# The 2021 ECFA Detector Research and Development Roadmap

Anna Colaleo, Leszek Ropelewski, Klaus Dehmelt, Barbara Liberti, Maxim Titov, Joao Veloso, Jocelyn Monroe, Auke-Pieter Colijn, Antonio Ereditato, Ines Gil Botella, et al.

► To cite this version:

Anna Colaleo, Leszek Ropelewski, Klaus Dehmelt, Barbara Liberti, Maxim Titov, et al. The 2021 ECFA Detector Research and Development Roadmap. CERN. 2021, 978-92-9083-614-8. 10.17181/CERN.XDPL.W2EX. in2p3-03877516

HAL Id: in2p3-03877516

https://hal.in2p3.fr/in2p3-03877516

Submitted on 29 Nov 2022

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

THE 2021 ECFA DETECTOR
RESEARCH AND DEVELOPMENT ROADMAP

The European Committee for Future Accelerators
Detector R&D Roadmap Process Group

<figure><img src="image_3.png" /></figure>

ECFA
European Committee
for Future Accelerators
<figure><img src="image_4.png" /></figure>
# The 2021 ECFA Detector Research and Development Roadmap

Prepared by the Detector R&D Roadmap Process Group of
the European Committee for Future Accelerators

<figure><img src="image_5.png" /></figure>

ECFA
European Committee
for Future Accelerators
Published: 21.10.2021

References: ECFA/RC/21/510
CERN-ESU-017

DOI: 10.17181/CERN.XDPL.W2EX

ISBN online: 978-92-9083-614-8

ISBN paperback: 978-92-9083-615-5

Copyright © CERN 2021

CC-BY-4.0
<figure><img src="image_6.png" /></figure>
<figure><img src="image_7.png" /></figure>
# Contents

<table><tr><td>Table of Contents</td><td>i</td></tr><tr><td>Introduction</td><td>1</td></tr><tr><td>References Introduction</td><td>8</td></tr><tr><td>1 Gaseous Detectors</td><td>9</td></tr><tr><td>1.1 Introduction</td><td>9</td></tr><tr><td>1.2 Main drivers from the facilities</td><td>10</td></tr><tr><td>1.2.1 Muon systems</td><td>10</td></tr><tr><td>1.2.2 Inner and central tracking with particle identification capability</td><td>13</td></tr><tr><td>1.2.3 Calorimetry</td><td>18</td></tr><tr><td>1.2.4 Photon detection</td><td>19</td></tr><tr><td>1.2.5 Time of Flight Systems</td><td>21</td></tr><tr><td>1.2.6 TPCs for rare event searches</td><td>22</td></tr><tr><td>1.3 Recommendations</td><td>24</td></tr><tr><td>1.3.1 Key technologies</td><td>24</td></tr><tr><td>1.3.2 Common challenges</td><td>25</td></tr><tr><td>1.3.3 R&amp;D environment and development tools</td><td>27</td></tr><tr><td>References Chapter 1</td><td>28</td></tr><tr><td>2 Liquid Detectors</td><td>37</td></tr><tr><td>2.1 Introduction</td><td>37</td></tr><tr><td>2.2 Main drivers from the facilities</td><td>40</td></tr><tr><td>2.3 Key technologies</td><td>42</td></tr><tr><td>2.3.1 Liquid Properties of Noble Liquids</td><td>43</td></tr><tr><td>2.3.2 Charge Collection in Noble Liquids</td><td>44</td></tr><tr><td>2.3.3 Purification, Cryogenics, Infrastructure and Integration for Noble Liquids</td><td>45</td></tr><tr><td>2.3.4 Light Collection in Noble and Other Liquids</td><td>47</td></tr><tr><td>2.3.5 Liquid Scintillator and Water Detectors</td><td>49</td></tr><tr><td>2.4 Observations</td><td>51</td></tr><tr><td>2.4.1 Neutrino Oscillation and Astro-particle Neutrino Detectors</td><td>51</td></tr><tr><td>2.4.2 Dark Matter and 0νββ Experiments</td><td>52</td></tr><tr><td>2.5 Recommendations</td><td>53</td></tr></table>
<table><tbody><tr><td>References Chapter 2</td><td>54</td></tr><tr><td><b>3 Solid State Detectors</b></td><td><b>57</b></td></tr><tr><td>3.1 Introduction</td><td>57</td></tr><tr><td>3.2 Main drivers from the facilities</td><td>59</td></tr><tr><td>3.3 Key technologies</td><td>62</td></tr><tr><td>3.3.1 CMOS sensors: MAPS and passive CMOS</td><td>62</td></tr><tr><td>3.3.2 Sensors for 4D-tracking</td><td>64</td></tr><tr><td>3.3.3 Silicon sensors for extreme fluences environments</td><td>66</td></tr><tr><td>3.3.4 Wide band-gap semiconductors</td><td>66</td></tr><tr><td>3.3.5 The future of interconnection technologies</td><td>68</td></tr><tr><td>3.3.6 Status and evolution of the simulation tools</td><td>70</td></tr><tr><td>3.4 Observations</td><td>71</td></tr><tr><td>3.4.1 Testing infrastructures</td><td>71</td></tr><tr><td>3.4.2 Industrialisation</td><td>72</td></tr><tr><td>3.4.3 Related fields</td><td>72</td></tr><tr><td>3.5 Recommendations</td><td>73</td></tr><tr><td>3.5.1 Detector R&amp;D Themes</td><td>73</td></tr><tr><td>3.5.2 "Blue-sky" Research</td><td>75</td></tr><tr><td>3.5.3 Further recommendations on industrialisation</td><td>76</td></tr><tr><td>References Chapter 3</td><td>76</td></tr><tr><td><b>4 Particle Identification and Photon Detectors</b></td><td><b>79</b></td></tr><tr><td>4.1 Introduction</td><td>79</td></tr><tr><td>4.2 Main drivers from the facilities</td><td>80</td></tr><tr><td>4.3 Key technologies: particle identification</td><td>82</td></tr><tr><td>4.3.1 RICH detectors</td><td>82</td></tr><tr><td>4.3.2 Detectors for Internally Reflected Čerenkov light</td><td>85</td></tr><tr><td>4.3.3 Time of flight detectors</td><td>86</td></tr><tr><td>4.3.4 Particle identification through dE/dx and TRD</td><td>88</td></tr><tr><td>4.4 Key technologies: photon detection</td><td>89</td></tr><tr><td>4.4.1 Vacuum-based photon detectors</td><td>89</td></tr><tr><td>4.4.2 Gas-based photon detectors</td><td>91</td></tr><tr><td>4.4.3 Semiconductor photon detectors</td><td>92</td></tr><tr><td>4.4.4 Superconducting photon detectors</td><td>96</td></tr><tr><td>4.4.5 Novel optical materials for fibre trackers and light collection</td><td>97</td></tr><tr><td>4.5 Observations</td><td>98</td></tr><tr><td>4.6 Recommendations</td><td>99</td></tr><tr><td>References Chapter 4</td><td>100</td></tr><tr><td><b>5 Quantum and Emerging Technologies Detectors</b></td><td><b>105</b></td></tr><tr><td>5.1 Introduction</td><td>105</td></tr><tr><td>5.2 Theory Motivation</td><td>107</td></tr><tr><td>5.3 Quantum Methodologies and Techniques</td><td>111</td></tr></tbody></table>
<table><tbody><tr><td>5.3.1 Clocks</td><td>111</td></tr><tr><td>5.3.2 Spin-based sensors for axions and axion-like-particles</td><td>112</td></tr><tr><td>5.3.3 Superconducting approaches</td><td>113</td></tr><tr><td>5.3.3.1 Dark matter searches with 3D microwave cavities</td><td>116</td></tr><tr><td>5.3.4 Optomechanical technologies</td><td>117</td></tr><tr><td>5.3.5 Atoms, molecules, ions and atom-interferometric probes</td><td>120</td></tr><tr><td>5.3.6 Metamaterials, low-dimensional materials, quantum materials</td><td>121</td></tr><tr><td>5.4 Observations and Prospects</td><td>123</td></tr><tr><td>References Chapter 5</td><td>124</td></tr><tr><td><b>6 Calorimetry</b></td><td><b>131</b></td></tr><tr><td>6.1 Introduction</td><td>131</td></tr><tr><td>6.2 Main drivers from the facilities</td><td>132</td></tr><tr><td>6.3 Key technologies</td><td>135</td></tr><tr><td>6.3.1 Silicon-based calorimeters</td><td>135</td></tr><tr><td>6.3.1.1 Challenges and requirements for future projects</td><td>135</td></tr><tr><td>6.3.1.2 Main R&amp;D Directions</td><td>136</td></tr><tr><td>6.3.2 Calorimetry Based on Liquified Noble Gases</td><td>137</td></tr><tr><td>6.3.2.1 Challenges and requirements for future projects</td><td>138</td></tr><tr><td>6.3.2.2 Main R&amp;D Directions</td><td>138</td></tr><tr><td>6.3.3 Calorimetry Based on Gaseous Detectors</td><td>140</td></tr><tr><td>6.3.3.1 R&amp;D needs for Gaseous Calorimeters</td><td>140</td></tr><tr><td>6.3.4 Calorimeters with light-based readout</td><td>141</td></tr><tr><td>6.3.4.1 State-of-the-art</td><td>141</td></tr><tr><td>6.3.4.2 Challenges and requirements for future projects</td><td>142</td></tr><tr><td>6.3.4.3 Main R&amp;D directions</td><td>143</td></tr><tr><td>6.3.5 Precision timing in calorimetry</td><td>143</td></tr><tr><td>6.3.5.1 Use of timing information for enhanced calorimetric performance</td><td>144</td></tr><tr><td>6.3.5.2 R&amp;D Needs</td><td>144</td></tr><tr><td>6.3.6 Readout Systems for Calorimetry</td><td>146</td></tr><tr><td>6.3.6.1 Breadth of challenges for calorimeter readout systems</td><td>146</td></tr><tr><td>6.3.6.2 Necessary ASIC developments</td><td>146</td></tr><tr><td>6.3.6.3 Discrete components for compact calorimeters</td><td>146</td></tr><tr><td>6.3.6.4 Connection technologies for large-area solid-state sensors</td><td>147</td></tr><tr><td>6.3.6.5 Readout integration and the new wave of FPGAs</td><td>147</td></tr><tr><td>6.4 Observations</td><td>147</td></tr><tr><td>6.5 Recommendations</td><td>148</td></tr><tr><td>References Chapter 6</td><td>150</td></tr><tr><td><b>7 Electronics and Data Processing</b></td><td><b>153</b></td></tr><tr><td>7.1 Introduction</td><td>153</td></tr><tr><td>7.2 Main drivers from the facilities</td><td>153</td></tr><tr><td>7.2.1 Technical requirements</td><td>153</td></tr></tbody></table>
<table><tbody><tr><td>7.2.2</td><td>The inheritance from HL-LHC</td><td>154</td></tr><tr><td>7.2.3</td><td>Industrial developments</td><td>154</td></tr><tr><td>7.2.4</td><td>Categorising New Developments</td><td>156</td></tr><tr><td>7.3</td><td>Technical Findings</td><td>156</td></tr><tr><td>7.3.1</td><td>Front-end ASICs</td><td>156</td></tr><tr><td>7.3.1.1</td><td>State-of-the-art</td><td>157</td></tr><tr><td>7.3.1.2</td><td>Technology choice and ASICs evolution</td><td>157</td></tr><tr><td>7.3.1.3</td><td>Identified R&amp;D themes</td><td>157</td></tr><tr><td>7.3.2</td><td>Links, Powering and Interconnects</td><td>158</td></tr><tr><td>7.3.2.1</td><td>State-of-the-art</td><td>159</td></tr><tr><td>7.3.2.2</td><td>Future challenges</td><td>159</td></tr><tr><td>7.3.2.3</td><td>Industry and other fields</td><td>160</td></tr><tr><td>7.3.2.4</td><td>Identified R&amp;D themes</td><td>160</td></tr><tr><td>7.3.3</td><td>Back-end Systems</td><td>161</td></tr><tr><td>7.3.3.1</td><td>State-of-the-art</td><td>161</td></tr><tr><td>7.3.3.2</td><td>Future challenges</td><td>161</td></tr><tr><td>7.3.3.3</td><td>Industry and other fields</td><td>162</td></tr><tr><td>7.3.3.4</td><td>Identified R&amp;D themes</td><td>162</td></tr><tr><td>7.4</td><td>Observations</td><td>163</td></tr><tr><td>7.4.1</td><td>Organisation and Collaboration</td><td>163</td></tr><tr><td>7.4.2</td><td>Systems Engineering</td><td>163</td></tr><tr><td>7.4.3</td><td>Tools and Technologies</td><td>164</td></tr><tr><td>7.4.4</td><td>Interactions outside HEP</td><td>164</td></tr><tr><td>7.4.5</td><td>Skills, Training and Careers</td><td>164</td></tr><tr><td>7.4.6</td><td>Common infrastructure</td><td>165</td></tr><tr><td>7.5</td><td>Recommendations</td><td>165</td></tr><tr><td>7.5.1</td><td>Themes for future R&amp;D</td><td>165</td></tr><tr><td>7.5.2</td><td>Approach to R&amp;D</td><td>168</td></tr><tr><td>7.5.2.1</td><td>Novel Developments</td><td>168</td></tr><tr><td>7.5.2.2</td><td>Horizon-Scanning</td><td>168</td></tr><tr><td>7.5.2.3</td><td>Software</td><td>169</td></tr><tr><td>7.5.3</td><td>Practical and Organisational Issues</td><td>169</td></tr><tr><td>7.5.3.1</td><td>Collaborative Model</td><td>169</td></tr><tr><td>7.5.3.2</td><td>Demonstrators and Common Developments</td><td>170</td></tr><tr><td>7.5.3.3</td><td>Infrastructure Needs</td><td>170</td></tr><tr><td>7.5.3.4</td><td>Interaction with Industry</td><td>171</td></tr><tr><td colspan="2">References Chapter 7</td><td>172</td></tr><tr><td>8</td><td>Integration</td><td>175</td></tr><tr><td>8.1</td><td>Introduction</td><td>175</td></tr><tr><td>8.2</td><td>Main drivers from the facilities</td><td>176</td></tr><tr><td>8.3</td><td>Key technologies</td><td>176</td></tr><tr><td>8.3.1</td><td>Novel magnet systems</td><td>176</td></tr><tr><td>8.3.2</td><td>Improved technologies and systems for cooling</td><td>179</td></tr></tbody></table>
<table><tbody><tr><td>8.3.2.1 Cooling systems</td><td>179</td></tr><tr><td>8.3.2.2 Local Cooling / Cooling contacts</td><td>180</td></tr><tr><td>8.3.3 Novel materials to achieve ultra-light, high precision mechanical structures</td><td>181</td></tr><tr><td>8.3.3.1 Machine-detector interface (MDI)</td><td>183</td></tr><tr><td>8.3.4 Monitoring</td><td>185</td></tr><tr><td>8.3.4.1 Environmental Monitoring</td><td>185</td></tr><tr><td>8.3.4.2 Beam and Radiation Monitoring</td><td>186</td></tr><tr><td>8.3.5 Calorimetry, Neutrino and Dark Matter Detectors</td><td>187</td></tr><tr><td>8.3.5.1 Calorimetry</td><td>187</td></tr><tr><td>8.3.5.2 Neutrino Detectors and Dark Matter Detectors</td><td>187</td></tr><tr><td>8.3.6 Robotic Systems, Survey</td><td>188</td></tr><tr><td>8.4 Observations and Recommendations</td><td>188</td></tr><tr><td>References Chapter 8</td><td>189</td></tr><tr><td><strong>9 Training</strong></td><td><strong>193</strong></td></tr><tr><td>9.1 Relevance of instrumentation training</td><td>193</td></tr><tr><td>9.1.1 Junior ECFA input</td><td>194</td></tr><tr><td>9.2 Status of instrumentation training in Europe</td><td>195</td></tr><tr><td>9.2.1 University programmes dedicated to HEP instrumentation training</td><td>196</td></tr><tr><td>9.2.2 Graduate schools, doctoral and post-doctoral programmes dedicated to HEP instrumentation training</td><td>197</td></tr><tr><td>9.2.3 Contribution of major laboratories</td><td>198</td></tr><tr><td>9.2.4 Status of accelerator training in Europe</td><td>200</td></tr><tr><td>9.3 The future of instrumentation training</td><td>201</td></tr><tr><td>9.3.1 A coordinated European training programme</td><td>202</td></tr><tr><td>9.3.2 The role of virtual laboratories</td><td>204</td></tr><tr><td>9.3.3 The role of major laboratories</td><td>204</td></tr><tr><td>9.3.4 Industry meets academia</td><td>205</td></tr><tr><td>9.4 Recommendations</td><td>206</td></tr><tr><td>References Chapter 9</td><td>207</td></tr><tr><td><strong>10 General Observations and Considerations</strong></td><td><strong>209</strong></td></tr><tr><td>References Observations</td><td>216</td></tr><tr><td><strong>11 Conclusions</strong></td><td><strong>219</strong></td></tr><tr><td><strong>Appendices</strong></td><td><strong>235</strong></td></tr><tr><td>A Glossary</td><td>235</td></tr><tr><td>B Authors</td><td>246</td></tr><tr><td>C Acknowledgements</td><td>248</td></tr></tbody></table>
# Introduction

The centrality of experimentation to the scientific method which has led to the discoveries that underpin our current understanding of the Universe, and the technological wonders which have flowed from this understanding, cannot be overstated. To appropriately test our current level of understanding requires very high accuracy instrumentation. The degree of confirmation that can be assigned to any hypothesis is determined both by the agreement between theoretical prediction and measurements and the level of precision achievable in each. Enabling future particle physics experiments to achieve the most accurate measurements possible is the fundamental target of all the proposed detector developments outlined in this report.

The European Strategy of Particle Physics is reviewed and updated every five to ten years with the most recent one concluded by CERN Council adopting the latest set of recommendations [Ch0-1] in June 2020. It provides an ambitious vision of the experimental programme for the future spanning many decades. Amongst the many recommendations, one requirement listed is: “organised by ECFA, a roadmap should be developed by the community to balance the detector R&D efforts in Europe, taking into account progress with emerging technologies in adjacent fields”. The purpose of this Roadmap more specifically should be to “identify and describe a diversified detector R&D portfolio that has the largest potential to enhance the performance of the particle physics programme in the near and long term”. The particle physics programme mentioned here is taken to consist of the projects listed in the Deliberation Document of the European Particle Physics Strategy Update (EPPSU) [Ch0-2] as either “*High-priority future initiatives*” or “*Other essential scientific activities for particle physics*”. The different priorities of the physics projects themselves is not a topic for this document, but the relative importance of proposed detector R&D activities to these projects is the main aspect that will be discussed. A grouped distillation of the EPPSU mentioned facilities/areas is listed below.

* Detector improvements required for full exploitation of the HL-LHC (R&D still needed for the next LHC Long Shutdown, LS3, upgrades and for experiment upgrades beyond then) including studies of flavour physics and quark-gluon plasma (where the latter topic also interfaces with nuclear physics);

* R&D for long baseline neutrino detectors (including aspects targeting astro-particle physics measurements) and supporting projects such as those at the CERN Neutrino Platform;
* Technology developments needed for detectors at $e^{+}e^{-}$ Higgs-EW-Top factories in all possible accelerator manifestations including instantaneous luminosities at 91.2 GeV of up to $5 \times 10^{36} \text{ cm}^{-2}\text{s}^{-1}$ and energies up to the TeV range;

* The long-term R&D programme for detectors at a future 100 TeV hadron collider with integrated luminosities targeted up to $30 \text{ ab}^{-1}$ and 1000 multiple interactions for 25 ns bunch crossing interval;

* Specific long-term detector technology R&D requirements of a muon collider operating at 10 TeV and with a luminosity of the order of $10^{35} \text{ cm}^{-2}\text{s}^{-1}$;

* Detector developments for accelerator-based studies of rare processes, DM candidates and high precision measurements (including strong interaction physics) at both storage rings and fixed target facilities, interfacing also with atomic and nuclear physics;

* R&D for optimal exploitation of dedicated collider experiments studying the partonic structure of the proton and nuclei as well as interface areas with nuclear physics;

* The very broad detector R&D areas for non-accelerator-based experiments, including dark matter searches (including axion searches), reactor neutrino experiments and rare decay processes, also considering neutrino observatories and other interface areas with astro-particle physics.

To create a time-ordered technology requirements driven R&D roadmap, focused on capabilities not currently achievable, ECFA set up the structure shown in Figure 1. One of the main routes for the community to shape the Roadmap has been through the six technology facing Task Forces (1-6) and three cross-cutting Task Forces (7-9) shown in the figure. These Task Forces are also composed of experts from the community covering the key sub-topics in the relevant technology areas including two convenors per Task Force. The Task Force convenors join the Chair, Scientific Secretary, ECFA appointed Coordinators and (ex-officio) representative from the Laboratories Directors Group (LDG), present ECFA Chair and previous ECFA Chair to make up the Detector R&D Roadmap Panel. Close contact is maintained with the LDG in particular, as their charge includes the development of the corresponding Roadmap for a coordinated and intensified programme of Particle Accelerator R&D including exploration of new technologies. This Accelerator R&D Roadmap is to be developed in parallel and on a similar timescale to the Detector R&D Roadmap.

The EPPSU report requires the Panel to take into account progress with emerging technologies in adjacent fields, which is here provided through the Advisory Panel with Other Disciplines (APOD), composed of the Chairs or Directors of the corresponding organisations. APPEC, ESA, LEAPS, LENS and NuPECC<fnref n="1" /> have provided expert contacts by Task Force area (as appropriate) within their organisations. The Roadmap

<footnote n="1">¹APPEC: Astro-Particle Physics European Consortium; ESA: European Space Agency; LEAPS: League of European Accelerator-based Photon Sources; LENS: League of advanced European Neutron Sources and NuPECC: Nuclear Physics European Collaboration Committee</footnote>
<figure><img src="image_8.png" /><figcaption>Figure 1: ECFA Detector R&D Roadmap Panel Organisation.</figcaption></figure>

Panel Chair or representative reports progress through Restricted ECFA (RECFA) and thence to the full Committee which is also kept fully informed by the ECFA Chair. To assist the process, ECFA has provided a list by country of National Contacts who are also consulted directly by the Task Forces. This report is developed with and submitted for endorsement by ECFA.

Figure 2 illustrates the process and timeline whereby this report is prepared. The different stages are shown in three blocks. The process started in May 2020 with the Coordinators team confirmed by ECFA. The Panel composition was completed by September and the organisational aspects were concluded by the end of that year. The first phase of input was through two days containing twelve presentations from leading scientists representing the eight facilities/areas listed above (for more details see [Ch0-3]). Their brief was to emphasise unmet detector R&D needs in a context that extends decades into the future, given the very long lead-times that some developments will inevitably require. These presenters have also agreed to continue as the primary contacts of the Panel and Task Forces for material in their area of expertise. Following these, the Task Forces were mainly occupied with collecting further inputs through surveys distributed largely through the National Contacts; consultation with the contacts provided by APOD<fnref n="2" /> and (also taking advantage of these) preparation of the corresponding day-long symposia discussed below.

Throughout the process, all the materials used to build the Roadmap have been publicly available through webpages [Ch0-3] that have been widely advertised within the community. Information on the nine topical Open Symposia (each devoted to a specific technology area or cross-cutting activity) was distributed at a national level through ECFA and the National Contacts; via APOD and using a large number of particle physics R&D related mailing lists. In compiling material for the Detector R&D

<footnote n="2">²ECFA provided a list of contacts by country, while APPEC, ESA, LEAPS, LENS and NuPECC have all provided expert contacts by Task Force area.</footnote>
<figure><img src="image_9.png" /><figcaption>Figure 2: Timeline of the ECFA Detector R&D Roadmap process.</figcaption></figure>

Roadmap, a group of 70 leading instrumentation scientists have both contributed their own expertise and been very active in organising this broad community consultation. The level of participation of the experimental physics community is reflected in the 1359 registrations for the symposia. More details on the level of engagement by Task Force topic can also be found at Reference [Ch0-3].

Following the last symposium, an intensive period of collating the inputs followed, including those from the expert presentations at the symposia, community feedback and the Input Session speakers, to structure the material both in terms of facility/area requirements but also by the sub-topics within each Task Force. For the six technology Task Forces in particular, the information has been first grouped by sub-technology area and then relevance to different current, planned or potential experiments evaluated along with the associated timeline for these. The driving principle has been to bring out synergies and stress interconnections between developments of similar technologies needed at different times by different programmes. In this context, for projects without fully defined schedules, the policy has been adopted to work in terms of the “earliest technically feasible start date” such that were the most optimistic scenario to be realised, the detector technology readiness could not become the limiting factor.

In the following chapters each Task Force (apart from Task Force 9 Training), has further sub-divided their technology area into broad sub-topics and taken the list of eight facilities/areas to help define the most pressing requirements as a function of time when needed (or could be needed where this is not yet defined). In addition, the Task Forces were asked to consider the following additional aspects when putting together their symposium agenda and seeking further community feedback.
* Facilities needed for detector evaluation, including test beams<fnref n="3" /> and different types of irradiation sources, along with the advanced instrumentation required for these;

* Infrastructures facilitating detector developments, including technological workshops and laboratories, as well as tools for the development of software and electronics;

* Networking structures in order to ensure collaborative environments, to help in the education and training, for cross-fertilisation between different technological communities, and in view of relations with industry;

* Overlaps with neighbouring fields and key specifications required for exploitation in other application areas;

* Opportunities for industrial partnership and technical developments needed for potential commercialisation.

The individual sections relating to different detector technologies in the document reflect the programme defined by first ensuring that the EPPSU identified "*High-priority future initiatives*" should not be potentially delayed by lack of availability of the required detector capabilities and also that the "*Other essential scientific activities for particle physics*" should not be compromised. As one looks further ahead, the required specifications could be matched by more than one development from current capabilities and the discussions are intended to reflect that, while focusing on the most promising options as viewed today. It is also accepted that serendipity has historically played an important role and that capabilities not widely emphasised even five years ago can now be regarded as highly desirable or essential to achieving the physics goals driving the updated particle physics strategy. It is not possible to foresee all the different ways current detector research activities may evolve to enable future experimental programmes but the need for greater coherence and coordination across Europe (and more widely) in many areas has come out clearly in discussions at the symposia. The Roadmap as presented (just as the European Particle Physics Strategy itself) is a snapshot of the perception of many of the leading experts at a particular time, and must itself also be subject to updating in the light of further developments. Nevertheless, it would also be a dereliction of the Roadmap Panel's obligations not to provide guidance on what are currently viewed as the most urgent topics to address if the future experimental programme is not to be compromised.

Detector technology development is a necessary but hardly sufficient basis for future progress in experimental particle physics. Talented and committed people are really the core requirement. They need to be enthused, engaged, educated, empowered and employed. The scientists, engineers and technicians who will build the future facilities need to be carefully nurtured and incentivised by appropriate and rewarding career opportunities. Without a constant infusion of bright individuals with a wealth of new

<footnote n="3">³These could be coupled with some of the demonstrators discussed in the corresponding Accelerator R&D Roadmap document.</footnote>
<figure><img src="image_10.png" /><figcaption>Figure 3: Large Accelerator Based Facility/Experiment Earliest Feasible Start Dates.</figcaption></figure>

ideas and the drive to realise these, the field of detector R&D for particle physics would wither and die. It has become evident in the process of compiling this Roadmap that many at early stages of their careers have a real passion for wholehearted engagement in the instrumentation aspects of particle physics, if only a number of real and perceived obstacles could be overcome and some negative attitudes of those who are able to shape their longer-term career prospects could be addressed. Recommendations in the following chapters and in the conclusions also reflect the all-important human aspects of ensuring a healthy future for the particle physics detector R&D activities which will be essential to allowing the ambitious envisaged future experimental programme to be realised.

Figure 3 shows an indicative future timeline for future collider and larger accelerator facilities, while Figure 4 shows that for non-accelerator and smaller accelerator experiments. Although the Roadmaps for both detector and accelerator R&D are focused on deliverables over the next five to ten years, they can only be developed and prioritised with some knowledge of the target dates for the final facilities. On the other hand, the projects shown in the diagrams are at differing stages of definition, approval and technical maturity. The dates shown in the diagram therefore have low precision, and are intended to represent the earliest ‘feasible start date’ (where a schedule is not already defined), taking into account the necessary steps of approval, development and construction for machine and civil engineering. They do not constitute any form of plan or recommendation, and indeed several options presented are mutually exclusive. Furthermore, the projects mentioned here are limited to those mentioned in the EPPSU, although it should be noted that detector R&D for other possible future facilities is usually aligned with that for programmes listed in Figure 3. For example, there are large overlaps and synergies between the R&D for the proposed CEPC in China and FCC-ee and in some cases also to the proposed experiments at either the ILC or CLIC. The timelines – and potentially the scope – of the projects will naturally change depending on future strategic decisions. The objective of the Roadmaps is to ensure that: (a) the basic R&D phase is not the limiting step, i.e. that R&D is started sufficiently early and prioritised correctly to meet the needs of the long-term European particle physics programme in its global context; and (b) that the outcomes of the R&D programme are able to provide the necessary information on the feasibility and cost of future deliver-
<figure><img src="image_11.png" /><figcaption>Figure 4: (Representative) Smaller Accelerator and Non-Accelerator Based Experiments Start Dates (*not intended to be at all an exhaustive list*).</figcaption></figure>

ables to allow such decisions to be made. In Figure 4 the information for a variety of smaller accelerator and non-accelerator based experiments is presented, where possible, grouped in the overarching physics themes they address. For each of the relevant Task Forces, a set of detector R&D aspects are identified which are required if the physics programmes of experiments at these facilities are not to be compromised.

In the ECFA Detector R&D Roadmap the focus has been on facilities targeting the properties and interactions of fundamental particles (including those that are undiscovered but theoretically motivated). It is appreciated that a number of particles increasingly play the role of cosmic messengers for phenomena happening far beyond our own galaxy which provides some of the exciting science opportunities in the neighbouring field of astro-particle physics, but the demanding detector requirements specific to this area are not generally within the scope of this document.

Throughout the following chapters, these figures inform the development of the Detector R&D Roadmap with a view to set concrete target timelines for the readiness of the recommended R&D thematic programmes emerging in each Task Force and summarised in Chapter 11. As the list of EPPSU facilities/areas shows, there are also many R&D requirements for upgrades to ongoing experiments or new experiments at existing facilities. It also discusses a wealth of future particle physics detector developments requiring R&D on highly specialised and advanced technologies at smaller accelerator and non-accelerator based experiments which are also described in the relevant sections.

The common introductory figures at the start of the following chapters are designed to capture where there are strong needs for new R&D activities in support of these programmes. The aim is not to provide an exhaustive list of all the running or future particle physics or particle physics related experiments, but to try to draw attention
to major areas of unmet detector requirements which may benefit from more general (rather than with high specificity to a single experiment) R&D programmes.

The introductory figures (“Detector Readiness Matrices”) intend to be a graphical representation focusing only on where significant R&D is required beyond incremental improvements to current capabilities. The colour coding is linked not to the intensity of the required effort but to the potential impact on the intended physics programme at the experiment. In some cases, absence of the proposed R&D would compromise the main motivation for the experiment as a whole (red, largest dot). In other cases, the R&D is required to ensure that many of the physics goals can be met (orange, large dot); or is desirable to enhance the physics reach and therefore gain maximum benefit from the investment in the facility (yellow, medium dot). In addition, in some cases the needs will be met from current R&D activities (green, small dot) while in others no further R&D is required or it is not relevant (blank). The tables should only be taken as a crude and imperfect summary of the much more detailed discussions in the text of each chapter but hopefully they illustrate how R&D needs are expected to broadly evolve with time.

It will be seen that in each chapter, the corresponding Task Forces have identified a number of recommendations particular to their technology area, also taking account of the community inputs they have received, and each provided a list of Detector R&D Themes (DRDTs) and Detector Community Themes (DCTs) which capture a number of the most pressing aspects of their R&D requirements. These are collated in the Conclusions (Chapter 11) along with a number of the most urgent related recommendations. Topics which have come up multiple times as being of more general significance to the community are also discussed in Chapter 10, “General Observations and Considerations”. From these, a number of high level proposals and recommendations, based on the Panel’s deliberations, are also offered for consideration in Chapter 11.

Finally, Appendix A contains a glossary of the most commonly used acronyms in the various sections; Appendix B lists those who have participated directly in the drafting of this document; while Appendix C acknowledges the many other contributors and provides further information on the Input Sessions, Symposia and the contact list for the Advisory Panel with Other Disciplines (APOD).

## References Introduction

[Ch0-1] https://europeanstrategy.cern.

[Ch0-2] The European Strategy Group, *Deliberation document on the 2020 Update of the European Strategy for Particle Physics*, CERN-ESU-014, 2020. https://cds.cern.ch/record/2720131.

[Ch0-3] https://indico.cern.ch/e/ECFADetectorRDRoadmap.
# Chapter 1

# Gaseous Detectors

## 1.1 Introduction

Gaseous Detectors (GDs) have been pivotal to the history of nuclear and, later, particle physics since the use of the first Geiger counters (dating back to 1908) and have represented the first approach to fine spatial resolution with the introduction of the MultiWire Proportional Chamber (MWPC) in 1968. Their role in particle physics experiments remains central, as testified by their use in the trigger and muon systems of all major LHC experiments (ALICE, ATLAS, CMS, LHCb) largely based on extended GD sets; while novel concepts within the same experiments are being developed for the next LHC data taking periods. The ongoing upgrades include the ALICE TPC with Gas Electron Multiplier (GEM) readout [Ch1-1]; the ATLAS muon system making use of Micro-mesh gaseous structure (Micromegas) [Ch1-2], small Thin Gap Chambers (sTGC) and Resistive Plate Chambers (RPC) [Ch1-3]; and the CMS muon system, where the endcaps are being instrumented with GEMs [Ch1-4] and improved RPCs [Ch1-5]. Large GD arrays, such as RPCs, have been widely used in large-scale experiments (including the ultra-high energy cosmic ray observatories [Ch1-6]), that demand low cost detector units able to operate reliably and cover large areas, with high efficiency and low power consumption, even in outdoor environments and with minimal maintenance.

The explanation of this persistent success is related to the main characteristics of GDs, namely the capability to cost-effectively instrument large areas, low material budget, operation in the presence of magnetic fields and radiation hardness; while their spatial and time resolution, along with high-rate capability performance, are continuously improving thanks to a world-wide community dedicated to R&D in this field. No other detector technology is providing a similar diversity of uses. The rich panorama of up-to-date GDs makes them adequate for a variety of applications in fundamental research domains and beyond, in spite of the operational complexity posed by the needs of high voltage and gas supplies. The portfolio of novel ideas promises innovative approaches and improved performance, while the experimental set-ups at the major future collider facilities as well as several future research programmes in the nuclear, astro-particle, neutrino and rare-event areas all require the use of GDs.
The main activity areas corresponding to the major drivers from future facilities, which will be analysed in this chapter, can be effectively summarised in the set of Detector Research and Development Themes listed below.

**DRDT 1.1** - Improve time and spatial resolution for gaseous detectors with long-term stability.

**DRDT 1.2** - Achieve tracking in gaseous detectors with $dE/dx$ and $dN/dx$ capability in large volumes with very low material budget and different read-out schemes.

**DRDT 1.3** - Develop environmentally friendly gaseous detectors for very large areas with high-rate capability.

**DRTD 1.4** - Achieve high sensitivity in both low and high-pressure TPCs.

The R&D needs for these areas are categorised schematically versus time in Figure 1.1. Based on these it can be seen that the R&D programmes for DRDT 1.1, DRDT 1.2 and DRDT 1.3 are linked to deliverables for multiple programmes in each of the half decades listed in Figure 3 and Figure 4 of the previous chapter up to the time of the FCC-ee; with DRDT 1.1 and DRDT 1.3 needed also for the FCC-hh/muon collider era. This is illustrated in Figure 11.1 with explanation of the diagram in the caption and corresponding text. As also discussed in Chapter 2, R&D for even longer-term facilities addressing topics in the nuclear, astro-particle, neutrino and rare-event areas will certainly be needed beyond the timelines shown in Figure 11.1 but it is not sensible today to attach dates to such further future projects.

## 1.2 Main drivers from the facilities

### 1.2.1 Muon systems

GDs will remain the primary choice for muon tracking and triggering at future facilities whenever cost-effective, large-area coverage with low material budget and high detection efficiency is required. Moreover, muon systems are often designed to provide a precise momentum measurement, usually in combination with an inner tracker. The use of fast gas mixtures, together with an optimisation of the geometry and the electric fields, has led to different design GDs (e.g. RPC, Cathode Strip Chambers (CSC), TGC, GEM) for trigger and bunch-crossing tagging in the major LHC experiments (ATLAS, CMS, ALICE and LHCb). Precision nanosecond-level timing also helps to mitigate pile-up effects and to reduce uncorrelated beam-induced backgrounds, while improving the sensitivity for heavy long-lived particle searches (e.g. slow muon-like particles with $\beta < 0.9$).

Moving from LHC to HL-LHC, the currently installed GDs in ATLAS [Ch1-3] and CMS [Ch1-7] (i.e. RPC, CSC, Drift Tubes, TGC) will remain in most of spectrometer
<figure><img src="image_12.png" /></figure>

Figure 1.1: Schematic timeline of categories of experiments employing GDs 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).

<footnote n="1">1) Large ton dual-phase (PandaX-4T, LZ, DarkSide -20k, Argo 200k, ARIADNE, ...)</footnote>

<footnote n="2">2) Light dark matter, solar axion, Onbb, rare nuclei&ions and astro-particle reactions, Ba tagging</footnote>

<footnote n="3">3) R&D for 100-ton scale dual-phase DM/neutrino experiments</footnote>
areas for muon tracking and triggering. Their performance, radiation hardness, and
longevity have been demonstrated for a wide range of operating conditions at different
irradiation facilities. Some detectors will undergo major electronics upgrades to cope
with higher trigger rates and bandwidth requirements (e.g. triggerless DAQ readout
in ALICE and LHCb). Other upgrades, targeting LHC running from 2022 (Run 3)
and beyond, are the addition of new muon stations, at different pseudo-rapidities, with
improved rate capability, spatial and timing resolution. These will complement exist-
ing GDs to ensure redundant tracking, triggering and momentum determination and
significantly decrease L1 muon trigger rates.

In particular, new RPCs with smaller gap size (1 vs 2 mm), High Pressure Lami-
nate (HPL) electrodes with reduced resistivity, and the latest generation of front-end
electronics ASICs (noise < 4000 e⁻) will be installed in the ATLAS Muon Barrel spec-
trometer during LS3 together with a new small diameter (15 vs 30 mm) Muon Drift
Tubes (sMDT) [Ch1-8]. Similarly, new double gap RPC with smaller gap size (1.4 vs.
2 mm) and reduced electrode resistivity will start operation in the CMS Muon endcap
for the LHC Run 4 (the beginning of HL-LHC running).

There have been major Micro-Pattern Gaseous Detectors (MPGD) developments
for ATLAS and CMS muon system upgrades (from Run 3 onwards), towards estab-
lishing technology goals, and addressing engineering and integration challenges. A big
step in the direction of large-scale applications has been obtained with both improved
conceptual consolidation and industrial plus cost-effective manufacturing of MPGDs by
developing new fabrication techniques: resistive Micromegas (to suppress destructive
sparks in hadron environments) and single-mask with self-stretching GEM techniques
(to enable production of large-size foils and significantly reduce detector assembly time).
Scaling up of MPGDs to very large single unit detectors of $O(m^2)$, has facilitated their
use in LHC upgrades: Micromegas in combination with sTGC will instrument an area of
$O(1000 \text{ m}^2)$ in the ATLAS New Small Wheel [Ch1-9], while GEMs will equip the stations
GE1/1 and GE2/1 to complement CSC in the CMS Muon endcaps [Ch1-10]. Exploiting
Micromegas, GEM, and Micro-Resistive WELL ($\mu$-RWELL) ability to measure both the
position and arrival time of the charge deposited in the drift gap, a novel $\mu$-TPC concept
has been developed; it permits achieving nearly constant spatial resolution over a wide
range of particle incident angles and allows 3D-track reconstruction with a single MPGD
layer [Ch1-11].

For HL-LHC operation from 2025 onwards, the CMS muon system (ME0 station)
[Ch1-12], based on GEMs with high granularity and spatial segmentation, will be in-
stalled during LS3 to ensure efficient matching of muon stubs and offline pixel tracks
at large pseudorapidities. Several solutions ($\mu$-RWELL) [Ch1-13], Micro Pixel Chamber
($\mu$-PIC) [Ch1-14] and small-pad resistive Micromegas [Ch1-15]) were initially considered
for the very forward muon tagger in the ATLAS Phase II Upgrade Muon TDR pro-
posal [Ch1-2]. Here, the main challenges are discharge protection and miniaturisation
of readout elements [Ch1-16], [Ch1-17], which can profit from the ongoing developments
on Diamond-Like Carbon technology [Ch1-18], [Ch1-19]. The $\mu$-RWELL approach is
also considered as one of the tracker options in LHCb beyond LS4 [Ch1-20]. GDs are
also proposed for the future CBM [Ch1-21] and PANDA experiments [Ch1-22] at the
FAIR facility. A GEM-based muon tracker with zigzag readout strips is foreseen at NA60+ [Ch1-23]. Zigzag strips can cover a readout area with fewer strips than regular straight ones while maintaining good spatial resolution (~100 µm) and rate capability (10 kHz/cm²). Addressing the short-term R&D needs for the existing muon systems beyond 2023 will require improvement of operation procedures to guarantee their longevity and stable, discharge-free, operation at high rates, and mitigation strategies for the use of greenhouse gases when it becomes mandatory (DRDT 1.3).

Muon systems at future $e^+e^-$ colliders (ILC, FCC-ee, CLIC, SCTF) or LHeC do not pose significant challenges in terms of particle fluxes and radiation environment. The IDEA detector layout concept at FCC-ee will require operation at rates up to 10 kHz/cm², spatial resolution of ~60–80 µm and a time resolution of 5-7 ns, with the total integrated charge < 100 mC/cm². Currently available technologies (RPC, sMDT, MPGD) can be used to instrument large-area $O(1000 \text{ m}^2)$ FCC-ee muon detectors. The ILC muon system/tail catcher technologies can include RPC or MPGD, as for Hadron Calorimetry (HCAL). Some engineering challenges are related to the operation in the high magnetic flux return (up to several Tesla). Generally, background rates in LHeC muon detector, which is based on the updated design of ATLAS Phase II Muon spectrometer, are lower than in pp colliders.

The muon tracking and triggering at a future hadron collider, such as FCC-hh, also requires large area coverage (~3000 m²), while particle rates do not exceed 0.5 kHz/cm² in the barrel and below 500 kHz/cm² in most of the endcaps. The existing technologies for HL-LHC (MPGD, RPC, sMDT, sTGC) are adequate in most of these muon spectrometer areas. However, major R&D would still be needed for the very forward endcap region at a radius below 1 m.

In a multi-TeV muon collider, the Beam Induced Background (BIB), due to muon decays, dominates in the endcap region, reaching a maximum flux of $O(\text{MHz/cm}^2)$. Based on the CLIC detector concept [Ch1-24], [Ch1-25], the muon system will be based on instrumented iron yoke plates with the option to use gaseous detectors in either or both of the barrel and endcap. A new generation of fast-timing GDs based on glass RPC, Multi-Gap RPC (MRPC), or fast timing MPGD (FTM) [Ch1-26], [Ch1-27] and PICOSEC [Ch1-28] are being developed, with a goal to achieve timing resolution of $O(100/\text{ps})$ and to reject off-time BIB hits. The main challenges at future facilities, particularly beyond 2030, include large area coverage with precision timing information (DRDT 1.1) to ensure correct track-event association, and the ability to cope with large particle fluxes using environmentally friendly gas mixtures (DRDT 1.3). Figure 1.2 summarises the main facilities, the proposed technologies to address the main challenges, and the most stringent conditions expected in muon systems.

### 1.2.2 Inner and central tracking with particle identification capability

The general requirement for central tracking with GDs are to ensure high-rate capability and excellent spatial resolution for precision momentum measurement and particle identification (PID) with a minimal material budget and lightweight mechanical support structures (DRDT 1.2). Straw tubes, drift chambers, planar or cylindrical MPGDs also
<table><thead><tr><th>Facility</th><th>Technologies</th><th>Challenges</th><th>Most challenging requirements at the experiment</th></tr></thead><tbody><tr><td>HL-LHC</td><td>RPC, Multi-GEM, resistive-GEM, Micromegas, micro-pixel Micromegas, μ-RWELL, μ-PIC</td><td>Ageing and radiation hard, large area, rate capability, space and time resolution, miniaturisation of readout, eco-gases, spark-free, low cost</td><td>(LHCb): Max. rate: 900 kHz/cm²<br>Spatial resolution: ~cm<br>Time resolution: O(ns)<br>Radiation hardness: ~2 C/cm² (10 years)</td></tr><tr><td>Higgs-EW-Top Factories (ee) (ILC/FCC-ee/CepC/SCTF)</td><td>GEM, μ-RWELL, Micromegas, RPC</td><td>Stability, low cost, space resolution, large area, eco-gases</td><td>(IDEA): Max. rate: 10 kHz/cm²<br>Spatial resolution: ~60-80 μm<br>Time resolution: O(ns)<br>Radiation hardness: <100 mC/cm²</td></tr><tr><td>Muon collider</td><td>Triple-GEM, μ-RWELL, Micromegas, RPC, MRPC</td><td>High spatial resolution, fast/precise timing, large area, eco-gases, spark-free</td><td>Fluxes: > 2 MHz/cm² (0-8°)<br>< 2 kHz/cm² (for 0>12°)<br>Spatial resolution: ~100μm<br>Time resolution: sub-ns<br>Radiation hardness: < C/cm²</td></tr><tr><td>Hadron physics (EIC, AMBER, PANDA/CMB@FAIR, NA60+)</td><td>Micromegas, GEM, RPC</td><td>High rate capability, good spatial resolution, radiation hard, eco-gases, self-triggered front-end electronics</td><td>(CBM@FAIR): Max. rate: <500 kHz/cm²<br>Spatial resolution: < 1 mm<br>Time resolution: ~ 15 ns<br>Radiation hardness: 10¹³ neq/cm²/year</td></tr><tr><td>FCC-hh (100 TeV hadron collider)</td><td>GEM, THGEM, μ-RWELL, Micromegas, RPC, FTM</td><td>Stability, ageing, large area, low cost, space resolution, eco-gases, spark-free, fast/precise timing</td><td>Max. rate 500 Hz/cm²<br>Spatial resolution = 50 μm<br>Angular resolution = 70 μrad (η=0) to get Δp/p≤10% up to 20 TeV/c</td></tr></tbody></table>

Figure 1.2: Main drivers for the Muon Systems at future facilities. The most stringent requirements for the future R&D activities are quoted in the last column.

using μ-TPC mode (Section 1.2.1), and TPCs with MPGD readout are widely employed.

## Straw trackers

The technique of self-supporting straw tubes with thin anode wire and an aluminised Mylar cathode wall offers a combination of short drift time, low mass, and high spatial resolution tracking by using long (a few meters) and small diameter (< 1 cm) straws, arranged in planar layers and mounted in a hexagonal array. Innovative straw detectors are foreseen at both future storage rings and fixed target facilities.

One example of the large-scale straw detector is the ATLAS Transition Radiation Tracker, based on 4 mm diameter tubes and ~250,000 straws. The state-of-the-art NA62 straw tracker, with a tube diameter of 9.8 mm and a wall thickness of 36 μm, leading to material budget of ~0.045% X/X₀, operates directly in the experiment's vacuum tank at rates up to 40 kHz/cm (500 kHz/straw), and requires ageing resistance up to ~1 C/cm/wire [Ch1-29]. NA62 has developed new construction techniques of ultrasonic welding to close the straw (and to keep them straight and withstand the vacuum pressure without breaking) - an important breakthrough for future experiments. The straw detector in the PANDA experiment at FAIR will be based on the 10 mm diameter and 1.5 m length tubes, made of a 27 μm thick Mylar foil, allowing to reduce detector thickness to ~1.2% X/X₀ (where 2/3 comes from tube walls and 1/3 from the gas), and to achieve spatial resolution of ~150 μm. A straw tracker has also been proposed for the NA62 upgrade, where a four times higher beam intensity is expected. This work has synergies with R&D for COMET Phase-II [Ch1-30] at JPARC and Mu2e-II [Ch1-31] at Fermilab, where the goal is to develop Mylar thin-walled (8 μm), narrow-gauge (less than 5 mm) straw tubes using the ultrasonic welding technique. This will
allow decreasing the material budget to $X/X_0 \sim 0.02\%$ per tube. The smaller diameter will lead to a reduction of the drift time and improvement in the trailing-edge time resolution from currently 30 ns to an ultimate goal of $\sim 6$ ns (per straw) and 1 ns (per track).

### Planar and cylindrical MPGDs

Due to the variety of geometries and flexible operating parameters, MPGDs fulfil the most stringent experimental constraints imposed by future nuclear, hadron physics experiments, and heavy ion facilities, i.e. Electron-Ion Collider (EIC), offering intrinsic high-rate capability ($\sim 10^6$ Hz/mm²), spatial resolution (down to 30 µm), multi-particle resolution ($\sim 500$ µm), and superior radiation hardness [Ch1-32]. Although normally used as planar detectors, GEM, Micromegas, and µ-RWELL can be bent to form cylindrically curved ultra-light inner tracking systems, without support and cooling structures, as implemented in KLOE-2, BESS-III, ASACUSA, MINOS, CMD-3, and proposed for the EIC and SCTF.

At the EIC [Ch4-2], all proposed detector concepts feature some form of large area MPGDs for barrel and forward tracking. Light-material planar Micromegas, µ-RWELL or GEMs, are the main options for forward tracking to achieve a momentum resolution of $\sigma(p_T)/p_T \sim 0.1\% \times p_T \oplus 2\%$ with a material budget of $X/X_0 \le 5\%$. Disk realisation with Cr-GEM foils is proposed where the Cu electrodes are reduced to the mere adhesive Cr-film with 50% less material compared to standard GEMs [Ch1-33]. Spatial resolution of 100 µm can be reached with a reduced number of readout channels thanks to a zigzag pad layout. Vigorous R&D is also needed for EIC barrel tracking, where multi-layer cylindrical MPGDs (Micromegas, GEM, or µ-RWELL) or a TPC with MPGD readout and an additional external cylindrical MPGD layer are being considered. The TPC approach has an advantage of high density of space points per track, which enables ionisation loss measurement for PID (DRDT 1.2); however, the TPC is slower and more challenging than MPGD-based tracking at rates $O(kHz/cm^2)$.

The requirements for central tracking at high luminosity future lepton colliders favour an ultra-light tracker to fully exploit the cleanliness of the $e^+e^-$ environment for precision electroweak physics, and for flavour physics, where the average charged particle momenta (few MeV to few GeV) are in a range over which the multiple scattering contribution to the momentum resolution is significant. The main technologies for tracking at “Higgs Factories” are: Drift Chambers (FCC-ee, CePC) and TPC with MPGD readout (ILC, CePC). The SCTF tracking system will comprise of Inner Tracker (TPC or cylindrical MPGDs) and a drift chamber [Ch1-34].

**Drift chambers.** DAFNE’s KLOE drift chamber [Ch1-35] and the recent version of it developed for the MEG2 experiment [Ch1-36] are the precursors of the next generation of ultralight weight central trackers for the future $e^+e^-$ colliders [Ch1-37]. The IDEA drift chamber at FCC-ee is based on a high granularity, low mass cylindrical drift chamber, coaxial to the 2 T solenoid field, which is expected to operate at a maximum rate of $\sim 25$ kHz/cm², with spatial (time) resolution < 100 µm ($\sim 1$ ns) in a very light gas mixture (He/iC₄ H₁₀). Particle separation is foreseen using a cluster counting technique at a $dE/dx < 3\%$ level and momentum resolution of $\sigma(p_T)/p_T \sim 0.40\%$ is required at
100 GeV/c. Main peculiarities of the drift chamber IDEA are its high transparency, in terms of radiation lengths, which leads to minimal multiple scattering and secondary interactions; the material budget in barrel (endcap) regions is designed to be equivalent to ~1.6%X₀ (~5%X₀).

Since the main contribution in terms of radiation length is related to tungsten wires, high transparency can be achieved thanks to a novel approach for the wiring and assembly procedures, stemming from the original ancestor KLOE drift chamber, and recently culminating in the construction of the MEG2 drift chamber. The main R&D is related to new wire materials: in particular carbon monofilaments obtained through high-power impulse magnetron sputtering techniques. The proposed drift chamber at SCTF will consist of O(100 k) thin wires, instrumented with new readout electronics implementing the cluster counting technique. The requirement for a maximum rate of 1 kHz/cm² is less stringent than for drift chamber at FCC-ee, however, spatial (< 100 µm), momentum (σp/p ~ 0.38% at 1 GeV) and dE/dx (~ 7.5%) resolutions, and radiation hardness (~ 1 C/cm) will be challenging.

Conventionally, drift chambers have been operated with hydrocarbon-based mixtures, which are not reliable for long-term, high-rate operation [Ch1-38], [Ch1-39]. A dedicated R&D is necessary to find an alternative hydrocarbon-free mixture adapted to the desired performance at future colliders.

## Time Projection Chamber (TPC)

The “ultimate” drift chamber is the TPC concept, which provides 3D precision tracking with low material budget and enables PID through dE/dx measurement or cluster counting $dN_{cl}/dx$ techniques; examples are continuous or pulsed mode (depending on beam structure) operation in magnetic fields that ranges from 0.5 T (ALICE) to 3.5 T (ILC). An important major innovation is related to the replacement of MWPC with MPGD for the TPC endplate readout, which allows greater geometrical design freedom, and offers many advantages: reduced pad-angle, track-angle, and negligible $E \times B$ track distortion effects, narrower pad response function, and intrinsic suppression of Ion Back Flow (IBF) [Ch1-40]. The use of Micromegas with resistive layers allows spark protection and further pad response function tuning. The upgraded ALICE TPC with GEM readout, which will operate from LHC Run 3 onwards, and T2K Near Detector TPC based on Micromegas represent the state-of-the-art in TPC technology.

Large TPC R&D efforts are ongoing within the LCTPC collaboration [Ch1-41]. The required ILC TPC performance in a 3.5 T field (with material budget below 10% X₀) is a momentum resolution $\Delta(1/p) < 10^{-4} \text{ GeV}^{-1}$, corresponding to a single-hit transverse (longitudinal) resolution better than 100 µm (1 mm) over more than 200 3D-space points and a dE/dx resolution superior to 5% [Ch1-42], [Ch1-43]. Three MPGD approaches are pursued for the ILC TPC: GEM, Micromegas, and GridPix. The GEM option is based either on a triple-GEM stack with a standard chemically etched thin-foil Cu-kapton-Cu sandwich or double-GEM with Liquid Crystal Polymer GEMs [Ch1-44]. Development of innovative encapsulated resistive anode Micromegas, based on a bulk technology and a Diamond-Like Carbon thin layer sputtered on a 50 µm thick insulator sheet [Ch1-45], [Ch1-46], profits from R&D synergies between the ILC TPC and the
T2K-II Near Detector TPC. A real breakthrough towards a pixel TPC is the development of an endplate with a 160 GridPix detectors (each ~ 2 cm²), corresponding to 10.5 million pixels, and read-out with the RD51 Scalable Readout System [Ch1-47]. One of the main TPC limitations is the field distortions in the drift volume coming from the space charge of ions due to the IBF process. MPGD-based readout allows suppression of IBF values to ~ 5 × 10⁻² (~ 2 × 10⁻³) in multi-GEM (Micromegas) and to below 1% in a cascaded GEM-Micromegas structure, originally developed to suppress sparks. The ILC beam bunch structure allows the implementation of a TPC gating scheme, based on large-aperture GEMs with honeycomb-shaped holes, while keeping high electron transparency [Ch1-48].

<table><thead><tr><th>Facility</th><th>Technologies</th><th>Challenges</th><th>Most challenging requirements at the experiment</th></tr></thead><tbody><tr><td>HL-LHC</td><td>MPGD</td><td>High spatial resolution, high rate/occupancy, radiation hardness, low mass</td><td>LHCb option: replace Scintillating Fibre tracker<br>Spatial resolution: 70 µm bending plane</td></tr><tr><td>Higgs-EW-Top Factories (ee) (ILC/FCC-ee/CepC/SCTF)</td><td>TPC+(multi-GEM, Micromegas, GridPix), Drift Chambers, Cylindrical layers of MPGD</td><td>Ultra-lightweight inner or central tracker, high spatial resolution, high rate/occupancy, radiation hardness, low mass, transparency, cluster counting, TPC continuous mode at high rate, (IBF x Gain) ~1</td><td>Inner tracker (SCTF)<br>Fluxes: ≥10 kHz cm⁻² s⁻<fnref n="1" /><br>Time resolution: 1 ns<br>X/X0 = 1%<br>Spatial resolution: ~100 µm<br>Central tracker (CepC)<br>Max. rate: >100 kHz/cm²<br>Spatial resolution: ~100 µm<br>Time resolution: ~ 100 ns<br>dE/dx: <5%<br>Particle separation with cluster counting at 2% level</td></tr><tr><td>Rare processes, atomic and nuclear physics (SPS Kaons: K⁺ Phase, K- Phase, Mu2eII/COMET-II, ELENA)</td><td>TPC, straw tubes</td><td>High spatial resolution, occupancy, fast/precise timing, radiation hardness, low mass, Gd-deposited MPGD detectors</td><td>Max rate = 500 kHz/straw<br>(Mu2e II): Thinner straw material: 8 µm<br>X/X0 ~ 0.02% per layer, X/X0 ~ 1% total<br>(COMET+): Diameter = 4.8 mm<br>Trailing time resolution = 1 ns per track</td></tr><tr><td>Hadron and nuclear physics (EIC, AMBER, PANDA and CMB@FAIR, PRES MAINZ, NA60+)</td><td>Micromegas, GEM, µ-RWELL, straw tubes</td><td>High spatial resolution, good timing, radiation hardness, tolerance to magnetic field</td><td>(EIC) Max rate = 100 kHz/cm²<br>Spatial resolution ~50 µm<br>X/X0 = 5%<br>dE/dx=12%, continuous running</td></tr></tbody></table>

Figure 1.3: Main drivers for the Inner and Central tracking at future facilities. The most stringent requirements for the future R&D activities are quoted in the last column.

The TPC at CepC has been inspired by the ILC-TPC development. Contrary to ILC, Z-pole running at CepC luminosity of ~ 10³⁶ cm⁻²s⁻<fnref n="1" />, does not allow gating mode TPC operation. In this situation, GridPix is an attractive option, which provides the high granularity needed to resolve individual electron clusters and to determine energy loss by the cluster counting technique, rather than by measuring the charge, with a precision of better than 3%. Baseline candidates for an inner tracker at the SCTF are cylindrical MPGD or TPC readout with GEM, Micromegas, or GridPix. Inspired by the PANDA-TPC development<fnref n="1" />, the latter aims to achieve transverse (longitudinal) spatial resolution in a 1 T field of about 50 – 100 µm (~ 300 µm), depending on the drift path. Figure 1.3 summarises the main facilities, the proposed technologies to address the main challenges, and the most stringent conditions expected in tracking systems.

<footnote n="1">¹The PANDA-TPC has been abandoned in favour of a straw-tube tracker.</footnote>
### 1.2.3 Calorimetry

R&D in particle-flow calorimetry is a major “paradigm shift” for high-resolution imaging calorimeters, which was originally an $e^{+}e^{-}$ linear collider driven effort, but nowadays is highly relevant for HL-LHC upgrades and future facilities (see Chapter 6). Particle-flow hadronic calorimeters with alternating layers of absorbers and sampling elements, based on GDs, are considered for ILC, FCC-ee, EIC, FCC-hh and muon collider [Ch1-49].

The main differences between circular and linear $e^{+}e^{-}$ colliders reside in the readout timing and the power pulsing, which if possible enables a larger effective density of the calorimeter and more compact particle showers. GD-based calorimeters with a typical cell size of $O(1 \text{ cm}^2)$, together with a large number of sampling layers ($\approx 50$ to contain hadronic showers) imply a very large channel count ($O(10\text{k})$ per m$^2$). This requires thin gas layers, which might affect signal amplification and timing resolution, and embedded electronics integrated in a very compact system. Moreover, production of high planarity, large-area PCBs for MPGDs and mechanical fabrication issues of very thin High Pressure Laminate RPCs represent additional challenges. Single-stage structures, based on RPC, resistive-plate WELL (RPWELL) and Micromegas also require a very uniform resistivity, gas gap thickness (down to micron level), and well-modelled gas distribution inside the detector volume in order to guarantee uniform response in terms of signal efficiency, rate capability and timing resolution. Last, but not least, hermetic calorimeters are usually extended down to small polar angles where beamstrahlung particles deposit several MGy of dose per year. Therefore, radiation hard gaseous detectors are needed.

<table><thead><tr><td>Facility</td><td>Technologies</td><td>Challenges</td><td>Most challenging requirements at experiment</td></tr></thead><tbody><tr><td>Higgs-EW-Top Factories (ee) (ILC/FCC-ee/CepC/SCTF)</td><td>RPC, Micromegas and GEM, μ-RWELL, GridPix, PICOSEC, FTM</td><td>High granularity, excellent hit timing, large area detectors, stability, uniform response, eco-gases</td><td>(ILC) Max. rate:1 kHz/cm<sup>2</sup><br>Granularity (~1 cm<sup>2</sup>)<br>Radiation hardness: no<br>Jet Energy resolution: 3-4 %<br>Power-pulsing, self-triggering readout</td></tr><tr><td>Muon collider</td><td>RPC, Micromegas and GEM, μ-RWELL, GridPix, PICOSEC, FTM</td><td>High granularity, radiation hardness, excellent hit timing, stability, uniform response, eco-gases</td><td>Granularity (~1cm<sup>2</sup>)<br>Fat jet identification<br>Time resolution= O(100ps)<br>Energy resolution =(5%)/sqrt(E) for fat-jet<br>High radiation hardness</td></tr><tr><td>Hadron physics (EIC)</td><td>RPC, Micromegas and GEM, μ-RWELL, GridPix, PICOSEC, FTM</td><td>High granularity, radiation hardness, excellent hit timing, stability, uniform response, eco-gases</td><td>(EIC option) DHCAL</td></tr></tbody></table>

Figure 1.4: Main drivers for Calorimeters at future facilities. The most stringent requirements for the future R&D activities are quoted in the last column.

Digital or semi-digital hadronic calorimeters, based on one- or two-bits ADC [Ch1-50], were advanced within the CALICE collaboration [Ch1-51]. In a semi-digital hadronic calorimeter, the energy measurement relies on the approximate linear relationship between the particle energy and the number of associated hits and requires sampling elements with high detection efficiency and a low pad hit multiplicity per traversing particle. Glass RPC [Ch1-52], GEM, RPWELL [Ch1-53] and Micromegas [Ch1-54] have been studied as potential sensing elements for the ILC Semi-Digital Hadronic Calorimeter. Larger area RPC prototypes were subsequently built to verify the scalability of the fabrication process and to address engineering challenges. Ultra-fast picosecond-timing
information with technologies like MRPC, PICOSEC and FTM can be used to resolve the development of hadron showers, resulting in a smaller “confusion term” and to improve jet energy resolution in hadronic calorimeters. The main challenges of the future R&D in the GD-based calorimetry is to ensure a uniform response over the large detector area (DRDT 1.1), and operation with eco-friendly gas mixtures (DRDT 1.3). Figure 1.4 summarises the main facilities, the proposed technologies to address the main challenges, and the most stringent conditions expected for calorimetry.

### 1.2.4 Photon detection

Gaseous photon detectors have been developed as single photon sensors for one of the most effective approaches to high-momentum particle identification, namely by Ring Imaging Čerenkov (CHerenkov) Counters (RICHs) with gas as the radiator medium (see Chapter 4). The driving considerations for this approach are the main features that characterise the GDs: cost-effectiveness, very low material budget and minimal sensitivity to magnetic fields. At future colliders, when particle identification is needed, as at the EIC or $e^{+}e^{-}$ factories, gaseous RICH based systems are the only possible options for PID at high momentum. This is particularly an issue at high pseudorapidity where PID is required for particles at high laboratory momenta.

Three generations of gaseous photon detectors have been developed [Ch1-55]: the GDs with converting vapours included in the gas mixture, open geometry MWPC with solid state CsI-photocathodes, and MPGD-based detectors with CsI-photocathodes. This historical development matches the need to provide progressively better solutions to the challenging requirements in this field, namely: (i) to reduce the photon feedback generated in the multiplication process which leads to spurious signals; (ii) to reduce the IBF rate because the ion bombardment destroys the proportional chamber and limits the lifetime of the detector (R&D line in common with TPC needs, DRDT 1.2) and (iii) to improve the detector performance in term of spatial and time resolution, along with fast response in order to open the way to high rate capabilities and precision measurements (DRDT 1.1).

MPGD-based photon detectors have been pioneered with the Hadron Blind Detector of the PHENIX experiment [Ch1-56]. Three layers of GEMs with a CsI layer as photoconverter were operated in a threshold Čerenkov counting mode. MPGDs are now in operation for single photon detection in the COMPASS RICH where a hybrid architecture formed by two THGEM layers is adopted [Ch1-57]. The first layer is coated with a CsI film, and a Micromegas acts as a third amplification stage. A quintuple GEM detector is considered for the photon detection at the EIC in a windowless RICH configuration [Ch1-58]. This new generation of gaseous photon detectors represents a major step forwards with respect to all three specific challenges: the intrinsic properties of MPGDs match the improved performance requirement (iii); a multi-layer GEM or THGEM architecture has no optical transparency and, therefore, removes photon feedback (i); IBF is reduced to a few percent because a significant fraction of ions are trapped in the intermediate layers (ii). Dedicated geometries derived from the GEM technology adding extra electrodes and, therefore, increasing the operation complexity, such as in
the double micromesh arrangement, in the Micro Hole and Strip Plate (MHSP) [Ch1-59] and in the COBRA [Ch1-60] design, allow for IBF reduction down to $O(10^{-4})$. In spite of this progress, the residual IBF poses limitations to photocathode durability and to the selection of photoconverting materials. CsI, with sizeable quantum efficiency (QE) only in the far UV region, has been used as a photocathode material because of its relatively high work function, which makes it more robust than others commonly used in vacuum-based detectors. In fact, CsI photocathodes, exposed to atmospheres with oxygen and water vapour contamination at a few ppm level, are robust at low integrated IBF and can preserve their QE over periods of years with a modest decrease. The use in GDs of solid state photoconverters in the visible range is desirable, even if challenging due to their fragility and chemical reactivity. Nevertheless, it is being pursued, even if maturity for adoption in an experiment is far from being accomplished. Novel, more robust photon converters are urgently needed. A systematic study of the hydrogenated nano-diamond powder photocathodes coupled to MPGD-based photon detectors is ongoing [Ch1-61]. The initial studies indicate its compatibility with gaseous atmospheres, reduced chemical reactivity and limited ageing by ion bombardment. Diamond-Like Carbon has also been proposed as a photoconverter candidate.

Nowadays, gaseous photon detectors are not restricted to Čerenkov counters. They are proposed for cryogenic noble liquid detectors in rare-event experiments (QUAX (Section 1.2.6)) and are able to detect the luminescent light produced by the multiplication processes in the gas (Section 1.3.1). They can provide fine time resolution response by detecting the Čerenkov light produced by ionising particles in compact radiators thanks to the intrinsic isochronism of the produced Čerenkov radiation (Section 1.3.1).

<table><thead><tr><th>Facility</th><th>Technologies</th><th>Challenges</th><th>Most challenging requirements at experiment</th></tr></thead><tbody><tr><td>Hadron and nuclear physics (EIC, AMBER, PANDA and CMB@FAIR)</td><td><strong>Gaseous-RICH</strong> with MPGD-based photon detector<br><strong>TRD</strong> with GEM or GridPix</td><td><ul><li><strong>RICH</strong>: Compact, single photon detection, high gain, fine spatial and time resolution, eco-friendly gas radiator, high pressure; limited IBF, novel photoconverters</li><li><strong>TRD</strong>: cluster counting technique, heavy gas for X-ray absorption, TRD photon -dE/dx separation.</li></ul></td><td><strong>(EIC-gaseous RICH)</strong><br>1 meter of radiator gas<br>High-gain: 10<sup>5</sup> - 10<sup>6</sup><br>Spatial resolution: O(1mm pitch)<br>Time resolution (even with small signals) ≤ 1n T<br>Tolerance to magnetic field (1.5 - 3 T)<br>Rad-hardness up to 10<sup>11</sup> neq/cm<sup>2</sup><br>option: High Pressure-Rich: Ar @ 3.5 bar<br><br><strong>(EIC-TRD)</strong><br>compactness 10<sup>-2</sup> rejection in 20-30 cm<br>improved MIP/x-ray identification</td></tr><tr><td>Higgs-EW-Top Factories (ee) (FCC-ee/CepC)</td><td><strong>Gaseous-RICH</strong> with MPGD-based photon detector</td><td><ul><li><strong>RICH</strong>: Compact, single photon detection, high gain, fine spatial and time resolution, eco-friendly gas radiator, high pressure; limited IBF, novel photoconverters</li></ul></td><td><strong>(Gaseous-RICH):</strong><br>High-gain: 10<sup>5</sup> - 10<sup>6</sup><br>Spatial resolution O(1mm pitch)<br>Time resolution (even with small signals) ≤ 1ns</td></tr></tbody></table>

Figure 1.5: Main drivers for the RICH and TRD. The most stringent requirements for the future R&D activities are quoted in the last column.

Transition Radiation Detectors (TRDs) offer electron identification at high momenta, by detecting soft X-rays. Traditionally, MWPCs and straw tubes (with Xe-based mixtures) have been used within various accelerator-based experiments. Ongoing R&D activities in the context of preparatory studies for the experiment at EIC are dedicated to establishing GEM detectors as active elements in a TRD counter [Ch1-62], with the capability to provide tracking information. The option of a GridPix detector as the TRD
active elements is also considered [Ch1-63]. A summary of the main challenges and proposed technologies at facilities for photon detection by GDs is presented in Figure 1.5.

### 1.2.5 Time of Flight Systems

Time of Flight (TOF) systems based on GDs are complementary to other PID techniques at low momenta in accelerator-based experiments, such as Čerenkov counters or energy loss $dE/dx$ measurements. The identification of charged particles with momenta larger than 1 GeV/c, by means of measuring their TOF, requires a very good time resolution and a long flight path. Large TOF systems in use today typically achieve $O(100 \text{ ps})$ resolution. Ongoing R&D aims to improve this significantly. Refining of time resolution leads either to the same discrimination power at higher momentum or to a more compact systems (reduced time of flight) at the same momentum range.

Gaseous TOF systems are currently based on MRPC technology. The existing ALICE TOF detector, covering an area of ~150 m², has proved to be robust, stable and reliable and achieved a time resolution of ~60 ps [Ch1-64]. Another fundamental MRPC parameter is the rate capability. For example, the large-area TOF in the CBM experiment at FAIR is designed to operate at particle fluxes up to 30 kHz/cm² with a time resolution of ~80 ps [Ch1-65]. Other TOF systems, based on MRPCs, include eTOF wall for the CEE experiment at HIRFL-CSR and the TOF detector in the SoLID experiment at JLAB. R&D has to continue towards an ultimate time resolution of ≤ 20 ps. This can be achieved by reducing the thickness of the gas gaps $O(100 \mu\text{m})$ and by increasing the number of gaps (up to order of tens) to maintain a high efficiency. In addition, time resolution below 15 – 20 ps is comparable to the avalanche jitter level, requiring novel very low noise front-end electronics. A rate capability up to 100 kHz/cm², necessary for systems in high radiation environments, could be achieved by thinner (better signal induction), low resistive electrodes (order of $10^7 \Omega \text{m}$). MRPCs are currently studied in order to fulfil these requirements thanks to low resistivity, radiation hard ceramic electrodes [Ch1-66], [Ch1-67].

<table><thead><tr><td>Facility</td><td>Technologies</td><td>Challenges</td><td>Most challenging requirements at experiment</td></tr></thead><tbody><tr><td>Hadron and nuclear physics (CMB@FAIR, SOLID@JLAB, CEE@HIRFL-CSR)</td><td>MRPC, MPGD with precise timing (PICOSEC, FTM)</td><td>Rate capability, radiation hardness, large area detectors, new material, eco-gas, thinner structures, FEE, system time distribution</td><td>(CMB) Max Rate = 30 kHz/cm²<br>Full system time resolution &lt; 80 ps<br>Occupancy &lt; 5%<br>Full system area = 120 m²<br>~100.000 channels, low power electronics</td></tr></tbody></table>

Figure 1.6: Main drivers for the TOF system. The most stringent requirements for the future R&D activities are quoted in the last column.

The front-end electronics needs to be improved with fast low-power amplification and continuous read-out. MPGD technologies for precision timing are FTM [Ch1-26], [Ch1-27] and PICOSEC [Ch1-28] with a goal to reach $\sigma(\text{MIP}) \sim 25 \text{ ps}$. Here the R&D is focused on scaling up to large area detectors which requires, in particular, to identify less expensive materials (radiators for PICOSEC) and very precise mechanical stability and uniformity ($\lesssim 10 \mu\text{m}$ flatness over $10 \times 10 \text{ cm}^2$). Radiation-hard photocathodes (CsI) are also needed for the anticipated particle fluxes. The main challenge for future R&D is
to be able to keep uniform response, in terms of high rate capability and time resolution (DRDT 1.1), over a large TOF detector area while operating with eco-friendly gases (DRDT 1.3). Figure 1.6 summarises the main challenges and proposed technologies at facilities using GD-based TOF systems.

### 1.2.6 TPCs for rare event searches

Gaseous TPCs are commonly used in rare event searches. Depending on the experimental needs one can vary the density of the gas to determine the track magnification or demagnification. Achieving high sensitivity in both low and high pressure TPCs, with differing readouts depending on the application, is a milestone for low-background rare event experiments (DRDT 1.4). Typically MPGD are used for the TPC amplification stage. Contrary to condensed phases, gaseous TPCs allow full 3D reconstruction of the nuclei recoils through elastic scattering together with the flexibility of choosing gas targets and operating pressures over a wide range. In particular, information about the direction of the nucleus can be obtained down to some 10's of keV (potentially allowing extraction of the apparent WIMP “wind” direction due to Earth’s motion) for operation well below atmospheric pressure (20-130 mbar).

The most popular technique for direct detection of WIMP Dark Matter in the 100 GeV – TeV mass range is to observe low energy nuclear recoils ($\mathcal{O}(1 - 100 \text{ keV})$) using different types of physical signals: ionisation electron charge (e.g. MIMAC [Ch1-68], NEWAGE [Ch1-69]) and negative ions at 20-40 mbar (e.g DRIFT [Ch1-70]). Recent experiments focus on the operation at near-atmospheric or even high pressure. In fact, electron ionisation and optically based readout at 1 bar in CYGNUS [Ch1-71] will allow exploration of WIMP masses below 15 GeV using He/CF$_4$/SF$_6$ based TPC. Operation at 1-10 bar in Ar or Ne mixtures are considered at TREX-DM [Ch1-72] and with the NEWS-G spherical detector [Ch1-73], using solely radio-pure materials ($\mu$Bq/cm$^2$) and purified light gases in order to achieve a low energy threshold ($\lesssim 1$ keV) for low mass WIMP (0.1-10 GeV) searches.

Going down into the WIMP MeV-mass range, the MIGDAL experiment uses 14.1 MeV neutrons at a rate of $10^{10}$ Hz on a target gas (CF$_4$) at low pressure ($< 100$ mbar) to detect visible MIGDAL [Ch1-74] electron tracks. Here the scintillation light from the electron multiplication in CF$_4$ is captured by a camera, while the amplified charge is collected at the indium tin oxide anode.

Solar axions conversions into low-energy $\mathcal{O}(\text{keV})$ photons can be detected in large TPC volumes operated inside strong magnetic fields. The future IAXO [Ch1-75], [Ch1-76] observatory, with an improvement of $\mathcal{O}(1-1.5)$ of magnitude in sensitivity to $g_\alpha^\gamma$ with respect to the CAST [Ch1-77], aims to explore a range of axion masses up to 0.25 eV. It will be equipped with a 6 T magnet able to focus signal photons into $\approx 0.2 \text{ cm}^2$ spots imaged by ultra-low-background “Microbulk” Micromegas, operating at atmospheric pressure.

In the field of low energy nuclear reactions, the next-generation active target multipurpose experiments will study very rare nuclear processes in inverse kinematics induced by low-intensity exotic beams. The TPC with an active target and a THGEM-Micromegas pad plane at the NSCL [Ch1-78] can operate under a 2 T magnetic field
<table><thead><tr><th>Facility</th><th>Technologies</th><th>Challenges</th><th>Most challenging requirements at experiment</th></tr></thead><tbody><tr><td>WIMP search (DRIFT, MIMAC, CYGNUS, MIGDAL, TREX-DM)</td><td>-TPC w/ MWPC/MPGD at 20-130 mbar, charge readout<br>-TPC w/ MPGD at 66 mb/1 bar, charge and optical readout<br>-TPC w/ MPGD at 1-10 bar, charge readout</td><td>High granularity, high gain, low background, very low noise level and fast electronics, self trigger capability, gas optimization</td><td>(CYGNUS)<br>Gain = O(10<sup>6</sup>)<br>Spatial resolution = O(100 µm)<br>Energy Threshold = 2 keVee<br>Energy Resolution = 20% at 5.9 keVee<br>Optical readout<br>He:SF<sub>6</sub> or He:CF<sub>4</sub> at P = 1 bar</td></tr><tr><td>Solar axion helioscope (IAXO)</td><td>-TPC w/ pixelated Micromegas, GridPix, charge readout</td><td>High granularity, low background, radiopure electronics, self-trigger capability</td><td>High efficiency in ROI (0-10 keV)<br>Spat. res = O(100 µm)<br>Background: 10<sup>-7</sup> c/keV/cm<sup>2</sup>/s<br>Xe at P = 1 bar<br>B = 6 T</td></tr><tr><td>Low energy nuclear physics general purpose active target (AT-TPC, ACTAR)</td><td>-TPC+MM at 0.05-3 bar, charge readout</td><td>Electronics with large dynamic range and flexible configuration. self-trigger capability, high pressure MPGD</td><td>(AT-TPC)<br>B = 2 T<br>P = 0.05-1 bar<br>3D-layout<br>Generic target gases (H<sub>2</sub>, He, Ar, CO<sub>2</sub>. . .)</td></tr><tr><td>Neutrino physics and Neutrino-less double beta decay (DUNE-ND, NEXT, PANDAX-II)</td><td>-TPC+SiPM+PM: electroluminescence readout, -TPC+MM: charge readout</td><td>low background, energy resolution and topological rejection factors, scale to large volume, transparency and long drifting distance, high pressure, Ba++ tagging</td><td>(NEXT)<br>P = 5-15 bar<br>3D-reconstruction of tracks through SiPM plane<br>Energy resolution &lt; 1%<br>Ba++tagging</td></tr><tr><td>Neutrinos and DM search (Dune, DarkSide-20k, Argo, PandaX-4T, LZ, ARIADNE, Darwin)</td><td>- Dual-Phase TPC+MPGD</td><td>Large volume (uniform and stability response), ultra-low background, energy resolution, low energy thresholds, high granularity, charge extraction from liquid to gas, background rejection by prompt scintillation light -S1/ signal from the charge -S2 optimisation; Xenon and Argon storage and recuperation techniques</td><td>(Darwin)<br>- 200 t x yr exposure<br>- Drift/diameter: 2.6 m / 2.6 m<br>- LXe Mass: 40 t<br>- Particle discrimination by S1/S2<br>- Low-energy threshold of ~1 keVnr<br>- Robust electrode design (up to 50kV)<br>- Ultra-low intrinsic radioactivity materials<br>- 222Rn: factor 100 reduction<br>- (a,n) neutrons (from PTFE)<br>- >99.98% Electron Recoil rejection at 30% Nuclear Recoil efficiency<br>- High light yield (QE) ~ 8 PE/keV<br>(Darkside-20k/Argo)<br>- 200 t x yr exposure /Argo = 3000 t x yr)<br>- Drift/diameter: 3.5 m / 3.5 m<br>- LAr Mass: 51.7 t/Argo - 350 t<br>- Particle discrimination by S1/S2 and pulse shape.<br>- Low-energy threshold of ~0.5 keVnr<br>- Highlander scintillation yield ~40 PE/keV<br>- Membrane cryostat like the ProtoDune<br>- Low radioactivity argon in underground CO<sub>2</sub> wells (UAr) with an activity 1400 times lower than atmospheric</td></tr></tbody></table>

Figure 1.7: Main drivers for TPCs used in rare event searches. The most stringent requirements for the future R&D activities are quoted in the last column.

and have similar size and complexity compared to TPCs at collider experiments. Gas and pressure (range of 0.1-3 bar) are adjustable to ensure an adequate interaction probability and stopping power, while amplification and electronics should be suitable for potentially relevant reactions. In nuclear astrophysics, the advent of high-intensity γ-ray beams opened a new opportunity to determine the C/O ratio at the end of the helium burning in stars; a low pressure CO₂ gas TPC with an active target and a GEM readout at ELI-NP facility is ideally suited for such studies [Ch1-79]. The most advanced 0νββ gaseous TPC is the one built for the NEXT experiment [Ch1-80]. It uses high-pressure enriched ¹³⁶Xe gas as both the source of the decay and the detection medium, and relies on the electroluminescence effect in order to approach the intrinsic energy resolution of the gas medium. The TPC performs 3D-track reconstruction through a SiPM plane with an energy resolution at Qββ of 1% FWHM. The aim is to consolidate the tech-
nology in view of future experiment stages (100 kg and 1 ton) by studying low-diffusion
mixtures (Xe/He, Xe/CH₄). Searches for 0νββ decay are also performed with the high-
pressure PandaX-III and R2D2 spherical TPCs. The detection of rare processes often
requires a dense target (to increase the interaction probability) and good background
discrimination. The R&D is focused towards NEXT-1Ton with Ba-tagging, i.e. identi-
fication of the ¹³⁶Ba daughter from ¹³⁶Xe 0νββ decay in tandem with electron energy
measurement techniques. It resorts to a coating consisting of a molecule that changes its
fluorescent properties after trapping a Ba⁺⁺ ion. The DUNE collaboration is exploring
a pressurised (10 bar) Ar-based TPC for its Near Detector to characterise precisely the
ν-beam energy and constrain nuclear effects in ν-Ar interactions with much lower mo-
mentum threshold for particle detection, compared to the adjacent LAr Near Detector.
Dual-phase detectors, based on gaseous TPC with a noble liquid, allow combined high
resolution tracking (MPGD for amplification and charge readout) with good calorimetric
response (electroluminescent signal) and a T0 signal for a trigger using primary scintilla-
tion. Large dual-phase multi-ton experiments, either based on LAr (Dune FD [Ch1-81],
ARIADNE [Ch1-82], DarkSide-20k and an ultimate ARGO [Ch1-83]) or LXe (PandaX-
4T [Ch1-84], LZ [Ch1-85] towards DARWIN [Ch1-86], [Ch1-87]) are under consideration
for detection of complex neutrino and WIMP interactions. In noble element detectors,
the target mixture or doping with other elements can influence detector sensitivity and
response (e.g. wavelength shifting and time-profile compression), enable optical readout
of scintillation light or negative ion charge transport, and improve detector stability by
quenching, etc. The proof of concept for this is successfully progressing in small size
systems. The main R&D challenges are related to scaling up in dimensions and com-
plexity to the future experiments (see in particular Chapter 2). Figure 1.7 summarises
the main challenges and proposed technologies in rare event search experiments.

1.3 Recommendations

1.3.1 Key technologies

Gaseous detectors offer a diversity of technologies to meet the challenges posed by future
facilities in various applications, as described in the previous sections, thanks to the
specific advantages that each GD concept provides. Future R&D should focus on pushing
the detector performance limits by overcoming the related technological challenges. In
several cases, the limitations are well understood, and R&D lines clearly defined, in
others, fully novel approaches are needed to meet the requirements.

For MPGDs, the main challenges remain large area, high rate, precise timing capabil-
ities (PICOSEC [Ch1-28], FTM [Ch1-26], [Ch1-27]), and stable discharge-free operation.
They are addressed by developing resistive electrodes and exploring novel materials, am-
plifying structures, and detector architectures. Precise assembly at an industrial scale
and integration of large area detector systems are additional items to be faced.

The focus for RPCs, MRPCs stays with the improvement of high-rate and precise
timing capabilities, uniform detector response, and mechanical compactness. Explo-
ration of novel materials [Ch1-88], [Ch1-89], [Ch1-90], [Ch1-91] and readout patterns,
development of low noise electronics [Ch1-92], [Ch1-93], optimisation of gas mixtures,
including eco-friendly ones, and mechanical designs [Ch1-94] are the main future R&D
lines.

Requirements for straw tubes include extended length and smaller diameter, low material budget, and operation in a highly challenging radiation environment. Primary efforts will focus on ultra-thin wall development, long and thin wire handling, precise mechanics, innovative designs, and over-pressure for vacuum operation. Additionally classical ageing issues should not be neglected.

Large volume drift chamber operation with a reduced material budget in a high-rate
environment requires searches for new wire materials and wiring procedures to increase
the detector granularity and optimisation of gas mixtures to mitigate classical ageing
effects. The application of cluster counting techniques, one of the motivation behind the
future adoption of this approach, requires the development of dedicated electronics.

IBF remains the main challenge for TPC applications in future facilities. The goal
is to minimise IBF while preserving good energy and spatial resolution, uniformity of
response, stable operation, and low material budget. Exploration of novel readout sensor
architectures and amplifying elements, including hybrid solutions with pixel ASICs (e.g.
GridPix), are the main directions in this domain.

IBF minimisation is also one of the recipes to extend the photocathode lifetime in gaseous photodetectors in RICH and PICOSEC, with a similar approach to the described above and an additional challenge of high gain stable operation. An alternative solution is the development of robust photocathodes by exploration of novel materials and photoconverter protection. The development of dedicated low noise electronics coping with high input capacitance and large dynamic range requirements are also essential in the future.

TPCs for rare event searches represent a specific class of applications probing fun-
damental physics with the properties of the gas molecules used for ionisation, charge
transfer and amplification, or light emission. Those applications have additional re-
quirements like radio-purity and ultra low noise electronics. In addition, they explore
core topics in detector physics, i.e. different amplifying structure designs for high/low-
pressure stable operation, ions as charge carriers to mitigate charge diffusion or gas
electroluminescence for optical readout, with potential for many novel applications.

1.3.2 Common challenges

To meet objectives as described in the DRDTs (Chapter 1.1) and to overcome common
challenges, new developments and novel approaches will be necessary. For many GDs,
the problem of greenhouse gases, widely used nowadays for detector operation, could be-
come a fundamental limitation; the European Commission regulations [Ch1-95] require
a substantial reduction of these emissions. The total amount of the most important
flourinated gases that are sold in the EU should be drastically reduced by 2030, their
prices could go up and future availability is unknown. Therefore, reduction of the use
of flourinated gases is fundamental for future GD applications. Flourinated gases will
be banned in the new installations where alternatives are available, and emission from
existing facilities need to be minimised by checks, optimisations, and recovery systems. Currently the most effective strategy to reduce greenhouse gases' emissions is gas recirculation in large detectors [Ch1-96]. This leads to the optimisation of gas consumption at the expense of operational complexity, with possible pressure and flow fluctuations requiring improved control and monitoring. The creation and accumulation of impurities, especially in F-based mixtures, requires gas purification techniques and could potentially affect long-term detector operation depending on luminosity and the recirculation fraction. Gas recuperation systems, even if very challenging, could permit the most valuable component to be extracted, stored, and re-used, according to the quality of recuperated gas. Unfortunately, gas leaks could make less effective this strategy for already installed gas systems [Ch1-97]. Possible alternatives to greenhouse gases (C₂H₂F₄, SF₆, CF₄) should remain the principal focus of the future R&D [Ch1-98], [Ch1-99], [Ch1-100]. New eco-friendly liquids/gases have been developed for industry [Ch1-101], [Ch1-102] as refrigerants and HV insulating medium. The ionisation and transport properties in these mixtures are not yet well known in high electric fields and should be simulated and measured. Performance studies of several eco-friendly mixtures for GDs are essential, together with a better understanding of the impurities creation under large irradiation fluences and their long-term ageing effects on GD performance [Ch1-103], [Ch1-104].

<table><caption>Figure 1.8: Summary of the R&D challenges for different applications.</caption><thead><tr><th>Muon System</th><th>Inner and Central tracking</th><th>Calorimetry</th><th>Photon detection</th><th>TOF</th><th>Rare decays</th></tr></thead><tbody><tr><td><ul><li>Radiation hardness and stability up to integrated charges of hundreds of C/m²:<ul><li>aging issues and discharges;</li><li>Operation in a stable and efficient manner with incident particle flows up to ~10 MHz/cm².</li><li>miniaturisation of readout elements needed to keep occupancy low</li></ul></li><li>Manufacturing, on an industrial scale, large detectors at low cost, by means of a process of technological transfer to the industry and identifies processes transferable to industries</li><li>Identification of eco-friendly gas mixture and mitigation of the issue related to the operation with high WGP gas mixture:<ul><li>gas tightness; gas recuperation system; accessibility for repairing</li></ul></li><li>Study of resistive materials (RPC and MPGD):<ul><li>higher gain in a single multiplication layer, with a remarkable advantage for assembly, mass production and cost</li><li>new material and production techniques for resistive layers for increasing the rate capability</li></ul></li><li>Thinner layers and mechanical precision over large area</li></ul></td><td><strong>Drift chambers</strong><ul><li>High rate, unique volume, high granularity, low mass</li><li>Hydrocarbon-free mixture for long-term and high-rate operation</li><li>Prove the cluster counting principle with the related electronics</li><li>Mechanics: new wiring procedure, new wire materials</li><li>Integration: accessibility for repairing</li></ul><strong>TPC</strong><ul><li>R&D on detector sensors to suppress the IBF ratio</li><li>Optimize IBF together with energy resolution</li><li>Gain optimization: IBF, discharge stability</li><li>Uniformity of the response of the sensors</li><li>Gas mixture: stability, drift velocity, ion mobility, aging</li><li>Influence of Magnetic field on IBF</li><li>High spatial resolution</li><li>Very low material budget (few %)</li><li>Mechanics: thickness minimization but robust for precise electrical properties for stable drift velocity</li><li>Integration: cooling of electronics</li></ul><strong>Straw chambers</strong><ul><li>Ultra-long and thin film tubes</li><li>"Smart" designs: self-stabilized straw module, compensating relaxation</li><li>Small diameter for faster timing, less occupancy, high rate capability</li><li>Reduced drift time, hit leading times and trailing time resolutions, with dedicated R&D on the electronics</li><li>PID by dE/dx with "standard" time readout and time-over-threshold</li><li>4D-measurement: 3D-space and (offline) track time</li><li>Over-pressurized tubes in vacuum: control the leakage rate to maintain the shape</li></ul></td><td><ul><li>Uniformity of the response of the area and dynamic energy range</li><li>Optimization of weights for different thresholds in digital calorimeters</li><li>Rate capability in detectors based on resistive materials; resistivity uniformity, discharge issue at high rate and in large area detector</li><li>R&D on sub-ns in active elements: resolution stables over wide range of fluxes</li><li>Gas homogeneity and stable over time</li><li>Eco-friendly gas mixture for RPC</li><li>Stability of the gas gain: fast monitoring of gas mixture and environmental conditions</li><li>Mechanics:<ul><li>large area needed to avoid dead zone: limitation on size and planarity of PCB is an issue</li><li>multi-gap with ultra-thin modules: very thin layer of glass and HPL electrodes, gas gap thickness uniformity few micron</li></ul></li></ul></td><td><ul><li>Preserve the photocathode and time resolution over large detector area</li><li>Gas radiator: alternative to CF₄</li><li>Gas tightness</li><li>Very low noise when coupling large capacitance</li><li>Large dynamic range of the FEE</li><li>Separate the TR radiation and the ionization process</li><li>In TRD use of cluster counting technique and improve it by means of a InGrid</li></ul></td><td><ul><li>Uniform rate capability and time resolution over large detector area</li><li>New material for high rate (low resistivity, radiation hardness)</li><li>uniform gas distribution</li><li>thinner structures: mechanical stability and uniformity</li><li>Eco-gas mixture</li><li>Electronics: Low noise, fast rise time, sensitive to small charge</li><li>Possibly optical readout</li><li>Precise clock distribution and synchronization over large area</li></ul></td><td><ul><li>Radio-purity of the materials</li><li>Low background</li><li>High granularity</li><li>For large volume detectors: transparency over large distance</li><li>Pressure stability and control</li><li>Electronics with large dynamic range and flexible configuration</li><li>Self-trigger capability</li><li>Low noise electronics</li><li>Fast electronics</li><li>Optical readout</li></ul></td></tr></tbody></table>

Large GD systems in future facilities will face integration challenges and the requirement for easy accessibility and replaceability in complex installations. The main challenges include gas tightness, overpressure operation and electronics cooling. In addition, engineering effort to ensure precision assembly of a large scale detector components will be needed.

Ageing phenomena constitute one of the most complex and serious potential prob-
lems which could limit or severely impair the use of gaseous detectors in unprecedented
harsh radiation environments and lead to operational instabilities [Ch1-38], [Ch1-39],
[Ch1-105], [Ch1-106], [Ch1-107]. To guarantee reliable and long-term operation in ex-
treme conditions for large area detectors using different technologies (resistive Micro-
megas, μ-RWELL, GEM, THGEM, FTM, RPC, MRPC, sMDT, sTGC, straws tubes,
et), further research on classical ageing effects and discharge suppression and mitigation
is mandatory [Ch1-108], [Ch1-109], [Ch1-12], [Ch1-110]. Such studies should include
fundamental gas properties and materials R&D, exploration of novel technologies, new
architectures and amplifying structures.

Gaseous detectors require dedicated front-end electronics development, both discrete and ASICs, focused on specific applications and technologies, while addressing diverse requirements such as: fast timing, TPC readout, large input capacitance, low noise, cluster counting, input discharge protection, cross-talk reduction, large pixel size, compactness, low power consumption and detector integration [Ch1-111]. At the same time, alternative and hybrid readout methods should be explored to grow the potential applications portfolio: optical readout with imaging sensors; hybrid (optical with electronic) readout and direct readout with pixelated ASICs (intensified TPX3Cam, GridPix, GEMPix). A summary of the main R&D topics needed for the different applications is shown in Figure 1.8.

1.3.3 R&D environment and development tools

It is imperative to create a friendly environment to facilitate detectors development and networking activity. Promoting R&D collaborations [Ch1-112], [Ch1-32] focused on detector technologies is very effective for an easy exchange of experience and resources; it allows building of a community with continuity and institutional memory and enhances support of generic R&D, education, and training of younger generation instrumentalists (see Chapter 9). Educational and outreach activities introduce high school students and teachers to research activities in a strong and intense collaboration with academic institutions (e.g. measurements of the cosmic ray radiation in the context of the EEE Project [Ch1-113]).

It is rather fundamental to secure institutional investment and maintenance sup-
port of infrastructures for detector development, testing and production laboratories;
equipped and maintained tests beam installations (trackers, DAQ, magnets); and pro-
duction and R&D facilities maintaining access to modern technologies through academia,
industry and upgrades of in-house infrastructure (see Chapter 10 and Chapter 11). Sup-
port for development and maintenance of the detector development tools, which should
include electronics R&D and software for detector physics simulations, is essential. It
is common practice to underestimate these important aspects and substantially delay
or practically prevent progress in the field. Development tools should be treated in the
same way as the essential hardware infrastructure.

Future projects will rely on large scale, industrial production. A suitable model
of effective academia-industry collaboration is largely missing. Institutional (or even
governmental level) policy to facilitate relations with industry, including mitigation of
administrative barriers which often prevent successful technology transfer, is crucial (see Chapter 10).

Supporting applications beyond fundamental research opens possibilities for industry involvement, increases visibility of detector R&D activities with a profound impact on society at large. The specific features of GDs allow for very challenging future applications of scientific, social and industrial interest. Many examples already exist using different concepts: GEMs and THGEM like-structures, Micromegas, RPCs, etc. The high spatial resolution, combined with large sensitive volume, energy resolving and time precision capabilities, enlarges the portfolio of GD applications for other science sectors, such as: Material Science, Energy, Space and Healthcare. Innovative approaches include: Material Science – neutron diffractometers [Ch1-114], energy dispersive x-ray fluorescence and imaging [Ch1-115]; Energy – Tokamak neutron and radioactive waste diagnostics [Ch1-116]; Medical – micro-dosimetry to qualify radiation fields for cancer therapy; 3D-ion beam monitoring and treatment plan verification in hadron therapy [Ch1-117], [Ch1-118]; monitoring of energy released in Intensity-Modulated Radiation Therapy; energy resolved radiography and Positron Emission tomography scanners [Ch1-119], [Ch1-120]; and muon based imaging - geophysics, geology, archaeological heritage [Ch1-121], civil engineering, and nuclear industry [Ch1-122]. Strategies, fostering portability and high detection efficiency, are of major interest to ensure competitiveness and a low level of complexity which facilitate large-scale production. In this context, common R&D efforts dedicated to: gas purification systems and the search for low outgassing materials (which are mandatory for sealed-system operation); improved photon detection efficiency at high gas pressure; compact and/or embedded dedicated readout, DAQ and biasing electronics “all in one”, should be advanced in order to facilitate the adequacy of GDs in applications beyond fundamental research.

## References Chapter 1

[Ch1-1] ALICE Collaboration, *Upgrade of the ALICE Time Projection Chamber*, CERN-LHCC-2013-020, ALICE-TDR-016, 2013. https://cds.cern.ch/record/1622286.

[Ch1-2] T. Kawamoto et al., *New Small Wheel Technical Design Report*, CERN-LHCC-2013-006, ATLAS-TDR-020, 2013. https://cds.cern.ch/record/1552862.

[Ch1-3] ATLAS Collaboration, *Technical Design Report for the Phase-II Upgrade of the ATLAS Muon Spectrometer*, CERN-LHCC-2017-017, ATLAS-TDR-026, 2017. https://cds.cern.ch/record/2285580.

[Ch1-4] A. Colaleo et al., *CMS Technical Design Report for the Muon Endcap GEM Upgrade*, CERN-LHCC-2015-012, CMS-TDR-013, 2015. https://cds.cern.ch/record/2021453.

[Ch1-5] CMS Collaboration, P. Kumari et al., *Improved-RPC for the CMS muon system upgrade for the HL-LHC*, JINST 15 C11012, arXiv:2005.11396 [physics.ins-det].
[Ch1-6] B. Bartoli et al., EAS age determination from the study of the lateral distribution of charged particles near the shower axis with the ARGO-YBJ experiment, Astroparticle Physics **93** (2017) 46-55.

[Ch1-7] CMS Collaboration, *The Phase-2 Upgrade of the CMS Muon Detectors*, CERN-LHCC-2017-012, CMS-TDR-016. https://cds.cern.ch/record/2283189.

[Ch1-8] ATLAS Collaboration, P. Rieck, *Design, Construction and Test of Small-Diameter Muon Drift Tube Chambers (sMDT) for the Phase-1 Upgrade of the ATLAS Muon Spectrometer*, ATL-MUON-PROC-2019-002, 2019. https://cds.cern.ch/record/2704673/.

[Ch1-9] I. Gnesi, *Micromegas chambers for the ATLAS New Small Wheel upgrade*, JINST **15** (2020) no. 09, C09019.

[Ch1-10] M. Gola, *CMS phase-II upgrade with GEM detector*, Journal of Physics: Conference Series **1498** (2020) 012054.

[Ch1-11] T. Alexopoulos et al., *Development of large size Micromegas detector for the upgrade of the ATLAS Muon system*, Nucl. Instrum. Meth. A **617** (2010) 161-165.

[Ch1-12] S. D. Butalla and M. Hohlmann, *Investigation and Mitigation of Crosstalk in the Prototype ME0 GEM Detector for the Phase-2 Muon System Upgrade of the CMS Experiment*, 2021. arXiv:2102.03342 [physics.ins-det].

[Ch1-13] G. Bencivenni et al., *The micro-Resistive WELL detector: a compact spark-protected single amplification-stage MPGD*, JINST **10** (2015) no. 02, P02008.

[Ch1-14] F. Yamane et al., *Development of the Micro Pixel Chamber with DLC cathodes*, Nucl. Instrum. Meth. A **951** (2020) 162938.

[Ch1-15] M. Alviggi et al., *Small-pads resistive micromegas*, JINST **12** (2017) no. 03, C03077.

[Ch1-16] D. Neyret et al., *New pixelized Micromegas detector with low discharge rate for the COMPASS experiment*, JINST **7** (2012) no. 03, C03006.

[Ch1-17] M. Alviggi et al., *Small-pad resistive Micromegas for high rate environment: Performance of different resistive protection concepts*, Nucl. Instrum. Meth. A **936** (2019) 408-411.

[Ch1-18] A. Ochi, *Introduction to Resistive DLC Collaboration, RD51 Mini-Week, 4-6 December 2018, CERN*, https://indico.cern.ch/event/761831/contributions/3236762/attachments/1765980/2867435/DLC_CP_Intro_ochi_181205_v2.pdf.

[Ch1-19] G. Bencivenni, *Current state of the art of DLC detectors in various users groups, DLC workshop, RD51 Mini week, CERN*, , 2020. https://indico.cern.ch/event/872501/contributions/3723333/attachments/1986092/3309414/Resistive_MPGDs-RD51-mini-week-Feb-2020.pdf.

[Ch1-20] G. Bencivenni et al., *The μ-RWELL layouts for high particle rate*, JINST **14** (2019) no. 05, P05014.

[Ch1-21] P. Senger, *Astrophysics in the Laboratory—The CBM Experiment at FAIR*, Particles **3** (2020) no. 2, 320-335.

[Ch1-22] A. Belias, *FAIR status and the PANDA experiment*, JINST **15** (2020) no. 10, C10001.
[Ch1-23] E. Scomparin, *The NA60+ experiment at the CERN SPS: physics goals and prospects*, 10th International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions 2020. http://cds.cern.ch/record/2721758.

[Ch1-24] L. L. et al., *Physics and Detectors at CLIC: CLIC Conceptual Design Report*, 2012. arXiv:1202.5940 [physics.ins-det].

[Ch1-25] Edited by D. Dannheim et al., *Detector Technologies for CLIC*, CERN-2019-001 (2019), arXiv:1905.02520 [physics.ins-det].

[Ch1-26] R. D. Oliveira, M. Maggi, and A. Sharma, *A novel fast timing micropattern gaseous detector: FTM*, 2015. arXiv:1503.05330 [physics.ins-det].

[Ch1-27] D. Abbaneo et al., *R&D on a new type of micropattern gaseous detector: The Fast Timing Micropattern detector*, Nucl. Instrum. Meth. A **845** (2017) 313-317.

[Ch1-28] J. Bortfeldt et al., *PICOSEC: Charged particle timing at sub-25 picosecond precision with a Micromegas based detector*, Nucl. Instrum. Meth. A **903** (2018) 317-325.

[Ch1-29] H. Danielsson, *Design, construction and testing of the straw tracker for the NA62 experiment*, 2013 IEEE Nuclear Science Symposium and Medical Imaging Conference (2013) 1-5.

[Ch1-30] N. Tsverava et al., *Development of Ultrathin 12 µm Thick Straw Tubes for the Tracking Detector of COMET Experiment*, 2019 IEEE Nuclear Science Symposium and Medical Imaging Conference (2019) 1-4.

[Ch1-31] Mu2e Collaboration, F. Abusalma et al., *Expression of Interest for Evolution of the Mu2e Experiment*, arXiv:1802.02599 [physics.ins-det].

[Ch1-32] S. D. Torre et al., *R&D Proposal, RD51 Extension Beyond 2018*, 2018. arXiv:1806.09955 [physics.ins-det].

[Ch1-33] B. Mindur et al., *Investigation of Copper-Less Gas Electron Multiplier Detectors Responses to Soft X-rays*, Sensors **20** (2020) no. 10, 2784.

[Ch1-34] A. Barnyakov et al., *The project of the Super Charm-Tau Factory in Novosibirsk*, Journal of Physics: Conference Series **1561** (2020) 012004.

[Ch1-35] KLOE Collaboration, A. Ferrari, *The KLOE drift chamber*, Nucl. Instrum. Meth. A **494** (2002) 163-172.

[Ch1-36] G. Tassielli et al., *The Drift Chamber of the MEG II experiment*, arXiv:2006.02378 [physics.ins-det].

[Ch1-37] F. Grancagnolo, *The Evolution of Drift Chambers at e+e- Colliders*, International Conference on Instrumentation for Colliding Beam Physics, Novosibirsk, Russia, 24-28 February 2020.

[Ch1-38] M. Titov, *Radiation damage and long-term aging in gas detectors*, Innovative Detectors for Supercolliders, Proceedings of the Workshop of the INFN ELOISATRON Project, Erice, Italy, Oct 2003 (2004), arXiv:0403055 [physics.ins-det].

[Ch1-39] M. Titov et al., *Summary and outlook of the international workshop on aging phenomena in gaseous detectors (DESY, Hamburg, October 2001)*, IEEE Trans. Nucl. Sci. **49** (2002) no. 4, 1609-1621.
[Ch1-40] D. S. Bhattacharya et al., *Experimental and numerical simulation of a TPC like set up for the measurement of ion backflow*, 2017. arXiv:1710.00624 [physics.ins-det].

[Ch1-41] LCTPC Collaboration. https://www.lctpc.org.

[Ch1-42] R. Diener, *Development of a TPC for an ILC Detector*, 2012. arXiv:1203.2074 [physics.ins-det].

[Ch1-43] T. Behnke et al., *The International Linear Collider Technical Design Report - Volume 4: Detectors*, 2013. arXiv:1306.6329 [physics.ins-det].

[Ch1-44] D. Attié et al., *A time projection chamber with GEM-based readout*, Nucl. Instrum. Meth. A **856** (2017) 109-118.

[Ch1-45] D. Attié et al., *Performances of a resistive Micromegas module for the Time Projection Chambers of the T2K Near Detector upgrade*, Nucl. Instrum. Meth. A **957** (2020) 163286, arXiv:1907.07060 [physics.ins-det].

[Ch1-46] D. Attié et al., *Characterization of resistive Micromegas detectors for the upgrade of the T2K Near Detector Time Projection Chambers*, arXiv:2106.12634 [physics.ins-det].

[Ch1-47] M. Lupberger et al., *Toward the Pixel-TPC: Construction and Operation of a Large Area GridPix Detector*, IEEE Trans. Nucl. Sci. **64** (2017) no. 5, 1159-1167.

[Ch1-48] G. Croci et al., *Gas Electron Multiplier (GEM) application for Time Projection Chamber (TPC) gating*, JINST **5** (2010) no. 03, P03001.

[Ch1-49] M. Aleksa et al., *Calorimeters for the FCC-hh*, 2019. arXiv:1912.09962 [physics.ins-det].

[Ch1-50] CALICE Collaboration, *Analogue, Digital and Semi-Digital Energy Reconstruction in the CALICE AHCAL*, CALICE-CAN-2014-003, 2014. https://cds.cern.ch/record/2669512.

[Ch1-51] CALICE Collaboration, *Calorimetry for Lepton Collider Experiments - CALICE results and activities*, arXiv:1212.5127 [physics.ins-det].

[Ch1-52] CALICE Collaboration, *Analysis of testbeam data of the highly granular RPC-steel CALICE digital hadron calorimeter and validation of Geant4 Monte Carlo models*, Nucl. Instrum. Meth. A **939** (2019) Pages 89-105.

[Ch1-53] S. Bressler et al., *Novel Resistive-Plate WELL sampling element for (S)DHCAL*, Nucl. Instrum. Meth. A **951** (2020) 162861.

[Ch1-54] M. Chefdeville et al., *Development of Micromegas detectors with resistive anode pads*, Nucl. Instrum. Meth. A **1003** (2021) 165268.

[Ch1-55] F. Tessarotto, *Evolution and recent developments of the gaseous photon detectors technologies*, Nucl. Instrum. Meth. A **912** (2018) 278-286.

[Ch1-56] W. Anderson et al., *Design, construction, operation and performance of a Hadron Blind Detector for the PHENIX experiment*, Nucl. Instrum. Meth. A **646** (2011) no. 1, 35-58.

[Ch1-57] J. Agarwala et al., *The MPGD-based photon detectors for the upgrade of COMPASS RICH-1 and beyond*, Nucl. Instrum. Meth. A **936** (2019) 416-419. Frontier Detectors for Frontier Physics: 14th Pisa Meeting on Advanced Detectors.
[Ch1-58] M. Blatnik et al., Performance of a Quintuple-GEM Based RICH Detector Prototype, IEEE Trans. Nucl. Sci. **62** (2015) no. 6, 3256-3264.

[Ch1-59] J. Veloso, J. D. Santos, and C. Conde, A proposed new microstructure for gas radiation detectors: The microhole and strip plate, Review of Scientific Instruments **71** (2000) 2371-2376.

[Ch1-60] A. Lyashenko et al., Efficient ion blocking in gaseous detectors and its application to gas-avalanche photomultipliers sensitive in the visible-light range, Nucl. Instrum. Meth. A **598** (2009) no. 1, 116-120.

[Ch1-61] F. Brunbauer et al., Nanodiamond photocathodes for MPGD-based single photon detectors at future EIC, JINST **15** (2020) no. 09, C09052.

[Ch1-62] F. Barbosa et al., A new Transition Radiation detector based on GEM technology, Nucl. Instrum. Meth. A **942** (2019) 162356.

[Ch1-63] J. Kaminski et al., GridPix detectors – introduction and applications, Nucl. Instrum. Meth. A **845** (2017) 233-235. Proceedings of the Vienna Conference on Instrumentation 2016.

[Ch1-64] ALICE Collaboration, N. Jacazio, PID performance of the ALICE-TOF detector at Run 2, arXiv:1809.00574 [physics.ins-det].

[Ch1-65] I. Deppner and N. Herrmann, The FAIR Phase 0 program of the CBM TOF, JINST **15** (2020) no. 10, C10030, arXiv:2006.08510 [physics.ins-det].

[Ch1-66] Y. Wang and Y. Yu, Multigap Resistive Plate Chambers for Time of Flight Applications, Appl. Sciences **11** (2020) no. 1, 111.

[Ch1-67] Z. Liu et al., Novel low resistivity glass: MRPC detectors for ultra high rate applications, Nucl. Instrum. Meth. A **959** (2020) 163483.

[Ch1-68] D. Santos et al., MIMAC: A micro-tpc matrix project for directional detection of dark matter, 2011. arXiv:1111.1566 [astro-ph.IM].

[Ch1-69] K. Miuchi et al., First underground results with NEWAGE-0.3a direction-sensitive dark matter detector, Physics Letters B **686** (2010) no. 1, 11-17.

[Ch1-70] J. Battat et al., Radon in the DRIFT-II directional dark matter TPC: emanation, detection and mitigation, JINST **9** (2014) no. 11, P11004.

[Ch1-71] S. E. Vahsen et al., CYGNUS: Feasibility of a nuclear recoil observatory with directional sensitivity to dark matter and neutrinos, 2020. arXiv:2008.12587 [physics.ins-det].

[Ch1-72] J. Castel et al., Background assessment for the TREX dark matter experiment, The European Physical Journal C **79** (2019) no. 9, 782.

[Ch1-73] NEWS-G Collaboration, Q. Arnaud et al., First results from the NEWS-G direct dark matter search experiment at the LSM, Astropart. Phys. **97** (2018) 54-62, arXiv:1706.04934 [astro-ph.IM].

[Ch1-74] M. Ibe et al., Migdal effect in dark matter direct detection experiments, Journal of High Energy Physics **2018** (2018) no. 3, 1.

[Ch1-75] J. K. V. et al., IAXO - The International Axion Observatory, 2013. arXiv:1302.3273 [physics.ins-det].
[Ch1-76] A. Abeln et al., *Conceptual Design of BabyIAXO*, the intermediate stage towards the International Axion Observatory, 2021. arXiv:2010.12076 [physics.ins-det].

[Ch1-77] E. Armengaud et al., *Conceptual Design of the International Axion Observatory (IAXO)*, JINST **9** (2014) no. FERMILAB-PUB-14-476-A, BNL-106253-2014-JA, T05002, arXiv:1401.3233 [physics.ins-det].

[Ch1-78] Y. Ayyad et al., *Next-generation experiments with the Active Target Time Projection Chamber (AT-TPC)*, Nucl. Instrum. Meth. A **954** (2020) 161341.

[Ch1-79] M. Gai et al., *Time Projection Chamber (TPC) Detectors for Nuclear Astrophysics Studies With Gamma Beams*, 2018. arXiv:1812.09536 [physics.ins-det].

[Ch1-80] A. Simón et al., *Boosting background suppression in the NEXT experiment through Richardson-Lucy deconvolution*, 2021. arXiv:2102.11931 [physics.ins-det].

[Ch1-81] DUNE Collaboration, *The DUNE Far Detector Interim Design Report Volume 1: Physics, Technology and Strategies*, 2018. arXiv:1807.10334 [physics.ins-det].

[Ch1-82] A. Lowe et al., *Optical Readout of the ARIADNE LArTPC Using a Timepix3-Based Camera*, Instruments **4** (2020) no. 4, 35.

[Ch1-83] P. Agnes et al., *Sensitivity of future liquid argon dark matter search experiments to core-collapse supernova neutrinos*, Journal of Cosmology and Astroparticle Physics **2021** (2021) no. 03, 043.

[Ch1-84] S. Wang, *PandaX-III high pressure xenon TPC for Neutrinoless Double Beta Decay search*, Nucl. Instrum. Meth. A **958** (2020) 162439. Proceedings of the Vienna Conference on Instrumentation 2019.

[Ch1-85] D. Khaitan, *Supernova neutrino detection in LZ*, JINST **13** (2018) no. 02, C02024.

[Ch1-86] J. Aalbers et al., *DARWIN: towards the ultimate dark matter detector*, Journal of Cosmology and Astroparticle Physics **2016** (2016) no. 11, 017.

[Ch1-87] Y. Wei et al., *Development and performance of a sealed liquid xenon time projection chamber*, JINST **16** (2021) no. 01, P01018.

[Ch1-88] A. Rocchi et al., *Linearity and rate capability measurements of RPC with semi-insulating crystalline electrodes operating in avalanche mode*, JINST **15** (2020) no. 12, C12004, arXiv:2006.00306 [physics.ins-det].

[Ch1-89] G. Aielli et al., *Performance of RPCs with phenolic glass electrodes*, JINST **11** (2016) no. 10, C10009.

[Ch1-90] X. Wang et al., *A new MRPC prototype with diamond-like-carbon coated glass electrodes*, JINST **16** (2021) no. 03, C03001.

[Ch1-91] L. M. S. Margato et al., *Towards high-rate RPC-based thermal neutron detectors using low-resistivity electrodes*, arXiv:2104.00695 [physics.ins-det].

[Ch1-92] L. Pizzimento et al., *Development of a new Front End electronics in Silicon and Silicon-Germanium technology for the Resistive Plate Chamber detector for high rate experiments*, JINST **14** (2019) no. 10, C10010, arXiv:1806.04113 [physics.ins-det].

[Ch1-93] R. Cardarelli, *Future RPC developments*, JINST **16** (2021) no. 05, C05004.
[Ch1-94] L. Lopes et al., Towards sealed Resistive Plate Chambers, JINST 15 (2020) no. 11, C11009, arXiv:2006.08291 [physics.ins-det].

[Ch1-95] M. Schulz and D. Kourkoulas, Regulation (eu) no 517/2014 of the european parliament and of the council of 16 April 2014 on fluorinated greenhouse gases and repealing regulation (ec) no 842/2006, Off. J. Eur. Union (2014) L150.

[Ch1-96] M. Capeans, R. Guida, and B. Mandelli, Strategies for reducing the environmental impact of gaseous detector operation at the CERN LHC experiments, Nucl. Instrum. Meth. A 845 (2017) 253-256. Proceedings of the Vienna Conference on Instrumentation 2016.

[Ch1-97] G. Pugliese, G. Aielli, and K. Kuznetsova, F-gases usage and reduction strategy plan in the ATLAS and CMS experiments, Mini-Workshop on gas transport parameters for present and future generation of experiments. https://tinyurl.com/ep335wyp.

[Ch1-98] B. Liberti et al., Further gas mixtures with low environment impact, JINST 11 (2016) no. 09, C09012.

[Ch1-99] M. Abbrescia et al., Eco-friendly gas mixtures for Resistive Plate Chambers based on tetrafluoropropene and Helium, JINST 11 (2016) no. 08, P08019.

[Ch1-100] G. Saviano et al., Properties of potential eco-friendly gas replacements for particle detectors in high-energy physics, JINST 13 (2018) no. 03, P03012.

[Ch1-101] J. G. Owens, Greenhouse gas emission reductions through use of a sustainable alternative to SF6, 2016 IEEE Electrical Insulation Conference (EIC) (2016) 535-538.

[Ch1-102] P. Billen et al., Replacing SF6 in Electrical Gas-Insulated Switchgear: Technological Alternatives and Potential Life Cycle Greenhouse Gas Savings in an EU-28 Perspective, Energies 13 (2020) no. 7, 1807.

[Ch1-103] R. Guida, GIF++: A new CERN Irradiation Facility to test large-area particle detectors for the High-Luminosity LHC program, PoS ICHEP2016 (2017) 260.

[Ch1-104] G. Rigoletti, B. Mandelli, and R. Guida, Characterization of RPC detectors with LHC-like background radiation and new eco-friendly gas mixtures, JINST 15 (2020) no. 11, C11003.

[Ch1-105] J. A. Merlin, Study of long-term sustained operation of gaseous detectors for the high rate environment in CMS. PhD thesis, Apr, 2016. https://cds.cern.ch/record/2155685. Presented 25 Apr 2016.

[Ch1-106] S. Bachmann et al., Discharge studies and prevention in the gas electron multiplier (GEM), Nucl. Instrum. Meth. A 479 (2002) no. 2, 294-308.

[Ch1-107] G. Bencivenni et al., The μ-RWELL for high rate application, JINST 15 (2020) no. 09, C09034.

[Ch1-108] R. Aly et al., Aging study on Resistive Plate Chambers of the CMS Muon Detector for HL-LHC, JINST 15 (2020) no. 11, C11002.

[Ch1-109] R. Arnaldi et al., Aging tests and chemical analysis of Resistive Plate Chambers for the trigger of the ALICE dimuon arm, Nucl. Instrum. Meth. A 533 (2004) no. 1, 112-115. Proceedings of the Seventh International Workshop on Resistive Plate Chambers and Related Detectors.
[Ch1-110] A. Marques et al., *Minimizing distortions with sectored GEM electrodes*, Nucl. Instrum. Meth. A **961** (2020) 163673.

[Ch1-111] *Topical workshop on "Front End Electronics for Gas Detectors, 15-17 June 2021*, 2021. https://indico.cern.ch/event/1051087/.

[Ch1-112] RD51 Collaboration. https://rd51-public.web.cern.ch.

[Ch1-113] P. L. Rocca et al., *The EEE Project: a sparse array of telescopes for the measurement of cosmic ray muons*, JINST **11** (2016) no. 12, C12056.

[Ch1-114] A. Muraro and G. Croci, *Review of MPGD applications for neutron detection*, JINST **14** (2019) no. 04, C04005.

[Ch1-115] J. Veloso and A. Silva, *Gaseous detectors for energy dispersive X-ray fluorescence analysis*, Nucl. Instrum. Meth. A **878** (2018) 24-39.

[Ch1-116] A. Curioni and others, *Measurements of 55Fe activity in activated steel samples with GEMPix*, Nucl. Instrum. Meth. A **849** (2017) 60-71.

[Ch1-117] J. Leidner, F. Murtas, and M. Silari, *Medical Applications of the GEMPix*, Applied Sciences **11** (2021) no. 1, 440. https://www.mdpi.com/2076-3417/11/1/440.

[Ch1-118] J. Leidner et al., *A GEMPix-based integrated system for measurements of 3D dose distributions in water for carbon ion scanning beam radiotherapy*, Medical Physics **47** (2020) no. 6, 2516-2525.

[Ch1-119] F. Brunbauer et al., *Radiation imaging with optically read out GEM-based detectors*, JINST **13** (2018) no. 02, T02006.

[Ch1-120] P. Martins et al., *Towards very high resolution RPC-PET for small animals*, JINST **9** (2014) no. 10, C10012.

[Ch1-121] D. Attié et al., *Current Status and Future Developments of Micromegas Detectors for Physics and Applications*, Applied Sciences **11** (2021) no. 12, 5362, https://www.mdpi.com/2076-3417/11/12/5362.

[Ch1-122] S. Procureur, *Muon imaging: Principles, technologies and applications*, Nucl. Instrum. Meth. A **878** (2018) 169-179.
# Chapter 2

# Liquid Detectors

## 2.1 Introduction

Liquid detectors today primarily employ target media of water, liquid scintillator, or cryogenic nobles, notably liquid argon (LAr), xenon (LXe) and helium (LHe). The physics applications of these detectors are in the domain of rare event searches, including neutrino physics, dark matter searches and astro-particle experiments.

The basic detection principle is that an interaction (e.g. of a neutrino or dark matter particle) with an atom in the liquid detector target produces final state particles which deposit energy in the liquid. The final state particle is often just the original scattering partner which obtains some measurable kinetic energy. This energy release may be electromagnetic, resulting in ionisation charge, scintillation and/or Čerenkov light, or heat. In water, Čerenkov emission dominates, typically producing $O(10^2)$ photons/MeV deposited in the visible wavelength range, with a characteristic sub-ns time scale. In scintillators, fluorescent molecules and wavelength-shifters produce $O(10^4)$ photons/MeV in the visible range, with ns to µs time scales. In noble liquids, excited atoms form dimers, which decay on time scales of ns to µs (for LAr and LXe), producing emission in the VUV (at 128 nm and 178 nm respectively), emitting $O(4×10^4)$ photons/MeV. At lower energies relevant for dark matter and $0νββ$ searches, the ionisation energy partition < 100 keV depends on the particle species and varies with energy, reaching ~30% for nuclear recoils, and thus the heat partition is an important detection channel. Above the MeV energy deposition scale, relevant for neutrino physics, the ionisation partition dominates.

The detector instrumentation strategy depends on the liquid employed. Water or scintillator targets are typically operated in single-phase detectors, in which a large open volume of liquid target is surrounded by photon sensors. R&D on liquid scintillators is exploring granular segmentation, similarly to solid scintillator detectors. Noble liquid targets are most often operated in time projection chamber (TPC) detectors. The operation principle is that charge produced in the interaction of a primary particle is drifted in an electric field into a charge collection region, which may contain gas in the case of dual-phase TPCs where amplification of the primary charge produces electro-
<figure><img src="image_13.png" /><figcaption>Figure 2.1: Current and near-future experiments addressing the physics drivers for liquid detectors (neutrino physics, neutrino astrophysics, $0\nu\beta\beta$, dark matter), grouped by detection modality, with liquid targets in purple. (Figure modified from L. Baudis, ECFA Plenary Input Session (see Appendix C).)</figcaption></figure>

luminescence, increasing the secondary signal. TPCs are typically instrumented with readout technologies to measure both charge and light (from primary or secondary emission produced in an amplification region). Dark matter experiments that are optimised to detect low-mass dark matter, below the GeV scale, additionally often benefit from instrumenting targets to detect a combination of the ionisation and heat energy partitions. For neutrinoless double beta decay searches, large detectors tend to detect the products of the decaying isotopes with liquid scintillator or with xenon TPCs (where the detector medium is both the source and the detection material). Liquid argon is also used as a veto for large crystal detectors [Ch2-1].

There are several large-scale and many small-scale running and planned experiments exploiting liquid targets. These are shown in Figure 2.1, grouped by energy partition measurement strategy. The major Detector R&D Themes for liquid detectors that have been identified in the framework of this Roadmap include in particular those listed below. It should be noted that R&D should be anticipated for facilities beyond the timelines illustrated in Figure 11.1 but it is not possible today to sensibly suggest dates for such even longer-term programmes requiring liquid detectors.

**DRDT 2.1 - Develop readout technology to increase spatial and energy resolution for liquid detectors.**

Developments should achieve readout of more highly pixelated detectors with greater photon collection capabilities. Advancing liquid detector readout technologies towards
greater quantum efficiency while still offering much higher granularity is a further objective.

### DRDT 2.2 - Advance noise reduction in liquid detectors to lower signal energy thresholds.

The expected performance of future liquid detectors requires R&D to achieve lower sensor and electronics noise, as well as developments to measure simultaneously more components of the energy partition: for example light, charge and heat.

### DRDT 2.3 - Improve the material properties of target and detector components in liquid detectors.

The R&D on material properties for liquid detectors aim to improve the emission properties of the target, for example through doping of Xe in Ar, H in Xe, Ne in Xe, Gd in H₂O, Xe, Te and Gd in liquid scintillator, and to achieve lower radiogenic backgrounds from the detector components, via target purification, material radioassay, and cryogenic distillation to change isotopic or atomic content.

### DRDT 2.4 - Realise liquid detector technologies scalable for integration in large systems.

Dedicated developments should achieve applications of the previous DRDTs in future detectors ten to a hundred times larger compared to the state of the art, and allow coping with increased noise from detectors with sensor areas reaching 10, 100 and ultimately 1000 m². This will have to proceed while addressing the step change in complexity, with decade-long construction, in underground or undersea environments, with handling of heat load, value engineering and industrial production.

Current noble liquid detectors are at the one to a hundred tonne scale of active mass, including DEAP-3600, ICARUS, MicroBooNE, ProtoDUNE (LAr), LZ, PandaX, XENONnT, XMASS (LXe) respectively. For the near future, detectors under construction include $O(100)$ tonne scale LAr TPCs for dark matter in DarkSide-20k and $O(10)$ kT-scale for neutrino physics in DUNE.

Broad R&D activities are being carried out on the basic properties of the liquid target media, discussed in Section 2.3.1. In addition to tailored doping components, work includes modifying the light emission spectrum and intensity, fast and ultra-fast light components, transparency, radiopurity, and developing novel light detection devices. R&D challenges include drifting charge over large distances in high-purity conditions, the electric rigidity of purified liquid nobles, and removal of electronegative compounds.

R&D directions associated with the realisation of alternative advanced ionisation detection methods (e.g. pixels replacing conventional wire planes), and the integrated collection of (possibly directional) light, over wavelengths ranging from NIR to UV and VUV, as well as charge, are discussed in Section 2.3.2.

R&D challenges associated with this massive scale up in size and complexity from current to near-future detectors is discussed in Section 2.3.3. In addition to widely-distributed small-scale R&D, the CERN Neutrino Platform is a major test facility that
has underpinned development and demonstration at the massive scale required for LAr
detectors.

R&D on new photodetectors is in common across many applications of liquid detectors, as described in Section 2.3.4, driven by the increasing size and complexity of the detectors and the need of a more “multifaceted” detection of the physics signals.

The key R&D aspects for water Čerenkov experiments, at the 55 kilotonne scale in Super-Kamiokande and the near-future Hyper-Kamiokande (Hyper-K) above the 200 kilotonne scale, are described in Section 2.3.5. Many of the ideas are in common with the liquid scintillator approach employed by experiments such as JUNO, SNO+, Theia and TAO. Technological synergies include target doping (e.g. Gd doping of water, quantum dots doping of scintillators), and the simultaneous detection of Čerenkov and scintillation light.

Synergies with other areas of high energy physics concern the physical properties of the media, for example the use of LAr calorimeters at high-energy colliders (Chapter 6); the detection technologies for charge (in Chapter 1 and Chapter 3) and light (in Chapter 4) produced by particle energy deposition in the target media; and the readout strategies for these signals, using electronics (Chapter 7) as well as quantum technologies (Chapter 5).

## 2.2 Main drivers from the facilities

Liquid detector development is driven by diverse science areas, of which some are linked to large common facilities, while others are pursued by groupings of individual experiments aiming at a coherent physics sensitivity. The main areas are categorised as: the DUNE programme; the Hyper-K programme; neutrino near detectors; neutrino telescopes; multi-tonne scale dark matter detectors; light dark matter detectors (0.001-10 kg scale); tonne-scale $0\nu\beta\beta$ experiments; and, low-energy scintillator neutrino detectors.

The R&D needs for these areas are categorised schematically as a function of time in Figure 2.2. The associations of DRDTs and R&D tasks are shown in Table 2.1. Note that, as discussed above, it is not practical to extend the horizon beyond 2035 although longer term facilities requiring R&D in liquid detectors should certainly be anticipated.

Long baseline accelerator-neutrino programmes are directly related to accelerator facilities (the FNAL PIP-II accelerator for the neutrino beam to LBNF, JPARC for the T2K and Hyper-Kamiokande neutrino beams and CERN for test beams). The physics drivers for these facilities are the discovery of CP violation in the neutrino sector through oscillation experiments, and the precision measurement of the PMNS matrix parameters. These large neutrino physics facilities also study solar and supernova neutrinos and proton decay; R&D addressing these topics is driven by the detector capabilities needed for new discoveries.

Near detector complexes for these long-baseline programmes measure the un-oscillated neutrino beam fluxes vs. energy and flavour, and provide constraints on neutrino interaction uncertainties, both essential to extract the neutrino oscillation parameters. Near detector complexes often combine several detector technologies, including water
<figure><img src="image_14.png" /><figcaption>Figure 2.2: Schematic timeline of categories of experiments employing liquid targets 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>

<table><caption>Table 2.1: Mapping of R&D tasks shown in the rows of Figure 2.2 (the “Detector Readiness Matrix”) to the Detector R&D Research Themes (DRDTs) for liquid detectors.</caption><thead><tr><th>DRDT</th><th>DRM (Figure 2.2) Row Numbering</th></tr></thead><tbody><tr><td>2.1 Readout Development</td><td>2.1.1. Higher Energy Resolution<br>2.1.2. Lower Energy Threshold<br>2.1.3. Expand wavelength sensitivity</td></tr><tr><td>2.2 Measurement Strategy</td><td>2.2.1. Fine granularity<br>2.2.2. dE/dx (combine modalities: charge, light, heat, acoustic)</td></tr><tr><td>2.3 Target Properties</td><td>2.3.1. Liquid doping and purification<br>2.3.2. High radiopurity</td></tr><tr><td>2.4 Scaling Up Challenges</td><td>2.4.1. Detector services &amp; integration<br>2.4.2. Large arrays (sensors)<br>2.4.3. Low power<br>2.4.4. Noise &amp; background mitigation</td></tr></tbody></table>
Čerenkov, LAr TPCs instrumented with pixels, high-pressure gaseous Ar TPCs, muon range detectors, electromagnetic calorimeters, etc. These near detector suites aim to characterise completely the final state particles produced in neutrino interactions in the GeV energy range; in addition they offer attractive environments to search for new physics processes.

Dark matter experiments are following two main avenues: (i) large increase in detector scale, to reach the so-called solar neutrino floor for dark matter candidates in the > 1 GeV mass regime, and (ii) new technology directions aiming to study particle dark matter down to the 1 eV scale or to measure new observables, such as direction sensitivity. Models addressed by (i) include not only WIMP dark matter but also high mass asymmetric dark matter, composite states, strongly-interacting massive particles, etc. The mass range addressed by (ii) includes candidates from a broad range of models that invoke either the freeze-in mechanism or non-thermal production, and feebly interacting massive particle candidates (FIMPs). R&D needs for dark matter searches addressing the wave-like dark matter candidate regime, below 1 eV mass, are addressed in Chapter 5.

Both high-mass and low-mass dark matter avenues require substantial technology development in readouts to reach very large areas, to expand wavelength sensitivity of photosensors into the VUV, to scale up target cryogenics and purification systems, to reach ultra-low levels of radioactivity, and to develop new readout technologies, such as quantum sensors, to measure the energy partition components down to the meV energy range in e.g. liquid He detectors.

Double beta decay experiments are driven by the stringent requirements on the background levels, by the need for excellent energy resolution and large increase in detector scale. Detector technologies employ diverse strategies, from liquid Xe or high-pressure Xe gas TPCs to cryogenic bolometers or quantum dots.

## 2.3 Key technologies

Enabling technologies for next generation liquid detectors have been identified along five main themes, that address either the inherent properties of the liquids themselves or the liquid handling and supporting infrastructures. For low-background experiments, development of supporting infrastructure additionally enables advances in radio-purity of detector materials, including liquid targets.

The global timescale for R&D developments, as illustrated in Figure 2.2, is relatively short compared to e.g. colliding beam experiments. The focused, near-term R&D described here is on a time scale of 5-10 years, and typically associated with existing experimental programmes. On longer timescales, transformative “blue-sky” ideas are being explored to enable new directions for future experiments.

The crucial technology developments needed to enable the next generation experiments in each of the five R&D themes are identified below. In addition, common interests and developments with HEP experiments, and potential collaborations with industrial partners are highlighted.
### 2.3.1 Liquid Properties of Noble Liquids

The intrinsic properties of liquefied noble gases combine high scintillation yield, high ionisation yield and long electron lifetime, i.e. the fact that the ionisation electrons released remain free to drift across long distances. The possibility of extracting electrons to the gas phase enables reaching very low deposited energy thresholds (DRDT 2.1), since the ionisation signal can be amplified through secondary scintillation or avalanche mechanisms. The key challenges to address the detector R&D themes (DRDTs) above are the following.

#### Microphysics of liquids

Improved understanding of the liquid microphysics will offer significant advances in detector resolution. For example, exploiting all available signal modalities in noble liquids (charge, light and heat) (DRTD 2.2), could enhance energy resolution and enable lower energy thresholds (DRDT 2.1). The use of dopants to modify the light emission properties is an active area of research, aimed at shifting the intrinsic VUV scintillation into the visible, where photodetectors have higher quantum efficiency. This can enable higher light collection (DRDT 2.1 and DRDT 2.2); building a strong understanding of how the scintillation wavelength spectrum changes when adding dopants to the liquid is needed. Currently, there is a high reliance on signal simulations to inform detector designs and understand physics performance; data-driven models are needed to reduce systematic uncertainties. A coordinated strategy of liquid properties measurements would allow more coherent progress and efficient collaboration within the community.

#### Light emission/detection

The intrinsic VUV scintillation in LAr (at 128 nm) and LXe (at 178 nm) presents a challenge for efficient light detection. A range of approaches beyond traditional photomultiplier tubes has been proposed to address this, from developing novel photon detectors, including aiming at VUV-sensitivity (DRDT 2.1, described in Section 2.3.4), to using wavelength shifters to modifying the scintillation light spectrum by introducing dopants in the liquid (DRDT 2.3). Other ideas for enhanced light collection exploit sub-dominant emissions, e.g. using the near infra-red light component in LAr to enhance signal discrimination factors (DRDT 2.1 and DRDT 2.2), and the possibility that LXe scintillation light patterns may offer some information on the directionality and the timing of the events via *superradiance*.

#### High voltages

The scale increase of noble liquid experiments - and more notably dual phase TPCs - is the high voltage needed to establish electron drift over long distances up to ~3 m and aiming as large as the 10 m scale. This necessitates power supplies that can deliver higher voltages (DRDT 2.4), new designs for the voltage deliveries (high voltage feedthroughs; also DRDT 2.4) and a much more detailed understanding of the dielectric properties of noble liquids under different conditions (DRDT 2.3). Amplification of a supplied voltage inside the cryostat as in Neutron EDM experiments is an interesting possibility under investigation. Development of new high-voltage schemes offers ample opportunity for collaboration with industry. Detailed models of light emission from high voltage
breakdown are also important for detector design.

### Calibration

To enhance physics capabilities, precise detector calibration methods are needed (DRDT 2.1 and DRDT 2.2). New methods are needed to ensure calibration capabilities at larger scales, particularly throughout large volumes. The use of intrinsic or added elements, such as $^{39}$Ar or $^{83m}$Kr, can be attractive for large-scale calibrations. Nuclear recoil calibration is challenging for large detectors and finding new isotopes and composites with spontaneous neutron emission is an important goal.

### 2.3.2 Charge Collection in Noble Liquids

Electric charges in noble liquids are generated by ionisation induced by charged particles traversing the medium. The most relevant quantity is the charge density, corresponding for 2.1 (4.0) MeV/cm to about 9000 (26000) electrons/cm for LAr (LXe). Most of this primary charge undergoes recombination. For practical detector implementations, e.g. a 3 mm pitch wire charge collection anode and a 1 kV/cm drifting field, the overall acquired charge corresponds to nearly 20000 electrons for LAr and over 50000 for LXe. The ionisation electrons drift towards the anode at a velocity in the range of about 2 mm/$\mu$s for a 1 kV/cm electric field. The electron distribution during the drift experiences charge diffusion, which ultimately impacts on the design granularity of the collecting anode device. The key challenges to address the detector R&D themes (DRDTs) above are the following.

#### Liquid impurities

Electronegative compounds reduce the net collected charge in an exponential way. The mean electron lifetimes for argon and xenon lie in the range of 1-10 ms for a suitably purified liquid at the level of 1 ppb impurity concentration. Modern technologies allow going well below such a value, strongly reducing the impact of this parameter on the performance of the detectors (Section 2.3.3).

#### Charge amplification

Charge amplification via several methods has been attempted and eventually exploited to acquire a more robust signal (DRDT 2.1 and DRDT 2.2), which eases requirements on the readout front-end electronics. Charge amplification of up to several orders of magnitude can be attained in the LAr gas phase or in LXe double phase devices [Ch2-2] via electroluminescence. Reaching amplification in the liquid itself is more challenging. A promising technique envisions doping LAr with small percentages of LXe, allowing charge amplification by a factor of a hundred or more [Ch2-3] (DRDT 2.3). Other promising R&D avenues include exploitation of localised electroluminescence, which is under investigation by DARWIN.

#### Charge readout structures

The “standard” TPC configuration employs planes of wires (with a pitch determined, as discussed above, by charge diffusion) possibly placed in different orientations to allow X-Y plane coordinate reconstruction, while the Z is determined by the measurement of the drift time, assuming accurate knowledge of the drifting field map. The use of wire planes
was brought to maturity by the ICARUS collaboration [Ch2-4] and successfully applied to neutrino physics by the MicroBooNE neutrino experiment at FNAL [Ch2-5]. Recent R&D conducted by the ArgonCube collaboration [Ch2-6] has led to the alternative use of pixel electrodes on PCBs, to directly measure an X-Y coordinate point, virtually eliminating the risk of reconstruction ambiguities, particularly relevant for “high luminosity” applications, such as is the case for a TPC close to a high-intensity neutrino source (as the near detector).

Additional related R&D directions address the stability of the charge amplification (if applied), the high-granularity charge-electroluminescence imaging (mostly in relation to cost and complexity) and, possibly, an integrated, simultaneous readout of light and ionisation charge.

### Cryogenic front-end electronics

Electronics are discussed in detail in Chapter 8. Specific to liquid detectors, projective event reconstruction implies a number of readout channels scaling with the number of wires, which can be as long as several meters, sufficient to cover large surfaces even for large-volume, surface-instrumented detectors. Challenges for the cryogenic front-end electronics include heat dissipation and the need of *in-situ* zero suppression to make manageable handling of the detector readout. Cryogenic optical transmission of signals is foreseen in DUNE and DarkSide-20k, which requires transmission driver and receiver development; this topic could benefit strongly from industrial engagement. Pixel schemes additionally include the requirement of independent front-end channel for each individual pixel, likely operating in cold (even immersed in the cryogenic liquid) with a potentially destructive dissipation heat.

Pixel electronics are being investigated and developed as a priority R&D direction for the five-year R&D timescale by several groups. The LArPiX electronics [Ch2-7] developed in the framework of ArgonCube has proven to be an excellent solution, allowing the operation of a series of increasing size detector prototypes and the full design of the LArTPC of the DUNE near detector [Ch2-8]. Other ideas in development include the option of time-to-charge conversion electronics [Ch2-9], or the integrated CCD readout approach [Ch2-10]. The handling of high bandwidths, the improvement of the S/N ratio, and the reduction of cost/channel will be crucial for envisioning pixel readout for very large-scale applications, such as those mentioned for DUNE.

### 2.3.3 Purification, Cryogenics, Infrastructure and Integration for No-ble Liquids

The large scale of next-generation detectors poses a unique challenge to the development of clean and radio-pure targets.

#### Purification

To accommodate the long drift distances foreseen by next-generation experiments employing TPCs (DRDT 2.4), an electron lifetime of > 1 ms per meter of drift length is required. This is achieved by removal of $O_2$ and $H_2O$ impurities, usually with standard commercial SAES hot getters, while alternatives are investigated. A disadvantage of
the use of hot gas-getters, is a limited system throughput. A new development in this field that is currently under investigation by the XENON collaboration is the removal of electronegative impurities directly from the LXe, which has the advantage of potentially allowing for much higher throughput of the purification system. Further investigation into the optimal getter materials, the optimal amount of getter material, getter regeneration strategies are a benefit to the full community.

## Target radiopurity

Target radiopurity of noble liquid targets is primarily of concern for the low-background experiments (DRDT 2.3 and DRDT 2.4). Requirements are diverse and specific, depending on the target. For LAr or LXe dark matter experiments, near absence of impurities like $^{39}$Ar, $^{222}$Rn, $^{85}$Kr is crucial, and prevention and purification of unwanted radioactive isotopes is an important field of R&D. In the case of LAr, sources of underground argon are used and in addition large-scale cryogenic distillation efforts for argon isotope are currently under development within the ARIA project [Ch2-11]. For LXe experiments suppression of events caused by beta or gamma radiation is even more important, due to lack of effective pulse shape discrimination. Through cryogenic distillation $^{85}$Kr can be removed to the sub-ppt level, which is sufficient for next-generation dark matter experiments. The focus for R&D is now on removal of $^{222}$Rn. As radon continuously emanates from all detector surface, continuous cryogenic distillation is needed, as is now pioneered for the XENONnT experiment.

## Cryogenics

Removal of electronegative impurities and establishing sufficiently radio-pure targets (DRDT 2.3 and DRDT 2.4) come at a cost of increasingly complex cryogenic systems (DRDT 2.4). Besides the cooling itself these systems need to facilitate circulation of LAr or LXe to maintain the cleanliness of the liquid targets. R&D into thermodynamic solutions necessitates development of ultra-clean compressors and heat exchangers. Some of the key supporting technologies, like clean hermetically sealed and magnetically driven gas/liquid pumps and vacuum insulation solutions find wider applications. The technologies needs are similar to those of particle accelerators, and detector cooling solutions in high energy physics experiments. This is potentially an area of R&D where the liquid detector community finds a natural synergy with the accelerator-based groups. For different reasons, both communities need clean, non-polluting components in their respective cryogenic and cooling systems. Within the rare event search community –and primarily for experiments using LXe– additional R&D is required to ensure low rates of $^{222}$Rn emanation.

Next generation experiments will make the transition from small scale to industrial scale enterprises, requiring high level professional system engineering of not only the core cryogenics systems, but also for auxiliary systems (DRDT 2.4). Extremely pure LAr and LXe are forming a large fraction of the capital cost of rare event searches. LAr and LXe storage components need to be developed for multi-ton experiments and beyond. These developments need seamless integration into the cryogenic environment of the experiment, employing rapid noble liquid recovery systems in case of emergency. Development of the hardware safety systems can benefit from the expertise in accelerator
and high-energy physics experiments.

### Radioassay

Assessment and control of radioactive background in the construction materials and components can make or break a low-background experiment (DRDT 2.3 and DRDT 2.4). Construction of such experiments is preceded by an extensive screening campaign, usually done in facilities in the low-background environment of an underground lab. In addition, highly specialised radon emanation measurements are performed. An area of R&D is in mitigation of emanation backgrounds by investigating material selection procedures and options for surface treatment in a systematic way. For next generation experiments, ultra-sensitive trace analysis capabilities need to be developed for a much larger scale; shared community resources for radioassay would be a clear benefit to both the LAr and LXe communities.

### 2.3.4 Light Collection in Noble and Other Liquids

Photon detectors used for light collection in noble liquids, water and organic scintillator detectors span PMTs, APDs, Hybrid PMTs, analogue and digital SiPMs.

The main challenges facing near to long-term future experiments are photosensor coverage over huge surfaces (DRDT 2.4), in extreme environments (e.g. underground, undersea and/or at cryogenic temperatures) and with single photon sensitivity (DRDT 2.1 and DRDT 2.2). Long term operation and stability of light collection and sensing technology are crucial as experiments may operate for lifetimes of 10 years or more between detector accesses. The transmission of signals from light collection/sensing technologies presents challenges in numbers of readout channels as well as data volume, since both ns and µs time scales are relevant given the fast and slow emission time constants in noble liquids, and precision measurement of the emission timing distribution is used for particle identification, e.g. [Ch2-12]. This naturally leads towards greater integration within photosensors, for example moving from discrete electronics chains of charge-sensitive preamplifiers or transimpedance amplifiers [Ch2-13], through shaping and ADC/TDC towards fully-integrated photon to digital conversion using embedded digitisation and digital signal processing [Ch2-14]. Radiopurity of light collection, sensing, front end electronics and signal transmission technologies is an essential requirement for dark matter experiments, and also important to lower threshold and backgrounds at low energy in neutrino experiments. The major R&D directions to address these challenges are the following.

#### Light Collection

Strategies employed to improve light collection efficiency (DRDT 2.1 and DRDT 2.4) include deployment of wavelength-shifting thin films (e.g. TPB, PEN); coating passive detector surfaces with high-reflectivity thin films (such as ESR) or materials (like Teflon); trapping light using dichroic filters to increase the probability that a photon is converted in a smaller photo-sensitive area (e.g. the ARAPUCA concept) [Ch2-15]; and, for silicon detectors, coatings and surface treatment optimisation to decrease the reflectivity at the silicon surface [Ch2-16]. R&D to maximise light collection includes developing large
<table><caption>Table 2.2: Area and type of photon detector arrays in use or under consideration in near- to far-future experiments.</caption><thead><tr><th>Experiment</th><th>Type</th><th>Photon detector</th><th>Area (m²)</th></tr></thead><tbody><tr><td>nEXO</td><td>LXe</td><td>SiPMs (FBK [Ch2-18], Hamamatsu [Ch2-19]), digital 3D-SiPM</td><td>5</td></tr><tr><td>DARWIN</td><td>LXe</td><td>PMTs, SiPMs or Hybrids (SIGHT, ABALONE)</td><td>8</td></tr><tr><td>TAO</td><td>LSci</td><td>FBK SiPMs</td><td>10</td></tr><tr><td>DarkSide-20k</td><td>LAr</td><td>SiPMs (FBK NUV-HD triple-dopant)</td><td>30</td></tr><tr><td>ARGO</td><td>LAr</td><td>SiPM is baseline option</td><td>200</td></tr><tr><td>DUNE</td><td>LAr</td><td>Light guide or trap + SiPM</td><td>10-1000</td></tr></tbody></table>

scale lamination of 25-50 µm thick polyethylene naphthalate films (PEN) together with enhanced specular reflector (ESR) films; engineering wavelength-shifting bars on PMMA substrate, for optimal matching at LAr temperature (87K); and alternatives such as doping of LAr targets with ppm-1000 pp admixtures of Xe or nanoparticles are under study to shift the VUV emission into the near UV or visible range before a photon reaches the photosensor [Ch2-17].

## Photosensitive Area

The active area of near-future experiments is at the $O(10)$ m² scale, whilst further future experiments foresee 100-1000 m² active areas, summarised in Table 2.2 (DRDT 2.4). SiPMs are widely used in large-area cryogenic applications. Beyond improvement in quantum efficiency, the main R&D efforts focus on reducing dark current rate, currently at the level of 0.01 Hz/mm² [Ch2-20]; reducing channel count through summing SiPM signals in arrays; increasing SNR through front-end electronics design to mitigate the large output capacitance of such arrays; and, timing resolution improvement, most relevant for TOF-PET and TOF-CT. Non-cryogenic experiments are similarly developing detectors composed of arrays of photosensors, such as DOMs in km³ water/ice detectors, made of 3” mPMTs with $4\pi$ angular acceptance, or $2\pi$ as planned in Hyper-K [Ch2-21]. Liquid scintillator detectors plan $4\pi$ SiPM coverage operated at -50°C in JUNO-TAO [Ch2-22], or $4\pi$ ultra-fast LAPPD coverage in the Theia WbLS detector [Ch2-23].

## Light detection

New strategies under development to enhance photon detection efficiency include increasing the sensitivity of silicon photosensors in the VUV range and NIR range [Ch2-20], through silicon doping and surface treatment (passivation and ion implantation), and pixel design (e.g. back-side illuminated (BSI) single-photon avalanche diodes [Ch2-24]); increasing the light collection with metalenses [Ch2-25], spectral photon sorting for water Čerenkov and liquid scintillator detectors [Ch2-26]; using color-sensitive sensors (dichroicons) [Ch2-27], and Winston cones together with dichroic filters; photocathode material optimisation for PMTs; and, working towards 10 ps timing resolution for temporal separation of Čerenkov and scintillation light. Light detection R&D in liquid
detectors can also find wider applications in medical imaging, e.g. for PET developments using LAr and LXe (DRDT 2.1, DRDT 2.2 and DRDT 2.3).

### Integration

As experiments grow in scale, reduction in number of readout channels and cost per channel become increasingly important (DRDT 2.4). Associated R&D includes reducing the DAQ channel cost through digital signal processing within the photosensor itself.

R&D on the 5-year horizon for greater integration include ASIC front-end electronics for PMTs, developing monolithic APD devices, and 3D vertical integration with photon sensing and signal processing electronics within a single silicon device. Specific to SiPMs, strategies under development for greater integration, moving from analog to digital SiPMs, include dedicated ASIC design; 3D-vertical integration R&D; and development of lower-power, larger-area and lower-radioactivity photodetection modules [Ch2-28]. A key challenge here is the need for cooperation with commercial foundries, as well as significant funding and time to develop and package a new device for qualification. Progress in this area needs common sensor or detector development across projects.

For the longer-term future, even greater integration of charge and light readout systems is under development (DRDT 2.1 and DRDT 2.2), via ideas like: making a large fraction of a TPC anode plane surface photo-sensitive; material engineering for pixel readout to collect both charge and UV photons; bubble-assisted liquid hole multipliers in LXe and LAr [Ch2-29]; and fully optical readout of THGEMs using SiPMs, CCDs and Timepix-based cameras [Ch2-10]. "Blue-sky" ideas for fully optical imaging of scintillation light include using segmented photodetectors coupled to coded aperture masks or UV-transparent lenses for 4D event reconstruction; and employing opaque scintillator together with wavelength-shifting fibres read by SiPMs to make calorimeter-TPC drifting light instead of charge [Ch2-30].

### 2.3.5 Liquid Scintillator and Water Detectors

Large detectors such as Hyper-K [Ch2-31] or JUNO [Ch2-32] take advantage of Čerenkov signals in ultra-pure water (WC) or liquid scintillator (LS). Metal-loaded LS is used in smaller (kT) targets as in the case of SuperK [Ch2-33] and SNO+ [Ch2-34] to search for neutrinoless double beta decay. These techniques are also used with or without Gd-doping in veto detectors as for XENONnT [Ch2-35], LZ [Ch2-36] DARWIN [Ch2-37] and SHiP [Ch2-38]. New ideas that go beyond the current state of the art, entering the demonstration phase, include hybrid Čerenkov/scintillation detectors as in Theia [Ch2-23]; cold LS with SiPM read-out as in TAO [Ch2-22]; opaque LS with fibre read-out as in LiquidO [Ch2-39]; LS doped with quantum dots for $0\nu\beta\beta$ searches as in NuDot [Ch2-40] and LS which becomes wax-like at room temperature.

While the requirements and needs are different depending on the physics goals, the main R&D topics are the following.

#### Radio-purity

A common requirement for all WC and LS detectors is to achieve sufficiently good radio-purity levels (DRDT 2.3), which is very demanding if the LS is the target of
an ultra low background experiment as for $0\nu\beta\beta$ decay, such as the current leading experiment KamLAND-ZEN [Ch2-41]. The state-of-the-art for water is $10^{-15}$g/g U/Th. Any improvement beyond this level would benefit not only the water-based detectors but also LS detectors since they use water to purify the liquid scintillator.

## Liquid composition

Liquid composition R&D aims to improve the optical properties of the liquid, the photon detection efficiency, and, closely related, to optimise the coupling of the light emitted to the photosensors (DRDT 2.3). To tune the photon emission spectrum, or wavelength in the liquid, there are proposals of water-based LS with organic micelles suspended in water, or doping with quantum dots. The light emission properties of the liquid are also under investigation to tune the fluorescence times and spectrum. This is particularly relevant for applications in which background is reduced using techniques of pulse shape discrimination, which can be improved by an appropriate choice of the liquid components.

R&D to improve event reconstruction addresses the development of highly transparent target media. This can be achieved either by a Water-based Liquid Scintillator (WbLS) [Ch2-42] or purification of the optically transparent scintillator components. WbLS detectors also offer an excellent capability for metal-loading of the scintillator material. While gadolinium-loading provides enhanced neutron detection, doping with tellurium and other candidate isotopes offers an avenue towards multi-ton $0\nu\beta\beta$ experiments.

The characterisation of the light properties is a key aspect. For instance, measuring the optical attenuation length of these purified liquids (≥ 30 m) requires long-arm setups. Alternatives to these systems based on interferometry of intensity-modulated light pulses in reflective cavities are proposed.

## Liquid doping/stratification

R&D for SNO+ has demonstrated the feasibility of loading LS targets with $0\nu\beta\beta$ sensitive dopants; R&D aims to reach loading levels of several percent with increased light output. Another direction for high loading is to operate the LS at low temperatures. R&D on the use of quantum dots is also underway. Related to stratification, there are proposals to explore alternative geometries for large scale LS detectors, for instance the use of dissimilar stratified liquids to separate scintillating and non-scintillating regions without the need of a physical barrier (such as an acrylic or nylon containment vessel). This would also lower detector radioactivity.

## Low temperatures

Another promising technique is related to the use of cold organic scintillators. Most liquid scintillators can be cooled down to -50°C which will increase the light yield and reduce the dark noise of SiPMs to an acceptable level. Since thousands of photoelectrons per MeV are collected, it is expected to achieve an energy resolution < 2% at 1 MeV. For that, it is important to preserve the liquid transparency and chemical stability at these low temperatures.
### Opacity

R&D is underway to exploit the opacity, rather than transparency, of the light medium. In an opaque LS detector, light emitted via scintillation remains locally, stochastically confined via an extreme level of light elastic-scattering, typically dominated by Mie and Rayleigh scatterings. The scattering mean-free-path is thus reduced by up to four orders of magnitude (up to order mm) relative to today's technology. In this heavily opaque medium, light displaces barely a few cm's, hence photons must be collected locally via a tight (order $\ge$ 1 cm) lattice of wavelength-shifting fibres arranged in an axial orientation. This technique aims for topological particle identification in the MeV range, enabling electron-positron discrimination.

### Hybrid detectors for event reconstruction

A medium able to separate Čerenkov from scintillation light signals using hybrid optical detectors offers improved capability for vertex reconstruction and background discrimination (DRDT 2.1 and DRDT 2.2). Čerenkov and scintillation signals can be separated based on the relative timing and/or wavelength of the arriving photons as well as the specific topology of Čerenkov rings. Separation by timing is aided by fast (sub-nanosecond) photo sensors (e.g. LAPPDs) or slow scintillating fluor [Ch2-43], wavelength discrimination by colour-sensitive sensors (e.g. dichroicons).

Complex event topologies, including hybrid Čerenkov and scintillation signals, demand the development of new reconstruction techniques including machine-learning and topological reconstruction. The main R&D goals here are the refinement of the current techniques and the realisation of the new concepts at the tonne scale (DRDT 2.4).

## 2.4 Observations

Expert input at the ECFA Plenary Input Sessions (see Appendix C) made the following observations on community, cross-cutting issues and interactions with neighbouring fields and industry.

### 2.4.1 Neutrino Oscillation and Astro-particle Neutrino Detectors

#### Community

The long-baseline neutrino community is growing, costs are increasing, and most funding agencies will be involved in both programmes of LHC and neutrino physics. Global planning and coordination is required. R&D efforts are already playing a role in this. There is significant emphasis on photon detectors, PCB-type readout technologies and related front-end microelectronics to increase detector resolution, for both near and far detectors (like pixels, fine grained detectors, magnetised trackers, wide-angle TPCs, high-pressure gas TPCs) as the field moves from statistical to systematic uncertainty dominated measurements.

For astro-particle neutrino experiments, a global neutrino network (GNN) is envisioned by the community, with greater interaction with particle physics centres, in particular CERN.
### Cross-cutting issues

Very complex detector integration and installation plans represent a step-change for the long-baseline neutrino field, with five years of components manufacturing across ~30 countries and almost one decade of construction. In both LAr and water Čerenkov programmes the complexity of the detectors in increasing by an order of magnitude, with much bigger far detectors, more diversified near detectors, and more dependencies from industrial projects / initiatives. Proper engineering approaches, both structural and electrical, are becoming more important, in particular as concerns integration and installation underground. This is reflected in DRDT 2.4.

The community has adopted the concept of test beams to test and qualify the performance of what has been built, e.g. the characterisation of ProtoDUNE at CERN and the planned Hyper-K Water Čerenkov Test Experiment.

For astro-particle neutrino experiments, there are overlaps with earth and sea sciences, with potentially relevant measurements for studies of climate change, marine life (e.g. noise pollution), and tsunami warnings. Cross-cutting R&D is exploring more pixelisation of photon detectors for increased resolution, as well as new technologies exploiting radio and acoustic detection of neutrinos.

### Interactions

Interactions with industry for the long-baseline programme include membrane cryostat technology. This technology from the liquified natural gas shipping industry has been adopted for very large cryostat vessels (~13,000 m³). It has been deployed in ProtoDUNE, and in the near term is being deployed in SBND, DarkSide-20k, and DUNE.

For astro-particle neutrino experiments interactions include commercial cabling and civil engineering of $O(km^3)$ detectors, which is a major challenge, e.g. in deployment of photosensor arrays from ice surface, or surface vessels (at sea). There is potential for involvement of offshore industry in a future cosmic ray detector on the sea floor.

## 2.4.2 Dark Matter and $0\nu\beta\beta$ Experiments

### Community

Simple extrapolations of existing technologies to larger scales are not sufficient to meet the physics ambition of next-generation dark matter and $0\nu\beta\beta$ detectors. Much of this community is in transition from small scale experiments towards large HEP size experiments, like DARWIN and ARGO.

There is a proliferation of creativity in detection schemes (new sensors, new detector ideas combining heat and ionisation partitions, integration with quantum technologies for readout etc.) in this community, with time scales that can be shorter than most areas of HEP, reflected by DRDT 2.1 and DRDT 2.2. Both incremental and transformative R&D efforts are essential to make progress in dark matter, because the physics parameter space is so large. Shared facilities across experiments are needed, for example for low-background screening and cryogenics facilities to test large scale equipment.
## Cross-cutting issues

The main technological challenges across different technologies is upscaling whilst reaching ultra-low backgrounds (DRDT 2.3 and DRDT 2.4). Material science is increasingly important (e.g. surface treatment to reduce backgrounds, cryogenic distillation on huge scale, new insulator or semiconductor target materials for bolometer development) reaching across fields (chemistry, materials science, etc.) Many new R&D efforts aim towards measuring recoil energies down to meV scales, to extend the sensitivity to MeV dark matter masses and below. Technological innovations benefit other fields (e.g. cryogenic distillation and medical isotopes for MRI). The need to keep the engineering/technical expertise at universities for efficient R&D progress is high.

## Interactions

Interactions with industry include photosensor development for lower radioactivity (PMTs) and dark count rate (SiPMs); vibration-isolated, cryogen-free dilution refrigerators, large cryogenic systems and cryogenic distillation.

## 2.5 Recommendations

Motivated by the unmet needs for short, medium and long-term applications of liquid detectors, the following are recommended, addressing both technology and organisation.

* Near-term R&D should facilitate x10 scale increase of dark matter and neutrino experiments over the next decade (DRDT 2.4). Depending on the physics drivers and detector specifics, this requires radiopurity reductions of 10-1000, an *O*(10) improvement in purification of targets, improvements of TPC pixel charge electronics to reduce heat dissipation and overall cost reduction. Fostering industry-academia collaboration from an early stage is an important requirement at this scale.

* Structural funding instruments are needed to stimulate coherent R&D which can be beyond the reach of relatively small individual experiments, e.g. a programme like AIDA for rare-event search detector development. This aligns with the 2021 APPEC dark matter strategy report recommendations [Ch2-44].

* Community-building is required to develop both science and technology with adjacent fields in:

- chemistry: to advance understanding/advantages of liquid doping, isotope extraction e.g. barium tagging, cryogenic distillation (DRDT 2.3 and DRDT 2.4);

- quantum technologies: readout development and also target doping, improvements in quantum efficiency (also relates with materials science) and energy resolution (DRDT 2.1, DRDT 2.2 and DRDT 2.3);

- optics and photonics: QE improvements, wavelength-shifting, meta-materials developments (DRDT 2.1, DRDT 2.2 and DRDT 2.3);
- engineering/materials science: liquid purification and radiopurity (DRDT 2.3 and DRDT 2.4);

- industry: semiconductors (photon detectors, VUV sensitivity, quantum dots) (DRDT 2.1); cryogenic infrastructure (large-scale cryostats from liquid gas industry, vacuum industry) (DRDT 2.4).

* Further future advances should explore how to combine detection of different modalities (DRDT 2.2), for example by:

- exploring the full light spectrum (from NIR to VUV) and establishing simultaneous Čerenkov-scintillation light detection;

- readout of electromagnetic and acoustic detection;

- establishing readout of light and charge in the same device.

* In general, it is important to highlight that both short term and long term R&D are important. In liquid detectors the present well-defined R&D horizon is $O(5)$ years; beyond this one must open the window to “blue-sky” developments, which could happen on the timescale of $O(10)$ years from now. This is important not only for technology progress but also for the healthy continuation of this research field.

## References Chapter 2

[Ch2-1] M. Agostini et al., LArGe - Active background suppression using argon scintillation for the GERDA $0\nu\beta\beta$-experiment, arXiv:1501.05762 [physics.ins-det].

[Ch2-2] A. Buzulutskov, Electroluminescence and Electron Avalanching in Two-Phase Detectors, Instruments 4 (2020) no. 2, 16, arXiv:2006.11017 [physics.ins-det].

[Ch2-3] J.G. Kim et al., Studies of electron avalanche behavior in liquid argon, IEEE Trans. Nucl. Sci. 49 (2002) no. 4, 1851-1856.

[Ch2-4] ICARUS Collaboration, S. Amerio et al., Design, construction and tests of the ICARUS T600 detector, Nucl. Instrum. Meth. A 527 (2004) 329-410.

[Ch2-5] R. Acciarri et al., Design and construction of the MicroBooNE detector, JINST 12 (2017) no. 02, P02017.

[Ch2-6] J. Asaadi et al., First Demonstration of a Pixelated Charge Readout for Single-Phase Liquid Argon Time Projection Chambers, Instruments 4 (2020) no. 1, 1.

[Ch2-7] D.A. Dwyer et al., LArPix: demonstration of low-power 3D pixelated charge readout for liquid argon time projection chambers, JINST 13 (2018) P10007.

[Ch2-8] DUNE Collaboration, A. Abed Abud et al., Deep Underground Neutrino Experiment (DUNE) Near Detector Conceptual Design Report, arXiv:2103.13910 [physics.ins-det].
[Ch2-9] The Qpix Consortium. https://indico.fnal.gov/event/21535/contributions/63303/attachments/\39668/48006/QpixConceptFEelect2.pdf.

[Ch2-10] A. Lowe et al., *Optical Readout of the ARIADNE LArTPC using a Timepix3-based Camera*, Instruments 4 (2020) no. 4, 35, arXiv:2011.02292 [physics.ins-det].

[Ch2-11] D. Agnes et al., *Separating $^{39}$Ar from $^{40}$Ar by cryogenic distillation with Aria for dark-matter searches*, Eur. Phys. J. C81 (2021) 395.

[Ch2-12] DEAP Collaboration, R. Ajaj et al., *Search for dark matter with a 231-day exposure of liquid argon using DEAP-3600 at SNOLAB*, Phys. Rev. D100 (2019) no. 2, 022004, arXiv:1902.04048 [astro-ph.CO].

[Ch2-13] D'Incecco et al., *Development of a Novel Single-Channel, 24 cm$^2$, SiPM-Based, Cryogenic Photodetector*, IEEE Trans. Nucl. Sci. 65 (2018) no. 1, 591-596.

[Ch2-14] J.-F. Pratte et al., *3D Photon-To-Digital Converter for Radiation Instrumentation: Motivation and Future Works*, Sensors 21 (2021) no. 2, 1.

[Ch2-15] A. A. Machado and E. Segreto, *ARAPUCA a new device for liquid argon scintillation light detection*, JINST 11 (2016) no. 02, C02004.

[Ch2-16] J. Heymes et al., *Comparison of Back-thinned Detector Ultraviolet Quantum Efficiency for Two Commercially Available Passivation Treatments*, in *IEEE NSS-MIC Symposium*. 2019.

[Ch2-17] M. Kuźniak and A. M. Szelc, *Wavelength Shifters for Applications in Liquid Argon Detectors*, Instruments 5 (2020) no. 1, 4, arXiv:2012.15626 [physics.ins-det].

[Ch2-18] https://www.fbk.eu/en.

[Ch2-19] https://www.hamamatsu.com.

[Ch2-20] A. Gola et al., *NUV-Sensitive Silicon Photomultiplier Technologies Developed at Fondazione Bruno Kessler*, Sensors 19 (2019) no. 2, 1.

[Ch2-21] B. Quilain et al., *Multi-PMT Modules for Hyper-Kamiokande*, JPS Conf. Proc. 27 (2019) 011017.

[Ch2-22] JUNO Collaboration, A. Abusleme et al., *TAO Conceptual Design Report: A Precision Measurement of the Reactor Antineutrino Spectrum with Sub-percent Energy Resolution*, arXiv:2005.08745 [physics.ins-det].

[Ch2-23] Theia Collaboration, M. Askins et al., *THEIA: an advanced optical neutrino detector*, Eur. Phys. J. C 80 (2020) no. 5, 416, arXiv:1911.03501 [physics.ins-det].

[Ch2-24] A. N. Otte, *Observation of VHE $\gamma$-Rays from the Vicinity of magnetized Neutron Stars and Development of new Photon-Detectors for Future Ground based $\gamma$-Ray Detectors*. PhD thesis, Munich, Tech. U., 2007.

[Ch2-25] A. A. L. Villalpando et al., *Improving the light collection efficiency of silicon photomultipliers through the use of metalenses*, JINST 15 (2020) no. 11, P11021, arXiv:2007.06678 [physics.ins-det].

[Ch2-26] T. Kaptanoglu et al., *Spectral Photon Sorting For Large-Scale Cherenkov and Scintillation Detectors*, Phys. Rev. D 101 (2020) no. 7, 072002, arXiv:1912.10333 [physics.ins-det].
[Ch2-27] C. Michel et al., *Dichroic and anti-reflective coatings for astronomical instrumentation*, Proc. SPIE Int. Soc. Opt. Eng. **10706** (2018) 107064Y.

[Ch2-28] DarkSide-20k Collaboration, C. E. Aalseth et al., *DarkSide-20k: A 20 tonne two-phase LAr TPC for direct dark matter detection at LNGS*, Eur. Phys. J. Plus **133** (2018) 131, arXiv:1707.08145 [physics.ins-det].

[Ch2-29] E. Erdal et al., *Bubble-assisted Liquid Hole Multipliers in LXe and LAr: towards “local dual-phase TPCs”*, JINST **15** (2020) no. 04, C04002.

[Ch2-30] LiquidO Collaboration, A. Cabrera, *LiquidO: First Opaque Detector for ββ Decay?*, PoS NOW2018 (2019) 028.

[Ch2-31] Hyper-Kamiokande Collaboration, K. Abe et al., *Hyper-Kamiokande Design Report*, arXiv:1805.04163 [physics.ins-det].

[Ch2-32] JUNO Collaboration, Z. Djurcic et al., *JUNO Conceptual Design Report*, arXiv:1508.07166 [physics.ins-det].

[Ch2-33] L. Marti et al., *Evaluation of gadolinium's action on water Cherenkov detector systems with EGADS*, Nucl. Instrum. Meth. A **959** (2020) 163549.

[Ch2-34] SNO Collaboration, J. Boger et al., *The Sudbury neutrino observatory*, Nucl. Instrum. Meth. A **449** (2000) 172-207, arXiv:nucl-ex/9910016 [physics.ins-det].

[Ch2-35] XENON Collaboration, E. Aprile et al., *The XENON1T Dark Matter Experiment*, Eur. Phys. J. C **77** (2017) no. 12, 881, arXiv:1708.07051 [astro-ph.IM].

[Ch2-36] D. Akerib and et al., *The LUX-ZEPLIN (LZ) experiment*, Nucl. Instrum. Meth. A **953** (2020) 163047.

[Ch2-37] DARWIN Collaboration, J. Aalbers et al., *DARWIN: towards the ultimate dark matter detector*, JCAP **11** (2016) 017, arXiv:1606.07001 [astro-ph.IM].

[Ch2-38] S. Alekhin et al., *A facility to Search for Hidden Particles at the CERN SPS: the SHiP physics case*, Rept. Prog. Phys. **79** (2016) no. 12, 124201, arXiv:1504.04855 [hep-ph].

[Ch2-39] A. Cabrera et al., *Neutrino Physics with an Opaque Detector*, arXiv:1908.02859 [physics.ins-det].

[Ch2-40] J. Gruszko, *NuDot: Double-Beta Decay with Direction Reconstruction in Liquid Scintillator*, Zenodo (2018).

[Ch2-41] KamLAND-Zen Collaboration, A. Gando et al., *Search for Majorana Neutrinos Near the Inverted Mass Hierarchy Region with KamLAND-Zen*, Phys. Rev. Lett. **117** (2016) no. 8, 082503.

[Ch2-42] J. R. Alonso et al., *Advanced Scintillator Detector Concept (ASDC): A Concept Paper on the Physics Potential of Water-Based Liquid Scintillator*, arXiv:1409.5864 [physics.ins-det].

[Ch2-43] S. D. Biller, E. J. Leming, and J. L. Paton, *Slow fluoros for effective separation of Cherenkov light in liquid scintillators*, Nucl. Instrum. Meth. A **972** (2020) 164106, arXiv:2001.10825 [physics.ins-det].

[Ch2-44] J. Billard et al., *Direct Detection of Dark Matter – APPEC Committee Report*, 2021. arXiv:2104.07634 [hep-ex].
# Chapter 3

# Solid State Detectors

## 3.1 Introduction

Solid State detectors (SSD) based on semiconductors, and in particular silicon detectors (planar pixels, planar strips, and 3D pixels), are used in almost all particle physics experiments. Since they can be easily segmented using standard photo-lithographic techniques, they can achieve superb position resolution and play a key role in measuring primary and secondary vertices and tracking charged particles. Silicon is also used as an active medium in particle flow calorimeters to associate showers with tracks and then to track showers as they develop in the calorimeter (see Chapter 6). Finally, as discussed in Chapter 4, silicon detectors are fundamental in photonics.

Revolutionary improvements of SSD performance are needed to match the requirements of future experiments. All-silicon trackers are required for future hadron colliders such as FCC-hh and are one of the most competitive options also for $e^{+}e^{-}$ Higgs factories. There are commonalities in the possible SSD technological choices since both hadrons and $e^{+}e^{-}$ colliders require low mass, low power, high-resolution trackers. Nonetheless, there are also differences since hadron colliders necessitate ultra-fast detectors, enabling 4D-tracking<fnref n="1" />, to deal with multiple interactions occurring within a bunch crossing (pile-up). Detectors at FCC-hh must also achieve unprecedented radiation hardness. The highest levels are reached in the forward calorimeters where the total ionising dose and the 1-MeV equivalent neutron fluence rise to values of 5000 MGray and $5 \times 10^{18} \text{ n}_{\text{eq}} \text{ cm}^{-2}$. Even in the innermost layer of the barrel vertex detectors the fluences approach $1 \times 10^{18} \text{ n}_{\text{eq}} \text{ cm}^{-2}$ after an integrated luminosity of $30 \text{ ab}^{-1}$.

After years of R&D, silicon sensors manufactured using mainstream CMOS imaging technologies are now being implemented in several high energy physics (HEP) experiments. CMOS MAPS (Monolithic Active Pixel Sensor) have now been installed in STAR and ALICE; they are planned for other experiments such as CBM, the LHCb tracker, and Mu3e. MAPS technologies are especially suited for applications requiring low-mass and excellent position resolution called for at electron machines.

<footnote n="1">¹Reconstructing the trajectory of a charged particle in three spatial dimensions plus time as a fourth dimension.</footnote>
Future flavour physics experiments will operate in a high-occupancy environment where event reconstruction will be very challenging. The physics programme enabled by the LHCb Upgrade II relies on an efficient and precise vertex detector with real time reconstruction of tracks from all LHC bunch crossings in the software trigger system, which also would highly benefits from having 4D-tracking. The higher occupancy expected in future running will also demand increased detector granularity for the LHCb tracker.

Reduction of material in the region close to the interaction point leading to significant improvements in tracking precision and efficiency at low transverse momentum, is critical to achieving the physics goals of Heavy Ion experiments, such as ALICE, and those planned for the EIC and particularly at future $e^+e^-$ colliders. Better position and timing resolution, and lower power consumption would also benefit the upgrades of Belle and NA62, which will occur during this decade. Devices with $\mathcal{O}(10 \text{ ps})$ timing resolution will be highly desirable for 4D-tracking reconstruction at the foreseen 1000 collision pile-up of the FCC-hh.

One aspect common to most future facilities is the requirement for the front-end electronics to perform very complex tasks, such as those required for 4D-tracking or by the transfer off-chip of very large data volumes. 3D-stacking is therefore a key technological development that needs to be included in future high-performing trackers.

Following these needs, Task Force 3 Solid State Detectors has identified the essential Detector R&D Themes (DRDT) which capture the most critical requirements.

### DRDT 3.1 - Achieve full integration of sensing and microelectronics in monolithic CMOS pixel sensors.

Developments of Monolithic Active Pixel Sensors (MAPS) should achieve very high spatial resolution and very low mass, aiming to also perform in high fluence environments. To achieve low mass in vertex and tracking detectors, thin and large area sensors will be crucial. For tracking and calorimetry applications MAPS arrays of very large areas, but reduced granularity, are required for which cost and power aspects are critical R&D drivers. Passive CMOS designs are to be explored, as a complement to standard sensors fabricated in dedicated clean room facilities, towards hybrid detector modules where the sensors is bonded to an independent ASIC circuit. Passive CMOS sensors are good candidates for calorimetry applications where position precision and lightness are not major constraints (see Chapter 6). State-of-the-art commercial CMOS imaging sensor (CIS) technology should be explored for suitability in tracking and vertex detectors.

### DRDT 3.2 - Develop solid state sensors with 4D-capabilities for tracking and calorimetry.

Understanding of the ultimate limit of precision timing in sensors, with and without internal multiplication, requires extensive research together with the developments to increase radiation tolerance and achieve 100%-fill factors. New semiconductor and technology processes with faster signal development and low noise readout properties should also be investigated.
### DRDT 3.3 - Extend capabilities of solid state sensors to operate at extreme fluences.

To evolve the design of solid state sensors to cope with extreme fluences it is essential to measure the properties of silicon and diamond sensors in the fluence range $1 \times 10^{16} \text{ n}_{\text{eq}} \text{ cm}^{-2}$ to $5 \times 10^{18} \text{ n}_{\text{eq}} \text{ cm}^{-2}$ and to develop simulation models which correspondingly include results from microscopic measurements of point and cluster defects. All technologies will need improved radiation tolerance for use at future hadron collider experiments. Exploration of alternative semiconductors and 2D-materials should already start, having as a target full functionality even after the extreme fluences present in the innermost parts of the detectors. A specific concern to be addressed is the associated activation of all the components in the detector. Exploration is desirable on alternative semiconductors and 2D-materials to further push radiation tolerance.

### DRDT 3.4 - Develop full 3D-interconnection technologies for solid state devices in particle physics.

3D-interconnection is commercially used, for instance in imaging sensors, to use the most appropriate technology process for the different functionalities of the devices. For particle physics detectors, this process would allow more compact and lighter devices with minimal power consumption. This approach also provides an alternative to the use of finer feature sizes to enable lower pitch and new digital features. An enhanced R&D effort towards building a demonstrator as a starting cornerstone is highly desirable. A demonstrator programme should be established to develop suitable silicon sensors, cost effective and reliable chip-to-wafer and/or wafer-to-wafer bonding technologies and to use these to build multi-layer prototypes with vertically stacking layers of electronics, interconnected by through-silicon vias (TSVs) and integrating silicon photonics capabilities.

The timelines for these R&D themes can be found at Figure 11.1 with explanation in the caption and associated text. R&D on DRDT 3.1, DRDT 3.2, and DRDT 3.4 is needed for multiple facilities listed in Figure 3 and Figure 4 of the Introduction, all the way through to the FCC-hh/muon collider era, as detailed below. For DRDT 3.3 there is fortunately more time to address the two orders of magnitude greater radiation hardness requirements of experiments at the FCC-hh with respect to those at HL-LHC.

## 3.2 Main drivers from the facilities

Figure 3.1 presents the Detector Readiness Matrix which summarises the requirements for future solid state detectors. The table reports on the horizontal axis the facilities while the vertical axis lists the quantities with the most demanding specifications, such as the spatial and temporal precision, power consumption, material contribution, and radiation tolerance, that must be achieved. The colour coding is explained in the caption.

Figure 3.2 complements Figure 3.1 by showing, in the same format, the required values of the parameters listed on the vertical axis as a function of time (and facility).
<figure><img src="image_15.png" /></figure>

Figure 3.1: Schematic timeline of categories of experiments employing solid state sensors 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).

<footnote n="1">1) HL-LHC Long shutdowns: LS3/LS4 2025/2031
(see https://lhc-commissioning.web.cern.ch/schedule/LHC-long-term.htm)</footnote>

<footnote n="2">2) LHCb/ATLAS/CMS consider Planar/3D sensors at the time of this document for rates and radiation tolerance. On longer term, pixelated LGADs could be considered for potentially higher timing precision.</footnote>

<footnote n="3">3) In trackers, coarser longitudinal granularities could be considered for MAPS. Thorough performance and cost comparison with passive CMOS would be needed. Pixelated LGADs could be considered for potentially higher timing precision.</footnote>

<footnote n="4">4) The size of wafers achievable can depend on technology (industrial process) with a general trend to benefits from larger areas.</footnote>

<footnote n="5">5) Ultrafast timing refers to ≤100 ps depending on technology and detector purpose.</footnote>

<footnote n="6">6) Two options exist for calorimetry: pads O(1) mm pitch with analog readout (applying to all technologies) and particle counting digital with MAPS O(50) µm pitch. LGADs could be considered for potentially higher timing precision.</footnote>

<footnote n="7">7) ToF, as compared to 4D-tracking, concerns dedicated layers for very high pile-up, beam induced background or particle identification with highest possible precision. Timing performance of sensors without amplification (MAPS, planar/3D/CMOS passive CMOS) is subject to R&D, while LGADs with amplification are at this stage expected to potentially provide higher precision.</footnote>