number of recommendations discussed in Chapter 10, while the technical R&D themes are detailed later in this section and summarised in Chapter 11. As discussed below, for the timeline illustrated in Figure 11.1 R&D on DRDT 7.1, DRDT 7.2, DRDT 7.3, and DRDT 7.5 is essential to realising the facilities listed in Figure 3 and Figure 4 of the Introduction, all the way through to the FCC-hh/muon collider era. For DRDT 7.4 there is the particular challenge of the two orders
of magnitude more extreme radiation hardness requirements for the FCC-hh.

### 7.2.4 Categorising New Developments

The industry roadmap is driven by commercial imperatives. The particle physics community needs to monitor technology evolution, and put in place structures and relationships providing access to advanced technologies in an affordable and effective way. This is particularly relevant where contracts, licenses or fees are needed to access technologies such as ASIC tool chains and libraries. For other technologies, including high-density interconnects, direct collaboration between industry and research institutes will be needed to gain the necessary access and experience. For yet others (for instance, advanced co-processing or network technology) particle physicists are effectively COTS end-users, and will need to maintain currency through test setups and deployment of new technology into existing experiments.

In some cases, the needs of HEP are far from the industry roadmap. An example is custom ultra-radiation-hard devices. Here, a long-term sustained R&D effort is critical to guarantee that the particle physics community remains abreast of new technologies and their capabilities. An appropriate organisation will need to be put in place to sustain critical mass of a very active but small and scattered community. In particular, the practical use of the most modern technologies may be restricted by cost or licensing, and this is likely to pose a barrier to meeting the needs of future experiments.

## 7.3 Technical Findings

### 7.3.1 Front-end ASICs

Over the last three decades, the crucial importance of custom chip design for successful experiments has become increasingly obvious. Microelectronics is likewise key for next-generation experiments, which cannot continue to be performed ‘the good old way’. ASIC technologies are evolving rapidly, bringing better performance and functionality, but at the expense of increased complexity and design cost. Design tools are becoming very elaborate, needing experts to handle them, and the specialisation and size of the teams needed to design, simulate and verify the next generations of ASICs will be much larger than in previous projects.

Front-end ASICs have become an integral part of modern detectors, and perform complex functions that determine overall system performance. For most detectors, sensor design is now closely intertwined with its readout electronics and the optimisation of their performance is a joint effort. This is particularly evident in the field of monolithic CMOS pixel sensors (see Section 3.3.1), where the front-end electronics is integrated in the same substrate as the sensing electrodes, setting even more demanding requirements on the design and layout of the pixel cell, which must be compatible with the operation of the sensor and requires similar tools and expertise as needed to design readout ASICs.
### 7.3.1.1 State-of-the-art

More and more functionality is now integrated inside the front-end ASICs, from low-noise, high-speed pre-amplifiers and discriminators to high-end ADCs and TDCs and digital processing [Ch7-5]. The trend is towards more channels, less power, higher speed, lower noise, and higher radiation-hardness. High-precision timing capability (< 50 ps) is becoming standard for devices currently in development [Ch7-6]. In addition, devices intended for calorimetry require large dynamic range and excellent linearity [Ch7-7].

### 7.3.1.2 Technology choice and ASICs evolution

The community currently bases most designs on 65 nm and 130 nm CMOS, which have been qualified by CERN for radiation-hardness. Despite being used for HL-LHC, these technologies are already fifteen to twenty years old, and a 28 nm CMOS technology has now been selected as the next node for highly integrated chips such as pixel readout. This will in particular enable the design of in-pixel precision TDCs. It will also allow for lower power digital processing, advanced programmability and configurability options and high-speed output links, but will come at a much higher cost and complexity, requiring specialised teams dedicated to design and verification tasks. A cheaper process should be preserved for less dense/demanding applications, and a CMOS Imaging Sensors (CIS) variant should be targeted for monolithic pixels. In the longer term, CMOS nodes below 28 nm will also need to be qualified for their potential use in future applications requiring extreme miniaturisation, high speed and low power.

More intelligence can be included in the front end ASICs to allow for data reduction and possibly an overall system power reduction, though there is a trade-off with the needs of software event reconstruction. As with today's trigger systems, complex processing having an irrevocable effect on the recorded data must be adaptable to changing experimental conditions. Ideally the ASICs would be programmable, and either "FPGA-like" or "CPU-like". Such programmability and configurability will ultimately enable the community to develop fewer ASICs of higher complexity and flexibility.

Interconnects represent an important area, where industrial technology now allows the integration of dissimilar technologies for sensor/analogue/digital/photonic functions. R&D in TSVs, ACF and bumpless techniques is needed, noting that 3D-integration is a key process in industry to overcome scaling limitations. CMOS image sensors are a good example, stacking two or three device layers fabricated in different technologies, with high density interconnects between them. Access to advanced interconnect design and fabrication processes is currently limited to selected customers and is a challenge which will need to be addressed.

### 7.3.1.3 Identified R&D themes

* High-granularity pixel readout chip with 10–100 ps timing and charge measurement capability in 28 nm CMOS, and highly programmable features (DRDT 7.1, DRDT 7.2, DRDT 7.4);
* Integration of “intelligent” signal processing into detector readout chips for data selection / reduction (DRDT 7.1, DRDT 7.2, DRDT 7.5);

* Readout structures for monolithic sensors (DRDT 7.1, DRDT 7.3, DRDT 7.5);

* LGAD/AC LGAD timing chip with 1–10 ps timing capability (DRDT 7.1, DRDT 7.3);

* 3D integration technologies (in collaboration with industry) for high density interconnection of stacked layers of sensors and readout electronics, or for connection of ASICs and specialised PCBs (DRDT 7.1, DRDT 7.5);

* Imaging/dual calorimeter readout chip for Si/SiPM readout (DRDT 7.1, DRDT 7.2, DRDT 7.3);

* Cryogenic readout chip for imaging LAr calorimetry (DRDT 7.1, DRDT 7.4);

* MPGD/RPC timing chip with < 10 ps resolution (DRDT 7.1, DRDT 7.3);

* Integration of readout ASICs with silicon photonics (DRDT 7.5);

* CMOS nodes beyond 28 nm for digital and mixed-signal applications, including characterisation of radiation hardness and analogue performance (DRDT 7.1, DRDT 7.4, DRDT 7.5).

### 7.3.2 Links, Powering and Interconnects

The raw data rates of modern experiments exceed exabytes per day. With current technologies, only a fraction of the raw data can be transferred from the front-end to the back-end electronics. Electrical data links are unavoidable in innermost detector layers, dictated by space constraints and radiation levels; elsewhere, optical data transmission is the dominant link technology. Tens of thousands of optical fibres are employed in the LHC experiments, and allow the required data throughput at minimum mass while preserving electromagnetic noise immunity and galvanic isolation.

Despite intensive efforts to minimise power, the front-end electronics of LHC subdetectors consumes tens to hundreds of kilowatts. Power distribution and cooling are and will remain major challenges for HEP detectors, particularly for highly granular silicon detectors.

The assembly of sensors, front-end ASICs as well as power conversion and data transmission devices into detector modules, and the combination of modules into larger units, continues to pose severe packaging and interconnect challenges. This is due to: the high density of connections between sensor and ASICs; the need for extremely low-mass assemblies; severe space constraints; and the need to operate sensors at low temperature.
### 7.3.2.1 State-of-the-art

Almost all HL-LHC optical links will use a common 10 Gbit s⁻¹ chipset developed at CERN combining the LpGBT ASIC (65 nm CMOS) with the VTRx+ optoelectronic module (multimode, VCSEL-based). The LpGBT [Ch7-8] aggregates lower-rate datasets from front-end ASICs to maximise the bandwidth utilisation of the multi-channel optics. 130,000 LpGBTs and 70,000 VTRx+ modules have been ordered by experiments. Tolerable radiation levels reach 2×10¹⁵ neq cm⁻² and 1 MGy, only just sufficient for HL-LHC.

Innovative powering concepts, DC-DC conversion and serial powering, were developed for the LHC upgrades to minimise losses in cables and minimise material. A key achievement, together with a comprehensive understanding of system reliability and safety, was the development of custom radiation-hard power conversion circuitry and regulators. For DC-DC conversion, dedicated chip sets and power modules were designed at CERN. For serial powering of pixel detectors, most of the circuitry is integrated in the readout chips. The specific requirements of high granularity calorimetry have also required specific developments in miniaturisation of components.

Packaging and interconnect technologies rely on sophisticated flip-chip and wire-bonding processes, fine-pitch flexible circuits, ultra-light high-precision carbon-fibre support structures, highly effective bi-phase cooling techniques, and materials with extreme thermal conductivity. The sophistication of packaging and interconnect technologies is reflected in densities of tens of thousands of pixels per cm², sensors and ASICs thinned to 50 µm, and cooling microchannels etched into silicon sensors and silicon modules of as little as 0.05% of a radiation length [Ch7-9].

### 7.3.2.2 Future challenges

Driven by increasing granularity and the use of precision timing, data rates will increase substantially at future colliders. The links will have to support this while complying with mass, radiation hardness, space, and power constraints, at an affordable cost. If the challenge can be met, advanced links may offer the opportunity to read out a larger fraction of the raw data than today, or even all the raw data, offering benefits for triggering and event selection. This represents an alternative solution to increased front-end data processing. System architectures using front-end data reduction or massive link capacity will thus have to be studied and compared. At the back end of the link, COTS optoelectronics will be used. Custom front-end developments will have to be compatible with COTS standards in terms of signalling rates, modulation formats, error correction schemes, and protocols, while maintaining radiation-hardness. This constraint will grow in importance as the technology gap between front- and back-end widens, and as data transmission and machine clocks may become asynchronous.

At future colliders, power distribution will remain a major concern. Even if the total power does not increase, currents delivered to ASICs will be higher due to reduced supply voltages. This will in turn drive interconnect and cooling specifications and will require packaging and integration studies as well as full scale developments. To reduce power dissipation, ASICs will need intelligent power management and multiple supply voltages,
increasing the complexity of DC-DC converters. The power efficiency of converters and regulators will need to remain high, and larger conversion ratios should be realised. Due to increased radiation exposure, and with more modules attached to a single DC-DC converter or a serial powering chain, remote control and monitoring circuits will need to achieve high robustness and reliability. For serial powering and in-chip regulators, circuit designs beyond 65 nm will be required, and this will need specialised expertise and qualification efforts to be sustained. Disruptive and unconventional power distribution approaches like power over fibre or wireless power transfer should be explored. In particular, DC optical power supply modules will become an essential building block for silicon photonics, should this technology become mature.

### 7.3.2.3 Industry and other fields

The commercial link technologies closest to HEP needs are deployed in data centres over tens of metres to a few kilometres. Data rates follow Ethernet standards from 10 GbE to 800 GbE. Above 25 Gbit s⁻¹, multiplexing over lanes, wavelengths (CWDM) or modulation levels (PAM) is used. VCSEL-based links are still used in the majority of cases, with highly optimised multimode fibres, but a new family of silicon-photonics-based single-mode systems is appearing in the high-rate/long-distance segment. Commercial FPGA IP cores implement these data rates and modulation formats, with corresponding processing and error correction logic. Attempts to co-package optoelectronics with FPGAs are being reported, but most designs still rely on pluggable optoelectronics. Silicon photonics may be a game-changing technology in this context thanks to its good integration density and integration synergies with microelectronics. In this diverse and dynamic environment, HEP will not be short of COTS components meeting its bandwidth needs at the back-end in the foreseeable future; the challenge will however be to develop custom front-end modules compatible with commercial standards.

DC-DC power conversion in HEP is a specialised technology. Commercial converters do not operate in strong magnetic fields or high radiation environments, while serial powering is barely used outside of HEP. Space applications have some commonalities with HEP, but overlapping requirements are difficult to find, and the particle physics community is required to develop its own solutions. However, efficient power distribution and conversion are hot topics everywhere, and promising technologies (GaN, SiC) are being aggressively introduced by industry.

### 7.3.2.4 Identified R&D themes

* Radiation-tolerant high-speed communication circuit blocks (SerDes, Driver, Receiver, high precision Clock/Timing, etc.) compatible with 25GbE and above, and with FPGA receivers (DRDT 7.1, DRDT 7.3, DRDT 7.4, DRDT 7.5);

* Silicon photonics as the successor to actively modulated VCSEL-based links, facilitating full-custom photonic integrated circuits (PICs) for HEP (DRDT 7.4, DRDT 7.5);
* Co-packaging of electronics and optics through multi-chip-assemblies, fibre-coupling, and cooling techniques compatible with HEP sensors (DRDT 7.4, DRDT 7.5);

* Low-power, low-mass wireline and wireless link technologies to maintain a diverse portfolio of data-transfer solutions adapted to multiple requirements (DRDT 7.1, DRDT 7.4, DRDT 7.5);

* High conversion factor DC-DC converters based on new processes and materials, and associated power management circuit blocks (DRDT 7.1, DRDT 7.2, DRDT 7.4, DRDT 7.5);

* Small form-factor power modules compatible with the HEP environment, including optimised coils, capacitors, cooling interfaces, connectors and packages (DRDT 7.1, DRDT 7.4, DRDT 7.5);

* Optical and wireless power transfer for reduction of cabling mass (DRDT 7.1, DRDT 7.4, DRDT 7.5);

* Low-mass and compact interconnect and assembly techniques, culminating in wafer-scale and 3D integration (DRDT 7.4, DRDT 7.5).

### 7.3.3 Back-end Systems

#### 7.3.3.1 State-of-the-art

Back-end systems usually comprise “trigger” and “DAQ”. The former uses partial information to make a selection decision within a defined maximum latency, during which data need to be buffered at the front-end. Today, triggers are overwhelmingly implemented in FPGAs, sometimes along with ASICs (see for instance [Ch7-10], [Ch7-11]). CPUs and DSPs are not often used due to latency constraints, but there is some adoption of GPGPUs. Specialised chips for “AI” (i.e. inference accelerators) are not yet in use. In some cases, FPGAs are physically near the front end, but for collider experiments this is prevented by radiation levels, and back-end electronics is situated up to several hundred metres from the detector with data transport via custom radiation-hard optolinks. The “DAQ” function typically comprises FPGA-based boards to terminate the optolinks, control and monitor front-end ASICs, distribute high precision clocks, buffer, validate and organise data, and convey them onto a COTS network and thence to commercial computing systems [Ch7-12], [Ch7-13]. Large amounts of buffer storage are used to decouple processing stages and to allow efficient resource usage under varying operational conditions. Large experiments use a dedicated subsystem to distribute clock, synchronisation and “fast” control information [Ch7-14].

#### 7.3.3.2 Future challenges

Power consumption and readout bandwidth will be limiting factors for future experiments. Moving processing traditionally done in the back-end closer to (or integrated with) the front-end can strongly reduce data transmission bandwidth. Standard COTS
form-factors (e.g. PCIe cards) cannot be easily used at the front-end, which increases cost and reduces flexibility for long-term hardware maintenance and upgrades. Hard-wiring complex data processing algorithms into ASICs also risks removing the flexibility necessary to address changing detector conditions and physics priorities.

Development of specialised AI and ML hardware in academia and industry is ex-
tremely rapid, and it is not clear how this can be reconciled with the extended develop-
ment cycles for large detector systems. Standardisation and modularisation of front-end
electronics and interfaces may offer a solution. It is unlikely that COTS components
will be suitable for use in the detector without adaptation, due to power, robustness
and radiation hardness constraints. The long-term radiation tolerance of any candidate
novel processing technologies must be established.

The FPGA layer connecting front-end links to COTS computing is a major cost
driver in large experiments, driven by both hardware and firmware developments. If
front-end links use a suitably optimised industry-standard protocol, this cost could be
at least partially removed. Today, and for the foreseeable future, this implies the use of
Ethernet, PCIe, or other packet-switched standards.

7.3.3.3 Industry and other fields

COTS system performance continues to increase through larger chips, increased power dissipation (> 300 W for high-end CPUs and GPUs), integrated memory controller, I/O and auxiliary functions (SoC), and multiple dies per package. Standard elements (CPUs, GPGPUs, FPGAs, ML ASICs, optoelectronics) have the highest volumes, and consequently the best price-performance ratio. They are typically deployed in standardised form-factors and platforms using relatively narrow I/O interfaces. Network backbone link speeds now reach 800 Gbits⁻¹ and credible roadmaps exist for another factor four in the coming years. The complexity of these devices and of their integration into full systems makes it very unlikely that they can be efficiently used outside their intended target platforms and applications, with the possible exception of FPGAs.

For large-scale users capable of the required investment, customisable IP for CPUs
and other elements (e.g. ARM architecture) allows development of “application opti-
mised” SoCs. These systems are often quite heterogeneous, which is hidden from the
user by sophisticated software. This approach may be a viable route for HEP in the
future, although lower-performance open IP (e.g. RISC-V) may be a more practical
proposition.

Other fields with large distributed real-time processing systems are also seeking to exploit the processing power of GPGPUs for parallelisable problems, with first-level signal processing also using FPGAs. The largest such systems are for SKA and ESRF. SKA shares power and throughput constraints, but of course has fewer issues related to packaging, radiation, and less stringent requirements for reliability [Ch7-15], [Ch7-16].

7.3.3.4 Identified R&D themes

* Tight integration of front-end and back-end using industry standard protocols and COTS components in extreme environments (DRDT 7.1, DRDT 7.4, DRDT 7.5);
* Integration of AI techniques closer to the front-end using modular, flexible designs using COTS chips or standard IP (DRDT 7.2);

* Use of FPGAs in extreme environments for data transmission and processing (DRDT 7.1, DRDT 7.4);

* Efficient use of modern compute hardware, use of GPGPUs and AI ASICs, cross-platform frameworks, and compatibility with industry standards, connected with high-speed software defined networks (DRDT 7.2, DRDT 7.5);

* Large-scale and realistic technology demonstrators, tracking relevant industry developments, with permanent integration and test-facilities (DRDT 7.1, DRDT 7.2, DRDT 7.5);

* Next-generation pico-second precision timing distribution systems to enable 4D detectors (DRDT 7.3, DRDT 7.4).

## 7.4 Observations

### 7.4.1 Organisation and Collaboration

The increasing cost, complexity and specialisation of developments (particularly in ASICs) will require changes in organisation, akin to past initiatives in computing and accelerators, as also discussed in Chapter 10. One model, extending arrangements for HL-LHC and emulating the organisation of large-scale computing developments, would be to organise design work around a number of well-resourced centres (e.g. CERN as “Tier-0” and national laboratories as “Tier-1”), directly supporting the more exploratory work of university groups (“Tier-2”). Larger centres can maintain the critical mass, access to tools and industry, and verification infrastructure, that are essential for successful distributed development. The full design and verification of a complex ASIC now exceeds the capacity of any single group, so means will be needed to recognise contribution and ownership of sub-components and circuit blocks, and to solve legal and practical issues of collaboration. Long-term positions and career paths for staff with specialised skills are vital, so that knowledge can be maintained (see Chapter 9). Young people will play a key role in generating ideas and providing design effort, as will subcontractors, but success will also rely on the supporting efforts of experienced people with familiarity with HEP-specific challenges. The same issues also apply in other areas of complex collaborative design such as firmware, embedded software and custom board design, as also picked up on in Chapter 11.

### 7.4.2 Systems Engineering

Intelligent and configurable systems will benefit from the rapid progress of commercial developments. In the future both front- and back-end parts, as well as any intermediate layers, will have to be designed in unison to ensure optimal performance. Contrary to current practices where groups drive developments quasi-independently, this will require
top level trade-offs in distribution of intelligence, power, storage, and data bandwidth between layers. System-level analysis, modelling, and simulation will become central themes for future R&D, and will form the starting point for future detector electronics developments, before a conceptual design is adopted. This crucial point applies even to the very next generation of detectors, where designs already exist.

### 7.4.3 Tools and Technologies

Access to advanced design tool kits and design technologies is crucial. Rising engineering costs dictate that tools and procedures must reach a level of sophistication such that the risk of failing a submission is kept extremely low. This is currently only achievable with commercial packages, with the issues of vendor lock-in and license costs, which can be prohibitive for certain EDA tools. The Europractice initiative is a very valuable facility in this area, underpinning almost every development; it must be supported, continued and if possible extended. A common support service should also provide and train the community with pre-configured design flows and kits so that every development centre uses a similar environment (see Chapter 10).

In parallel, there should be a new emphasis on open-source tools, which have had profound impact on software development. Such open-source projects benefit the entire community. Similarly, the *OpenHW* initiative provides IP blocks free of license constraints, ideal for quick or smaller projects. These approaches support those who cannot access commercial licenses, and are used outside HEP. However, open-source tools are not free, or even of lower short-term cost than commercial tools: development, maintenance and support time needs to be invested. Nonetheless, this is a potentially valuable way of reducing long-term risk, and may provide a route to new innovation in design optimisation.

### 7.4.4 Interactions outside HEP

HEP instrumentation has similarities to that in astro-particle and astrophysics, hadron and heavy-ion physics, nuclear physics, photon science, space, and medical fields. All seek an increasing channel count, and higher data rates and bandwidths. The take-up of HEP-developed microelectronics is often hampered by lack of documentation and support, non-use of common standards, and lack of open designs with clear licensing conditions. However, there are very successful examples demonstrating that it is possible to break the HEP bubble and develop ASIC families of interest for multiple fields (Medipix, Timepix, Velopix, etc.). Working more closely with projects targeting other applications may help designers to maintain and develop expertise and know-how on advanced technologies and design techniques. This may then provide innovative solutions applicable to HEP.

### 7.4.5 Skills, Training and Careers

Advanced electronics development relies on well-trained specialists and engineers who are highly sought-after in industry, and who must cope with ever-more-complex tools
and designs. They also need a mature understanding of physics requirements and ex-
perimental conditions. While the community continually educates and trains highly
skilled people, the availability of advanced electronics skills in the field is now clearly
insufficient. As in other areas, the non-specialist will find it increasingly difficult to
make a satisfactory professional contribution without a large investment of time. It is
vital to maintain a specialist talent pool in academia, perhaps attracting them with
the multi-disciplinary and “grand challenge” nature of the task compared to industry.
Collaboration with electrical engineering departments, early recruitment into attractive
graduate programmes in HEP and later into permanent posts with competitive salaries,
plus a significantly larger number of technical positions at national research centres and
universities, along with a visible career path leading to senior positions, are required (see
Chapter 9, Chapter 10 and Chapter 11).

7.4.6 Common infrastructure

Access to radiation-test facilities remains vital, is a current bottleneck, and if anything
will become only more important as radiation-hardness requirements increase. However,
increasingly complex and costly developments call also for other shared and expensive
facilities. For instance, testing very fast and high-timing-precision optical and electrical
circuits requires test-equipment and laboratory facilities beyond the reach of most insti-
tutes, as do specialised developments in optoelectronics and wireless technology. Also,
large and complex systems require sophisticated test infrastructure which will increas-
ingly need to be shared (see Chapter 10).

7.5 Recommendations

7.5.1 Themes for future R&D

The future R&D topics for electronics have been organised into a small number of co-
herent, but necessarily overlapping, themes. Each of these themes could form the basis
of one or more focused RD collaborations.

DRDT 7.1 - Advance technologies to deal with greatly increased data density.

* High data rate ASICs and systems:
  More channels and more bits per sample require higher data rates inside and outside the ASIC. Low noise, low power, high speed ADCs and TDCs, high speed serialisers / deserialisers, PLLs, NRZ and PAM driver blocks need to be developed in advanced technology nodes. Data rates, FEC and modulation formats must be selected at system level, in line with available COTS at the back-end;

* New link technologies (fibre, wireless, wireline, etc.):
  Novel link technologies must be developed to cope with higher data rates, to con-
  nect neighbouring detector layers for advanced data reduction techniques, and to
do so with reduced mass and power. Critical technologies include radiation-hard optical links, wireline, wireless, and free-space optics;

*   **Power and readout efficiency:**
    Low-power design techniques are needed at the front-end, including novel architectures. Efficient power distribution, power converter and regulator devices, and protection circuits are required to minimise detector mass and heating. Efficient readout controllers must work in concert with DAQ to optimally aggregate, buffer and transmit data to maximise the utilisation factor of very high bandwidth off-detector links.

**DRDT 7.2 - Develop technologies for increased intelligence on the detector.**

*   **Front-end programmability, modularity and configurability:**
    Radiation-tolerant processors and programmable logic elements must be made available as circuit blocks in order to allow fewer, more versatile front-end ASICs. Common interfaces and protocols must allow re-use of modules;

*   **Intelligent power management:**
    Increased ASIC power-performance ratio must be achieved by developing improved power management schemes. Clock gating, power pulsing, dynamic voltage supply and other techniques will be key to efficiency;

*   **Advanced data reduction techniques (ML/AI):**
    COTS AI circuits cannot easily be integrated into the front-end, and will not be radiation hard. Standardised, shareable blocks for AI functions, implementable in ASICs or FPGAs, can leverage the enormous intellectual investment in the AI/ML revolution for intelligent data reduction.

**DRDT 7.3 - Develop technologies in support of 4D-and 5D-techniques.**

High 4D-(timing as well as spatial) resolution requires developing solutions to improve the noise-speed-resolution trade-offs in advanced technologies with low supply voltage and high transistor density, along with achieving an unprecedented precision for the distribution of frequency and time references. Combination with accurate measurement of the energy deposited gives the additional possibility of “5D”-capabilities.

*   **High-performance sampling (TDC, ADC):**
    High-4D resolution requires a solution to the difficult noise-speed-resolution trade-offs in advanced technologies with low supply voltage and high transistor density;

*   **High-precision timing distribution:**
    Distribution of precise frequency and time references remains vital for all readout-systems. The performance of these systems will be pushed to unprecedented levels by 4D sensors, for which they are a limiting factor. There are no ready-made solutions at hand, and the challenge is even bigger in radiation environments;
* **Novel on-chip architectures:**
  Adding per-hit timing information to high channel count ASICs results in a large increase in complexity. Novel architectures must be developed to manage this challenge as circuit size increases to cover larger detector areas.

**DRDT 7.4 - Develop novel technologies to cope with extreme environments and required longevity.**

* **Radiation-hardness:**
  In future particle physics experiments, particularly at energy-frontier colliders, particle fluences are extreme. ASICs, optoelectronics, powering devices, and on- or near-detector FPGAs must be designed and qualified for radiation-hardness;

* **Cryogenic conditions:**
  Cryogenic detectors offer high sensitivity and resolution for future neutrino and dark matter experiments, but are challenging for the operation of microelectronics. Readout of new sensor types (some operating at mK) requires thorough characterisation and modelling of ASIC technologies, exploration of new data transfer concepts, development of multiplexing technologies, and novel readout and control;

* **Reliability, fault tolerance, detector control:**
  Excellent monitoring and reliable control are crucial for detector performance and long-term stability. In harsh environments it is an open problem to achieve reliable control and monitoring in inaccessible areas without violating power, cooling or space constraints. Wireless communication may be a solution, but reliability in extreme temperatures, radiation and/or magnetic fields will be a challenge;

* **Cooling:**
  Sub-detector systems may consume tens or hundreds of kilowatts, predominantly in the front-end ASICs. At the same time sensors must be cooled to minimise leakage current and noise and to avoid thermal runaway. Critical technologies are micro-channels in silicon and novel heat-conducting materials.

**DRDT 7.5 - Evaluate and adapt to emerging electronics and data processing technologies.**

* **Novel microelectronic technologies, devices, materials:**
  A rolling R&D campaign is necessary to keep up with fast-moving and emerging technologies. Over the long timescales of HEP developments, one must not lose track of commercial evolution. For example, transistor structures in advanced nodes (FinFETs, Gate-all-around devices) will require thorough study of their behaviour and of the techniques for designing high performance circuits;

* **Silicon photonics:**
  Silicon photonics is at an early stage of maturity and is new to the HEP community. A robust R&D programme is required to qualify the technology. In case of success
an entire design ecosystem must be put in place to enable its coordinated use across institutes and laboratories;

*   **3D integration and high density interconnects:** Increasing channel density combined with enhanced functionality per pixel can only be managed with sophisticated 3D integration and interconnect technologies. 3D stacking must urgently be explored to combine analogue, digital and photonic functions. 3D integration of sensors with ASICs is critical and even more demanding. Access to reliable and affordable flip-chip processes, redistribution layers, stitching and ACF / ACP will also be important;

*   **Keeping pace with, adapting, and interfacing with COTS:** COTS computing (CPUs, GPGPUs, FPGAs, AI accelerators) and networking equipment increases performance at breathtaking pace. Since it is targeted mostly at cloud data centres, use in HEP requires adaptation and integration both at the hardware and software level. This is challenging work which needs to be repeated for every new generation of COTS.

### 7.5.2 Approach to R&D

#### 7.5.2.1 Novel Developments

An R&D roadmap by definition must provide a planned and prioritised route to produce well-understood deliverables. As indicated throughout, the increasing costs and complexity of HEP microelectronics and computing mandate such an approach. However, transformational “blue-sky” R&D must not be lost from the field of particle physics. Excessive planning and rigidity in the allocation of resources therefore also poses risks. A strong point in the particle physics community always has been tolerance and relative freedom for bottom-up initiatives including “crazy ideas” pursued quickly using the expertise, infrastructure and resources of well-equipped institutes. Scientific as well as technical staff have throughout the decades been highly motivated to find solutions for impossible tasks, or simplifications and cost savings for overly complex and expensive plans, and this has led to success. The Roadmap must provide sufficient freedom for new developments to be pursued as they arise, for rapid solutions to be found to urgent problems, and for clever ideas that have been promoted for many years, but never sufficiently prioritised, to be followed up (as also emphasised in Chapter 10).

#### 7.5.2.2 Horizon-Scanning

Even established technologies are never static, and emerging ones can quickly become mainstream. It is therefore important to maintain a rolling survey of the technological environment and constantly question particle physics R&D directions. This naturally starts with attending conferences and workshops inside and outside HEP, but does not stop there. Building concrete hardware and software demonstrators to evaluate new proposals and assess their relevance to the field of particle physics is a mandatory part of the Roadmap.
### 7.5.2.3 Software

In industry, software frequently drives innovation. Many impressive recent chip developments are made to run specific software, e.g. matrix / tensor operations required by ML frameworks. Conversely, good software is crucial for the market success of hardware. Since skilled developer resources are scarce and expensive, software frameworks that increase productivity and allow harnessing of full hardware potential have a tremendous advantage, and this will usually trump notionally better hardware. There are obvious improvements to be made in HEP software development: in training, in component re-use, and in avoidance of duplication. Most importantly, the importance of long-term software support must be recognised and funded. Making software re-usable beyond a specific experiment or project is a significant extra effort. However, re-use of software within and outside HEP (e.g. as for ROOT or Geant4) is a tremendous gain for the whole scientific community. This trade-off and long-term model must be taken into account when resourcing larger software projects.

## 7.5.3 Practical and Organisational Issues

### 7.5.3.1 Collaborative Model

The Roadmap encompasses significant new short- and long-term activities, requiring concentration of investment and expertise. The international working model must be improved to avoid dispersed, uncoordinated, and parallel activities. An emphasis on open common developments, which can be implemented and specialised in the context of particular projects, is needed. This approach has sometimes been in tension with other drivers: a focus on specific projects as opposed to R&D; a wish to pursue "important" or "sole supplier" projects as opposed to shared or incremental work; and a desire to re-use private in-house IP and tools across projects. While understandable, these issues have sometimes led to grossly inefficient use of resources, redundant work, and even failure of developments. In the worst cases, effectively identical developments arise even within single experiments, implying a weaker effort by all parties, and a multiplication of ongoing support and maintenance load. This will not be tenable in the future (as also discussed in Chapter 10).

The community worked together to address the huge challenges of LHC via the CERN RD programme. The structure of long-lived semi-formal R&D collaborations, self-organised but with governance and support (financial and technical) from large laboratories, is still relevant: RD53 has demonstrated success for the most significant technical challenges posed by HL-LHC. This "RD nouveau" approach must encompass the full spectrum: not just for near-term collider projects, but for non-collider experiments and (crucially) long-term projects where basic feasibility has to be demonstrated. CERN has a central role, but in the "Tier" model, not as main sponsor of every development.

Developments in the coming decade will exceed the capabilities of even the largest groups, using entirely new technologies. An open collaborative model is vital. This will depend on the establishment of trust, best achieved within RD collaborations, and on a collaborative framework for open hardware, firmware, software and documentation,
including a common licensing strategy. A key aspect, with a long and successful tradition
in HEP, is the definition of common standards, implying adoption of relevant commercial
standards (e.g. the use of xTCA hardware) and definition of standards for custom
developments. Examples of the latter include: upper-layer readout and control link
protocols; interfaces between ASICs; interface definitions and design rules for circuit
blocks; and common interfaces to control software.

7.5.3.2 Demonstrators and Common Developments

Tracking industry developments and advancing the state-of-the-art requires an incre-
mental, continuous approach, where demonstrators and proof-of-concept systems are
regularly developed. It is important to continuously learn and gain experience, as it is
very difficult to skip generations of technologies, when important changes accumulate.
Resources must be provided to allow this, even outside the upgrade cycles of major
experiments, and without firm commitments of experiments to use the exact same prod-
uct. For such developments, documentation, support and dissemination are even more
important than for projects directly targeting a specific facility.

In this context, common developments (i.e. joint work undertaken with intended
application to a number of projects) have obvious advantages: they can rely on a broader
base of resources, both material and human; they can leverage more expertise; and they
have a larger potential user-base for testing. On the other hand, common developments
are not without risk. Broader applicability leads to more generic and flexible designs,
which can increase cost and complexity. Re-usability also requires that projects follow
well-defined development practices, so that information can be understood, updated and
maintained outside the immediate team. A common development calls for a substantial
long-term support effort since an entire community may rely on it. This is a frequently
forgotten dimension that in the past has led to failure of such developments after initial
enthusiasm. Involving larger laboratories can help, but such maintenance effort always
needs to be justified and funded. This implies measurement of take-up and added value
in the community. Conversely, and yet more dangerously, several common projects for
LHC upgrades have ended up with only one institute carrying the projects to completion.
The optical links and DC-DC converter projects for the LHC are an example, depending
entirely on a single CERN group. It is important to spread the load, risk and expertise
better across the community in the future, and to fairly support and acknowledge the
invested effort. A "tiered" model may help in this respect.

7.5.3.3 Infrastructure Needs

HEP electronics R&D depends on the availability of well-funded and extremely well-equipped laboratories, and the field is fortunate to have many institutes which have historically made such investments. However, the cost (both purchase and maintenance) of very-high-end test equipment and assembly facilities now presents a challenge even at national laboratories and CERN. This will become yet more significant as HEP embraces new technologies in photonics, wireless, etc. It is essential to ensure that, where such investments are made, facilities are available for a wide range of users and developments.
It would also be beneficial to ensure that investments in large-scale facilities are made
in a coordinated way, reflecting the specialisms of institutes, such that good coverage
is obtained collectively. A prominent example of the above is the availability of ASIC
high end test, verification and radiation exposure facilities. These are barely sufficient
to support the collective development effort on ASICs at the present time, leading to
delays and compromises across projects, and this problem will soon become even more
acute.

Distributed common projects also need well-supported IT infrastructure, including
services for collaboration support, source code repositories, and infrastructure for con-
tinuous integration and testing. These systems need software licenses and massive CPU
capacity for the simulation of ASIC and FPGA designs, and are most easily hosted in
large laboratories with full-time IT teams.

7.5.3.4 Interaction with Industry

HEP laboratories and experiments are rarely a commercially important customer for
industry. However, industry is often willing to engage in collaboration based on the
intrinsic technical interest of particle physics problems, the reputation of the field, and
access to highly talented young people for recruitment. Large laboratories such as CERN
are clearly well-placed here, and have an important “door-opening” function with indus-
try. In offline computing, the CERN OpenLab [Ch7-17] is a good example of how this
can benefit the community when multi-party projects are hosted, and most of the aca-
demic work is done by institutes other than CERN. When dealing with global industrial
partners, it is important that the HEP community speaks with a single coordinated
voice.

While key components like chips, optical fibres, and back-end electronics are pro-
duced commercially, access to industry know-how and proprietary information is critical
for obtaining optimum performance in extreme experimental conditions. Some specific
packaging and interconnect requirements of HEP (e.g. ultra-compact or very-low-mass
assemblies), are however not a focus for the microelectronics industry, and progress in
these fields relies on close and continuous collaboration with a limited number of spe-
cialised research institutions.

Co-design with industry is attractive and important, but it has to be kept in mind
that industry's market focus and short time cycles also induce the risk that technologies
can be obsoleted at very short notice. Since particle physics developments often need to
be aligned with long-term schedules, it is important to have a long-term forward view of
industry developments. This is usually only possible through long-term partnerships and
in most cases requires legal agreements such as non-disclosure agreements (NDA) with
relevant firms. Due to the distributed nature of particle physics collaborations, it would
be very useful to define a legal framework where only a single point of contact engages
with firms and then audits its collaborating members internally to make information
available, without the need for multi-lateral NDAs or individual negotiation of licenses,
NDAs and SLAs, thus avoiding much of the repetitive legal work.
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[Ch7-13] CMS Collaboration, *The Phase-2 Upgrade of the CMS DAQ Interim Technical Design Report*, CERN-LHCC-2017-014, CMS-TDR-018, 2017. https://cds.cern.ch/record/2283193. This is the CMS Interim TDR devoted to the upgrade of the CMS DAQ in view of the HL-LHC running, as approved by the LHCC.
[Ch7-14] E. Mendes et al., *A Timing, Trigger, and Control System With Picosecond Precision Based on 10 Gbit/s Passive Optical Networks for High-Energy Physics*, IEEE Trans. Nucl. Sci. **68** (2021) no. 4, 447-457.

[Ch7-15] R. Wang et al., *Processing Full-Scale Square Kilometre Array Data on the Summit Supercomputer*. IEEE Press, 2020.

[Ch7-16] S. Chilingaryan, A. Kopmann, A. Mirone, and T. dos Santos Rolo, *A GPU-based architecture for real-time data assessment at synchrotron experiments*, in *2010 17th IEEE-NPSS Real Time Conference*, pp. 1-8. 2010.

[Ch7-17] https://home.cern/science/computing/cern-openlab.
# Chapter 8

# Integration

## 8.1 Introduction

This chapter is concerned with R&D topics of integration, including tracker mechanics and general detector mechanics topics, cooling systems and on-detector cooling contacts, detector magnets, machine detector interface, monitoring systems of all sorts as well as purification systems for liquids and robotic systems.

Here, a clear distinction is made between “R&D topics” and “engineering challenges and prototyping”. The R&D topics listed in this section focuses on items that need major development before deciding whether and how to implement them in a given system. Many other items of future detectors represent significant engineering challenges and need extensive prototyping, but the state of the art is sufficiently mature that extensive further R&D is not required to allow implementation at the time of detector realisation. Examples are dry gas supply, e.g. membrane plant from industry to provide oxygen depleted air, dewpoint measurement - sniff and measure with commercial dewpoint meters outside volume, leak cables, survey, 3D laser scanning, virtual/augmented reality, neutron moderators, cooling transfer lines (triple-jacketed vacuum pipes, capillaries), thermal shields, thermal insulation, large movement systems for the neutrino near detectors, safety systems and gas chromatographs, sonar systems to measure gas mixtures.

Gas recuperation systems closely coupled to the specific detector gases are discussed in Chapter 1. Accessibility, repair and partial exchange capability have to be thoroughly taken into account at early stages of specific system design/engineering and also studied at early mock-ups. Also cables, especially low impedance and micro-cables/flexes and their respective connectors should be engineered and prototyped from the beginning but no genuine common R&D is proposed. 3D printing enters the particle physics field at many places and should be exploited for R&D and early prototyping and access to such facilities will be largely beneficial.

The main Detector Research and Development Themes DRDTs for the integration aspects are:
**DRDT 8.1 - Develop novel magnet systems.**

**DRDT 8.2 - Develop improved technologies and systems for cooling.**

**DRDT 8.3 - Adapt novel materials to achieve ultralight, stable and high precision mechanical structures. Develop Machine Detector Interfaces.**

**DRDT 8.4 - Adapt and advance state-of-the-art systems in monitoring including environmental, radiation and beam aspects.**

The timelines for the above themes can be found at Figure 11.1 showing that for all the half-decade blocks shown in Figure 3 and Figure 4 of the Introduction, there are facilities requiring R&D in each of these areas. Indeed, these are the four key research developments without which future experiments cannot function and which are in need of major development effort. Interfaces to cold detectors and purification systems are further demanding necessary developments, as particularly emphasised in Chapter 2.

## 8.2 Main drivers from the facilities

The main drivers for developments in the near future are the existing and approved facilities like the HL-LHC, Belle II and EIC. The requirements for detectors at these facilities fall broadly into two categories, namely “radiation dominated” experiments like ATLAS, CMS, LHCb where radiation hardness dominates most of the choices and “lower radiation exposed” experiments like ALICE, Belle II, Mu3e and EIC where luminosity levels are moderate and precision vertexing and PID are key elements. The detector developments for the HL-LHC hence form a preparation for a future high-energy hadron collider, while ALICE, EIC, Belle II and Mu3e are natural stepping stones for R&D towards a future $e^{+}e^{-}$ “Higgs factory” collider.

Figure 8.1 puts these developments into perspective of the future facilities.

## 8.3 Key technologies

### 8.3.1 Novel magnet systems

The LHC detectors ATLAS and CMS are currently operating large scale superconducting magnets that are important references for the development of future experiment magnets. Table 8.1 lists some examples of future magnet systems that represent the spectrum of engineering challenges and R&D needs for this topic. Detector proposals at future colliders like ILC, FCC-ee, CLIC, FCC-hh or Muon colliders use superconducting solenoids for the momentum spectroscopy. In addition to the choice of size and magnetic field for these solenoids there is the principle choice of whether to place the coil in front of the calorimeter system or behind the electromagnetic or hadronic calorimeter. Proposed detector systems placing the coil behind the calorimeter result in very large magnets of 3-
<table><caption>Table 8.1: Examples of magnets for future experiments that represent the engineering and R&D challenges. The dimensions and fields refer to the free bore. The magnets for ATLAS and CMS are given for reference.</caption><thead><tr><th>Accelerator</th><th>Detector</th><th>B [T]</th><th>R[m]</th><th>L[m]</th><th>I [kA]</th><th>E [GJ]</th><th>comment</th></tr></thead><tbody><tr><td>LHC</td><td>CMS</td><td>4</td><td>3</td><td>13</td><td>20</td><td>2.7</td><td>scaling up</td></tr><tr><td>LHC</td><td>ATLAS</td><td>2</td><td>1.2</td><td>5.3</td><td>7.8</td><td>0.04</td><td>scaling up</td></tr><tr><td></td><td>solenoid</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>FCC-ee</td><td>CLD</td><td>2</td><td>3.7</td><td>7.4</td><td>20-30</td><td>0.5</td><td>scaling up</td></tr><tr><td>[Ch8-1]</td><td>IDEA</td><td>2</td><td>2.1</td><td>6</td><td>20</td><td>0.2</td><td>ultra light</td></tr><tr><td>CLIC</td><td>CLIC-detector</td><td>4</td><td>3.5</td><td>7.8</td><td>20</td><td>2.5</td><td>scaling up</td></tr><tr><td>[Ch8-2]</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>FCC-hh</td><td>main</td><td>4</td><td>5</td><td>19</td><td>30</td><td>12.5</td><td>new scaling up</td></tr><tr><td>[Ch8-3]</td><td>solenoid</td><td></td><td></td><td></td><td></td><td></td><td>scaling up</td></tr><tr><td></td><td>forward</td><td>4</td><td>2.6</td><td>3.4</td><td>30</td><td>0.4</td><td>scaling up</td></tr><tr><td></td><td>solenoid</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>IAXO</td><td>8 coil toroid</td><td>2.5</td><td>8x0.6</td><td>22</td><td>10</td><td>0.7</td><td>new toroid</td></tr><tr><td>[Ch8-4]</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MadMax</td><td>dipole</td><td>9</td><td>1.3</td><td>6.9</td><td>25</td><td>0.6</td><td>large volume</td></tr><tr><td>[Ch8-5]</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

5 m bore radius and they represent scaled versions of the CMS coil, the largest ones being the FCC-hh coil. The development of next generation Al-stabilised high yield-strength Rutherford cable conductors for 30-40 kA and prototyping are needed for realising these magnets. Coils placed in front of the calorimeters have to be ultra-thin and represent < 1 $X_0$. Preliminary designs show that thin conductors based on Al/Cu/NbTi together with a cryostat made from an Al honeycomb structure can achieve this goal for a coil of 4 m bore diameter and 2 T field. R&D on dedicated conductors and prototyping is needed to achieve these goals. In the long term, the development of high temperature superconductors for coils and current leads would remove the need for He temperatures and allow operation at 30-40 K. Some detector proposals use dual solenoids instead of iron yokes for shielding of the magnetic field and the R&D for assemblies of this size still has to be performed.

The magnets for non-collider experiments are mainly related to axion searches that either make use of existing accelerator magnets or propose dedicated large volume magnets, the largest one being MadMAX with a 9 T/1.3 m bore magnet, which requires extensive R&D.

Development of quench protection, energy extraction and high voltage designs for coils with high energy/mass ratios is also needed.
<figure><img src="image_23.png" /><figcaption>Figure 8.1: Schematic timeline of categories of diverse R&D topics of integration 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).</figcaption></figure>
8.3.2 Improved technologies and systems for cooling

Cooling is a central topic for all future detectors with very different requirements. Scaling up of current cooling systems will work for some applications, but others need long-term R&D for refrigeration systems, detector intrinsic cooling contacts and transfer/distribution lines. Cooling needs to be applied to electronics (Chapter 7), irradiated semiconductor sensors to avoid thermal runaway (Chapter 3) and to silicon photomultipliers (SiPMs) decreasing dark count rate and in general sensor noise (Chapter 4). Cryogenic systems without significant heat load are typical examples where industrial solutions exist and engineering scale-up is possible, e.g. for liquid calorimeter, Neutrino and dark matter detectors (Chapter 2). No R&D is advised for any fluorocarbon or other greenhouse cooling liquid.

Transfer lines and detector internal capillaries must be considered at early stages of
system design but, as long heat insulation and pressure can be accommodated, this is
considered an engineering challenge and triple-jacketed vacuum pipes can run long and
complicated paths.

**8.3.2.1 Cooling systems**

The key parameters are minimum temperature (operation), highest expected temper-
ature (annealing/start-up/commissioning) and especially power density. Expertise in
standard mono-phase and bi-phase operation exists in the community, while operation
in supercritical mode (special monophase cooling with favourable properties for heat
and mass transfer due to very high heat capacity and very low viscosity) has been less
explored so far.

Above heat loads > 0.15 W/cm² bi-phase cooling is the preferred choice and in HEP,
temperatures down to -45°C are today's standard with CO₂ scaled to several hundreds of
kilowatts – 2PACL [Ch8-6], [Ch8-7] and adapted industrial R744 systems [Ch8-8], [Ch8-9].
For CO₂ additional R&D would be necessary to go above 1 W/cm² or to higher tem-
peratures (above 20°C). For temperatures lower than -45°C different cooling media and
cycling technologies needs to be explored. Figure 8.2 gives an overview of the different
temperature regimes, potential use cases and promising liquids.

LHCb is seriously looking into this regime for VELO-3 during LS4 and FCC-hh is interested in this as well. For cooling systems using Krypton (evaporative and supercritical) a new cycle technology is needed as cool down starts from gas phase. Mixtures of N₂O/CO₂ could be considered for lower heat fluxes, e.g. for SiPM applications, as N₂O has the same properties as CO₂ but at lower freezing point (100% CO₂ > -55 °C / 100% N₂O > at -90 °C).

For low heat loads below 0.15 W/cm², standard monophase, supercritical or even air flow cooling can be considered. In addition, Lepton and Ion collider are less affected by radiation thus can be cooled to only moderate temperatures. Monophase water or Novec<fnref n="1" /> cooling with ultrathin pipes could be engineered. Warm evaporative or supercritical CO₂ cooling systems would need dedicated R&D.

<footnote n="1">¹Fluid with thermo-physical properties similar to C₆F₁₄ and a very low global warming potential.</footnote>
<figure><img src="image_24.png" /><figcaption>Figure 8.2: Potential liquids for the different temperature and power density regimes.</figcaption></figure>

Another very promising path is air cooling (or e.g. Helium gas cooling) where the full geometrical design of the structure must be entirely designed to this goal from the very beginning, to guarantee that the cooling flow reaches all relevant surfaces in the detector. Collider detectors with moderate radiation load like ALICE, EIC can be stepping stones to inform later lepton experiments. The development of early small-scale generic systems will be useful to inform later dedicated engineering prototypes – a good use case for 3D printing. In addition, dedicated R&D is necessary for the cryogenic tracker in the AMBER experiment [Ch8-10] to achieve temperatures below 1.8 K with power dissipation of 10 W.

### 8.3.2.2 Local Cooling / Cooling contacts

Thermal management within the detector, tying in with the requirement to minimise material budget, is one of the biggest challenges for tracking detectors. The key parameters are heat produced at the source, maximum temperature of the source, uniformity and stability, thermal interface/contact, space and material budget considerations, reliability and lifespan. The important factor to minimise is the Thermal Figure of Merit $TFM = \frac{\Delta T_{sensor-fluid}}{SurfacePowerDensity}$ competing with the amount of contact material (radiation length $X_0$).

As discussed in the previous section for air cooling the aerodynamic systems properties are important. With substantial R&D these can be improved by liquid assisted air cooling, guiding micro-fluid (e.g. CO₂ or Novec) through porous carbon foam as
radiator/heat exchanger. Development on cooling to foam interface and miniaturisation
seems important as well as choice of the foam itself.

In dense environments Polyimide or Carbon Tubes embedded in light-weight sup-
port structures, e.g. high conductivity carbon plates are an established concept to be
engineered to the specific system. A common database of tubes properties (material,
diameter, wall thermal resistance, deformation under pressure, heat transfer coefficient,
cooling compatibility bending, radiation tolerance, possible connections, 3D printable,
source, etc.) as well as thermal interface materials is important and should be system-
atically realised and continuously updated.

Three more complex concepts of thermal interfaces need to be further developed, Thermo Electric Coolers (TECs), micro-Pulsating Heat Pipes (PHPs) and microchannel cooling, the probably ultimate low TFM. Examples of these concepts are displayed in Figure 8.3.

TECs, micro-Peltier elements, are available in industry, and are useful to locally de-
crease the temperature further. Advantages include: no moving parts, precise tempera-
ture control, radiation tolerance, no maintenance. Disadvantages include: low efficiency,
high X₀ and the need of an additional heat sink. Improvement on efficiency and more
miniaturisation would be beneficial. They are the baseline choice for the Phase-II CMS
Barrel Timing Detector cooling SiPMs.

PHPs work without wick/capillaries and need no return flow but need power at the
source. PHPs have potential as secondary cooling circuits, pre-integrated into an ultra-
light support structure, potentially also 3D printed, e.g. connected directly to pixel
sensors [Ch8-11], [Ch8-12], [Ch8-13].

Microchannel cooling [Ch8-14], [Ch8-12] comes in a large variety and seems to have
the potential to minimise the TFM but R&D is necessary to overcome current drawbacks
and to develop standard solutions which can then be highly customised. Microchannel
cooling is already applied in NA62 (liquid FC-72) [Ch8-15] and LHCb-VELO (bi-phase
CO₂) [Ch8-16]. In general, the micro hydraulic connectors are fundamental bottle-
necks. Current thermo-mechanical structures can be realised in silicon (potentially in-
tegrated/etched directly into sensor/CMOS wafer), ceramic composites, metal alloys or
ultra-thin polymer pipes in carbon/graphite matrices but future developments should
not be limited to these. It should be investigated how far 3D printing can be exploited
here. The concept allows monophase, bi-phase or supercritical operation directly con-
nected to the full heat source surface. For silicon microchannel cooling, well established
in MEMS technology, the brittleness, cost and size limitations should be overcome.

8.3.3 Novel materials to achieve ultra-light, high precision mechanical structures

Ultra-lightweight support mechanics are the big challenges for tracking detectors and cryostats for calorimeters and magnets in all collider experiments to minimise multiple scattering of particles, brems-strahlung, photon conversions and nuclear interactions. Aspects to be taken into account are a high radiation environment, dynamic stability under external vibration, stability under temperature and humidity variations, high
<figure><img src="image_25.png" /><figcaption>Figure 8.3: Local Cooling Concepts to develop further: Microchannel, micro-Pulsating Heat Pipe and Thermo Electric Coolers.</figcaption></figure>

required thermal conductivity, non-flammability, minimum mass (low $X/X_0$), non-out-gassing and sometimes the need for electrical insulation, and all inherently linked to the cooling requirements to minimise TFM. Low $X/X_0$ is even more important for lepton (ILC, FCC-ee, CLIC) and ion experiments compared to hadron experiments (HL-LHC, FCC-hh). References [Ch8-17], [Ch8-18], [Ch8-19], [Ch8-1] give examples of requirements and provide perspective for this section.

Also here, a common updated database with full characterisation of all potential materials; being it for support or cooling tubes, or microchannels (Thermoplastics, Polyimides, ceramic composites, CF sheets, Carbon foams, Graphene, etc.) but also resins and potting or insulation material; is key to success. The database should also cross reference which materials can be easily connected/embedded together. R&D improvements on lighter thin pipes and channels, carbon microvascular plates, CF plates, thermal pyrolytic graphite and carbon foam is encouraged. Novel materials like nano-material, thermal conductive carbon nanotubes fibres should be investigated profiting from industrial advances. For resin systems, thermoplastics and nano filled resins (nano composites) seem promising. Carbon foams can be used as radiators in gas cooling applications and many foams are available from Aerospace to be characterised.

Easy access to wind tunnel testing facilities seem necessary to validate gas cooling on mock-ups, checking mechanical stability and oscillations for optimisation and final validation. Another aspect to explore is light weight mechanics that can be partially transparent to gas flow, as specifically needed for self-supporting silicon devices, e.g. DEPFET in Belle II and bent sensors for the ALICE upgrade during LS3.

Entry windows for fixed target experiments and inner detectors walls in front of the first layer should be minimised. Studies on very thin Beryllium, Aluminium, AlBeMet, etc. should be conducted, and also carbon composites should be investigated. Studies on non-out-gassing material for devices within vacuum applications is mandatory.

R&D on fabrication methods should be pursued together with standardisation; key considerations include micro-fabrication, additive manufacturing, and inter-connectivity. As of today, modules are attached to long cooling pipes while a “LEGO” concept, with
partial cooling pipes embedded and then attached together, would facilitate large systems in a leak tight fashion.

Lighter and thinner cryostats for superconducting magnets and calorimeters must be investigated, as well as lightweight coils themselves. This is also relevant for some neutrino detectors, i.e. in between sensitive volume and downstream (muon) detectors. Aluminium honeycomb panels represent an attractive solution. Potential advances in industry and aerospace can be exploited, e.g. of developments for liquid oxygen and hydrogen tanks. Aluminium could potentially be replaced by Ti, carbon composites, high and ultra-high modulus Carbon Fibre Reinforced Polymer (CFRP) or carbon sandwich structures thereof. R&D in industry on sandwich/flute designs, thin plies, out of autoclave materials and winding/tape deposition as well as Ultra-High Vacuum (UHV) seal joints should be evaluated and tested. Figure 8.4 shows state-of-art examples of cooling pipes, new materials and cryostats.

<figure><img src="image_26.png" /><figcaption>Figure 8.4: State-of-art examples of pipes, embedding, new materials for beampipes, entry windows and support structures and also cryostats systems.</figcaption></figure>

### 8.3.3.1 Machine-detector interface (MDI)

Machine-detector interfaces differ significantly between the different accelerator types as indicated in Tables 8.2 and 8.3. An important difference between hadron machines and $e^{+}e^{-}$ machines concerns the position of the last machine elements i.e. the final focusing magnets, that determines the space for detector elements. For the FCC-hh these magnets are placed at a distance of 40 m from the IP, while for the FCC-ee this distance is just 2.2 m. For $e^{+}e^{-}$ accelerators the integration of detector and machine elements is therefore a prime challenge. For circular $e^{+}e^{-}$ machines the synchrotron
radiation hitting these machine and detector elements is a significant source of background and is therefore strongly interleaved with this integration question. The further development and application of reliable simulation tools for background simulations and benchmarking at existing facilities are therefore of utmost importance.

The central beampipe is the most direct interface between accelerator and detector. Beampipes with minimum amount of material as well as the smallest possible radius are key elements for the physics exploitation at all machines. Impedance and vacuum quality are key parameters for the machine performance. In addition, for $e^{+}e^{-}$ machines these beampipes have to be cooled. Engineering, simulation of background, impedance and vacuum as well as prototyping are key R&D topics in this context. Vertex trackers in secondary vacuum vessels that are dynamically moved close to the beam after injection are a very effective possibility for significant improvement of the tracking and vertexing performance. LHCb is exploiting this already at the LHC and future proposals like the ALICE 3 detector are planning for such installations. While the development of the focusing magnets and other machine elements is typically part of the accelerator project, the support and alignment of these is an integration challenge that concerns the detector, and topics like remote vacuum connection have to be designed together.

<table><caption>Table 8.2: Examples of accelerator parameters for future hadron accelerators machines that represent the MDI R&D challenges [Ch8-20], [Ch8-3], [Ch8-21], [Ch8-22].</caption><thead><tr><th></th><th></th><th>LHC</th><th>HL-LHC</th><th>FCC-hh</th><th>EIC</th><th>LHeC</th></tr></thead><tbody><tr><td>E</td><td>GeV</td><td>7000</td><td>7000</td><td>50000</td><td>10/275</td><td>49.19/7000</td></tr><tr><td>L</td><td>10<sup>34</sup>cm<sup>-2</sup>s<sup>-1</sup></td><td>2.1</td><td>5</td><td>5-30</td><td>1</td><td>23</td></tr><tr><td>Angle</td><td>mrad</td><td>0.26</td><td>0.5</td><td></td><td>25</td><td>0</td></tr><tr><td>Bunch</td><td>ns</td><td>25</td><td>25</td><td>25</td><td>10</td><td>50</td></tr><tr><td>L*</td><td>m</td><td>23</td><td>23</td><td>40</td><td>4.5</td><td>10</td></tr><tr><td>B det</td><td>T</td><td>2/4</td><td>2/4</td><td>4</td><td>1.4</td><td>3.5</td></tr><tr><td>r<sub>pipe</sub></td><td>cm</td><td>2.35/2.1</td><td>2.35/2.1</td><td>2.5</td><td>ell.</td><td>ell.</td></tr></tbody></table>

<table><caption>Table 8.3: Examples of accelerator parameters for future $e^{+}e^{-}$ machines that represent the MDI R&D challenges [Ch8-23], [Ch8-1], [Ch8-24], [Ch8-2].</caption><thead><tr><th></th><th></th><th>KEKB</th><th>FCC-ee</th><th>ILC</th><th>CLIC</th></tr></thead><tbody><tr><td>E</td><td>GeV</td><td>4/7</td><td>45.5/120/182.5</td><td>125/250</td><td>190/1500</td></tr><tr><td>L</td><td>10<sup>34</sup>cm<sup>-2</sup>s<sup>-1</sup></td><td>80</td><td>230/8.5/1.6</td><td>1.4/1.8</td><td>1.5/6</td></tr><tr><td>Angle</td><td>mrad</td><td>83</td><td>30</td><td>14</td><td>16.5/20</td></tr><tr><td>Bunch</td><td>ns</td><td>4</td><td>20</td><td>554, 5Hz</td><td>0.5, 50Hz 231</td></tr><tr><td>L*</td><td>m</td><td>0.77/1.11</td><td>2.2</td><td>4.1</td><td>6</td></tr><tr><td>B det</td><td>T</td><td>1.5</td><td>2</td><td>5</td><td>3.5-5</td></tr><tr><td>r<sub>pipe</sub></td><td>cm</td><td>1</td><td>1.5(1)</td><td>1</td><td>3</td></tr></tbody></table>

Radiation simulations are crucial for the following MDI topics: for $e^{+}e^{-}$ machines, shielding is key for suppression of synchrotron radiation background. For hadron colliders
the shielding is key for suppression of neutron background in the experiment cavern and for minimisation of energy deposit in the superconducting magnets. The activation of the elements has an important impact on access and detector maintenance. For muon colliders the shielding cones on both sides of the detector that are absorbing the muon decay electrons and positrons are a key MDI element that has to be optimised and simulated in detail. Multi-turn tracking of IP collision debris for optimisation of collimation schemes is important as well.

### 8.3.4 Monitoring

#### 8.3.4.1 Environmental Monitoring

The community has established significant experience with environmental sensors and existing solutions can be scaled-up and engineered to the specific use case. Still there is room for improvement and standardisation, especially for radiation tolerant and small package devices. As safety systems must be simple and reliable, it is argued here that the usual overheating/-voltage/-current protection, RTDs (Resistance Temperature Detector) plus commercial dewpoint meters checking inlet gas quality and gas sniffed/pumped from the volume is adequate and no further R&D is recommended.

As many sensors, including radiation tolerant ones, exist and are in use at different experiments, a database with full characterisation and specification of sensors seems mandatory to avoid duplication of developments and characterisation. This is especially true for MEMS (Micro-Electro-Mechanical Systems), as they are available in industry but not systematically designed/tested for radiation tolerance [Ch8-25], e.g. pressure-, vibration- and flow sensors.

Opportunities are recognised for further embedding and for increased granularity of such sensors, eventually implemented in 3D printed metal tubes/tanks/feedthroughs, and for heat exchangers which could also be realised in carbon fibres (CF) structures. Fibre-Optical Sensors (FOS), e.g. Fibre-Bragg Grating Sensors (FBGS) are a well-established technology to measure e.g. temperature, deformation, stability and relative humidity [Ch8-26] if the correct coating is used. These sensors are established to be sufficiently radiation tolerant [Ch8-27] for the HL-LHC environment. There is ample development opportunity to standardise FOS and to provide templates for later custom solutions. The following challenges merit further development: increased precision, e.g. 0.1°C and utilisation in cryogenic environments, and increased sampling rate to achieve dynamic measurements, e.g. oscillations but maybe also flow and pressure measurements. A key target would be the embedding in CF to achieve real-time information on structure deformation/vibration at sub-µm level.

No adequate sensors for flow and vapour quality measurement in 2-phase cooling systems exist and these should therefore be developed. Also miniaturised gas flow meters embedded in the detector for gas cooled systems would be very useful, e.g. MEMS with vanes or sense hairs.

Position survey tools are mature and industry standard. The development of an easy to use and fast to deploy system for maintenance and re-assembly procedures would be
very useful, as well as radiation tolerant and miniaturised distance measurement devices/cameras for opening/closing systems. It has to be mentioned that some systems e.g. the BEAMCAL luminometer [Ch8-28] have severe requirements on placement precision, i.e. 50 µm for linear and 1 µm for circular colliders.

### 8.3.4.2 Beam and Radiation Monitoring

#### Beam Loss, Beam Induced Background, Luminosity

Beam loss protection devices exist and should continue to be improved and adapted to specific use cases. Beam loss monitors need to be radiation tolerant and of small size as they must be located in the inner detector. They must be 100% available and reliable, fast enough to provide beam aborts/interlocks, and neither sensors nor electronics should be susceptible to saturation. Diamond and sapphire detectors are good candidates.

Beam condition/background monitors must be radiation hard and small due to the preferred closeness to the beampipe and high-rate capable to allow bunch-by-bunch information. 4D-tracking (time & space) as well as directional measurement devices able to discriminate between collision and Beam Induced Background (BIB) should be developed. BIB measurements with timing capability seem especially important for a future Muon collider with ~ $10^8$ BIB particles in a single event.

Luminosity determination is a key challenge at all collider experiments and merits further development to achieve a bunch-by-bunch error below 1% (goal for HL-LHC). Individual luminometers need at least a 0.7% precision on calibration and linearity with good stability and an understanding of systematic errors at the per mille level. Linearity is key to calibrate with Van-der-Meers scans from multiple interaction per bunch crossing of 0.5 to 200 (HL-LHC) or even 1000 (FCC-hh). R&D addressing the system level must either produce fast analogue front-end electronics or a low occupancy, thus high granularity, system or both. A specific calometeric luminometer BEAMCal is described in Chapter 6.

#### Polarisation measurement at ILC, FCC-ee and CLIC

The polarisation measurement of the up- and downstream $e^{+}e^{-}$ beams is an essential combined experiment and accelerator R&D endeavour. The complete systems (magnet, laser, alignment system, vacuum guiding, position sensitive Compton electron detector, e.g. quartz Čerenkov) has to be developed as a package. Specification and radiation tolerance of the system has to be defined by the experiments. More details can be found [Ch8-29] for FCC-ee and [Ch8-30] for ILC.

#### Radiation Monitoring

The community has a vast experience with radiation monitoring and this needs to be continuously improved, especially in view of the future higher levels (HL-LHC and FCC-hh) and higher energies. The challenge is threefold: improve simulation, measurement devices and cross-calibration of irradiation facilities.
There is an important need to maintain and improve the code for simulations of the underlying physics processes, e.g. cross sections, event generation at higher energies, etc. These tools are of utmost importance to understand longevity of detectors, remaining activation during maintenance periods but also to understand particle rates in systems or effectiveness of shielding, etc.

Further R&D on spectroscopic particle detectors should be supported, e.g. Medipix based detectors with specialised conversion targets. Also radiation-hard, small devices with low power consumption or on-chip dosimetry and fluence measurement is very interesting. Long term there is the need for devices to measure integrated values of $1×10^{18} n_{eq} cm^{-2}$ and 300 MGy Total ionising Dose (TID) not achievable with conventional p-in-n diodes. Radiation dependent resistors as NIEL monitor or Fibre based dosimeters (RaDFOS) [Ch8-31] might be candidates to explore further. For any new detector material NIEL (Non Ionising Energy Loss) studies (spectra) for different particle types and energy and interpolated to the mixed fields of future experiments are of the utmost importance.

To allow the thorough evaluation of such devices as well as other sensors, the network of radiation facilities should be fully utilised and improving standard methods, e.g. fluence measurements, cross-calibration and exchange of information (database) is strongly recommended (see also Chapter 10).

## 8.3.5 Calorimetry, Neutrino and Dark Matter Detectors

### 8.3.5.1 Calorimetry

The integration challenge for calorimeters has many aspects that are often tied to the specific detector design and that are therefore part of the project prototyping and construction effort (see Chapter 6). Many topics will also profit from progress in industry like e.g. 3D printing techniques. Collaboration on this technology will allow highly optimised absorber structures. There are however some general items that require R&D in order to be able to make specific choices for next generation calorimeters. Calorimeters using cryogenic liquids (see also Chapter 2) require cryostats that need to support the heavy weights but at the same time have to represent a minimum amount of material, as discussed in the previous section. The need of high granularity in future calorimeters also requires a large number of signals and services which calls for the development of compact data concentration and powering solutions and, for cryogenic detectors, of high-density signal feedthroughs. Access to cryo-laboratory facilities for generic testing and development is essential (see Chapter 10). Another topic that differs from industry needs is the radiation load, specifically for the FCC-hh with hadron fluence numbers up to $1×10^{18} n_{eq} cm^{-2}$. Radiation hard insulators, glues and other relevant materials have therefore to be identified and characterised.

### 8.3.5.2 Neutrino Detectors and Dark Matter Detectors

The next generation large scale neutrino detectors like DUNE or HYPER-K represent significant engineering challenges that are specific to the detector design. The large scale
components for these facilities are developed together with industry. For prototyping of
these detectors, facilities like the CERN neutrino platform are essential. Common R&D
needs refer to purifiers for the production of ultra pure noble liquids with < 30 ppt of O₂-
equivalent impurities and purifiers for ultra-pure water. Radio purity of these systems
is essential to enable low energy physics studies (see Chapter 2). Reliable programmes
for CFT calculations for evaluation of the distribution of ions, impurities and dopants
have to be developed. R&D on scaled up HV systems, electrical insulation and cryostat
feedthroughs for supply of the field-cage potentials is also needed. 3D printing of e.g.
scintillator cubes or optical components like Winston cones are an interesting option for
affordable large scale applications.

Radio purity is also a key topic for dark matter detectors. Access to radon free cleanroom facilities is therefore essential. The assembly of photosensors into highly efficient large area arrays is a crucial engineering challenge for dark matter detectors (see Chapter 4).

8.3.6 Robotic Systems, Survey

Robotic systems as well as positioning systems and sensors are being used and developed
in industry at a large scale and the R&D efforts relate to their customisation for applica-
tion in high energy physics. Integration of systems for heavy mass movement can make
use of industrial solutions and the R&D is related to the development of the interfaces
to the detector structures. Positioning systems and positioning sensors are available on
the market and have to be customised in collaboration with industry. For magnetic field
mapping and radiation mapping, R&D is required to allow the use of drones or blimps
in the presence of magnetic fields and radiation. The development of robotic systems is
certainly also driven by industry and the R&D is on the adaption to the radiation and
magnetic field environment. A potential use-case would be regular inspection and fault
detection in very confined, inaccessible and highly radioactive areas. Robotic systems
for cryogenics environments, like the large Liquid Argon TPCs, are not available on the
market and dedicated R&D is needed.

Finally, the significant radiation levels at future hadron colliders are calling for auto-
mated detector opening systems and automated insertion of radiation shields to allow
the safe access of personnel for maintenance.

8.4 Observations and Recommendations

Many integration aspects require continuous R&D and the synergies unfold for the dif-
ferent detectors, e.g. local cooling contacts DRDT 8.2, lightweight mechanics DRDT
8.3 and all aspects of monitoring DRDT 8.4. Magnets DRDT 8.1 are very specific with
some crucial R&D but long lead time items, e.g. the conductors.

There is a general sequence of use cases for LHCb Upgrade II, CMS/ATLAS partial
pixels replacement in LS5 leading towards FCC-hh and on the other hand Belle II,
ALICE 3 and EIC experiments clearly spearhead needs for future e⁺e⁻ “Higgs factories”.
Discussions during and around the symposium showed the usefulness of 3D-printing for many aspects, especially for prototyping, and there is the need to continuously monitor and exploit advances in industry.

All aspects of integration are in high demand of continuous access to engineering resources and close collaboration with industry during R&D and also later design and realisation of the experiments.

Ultra-light cryostats are mandatory for noble liquid calorimeters, thin magnets in front of the calorimeters, and possibly for Neutrino experiments. Depending on the success on these in combinations with ultra-light solenoids, one could later reconsider experiment configurations toward smaller bore magnets.

Many developments will profit from central collections of information (databases) on items including their properties, e.g. glue, resin, lightweight materials, sensors, radiation monitors, cables, etc. Also all items from industry, characterised by the particle physics community must enter here.

Another crucial aspect is to continuously maintain and improve simulation tools, e.g. for detector development [Ch8-32], [Ch8-33], [Ch8-34], radiation, background simulation, activation [Ch8-35], [Ch8-36], gas flow dynamics and thermal management.

The necessity to have regular and easy access to test beams [Ch8-37] and radiation facilities [Ch8-38] cannot be emphasised enough. But also more generic access to wind tunnel experimental areas, cryogenic facilities and radon free clean rooms are essential.

It is mandatory to establish and maintain fora with regular intellectual exchange on these topics, e.g. the "Forum on Tracking Detector Mechanics", as such gatherings and exchanges are less common than sensor conferences.

It should be emphasised that aspects of detector integration cannot be done in isolation. There must be strong cohesion and collaboration with all detector Task Forces and technologies and especially with the second horizontal Task Force 7 Electronics (Chapter 7). For mechanics and cooling (DRDT 8.2 and DRDT 8.3 combining) effort with Task Force 3 Solid state detectors (Chapter 3) is especially important. Obviously, this becomes mandatory as soon as concrete system designs and their realisation starts.

## References Chapter 8

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[Ch8-2] CLICdp Collaboration, CLIC Collaboration, P. N. Burrows et al., *The Compact Linear Collider (CLIC) - 2018 Summary Report*, CERN Yellow Reports: Monographs 2 (2018) https://cds.cern.ch/record/2652188.

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[Ch8-10] https://amber.web.cern.ch.

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[Ch8-12] A. Mapelli, Microfabricated silicon substrates for pixel detectors assembly and thermal management a.k.a. Silicon Microchannel Cooling Plates, Nucl. Instrum. Meth. A **958** (2020) 162142. Proceedings of the Vienna Conference on Instrumentation 2019.

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[Ch8-14] M. Vos, Micro-channel cooling in high energy physics, PoS Vertex2016 (2017) 037.

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[Ch8-17] G. Contin et al., The STAR MAPS-based PiXeL detector, Nucl. Instrum. Meth. A **907** (2018) 60-80. Advances in Instrumentation and Experimental Methods (Special Issue in Honour of Kai Siegbahn).

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[Ch8-21] E. C. Aschenauer et al., eRHIC Design Study: An Electron-Ion Collider at BNL, arXiv:1409.1633 [physics.acc-ph].

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[Ch8-25] H. R. Shea, Effects of radiation on MEMS, in Reliability, Packaging, Testing, and Characterization of MEMS/MOEMS and Nanodevices X, S. Garcia-Blanco and R. Ramesham, eds., vol. 7928, pp. 96 – 108, International Society for Optics and Photonics. SPIE, 2011. https://doi.org/10.1117/12.876968.

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[Ch8-37] http://test-beam-facilities.web.cern.ch.

[Ch8-38] http://irradiation-facilities.web.cern.ch.
# Chapter 9

# Training

## 9.1 Relevance of instrumentation training

Particle physics experiments demand technology well beyond the state-of-the-art, with ever increasing complexity. Establishing novel technologies requires decades from conception to application. The detector development programme of particle physics experiments must be accompanied by a well balanced training programme aimed at preparing the next generations of detector developers required by the field and by industry. This should also contribute to train and motivate those who commit to the continuity in operation of existing complex detector systems. A successful training concept needs to be based on the following key points:

* Stimulate and recognise the field of instrumentation in particle physics and specifically the importance of innovation, detector development and operation;

* Attract and train outstanding talented individuals in physics and engineering;

* Recognise the diversity of skills needed in the field;

* Find an appropriate balance between specialisation and breadth.

Detector instrumentation training spans all career levels, from university courses, to (post-)graduate programmes, to specialised courses for professionals. An adequate and well structured programme needs to be developed which is well balanced between classroom, online and hands-on courses. It must provide access to modern infrastructures and also address geographic, nationality and gender diversity.

Scientists pursuing a career in instrumentation should be able to count on: recognition at all stages, e.g. dedicated scholarships, stipends, awards; opportunity for publications in high-ranked journals of technology and experimental methods; and attractive career prospects. Particularly this last point is of great importance for Early Career Researchers (ECRs) and has not been sufficiently addressed.

Establishing a successful long-term training programme in instrumentation is crucial for particle physics, since it ensures the continuity of highly qualified detector experts
from R&D to construction and to operation of HEP detectors. It has also benefits for society, by providing a talent pool for industry and other sectors.

### 9.1.1 Junior ECFA input

As training is primarily dedicated to the education of less senior physicists, it was recognised that the input and opinions of Early Career Researchers (ECRs) were key elements in appreciating their perception of the existing provision, as well as to identifying improvements to better meet their needs. The ECFA - Early Career Researchers Panel was thus approached to provide direct input to the symposium. A working group was formed, and a survey was distributed about training in instrumentation and related issues. In total, 473 answers were collected in a very limited time, showing the strong interest of the ECRs in this topic. 80% of the participants were from Europe. A complete analysis document was produced [Ch9-1]. Here only a brief account of the most significant outcomes is possible.

A first message from the ECRs survey and the symposium debate is related to the general recognition and perception of instrumentation work by the community. Many junior physicists think that work in instrumentation, although of prime interest to them and moreover to their experiments, is not sufficiently recognised and could even be detrimental to their career. This negative perception needs to be further investigated by ECFA. If confirmed, corrective measures should be taken, and recommendations made. ECRs need to be deeply convinced that if they engage in instrumentation training and related subsequent commitments, this is not against their prime career interests. The same also prevails for ECRs who participate as trainers in instrumentation programmes. There is a wide belief among ECRs (with the exception perhaps of assistant professors - yet at a later career stage) that acting as trainers in instrumentation is not sufficiently rewarded. Unless this is addressed, it will be difficult to justify involving more ECRs in designing and implementing training programmes, which is one of the very fruitful measures to make such training more attractive (as horizontal training - ECRs to ECRs).

Out of the 473 respondents, more than 70% indicated that they are active in instrumentation (more than 20% of their time spent working on instrumentation). This is a very significant fraction that makes the survey findings quite relevant for planning training evolution. There were apparently no perceived differences between respondents based at universities compared to those at laboratories, which is contrary to the expectations expressed by some in the symposium.

Only 40% of the respondents felt that they had access to sufficient training in instrumentation, and in general ECRs feel that they are insufficiently informed of what is available. The global satisfaction of those who took instrumentation training, in all its diverse forms, is rather high. It is interesting to note that 60% of those who followed remote training were positive about it. Apparently, many feel that they lack training in the use of Open Source software tools that are frequently deployed for operating experiments. The same is true with respect to networking opportunities in instrumentation. This is one of the areas where ECRs working on instrumentation would benefit from significant improvements, as only 20% of the respondents felt satisfied with the network-
ing opportunities. Developing networks or forums in instrumentation as well as training ECRs on “how to network” could be considered.

The ECR Panel report also mentions difficulty reaching young engineers who were not included in the initial discussions. It was highlighted in the ECR Panel’s town-hall meeting that those involved in instrumentation can have a range of backgrounds involving physics and engineering, but the number of engineers responding to the survey was relatively low. A subsequent survey with an adapted questionnaire could be undertaken to collect their inputs on the issues discussed here.

Human and geographic diversity and inclusion issues are serious concerns in the ECR community that need to be properly addressed especially at the training level. Many of the respondents called for a wider inclusivity in training opportunities to help further improve diversity in the experimental community.

Last but not least, it is interesting to note the apparent division between analysis and detector physicists, which is perceived as a particularly pronounced feature of the LHC experiments, is really disapproved of by many ECRs. ECFA should take actions to remind the community that experimental particle physics is a highly varied activity that includes many career profiles, and ensure that the opportunities and recognition for training and work in instrumentation are appropriate in light of this variety.

In summary, the survey revealed that there is much interest and enthusiasm in the ECR community for instrumentation work, however there are also barriers. The ECR community is eager to work with ECFA and the rest of the community to improve accessibility to training and networks in instrumentation, and on better recognition for those who design and deliver training programmes.

## 9.2 Status of instrumentation training in Europe

The design and delivery of detector instrumentation and accelerator projects in high energy physics relies heavily on the expertise and experience of senior scientists, a specialised technical workforce, cutting edge facilities and a large number of students, post-doctoral researchers and early career scientists.

In addition to the main HEP instrumentation fields, such as: detector science, electronics, mechanics, materials science, programming (including that of FPGAs and for DAQ) and accelerators; scientists require continuous training in emerging topics. Examples include: quantum computing, detectors at the quantum limit, condensed matter, quantum dots, novel methods of acceleration and cryogenic detectors.

In order to attract young people to the field, early recruitment has to happen, starting with programmes designed for high schools, and then through instrumentation studentships and detector schools, or through graduate training and dedicated schools in the case of accelerators. In this section, the existing landscape of instrumentation and accelerators is reviewed in terms of: university courses (Section 9.2.1); dedicated graduate schools for HEP instrumentation in Europe and worldwide (Section 9.2.2); and specific programmes offered by the major European laboratories (Section 9.2.3). The status of accelerator training is reviewed in Section 9.2.4.
### 9.2.1 University programmes dedicated to HEP instrumentation training

Future instrumentalist in HEP typically enter the field through two main paths: Bachelor/Master in physics or in engineering. The challenges are different for the two paths. Typically, the physics students do not have sufficient access to training on detectors and technology, while the engineering students have limited knowledge of HEP to be attracted to this domain. To begin with, students need to be attracted to physics studies. Here, instrumentation activities at universities and visits to see local, national or international research infrastructures is highly valuable. It is important to have early opportunities for high school students to get in touch with experimental work. In the programme “Beamline 4 Schools” offered by CERN, high school classes may collaborate with local universities in preparation of the bids or experiments for the programme.

Once students have started physics studies opportunities for training in instrumentation vary between universities within each country and even more between countries.

To provide hands-on experience in instrumentation to students is one of the major challenges for university programmes. Few groups have access to infrastructure for hands-on training. Equipment used in nuclear physics laboratory courses can be used to give the first basic experience in instrumentation. Some universities have access to accelerator and state-of-the-art equipment on site that can be used in a coordinated way by a cluster of universities in the region. Bigger infrastructures at national and international laboratories should in particular be more intensively used for training.

Instrumentation has to be introduced already at bachelor level, if one wants to efficiently attract students to this area. There are many advantages with hands-on training compared to teaching in a classroom. Instrumentation is by nature multi-disciplinary, connecting theory with experiments, and where progress at the technology frontier requires teamwork amongst experts from differing backgrounds. Working on instrumentation also stimulates creativity and a healthy teamwork spirit. The successful experience of problem solving is often contagious and helps attract bright minds to particle physics.

At the ECFA symposium, a number of good examples of training programmes at undergraduate level were presented. In Germany there are several examples like those at the Karlsruhe Institute of Technology and at Heidelberg University, where the masters in physics offers an optional track towards instrumentation. These programmes integrate the students with the local research infrastructures and activities. In the Netherlands, instrumentation programmes connect to a broad range of application ranging from medical to cosmology. This clearly emphasises the multidisciplinary reach of the instrumentation field for the students. In Paris, an inter-university master is offered in the field of large instruments, plasmas, accelerators, etc. A programme called “Excellence by Experiments” provides eight experimental platforms in fields of nuclear physics, particle physics, astro-particle physics and astrophysics. Three of the platforms utilise the local 4 MeV accelerator. More examples certainly exist, but a clear overview is missing. Also, as far is known, an exchange between these programmes similar to the Erasmus initiative, but dedicated to instrumentation courses is not foreseen at present. This is why it is proposed to create and maintained a list of master programmes and schools
<figure><img src="image_27.png" /><figcaption>Figure 9.1: Students at CERN’s S’Cool LAB [Ch9-2].</figcaption></figure>

dedicated to instrumentation training. When compiled, this list will become accessible
via the ECFA Detector Panel web pages at https://ecfa-dp.desy.de/public_documents/.

### 9.2.2 Graduate schools, doctoral and post-doctoral programmes dedicated to HEP instrumentation training

The content of this section draws heavily from the presentation of A. S. Navarro at the Symposium of Task Force 9 Training (see Appendix C), as well as input from the ECFA appointed National Contacts (as discussed in the Introduction).

Instrumentation schools play a fundamental role in the training of Masters/PhD students, postdoctoral researchers and ECRs, with Europe being at the forefront of organising such programmes. Many international and national schools exist, ranging from those with a very specialised focus, through to those with a more broad and/or interdisciplinary remit, which can also serve as an opening for students to access other fields and related synergies. Table 9.1 lists selected examples of detector international schools, illustrating some of the variations on offer, in terms of format, target audience and breadth versus specificity. The corresponding information for accelerator schools, which form a key part of the education of the personnel in the field, can be seen in Table 9.2 in Section 9.2.4.

The available schools typically offer a balance of theoretical (e.g. via lectures, classes, tutorials) and experimental (e.g. via laboratory experiments) learning, with both elements considered essential to fully train future instrumentalists. The schools are typically one to two weeks in duration, though some are longer and extend to periods of several weeks, e.g. ESIPAP. A few also include a dedicated period of pre-school preparation, in
order to maximise the efficiency of learning during the school itself. Participant numbers generally range from about 30 to 100, which is primarily limited by hands-on laboratory capacity and/or available staff. Several schools also offer awards and/or academic accreditation to participants. In contrast, as far as is known, there is little or no routine recognition of training-staff and organisers. Experience during the recent COVID-19 pandemic has shown that, in some cases, virtual schools can be effective (e.g. those with a strong software focus), and can widen participation, but should be used appropriately and in moderation, due to the importance of hands-on experience.

A point of note is that, while a variety of examples of international schools exist, they tend to be independently organised and not sufficiently publicised to ECRs. A better international coordination and a wider advertising effort would greatly improve that situation. In addition to the international and European-wide opportunities, there are a variety of national programmes that offer graduate-level training. For instance, some countries offer doctoral programmes in which instrumentation is key, for example: INFN funding for ~60 PhD studentships per year, most on instrumentation topics; KSETA (Karlsruhe School of Particle and Astroparticle Physics) [Ch9-3]; the German-Norwegian PhD research school [Ch9-4]; and the UK STFC centres for postdoctoral training [Ch9-5], which include industrial or major laboratory placements. The “Terascale Detector Workshop” [Ch9-6] is a specific example of a national workshop targeted at PhD students and postdocs from German groups who are active in the area of detector development. National schools are also on offer in several countries, such as: the INFN school on detectors for graduate students [Ch9-7]; the Mainz biennial school on instrumentation for Masters/PhD students [Ch9-8]; and the Spanish “Phy6cool” school on particle, astro-particle physics and cosmology [Ch9-9]. Many countries also benefit from traditionally close collaborations with large national or international laboratories, which serve as a hub for training opportunities, for instance in the form of short visits or extended stays, and/or via shared studentships between those laboratories and university groups that strategically target R&D and detector construction projects (e.g. INFN in Italy, DESY in Germany, RAL in the UK). Overall, it is nevertheless apparent that the programmes on offer vary widely from country to country.

### 9.2.3 Contribution of major laboratories

Major laboratories act as hubs of scientific activity for a diverse international community. As such, they have well-established research infrastructures that allow for specialised training in instrumentation work. At the ECFA symposium, CERN and DESY were identified as two prominent examples in Europe. Both play a crucial role in training the next generation by providing ample training opportunities in instrumentation that complement programmes at universities.

To some extent, access to these programmes already starts at the high school level. For instance, “S’Cool LAB” (see Figure 9.1), CERN’s hands-on laboratory, offers highly popular workshops through which high-school students are introduced to fundamental principles of particle detectors and particle accelerators [Ch9-18]. In addition, “Beamline 4 Schools”, an international competition for high-school students from all around the