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# Technical Design Report for the Phase-II Upgrade of the ATLAS TDAQ System

N. Berger, A.M. Burger, O. Dartsi, M. Delmastro, L. Di Ciaccio, P.J. Falke, S. Falke, C. Goy, T. Guillemin, T. Hryn'ova, et al.

► To cite this version:

N. Berger, A.M. Burger, O. Dartsi, M. Delmastro, L. Di Ciaccio, et al. Technical Design Report for the Phase-II Upgrade of the ATLAS TDAQ System. CERN-LHCC-2017-020, CERN. 2018. hal-03990850

HAL Id: hal-03990850

https://hal.science/hal-03990850

Submitted on 15 Feb 2023

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ATLAS
EXPERIMENT

ATL-TDR-029 · LHCC-2017-020

0100101101110101 0100101101110101
0100101101110101 0100101101110101

# ATLAS
TDAQ Phase-II Upgrade

<figure><img src="image_4.png" /></figure>

Technical Design Report
<figure><img src="image_5.png" /></figure>
Technical Design Report for the Phase-II
Upgrade of the ATLAS Trigger and Data
Acquisition System

The ATLAS Collaboration

Reference: CERN-LHCC-2017-020
ATLAS-TDR-029

Created: 15 June 2018

Last modified: 15 June 2018

Prepared by: The ATLAS Collaboration
<figure><img src="image_6.png" /></figure>
Abstract

This Technical Design Report documents the plans to upgrade the ATLAS Trig-
ger and Data Acquisition system for the High Luminosity LHC (HL-LHC). The
HL-LHC is expected to start operations in the middle of 2026, to ultimately reach
a peak instantaneous luminosity of $\mathcal{L} = 7.5 \times 10^{34} \text{ cm}^{-2}\text{s}^{-1}$, corresponding to
approximately 200 inelastic proton-proton collisions per bunch crossing, and
to deliver more than ten times the integrated luminosity of the LHC Runs 1-3
combined (up to 4000 fb}^{-1}).

Meeting these requirements poses significant challenges to the Trigger and to the Data Acquisition system to fully exploit the physics potential of the HL-LHC. A baseline architecture, based on a single-level hardware trigger with a maximum rate of 1 MHz and 10 µs latency, is proposed and documented. With the help of a hardware-based tracking sub-system as co-processor, software-based reconstruction follows to achieve further rejection. Up to 10 kHz event data are sent into storage.

The Report describes in detail the physics motivations, the requirements, the
fundamental parameters, the technical design implementation, and the expec-
ted performance of the proposed upgrade. The Report also documents the
organisation of the Upgrade Project, its management structure, planning and
scheduling with a review of the major milestones, and costing information.
<figure><img src="image_7.png" /></figure>
# ATLAS Institutions

## Argentina

Departamento de Física, Universidad de Buenos Aires, Buenos Aires
Instituto de Física La Plata, Universidad Nacional de La Plata and CONICET, La Plata

## Armenia

Yerevan Physics Institute, Yerevan

## Australia

Department of Physics, University of Adelaide, Adelaide
School of Physics, University of Sydney, Sydney
School of Physics, University of Melbourne, Victoria

## Austria

Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität, Innsbruck
Fachhochschule Wiener Neustadt, Wiener Neustadt

## Azerbaijan

Institute of Physics, Azerbaijan Academy of Sciences, Baku

## Belarus

B.I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, Minsk
Research Institute for Nuclear Problems of Byelorussian State University, Minsk

## Brazil

*Brazil Cluster:* Departamento de Engenharia Elétrica, Universidade Federal de Juiz de Fora (UFJF), Juiz de Fora; Universidade Federal do Rio De Janeiro COPPE/EE/IF, Rio de Janeiro; Universidade Federal de São João del Rei (UFSJ), São João del Rei; Instituto de Física, Universidade de São Paulo, São Paulo

## Canada

Department of Physics, Simon Fraser University, Burnaby BC
Department of Physics, University of Alberta, Edmonton AB
Department of Physics, McGill University, Montreal QC
Group of Particle Physics, University of Montreal, Montreal QC
Department of Physics, Carleton University, Ottawa ON
Department of Physics, University of Toronto, Toronto ON
Department of Physics, University of British Columbia, Vancouver BC
TRIUMF, Vancouver BC; Department of Physics and Astronomy, York University, Toronto ON
Department of Physics and Astronomy, University of Victoria, Victoria BC

## CERN
European Organization for Nuclear Research, Geneva, Switzerland

## Chile

*Chile Cluster:* Departamento de Física, Pontificia Universidad Católica de Chile, Santiago; Departamento de Física, Universidad Técnica Federico Santa María, Valparaíso

## China

*China IHEP-NJU-THU Cluster:* Institute of High Energy Physics, Chinese Academy of Sciences, Beijing; Physics Department, Tsinghua University, Beijing; Department of Physics, Nanjing University, Nanjing

*China USTC-SDU-SJTU Cluster:* Department of Modern Physics and State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China, Hefei; Institute of Frontier and Interdisciplinary Science and Key Laboratory of Particle Physics and Particle Irradiation (MOE), Shandong University, Qingdao; School of Physics and Astronomy, Shanghai Jiao Tong University, KLPPAC-MoE, SKLPPC, Shanghai; Tsung-Dao Lee Institute, Shanghai

*Hong Kong Cluster:* Department of Physics, Chinese University of Hong Kong, Shatin, N.T., Hong Kong; Department of Physics, University of Hong Kong, Hong Kong; Department of Physics and Institute for Advanced Study, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

## Colombia

Centro de Investigaciones, Universidad Antonio Nariño, Bogota

## Czech Republic

Palacký University, RCPTM, Joint Laboratory of Optics, Olomouc

Charles University, Faculty of Mathematics and Physics, Prague

Czech Technical University in Prague, Prague

Institute of Physics, Academy of Sciences of the Czech Republic, Prague

## Denmark

Niels Bohr Institute, University of Copenhagen, Copenhagen

## France

LAPP, Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS/IN2P3, Annecy

LPC, Université Clermont Auvergne, CNRS/IN2P3, Clermont-Ferrand

IRFU, CEA, Université Paris-Saclay, Gif-sur-Yvette

LPSC, Université Grenoble Alpes, CNRS/IN2P3, Grenoble INP, Grenoble

CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille

LAL, Université Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, Orsay

LPNHE, Sorbonne Université, Paris Diderot Sorbonne Paris Cité, CNRS/IN2P3, Paris

## Georgia

*Georgia Cluster:* E. Andronikashvili Institute of Physics, Iv. Javakhishvili Tbilisi State University, Tbilisi; High Energy Physics Institute, Tbilisi State University, Tbilisi
## Germany

Institut für Physik, Humboldt Universität zu Berlin, Berlin

Physikalisches Institut, Universität Bonn, Bonn

Lehrstuhl für Experimentelle Physik IV, Technische Universität Dortmund, Dortmund

Institut für Kern- und Teilchenphysik, Technische Universität Dresden, Dresden

Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Freiburg

II. Physikalisches Institut, Justus-Liebig-Universität Giessen, Giessen

II. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen

Deutsches Elektronen-Synchrotron DESY, Hamburg and Zeuthen

Kirchhoff-Institut für Physik, Ruprecht-Karls-Universität Heidelberg, Heidelberg;

Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg

Institut für Physik, Universität Mainz, Mainz

Fakultät für Physik, Ludwig-Maximilians-Universität München, München

Max-Planck-Institut für Physik (Werner-Heisenberg-Institut), München

Department Physik, Universität Siegen, Siegen

Fakultät für Mathematik und Naturwissenschaften, Fachgruppe Physik, Bergische Universität Wuppertal, Wuppertal

Fakultät für Physik und Astronomie, Julius-Maximilians-Universität Würzburg, Würzburg

## Greece

National Centre for Scientific Research "Demokritos", Agia Paraskevi

Physics Department, National and Kapodistrian University of Athens, Athens

Department of Physics, Aristotle University of Thessaloniki, Thessaloniki

Physics Department, National Technical University of Athens, Zografou

## Israel

Department of Physics, Technion, Israel Institute of Technology, Haifa

Department of Particle Physics, Weizmann Institute of Science, Rehovot

Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv

## Italy

INFN Sezione di Bologna; Dipartimento di Fisica e Astronomia, Università di Bologna, Bologna

INFN e Laboratori Nazionali di Frascati, Frascati

INFN Sezione di Genova; Dipartimento di Fisica, Università di Genova, Genova

INFN Sezione di Lecce; Dipartimento di Matematica e Fisica, Università del Salento, Lecce

INFN Sezione di Milano; Dipartimento di Fisica, Università di Milano, Milano

INFN Sezione di Napoli; Dipartimento di Fisica, Università di Napoli, Napoli

INFN Sezione di Pavia; Dipartimento di Fisica, Università di Pavia, Pavia

INFN Sezione di Pisa; Dipartimento di Fisica E. Fermi, Università di Pisa, Pisa
INFN Gruppo Collegato di Cosenza, Laboratori Nazionali di Frascati; Dipartimento di
Fisica, Università della Calabria, Rende

INFN Sezione di Roma; Dipartimento di Fisica, Sapienza Università di Roma, Roma

INFN Sezione di Roma Tor Vergata; Dipartimento di Fisica, Università di Roma Tor
Vergata, Roma

INFN Sezione di Roma Tre; Dipartimento di Matematica e Fisica, Università Roma Tre,
Roma

INFN-TIFPA; Università degli Studi di Trento, Trento

INFN Gruppo Collegato di Udine, Sezione di Trieste, Udine; ICTP, Trieste; Dipartimento
di Chimica, Fisica e Ambiente, Università di Udine, Udine

Japan

Research Center for Advanced Particle Physics and Department of Physics, Kyushu
University, Fukuoka

Faculty of Applied Information Science, Hiroshima Institute of Technology, Hiroshima

Graduate School of Science, Kobe University, Kobe

Faculty of Science, Kyoto University, Kyoto

Kyoto University of Education, Kyoto

Department of Physics, Shinshu University, Nagano

Nagasaki Institute of Applied Science, Nagasaki

Graduate School of Science and Kobayashi-Maskawa Institute, Nagoya University,
Nagoya

Faculty of Science, Okayama University, Okayama

Graduate School of Science, Osaka University, Osaka

Department of Physics, Tokyo Institute of Technology, Tokyo

Graduate School of Science and Technology, Tokyo Metropolitan University, Tokyo

International Center for Elementary Particle Physics and Department of Physics,
University of Tokyo, Tokyo

Waseda University, Tokyo

Division of Physics and Tomonaga Center for the History of the Universe, Faculty of Pure
and Applied Sciences, University of Tsukuba, Tsukuba

KEK, High Energy Accelerator Research Organization, Tsukuba

Morocco

*Morocco Cluster:* Centre National de l'Energie des Sciences Techniques Nucleaires (CNESTEN), Rabat; Faculté des Sciences Ain Chock, Réseau Universitaire de Physique des Hautes Energies - Université Hassan II, Casablanca; Faculté des Sciences Semlalia, Université Cadi Ayyad, LPHEA-Marrakech; Faculté des Sciences, Université Mohamed Premier and LPTPM, Oujda; Faculté des sciences, Université Mohammed V, Rabat

Netherlands

Nikhef National Institute for Subatomic Physics and University of Amsterdam,
Amsterdam
Institute for Mathematics, Astrophysics and Particle Physics, Radboud University
Nijmegen/Nikhef, Nijmegen

Norway

Department for Physics and Technology, University of Bergen, Bergen
Department of Physics, University of Oslo, Oslo

Poland

Institute of Nuclear Physics Polish Academy of Sciences, Krakow
AGH University of Science and Technology, Faculty of Physics and Applied Computer
Science, Krakow; Marian Smoluchowski Institute of Physics, Jagiellonian University,
Krakow

Portugal

*Portugal Cluster*: Laboratório de Instrumentação e Física Experimental de Partículas - LIP;
Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Lisboa;
Departamento de Física, Universidade de Coimbra, Coimbra; Departamento de Física,
Universidade do Minho, Braga; Departamento de Física Teorica y del Cosmos,
Universidad de Granada, Granada (Spain); Dep Física and CEFITEC of Faculdade de
Ciências e Tecnologia, Universidade Nova de Lisboa, Caparica

Romania

*Romania Cluster*: Transilvania University of Brasov, Brasov; Horia Hulubei National
Institute of Physics and Nuclear Engineering, Bucharest; National Institute for Research
and Development of Isotopic and Molecular Technologies, Physics Department,
Cluj-Napoca; Department of Physics, Alexandru Ioan Cuza University of Iasi, Iasi;
University Politehnica Bucharest, Bucharest; West University in Timisoara, Timisoara

Russia

D.V. Skobeltsyn Institute of Nuclear Physics, M.V. Lomonosov Moscow State University,
Moscow
Institute for Theoretical and Experimental Physics (ITEP), Moscow
National Research Nuclear University MEPhI, Moscow
P.N. Lebedev Physical Institute of the Russian Academy of Sciences, Moscow
Novosibirsk State University Novosibirsk; Budker Institute of Nuclear Physics, SB RAS,
Novosibirsk
State Research Center Institute for High Energy Physics, NRC KI, Protvino
Konstantinov Nuclear Physics Institute of National Research Centre "Kurchatov Institute",
PNPI, St. Petersburg
Tomsk State University, Tomsk

JINR

Joint Institute for Nuclear Research, Dubna, Russia

Serbia
Institute of Physics, University of Belgrade, Belgrade

**Slovak Republic**

*Slovak Republic Cluster*: Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava; Department of Subnuclear Physics, Institute of Experimental Physics of the Slovak Academy of Sciences, Kosice

**Slovenia**

Department of Experimental Particle Physics, Jožef Stefan Institute and Department of Physics, University of Ljubljana, Ljubljana

**South Africa**

*South Africa Cluster*: Department of Physics, University of Cape Town, Cape Town; Department of Mechanical Engineering Science, University of Johannesburg, Johannesburg; School of Physics, University of the Witwatersrand, Johannesburg

**Spain**

Institut de Física d'Altes Energies (IFAE), Barcelona Institute of Science and Technology, Barcelona

Departamento de Física Teorica C-15 and CIAFF, Universidad Autónoma de Madrid, Madrid

Instituto de Física Corpuscular (IFIC), Centro Mixto Universidad de Valencia - CSIC, Valencia

**Sweden**

Fysiska institutionen, Lunds universitet, Lund

Department of Physics, Stockholm University; Oskar Klein Centre, Stockholm

Physics Department, Royal Institute of Technology, Stockholm

Department of Physics and Astronomy, University of Uppsala, Uppsala

**Switzerland**

Albert Einstein Center for Fundamental Physics and Laboratory for High Energy Physics, University of Bern, Bern

Département de Physique Nucléaire et Corpusculaire, Université de Genève, Genève

**Taiwan**

Department of Physics, National Tsing Hua University, Hsinchu

Institute of Physics, Academia Sinica, Taipei

**Turkey**

*Ankara Cluster*: Department of Physics, Ankara University, Ankara; Istanbul Aydin University, Istanbul; Division of Physics, TOBB University of Economics and Technology, Ankara

*Bogazici Cluster*: Bahcesehir University, Faculty of Engineering and Natural Sciences, Istanbul; Istanbul Bilgi University, Faculty of Engineering and Natural Sciences, Istanbul;
Department of Physics, Bogazici University, Istanbul; Department of Physics Engineering, Gaziantep University, Gaziantep

## United Kingdom

School of Physics and Astronomy, University of Birmingham, Birmingham

Department of Physics and Astronomy, University of Sussex, Brighton

Cavendish Laboratory, University of Cambridge, Cambridge

Department of Physics, University of Warwick, Coventry

Particle Physics Department, Rutherford Appleton Laboratory, Didcot

SUPA - School of Physics and Astronomy, University of Edinburgh, Edinburgh

Department of Physics, Royal Holloway University of London, Egham

SUPA - School of Physics and Astronomy, University of Glasgow, Glasgow

Physics Department, Lancaster University, Lancaster

Oliver Lodge Laboratory, University of Liverpool, Liverpool

Department of Physics and Astronomy, University College London, London

School of Physics and Astronomy, Queen Mary University of London, London

School of Physics and Astronomy, University of Manchester, Manchester

Department of Physics, Oxford University, Oxford

Department of Physics and Astronomy, University of Sheffield, Sheffield

## United States of America

Physics Department, SUNY Albany, Albany NY

Department of Physics and Astronomy, University of New Mexico, Albuquerque NM

Department of Physics and Astronomy, Iowa State University, Ames IA

Department of Physics, University of Massachusetts, Amherst MA

Department of Physics, University of Michigan, Ann Arbor MI

High Energy Physics Division, Argonne National Laboratory, Argonne IL

Department of Physics, University of Texas at Arlington, Arlington TX

Department of Physics, University of Texas at Austin, Austin TX

Physics Division, Lawrence Berkeley National Laboratory and University of California, Berkeley CA

Department of Physics, Indiana University, Bloomington IN

Department of Physics, Boston University, Boston MA

Laboratory for Particle Physics and Cosmology, Harvard University, Cambridge MA

Enrico Fermi Institute, University of Chicago, Chicago IL

Ohio State University, Columbus OH

Physics Department, Southern Methodist University, Dallas TX

Department of Physics, Northern Illinois University, DeKalb IL

Department of Physics, Duke University, Durham NC

Department of Physics and Astronomy, Michigan State University, East Lansing MI

Center for High Energy Physics, University of Oregon, Eugene OR

University of Iowa, Iowa City IA

Department of Physics and Astronomy, University of California Irvine, Irvine CA
Nevis Laboratory, Columbia University, Irvington NY

Department of Physics, University of Wisconsin, Madison WI

Department of Physics and Astronomy, Tufts University, Medford MA

Department of Physics, Yale University, New Haven CT

Department of Physics, New York University, New York NY

Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma,
Norman OK

Department of Physics, University of Pennsylvania, Philadelphia PA

Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh PA

Physics Department, University of Texas at Dallas, Richardson TX

Louisiana Tech University, Ruston LA

Santa Cruz Institute for Particle Physics, University of California Santa Cruz, Santa Cruz
CA

Department of Physics, University of Washington, Seattle WA

SLAC National Accelerator Laboratory, Stanford CA

Department of Physics, Oklahoma State University, Stillwater OK

Departments of Physics and Astronomy, Stony Brook University, Stony Brook NY

Department of Physics, University of Arizona, Tucson AZ

Physics Department, Brookhaven National Laboratory, Upton NY

Department of Physics, University of Illinois, Urbana IL

Department of Physics, Brandeis University, Waltham MA
# Contents

Executive Summary 1

I General Overview of the TDAQ Phase-II Upgrade Project 3

1 Introduction 5

1.1 LHC Roadmap 6

1.2 Physics Drivers for the HL-LHC Upgrades 9

1.3 ATLAS Upgrade Strategies for the HL-LHC 11

1.4 Baseline Architecture and Structure of the Upgrade Project 13

1.5 Outline of this Report 16

2 Physics Motivation 19

2.1 Physics Signatures with Single-Electron and Single-Muon Triggers 20

2.2 Physics Signatures with Two Leptons 24

2.3 Physics Signatures with Photons 24

2.4 Physics Signatures with Hadronically Decaying Tau Leptons 26

2.5 Physics Signatures with Jets 27

2.6 Physics Signatures with Missing Transverse Energy 30

2.7 Physics Signatures with Forward Electrons 31

2.8 Physics Signatures with Exotic Objects 32

2.9 Physics with an Inclusive Vector Boson Fusion (VBF) Trigger 33

2.10 B-Physics Signatures 34

2.11 Physics Signatures for Heavy Ion Collision 35

2.12 Summary of Requirements and Motivation for the Upgrade 37

3 Challenges and Limitations of the Run 3 TDAQ system 39

3.1 Overview of the Phase-I Trigger and Readout Architecture 40

3.2 Features and Limitations of the Run 3 Level-1 Trigger System 44

3.2.1 Level-1 Calorimeter Trigger Limitations 44

3.2.2 Level-1 Muon Trigger Limitations 53

3.2.3 Level-1 Topological Trigger Limitations 58

3.2.4 Level-1 Trigger Menu up to Run 3 59

3.2.5 Limitations in the Level-1 CTP and TTC systems 62

3.3 Limitations of the Run 3 DAQ System 63

3.3.1 Detector Readout 63
<table>
  <tbody>
    <tr>
      <td>3.3.2</td>
      <td>Dataflow, Storage and Networking</td>
      <td>65</td>
    </tr>
    <tr>
      <td>3.4</td>
      <td>Limitations of the Run 3 HLT System</td>
      <td>68</td>
    </tr>
    <tr>
      <td>3.4.1</td>
      <td>Estimates of Rejection Factors in the Run 3 HLT</td>
      <td>68</td>
    </tr>
    <tr>
      <td>3.4.2</td>
      <td>Fast Tracking</td>
      <td>68</td>
    </tr>
    <tr>
      <td>3.4.3</td>
      <td>Features and Limitations of Event Selection in the Run 3 HLT</td>
      <td>70</td>
    </tr>
    <tr>
      <td>4</td>
      <td>Architectural and Functional Requirements</td>
      <td>75</td>
    </tr>
    <tr>
      <td>4.1</td>
      <td>Constraints from the Detectors</td>
      <td>75</td>
    </tr>
    <tr>
      <td>4.1.1</td>
      <td>New Inner Tracker</td>
      <td>75</td>
    </tr>
    <tr>
      <td>4.1.2</td>
      <td>Calorimeter Detectors</td>
      <td>77</td>
    </tr>
    <tr>
      <td>4.1.3</td>
      <td>Muon System</td>
      <td>77</td>
    </tr>
    <tr>
      <td>4.1.4</td>
      <td>Other Detectors</td>
      <td>79</td>
    </tr>
    <tr>
      <td>4.2</td>
      <td>Level-0 Trigger Requirements</td>
      <td>79</td>
    </tr>
    <tr>
      <td>4.3</td>
      <td>DAQ Requirements</td>
      <td>83</td>
    </tr>
    <tr>
      <td>4.4</td>
      <td>Event Filter Requirements</td>
      <td>84</td>
    </tr>
    <tr>
      <td>5</td>
      <td>Description of the Baseline System</td>
      <td>87</td>
    </tr>
    <tr>
      <td>5.1</td>
      <td>Functional overview</td>
      <td>87</td>
    </tr>
    <tr>
      <td>5.2</td>
      <td>The Level-0 Trigger System</td>
      <td>90</td>
    </tr>
    <tr>
      <td>5.2.1</td>
      <td>Level-0 Calorimeter Trigger</td>
      <td>90</td>
    </tr>
    <tr>
      <td>5.2.2</td>
      <td>Level-0 Muon Trigger</td>
      <td>92</td>
    </tr>
    <tr>
      <td>5.2.3</td>
      <td>MUCTPI</td>
      <td>94</td>
    </tr>
    <tr>
      <td>5.2.4</td>
      <td>Global Trigger</td>
      <td>94</td>
    </tr>
    <tr>
      <td>5.2.5</td>
      <td>Central Trigger Processor</td>
      <td>95</td>
    </tr>
    <tr>
      <td>5.2.6</td>
      <td>Trigger, Timing and Control System</td>
      <td>96</td>
    </tr>
    <tr>
      <td>5.2.7</td>
      <td>Technical implementation and system size</td>
      <td>98</td>
    </tr>
    <tr>
      <td>5.2.8</td>
      <td>Level-0 Trigger Latency</td>
      <td>100</td>
    </tr>
    <tr>
      <td>5.3</td>
      <td>Data Acquisition System</td>
      <td>101</td>
    </tr>
    <tr>
      <td>5.3.1</td>
      <td>Readout</td>
      <td>101</td>
    </tr>
    <tr>
      <td>5.3.2</td>
      <td>Dataflow</td>
      <td>106</td>
    </tr>
    <tr>
      <td>5.3.3</td>
      <td>Network</td>
      <td>109</td>
    </tr>
    <tr>
      <td>5.4</td>
      <td>Event Filter System</td>
      <td>110</td>
    </tr>
    <tr>
      <td>5.4.1</td>
      <td>Use of Tracking in the Event Filter</td>
      <td>111</td>
    </tr>
    <tr>
      <td>5.4.2</td>
      <td>Event Filter System Overview</td>
      <td>113</td>
    </tr>
    <tr>
      <td>5.4.3</td>
      <td>Event Filter Farm Hardware</td>
      <td>114</td>
    </tr>
    <tr>
      <td>5.4.4</td>
      <td>Event Filter Software</td>
      <td>116</td>
    </tr>
    <tr>
      <td>5.4.5</td>
      <td>Hardware-based Tracking for the Trigger Subsystem</td>
      <td>117</td>
    </tr>
    <tr>
      <td>6</td>
      <td>Expected Performance</td>
      <td>123</td>
    </tr>
    <tr>
      <td>6.1</td>
      <td>Performance Estimation Procedures</td>
      <td>123</td>
    </tr>
    <tr>
      <td>6.2</td>
      <td>Topological Clusters</td>
      <td>124</td>
    </tr>
    <tr>
      <td>6.3</td>
      <td>Electrons and Photons</td>
      <td>124</td>
    </tr>
    <tr>
      <td>6.4</td>
      <td>Muons</td>
      <td>128</td>
    </tr>
  </tbody>
</table>
<table><tbody><tr><td>6.5 Tau Leptons</td><td>132</td></tr><tr><td>6.6 Jets</td><td>135</td></tr><tr><td>6.7 Missing Transverse Energy</td><td>141</td></tr><tr><td>6.8 Boosted-object Triggers</td><td>145</td></tr><tr><td>6.9 Exotic-object Triggers</td><td>147</td></tr><tr><td>6.10 Inclusive Vector Boson Fusion Trigger</td><td>148</td></tr><tr><td>6.11 Example Run 4 Trigger Menu</td><td>149</td></tr><tr><td>6.12 Tracking Needs</td><td>151</td></tr><tr><td>6.13 Impact of the System on HH → 4b</td><td>156</td></tr><tr><td>6.14 System Flexibility and Dataset Quality</td><td>158</td></tr><tr><td><b>II Detailed Description of System Components</b></td><td><b>161</b></td></tr><tr><td><b>7 Level-0 Calorimeter Trigger</b></td><td><b>163</b></td></tr><tr><td>7.1 Evolution of the Hardware Calorimeter Trigger</td><td>163</td></tr><tr><td>7.1.1 Enhancements and Interface changes of the Phase-I Level-1 Calorimeter Trigger</td><td>163</td></tr><tr><td>7.1.2 Requirements for the Level-0 Calorimeter Trigger</td><td>164</td></tr><tr><td>7.2 Performance of the Level-0 Calorimeter Trigger</td><td>164</td></tr><tr><td>7.3 Architecture and Hardware Realisation</td><td>165</td></tr><tr><td>7.3.1 Overview</td><td>165</td></tr><tr><td>7.3.2 Input Signals</td><td>168</td></tr><tr><td>7.3.3 Processing System</td><td>170</td></tr><tr><td>7.3.4 The forward FEX System (fFEX)</td><td>171</td></tr><tr><td>7.3.5 Output Signals</td><td>173</td></tr><tr><td>7.3.6 Latency</td><td>174</td></tr><tr><td>7.3.7 Readout and Monitoring</td><td>174</td></tr><tr><td>7.4 Firmware</td><td>175</td></tr><tr><td>7.4.1 Algorithmic Firmware</td><td>175</td></tr><tr><td>7.4.2 Infrastructure Firmware</td><td>175</td></tr><tr><td>7.5 R & D Programme</td><td>175</td></tr><tr><td>7.6 Commissioning</td><td>176</td></tr><tr><td><b>8 Level-0 Muon Trigger</b></td><td><b>177</b></td></tr><tr><td>8.1 Introduction</td><td>177</td></tr><tr><td>8.2 Overview of the Current System and the Limitations</td><td>177</td></tr><tr><td>8.3 Overview of the Upgrade</td><td>179</td></tr><tr><td>8.3.1 Extension of the Latency and the Rate</td><td>180</td></tr><tr><td>8.3.2 Improvement of the Trigger Performance</td><td>181</td></tr><tr><td>8.4 Sector Logic</td><td>182</td></tr><tr><td>8.4.1 Trigger Scheme and Performance</td><td>183</td></tr><tr><td>8.4.2 Hardware Design</td><td>196</td></tr></tbody></table>
<table><tr><td>8.4.3</td><td>Firmware Design</td><td>198</td></tr><tr><td>8.4.4</td><td>Detector Signal Inputs</td><td>199</td></tr><tr><td>8.4.5</td><td>Realtime Output Data Format</td><td>200</td></tr><tr><td>8.5</td><td>NSW Trigger Processor</td><td>202</td></tr><tr><td>8.5.1</td><td>Motivation for the Upgrade</td><td>202</td></tr><tr><td>8.5.2</td><td>Performance of the Segment Reconstruction</td><td>204</td></tr><tr><td>8.5.3</td><td>Hardware Design</td><td>204</td></tr><tr><td>8.5.4</td><td>Firmware Design</td><td>205</td></tr><tr><td>8.5.5</td><td>Detector Signal Inputs</td><td>206</td></tr><tr><td>8.5.6</td><td>Realtime Output Data Format</td><td>207</td></tr><tr><td>8.6</td><td>MDT Trigger Processor</td><td>207</td></tr><tr><td>8.6.1</td><td>Motivation for the Implementation</td><td>207</td></tr><tr><td>8.6.2</td><td>Trigger Performance</td><td>208</td></tr><tr><td>8.6.3</td><td>Hardware Overview</td><td>212</td></tr><tr><td>8.6.4</td><td>Hit Extraction</td><td>213</td></tr><tr><td>8.6.5</td><td>Segment Reconstruction</td><td>216</td></tr><tr><td>8.6.6</td><td>Transverse Momentum Evaluation</td><td>223</td></tr><tr><td>8.6.7</td><td>Realtime Output Data Format</td><td>225</td></tr><tr><td>8.6.8</td><td>Readout Path</td><td>225</td></tr><tr><td>8.6.9</td><td>Resource Estimates</td><td>226</td></tr><tr><td>8.7</td><td>Latency Estimates</td><td>226</td></tr><tr><td>8.8</td><td>R&D Items</td><td>229</td></tr><tr><td>9</td><td>Global Trigger</td><td>235</td></tr><tr><td>9.1</td><td>Introduction</td><td>235</td></tr><tr><td>9.1.1</td><td>Requirements for the Global Trigger System</td><td>236</td></tr><tr><td>9.1.2</td><td>Overview of Global Trigger System Design</td><td>237</td></tr><tr><td>9.2</td><td>Global Trigger Interfaces</td><td>239</td></tr><tr><td>9.2.1</td><td>Calorimeter Subdetector Inputs</td><td>240</td></tr><tr><td>9.2.2</td><td>L0Calo Inputs</td><td>240</td></tr><tr><td>9.2.3</td><td>L0Muon Inputs</td><td>240</td></tr><tr><td>9.2.4</td><td>Central Trigger Processor Outputs</td><td>240</td></tr><tr><td>9.2.5</td><td>Front-End L1Link eXchange (FELIX) Inputs and Outputs</td><td>241</td></tr><tr><td>9.2.6</td><td>Configuration and Control, Interface to Detector Control System</td><td>241</td></tr><tr><td>9.3</td><td>Trigger Strategy and Algorithms</td><td>241</td></tr><tr><td>9.3.1</td><td>Algorithm Performance</td><td>242</td></tr><tr><td>9.3.2</td><td>Algorithm Implementation</td><td>242</td></tr><tr><td>9.3.3</td><td>Topological Clustering</td><td>243</td></tr><tr><td>9.3.4</td><td>Anti-k<sub>t</sub> Jet Clustering</td><td>249</td></tr><tr><td>9.4</td><td>Physical Realisation</td><td>255</td></tr><tr><td>9.4.1</td><td>Architecture</td><td>255</td></tr><tr><td>9.4.2</td><td>Required Number of Global Common Modules (GCMs)</td><td>255</td></tr><tr><td>9.4.3</td><td>Required Number of Links on each GCM</td><td>256</td></tr></table>
<table><tbody><tr><td>9.4.4</td><td>GCM Requirements and Preliminary Design</td><td>258</td></tr><tr><td>9.4.5</td><td>Fibre Plant</td><td>259</td></tr><tr><td>9.4.6</td><td>Production Firmware Deployment Module</td><td>260</td></tr><tr><td>9.5</td><td>Latency</td><td>261</td></tr><tr><td>9.5.1</td><td>Data arrival time at Multiplexer Processor</td><td>262</td></tr><tr><td>9.5.2</td><td>Processing latency and budget</td><td>262</td></tr><tr><td>9.6</td><td>Firmware</td><td>263</td></tr><tr><td>9.6.1</td><td>Firmware Blocks</td><td>264</td></tr><tr><td>9.6.2</td><td>FPGA Resource Usage</td><td>265</td></tr><tr><td>9.6.3</td><td>Firmware Development</td><td>266</td></tr><tr><td><b>10</b></td><td><b>Central Trigger System</b></td><td><b>269</b></td></tr><tr><td>10.1</td><td>Level-0 CTP</td><td>271</td></tr><tr><td>10.1.1</td><td>CTPMI: Machine Interface</td><td>272</td></tr><tr><td>10.1.2</td><td>CTPIN: Trigger Inputs</td><td>273</td></tr><tr><td>10.1.3</td><td>CTPCORE</td><td>274</td></tr><tr><td>10.2</td><td>Distribution of Timing, Trigger, and Control Signals (TTC)</td><td>278</td></tr><tr><td>10.2.1</td><td>Local Trigger Interface</td><td>279</td></tr><tr><td>10.3</td><td>Level-0 MUCTPI</td><td>281</td></tr><tr><td>10.4</td><td>Monitoring, Control, Configuration, Offline Software</td><td>283</td></tr><tr><td>10.5</td><td>Latency</td><td>284</td></tr><tr><td>10.6</td><td>Project Planning</td><td>285</td></tr><tr><td><b>11</b></td><td><b>Data Acquisition</b></td><td><b>289</b></td></tr><tr><td>11.1</td><td>Introduction</td><td>289</td></tr><tr><td>11.2</td><td>Overview of System Functionality</td><td>290</td></tr><tr><td>11.3</td><td>Event Data Format</td><td>290</td></tr><tr><td>11.4</td><td>Detector Readout</td><td>291</td></tr><tr><td>11.4.1</td><td>FELIX</td><td>291</td></tr><tr><td>11.4.2</td><td>Data Handler</td><td>294</td></tr><tr><td>11.4.3</td><td>R&amp;D leading to the final design and implementation of the Readout System</td><td>296</td></tr><tr><td>11.5</td><td>Dataflow</td><td>298</td></tr><tr><td>11.5.1</td><td>Event Builder</td><td>299</td></tr><tr><td>11.5.2</td><td>Storage Handler</td><td>300</td></tr><tr><td>11.5.3</td><td>Event Aggregator</td><td>300</td></tr><tr><td>11.5.4</td><td>Storage System Sizing &amp; Technology</td><td>300</td></tr><tr><td>11.5.5</td><td>Factors Affecting Implementation</td><td>301</td></tr><tr><td>11.5.6</td><td>System Workflow</td><td>302</td></tr><tr><td>11.6</td><td>Network</td><td>306</td></tr><tr><td>11.6.1</td><td>Network Domain Description</td><td>306</td></tr><tr><td>11.6.2</td><td>Network Technology Choice</td><td>307</td></tr><tr><td>11.6.3</td><td>Network Architecture</td><td>308</td></tr></tbody></table>
<table><tbody><tr><td>11.6.4 Control Network</td><td>309</td></tr><tr><td>11.6.5 Network Installation</td><td>310</td></tr><tr><td>11.7 Online Software</td><td>310</td></tr><tr><td>11.7.1 System Overview</td><td>310</td></tr><tr><td>11.7.2 EF Farm Management</td><td>312</td></tr><tr><td>11.7.3 Operational Monitoring of the Online System</td><td>316</td></tr><tr><td>11.7.4 Physics Monitoring</td><td>318</td></tr><tr><td>11.7.5 System Administration</td><td>323</td></tr><tr><td>11.8 Standalone DAQ Mode &amp; Commissioning</td><td>324</td></tr><tr><td><strong>12 Event Filter</strong></td><td><strong>329</strong></td></tr><tr><td>12.1 Event Filter Hardware</td><td>331</td></tr><tr><td>12.2 Selection software</td><td>332</td></tr><tr><td>12.3 Study of GPGPU usage in the Event Filter (EF)</td><td>334</td></tr><tr><td>12.4 Model for CPU estimation</td><td>337</td></tr><tr><td><strong>13 Hardware-based Tracking for the Trigger (HTT)</strong></td><td><strong>343</strong></td></tr><tr><td>13.1 Overview of the HTT Architecture</td><td>345</td></tr><tr><td>13.2 Interface with Event Filter</td><td>347</td></tr><tr><td>13.3 Comparison to FTK</td><td>348</td></tr><tr><td>13.4 Functional description of HTT</td><td>349</td></tr><tr><td>13.4.1 Data preparation</td><td>349</td></tr><tr><td>13.4.2 Pattern matching</td><td>349</td></tr><tr><td>13.4.3 Track fitting</td><td>352</td></tr><tr><td>13.4.4 Duplicate removal</td><td>354</td></tr><tr><td>13.5 HTT Performance Studies</td><td>354</td></tr><tr><td>13.5.1 Associative Memory (AM) Pattern-Matching Performance</td><td>354</td></tr><tr><td>13.5.2 Track-Fitting Performance</td><td>356</td></tr><tr><td>13.5.3 Muon and Electron Track-finding Efficiencies</td><td>358</td></tr><tr><td>13.5.4 Resolutions of Track Parameters</td><td>358</td></tr><tr><td>13.5.5 Simulated HTT Data Size</td><td>360</td></tr><tr><td>13.6 Description of the HTT Hardware and Firmware</td><td>362</td></tr><tr><td>13.6.1 Dataflow requirements</td><td>363</td></tr><tr><td>13.6.2 Tracking Processor (TP)</td><td>367</td></tr><tr><td>13.6.3 Pattern Recognition Mezzanine (PRM)</td><td>370</td></tr><tr><td>13.6.4 Associative Memory (AM) for Phase-II</td><td>376</td></tr><tr><td>13.6.5 Track Fitter Mezzanine (TFM)</td><td>383</td></tr><tr><td>13.6.6 The HTT Interface (HTTIF) Infrastructure</td><td>386</td></tr><tr><td>13.6.7 Dataflow summary</td><td>387</td></tr><tr><td>13.6.8 Size and power consumption of the HTT system</td><td>387</td></tr><tr><td>13.7 Project Milestones</td><td>390</td></tr></tbody></table>
<table><thead><tr><th></th><th></th></tr></thead><tbody><tr><td>14 Evolution Scenario</td><td>393</td></tr><tr><td>14.1 Criteria for Evolution</td><td>393</td></tr><tr><td>14.1.1 Uncertainty in Hadronic Trigger Rate Estimates</td><td>394</td></tr><tr><td>14.1.2 Uncertainty in ITk Pixel Detector Occupancies</td><td>395</td></tr><tr><td>14.2 Requirements for the Evolved System</td><td>396</td></tr><tr><td>14.2.1 ITk Pixel Detector</td><td>398</td></tr><tr><td>14.2.2 ITk Strip Detector</td><td>398</td></tr><tr><td>14.2.3 NSW</td><td>399</td></tr><tr><td>14.3 Overview of the Evolved System Architecture</td><td>399</td></tr><tr><td>14.4 Design Implications for the TDAQ Sub-systems</td><td>401</td></tr><tr><td>14.4.1 Level-0 Calo Trigger</td><td>401</td></tr><tr><td>14.4.2 Level-0 Muon Trigger</td><td>401</td></tr><tr><td>14.4.3 Global Trigger</td><td>401</td></tr><tr><td>14.4.4 Trigger, Timing, and Control</td><td>403</td></tr><tr><td>14.4.5 Central Trigger System</td><td>404</td></tr><tr><td>14.4.6 Summary of Hardware Trigger Latency Estimates in the Evolved System</td><td>407</td></tr><tr><td>14.4.7 Data Acquisition</td><td>407</td></tr><tr><td>14.4.8 Event Filter</td><td>409</td></tr><tr><td>14.4.9 HTT</td><td>411</td></tr><tr><td>14.5 Physics Opportunities with the Evolved System</td><td>416</td></tr><tr><td>14.6 Challenges of the Evolved System</td><td>418</td></tr><tr><td>15 DCS and TDAQ interfaces</td><td>421</td></tr><tr><td>15.1 Interfaces for On-Detector Front End (FE) Components</td><td>422</td></tr><tr><td>15.1.1 Exclusive DCS On-Detector Control Path</td><td>423</td></tr><tr><td>15.1.2 On-Detector Control via the Readout Path</td><td>424</td></tr><tr><td>15.2 Interfaces for Off-Detector Front-End Components</td><td>425</td></tr><tr><td>15.2.1 Devices with Proprietary Interfaces</td><td>426</td></tr><tr><td>15.2.2 Off-detector Electronics based on ATCA and VME Standards</td><td>426</td></tr><tr><td>15.2.3 Interfaces for Other Purpose-Built Off-Detector Front-Ends</td><td>427</td></tr><tr><td>15.3 Back-End and Middleware</td><td>427</td></tr><tr><td>15.3.1 Back-End Hardware and Software</td><td>427</td></tr><tr><td>15.3.2 Standard Middleware OPC UA</td><td>428</td></tr><tr><td>15.3.3 Network</td><td>430</td></tr><tr><td>15.4 DCS of TDAQ Sub-Systems</td><td>430</td></tr><tr><td>16 System Integration, Installation and Commissioning</td><td>433</td></tr><tr><td>16.1 Overview of the TDAQ Phase-II Integration and Installation</td><td>433</td></tr><tr><td>16.1.1 Location of the TDAQ Phase-II Upgrade Project's Components</td><td>434</td></tr><tr><td>16.1.2 Installation and Commissioning Planning</td><td>434</td></tr><tr><td>16.2 Laboratories and Facilities</td><td>437</td></tr><tr><td>16.2.1 Building 4 surface testing facility</td><td>437</td></tr></tbody></table>
<table><tr><td>16.2.2 Lab4 Testbed Infrastructure</td><td>437</td></tr><tr><td>16.2.3 TDAQ Maintenance Facility</td><td>438</td></tr><tr><td>16.2.4 Safety</td><td>438</td></tr><tr><td>16.3 Production Firmware Deployment</td><td>440</td></tr><tr><td>16.4 Validation and Initial System Integration at CERN of custom Hardware sub-systems</td><td>441</td></tr><tr><td>16.4.1 Quality Assurance and Quality Control at the production sites</td><td>441</td></tr><tr><td>16.4.2 Acceptance validation tests at CERN</td><td>442</td></tr><tr><td>16.4.3 Level-0 Trigger Sub-system Integration Tests</td><td>442</td></tr><tr><td>16.4.4 HTT specific Integration and Initial Commissioning</td><td>443</td></tr><tr><td>16.4.5 DAQ Readout Sub-system: FELIX and Data Handlers</td><td>443</td></tr><tr><td>16.5 Installation in USA15</td><td>444</td></tr><tr><td>16.6 Integration with the Detector Systems and Early Commissioning</td><td>449</td></tr><tr><td>16.7 Installation and Commissioning Plans of DAQ/EF components in SDX1</td><td>450</td></tr><tr><td><b>III Project Management and Organisation</b></td><td><b>455</b></td></tr><tr><td><b>17 Project Management and Organisation</b></td><td><b>457</b></td></tr><tr><td>17.1 Overview of the ATLAS Upgrade Organisation</td><td>457</td></tr><tr><td>17.1.1 ATLAS management</td><td>458</td></tr><tr><td>17.1.2 Executive Board</td><td>458</td></tr><tr><td>17.1.3 Upgrade Coordinator</td><td>459</td></tr><tr><td>17.1.4 Upgrade Steering Committee</td><td>459</td></tr><tr><td>17.1.5 Upgrade Advisory Board</td><td>459</td></tr><tr><td>17.1.6 Interactions with External Committees and Organisations</td><td>460</td></tr><tr><td>17.1.7 Upgrade Projects</td><td>460</td></tr><tr><td>17.2 Organisation and management of the TDAQ systems and the TDAQ Upgrades</td><td>461</td></tr><tr><td>17.2.1 TDAQ Institutional Board</td><td>462</td></tr><tr><td>17.2.2 TDAQ Management Team</td><td>463</td></tr><tr><td>17.2.3 TDAQ Steering Group</td><td>463</td></tr><tr><td>17.2.4 TDAQ Resources</td><td>463</td></tr><tr><td>17.2.5 TDAQ system and TDAQ Upgrade Projects</td><td>464</td></tr><tr><td>17.3 TDAQ UPR Organisation and Upgrade Project Management Plan</td><td>465</td></tr><tr><td>17.3.1 UPR Management Team</td><td>465</td></tr><tr><td>17.3.2 Upgrade Project Leader (UPL)</td><td>467</td></tr><tr><td>17.3.3 Deputy Upgrade Project Leaders</td><td>467</td></tr><tr><td>17.3.4 Extended TDAQ Steering Group</td><td>468</td></tr><tr><td>17.3.5 UPR Level-0 Trigger Coordinator</td><td>468</td></tr><tr><td>17.3.6 UPR DAQ Coordinator</td><td>469</td></tr><tr><td>17.3.7 UPR EF & Performance Coordinator</td><td>469</td></tr><tr><td>17.3.8 TDAQ Upgrade Project Office</td><td>470</td></tr><tr><td>17.3.9 Organisation of the TDAQ UPR Systems and Sub-systems</td><td>470</td></tr></table>
<table><thead><tr><th></th><th></th></tr></thead><tbody><tr><td>18 Project Cost Estimates</td><td>477</td></tr><tr><td>18.1 Overview of the Cost Management Plan</td><td>477</td></tr><tr><td>18.2 CORE Costing Policy</td><td>478</td></tr><tr><td>18.3 Costing Methodology</td><td>479</td></tr><tr><td>18.3.1 Estimate Uncertainties</td><td>481</td></tr><tr><td>18.4 UPR Project Breakdown Structure (PBS)</td><td>482</td></tr><tr><td>18.5 UPR CORE Costing Tables</td><td>486</td></tr><tr><td>18.5.1 Level-0 Trigger</td><td>487</td></tr><tr><td>18.5.2 Data Acquisition System</td><td>488</td></tr><tr><td>18.5.3 Event Filter</td><td>488</td></tr><tr><td>18.6 Costing Profiles</td><td>491</td></tr><tr><td>18.6.1 Spending profile of the Level-0 Trigger System</td><td>493</td></tr><tr><td>18.6.2 Spending profile of the DAQ System</td><td>493</td></tr><tr><td>18.6.3 Spending profile of the EF Trigger System</td><td>494</td></tr><tr><td>19 Planning and Schedule</td><td>497</td></tr><tr><td>19.1 Overview of the Schedule Management Plan</td><td>497</td></tr><tr><td>19.2 Design, Production Plan and Hardware/Firmware Integration</td><td>499</td></tr><tr><td>19.2.1 Specification Review</td><td>500</td></tr><tr><td>19.2.2 Preliminary Design Review</td><td>501</td></tr><tr><td>19.2.3 Final Design Review</td><td>501</td></tr><tr><td>19.2.4 Production Readiness Review</td><td>502</td></tr><tr><td>19.2.5 Internal TDAQ Reviews</td><td>503</td></tr><tr><td>19.3 Production Schedule and Milestones</td><td>504</td></tr><tr><td>19.3.1 Level-0 Trigger System's Milestones</td><td>507</td></tr><tr><td>19.3.2 DAQ System's Milestones</td><td>509</td></tr><tr><td>19.3.3 EF System's Milestones</td><td>509</td></tr><tr><td>20 Resources Requirements and Institutional Responsibilities</td><td>513</td></tr><tr><td>20.1 Resource Management Plan</td><td>513</td></tr><tr><td>20.2 Required Manpower Estimate</td><td>514</td></tr><tr><td>20.2.1 Level-0 Trigger System</td><td>514</td></tr><tr><td>20.2.2 DAQ System</td><td>514</td></tr><tr><td>20.2.3 EF System</td><td>517</td></tr><tr><td>20.3 Participating Institute Responsibilities</td><td>524</td></tr><tr><td>21 Risk Analysis and Mitigation Strategies</td><td>533</td></tr><tr><td>21.1 Risk Management Plan</td><td>533</td></tr><tr><td>21.2 Risk Register</td><td>535</td></tr><tr><td>21.2.1 Detector Readout Limitations</td><td>537</td></tr><tr><td>21.2.2 Projected rates for hadronic trigger signatures</td><td>537</td></tr><tr><td>21.2.3 Resource availability of the EF Processing Unit farms</td><td>537</td></tr><tr><td>21.2.4 Hardware Tracking ASIC</td><td>538</td></tr></tbody></table>
<table><tr><td>21.2.5 FPGA resource usage</td><td>538</td></tr><tr><td>21.2.6 Interfaces and Link speeds</td><td>538</td></tr><tr><td>21.2.7 Latency limitations</td><td>538</td></tr><tr><td>21.2.8 FPGA obsolescence</td><td>539</td></tr><tr><td>21.2.9 Commissioning time of the Level-0 Muon Trigger sub-system</td><td>539</td></tr><tr><td>21.2.10 Commissioning time of the Global Trigger sub-system</td><td>539</td></tr><tr><td>21.2.11 Readout's I/O bandwidth margins</td><td>539</td></tr><tr><td>21.2.12 Data Handler processing capabilities for detector-specific functionality</td><td>540</td></tr><tr><td>21.2.13 Event Size</td><td>540</td></tr><tr><td>21.2.14 Simulation software schedule</td><td>540</td></tr><tr><td>IV Appendix: Glossary</td><td>543</td></tr><tr><td>The ATLAS Collaboration</td><td>557</td></tr></table>
# Executive Summary

This Technical Design Report documents the strategy and the design of the ATLAS Trigger and Data Acquisition System (TDAQ) system for the High Luminosity upgrade of the Large Hadron Collider (HL-LHC). The Phase-II upgrade of the TDAQ system will enable the broad physics programme planned for the HL-LHC, including a detailed exploration of the mechanism of electroweak symmetry breaking through the properties of the Higgs boson, searches for new physics through the study of rare Standard Model processes, searches for new heavy states, and measurements of the properties of any newly discovered particles.

The HL-LHC is expected to start operations in the middle of 2026, and to reach nominally a peak instantaneous luminosity of $\mathcal{L} = 5 \times 10^{34} \text{cm}^{-2}\text{s}^{-1}$, corresponding to approximately 140 inelastic proton-proton collisions per bunch crossing, which could be maintained for the duration of the HL-LHC project. In this configuration, approximately $250 \text{ fb}^{-1}$ will be delivered per year, for a total of $3000 \text{ fb}^{-1}$ by the end of Run 6, ten times the integrated luminosity of the LHC Runs 1-3 combined. The design of the HL-LHC accommodates large safety margins with respect to heat deposition and integrated radiation dose. The performance of the HL-LHC could be accordingly increased to its *ultimate* configuration, where the instantaneous luminosity is levelled at $\mathcal{L} = 7.5 \times 10^{34} \text{cm}^{-2}\text{s}^{-1}$ and the experiments need to cope with pile-up of up to 200 inelastic collisions per bunch crossing. The *ultimate* performance configuration, as defined in the HL-LHC Technical Design Report, would provide more than $300-350 \text{ fb}^{-1}$ per year and a total integrated luminosity up to $4000 \text{ fb}^{-1}$. This is the configuration for which the HL-LHC TDAQ upgrade is designed.

Meeting these requirements poses significant challenges to the ATLAS TDAQ system to fully exploit the physics potential of the HL-LHC. The ATLAS collaboration described its initial plans and goals for the corresponding Phase-II upgrade of the detector in the Letter of Intent in 2012, and in the Scoping Document in 2015. Two "custom-hardware" trigger levels were proposed in the Scoping Document that allowed for data streaming off-detector either after an initial trigger decision, or in some cases, at the full 40 MHz bunch crossing rate. Since then, the design of the upgraded architecture of the TDAQ system has developed further, resulting in a baseline architecture with a single-level hardware trigger that features a maximum rate of 1 MHz and 10 µs latency.

The hardware-based Level-0 Trigger system is composed of the Level-0 Calorimeter Trigger (L0Calo), the Level-0 Muon Trigger (L0Muon), the Global Trigger and the Central Trigger sub-systems. In the L0Calo sub-system, the Phase-I calorimeter feature extraction trigger processors will be complemented by a new forward Feature EXtractor (fFEX) to reconstruct
forward jets and electrons, matching the pseudo-rapidity coverage of the new tracker sys-
tem. The new L0Muon sub-system will use upgraded Barrel and Endcap Sector Logic and
New Small Wheel (NSW) Trigger Processors, for the reconstruction of muon candidates in
the barrel Resistive Plate Chambers (RPCs), in the endcap Thin Gap Chambers (TGCs) and
NSW detectors, respectively. In addition, Monitored Drift Tube (MDT) information will be
used in new dedicated processors to improve robustness and efficiency of the Muon Trigger,
its $p_T$ resolution and selectivity. The Global Trigger will replace and extend the Run 2
and Phase-I Topological Processor, by accessing full-granularity calorimeter information to
refine the trigger objects calculated by L0Calo, perform offline-like algorithms, and calculate
event-level quantities before applying topological selections. The final trigger decision
is made by the Central Trigger Processor (CTP), which can apply flexible prescales and
vetoes to the trigger items. The CTP also drives the Trigger, Timing and Control system
network to start the readout process of the detectors.

The result of the Level-0 trigger decision is transmitted to all detectors and trigger pro-
cessors, upon which the resulting detector and trigger data, respectively, are transmitted to
the Data Acquisition system at 1 MHz through the Readout and the Dataflow sub-systems.
Both sub-systems are based on commodity PC servers and standard networking infrastruc-
ture. The Readout sub-system includes custom input/output cards to convert the detector
front-end protocol data into standard network packets.

The upgraded EF system provides high-level trigger functionality, and consists of a CPU-
based processing farm complemented by Hardware-based Tracking for the Trigger (HTT)
co-processors. The EF system refines the trigger objects in order to achieve a maximum
output event rate of 10 kHz. The HTT includes regional (rHTT) and full-scan (gHTT) track
reconstruction capabilities. The EF trigger decision enables the transfer of data correspond-
ing to selected events from the Data Acquisition System (DAQ) to permanent storage.

Each system and sub-system will be capable of evolving to a dual-level hardware trigger
architecture as a mitigation strategy in case pile-up conditions at the HL-LHC either chal-
lenge the readout capabilities of detectors to the limits of the bandwidth available, or in
case the rates of hadronic trigger signatures with the needed thresholds exceed our cur-
rent allocations. The evolved two-level architecture specifies a Level-0 trigger rate up to
2-4 MHz and 10 µs latency, followed by a Level-1 trigger rate of 600-800 kHz and latency up
to 35 µs. Hardware-based track reconstruction is implemented in the Level-1 trigger system
by reconfiguring part of the HTT. The reconstructed tracks are combined with calorimeter-
and muon-based trigger objects in the Global Trigger, after which the Central Trigger forms
the Level-1 decision.

A management plan is in place to deliver the baseline TDAQ upgraded system fully com-
missioned by the end of Long Shutdown 3.
# Part I

## General Overview of the TDAQ Phase-II Upgrade
Project
<figure><img src="image_8.png" /></figure>
# 1 Introduction

This Technical Design Report describes the upgrade of the ATLAS Trigger and Data Acquisition (TDAQ) system for operation at the High Luminosity LHC (HL-LHC). The overall goal of the TDAQ Phase-II Upgrade Project is to design, build, and install new trigger and data acquisition hardware with its firmware and needed software during the third long shutdown of the LHC in 2024. In this document, the scientific motivation and the technical implementation of the upgrade are detailed.

The Phase-II upgrade of the ATLAS TDAQ system must satisfy the broad ATLAS physics programme planned for the decade-long HL-LHC, while coping with ultimate HL-LHC conditions: a peak instantaneous luminosity seven times above the original LHC design with up to 200 inelastic proton-proton collisions per beam crossing. During this period, ATLAS aims to collect a total dataset of 4000 fb⁻¹, allowing for a detailed exploration of the mechanism of electroweak symmetry breaking through the properties of the Higgs boson, searches for new physics through the study of rare Standard Model processes, searches for new heavy states, and measurements of the properties of any newly discovered particles. Both the necessity of a highly efficient selection of events with Higgs bosons in decay modes useful for e.g. self-coupling measurements and of events accessing new physics scenarios in regions of parameter space yet unexplored, requires exceptional trigger and data acquisition performance over the decade-long HL-LHC run.

This upgrade design takes advantage of and surpasses the Phase-I upgrade of the ATLAS TDAQ system, which will be installed during the second long shutdown of the LHC starting in 2019. The Phase-I TDAQ upgrade was designed to efficiently select and record events of interest at instantaneous luminosities that are up to twice that of the nominal LHC design luminosity with up to 80 proton-proton collisions per beam crossing, while maintaining trigger thresholds close to those used in Run 1. The HL-LHC is expected to start operations in the middle of 2026 and the performance increase to its ultimate configuration, where the instantaneous luminosity is levelled at $\mathcal{L} = 7.5 \times 10^{34} \text{cm}^{-2}\text{s}^{-1}$ and the experiments need to cope with a pile-up up to 200 inelastic collisions per bunch crossing. The Phase-II TDAQ upgrade is designed to cope with this LHC ultimate luminosity.

The upgraded trigger system will take advantage of increased granularity provided by the calorimeters, will improve efficiency for muon-based triggers and perform hardware-based tracking profiting from the extended coverage of the planned Inner Tracker (ITk) [1.1]. A factor of 10 higher trigger rates than the ones expected in Run 3 are needed to retain the events to perform the physics programme in Run 4. The upgrade of the TDAQ system will
<figure><img src="image_9.png" /><figcaption>Figure 1.1: *Baseline upgrade programme of the LHC accelerator complex over the next two decades.* The plan shows the scheduled long shutdowns for the machine and experiment upgrades, and the projected luminosities delivered by the LHC in a three-year run period following each long shutdown [1.2].</figcaption></figure>

provide the required bandwidth and processing capacity to efficiently select events at high luminosity.

### 1.1 LHC Roadmap

The LHC and HL-LHC project schedule is shown in Fig. 1.1. After the consolidation of the machine elements during the Long Shutdown 1, the LHC operated at a centre-of-mass energy of $\sqrt{s} = 13$ TeV during Run 2, and delivered in the second half of 2017 an instantaneous luminosity in excess of $\mathcal{L} = 2.0 \times 10^{34} \text{ cm}^{-2}\text{s}^{-1}$, and an average of up to $\langle \mu \rangle \simeq 60$ minimum bias collisions per bunch crossing. By the end of Run 2 the LHC is expected to have delivered a total of about $150 \text{ fb}^{-1}$ to each of the experiments. During the Long Shutdown 2, scheduled for 2019-2020, the luminosity production will be consolidated and the upgraded (Phase-I) experiments will be able to record up to approximately $300 \text{ fb}^{-1}$ during Run 3 (2021-2023). The HL-LHC project [1.2] is planned to begin collisions in the second half of 2026 and will allow ATLAS [1.1][1.3][1.4] to collect an integrated luminosity of $3000 \text{ fb}^{-1}$ after ten years of operation. The HL-LHC upgrades of both the machine and the experiments will occur in the Long Shutdown 3 between 2024-2026, allowing full exploitation of the LHC physics programme.

The high luminosity configuration of the LHC will target an integrated luminosity a factor of 10 higher than that expected to be delivered by the end of Run 3. Figure 1.2a shows the possible evolution of the luminosity during the HL-LHC era. In Run 4 the luminosity will progressively increase potentially attaining a levelled instantaneous peak luminosity
<table><caption>Table 1.1: Comparison of the nominal LHC parameters with those of three possible HL-LHC schemes. The levelled luminosity is assumed for $\mu \simeq 140$. The levelling time assumes no emittance growth.</caption><thead><tr><th rowspan="2">Parameter</th><th>Nominal LHC</th><th colspan="3">Nominal HL-LHC 25ns</th></tr><tr><th>[Design Report]</th><th>[standard]</th><th>[BCMS]</th><th>[8b4e]</th></tr></thead><tbody><tr><td>Beam energy in collision [TeV]</td><td>7</td><td>7</td><td>7</td><td>7</td></tr><tr><td>Number of protons per bunch [×10<sup>11</sup>]</td><td>1.15</td><td>2.2</td><td>2.2</td><td>2.3</td></tr><tr><td>n<sub>b</sub></td><td>2808</td><td>2748</td><td>2604</td><td>1968</td></tr><tr><td>Number of collisions in IP1 and IP5</td><td>2808</td><td>2736</td><td>2592</td><td>1960</td></tr><tr><td>Beam current [A]</td><td>0.58</td><td>1.09</td><td>1.03</td><td>0.82</td></tr><tr><td>crossing angle [μrad]</td><td>285</td><td>590</td><td>590</td><td>554</td></tr><tr><td>beam separation [σ]</td><td>9.4</td><td>12.5</td><td>12.5</td><td>12.5</td></tr><tr><td>β* [m]</td><td>0.55</td><td>0.15</td><td>0.15</td><td>0.15</td></tr><tr><td>ε<sub>n</sub> [μm]</td><td>3.75</td><td>2.50</td><td>2.50</td><td>2.2</td></tr><tr><td>ε<sub>L</sub> [eV]</td><td>2.5</td><td>2.5</td><td>2.5</td><td>2.5</td></tr><tr><td>Levelled luminosity [×10<sup>34</sup>cm<sup>-2</sup>s<sup>-1</sup>]</td><td>-</td><td>5.32</td><td>5.02</td><td>5.03</td></tr><tr><td>Events / crossing</td><td>27</td><td>140</td><td>140</td><td>140</td></tr><tr><td>Levelling time [hours]</td><td>-</td><td>8.3</td><td>7.6</td><td>9.5</td></tr></tbody></table>

of $\mathcal{L} = 5 \times 10^{34} \text{cm}^{-2}\text{s}^{-1}$ ($\langle \mu \rangle \simeq 140$ $^1$), which would be maintained for the duration of the HL-LHC lifetime. In this configuration, approximately $250 \text{ fb}^{-1}$ will be delivered to each experiment per year, for a total of $3000 \text{ fb}^{-1}$ by the end of Run 5. The configuration assumes 160 days of physics operations of the LHC complex per year and a physics efficiency of 50%.

The main parameters of the HL-LHC have been re-optimised in 2016 [1.2]. Table 1.1 compares the nominal parameters of the LHC with three possible HL-LHC configuration schemes, in operation in times in 2017:

* the standard 25 ns configuration,

* the "Batch Compression Merging and Splitting (BCMS)" scheme: a different configuration with reduced transverse emittance which may be utilised if operations with high beam intensities result in unforeseen emittance blow up, and

* the "8b4e" configuration consisting of 8 colliding bunches followed by 4 empty (colli-sionless) bunches which may be implemented to mitigate the impact of electron-cloud effects.

<footnote n="1">¹ The average number of interactions per bunch crossing values are calculated using the formula $\langle \mu \rangle = \mathcal{L} \times \sigma_{\text{inel}}/n_b \times f_{\text{rev}}$, where the total inelastic proton-proton cross section is assumed to be $\sigma_{\text{inel}} = 85 \text{ mb}$ at $\sqrt{s} = 14$ TeV, the LHC revolution frequency is $f_{\text{rev}} = 11.245 \text{ kHz}$, and $n_b$ is the number of colliding bunches at the ATLAS interaction point.</footnote>
<figure><img src="image_10.png" /><figcaption>Figure 1.2: Planned instantaneous (●) and integrated (—) luminosity profiles for two HL-LHC configurations [1.2].</figcaption></figure>
<table><caption>Table 1.2: Comparison between the planned HL-LHC nominal and ultimate luminosity parameters.</caption><thead><tr><th>Configuration</th><th>L<sub>inst</sub><br>[10<sup>34</sup>cm<sup>-2</sup>s<sup>-1</sup>]</th><th>&lt;μ&gt;</th><th>∫ L per year<br>[fb<sup>-1</sup>]</th></tr></thead><tbody><tr><td>Baseline</td><td>5</td><td>140</td><td>250</td></tr><tr><td>Ultimate</td><td>7.5</td><td>200</td><td>&gt;300</td></tr></tbody></table>

If the performance of the HL-LHC can be pushed further and the experiments are able to cope with pile-up up to $\langle\mu\rangle \simeq 200$ $^2$, the ultimate HL-LHC scenario shown in Fig. 1.2b could be realised. Table 1.2 presents a comparison between the two configurations. After the Long Shutdown 4 (2030) the instantaneous levelled luminosity could reach $\mathcal{L} = 7.5 \times 10^{34} \text{cm}^{-2}\text{s}^{-1}$, resulting to more than 300 fb$^{-1}$ per year and up to 4000 fb$^{-1}$ at the end of the HL-LHC lifetime.

## 1.2 Physics Drivers for the HL-LHC Upgrades

The Phase-II TDAQ upgrade must support the broad ATLAS physics programme for the HL-LHC; this programme has been presented and discussed in detail in several documents, here listed in chronological order: (i) the Phase-II Upgrade Letter of Intent [1.4], dating from 2012, (ii) the two reports submitted to the European Committee for Future Accelerators (ECFA) [1.5][1.6], published in 2013 and 2014 respectively, and (iii) the Scoping Document [1.1] released in late 2015. Table 1.3 presents the wide spectrum of physics goals and a representation of analyses that will be carried out by ATLAS to exploit the full potential of the HL-LHC. Also given are the corresponding trigger signatures. These goals include unveiling the paradigm of electroweak symmetry breaking through precision measurements of the properties of the Higgs boson, improved measurements of all relevant Standard Model parameters including the study of rare Standard Model processes, searches for Beyond the Standard Model (BSM) signatures and flavour physics. The trigger has to address also specific challenges of the heavy-ion physics programme. This Technical Design Report (TDR) documents a subset of analyses in Chapter 2, and Table 1.3 includes references to the relevant sections in Chapter 2 for those selected analyses, focusing on the enhancements of the physics object measurements enabled by the upgrades. Unless otherwise specified, the trigger performance studies and the benchmark physics analyses have been carried through assuming the pile-up conditions for the ultimate HL-LHC configuration, $\langle\mu\rangle \simeq 200$.

<footnote>$^2$ A benchmark scenario with a $\langle\mu\rangle$ of approximately 200 is obtained by assuming $n_b = 2808$ and a peak instantaneous luminosity of $\mathcal{L} = 7.5 \times 10^{34} \text{cm}^{-2}\text{s}^{-1}$. For more details, see the HL-LHC Technical Design Report [1.2].</footnote>
<table><caption>Table 1.3: Outline of the main physics drivers relevant for the TDAQ system at the HL-LHC, including examples of final state processes and their corresponding trigger signatures. Only a limited sub-set of those processes are documented in this TDR, reported in the sections referenced in the last column. The abbreviation “diff” stands for differential.</caption><thead><tr><th>Physics Drivers @ HL-LHC</th><th>Processes</th><th>Trigger Signatures</th><th>TDR Sect.</th></tr></thead><tbody><tr><td rowspan="5">Precision measurements of the properties of the Higgs Boson</td><td>Couplings to fermions</td><td>H → ττ, H → μμ, t<span style="text-decoration: overline">t</span>H, H → bb</td><td>single/di-e or μ / di-τ</td><td>2.2, 2.4</td></tr><tr><td>Couplings to W/Z, diff. cross-sections</td><td>H → γγ, H → W<sup>+</sup>W<sup>-</sup> → <span style="text-decoration: overline">ℓ<sup>+</sup>νℓ<sup>-</sup>ν</span>, H → ZZ<sup>(*)</sup> → <span style="text-decoration: overline">ℓ<sup>+</sup>ℓ<sup>-</sup>ℓ<sup>+</sup>ℓ<sup>-</sup></span></td><td>e/μ, di-γ</td><td>2.3</td></tr><tr><td>Self-coupling</td><td>HH → bbττ / bbγγ / 4b</td><td>di-τ/γ, multi-jets</td><td>2.5</td></tr><tr><td>Scalar Higgs boson vs. BSM composite</td><td>H → <span style="text-decoration: overline">ℓℓ'</span>, ZH → <span style="text-decoration: overline">ℓℓ</span> + (inv)</td><td>e/μ</td><td></td></tr><tr><td>Forward/backward asymmetry</td><td>Z → e<sup>+</sup>e<sup>-</sup>, μ<sup>+</sup>μ<sup>-</sup></td><td>single e/μ</td><td></td></tr><tr><td rowspan="2">Precision Standard Model Measurements</td><td>Vector-boson scattering</td><td>WWjj, WZjj</td><td>single e/μ</td><td></td></tr><tr><td>Precision top mass and cross-sections</td><td>t<span style="text-decoration: overline">t</span> production</td><td>e/μ, large R-jets/multi-jets</td><td>2.1</td></tr><tr><td rowspan="5">Searches for BSM Signatures</td><td>Searches for new vector bosons</td><td>Vector Boson Fusion (VBF) Z' → <span style="text-decoration: overline">ℓℓ'</span></td><td>high-p<sub>T</sub> single e/μ</td><td></td></tr><tr><td>Searches for electroweak SUSY</td><td>χ<sub>1</sub><sup>+</sup>χ<sub>2</sub><sup>0</sup> → WHχ<sub>1</sub><sup>0</sup>χ<sub>1</sub><sup>0</sup></td><td>E<sub>T</sub><sup>miss</sup>, single/di-e, μ, τ</td><td>2.2, 2.6</td></tr><tr><td>SUSY top partners</td><td>t̃<sub>1</sub> → tχ<sub>1</sub><sup>0</sup></td><td>large R-jets/multi-jets+E<sub>T</sub><sup>miss</sup></td><td></td></tr><tr><td>Dark matter</td><td>ISR+χ<sub>1</sub><sup>0</sup>χ<sub>1</sub><sup>0</sup></td><td>jets+E<sub>T</sub><sup>miss</sup></td><td></td></tr><tr><td>New resonances, SUSY</td><td>Z', χ<sub>1</sub><sup>0</sup> → jjj</td><td>jets, large R-jets, e/μ, γ, E<sub>T</sub><sup>miss</sup></td><td>2.3, 2.5</td></tr><tr><td rowspan="3">Flavour Physics</td><td>Long-lived particles</td><td>g̃ → q<span style="text-decoration: overline">q</span>χ̃<sub>1</sub><sup>0</sup>, χ̃<sub>1</sub><sup>±</sup> → π<sup>±</sup>χ̃<sub>1</sub><sup>0</sup></td><td>high impact parameter, E<sub>T</sub><sup>miss</sup></td><td></td></tr><tr><td>Lepton Flavour Violation</td><td>τ → μμμ</td><td>low p<sub>T</sub> di-μ</td><td></td></tr><tr><td>Searches for FCNC in top decays</td><td>t → u/c + H/Z</td><td>single e/μ</td><td></td></tr><tr><td rowspan="3">Heavy-Ion Physics</td><td>Rare B-meson decays</td><td>B → μμ, B<sub>s</sub> → J/Ψ + Φ</td><td>low p<sub>T</sub> di-μ</td><td>2.10</td></tr><tr><td>Light-by-light scattering</td><td>γγ → γγ</td><td>low p<sub>T</sub> di-photons</td><td></td></tr><tr><td>Electroweak production</td><td>W/Z/t</td><td>single e/μ</td><td></td></tr><tr><td rowspan="2"></td><td>In-medium parton energy loss (jets in PbPb)</td><td>mono-jets</td><td>jet, minbias</td><td></td></tr><tr><td>Quarkonia production</td><td>J/Ψ, Y</td><td>low-mass di-e/μ</td><td>2.11</td></tr></tbody></table>
<table><caption>Table 1.4: ATLAS HL-LHC Upgrade plans and reference to the detector system TDRs.</caption><thead><tr><th>Detector System</th><th>Upgrade scope</th><th>CDS Reference</th></tr></thead><tbody><tr><td>ITk Pixel Detector</td><td>Sensors, modules, mechanics, FE electronics</td><td>CERN-LHCC-2017-021</td></tr><tr><td>ITk Strip Detector</td><td>Sensors, modules, mechanics, FE electronics</td><td>CERN-LHCC-2017-005</td></tr><tr><td>LAr Calorimeter</td><td>FE and BE electronics</td><td>CERN-LHCC-2017-018</td></tr><tr><td>Tile Calorimeter</td><td>Mechanics, FE and BE electronics FE electronics</td><td>CERN-LHCC-2017-019</td></tr><tr><td>Muon Spectrometer</td><td>Inner Barrel MDT chambers Inner Barrel RPC stations</td><td>CERN-LHCC-2017-017</td></tr><tr><td>TDAQ</td><td>On-detector readout and trigger electronics (this document)</td><td>CERN-LHCC-2017-020</td></tr></tbody></table>

## 1.3 ATLAS Upgrade Strategies for the HL-LHC

The overall upgrade strategy of the ATLAS collaboration for the HL-LHC upgrades is described in detail in the Phase-II Letter of Intent [1.4] and, more recently, in the Scoping Document [1.1].

In most cases, the design and techniques proposed for the Phase-II upgrades represent an evolution from the new designs and technologies already introduced during the Long Shutdown 1 consolidation programme, and from the Phase-I upgrades now being prepared for installation during Long Shutdown 2. This evolutionary approach leads to a relatively solid understanding of the technical challenges, as well as the uncertainties on the costs and the required resources, even at the present early stage of the Phase-II upgrade activities.

A synoptic view of the upgrade plans, with the references to the TDRs of each system, is presented in Table 1.4. An internal ATLAS document [1.7] defines the trigger and readout requirements on the interfaces to the front-end electronics systems of each detector in Table 1.4.

### Inner Tracker

New silicon strip and pixel detectors will be installed during Long Shutdown 3 with a geometrical acceptance up to $|\eta| = 4.0$. The final layout of the detectors has evolved with respect to the layout presented in [1.1]. For example, five pixel layers and four strip layers are planned for the ITk barrel. The full details of the ITk design are described in the ITk strip [1.8] and pixel [1.9] TDRs.
## Calorimeters

The readout electronics of both the LAr and Tile calorimeters need to be upgraded because of radiation tolerance limits, and because the on-detector front-end electronics cannot operate with the trigger rates and latencies required for the Phase-II luminosities. The current FCal calorimeter will remain in Phase-II (contrary to the strategy presented in [1.1]). ATLAS has evaluated that the risks of replacing the forward LAr calorimeter outweighed the possible performance and physics benefits.

## Muon Spectrometer

The scope of the Muon Spectrometer upgrade focuses primarily on the improvement of the performance of the muon trigger chambers. The Level-0 trigger electronics of the RPC and TGC chambers will be upgraded. New RPC detectors will be added in the barrel ($|η| < 1$) to significantly increase the solid angle coverage and the redundancy of the Level-0 trigger system. The replacement of the MDT front-end readout will address the trigger rate and latency requirements of the TDAQ system in Phase-II and allow the use of MDT hit information to improve the muon $p_T$ resolution in the Level-0 trigger.

## Other detectors

One new detector currently expressing interest as an upgrade for Phase-II but not having reached the maturity of providing a TDR is the High-Granularity Timing Detector (HGTD), a detector intended to distinguish between collisions occurring very close in space but well-separated in time. The current proposed detector design is based on low-gain avalanche detector technology that will cover the pseudorapidity region between 2.4 and 4.0, with a timing resolution of 30 ps for minimum-ionising particles. High-precision timing will improve the track-to-vertex association in the forward region, impacting jet and lepton reconstruction, as well as offering unique capabilities for online and offline luminosity determination. Should this detector be approved, it may impose requirements on the TDAQ system as described in Section 4.1.4.

## Trigger and Data Acquisition System

Since the ATLAS Phase-II Upgrade Scoping Document<fnref n="3" /> [1.1], the design of the architecture of the TDAQ system has developed, resulting in a single-level hardware trigger in the baseline design. This decision was reached through several approval steps:

<footnote n="3">³ The TDAQ scenario in the scoping document described two "custom-hardware" trigger levels that allowed for data streaming off-detector either after an initial trigger decision, or in some cases, at the full 40 MHz bunch crossing rate.</footnote>
* The TDAQ system prepared an IDR document that was reviewed in April 2016; the resulting report was approved by the ATLAS Executive Board in June 2016.

* A sub-committee of the ATLAS Upgrade Steering Committee was appointed by the ATLAS Upgrade Coordinator to address ATLAS-wide aspects of the trigger rates and assess the capabilities and limitations of the detector systems' readout architectures. This sub-committee's final report was discussed and approved by the ATLAS Upgrade Steering Committee in November 2016, recommending a single-hardware-level trigger architecture capable of evolving into a two-level (Level-0 and Level-1) hardware trigger system based on the use of tracking reconstruction in the Level-1 trigger.

* The ATLAS Executive Board approved the TDAQ Phase-II Upgrade Project in December 2016; this was endorsed by the ATLAS Collaboration Board in February 2017.

The resulting baseline architecture design is based on a single Level-0 hardware trigger with a detector readout rate of 1 MHz and a maximum latency of 10 µs. The Level-0 trigger decision is formed using calorimeter and muon information. The Phase-I calorimeter trigger processors will be maintained during the HL-LHC operations, and their firmware optimised for the pile-up conditions and the extra latency budget available. The processors will be complemented by additional processors that will implement more sophisticated, offline-like algorithms to provide additional background rejection. The EF system will further select events based on a commodity processor farm and a custom HTT to reduce the overall CPU requirements. The system is designed to accommodate an output rate of 10 kHz to permanent storage, averaged over a fill.

Each system and sub-system is designed to be capable of evolving to a dual-level hardware-based trigger architecture as a mitigation strategy in case pile-up conditions at the HL-LHC either challenge the readout capabilities of certain detectors (for example of the innermost layers of the ITk) to the limits of the bandwidth available, or in case the rates of hadronic trigger signatures surpass the current predictions. This two-level hardware trigger design specifies a Level-0 trigger rate up to 4 (2) MHz and 10 µs latency, followed by a Level-1 trigger rate of 600 (800) kHz and latency up to 35 µs. In this design, hardware-based track reconstruction is implemented in the Level-1 trigger system by reconfiguring part of the HTT; these reconstructed tracks are combined with calorimeter- and muon-based trigger objects to form the Level-1 decision.

## 1.4 Baseline Architecture and Structure of the Upgrade Project

A high-level functional description of the architecture is presented in Fig. 1.3. The three main systems are the Level-0 Trigger System, the Data Acquisition (DAQ) System, and the Event Filter System<fnref n="4" />.

<footnote n="4">⁴ They are at the second level of the PBS in the project organisation.</footnote>
<figure><img src="image_11.png" /><figcaption>Figure 1.3: Design of the TDAQ Phase-II upgrade architecture, highlighting the organisation of the Upgrade Project in three main systems: Level-0 Trigger, DAQ (Readout and Dataflow subsystems), and Event Filter. Direct connections between each Level-0 trigger component and the Readout system are suppressed for simplicity.</figcaption></figure>
## Level-0 Trigger System

The hardware-based Level-0 Trigger system is composed of the Level-0 Calorimeter Trigger (L0Calo), the Level-0 Muon Trigger (L0Muon), the Global Trigger and the Central Trigger sub-systems. The L0Calo sub-system is composed of the Phase-I electron Feature EXtractor (eFEX), jet Feature EXtractor (jFEX), and global Feature EXtractor (gFEX) complemented by a new fFEX component to reconstruct forward jets and electrons. The L0Muon sub-system comprises the Barrel and Endcap Sector Logic processors (for muon candidates from the barrel RPC and TGC, respectively), the NSW Trigger Processor, and the MDT Trigger Processor. The MUCTPI component interfaces the L0Muon sub-system with the CTP. The Global Trigger uses full-granularity calorimeter information to perform offline-like algorithms, refines the trigger objects calculated by L0Calo and L0Muon, calculates event-level quantities, and executes topological algorithms. The CTP forms triggers based on combinations of inputs or conditions received from the Global Trigger and other sources; it applies pre-scale factors and introduces deadtime when necessary to avoid saturation in the front-end and readout systems; and it ultimately makes the final decision on whether the event is accepted, driving the Trigger, Timing, and Control system (TTC) network to start the readout process of the detectors.

## Data Acquisition (DAQ)

The result of the Level-0 trigger decision is transmitted to all detectors, upon which the resulting trigger data and detector data are transmitted at 1 MHz through the Readout subsystem (Readout) sub-system, which contains the Front-End LInk eXchange (FELIX) and Data Handler components, and the Dataflow subsystem (Dataflow) sub-system, which contains the Event Builder, Storage Handler, and Event Aggregator components. Together these compose the DAQ system. They are based on commodity Personal Computer (PC) servers and standard networking infrastructure. The Readout system includes custom input/output cards to convert the detector front-end protocol data into standard network packets.

## Event Filter (EF)

The EF system consists of a CPU-based processing farm and a HTT co-processor. The main function of the EF system is to refine the trigger objects in order to get down to the final output rate. The HTT includes regional (rHTT) and global (gHTT) track reconstruction capabilities. Events will be rejected as early as possible during their processing. The EF trigger decision is communicated to the Dataflow system, which transfers accepted events to permanent storage.
The planned physical locations of the Phase-II TDAQ Upgrade components are described in Section 16.1.1. The baseline choice is to install all of the Level-0 Trigger modular electronics, the DAQ Readout components (FELIX and the Data Handlers) and the HTT electronics in the main underground electronics cavern, USA15. The Dataflow and EF servers will be installed in the surface-level counting room, SDX1.

## 1.5 Outline of this Report

This document is organised into three main parts: a general overview, detailed technical descriptions of the sub-systems, and the project management and organisation.

Part I of this document contains this introduction, a description of the physics motivation for the upgrade (Chapter 2), the limitations of the Run 3 TDAQ system (Chapter 3) and the subsequent requirements for the Run 4 TDAQ system (Chapter 4), followed by a description of the proposed TDAQ system (Chapter 5). Chapter 6 contains the expected performance of the Phase-II TDAQ System.

Part II of this document contains detailed descriptions of each of the components of the proposed system, including hardware specifications and requirements for each component (Chapters 7-13), as well as a discussion of the plan for a potential evolution of the system (Section 14), should it be necessary. A system-by-system guide to this document is presented in Table 1.5, along with a list of each system's components and the relevant section of the document that contains their detailed technical descriptions.

Originally the Detector Control System (DCS) was a fully integrated component of the TDAQ system and reporting to the TDAQ Management. After installation and commissioning completed in 2008, ATLAS decided to support DCS operations through Technical Coordination, with the scientific and technical personnel affiliated to the Technical Coordination Project Office. Tight integration between TDAQ and DCS groups will be required for the Phase-II upgrades, as DCS functions will be carried through the Readout sub-system in a few of the upgrade projects, namely ITk-strips and NSW. Outlines of the general DCS plans and of the TDAQ interfaces are presented in Chapter 15 of this report. Nevertheless, it should be highlighted that DCS is not part of the TDAQ Phase-II Upgrade Project scope, and, consequently, no aspect related to its Project Management will be addressed in this TDR.

The strategy for system integration, installation, and commissioning is described in Chapter 16. And finally, Part III of this document focuses on the upgrade project management aspects, covering:

* the organisation of the project and initial project management (Chapter 17);

* the product breakdown structure, CORE costs (presented for the baseline TDAQ architecture), and spending profile (Chapter 18);

* the work breakdown structure, planning, and scheduling (Chapter 19);
<table><caption>Table 1.5: Organisation of the ATLAS TDAQ system, mapped to the detailed technical descriptions in Part-II of this TDR.</caption><thead><tr><th>System</th><th>Sub-system</th><th>Component</th><th>Technical Description</th></tr></thead><tbody><tr><td rowspan="10">Level-0 Trigger</td><td rowspan="4">L0Calo</td><td>eFEX</td><td rowspan="4">Chapter 7</td></tr><tr><td>jFEX</td></tr><tr><td>gFEX</td></tr><tr><td>ffEX</td></tr><tr><td rowspan="2">L0Muon</td><td>Barrel Sector Logic</td><td rowspan="2">Chapter 8</td></tr><tr><td>Endcap Sector Logic</td></tr><tr><td>MUCTPI</td><td>–</td><td>Section 10.3</td></tr><tr><td>Global Trigger</td><td>–</td><td>Chapter 9</td></tr><tr><td>CTP</td><td>–</td><td>Section 10.1</td></tr><tr><td>TTC</td><td>–</td><td>Section 10.2</td></tr><tr><td rowspan="2">DAQ</td><td>Readout</td><td>FELIX<br>Data Handlers</td><td>Section 11.4</td></tr><tr><td>Dataflow</td><td>Event Builder<br>Storage Handler<br>Event Aggregator</td><td>Section 11.5</td></tr><tr><td rowspan="2">EF</td><td>Processor Farm</td><td></td><td>Chapter 12</td></tr><tr><td>HTT</td><td>rHTT<br>gHTT</td><td>Chapter 13</td></tr></tbody></table>

* the required human and financial resources (Chapter 20); and

* the risk analysis and mitigation strategies (Chapter 21).

The glossary in Appendix IV defines many of the common acronyms and terms used in this document.

## References

[1.1] ATLAS Collaboration, *ATLAS Phase-II Upgrade Scoping Document*, Tech. Rep. CERN-LHCC-2015-020. LHCC-G-166, CERN, Geneva, Sep, 2015. https://cds.cern.ch/record/2055248.

[1.2] G. Apollinari et al., *High-Luminosity Large Hadron Collider (HL-LHC): Technical Design Report V. 0.1*. CERN Yellow Reports: Monographs. CERN, Geneva, 2017. http://cds.cern.ch/record/2284929.

[1.3] ATLAS Collaboration, *The ATLAS Experiment at the CERN Large Hadron Collider*, JINST 3 (2008) S08003.
[1.4] ATLAS Collaboration, Letter of Intent for the Phase-II Upgrade of the ATLAS Experiment, Tech. Rep. CERN-LHCC-2012-022. LHCC-I-023, CERN, Geneva, Dec, 2012. https://cds.cern.ch/record/1502664.

[1.5] ECFA High Luminosity LHC Experiments Workshop: Physics and Technology Challenges. 94th Plenary ECFA meeting, ECFA-13-284. https://cds.cern.ch/record/1631032.

[1.6] ECFA High Luminosity LHC Experiments Workshop - Physics and Technology Developments Summary submitted to ECFA 2014. 96th Plenary ECFA meeting, ECFA-15-289. https://cds.cern.ch/record/1983664.

[1.7] F. Lanni et al., Interfaces with Detector Front-End Systems. Requirement Document for HL-LHC, Tech. Rep. ATL-DA-ES-0051 v.1, EDMS Id 1563801 v.1, CERN, Geneva, March, 2016. https://edms.cern.ch/document/1563801.

[1.8] ATLAS Collaboration, Technical Design Report for the ATLAS Inner Tracker Strip Detector, Tech. Rep. CERN-LHCC-2017-005, CERN, Geneva, Apr, 2017. https://cds.cern.ch/record/2257755.

[1.9] ATLAS Collaboration, Technical Design Report for the ATLAS Inner Tracker Pixel Detector, Tech. Rep. CERN-LHCC-2017-021, CERN, Geneva, (in preparation).
## 2 Physics Motivation

The Phase-II TDAQ upgrade is designed to enable the HL-LHC programme which includes a large and rich programme of studies that explores a wide range of topics including rare and precision Higgs boson and Standard Model searches and measurements, searches beyond the Standard Model including SUSY, heavy-flavour physics, and heavy-ion physics.

This chapter describes a subset of the physics measurements that represent the breadth of the planned HL-LHC physics programme. The representative measurements used to motivate the trigger requirements are selected to be both challenges to the trigger system and central to the scientific goals. Beyond these specific examples, it is important that the system be flexible and the requirements sufficiently general to support new experimental analysis techniques and theoretical ideas in addition to the baseline programme.

The physics requirements for the trigger are a set of $p_T$ thresholds requirements for the relevant objects ($e, \mu, \tau, \text{jet, ...}$) and combinations thereof (invariant masses, etc.). Ideal triggers select the same events as the offline object-level reconstruction with good efficiency above the threshold. The effective offline threshold is considered to be the offline reconstructed $p_T$ where the trigger has reached approximately 90-95% of the plateau efficiency (maximum efficiency for all $p_T$); the exact definition relevant for each trigger varies. In general, systematic uncertainty-limited analyses (e.g., precision studies) do not use data where the trigger is lower than 90-95% of the plateau efficiency because of the additional systematic uncertainty incurred.

For each physics signature described in this chapter, a comparison is made between the thresholds that would be possible in Run 4 conditions with and without the Phase-II upgrade. The no-upgrade scenario, assumes a ~100 kHz Level-0 output rate, the Phase-I hardware selections unchanged, and rate allocations comparable to those planned for Phase-I (which has 100 kHz output rate for the first level). For the no-upgrade scenario, the triggers are assumed to be completely limited by the hardware trigger and no additional Event Filter limits are added. Of particular importance for the system design are the single lepton, di-hadronic $\tau$, multi-jet, and missing transverse energy triggers.

The TDAQ upgrade takes advantage of the upgrades of the other ATLAS detector systems to efficiently select physics within or beyond the Standard Model. This is achieved through a cost-effective high-bandwidth readout and the ability to run sophisticated algorithms to distinguish signal from background. The performance of the upgraded system comes from a combination of the high-bandwidth DAQ, which accepts 1 MHz from the Level-0 hardware trigger, and improved object identification delivered at Level-0 by the upgraded
calorimeter, muon, and global trigger systems. In order to support the high rate Level-0, the software-based Event Filter uses hardware-based regional and global tracking systems, rHTT and gHTT respectively, for track finding and fitting. Regional tracking is used to do early rejection to reduce the rate below 400 kHz. High-precision global tracking is used in the final selection to reduce the rate to 10 kHz.

The interplay between the Phase-II TDAQ system design (described in detail in Section 5) and representative physics goals, mapped through the corresponding triggers, is summarised in Fig. 2.1. For example, the Global Trigger enables a low-$p_T$ electron trigger at Level-0. Regional-tracking is then used to reduce the high rate early in the Event Filter processing. The L0Muon system provides good efficiency at a moderate rate supporting a low threshold single muon trigger. A final example is that the Global Trigger enables low thresholds for multi-jet and $E_T^{miss}$ triggers at Level-0, after which regional tracking and then full-detector tracking are used to reduce the background acceptance rate while preserving the physics acceptance.

The kind of studies used to motivate the trigger thresholds can be classified into three categories: 1) the dependence of the sensitivity on the $p_T$ threshold for an analysis based on simulated data including background modeling and systematic uncertainty estimates, 2) the dependence of signal acceptance on the trigger threshold, and 3) the impact of the trigger on a Run 2 result. The first category provides the most direct information, however analysis techniques may improve over the long period before the end of the HL-LHC data-taking. The second category shows the potential gain from the improved trigger acceptance, but does not investigate how much of this gain can be exploited in the full, optimised analysis. The third category is instructive, but does not fully take into account the increased challenges associated with higher instantaneous luminosities or the potential analysis optimisations that can be made with the benefit of two orders of magnitude more data.

The expected performance of the various trigger objects after the Phase-II upgrade is described in Chapter 6, including how trigger acceptance rates depend on thresholds and hardware configuration. Section 6.11 presents the allocation of the DAQ bandwidth resources, describing the resulting list of trigger thresholds and acceptance rates referred to as the trigger “menu”. The Phase-II trigger menu meets the physics requirements presented in the following sections.

## 2.1 Physics Signatures with Single-Electron and Single-Muon Triggers

Single-electron and single-muon signatures play a central role in LHC data analyses, since leptons are clear signals of electroweak physics. It is therefore critical to record as many events containing single leptons as possible, including W and Z events produced near threshold. The low mass of the Higgs boson, for which precision measurements are a
<figure><img src="image_12.png" /><figcaption>Figure 2.1: Schematic summary of the flow from the representative set of physics goals described in this section (left column) to the hardware systems (right column) needed to achieve these goals. The middle column lists the corresponding triggers required.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Figure 2.2: The integrated acceptance as a function of the single lepton $p_T$ threshold for four representative channels: $W \to l\nu$, $H \to \tau\tau b\bar{b}$, $t\bar{t}$, and a compressed spectrum SUSY model relevant for “Well-tempered Neutralino” motivated models. The Phase-II TDAQ upgrade would enable lowering the single lepton Level-0 threshold to 20 GeV from 50 GeV, the projected threshold without the upgrade.</figcaption></figure>

primary motivator for the Phase-II physics programme, also requires these single-electron and single-muon thresholds to remain low.

Among the physics processes selected by the single-lepton triggers are $t\bar{t}$ production, inclusive $W \to l\nu$, $HH \to \tau\tau b\bar{b}$ with at least one $\tau \to e$ or $\mu$, and electroweak SUSY signatures with low-$p_T$ leptons. The acceptance for each of these four processes as a function of the lepton $p_T$ threshold is shown in Fig. 2.2. The SUSY model is a “Well-tempered Neutralino” model that is designed to be consistent with the dark matter relic density [2.1]. A threshold of 20 GeV provides good acceptance for $WH$, $t\bar{t}$, and $\tilde{\chi}_1^\pm\tilde{\chi}_2^0 \to W^\pm\tilde{\chi}_1^0Z\tilde{\chi}_1^0$ with significant losses if the thresholds are raised to the no-upgrade scenario.

The search for non-resonant $HH$ production and anomalous Higgs boson self-couplings are key goals of the HL-LHC programme. Modification of the Higgs boson self-coupling can lead to changes the cross-section of order unity [2.2]. Specially, because of destructive interference removing the coupling approximately doubles the $HH$ cross-section. Figure 2.3 shows the sensitivity of the search for $HH \to \tau\tau b\bar{b}$, with one $\tau \to e$ or $\mu$ and one $\tau$ decaying hadronically, as a function of the offline lepton $p_T$ requirement which is determined by the trigger threshold. The points in the figure show the estimated sensitivity based on fully simulated signals and backgrounds scaling from the Run 2 result to the HL-LHC luminosity and centre-of-mass energy of 14 TeV. The analysis also includes a data-driven estimate of the jets misidentified as $\tau$ leptons (fake-$\tau$ background), which leads the result to be limited by the required minimum lepton $p_T$ (27 GeV) corresponding to the Run 2 trigger
<figure><img src="image_14.png" /><figcaption>Figure 2.3: Exclusion limit on the ratio of the cross-section to the Standard Model cross-section, $\sigma/\sigma_{SM}$, as a function of the leading lepton $p_T$ requirement for $HH \to \tau\tau b\bar{b}$ with one $\tau \to e$ or $\mu$ and one $\tau$ decaying hadronically. As discussed in the text, modifications of the Higgs self-coupling can modify the cross-section by factors of order unity. The dots are based on scaling signals and backgrounds from the Run 2 result and are thus limited by the lepton $p_T$ requirement of that analysis to only show above 27 GeV. The acceptance shown in Fig. 2.2 has been used to extrapolate up and down from 30 GeV. Above 30 GeV, the acceptance extrapolation shows good agreement with the Run 2 scaled result.</figcaption></figure>

threshold. Because the fully simulated result cannot be produced for a lepton $p_T$ threshold below 27 GeV, the acceptance shown in Fig. 2.2 has been used to extrapolate (solid line) up and down from 30 GeV assuming the expected limit just depends on the square-root of the acceptance. Above 30 GeV, the acceptance extrapolation shows good agreement with the Run 2 scaled result. The main background that would increase at lower lepton $p_T$ is the fake $e$ or $\mu$ leptons background, but they are not a major background in the analysis. The acceptance scaling is therefore expected to be a reliable estimate of the gain from reducing the threshold to 20 GeV, giving a ≈ 70% gain in sensitivity, equivalent to having ≈ 3× more data than in the no-upgrade scenario with a 50 GeV trigger threshold. The $HH \to \tau\tau b\bar{b}$ with one $\tau \to e$ or $\mu$ and one $\tau$ decaying hadronically channel will be combined with other $HH$ channels including $HH \to \tau\tau b\bar{b}$ with hadronic taus (discussed in Section 2.4), $HH \to \gamma\gamma b\bar{b}$ (Section 2.3), and $HH \to 4b$ (Section 2.5). Each channel contributes significantly and further possibilities are under investigation.
### 2.2 Physics Signatures with Two Leptons

Even with a low single-lepton $p_T$ threshold, dilepton triggers are needed to augment the acceptance in certain channels. Figure 2.4 shows the acceptance as a function of the leading and subleading lepton $p_T$ thresholds for $VBF H \to \tau\tau$ with both $\tau$ leptons decaying to $e$ or $\mu$ leptons and for the same compressed spectrum SUSY model, but with a more difficult mass splitting. The SUSY model acceptance is relative to a preselection of two leptons with $p_T > 3$ GeV and $E_T^{miss} > 50$ GeV that is used for simulation in the current (Run 2) analysis. A dilepton trigger with a threshold of 10 GeV on each lepton gives a substantial gain in acceptance in the SUSY model. The relative acceptance gains compared to a single-lepton trigger for the SUSY model is 39% while for $VBF H \to \tau\tau$ it is only about 7%$^1$. Including a moderately low dilepton threshold will improve the sensitivity to the broad, interesting class of signatures with small mass splittings similar to the SUSY model shown. Figure 2.4 also shows that in the no-upgrade scenario, a dilepton trigger does not have a high acceptance for these channels and will not effectively recover acceptance loss of the high single lepton trigger threshold. Finally, without the upgrade, the muon efficiencies would be significantly reduced, which has a large effect on the dimuon triggers because they are impacted by the square of the efficiency.

### 2.3 Physics Signatures with Photons

Studies of processes involving $H \to \gamma\gamma$ will enter a new regime of precision in the HL-LHC. These processes will be a workhorse of precision inclusive and differential cross-section measurements for the various production modes as well as from some beyond the Standard Model processes involving a Higgs boson. Photon triggers, and in particular di-photon triggers, are essential to performing these measurements. Additionally, the $HH \to b\bar{b}\gamma\gamma$ process is one of the key contributing channels for the overall $HH$ sensitivity, which is a key goal of the HL-LHC programme. Figure 2.5 shows the acceptance for $VBF H \to \gamma\gamma$ and $HH \to b\bar{b}\gamma\gamma$ as a function of the subleading photon $p_T$ threshold. The target threshold of 25 GeV preserves 22% more acceptance than the no-upgrade scenario with a 35 GeV threshold.

Single-photon triggers also play a significant role through Initial State Radiation (ISR), such as in searches for low-mass di-jet resonances, as seen in Fig. 2.6. In particular, low-mass di-jet resonances are more accessible with photon ISR selections than with jet ISR selections. Fig. 2.7 shows the di-jet invariant mass distributions for low-mass resonance search in partial Run 2 data [2.3], for the photon ISR + di-jet signature (left) and jet ISR + di-jet signature

<footnote n="1">¹ One can get these numbers by comparing the (20 GeV, 10 GeV) point to the (10 GeV, 10 GeV) point. For example, for the SUSY model, 16% of events pass the (20 GeV, 10 GeV) selection and 27% pass the (10 GeV, 10 GeV) selection, which means that 11% acceptance is added. That can then be compared to the (20 GeV, 0 GeV) which corresponds to the single lepton trigger and has an acceptance of 29%, giving a relative gain of 11%/28% = 39%.</footnote>
<figure><img src="image_15.png" /><figcaption>(a) VBF $H \to \tau\tau$ where both $\tau$ leptons decaying to e or $\mu$. The acceptance at the target is 45% and the target is 27% and the acceptance in the no-upgrade scenario is 4%. (b) Compressed Spectra SUSY. The acceptance at the target is 45% and the acceptance in the no-upgrade scenario is 1%.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Figure 2.5: Acceptance for VBF $H \to \gamma\gamma$ and $HH \to b\bar{b}\gamma\gamma$ for di-photon triggers.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>Figure 2.6: Summary plot of ATLAS bounds in the coupling-mediator mass plane from di-jet resonance searches using 2015 and 2016 data. Observed (solid lines) and expected (dotted lines) 95% CL upper limits on the coupling ($g_q$) as a function of the resonance mass ($m_{Z'}$) for a leptophobic $Z'$ model [2.4]. The darker blue curves show results from a search trigger is associated with a jet from the candidate di-jet resonance [2.5]. The red and purple curves show results from a search triggering on an ISR photon or jet, respectively [2.3]. Finally, the lighter blue curves show results from a trigger-level analysis (Trigger-object Level Analysis (TLA)) [2.6].</figcaption></figure>

(right). The mass range for the photon is wider than the mass range for jets, both because the trigger thresholds on the ISR photon are lower than those on the ISR jet and because in the photon ISR + di-jet case there is less ambiguity in the jet assignment as in the case when there are three jets where one is ISR and the other two are from the resonance. The two searches are complementary in the range where they overlap, due to the increased rate of jet ISR with respect to photon ISR. The sensitivity and mass range of both searches are driven by the trigger thresholds on photon and jet. More information on the motivation for low-coupling-constant di-jet resonance searches is given in Section 2.5. A low photon trigger threshold will benefit the reach of these analyses both in coupling constant and the minimum resonance mass accessible.

## 2.4 Physics Signatures with Hadronically Decaying Tau Leptons

With 64% of tau leptons decaying to hadrons, a dedicated tau trigger to discriminate between these tau decays and quark- or gluon-initiated jets provides important support to the ATLAS
<figure><img src="image_18.png" /><figcaption>Figure 2.7: The reconstructed di-jet mass distribution (filled points) for events containing (left) a photon with $p_T > 150$ GeV and two jets with $p_T > 25$ GeV and (right) a jet with $p_T > 430$ GeV and two additional jets with $p_T > 25$ GeV. The open points in both plots show the expected distributions of a $Z'$ signal. The photon-triggered sample has a much better mass resolution because of the lower trigger threshold of the photon trigger compared to the single-jet trigger.</figcaption></figure>

physics programme. Additionally, these decays carry information about their parent particles, providing the ability to perform spin and CP measurements of heavy resonances that decay to taus. Within the Standard Model, the mass dependence of the Higgs boson couplings means that $H \to \tau\tau$ is a process with a substantial rate and will be the most precisely measured leptonic branching fraction of the Higgs boson decay channels. In addition, the $HH \to \tau\tau b\bar{b}$ process will be a significant contribution to the measurement of non-resonant $HH$ production. Figure 2.8 shows the acceptance for VBF $H \to \tau\tau$ and $HH \to \tau\tau b\bar{b}$ where both tau leptons decay hadronically. Comparison to the no-upgrade scenario shows the substantial gain in acceptance for the achievable target threshold of leading tau lepton $p_T > 40$ GeV and sub-leading tau lepton $p_T > 30$ GeV; for VBF $H \to \tau\tau$ the gain is a factor of ~3.7 and for $HH \to \tau\tau b\bar{b}$ the gain is a factor of ~2.5. Further acceptance improvement for hadronic tau leptons could be made in the split hardware-level trigger system as described in Section 14.5.

## 2.5 Physics Signatures with Jets

Jet triggers are used in a variety of topics including multi-jet triggers for the $HH \to 4b$ signature and SUSY particles decaying to two Higgs bosons and $E_T^{miss}$. Single jet triggers are relevant for di-jet resonance searches and searches where an ISR jet is used. As compared to the previously mentioned photon ISR, jets provide higher rates, but also require higher
<figure><img src="image_19.png" /><figcaption>(a) VBF $H \to \tau\tau$ acceptance using $\tau\tau$ triggers where both $\tau$s decay hadronically. The acceptance at the target is 30% and the acceptance in the no-upgrade scenario is 8%</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>(b) $HH \to \tau\tau b\bar{b}$ acceptance using $\tau\tau$ triggers where both $\tau$s decay hadronically. The acceptance at the target is 32% and the acceptance in the no-upgrade scenario is 13%</figcaption></figure>

Figure 2.8: Acceptance for VBF $H \to \tau\tau$ and $HH \to \tau\tau b\bar{b}$ for dihadronic tau ($\tau\tau$) triggers where both $\tau$s decay hadronically.

$p_T$ selections. As shown in Fig. 2.6, the two methods are complementary; photon-triggered ISR events are sensitive at low $Z'$ masses, while jet-triggered ISR events are more sensitive at higher $Z'$ masses.

For the $HH$ non-resonant process, a dedicated study [2.7] of the sensitivity as a function of the offline jets $p_T$ threshold has been performed. The results are shown in Fig. 2.9 assuming that systematics are not a strong limitation on the result. This shows that the loss due to the trigger requirements below 50–60 GeV moderate (but non-negligible), and above ≈ 65 GeV the sensitivity in this important analysis degrades rapidly. The current estimated achievable threshold for the upgraded system is 65 GeV. A no-upgrade scenario would require a threshold of approximately 100 GeV, leading to a loss approximately a factor of two on the $\sigma(HH \to 4b)/\sigma(HH \to 4b)_{SM}$ limit. Results with more pessimistic systematics assumptions show similar trigger impacts. For a more detailed discussion of the analysis and the impact of the system design on it, see Section 6.13. The Standard Model $HH \to 4b$ non-resonant process cannot be observed with this channel alone, but the analysis would be sensitive to an enhanced cross-section due to anomalous Higgs self-couplings.

Di-jet resonance searches can be motivated by a variety of physics models. In particular, resonances with relatively small couplings to visible matter have been motivated by recent dark matter models [2.4][2.8]. Figure 2.6 shows a summary of the ATLAS bounds on the coupling $g_q$ as a function of the $Z'$ resonance mass. At high mass, above ~ 1.5 TeV (dark blue line), the single jet trigger is used. For di-jet resonances with masses below ~ 1 TeV, triggers are designed to select an ISR jet (purple line) or photon (red line, see also Section 2.3) present in the process rather than the decay products of the resonance. The jet $p_T$ threshold affects the lower bound of the di-jet+ISR search with ISR jet. This is because
<figure><img src="image_21.png" /><figcaption>Figure 2.9: Expected 95% C.L. upper limit on the cross-section ratio $\sigma(HH \to 4b)/\sigma(HH \to 4b)_{SM}$ as a function of the minimum $p_T$ requirement applied to the fourth-leading jet, assuming that systematics are not a strong limitation on the result. As discussed in Section 2.2, modifications of the Higgs self-coupling can modify the cross-section by factors of order unity. Results with systematics show similar trigger impacts. For a more detailed discussion, see Section 6.13.</figcaption></figure>

the trigger selection of a high-$p_T$ ISR jet biases the distribution of the invariant mass of the second and third leading jet for the QCD multijet background, which is problematic for the background estimation used in these searches.

The single-jet $p_T$ threshold is driven by the Event Filter output limitation. The rate of high-$p_T$ single-jet events that can be accepted by the Level-0 and used for combinations with other signatures at the Event Filter is on the order of 25 kHz for HL-LHC; this corresponds to a $p_T$ threshold of 180 GeV. Such a rate is much larger than the maximum recording rate of 10 kHz. An alternative is to reduce the amount of recorded data for these events by recording only the reconstructed objects in the Event Filter instead of the full detector data. In order for this to work, the reconstruction in the Event Filter needs to be as close as possible to a well understood offline reconstruction. This has been implemented in Run 2 and the result is shown by the light blue line in Fig. 2.6 (named TLA in the legend). It provides a substantial gain in sensitivity in the ~450–1000 GeV $Z'$ mass range (purple line in the figure). This illustrates the value of a low single-jet threshold and reconstruction in the Event Filter that closely follows the offline reconstruction (including having tracking available for pile-up mitigation and calibrations). The mass range of this search is ultimately limited by the Level-0 thresholds, and by the CPU requirements for obtaining tracks associated to trigger jets that can guarantee a good pile-up suppression performance.
<figure><img src="image_22.png" /><figcaption>Figure 2.10: Separation between (a) the W and b-quark as a function of the top-quark $p_T$ and (b) the two quarks from the W as a function of the W $p_T$ in $Z' \to t\bar{t}$ events.</figcaption></figure>

For boosted W, Z, and H bosons and top quarks, the jets from the final state partons begin to merge into a single large jet which requires a dedicated trigger. Analyses address this by first reconstructing the entire heavy object as one large-radius jet reconstructed with the anti-$k_t$ algorithm with a radius parameter of $R = 1.0$. In order to efficiently trigger on these signatures, the trigger must similarly accept events with high-$p_T$ large-radius jets. Figure 2.10 shows the angular separation between the W and b-quark as a function of top quark $p_T$ in $Z'$ to $t\bar{t}$ events and the separation between the two quarks from the W as a function of the W $p_T$ in same events. They have separations smaller than the standard $R = 0.4$ jets radius around 300-400 GeV, at which point the multijet trigger will begin to lose efficiency; thus, 300-400 GeV is the appropriate target range for a large-$R$ jet trigger. The target threshold has been set at 300 GeV (see Section 6.8) while in the no-upgrade scenario, the threshold would be ≈ 460 GeV.

## 2.6 Physics Signatures with Missing Transverse Energy

Missing transverse momentum ($E_T^{\text{miss}}$) is a critical trigger signature for Standard Model measurements (e.g., $ZH \to vवb\bar{b}$), searches for SUSY, and in searches for other new physics that result in events that do not interact with the detector (e.g. dark matter) This also includes processes such as “disappearing tracks” where a charged particle is produced and decays in the detector tracking volume. Figure 2.11 shows the acceptance as a function of the missing energy threshold for $ZH \to vवb\bar{b}$ and a compressed SUSY scenario. In the compressed SUSY scenario, the momentum of the invisible particles (observed as $E_T^{\text{miss}}$) is primarily due to the boost of the invisible system due to ISR. The $E_T^{\text{miss}}$ distributions will therefore be similar for all models in which the $E_T^{\text{miss}}$ is due to ISR with only a weak depend-
<figure><img src="image_23.png" /><figcaption>Figure 2.11: The integrated acceptance as a function of the missing transverse energy threshold for representative channels. A representative compressed SUSY model and $ZH \to vवbб$ are shown.</figcaption></figure>

ence on the mass of the invisible system. The typically achievable $E_T^{\text{miss}}$ trigger threshold is ~200 GeV for the current Run 2 trigger system. Operating the Phase-I system in Phase-II conditions with no-upgrade would result in a threshold of 300 GeV. Each 50 GeV reduction in threshold corresponds to a gain of approximately 50% – 100% in acceptance for the models shown. As discussed in Section 6.8, a target threshold of ~200 GeV is achievable in the baseline upgrade scenario.

## 2.7 Physics Signatures with Forward Electrons

The capability for triggering on forward electrons adds acceptance to the trigger system for Phase-II. The acceptance for lepton triggers is generically enhanced, but the forward trigger in particular extends the coverage for high-rapidity electroweak bosons. Central (C) and forward (F) electrons are defined to be those with $|\eta| < 2.5$ and $|\eta| > 2.5$, respectively. Candidate $Z \to ee$ events can be classified as CC, CF, and FF. Figure 2.12 shows the distribution of Z boson rapidities accessible by the central trigger (CC and CF) and those only accessible with a forward trigger (FF). This extended reach in boson rapidity translates into an increased reach for determining the proton Parton Distribution Function (PDF). In particular, the large and small $x$ regions are significantly impacted. The impact will be similar to the impact of recent LHCb [2.9] measurements on PDFs [2.10], but with significantly greater statistical power (and a systematic benefit of measuring the central and forward regions in the same experiment).
<figure><img src="image_24.png" /><figcaption>Figure 2.12: Unique Z boson rapidity ($y_{m_{ee}}$) reach enabled by a forward electron trigger. The labels CC, CF, and FF stand for central-central, central-forward, and forward-forward pseudorapidity electrons, respectively. The y-axis is the yield with an arbitrary normalisation.</figcaption></figure>

## 2.8 Physics Signatures with Exotic Objects

Some new physics scenarios produce signatures which are not identified by the standard set of reconstructed objects ($e, \mu, \tau, \text{jet},$...). For example, long-lived particles (LLPs) might not have tracks associated with them, or might not point back to the beamline as prompt objects do. In addition, models such as $\tau \to \mu\mu\mu$ and muon-jets resulting from Hidden Valley models [2.11] can result in close-by muons that are not identified with high efficiency. Figure 2.13, shows the opening angle between muons in a Hidden Valley model which is much smaller than the current system can resolve (approximately 0.2 in $\Delta\phi(\mu, \mu)$). In the no-upgrade scenario, these can only be recorded by the single muon trigger (estimated to be 25 GeV, with the poor acceptance described in Section 3.2.2). In the upgraded scenario, a dedicated trigger is under development for a dimuon trigger with a $p_T$ threshold of $\approx 10$ GeV. The near-by muon conditions would be implemented in the new muon system Sector Logic, NSW Trigger Processor, and MDT trigger processor described in Chapter 8.

Dedicated signature-based triggers will be needed to address these exotic signatures, and such work is ongoing. In general, the system must be flexible enough to allow the development of a wide variety of dedicated triggers and accommodate new ideas introduced between now and the end of HL-LHC data-taking. In addition to supporting such possibil-
<figure><img src="image_25.png" /><figcaption>Figure 2.13: Example distribution of the muon momentum (y-axis) vs angular separation (x-axis) for muons in muon-jets resulting from Hidden Valley models [2.11]. The Phase-I system cannot find a second muon with $p_T$ ≈ 0.2 GeV can be used significantly reducing the acceptance.</figcaption></figure>

ities, it is important to maintain inclusive triggers such as the single-object (jet and photon) triggers described above as well as $E_T^{miss}$ and VBF inclusive triggers in order to maintain sensitivity to exotic physics signatures.

## 2.9 Physics with an Inclusive Vector Boson Fusion (VBF) Trigger

The VBF process plays an important role in Higgs boson physics and beyond. After the gluon-gluon fusion process it is the next largest Standard Model production cross-section. Using the full granularity in the forward calorimeters in the fFEX and the topological capability of the Global Trigger allows for the possibility of combining two jets with a large $\eta$ separation to make an inclusive VBF trigger. This is particularly interesting for Higgs boson decays that are difficult to trigger on, e.g. $H \to bb$ in the Standard Model. In addition, there are a wide variety of possible BSM Higgs decay modes summarised in [2.12]. Many of those modes, such as a single Higgs boson decaying to four b-quarks, are nearly impossible to trigger on directly, because of the low resulting jet energies. Another option is to trigger on the Higgs objects associated with the production mechanism as opposed to its decay products. The WH and ZH processes provide single leptons (and motivate a low single lepton threshold), but because they are lower cross-section than VBF and require the boson
to decay leptonically, a VBF trigger such as the one described in Section 6.10 is a promising option.

### 2.10 B-Physics Signatures

The B-physics analyses search for new physics effects indirectly, namely through possible deviations from Standard Model predictions of precision measurements or by studying very rare processes. To cope with high pile-up and the relatively low $p_T$ of the produced b-hadrons, the ATLAS B-physics programme focuses on fully reconstructable b-hadron decays with muons in the final state. In order to maximise signal yield, the muon trigger $p_T$ thresholds need to be kept low. In Run 2 the lowest unprescaled di-muon trigger for B-physics requires two muons, each with $p_T > 6$ GeV. An acceptable rate of this trigger is kept by additional topological selections at Level-1 (which is the first hardware level in Run 2), applying loose cuts on the di-muon system invariant mass and opening angle [2.13].

When no additional measures (as e.g. the Level-1 topological selections) are taken, the lowest unprescaled di-muon trigger in the upgrade menu requires two muons with $p_T > 10$ GeV. This would lead to a loss of signal yields in flagship B-physics analyses as seen in Fig. 2.14. A dedicated di-muon trigger with topological selections is under investigation.

<figure><img src="image_26.png" /><figcaption>Figure 2.14: The relative yield of signal B-meson decays (left: $B_s^0 \to \mu^+\mu^-$, right: $B_s^0 \to J/\psi(\mu^+\mu^-)\phi(K^+K^-)$) as a function of the $p_T$ thresholds applied on the muon candidate tracks. The yields are normalised to the number of signal events passing di-muon $p_T$ trigger thresholds of 6 GeV on both the muon candidates. This particular configuration corresponds to the lowest unprescaled di-muon trigger in Run 2 used for B-physics analyses. Run 1/2 baseline offline cuts are applied to the reconstructed B-meson decays [2.14][2.15].</figcaption></figure>
Another important aspect is the limited ability of the Run 2 Level-1 muon trigger to distinguish muons too close to one another in the momentum direction. The opening angle between the two muons in the b-decays depends on the b-hadron momenta and the invariant mass of the two muons. Figure 2.15 demonstrates this effect, showing signal events fraction under the condition that the two muons are separated either in $|\Delta\eta(\mu^+\mu^-)| > 0.2$ or $|\Delta\phi(\mu^+\mu^-)| > 0.2$ rad (typical Level-1 muon trigger granularity). This limitation becomes critical for B decays with small invariant mass di-muon pairs (namely $B \to \mu^+\mu^-X$ and $B \to J/\psi(\mu^+\mu^-)X$ decays). A study described in Section 6.9 is underway to improve the trigger acceptance for near-by muons.

<figure><img src="image_27.png" /><figcaption>Figure 2.15: The fraction of signal B-meson decays (left: $B_s^0 \to \mu^+\mu^-$, right: $B_s^0 \to J/\psi(\mu^+\mu^-)\phi(K^+K^-)$) with muons separated well enough for the Run 2 Level-1 trigger, as a function of the $p_T$ thresholds applied on the muon candidate tracks. The separation condition used here is that the opening angle of the two muons is either $|\Delta\eta(\mu^+\mu^-)| > 0.2$ or $|\Delta\phi(\mu^+\mu^-)| > 0.2$ rad. Run 1/2 baseline offline cuts are applied to the reconstructed B-meson decays [2.14][2.15].</figcaption></figure>

## 2.11 Physics Signatures for Heavy Ion Collision

The ATLAS HL-LHC programme will also include a study of Heavy-Ion (HI) collisions aimed at understanding the nature of a Quark-Gluon Plasma (QGP) and associated many-body QCD, which have unique triggering requirements. Studies of the HI performance are underway, but not yet complete. Described below is a qualitative summary of the selections used for HI programme. The system design is expected to provide similar performance gains to this programme as the pp programme, with significant improvements in the tracking-based triggers that are computing-limited in Run 2.

**Signatures with Jets** Jets, di-jets, multi-jets and b-tagged jets are used to study the interactions of colour charges in the QGP. The dominant issue for jet measurements in HI
collisions is the presence of a large amount of additional energy coming from the Underlying Event (UE). The properties of this energy depend on the impact parameter, or minimum distance, between the two colliding nuclei. Jet reconstruction in HI events needs to correct for the contributions from the large UE. The proposed system allows for such corrections optimised for HI in the Level-0 trigger.

**Signatures with Colourless Probes: Electrons, Muons, and Photons** Prompt photons, colourless probes, are an important tool for the study of the QGP. They probe the very initial stages of the collision because due to their colourless nature they are transparent to the subsequent evolution of the matter. The production rate of prompt photons is sensitive to the collision geometry and to modifications of the parton distribution functions of nucleons bound in a nucleus with respect to free nucleon. The complementary information is provided by measurement of electrons originating from decays of W and Z bosons. Furthermore, the photons or Z bosons provide a precise energy reference for the recoiling jet in γ-jet and Z-jet measurements. These measurements are essential to the understanding of interactions of colour charges with deconfined matter and are statistically limited with current datasets. Two-photon reactions or photo-nuclear reactions in ultra-peripheral heavy-ion collisions can be used to measure processes like light-by-light scattering that is potentially sensitive to BSM physics or nuclear PDFs, respectively. The sophisticated algorithms used for photon, electron, and muon identification in pp collision are also well suited to these applications, and this physics benefits similarly from the upgrade.

**Signatures with Muons** In addition to the colourless probes discussed above, muon triggers are used to select events where vector mesons are produced. Particularly $J/\Psi$ and Y mesons directly probe the Debye screening of colour charges in the QGP. As the muon triggers are not effective by the UE, no specific treatment of the muon triggers for the HI programme is needed, however a low-threshold muon trigger is of particular interest. The L0Muon system upgrades and the high DAQ rate will allow for strong performance.

## Track-based signatures for HI physics

**Ultra-peripheral Collisions** Vector mesons produced in diffractive ultra-peripheral collisions effectively image the geometric structure of the nucleus. As the different vector mesons have different binding energies and different effective “sizes”, repeating such a measurement with as many vector mesons as possible $(\rho, J/\Psi,..)$ would allow imaging the target with projectiles of various size. Furthermore, there is an open question of whether the $J/\Psi$ meson can be produced perturbatively in reactions involving two-gluon exchange. To trigger on these, low activity events, based on Level-0 calorimetry, are processed with full-detector tracking.
**High multiplicity Events** An enhancement in the production of particles with small azimuthal-angle separation extending over a wide range of pseudorapidity differences, usually referred as “ridge”, has been observed in p+Pb. The data used in these measurements are selected using several high-multiplicity triggers. These rely on the a Level-0 requirement on the total transverse energy in the calorimeters and on the requirement on the multiplicity of reconstructed charged-particle tracks in the Event Filter. Full-detector hardware tracking will be a powerful tool for facilitating these triggers.

**Low $p_T$ Heavy Flavours and $J/\Psi$** In decays of low-$p_T$ $J/\Psi$ decays one of the two muons may have insufficient momentum to reach the muon spectrometer and, as a result, is not detected. Those muons can be recovered by looking at their tracks in the inner detector, specifically full-detector tracking is again a powerful tool.

## 2.12 Summary of Requirements and Motivation for the Upgrade

The set of benchmark signatures discussed above leads to a list of thresholds and corresponding physics requirements needed to exploit the full potential of the ATLAS HL-LHC programme. Broadly, this includes maintaining low single-lepton, di-lepton, di-hadronic $\tau$, multi-jet, and missing transverse energy thresholds, as well as a complementary set of other triggers which complete the full programme. In particular, the ability to make selections not just with object momentum thresholds and multiplicity, but also the angular information (known as topological triggers) allows specific scenarios to be addressed. The following chapters describe the limitations of the Run 3 system in meeting those requirements, followed by the TDAQ Phase-II upgrade plans to address the requirements. Chapter 6 then describes the expected performance of the upgrades, including an example set of trigger object $p_T$ thresholds (trigger “menu”) that achieve these requirements.

## References

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[2.2] R. Frederix, S. Frixione, V. Hirschi, F. Maltoni, O. Mattelaer, P. Torrielli, E. Vryonidou, and M. Zaro, *Higgs pair production at the LHC with NLO and parton-shower effects*, Phys. Lett. **B732** (2014) 142–149, arXiv:1401.7340 [hep-ph].

[2.3] ATLAS Collaboration, *Search for new light resonances decaying to jet pairs and produced in association with a photon or a jet in proton-proton collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector*, ATLAS-CONF-2016-070, 2016.
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[2.5] ATLAS Collaboration, Search for new phenomena in dijet events using 37 fb⁻¹ of pp collision data collected at √s = 13 TeV with the ATLAS detector, Phys. Rev. D96 (2017) no. 5, 052004, arXiv:1703.09127 [hep-ex].

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[2.7] ATLAS Collaboration, *Projected sensitivity to non-resonant Higgs boson pair production in the b¯b b final state using proton-proton collisions at HL-LHC with the ATLAS detector*, Tech. Rep. ATL-PHYS-PUB-2016-024, CERN, Geneva, Oct, 2016.
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# 3 Challenges and Limitations of the Run 3 TDAQ system

The pile-up conditions dictated by the expected design luminosity of the HL-LHC indicate a need for a factor of ten increase in trigger rates compared to those expected in Run 3. This increase in trigger rate exposes the following fundamental limitations in the Run 3 TDAQ system:

* The readout bandwidth is limited by the detector front-end electronics and the available radiation-hard technology at the time of their construction.

* The Level-1 trigger rate in the Run 3 system cannot be increased beyond the 100 kHz rate for which it was designed without an unacceptable increase in deadtime.

* The maximum latency of the Run 3 system is 2.5 µs; this latency is insufficient to implement more powerful selection algorithms in order to reduce the trigger rate. This limitation and the previous two will be ameliorated by the planned upgrades to the detector front-end systems described in [3.1, 2, 3, 4], and [3.5].

* The Run 3 muon trigger system will have limited muon acceptance and reduced efficiency due to chamber integrated charge limits in the barrel region and high background rates in the endcap region when operated in Run 4 conditions.

* The readout and dataflow components of the Run 3 TDAQ system do not have the flexibility or scalability required to cope with the factor of more than twenty increase in bandwidth requirement resulting from the increased event size and higher data rates.

* Finally, the extremely high pile-up environment presents a special challenge for charged particle tracking algorithms; the Run 3 hardware-based tracking system will not be able to maintain its high efficiency for track reconstruction at $\langle\mu\rangle \simeq 200$.

Thus, the Run 3 TDAQ system will not be able to cope with these high rates while keeping the same requirements on physics objects such as electrons, muons, tau leptons, and jets. As a result, without an upgrade of the TDAQ system, the Run 4 ATLAS physics programme will be significantly degraded.

The Phase-I TDAQ System was designed to maximise the performance of the system within tight constraints, as described above. These limitations are discussed in further detail in this chapter. First, a brief description of the TDAQ Run 3 system architecture is presented in Section 3.1. In the following three sections, the inherent limitations of each aspect of the Phase-I System (the Level-1 Trigger, DAQ, and High-Level Trigger (HLT)) are described.
Discussions of the hardware components and strategies that will be reused for the Phase-II System are also included. These restrictions steer the definition of the requirements for the TDAQ architecture at HL-LHC, as described in Chapter 4.

### 3.1 Overview of the Phase-I Trigger and Readout Architecture

The ATLAS Run 3 TDAQ system [3.6] is designed to cope with a LHC luminosity of $L = 3 \times 10^{34} \text{ cm}^{-2}\text{s}^{-1}$ and 80 interactions per bunch crossing provided on-detector electronics in the tracking detectors cope with it. The Phase-I upgrade was required to integrate adiabatically new elements of the trigger architecture, within the constraints and the limits of the existing detector systems, including an average trigger acceptance rate of 100 kHz at the output of the Level-1 trigger system, a maximum Level-1 latency of 2.5 µs, and a 1 kHz rate at the output of the HLT.

The three main objectives of improvements in the Run 3 TDAQ system design are to:

* increase the background rejection for electron/photon trigger candidates through the upgrade of the electromagnetic calorimeter trigger electronics [3.7] and the corresponding trigger processors;

* improve muon reconstruction and the rejection of "fake" candidates in the muon endcap spectrometer by installing new precision and high efficiency detectors in the NSW [3.8]; and to

* make use of full-event hardware-based charged particle track reconstruction (Fast TracKer (FTK)) during event processing at the HLT CPU farms, e.g. boosting selectivity for certain signatures, as in events with b-jets or $\tau$ leptons, and improving object isolation algorithms [3.9].

Run 3 TDAQ components are required, wherever possible, to be forward compatible with the expected conditions at the HL-LHC. Figure 3.1 is a high-level architectural diagram of the Run 3 TDAQ System. Event data move from the ATLAS on-detector electronics into the front-end buffers at the bunch crossing rate of 40 MHz.

The Level-1 trigger system, built using custom electronics, processes data from the calorimeters and the muon detectors, searching for signatures such as large electromagnetic energy deposits or high-$p_T$ muon tracks. These signatures, as well as global event quantities such as missing transverse energy, are combined to form multiplicities and flags indicating if topological or global event criteria have been satisfied. These multiplicities and flags are then combined in the CTP with reference to up to 512 programmable trigger menu items; the CTP box in Fig. 3.1 includes the local trigger processors and the TTC distribution system. The main new components for Run 3 are highlighted in the architecture diagram: the eFEX, jFEX and gFEX processors, the Level-1 Muon Trigger sector logic for the endcap, a NSW trigger processor for muon reconstruction in the endcap region, the inclusion of Tile Calorimeter information in the muon trigger, an improved MUCTPI capable of providing
<figure><img src="image_28.png" /><figcaption>Figure 3.1: Schematic overview of the TDAQ system after the Phase-I upgrade, with an average trigger acceptance rate of 100 kHz at the output of the Level-1 trigger system and a maximum Level-1 latency of 2.5 µs. The main objective of the system is to filter events and select up to approximately one thousand events per second for recording to permanent storage.</figcaption></figure>
precision muon inputs to the Level-1 Topological Processor (L1Topo), and an upgraded
version of L1Topo.

If the trigger conditions are met, a Level-1 Trigger Accept (L1A) is issued, initiating readout and processing in the ReadOut Drivers (RODs), and subsequent transfer to the ReadOut System (ROS). Readout data is transmitted from each of L1Calo, L1Muon, MUCTPI, and L1Topo; these lines are suppressed from Fig. 3.1 for simplicity. At the same time, Regions of Interest (RoIs) (small data packets describing event feature locations and energies) are built and sent to the HLT to steer further event processing.

After a L1A is issued, the DAQ system is responsible for the transport and assembly of the event data all the way from the sub-detector RODs to the logging to disk. While the readout path remains unchanged for most systems, the new and upgraded subdetector systems, such as the Muon NSW, L1Calo and the LAr calorimeter, deploy a FELIX-based readout system, the first element of the DAQ system receiving front-end detector data and routing it using commodity multi-gigabit network to further processing. The data are passed and buffered in the ROS, till requested by the HLT farm, where they are assembled into events (event building) and ultimately recorded to disk once accepted by the HLT.

The HLT selection is implemented with software running on a large farm of commercial computer processors. The HLT executes chains of reconstruction and signature algorithms that analyse the properties of the events, running essentially offline algorithms to make a final decision within approximately one second. At this stage, a rejection factor of 100 is required, resulting in a recording rate of approximately 1 kHz. Wide-ranging software upgrades will be needed for core software, trigger menus and algorithms. Furthermore, the HLT system exploits the capabilities of the FTK, which is designed to provide charged particle track reconstruction within 100 µs of every event accepted by the Level-1 Trigger.

The trigger rates and trigger latency are critical parameters for the Phase-I TDAQ system design. These parameters are described in the following, and summarised in Table 3.1. The Level-1 trigger latency is defined as the time from the collision to the output of the corresponding L1A signal at the output of the CTP. The Level-1 readout latency is defined as the time it takes for the L1A signal to reach the sub-detector front-ends. As no major changes in the Level-1 readout latency are possible during the Phase-I upgrade, because of constraints from detector front-end electronics that will not be replaced in the Phase-I upgrade, the Level-1 trigger latency must remain within a strict budget of 1.9 µs and may generate at most a sustained 100 kHz rate of accepted events. Thus, the overall Level-1 trigger latency for the new Level-1 trigger components, such as the L1Calo Feature EXtractors (FEXs) and L1Topo, are highly constrained.

While the Level-1 trigger latency is common to all trigger signals and is determined by
the slowest one, the Level-1 readout latency depends on the particular sub-detector. Many
of the new Run 3 components use high-speed digital optical links that are significant con-
sumers of the latency budget. However, several ATLAS detector subsystems, including
<table><caption>Table 3.1: The main design parameters of the Run 3 TDAQ system, assuming a peak luminosity of $\mathcal{L} = 3 \times 10^{34} \text{ cm}^{-2}\text{s}^{-1}$ and 80 interactions per bunch crossing. Specialised calibration data is not taken into account.</caption><thead><tr><td>Parameter</td><td>Run 3 Value</td></tr></thead><tbody><tr><td>Level-1 trigger latency</td><td>≈ 1.9 μs</td></tr><tr><td>Maximum Level-1 readout latency</td><td>2.5 μs</td></tr><tr><td>Level-1 trigger rate</td><td>100 kHz</td></tr><tr><td>Detector system readout deadtime (1.9 μs latency)</td><td>&lt; 2%</td></tr><tr><td>Number of detector readout data sources</td><td>~ 100</td></tr><tr><td>Bandwidth of detector ROLs</td><td>1.28-9.6 Gb/s</td></tr><tr><td>Average readout event size at peak luminosity</td><td>≈ 2.9 MB</td></tr><tr><td>ROLs bandwidth at peak luminosity</td><td>~ 290 GB/s</td></tr><tr><td>Maximum readout fraction of the ROS</td><td>50%</td></tr><tr><td>Average data collection bandwidth at peak luminosity</td><td>≈ 25 GB/s</td></tr><tr><td>Average output rate at peak luminosity</td><td>1.5 kHz</td></tr><tr><td>Number of data loggers</td><td>~ 10</td></tr><tr><td>Average output event size (raw) at peak luminosity</td><td>≈ 3.5 MB</td></tr><tr><td>Average output bandwidth at peak luminosity</td><td>≈ 3.2 GB/s</td></tr></tbody></table>

large parts of the Inner Detector and the LAr Calorimeter, continue to use the same front-end and readout electronics up to the end of Run 3.

The Level-1 trigger rate was specified to be 100 kHz in Run 2, and is maintained at that rate in Run 3. The detector system readout deadtime is also a Run 2 parameter, which was obtained through the introduction of a complex deadtime algorithm customised to each detector's needs. Overcoming both the maximum trigger rate and deadtime limitations can only be accomplished by new front-end requirements and implementations.

The number of detector readout data sources includes the legacy readout links and the new sources from the FELIX systems; the bandwidth of the detector ReadOut Links (ROLs) is also specified in Table 3.1. The average readout event size includes the FELIX systems of the NSW and LAr Calorimeter, but not all L1Calo data fragments. Finally, the ROS maximum readout fraction is an internal limitation of the ROS, from its internal data paths and processing power.

The bandwidth of data fragments received by the HLT is given by the average data collection bandwidth parameter. The average output event size is larger than the average readout size due to the additional information provided by the HLT. Ten data loggers are estimated to transfer data from the HLT to permanent storage. Finally, the average output bandwidth is given, assuming a compression factor of 60% on the output event fragments.
<figure><img src="image_29.png" /><figcaption>Figure 3.2: The complex deadtime fraction as a function of the Level-1 output rate for the LAr calorimeter (left) and Semiconductor Tracker (SCT) (right), at the current latency of ≈ 1910 ns. The different curves, created with simulation and cross-checked with data, show variations of the “Leaky Bucket” (“Sliding Window”) algorithm parameters implemented for LAr (SCT). A very steep increase in deadtime can be observed when the Level-1 output rate increases above 100 kHz.</figcaption></figure>

## 3.2 Features and Limitations of the Run 3 Level-1 Trigger System

The main limitations of the Phase-I Level-1 Trigger system are as follows. The first limitation results from the front-end detector electronics. These electronics systems were designed with the best available radiation-hard technology at the time of their construction, but the 100 kHz trigger rate for which it was designed presents a fundamental limitation to the DAQ system (as described in Section 3.3) as well as a fundamental limitation to the Level-1 Trigger System. An appreciable increase in this rate would result in an unacceptable increase in deadtime, or the fraction of the data acquisition time in which no events can be recorded, as shown in Fig. 3.2. The detector electronics also present the limitation on the maximum latency: 2.5 µs. Without a higher rate or longer latency, the Level-1 Trigger algorithms cannot be improved enough to cope with HL-LHC conditions.

The planned Phase-II upgrades to the detector electronics allow the design of the Phase-II TDAQ system to overcome the remaining limitations from the Phase-I system. These include the limited granularity from the calorimeters, the muon trigger acceptance in the barrel region, and the low background rejection of the muon trigger in the endcap regions.

### 3.2.1 Level-1 Calorimeter Trigger Limitations

The Phase-I FEXs are designed to match the Phase-I calorimeter trigger granularity and, thus, they do not have the available input fibres to handle finer granularity calorimeter information. They are therefore limiting the redefinition of the calorimeter-based trigger
object selection in order to improve the trigger object identification and background rejec-
tion at $\langle\mu\rangle \simeq 200$. However, the coarser granularity algorithms will be utilised in Phase-II
as pre-processors to the fine granularity algorithms anticipated. The general purpose of
each Phase-I FEX will remain intact: $e/\gamma$ and $\tau$ triggers will be handled by the eFEX sub-
system, single-jet triggers by the jFEX subsystem, and large-$R$ (or multi-jet) triggers plus
the calculation of global quantities will be handled by the gFEX subsystem.

Electron and Photon Triggers

There are two key elements to an e/γ trigger: efficient cluster formation and strong jet rejec-
tion. A cluster algorithm, which is large enough in area to contain the energy of an electro-
magnetic shower, but small compared with a typical jet in order to reduce the background
rate, provides good e/γ pT resolution and a sharp efficiency curve. In addition, additional
discriminating variables, e.g. based on shower shape and activity around the cluster core,
are necessary at low pT for additional rejection against jets and reduce the overall trigger
rates. The clustering algorithm used in the eFEX approximates the clustering used in the
ATLAS HLT and offline, but is based on super cell calorimeter granularity that provides
information for each calorimeter layer with granularity up to Δη × Δφ = 0.025 × 0.1 seg-
mentation in the front and middle layers. The basic steps of the algorithms developed for
Run 3 are:

* Electromagnetic clusters are built around a "seed" - a super cell in the middle layer of the electromagnetic calorimeter, where most of the shower energy is deposited and which is more energetic than any of the surrounding super cells.

* A cluster is formed by summing this cell with the more energetic of its neighbours in the $\phi$ direction, then summing both neighbours in the $\eta$ direction, to form a cluster of 3 × 2 super cells.

* The corresponding cells from the front later are added, plus the pair of cells in the presampler and three corresponding to the towers containing the seed and its neighbour in $\phi$.

The main part of the cluster slides in steps of 0.025 in $\eta$, providing good containment of
the showers wherever they are within a tower, and resulting in a turn-on curve with less
dependence on the position of the shower within the tower than the Run 2 trigger. Without
any additional vetoes, the Run 3 system produces a ~20% lower rate for the same efficiency
of the Run 2 system.

The finer granularity of the super cell and the depth information of the calorimeters, enables
the eFEX firmware to perform a sophisticated rejection of jet backgrounds through the use
of shower-shape variables. The most discriminating of these is defined as follows. Given a
3 × 2 group of Super Cells in $\eta \times \phi$ centred on the highest-energy Super Cell in the middle
<figure><img src="image_30.png" /><figcaption>Figure 3.3: $R_η$ distributions for clusters matched to reconstructed electrons and for clusters found in minimum-bias events with Run 3 conditions ($\langle\mu\rangle = 80$). (b) Electron efficiency vs rejection of clusters in minimum bias events for two different $R_η$ variables. All clusters were required to pass a $E_T > 20$ GeV threshold.</figcaption></figure>

layer, $R_η$ is defined as the transverse energy measured in the 3 × 2 group divided by the transverse energy measured in a 7 × 2 group:

$$R_{\eta} = \frac{E_{T, \Delta\eta \times \Delta\phi}^{(2)}}{E_{T, \Delta\eta \times \Delta\phi}^{(2)}}. \qquad (3.1)$$

The distribution of $R_η$ for clusters matched to reconstructed electrons compared to clusters found in minimum-bias events is shown in Fig. 3.3. The shower-shape variables implemented in the eFEX in Phase-I are sufficient to provide the background rejection factors required for Run 3 operating conditions, but the super cell granularity is a limiting factor for increasing the background rejection further in Run 4. The eFEXs can only consider a 0.3 × 0.3 algorithm window size; this narrow field of view may limit the use of offline-like algorithms. However, the electron and photon trigger candidates found by the eFEX can be utilised as a starting point for further refinement with full calorimeter granularity in the Phase-II trigger system.

### Forward Electron Triggers

The availability of tracking information at larger $\eta$ for the Phase-II ITk introduces many advantages in terms of physics objects reconstruction, and in terms of pile-up mitigation by linking objects to the primary vertex corresponding to the hard-scatter of interest. Since the ITk extends tracking up to $|\eta| < 4.0$, the geometrical acceptance of the Phase-I Level-1 Trigger system is a significant limiting factor for the ATLAS discovery potential. The Phase-I eFEX system, dedicated to $e/\gamma$ identification, covers $|\eta| < 2.5$, the coverage of the original ATLAS inner detectors. Without an upgrade, there is no recourse for identifying electrons in the forward regions.
<figure><img src="image_31.png" /><figcaption>Figure 3.4: $R_{\text{core}}$ distributions for truth-matched tau clusters and clusters from minimum-bias background events.</figcaption></figure>

## τ-lepton Triggers

Hadronic $\tau$ triggering essentially involves the selection of narrow mixed electromagnetic and hadronic clusters and separating these from the jet background. It is particularly challenging since hadronic tau decays include a range of different states, are less distinctive than electrons, and also typically have a lower observable $E_T$ due to the neutrinos present in the decays. The Run 3 approach used in triggering on $\tau$ leptons is to use super cell electromagnetic (EM) calorimeter granularity information in forming clusters and jet discriminants in the eFEX.

The eFEX implementation of $\tau$ identification is based on a cluster calculated in a region of 5 × 2 super cells in the middle layer of the EM calorimeter (0.125 × 0.2), 3 in the front layer, 0.1 × 0.2 in the presampler, and 0.2 × 0.2 clusters in the back layer and in the hadronic calorimeter. In addition, a 3 × 3 tower cluster (0.3 × 0.3, EM+hadronic) has been studied using towers built from super cell. Jet rejection relies on the narrower width of the $\tau$ cluster compared to a jet. This can be exploited via isolation, such as the $R_{\text{core}}$ variable defined as:

$$R_{\text{core}} = \frac{E_{T, \Delta\eta \times \Delta\phi}^{(2)}}{E_{T, \Delta\eta \times \Delta\phi}^{(2)}}. \qquad (3.2)$$

The resulting distribution, comparing truth-matched $\tau$ clusters to clusters from minimum-bias events, is shown in Fig. 3.4.

This algorithm is effective for identifying narrow showers resulting from a hadronic $\tau$ lepton decay using super cell granularity. The jFEX could also be used to search for tau leptons, implementing an algorithm that considers a larger $\eta - \phi$ area, but without the super cell granularity. Neither the narrow window of consideration of the eFEX nor the larger window, but coarse granularity of the jFEX are optimal for triggering on hadronic taus, and
thus are a limitation of the Run 3 system. However, either option could be used to seed a fine-granularity, large-area hadronic tau trigger identification algorithm in Run 4.

### Single-jet Triggers

In the Phase-I Level-1 Calorimeter Trigger System, the jFEX is responsible for identifying isolated $R = 0.4$ jets for the single-jet trigger. To accomplish this, the inputs are towers of $\sim 0.1 \times 0.1$ in the region $|\eta| < 2.5$ and $0.2 \times 0.2$ between $2.5 < |\eta| < 3.2$. In the FCal region the $\phi$ granularity is $\sim 0.4$, while the $\eta$ granularity varies between 0.1 and 0.4. The $E_T$ in each tower is summed over all of the calorimeter layers.

Iterative algorithms used in the HLT and offline jet reconstruction are not suitable in the jFEX because of the limited and fixed latency. Level-1 jet trigger algorithms therefore must be executed in parallel within a set of overlapping window environments, and cannot use information from outside those environments. The size of the environment is limited by the available bandwidth and the necessary interconnectivity to share data between FPGAs, modules and crates. In the baseline design the environment is 9 × 9 towers, which corresponds to an area slightly larger than a $\Delta R = 0.4$ cone, though options that would allow this to be increased are being investigated.

The baseline jet-finding technique for the jFEX is a simple circular sliding window algorithm, where towers with energy above $4\sigma$ are considered. The performance of the single-jet trigger algorithm implemented in the jFEX is shown in Fig. 3.5. Though there is additional room for improvement between the jFEX algorithm and the offline reconstruction, the algorithm does not provide an inherent limitation for isolated single-jet identification. Further improvements in the trigger performance may be obtained by a so-called "Gaussian filter" algorithm, where towers are summed with a weight that decreases as a Gaussian function of the distance from the centre of the cluster. Gaussian jet algorithms may have different responses for quark and gluon jets, etc. The key parameter in determining the performance is the value used for $\sigma$ (the Gaussian width) large values ($0.3 - 0.4$) give good inclusive jet performance, while small values ($\sim 0.1$) are good for identifying nearby jets, and perhaps for jet substructure. This option is under investigation. Furthermore, once the tight latency window required in the Phase-I system is relaxed, the jFEX may also include an offline-like jet calibration.

### Multi-jet and Boosted Object Triggers

The gFEX is intended to enhance the selectivity of the Level-1 trigger and increase sensitivity to key physics channels. The gFEX identifies large-radius jets, typical of Lorentz-boosted objects, by means of wide-area jet algorithms refined with subjet information. The architecture of the gFEX permits event-by-event local pile-up suppression for these jets using baseline subtraction techniques comparable to those developed for offline analyses.
<figure><img src="image_32.png" /><figcaption>Figure 3.5: Trigger rate for a 95% efficiency on the leading jet vs. offline $p_T$ thresholds for jets reconstructed in the jFEX ($|\eta| < 3.2$). The efficiency is evaluated using HH → bbbb signal events, and the trigger rate is evaluated based on minimum bias background events at $\langle\mu\rangle \simeq 200$ and correspond to a luminosity of $\mathcal{L} = 5 \times 10^{34} \text{cm}^{-2}\text{s}^{-1}$. The jFEX algorithm, the offline anti-$k_t$ algorithm (run over $\eta \times \phi = 0.1 \times 0.1$ towers), and the full offline reconstruction are compared.</figcaption></figure>

The gFEX architecture is also suitable for other global event algorithms such as $E_T^{\text{miss}}$ and centrality-related observables.

A key feature of the gFEX is that the entire calorimeter is available in a single module, which enables the use of algorithms that can scan the entire $\eta$ range of the calorimeter. However, the granularity available is $\eta \times \phi = 0.2 \times 0.2$ towers summed over all of the calorimeter layers. One of these full-scan algorithms involves the identification of boosted hadronic topologies that are characteristic of new physics scenarios. At high transverse momentum, the hadronic decay products of energetic bosons and top quarks tend to be highly collimated. In the gFEX, large-radius jets can be used to capture the boson or top quark decay products, whereas in Run 2, algorithms identify significant local energy deposits in a limited region of interest. Once these large-radius jets are identified, exploiting the internal structure of these jets allows for differentiation of multi-prong objects from the dominant single-prong jet backgrounds. Thus the gFEX will allow the efficient identification of boosted objects while maintaining a suitable rate in the expected LHC environment after the Phase-I upgrade. Figure 3.6a illustrates the large-$R$ jet reconstruction by the gFEX in an event with top quark produced at high $p_T$. The jet-finding algorithm in the gFEX consists of building $\eta \times \phi = 0.6 \times 0.6$ fully overlapping regions, as illustrated in Fig. 3.6b, in order to maintain high efficiency for multi-jet triggers.
<figure><img src="image_33.png" /><figcaption>Figure 3.6: (a) Fully-simulated large-radius jet from a top quark produced at high transverse momentum. Subjets of a particular category have the same fill color and their extent represents the subjet active catchment area. Jet constituents are shown as black dots. The red circle represents a $R = 0.4$ jet, whereas the black circle represents the triggers provided by the gFEX. (b) Illustration of the gFEX jet-finding algorithm, building multiple jets of size $\eta \times \phi = 0.6 \times 0.6$. Note that jets are allowed to overlap.</figcaption></figure>

Models of new physics often result in final states with stable, invisible particles. These include many dark matter (DM) models, supersymmetry (SUSY) and beyond the Standard Model decays of the Higgs boson. Typically, these events are searched for by requiring a large momentum imbalance, $E_T^{\text{miss}}$. The $E_T^{\text{miss}}$ in the detector is reconstructed using inputs of the visible momenta perpendicular to the beam axis. In the Level-1 trigger system, the $E_T^{\text{miss}}$ is computed using only calorimeter information.

The jFEX has a significant inefficiency for event configurations with multiple jets where some of the jets are close together such as might be found in $t\bar{t}$, Higgs, or SUSY events. This has been one of the primary motivations to develop the gFEX system and it is illustrated in Fig. 3.7.

Finally, because of the granularity of the jet constituents at their inputs, the jFEX and gFEX systems cannot trigger efficiently on jets with $p_T < 50$ GeV at HL-LHC. In general, in order to retain similar acceptance to offline for jets and hadronic tau decays higher granularity information is required as the pile-up increases so that cluster identification and separation is not diluted by the effects of pile-up on the fluctuations of the calorimeter energy deposits. Such intrinsic limitations may be overcome by using fine calorimeter granularity and the implementation of offline-like topological clusters and iterative jet reconstruction algorithms (e.g., anti-$k_t$). However, the jFEX and gFEX may be used to pre-process events of interest and restrict the area over which iterative jet-finding is performed.
<figure><img src="image_34.png" /></figure>

Figure 3.7: Sub-jet reconstruction trigger efficiency turn-on curves for t$t$ events are shown on the left plot as a function of uncalibrated jet $p_T$. The middle and right plots show the trigger efficiency curves for t$t$ and WH → $lvb\bar{b}$ events respectively. The red dots correspond to the results for a gFEX-like algorithm reconstructing fat jets with $p_T > 140$ GeV while those corresponding to a jFEX-like algorithm reconstructing jets with $p_T > 100$ GeV are shown by open blue dots.

## Forward Jet Triggers

The L1Calo Phase-I system is capable of reconstructing jets in the range $|η| < 4.9$ but the granularities of both the jFEX and gFEX is extremely coarse in the forward region (as seen in Fig. 3.6b). Thus, no Phase-I Level-1 Trigger component is capable of detailed forward jet reconstruction that is needed to reject pile-up energy depositions in the forward region at the HL-LHC.

Furthermore, even the Phase-I super cell granularity is rather irregular in the forward region ($3.2 < |η| < 4.9$) and particularly coarse in $\phi$. Therefore, forward electron identification, jet and missing-energy measurements may degrade significantly due to the excessive pile-up noise. Figure 3.8 illustrates qualitatively the concept by showing the energy flow in the first module of the FCal calorimeter by a single jet with $p_T \sim 50$ GeV ($E = 2.6$ TeV) at HL-LHC conditions either (a) using super cell information, or (b) the full granularity of the FCal detector.

Finally, Fig. 3.9 (see also Ref. [3.10]) illustrates the extent of the challenge of rejecting pile-up in the forward region. Even with full granularity and offline topological cluster reconstruction at low pile-up ($< \mu > = 30$), the topoclusters in the FCal tend to merge into a single cluster. A dedicated trigger solution for Phase-II is needed to optimise the reconstruction of forward jet trigger objects.
<figure><img src="image_35.png" /><figcaption>Figure 3.8: (a): Energy flow of a single jet (with $E = 2.6$ TeV, $p_T \sim 50$ GeV) in the EM layer of the FCal detector at HL-LHC ($\mu \simeq 200$). The regions indicate the super cell readout granularity developed for the Phase-I system. (b): Energy flow for the same single jet, but the regions indicate the full granularity information of the FCal detector. The red circle in each figure indicates the truth jet position.</figcaption></figure>

<figure><img src="image_36.png" /><figcaption>Figure 3.9: Number of topoclusters in the FCal for $< \mu >$= 30, with pile-up generated from overlaid minimum bias data events.</figcaption></figure>
<figure><img src="image_37.png" /><figcaption>Figure 3.10: The Phase-I Level-1 muon trigger scheme in the barrel region ($|η| < 1.05$).</figcaption></figure>

## 3.2.2 Level-1 Muon Trigger Limitations

The ATLAS muon spectrometer has shown that it is able to achieve good muon identification and momentum resolution (~10% at 1 TeV). This performance will be maintained through the HL-LHC.

The Phase-I Level-1 muon trigger is sufficiently selective to provide a rate that would take a large but not prohibitive fraction of the 100 kHz Level-1 trigger rate and a relatively small fraction of the proposed Phase-II 1 MHz Level-0 trigger rate. However, the original RPC and TGC systems are designed for maximum latencies of 6.4 µs and 3.2 µs, respectively, and for a maximum rate of 100 kHz. These limitations arise from the depth of the readout buffer to store the hit data before the arrival of the trigger accept signals and also from the readout bandwidth. The momentum resolution for low-$p_T$ muon also limits the system performance, since poorly-measured muons below the trigger threshold have an appreciable impact on the trigger rates. Furthermore, there are limitations on the muon acceptance and efficiency, as described below.

### Level-1 Muon Triggers in the Barrel Region

The Muon trigger in the barrel region ($|η| < 1.05$) is based on three trigger stations. Two stations are used for low-$p_T$ (6-10 GeV) muon triggers, while the third station is used in addition for high-$p_T$ (8-40 GeV) triggers as shown in Fig. 3.10.
Each station is composed of two detector planes, and each detector plane is read out in two orthogonal projections, $\eta$ and $\phi$, that will be referred to as the bending and non-bending<fnref n="1" /> projections, respectively. The muon trigger $p_T$ resolution is dominated by the information read out from the detectors in the bending projection. However, the information in the non-bending view helps to reduce the background trigger rate from noise hits in the chambers produced by low-energy photons, neutrons and charged particles, as well as localising the track candidates in space as required for the HLT trigger. In addition, the trigger chamber information in the non-bending view provides a second coordinate measurement for offline reconstruction of muons (the precision chambers give information only in the bending projection).

The basic principle of the muon trigger identification algorithm in Phase-I is to require a coincidence of hits in the different chamber layers within a readout. The width of the readout is related to the $p_T$ threshold to be applied. Space coincidences are required in both views, within a time window close to the bunch-crossing period (25 ns). The coincidence requirement allows for missing layers due to detector inefficiencies, dead regions, etc. For the low-$p_T$ trigger, hits are required within the readout in at least three of the four layers, in each of the two projections. For the high-$p_T$ trigger, an additional requirement is made, demanding hits in at least one of the two layers in each of the two projections of the third station. A system of programmable coincidence logic allows concurrent operation with a total of six thresholds, three associated with the low-$p_T$ trigger and three associated with the high-$p_T$ trigger. Each of the six thresholds is independently programmable.

The muon efficiency for muons with $p_T > 25$ GeV is only ~70% in the barrel ($|\eta| < 1.05$), as illustrated in the left plot of Fig. 3.11, based on Run 2 data. This is, by design, limited by the presence of the ATLAS mechanical structure of the air toroid in the barrel. However, it is well below the desired efficiency of 95% that has been achieved with the electron trigger. An additional intrinsic RPC inefficiency may originate from the reduction of the electric field in the gas volume, aimed to reduce the effects of chamber ageing, the use of new low greenhouse-effect mixture, and the inaccessibility of chambers for repairs. The Muon Phase-II upgrade [3.3] introduces a Barrel Inner (BI) layer in the central region, presenting an opportunity for a new flexible trigger logic that does not require the strict 3-out-of-3 chamber coincidence present in the Phase-I system.

### Level-1 Muon Triggers in the Endcap Region

The Phase-I Level-1 muon trigger in the endcap region ($1.05 < |\eta| < 2.4$) combines TGC muon trigger information with that from the NSW, which is composed of small-strip TGC (sTGC) and Micromega (MM) chambers. A schematic representation of the algorithm is shown in Fig. 3.12. The TGC muon segment is primarily seeded on track segments in the TGC chambers of the middle muon station, located after the endcap toroid magnet. The

<footnote n="1">¹ However, for central muons there is some bending even in the “non-bending projection”.</footnote>
<figure><img src="image_38.png" /><figcaption>Figure 3.11: Product of geometrical acceptance and efficiency as a function of $p_T$ as measured in an offline analysis for the present Level-1 and HLT single-muon trigger, for the barrel (left) and the endcap (right) regions. “L1 MU20” labels the Level-1 muon trigger with a $p_T$ > 20 GeV threshold. The blue points show the fractions of muons that were reconstructed by the HLT as isolated muons with $p_T$ > 26 GeV or as non-isolated muons with $p_T$ > 50 GeV. The red points show the fraction of Level-1 triggers that pass the HLT trigger requirements.</figcaption></figure>

endcap muon trigger object is formed by matching the track positions of the TGC and NSW muon candidates; this correlation substantially suppresses the rate of fake triggers due to background particles emerging from the endcap toroids and calorimeters. The muon trigger object $p_T$ is determined by the angle of the segments with respect to a straight line pointing to the nominal interaction point.

The strategy for separating trigger and precision-measurement muon detectors has been very successful, but carries a limited momentum measurement resolution of muon candidates passing the $p_T$ nominal threshold. Consequently, the Phase-I system has a low rejection of muons below threshold. The limitation primarily arises from the short latency in the Phase-I system; a longer latency would allow for inclusion of the MDT trigger information, thus improving the momentum resolution and reducing the muon rate.

The detector layout in the transition region between the barrel and the endcap regions of the muon spectrometer, at $1.0 < |\eta| < 1.3$, has a complicated structure due to the presence of the barrel toroid magnets. About 70% of the transition region in azimuth is covered by the EIL4 TGC doublets and the EIL4 MDT chambers, corresponding to the large sectors of the spectrometer in-between barrel toroid coils. In small sectors, integrated chambers (BIS78), with RPC and Small-diameter Monitored Drift Tube (sMDT) detectors for trigger and tracking respectively, are installed as part of the ATLAS Phase-I upgrade programme. The trigger logic is different between the systems based on the BIS78 RPC and the EIL4 TGC chambers. In the case of the BIS78 RPC triplet chambers, a majority logic of 2-out-of-3 layers can be applied. In the case of the EIL4 TGC doublets, the only possible choice is a logical OR of the two layers. In either cases the trigger information is combined with the
<figure><img src="image_39.png" /><figcaption>Figure 3.12: Block diagram of the endcap Trigger in Phase-I</figcaption></figure>

endcap TGC information and with the information of the outer layer of the Tile Calorimeter to form an additional coincidence.

As shown in the right plot of Fig. 3.11, based on Run 2 data the muon efficiency for muons with $p_T > 25$ GeV is ≈ 85% in the endcap ($1.05 < |\eta| < 2.4$), also below the electron trigger efficiency.

### Level-1 Di-muon Triggers

Heavy flavour physics (e.g. $B_s \to \mu^+\mu^-$) is a particular challenge for the TDAQ system, since it depends critically on low $p_T$ thresholds in the di-muon trigger. Low di-muon thresholds, in addition to topological selections such as invariant mass and angular criteria, are of paramount importance to maintain sensitivity to B-meson decays.

The selection of close-by muon candidates in the trigger is a particular challenge; a required separation of $\Delta\eta \times \Delta\phi = 0.2 \times 0.2$ between two trigger muon candidates results in dramatic inefficiencies for $B_s \to \mu^+\mu^-$ and $B_s \to J/\psi\phi$, as shown in Table 3.2.

Distributions of the $\Delta\eta$ vs. $\Delta\phi$ for the two muon candidates in $B_s \to \mu^+\mu^-$ and $B_s \to J/\psi\phi$ decays are shown for four muon thresholds respectively in Figs. 3.13 and 3.14. Figure 3.15 shows the fraction of events where the two final state muons are separated by less than $\Delta\eta \times \Delta\phi = 0.2 \times 0.2$ (Phase-I granularity of the di-muon trigger) as a function of the muon $p_T$ thresholds.
<table><caption>Table 3.2: Di-muon trigger inefficiencies due to a required separation of $\Delta\eta \times \Delta\phi = 0.2 \times 0.2$ in the trigger, for several trigger muon $p_T$ thresholds.</caption><thead><tr><th colspan="2">trigger muon threshold</th><th colspan="2">inefficiency</th></tr><tr><th>$p_{T,1}$</th><th>$p_{T,2}$</th><th>$B_s \to \mu^+\mu^-$</th><th>$B_s \to J/\psi\phi$</th></tr></thead><tbody><tr><td>6 GeV</td><td>4 GeV</td><td>0.5%</td><td>5.7%</td></tr><tr><td>6 GeV</td><td>6 GeV</td><td>1.2%</td><td>13.7%</td></tr><tr><td>10 GeV</td><td>6 GeV</td><td>2.3%</td><td>23.1%</td></tr><tr><td>10 GeV</td><td>10 GeV</td><td>8.2%</td><td>58.8%</td></tr></tbody></table>

<figure><img src="image_40.png" /><figcaption>(a) Trigger muon $p_{T,1} > 6$ GeV, $p_{T,2} > 4$ GeV.</figcaption></figure>

<figure><img src="image_41.png" /><figcaption>(c) Trigger muon $p_{T,1} > 10$ GeV, $p_{T,2} > 6$ GeV.</figcaption></figure>

<figure><img src="image_42.png" /><figcaption>(b) Trigger muon $p_{T,1} > 6$ GeV, $p_{T,2} > 6$ GeV.</figcaption></figure>

<figure><img src="image_43.png" /><figcaption>(d) Trigger muon $p_{T,1} > 10$ GeV, $p_{T,2} > 10$ GeV.</figcaption></figure>

<caption>Figure 3.13: Angular separation of muon candidates for $B_s \to \mu^+\mu^-$ decays as a function of di-muon trigger $p_T$ thresholds.</caption>
<figure><img src="image_44.png" /><figcaption>(a) Trigger muon $p_{T,1} > 6$ GeV, $p_{T,2} > 4$ GeV.</figcaption></figure>

<figure><img src="image_45.png" /><figcaption>(c) Trigger muon $p_{T,1} > 10$ GeV, $p_{T,2} > 6$ GeV.</figcaption></figure>

<figure><img src="image_46.png" /><figcaption>(b) Trigger muon $p_{T,1} > 6$ GeV, $p_{T,2} > 6$ GeV.</figcaption></figure>

<figure><img src="image_47.png" /><figcaption>(d) Trigger muon $p_{T,1} > 10$ GeV, $p_{T,2} > 10$ GeV.</figcaption></figure>

Figure 3.14: Angular separation of muon candidates for $B_s \to J/\psi\phi$ decays as a function of di-muon trigger $p_T$ thresholds.

### 3.2.3 Level-1 Topological Trigger Limitations

The original L1Topo system was designed to perform topological trigger algorithms for Run 2. These algorithms incorporate the geometric and kinematic relationships between trigger objects, such as the angular separation between a trigger muon candidate and a jet. Similar functionality will be employed for the Run 3 L1Topo system. For Run 2, the L1Topo system sends only simple flags to the CTP, indicating the results of individual algorithms; multiplicity information for electrons/photons, taus and jets, was provided separately by L1Calo. For Run 3, L1Topo will provide multiplicities of trigger objects passing the requirements of certain algorithms. In order to make the Run 3 L1Topo compatible with the L1Calo FEX modules designed for Run 3, this system will accommodate a larger number of inputs and more resourceful FPGAs.
<figure><img src="image_48.png" /><figcaption>Figure 3.15: Fraction of events where the two final state muons are separated by more than $\Delta\eta \times \Delta\phi = 0.2 \times 0.2$ as a function of the muon $p_T$ thresholds.</figcaption></figure>

However, the Run 3 L1Topo architecture cannot accommodate the required number of fibre inputs ($\mathcal{O}(2000)$) to be able to take advantage of the full granularity calorimeter information and high-resolution full-detector muon system information that will be provided by the Phase-II detector upgrades. Furthermore, the Run 3 L1Topo cannot handle the number of expected trigger objects in the high-pile-up environment in Run 4.

### 3.2.4 Level-1 Trigger Menu up to Run 3

An example of how the Level-1 trigger menus have evolved through Run 1 and Run 2, and how they will evolve to cope with the LHC conditions after the Phase-I upgrades (Run 3) is given in Table 3.3. The Run 1 and Run 2 menus have been used in the 2012 and 2015 data taking for the LHC fills with the largest luminosity delivered. The Run 3 menu is expected to evolve as more sophisticated algorithms are being developed, some of which have been documented in the TDAQ and in the companion LAr Calorimeter Phase-I TDR [3.6][3.7]. The thresholds and the rates in the Run 3 column assume an instantaneous luminosity of $3 \times 10^{34} \text{ cm}^{-2} \text{ s}^{-1}$ and $\sqrt{s} = 14$ TeV, corresponding to $\langle \mu \rangle \simeq 80$.

In general a Level-1 trigger item in Table 3.3 is indicated by a "signature" key, i.e. "EM" for electrons and photons, "MU" for muons, "TAU" for taus, "J" for jets and "XE" for $E_T^{\text{miss}}$ triggers, followed by the object's online threshold, and by a generic key indicating whether further algorithms have been applied to improve selectivity as follows: an "H" denotes hadronic isolation, i.e. limiting the energy deposition in the hadronic calorimeter; an "I" indicates electromagnetic isolation (denoted by "I"), i.e. the object has passed selection if the energy deposited in a fixed-size cone in the electromagnetic calorimeter around the object's is below a given value; "R" indicates the use of a shower shape algorithms in the
electromagnetic and τ-triggers (possible only after the Phase-I upgrades); a "V" denotes varying thresholds with η to account for energy loss. Finally, "L", "M", and "T" denote loose, medium, and tight criteria depending on the values of the isolation thresholds applied.

A comparison among the three menus unveils the general strategy adopted in the past
several years and planned for Run 4:

* Maintain thresholds sufficiently low to maximise signal acceptance for all the relevant electroweak signatures with leptons, allocating approximately 50-60% of the total bandwidth to single and di-lepton triggers.

* Control rates increasing with luminosity by deploying more sophisticated algorithms. For example, applying isolation to lepton trigger objects and implementing discriminants based on shower shape in the calorimeters.

* Adapt to the different luminosity conditions and optimise the menu items for hadronic signatures and topological triggers for the remaining ~40% Level-1 bandwidth.

More specifically for each signature:

* Selection based on shower shapes allows the offline threshold to be maintained around 30 GeV to be compared to the 25 GeV for single electrons in Run 1.

* During Run 2, before the introduction of the NSW, one may be able to maintain a trigger threshold of 20 GeV, but only by significantly cutting bandwidth from other triggers, such as those in the topological processor. After the Phase-I upgrade, there will be additional bandwidth available to design many specialised topological triggers.

* The Phase-I improvements will maintain approximately the offline thresholds of the di-tau triggers to 40 GeV, crucial for example for the reach of the $H \to \theta\theta$ analysis, and will contain the threshold increase on the single $\tau$-trigger.

* In Table 3.3 it has been conservatively assumed that the jet thresholds and rates will be unchanged, but it is expected that after the Phase-I improvements, the offline $E_T^{\text{miss}}$ cut can be lowered from 250 GeV to 200 GeV [3.7].

Up to 256 menu trigger items were available in Run 1 and up to 512 are and will be available
in Run 2 and Run 3. While the limitation of 512 items is not problematic, there are other
reasons for replacing the CTP in the Phase-II upgrade, and additional flexibility in the menu
would be highly desirable for Run 4 and beyond to benefit from enhancements elsewhere
in the system. Only a few representatives of the trigger items are shown in Table 3.3. They
correspond to the lowest, i.e. smallest $p_T$ threshold, unprescaled triggers of the principal
single and di-object signatures. The total bandwidth allocated to a signature, as indicated in
Table 3.3, is only an approximated value because of the existing correlations and recurring
overlaps among different triggers that needs to be taken into account. A single energy
deposition can give rise to several triggers that overlap in the same region of $\phi$ and $\eta$, for
example, deposition of energy in the electromagnetic layers of the calorimeter can cause
overlapping EM, tau and jet triggers. There is no overlap removal in the Level-1 trigger
between multi-objects, except for some specialised instances in the topological processor.
<table><caption>Table 3.3: Level-1 Trigger menus for used configurations during Run 1 and Run 2, and example menu for Run 3. The offline thresholds typically correspond to the point at which the trigger efficiency turn-on curve reaches 90-95% of its plateau value. The items listed in this table assume no overlap removal.</caption><thead><tr><th colspan="3">Run 1</th><th colspan="3">Run 2</th><th colspan="3">Run 3</th></tr><tr><th>Run No.</th><td colspan="2">215414</td><th>Run No.</th><td colspan="2">332303</td><th colspan="2">L [×10<sup>34</sup>cm<sup>-2</sup>s<sup>-1</sup>]</th><td>3</td></tr><tr><th>Date</th><td colspan="2">Nov 2012</td><th>Date</th><td colspan="2">Aug 2017</td><th colspan="2">L [×10<sup>34</sup>cm<sup>-2</sup>s<sup>-1</sup>]</th><td>80</td></tr><tr><th>L [×10<sup>34</sup>cm<sup>-2</sup>s<sup>-1</sup>]</th><td colspan="2">0.73</td><th>L [×10<sup>34</sup>cm<sup>-2</sup>s<sup>-1</sup>]</th><td colspan="2">1.7</td><th colspan="2">L [×10<sup>34</sup>cm<sup>-2</sup>s<sup>-1</sup>]</th><td></td></tr><tr><th>⟨μ⟩</th><td colspan="2">34</td><th>⟨μ⟩</th><td colspan="2">48</td><th colspan="2">⟨μ⟩</th><td></td></tr><tr><th>n<sub>bunches</sub></th><td colspan="2">1368</td><th>n<sub>bunches</sub></th><td colspan="2">2544</td><th colspan="2"></th><td></td></tr><tr><th></th><th>Offline p<sub>T</sub><br>Threshold<br>[GeV]</th><th>Rate<br>[kHz]</th><th></th><th>Offline p<sub>T</sub><br>Threshold<br>[GeV]</th><th>Rate<br>[kHz]</th><th></th><th>Offline p<sub>T</sub><br>Threshold<br>[GeV]</th><th>Rate<br>[kHz]</th></tr></thead><tbody><tr><td>EM18VH</td><td>25</td><td>19</td><td>EM22VHI<sup>a</sup></td><td>27</td><td>27.8</td><td>EM25VHR</td><td>32</td><td>14</td></tr><tr><td>EM30</td><td>37</td><td>7.3</td><td></td><td></td><td></td><td>EM80</td><td>100</td><td>2.5</td></tr><tr><td>2EM10VH</td><td>2×17</td><td>6.5</td><td>2EM15VHI</td><td>2×18</td><td>1.7</td><td>2EM12VHR</td><td>2x19</td><td>5.0</td></tr><tr><td>EM total</td><td></td><td>~25</td><td></td><td></td><td>~30</td><td></td><td></td><td>~20</td></tr><tr><td>MU15</td><td>25</td><td>9.3</td><td>MU20</td><td>27</td><td>15.5</td><td>MU20</td><td>25</td><td>15</td></tr><tr><td>2MU10</td><td>2×12</td><td>0.8</td><td>2MU10</td><td>2×15</td><td>1.8</td><td>2MU11</td><td>2x13</td><td>4.0</td></tr><tr><td>2MU6_MU10</td><td>8,12</td><td>1.9</td><td>2MU6_MU10</td><td>8,12</td><td>5.2</td><td></td><td></td><td></td></tr><tr><td>Muon total</td><td></td><td>~20</td><td></td><td></td><td>~22</td><td></td><td></td><td>~19</td></tr><tr><td>EM10VH_MU6</td><td>17,6</td><td>2.9</td><td>EM15VH_MU10</td><td>18,15</td><td>2.0</td><td>EM10VHR_MU10</td><td>17,12</td><td>3.0</td></tr><tr><td>TAU40</td><td>100</td><td>5.5</td><td>TAU100</td><td>170</td><td>1.2</td><td>TAU80VR</td><td>180</td><td>3.2</td></tr><tr><td></td><td></td><td></td><td>TAU60_2TAU40</td><td>140,100</td><td>2.8</td><td>2TAU40VR</td><td>2x100</td><td>3.9</td></tr><tr><td>2TAU111_TAU15</td><td>30,40</td><td>13.6</td><td>TAU20IM_2TAU12IM_4J12</td><td>40,30,40</td><td>3.2</td><td>2TAU15VR_3J15</td><td>2x40,50</td><td>8.1</td></tr><tr><td>TAU15_XE35</td><td>40,80</td><td>2.5</td><td>TAU20IM_2J20_XE45</td><td>50,2x60,100</td><td>2.4</td><td>TAU15VR_XE40_3J15</td><td>40,90,50</td><td>5.0</td></tr><tr><td>Tau total</td><td></td><td>~20</td><td></td><td></td><td>~10</td><td></td><td></td><td>~20</td></tr><tr><td>J75</td><td>150</td><td>2.1</td><td>J100</td><td>460</td><td>3.3</td><td>J100</td><td>200</td><td>7.0</td></tr><tr><td></td><td></td><td></td><td>J100</td><td>235<sup>b</sup></td><td></td><td></td><td></td><td></td></tr><tr><td>4J15</td><td>4×55</td><td>1.8</td><td>3J50</td><td>120</td><td>0.5</td><td>4J25</td><td>4x60</td><td>3.3</td></tr><tr><td></td><td></td><td></td><td>4J15</td><td>4x45<sup>c</sup></td><td>5.0</td><td>J75_XE40</td><td>150,150</td><td>8.3</td></tr><tr><td>XE40</td><td>120</td><td>5.2</td><td>XE50</td><td>200</td><td>5.0</td><td>XE70</td><td>200</td><td>13</td></tr><tr><td>Jet/E<sup>miss</sup> total<sup>d</sup></td><td></td><td>~10</td><td></td><td></td><td>~14</td><td></td><td></td><td>~25</td></tr><tr><td>Topological triggers</td><td></td><td>-</td><td></td><td></td><td>~5</td><td></td><td></td><td>~15</td></tr><tr><td>Total</td><td></td><td>~75</td><td></td><td></td><td>~75</td><td></td><td></td><td>~100</td></tr></tbody></table>

For example, the trigger TAU20IM_2TAU12IM_4J12 is designed to trigger events with four jets, and two (medium) isolated taus, where the taus may also satisfy the jet triggers.

<footnote>a Isolation is not required for electromagnetic clusters with $E_T$ above 50 GeV.</footnote>

<footnote>b Single b-jet trigger.</footnote>

<footnote>c Trigger requiring four b-jets and operating below the efficiency plateau of the Level-1 trigger.</footnote>

<footnote>d $E_T^{\text{miss}}$ triggers in the Run 1 menu are assumed to be vetoed on the first 3 bunches to avoid huge bunch train effects giving a 15% inefficiency. Also for Run 1 some of the offline jet thresholds were set at the point where the efficiency reached 99% of its plateau value.</footnote>
<table><caption>Table 3.4: Level-1 $p_T$ thresholds of few key lepton triggers at the Phase-I luminosity and at the planned peak HL-LHC luminosity of $\mathcal{L} = 7.5 \times 10^{34} \text{ cm}^{-2}\text{s}^{-1}$ assuming the Phase-I system with no upgrade. The second column indicates the Run 3 estimated rates. The thresholds indicated in the third column are the ones required to satisfy a maximum of $\sim 100 \text{ kHz Level-1 trigger rate that the Phase-I system can sustain.}$</caption><thead><tr><th>Trigger</th><th>Level-1 p<sub>T</sub> Threshold [GeV]<br>at L = 3 &times; 10<sup>34</sup> cm<sup>-2</sup>s<sup>-1</sup></th><th>Level-1 Rate<br>[kHz]</th><th>Level-1 p<sub>T</sub> Threshold [GeV]<br>at L = 7.5 &times; 10<sup>34</sup> cm<sup>-2</sup>s<sup>-1</sup></th></tr></thead><tbody><tr><td>isolated single e</td><td>32</td><td>14</td><td>50</td></tr><tr><td>di-e</td><td>19</td><td>5</td><td>35</td></tr><tr><td>single &mu;</td><td>25</td><td>15</td><td>40 (with low efficiency)</td></tr></tbody></table>

Without an upgrade of the TDAQ Phase-I system, the expected trigger rates at a peak luminosity of $\mathcal{L} = 7.5 \times 10^{34} \text{ cm}^{-2}\text{s}^{-1}$ are incompatible with the Phase-I readout system. Table 3.4 shows for a few key signatures the Level-1 $p_T$ trigger thresholds in Phase-I and the equivalent in Phase-II if we were to keep the Phase-I readout system with the required maximum Level-1 rate of 100 kHz. It can be observed that $p_T$ thresholds would need to be increased significantly thus affecting the physics potential of ATLAS. Equivalently, the total trigger rate corresponding to the Phase-I $p_T$ trigger thresholds would result in a factor of 10 times the allowed rate for the Phase-I readout system.

To maintain the single-electron and single-muon trigger rates at the levels achieved in Runs 1-3, leaving sufficient bandwidth for other important triggers in the trigger menu, the thresholds for these key triggers would have to be more than 50 GeV for electrons and more than 40 GeV for muons, as shown in Fig. 3.16. These high thresholds would significantly degrade the potential for the physics programme planned for Run 4, as shown Section 2.1. Furthermore, the improvements expected for tau lepton triggers from the Phase-I Level-1 calorimeter upgrade will be insufficient in Run 4, severely impacting the physics acceptance.

### 3.2.5 Limitations in the Level-1 CTP and TTC systems

The CTP that was installed at the start of Run 2 will support the Phase-I system through Run 3. However, it is not adapted to the technical requirements of Run 4, such as providing readout at rates of 1 MHz or beyond, and driving the new TTC system.

The TTC system that will be used up to the end of Run 3 was conceived in the 1990s and is constrained by the technology that was available at that time. With a line rate of only 80 Mb/s, its capabilities for transmitting data with each positive trigger decision are limited. For this reason, for example, it relies on local counters in the front-end receiver ASICs for the detection of synchronisation errors. This and other considerations led ATLAS to envisage a much improved system that would be provided to all detector systems in the Phase-II upgrade, that would also be capable of supporting a two-level trigger scheme.
<figure><img src="image_49.png" /><figcaption>Figure 3.16: Single-lepton trigger rates at Level-1 versus $p_T$ threshold expected with the Phase-I trigger system at a luminosity of $\mathcal{L} = 7.5 \times 10^{34} \text{ cm}^{-2}\text{s}^{-1}$ as extrapolated from Run 2 data. Left plot electron rates and right plot muon rates.</figcaption></figure>

The Run 2 trigger menu uses 406 inputs and 501 items, including triggers for monitoring, control samples, etc. At most about 250 out of the 501 items are enabled concurrently. It is convenient to implement a large trigger menu in which only a subset of the items is actively used at any given time, allowing rapid switching according to the instantaneous luminosity, and simplifying monitoring and offline use of trigger information. Noting that the CTP needs to be replaced for other reasons, it is clearly prudent to foresee support for larger trigger menus from Run 4. This will give flexibility to benefit from enhancements elsewhere in the trigger system, to profit from new ideas for triggering, and to react should new physics require additional trigger items.

## 3.3 Limitations of the Run 3 DAQ System

As discussed earlier in this chapter, the detector electronics readout is one of the main limitations of both the Run 3 trigger and DAQ systems. Furthermore, the readout and Dataflow components of the Run 3 DAQ system do not have the flexibility or scalability required to cope with the factor of more than twenty increase in bandwidth expected in Run 4 conditions. This large increase in bandwidth results from the increased event size and higher data rates, as discussed in this section.

### 3.3.1 Detector Readout

Most of the present detector readout and trigger elements have hard limits on the maximum first-level trigger latency and rate they can operate, and therefore they have to be replaced
<figure><img src="image_50.png" /></figure>

Figure 3.17: Block diagram of the FEB architecture, depicting the data flow for four of the 128 readout channels per FEB. The shaped signals are sampled at the LHC bunch crossing frequency of 40 MHz by Switched Capacitor Array (SCA) analog pipeline chips, which store the signals in analog form during the Level-1 trigger latency. The limitation on the maximum latency is coming from the SCA.

for operation in Phase-II. Only some of the on-detector front-end electronics will remain in
limited cases, such as the LAr Calorimeter cold electronics in the Hadronic Endcaps and the
RPC front-end electronics for the middle and outer stations. Note that systems designed for
Phase-I have been designed with the aim of being used without modification in Phase-II.

**Front-end Pipeline Limitations** The original ATLAS requirements for on-detector front-end electronics stipulated that detector signals were sampled at the LHC bunch crossing frequency of 40 MHz. These original requirements also stated that signals must be stored during the latency of the Level-1 trigger of up to 2.5 µs (100 bunch crossings), and for triggered events, that the readout must be accomplished without significant dead-time for a maximum mean Level-1 trigger rate of 75 kHz. The radiation tolerance issues required that all electronics components selected for use on the detector be subjected to an extensive radiation qualification process. This led to the development of a number of custom AS-ICs in specialised, radiation-tolerant semiconductor processes, and to a very limited use of commercial components. The specifications of these ASICs were pushed to the limits of the technology available at the time. However, these original specifications are a limiting factor for the detector front-end electronics (with the exceptions listed above).

One example is the LAr calorimeter front-end electronics system, which includes Front End Boards (FEBs) that perform the amplification, shaping, sampling, storage, digitisation, and readout of the calorimeter signals. In Fig. 3.17 the block diagram of the LAr FEB shows the main limitation on the maximum allowed latency due to the analog pipeline, which stores the shaper output samples for a maximum latency of 144/40 MHz = 3.6 µs. Similar limitations are built-in for all detector front-ends and they can be overcome only through re-design and by building new readout systems.

**Deadtime Limitations** Given the latency involved in propagating BUSY signals, some form of complex deadtime is mandatory, e.g. leaky bucket or sliding window described
below. This is in addition to the simple deadtime where a minimum time interval between successive L1As signals is required. The goal foreseen up to Run 3 was to achieve less than 2% deadtime on each detector readout system. Several Level-1 Trigger complex deadtime algorithms have been investigated as a protective measure for shaping the Level-1 dead-time; the two most successful algorithms implemented to date are:

**"Leaky Bucket" algorithm** This algorithm is driven by the data integrity versus latency. Even varying the algorithm parameters, the deadtime depends steeply on the trigger rate. This algorithm is best suited for detectors, like LAr, where constraints are based on whole event buffers (fixed-size event fragments).

**"Sliding window" algorithm** This algorithm is more suited for detectors, like Pixels, where the event fragment size distribution matters, with very rare large event fragments.

Deadtime estimations and measurements from Run 2 are shown for LAr and Pixel detectors in Fig. 3.2, where the effectiveness of the protection with different parameter settings is shown. A very steep increase in deadtime can be observed when the Level-1 output rate increases over 100 kHz. This is a hard limitation due to design issues which are explained in the next subsection, which may be overcome only by re-designing the detector readout systems.

**Readout Bandwidth Saturation** Figure 3.18 shows the number of minimum bias interactions per crossing at which the occupancy of the readout link of each detector will exceed a 90% threshold during the 2017 Run 2 data as function of pile-up. Notice that already during the Run 2 operations, the Transition Radiation Tracker (TRT) will start saturating the bandwidth available, and most of the inner detector will start saturating the readout during the Run 3 data taking. The hard limitation is due to the bandwidth limit (S-Link @ 1 Gbps by design) of the readout link between the ROD and the ROS systems used by the detectors. Furthermore, the ROD/ROS technology is not viable for the readout system of all the detectors at the HL-LHC. A different approach based on modern technologies is needed.

Beyond the ROL occupancy limitation, the ROS is currently limited to a maximum readout fraction of ~50%. In order to achieve a higher readout fraction, the firmware of the Peripheral Component Interconnect Express (PCIe) card would need to be rewritten. The next bottleneck to be solved would be the CPU processing speed, and the networking would ultimately limit the readout bandwidth.

### 3.3.2 Dataflow, Storage and Networking

The estimated event size for each detector system during the Run 3 operations after the Phase-I upgrades is estimated by a linear regression of the Run 2 data as shown in Fig. 3.19. This estimate is summarised in the second column in Table 3.5 while the third column shows the event size as estimated by each detector at the ultimate luminosity during the
<figure><img src="image_51.png" /><figcaption>Figure 3.18: Number of interactions per crossing (“pile-up”) at which 90% of the ROL occupancy for each ATLAS detector system is reached. The density scale indicates the maximum Level-1 trigger rate.</figcaption></figure>

HL-LHC operations. The large increase of the Pixel event size in Phase-II with respect to Phase-I is due to the higher detector occupancy and the significant increase of detector readout channels. A total event size of 5.2 MB is estimated in Run 4. Note that there is ~0.1 MB included in the TDAQ size for both Run 3 and Run 4, to accommodate the data from LAr trigger electronics, since super cell information from LAr is needed for the validation of the L1Calo FEX trigger logic. Furthermore selected ADC data further allows confirmation of energy assignment to individual bunch crossings.

For the Run 3 system, the network and storage resources have been determined using the trigger/readout parameters described in Table 3.1. There are no intrinsic limitations in the network and storage systems, which fully scale linearly with the parameters of the trigger and readout architecture of Table 3.1 and with the average event size. Therefore, the size of the system would have to be increased by a factor of approximately 2.5 just to account for the increase in event size expected in Run 3 compared to Run 4. This is without considering the additional increase in rate.
<figure><img src="image_52.png" /></figure>

Figure 3.19: Run 2 event size (in kB) as a function of the number of interactions per bunch crossing (“pile-up”). A linear fit is applied in order to extrapolate to the Run 3 conditions after the Phase-I upgrades. Note that the extrapolation does not include the event size of the new detector systems to be introduced at the Phase-I upgrade, which is expected to contribute ≈0.7 MB in addition, at pileup of 80.

Table 3.5: Average Event Size (before HLT selection) as extrapolated from Run 2 data to the Phase-I conditions (including the new detector systems introduced by Phase-I upgrade), and estimated by the detector systems after the Phase-II upgrades. Forward detectors are not listed since the associated event size is negligible.

<table><thead><tr><th rowspan="2">Detector System</th><th colspan="2">Extrapolated Data Size [MB]</th></tr><tr><th>Phase-I</th><th>Phase-II</th></tr></thead><tbody><tr><td>Pixel</td><td>0.3</td><td>2.4</td></tr><tr><td>Strip</td><td>0.2</td><td>0.5</td></tr><tr><td>TRT</td><td>0.5</td><td>-</td></tr><tr><td>LAr</td><td>0.7</td><td>0.7</td></tr><tr><td>Tile</td><td>0.1</td><td>0.2</td></tr><tr><td>Muon</td><td>0.5</td><td>0.8</td></tr><tr><td>TDAQ</td><td>0.6</td><td>0.6</td></tr></tbody><tfoot><tr><td><strong>Total</strong></td><td><strong>2.9</strong></td><td><strong>5.2</strong></td></tr></tfoot></table>
# 3.4 Limitations of the Run 3 HLT System

The Run 3 HLT system will not be able to cope with the increased event input rate and the rise in algorithm execution times resulting from the increased level of pile-up at the HL-LHC. Furthermore, the hardware-based tracking system in Run 3 has been designed to cope with the pile-up conditions and Level-1 trigger rate of the Phase-I TDAQ system, and cannot perform at the same rate with the same efficiency in the HL-LHC.

### 3.4.1 Estimates of Rejection Factors in the Run 3 HLT

For the HLT system to provide sufficient background rejection and reasonable rates, it is necessary to implement physics analyses requirements in the HLT. Therefore, the quality of trigger object reconstruction in the HLT needs to approach that of the offline reconstruction. This statement is true for all objects including leptons, (b-tagged) jets, and $E_T^{miss}$. Thus full-event reconstruction in the calorimeters and tracking is required for most events that pass a single-electron selection. Additionally, at HL-LHC conditions where much of the offline-like selection will be implemented in the hardware-based Level-0 trigger system, background rejection becomes even more challenging.

Table 3.6, extracted from [3.6], shows the rejection at the HLT for single-lepton and di-$\tau$ triggers during Run 1 and Run 2, and the expected rejection in Run 3 after the Phase-I upgrades. As the pile-up increases, the HLT selection will be pushed at the limit to maintain reconstruction efficiency and rejection capability: (i) selection parameters will be tuned and optimised with respect to luminosity (e.g., noise thresholds), (ii) object isolation will be tightened for all the trigger signatures, and, (iii) selection algorithms at the HLT will be made very similar or identical in performance to the offline reconstruction algorithms.

Therefore, an overall rejection of $\sim 100$ at HLT will be a limiting value during the Run 3 data taking, and a target value for the Phase-II upgrades.

### 3.4.2 Fast Tracking

In the expected high pile-up environment, charged particle tracking can greatly assist in identifying trigger objects. However, tracking algorithms are challenging with respect to processing time; as shown in Fig. 3.20, the processing time for the existing HLT software tracking increases faster than linearly with luminosity, making it prohibitively expensive to run on every event.

A dedicated hardware-based tracking system has been installed in ATLAS in Run 2: the Fast TracKer system (FTK), that will continue to operate in Run 3. It is a massively parallel hardware system using tracking information from a region $|\eta| < 2.5$ and designed to cope
<table><caption>Table 3.6: Examples of background rejection factors at the HLT. The first column gives the rejection of the HLT achieved in Run 1 and Run 2. The second column gives the expected rejection if the Run 1/2 selection criteria were run on the Phase-I Level-1 output. The third column gives the fraction of Run 1/2 events which are selected by the corresponding offline selection. The last column gives the desired rejection factor of the HLT in Phase-I. Because of the high purity of the single-object triggers, additional rejection must be obtained using event properties such as jets reconstructed in the HLT.</caption><thead><tr><th rowspan="2">Signature</th><th>Run 1/2 selection</th><th>Run 1/2 selection</th><th>Run 1/2 fraction</th><th>Phase-I</th></tr><tr><th>HLT rejection</th><th>used at Phase-I<br>HLT rejection</th><th>selected offline</th><th>(stipulated)<br>HLT rejection</th></tr></thead><tbody><tr><td>single-<i>e</i></td><td>180</td><td>60</td><td>~ 80%</td><td>60</td></tr><tr><td>single-<sup>-</sup></td><td>80</td><td>25</td><td>~ 95%</td><td>25</td></tr><tr><td>di-<i>θ</i></td><td>500</td><td>250</td><td>~ 70%</td><td>300</td></tr></tbody></table>

<figure><img src="image_53.png" /><figcaption>Figure 3.20: The trigger track reconstruction time for the beamspot trigger for simulated 14 TeV tt events with 46, 69 and 138 interactions per bunch crossing with the Run 2 detector simulation, measured on a 2.4 GHz Intel Xeon CPU. The software version used corresponds to the 2016 online trigger system. Statistical uncertainties are shown. A second-order polynomial is fitted to the points.</figcaption></figure>

with an average pile-up of $\mu \simeq 80$ and at a L1A rate of 100 kHz [3.9]. The FTK is based on the CDF Silicon Vertex Trigger [3.11][3.12] and much of it is VME-based.

The expected Run 4 conditions will lead to higher occupancy in the ITk, which would result in an increase in the dataflow (considering the event size and the rate increase) by more than a factor of 20 into the FTK. Furthermore, the event complexity due to pile-up would significantly increase the processing power requirements for the pattern-matching stage in the Associative Memory (AM) chips and on the processing FPGAs that fit the track candidates. Processing events with $\mu \simeq 140-200$ would lead to long tails in the event processing time distribution, which intrinsically limit the maximum operating rate of FTK well below the 100 kHz L1A rate. To compensate for this, the processing time could be reduced, and