Commissioning of New Trigger Readout
System of Liquid Argon Calorimeter
towards the LHC-ATLAS Run3
Experiment(LHC-ATLAS Run3 実験に向けた液体アルゴンカロリメータの新型トリガー読み出しシステムのコミッショニング)藏 嘉琦Jiaqi Zang
Department of Physics
Graduate School of Science
The University of Tokyo
This dissertation is submitted for the degree ofMaster of Science
January 2022

Acknowledgements

First and foremost, I would like to express my great gratitude to my advisor Prof. Junichi
Tanaka, who provided me with such a chance to participate in the ATLAS experiment.
Tanaka-sensei has helped me a lot since I came to Japan, teaching me many things about
experiments, data analysis, and programming. He also supported me a lot in my research lifeand gave me much advice for writing the thesis. I am also grateful to Assistant Prof. Yuji
Enari. He taught me a lot about the LAr calorimeter, especially about the new digit trigger
readout electronics and the online software development. When I encountered difficulties in
my research, he would actively provide me with constructive ideas and suggestions. He also
gave me much valuable advice for this master’s thesis.
Thanks to the experts in the LAr group at CERN. Project leader George Aad supervisedand helped me with the qualification task. He also gave me many constructive suggestions
on the connectivity scan and BCID calibration. Postdoctoral researcher Ellis Kay from the
University of Victoria helped me develop the automation script for the SSW scan analysis. Postdoctoral researcher Konie Al Khoury from Columbia University helped with thevalidation of LTDBs.
In addition, I would like to acknowledge the senior students and my friends in Tanaka
Lab: Gen Tateno, Reiyo Oishi, Tingyu Zhang, Ryunosuke Okubo, and Marin Furukawa.
Tateno provided me with a lot of technical advice on programming. In addition, the video
tutorials he made about the LAr calorimeter helped me deepen my understanding of theexperiments to a great extent. Oishi taught me a lot about LATOME firmware development.
Tingyu provided a lot of help and advice for my data analysis. Okubo has given me many
interesting thoughts about his research on quantum computers. Furukawa added a lot ofcolor to my daily life during this COVID-19 pandemic. Her positive attitude towards life andresearch is an inspiration to me.
I am also grateful to the other contemporary students for the discussions and presentations
during the various seminars. I have benefited a lot from these experiences. And thanks to the
University of Tokyo and ICEPP for providing such a profound academic atmosphere.

iv
Thanks to my other friends who study at the University of Tokyo for their help and mental
support in my daily life. Despite the coronavirus outbreak all over the world, I still have faithin an open and hopeful future.
At last, I would like to express my heartfelt gratitude to my family, who supported myresearch life in Japan.

Abstract

The world’s highest energy proton-proton colliding accelerator, the Large Hadron Collider(LHC) experiment at CERN, was proposed and started in 2008 to extend the frontiers ofparticle physics. The ATLAS experiment at LHC is designed to record the particle collisions
at the LHC and search for Higgs bosons and physics beyond the Standard Model. Due to theneed to record a large amount of collision data, the trigger system is applied. The trigger
system uses the information provided by the readout system for object recognition, such as
position and energy. So that only the events that pass a set of criteria are selected. During
the Run-2 period (the year 2015-2018), the LHC collided beams at center-of-mass energy√s = 13 TeV, and ATLAS recorded about 139 fb−1 of proton-proton collision data. Since
Run-2 ended in 2018, the LHC has been upgraded and will start Run-3 in April 2022. Inorder to accommodate the update, the ATLAS trigger system requires higher efficiency andbetter resolution. Hence the phase-I upgrade for the ALTAS trigger system is performed.
The ATLAS Liquid Argon Calorimeter phase-I upgrade provides the new trigger system
to obtain better energy resolution and efficiency for selecting electrons, photons, τ leptons,
jets, and missing transverse energy. The new readout system applies over 34000 supercellswith 10 times higher granularity than the old readout cells and uses optical fibers to transmit
the digitized data from the front-end to the back-end. The supercells with multi-layer structure
cover the region of |η| < 4.9. The signals read out from the supercells are digitized by the
LTDB at the front-end and transmitted to the back-end via optical fibers, where they arecalculated by the LATOME board equipped with a high-speed FPGA for energy and timing
reconstruction. The computation result is then sent to FEX for object recognition throughoptical fibers.
In this thesis, several commissioning works to validate the new trigger system arepresented. The first work is connectivity check. It’s performed to check whether thesupercells are connected correctly from the detector to the back-end. The SSW scan is used
to check the mapping of all the 34048 supercells, and the corresponding analysis framework
for the SSW scan is developed in this research. Also, by utilizing such an analysis tool, someother channel issues like bad links or bad ADC configuration are also investigated.

vi
Besides checking the connectivity of supercells, the system latency of the new trigger
system is also calibrated in the study. The system latency is required to be well-tuned andfixed for a stable system. The energy and timing computation values cannot be obtainedcorrectly without the calibrated system latency. In the ATLAS experiment, the bunch crossing
identification (BCID) is used for adjusting the system latency. Such work of adjusting the
BCID of over 4000 fibers on the LATOME board is done by aligning the BCID of the new
trigger system to the BCID of the reference main readout system. And the result of BCID
calibration is given at last.
The research also utilizes the pilot run data taken in October 2021 to perform a fullpowered new readout system test with real particles. In the pilot run, the detector managed
to take both beam splash and collision data. With these data, the comparison of BCID
and the reconstructed ET between the two readouts is made, where ET is computed by the
optimal filtering algorithm with the calibrated optimal filtering coefficient. And through thiscomparison, the thesis gives the evaluation of the commissioning works and the conclusion
on whether the new trigger system can achieve the expected performance.

Table of contents
List of figuresix

List of tables

xiii

Nomenclature

xv

1

Introduction
1.1 Theoretical Background . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.1 The Standard Model . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.2 Beyond the Standard Model . . . . . . . . . . . . . . . . . . . . .
1.2 Research Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 Thesis Constitutes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1
1
1
1
3
3

2

LHC-ATLAS Experiment
2.1 Large Hadron Collider . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 ATLAS Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Inner Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2 Calorimeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3 Muon Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.4 Data Acquisition and Trigger System . . . . . . . . . . . . . . . .
2.3 The LHC Upgrade Project . . . . . . . . . . . . . . . . . . . . . . . . . .

5
5
6
9
10
14
15
16

3

LAr Phase-I Upgrade Project3.1 Signal Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 New Readout and Trigger System . . . . . . . . . . . . . . . . . . . . . .
3.2.1 Super Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2 Front-End Electronics . . . . . . . . . . . . . . . . . . . . . . . .
3.2.3 Back-End Electronics . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.4 Energy and Timing Reconstruction in Run-3 . . . . . . . . . . . .

21
21
22
22
23
29
33

viii

Table of contents
3.3

Installation Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

36

4Connectivity Check and BCID Calibration
4.1 Shaper Switch Scan and Connectivity Check . . . . . . . . . . . . . . . . .
4.1.1 Workflow of Connectivity Check . . . . . . . . . . . . . . . . . . .
4.1.2 Pulse Detection Criteria . . . . . . . . . . . . . . . . . . . . . . .
4.1.3 Scan Validation Algorithm . . . . . . . . . . . . . . . . . . . . . .
4.1.4 Validation Status and Auto-processing System . . . . . . . . . . .
4.2 BCID Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1 Timing Alignment by Calibrating the Peak Position . . . . . . . . .
4.2.2 Instability of the Calibration Value . . . . . . . . . . . . . . . . . .
4.2.3 Optimization for Mitigating the Fluctuation . . . . . . . . . . . . .

39
41
42
43
45
47
53
53
55
56

5

Calibration Run and Analysis for the LHC Pilot Run Data
5.1 Test Pulse System and OFC Calibration . . . . . . . . . . . . . . . . . . .
5.1.1 Optimal Filtering Coefficient . . . . . . . . . . . . . . . . . . . . .
5.1.2 Calibration Run . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.3 OFC calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Data Analysis with LHC Pilot Run Data . . . . . . . . . . . . . . . . . . .
5.2.1 LHC Pilot Run . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.2 Peak Sample and BCID Delay . . . . . . . . . . . . . . . . . . . .
5.2.3 Energy and Timing Reconstruction. . . . . . . . . . . . . . . . . .

59
59
59
60
61
63
63
65
68

6

Conclusion and Outlook
6.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2 Outlook on Works towards Run-3 . . . . . . . . . . . . . . . . . . . . . .

73
73
74

References

77List of figures
1.1

Elementary Particles in SM . . . . . . . . . . . . . . . . . . . . . . . . . .

2

2.1 Overall view of the LHC . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Schematic view of the ATLAS experiment . . . . . . . . . . . . . . . . . .
2.3 Coordinate system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Pseudorapidity η . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5 3D vision of the barrel of the Inner Detector. . . . . . . . . . . . . . . . . .
2.6 Cut-away view of the calorimeter . . . . . . . . . . . . . . . . . . . . . . .
2.7 Accordion structure of the calorimeters . . . . . . . . . . . . . . . . . . .
2.8 Geometry of the Tile calorimeter module . . . . . . . . . . . . . . . . . . .
2.9 Schematic view of HEC . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.10 FCal modules location in the end-cap cryostat . . . . . . . . . . . . . . . .
2.11 Cut-away view of Muon Spectrometer . . . . . . . . . . . . . . . . . . . .
2.12 Workflow of the L1Calo system in Run-3. . . . . . . . . . . . . . . . . . .
2.13 Schematic view of the ALTAS DAQ and trigger system after the phase-I
upgrade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.14 Schedule for the LHC upgrade plan. . . . . . . . . . . . . . . . . . . . . .

7
8
9
10
11
12
13
14
15
16
17
18

3.1
3.2
3.3
3.4

22
23
24

3.5
3.6
3.7
3.8

Accordion structure of the EMB . . . . . . . . . . . . . . . . . . . . . . .
Schematic diagram of the tracks from the splashing particles in ATLAS . .
LAr calorimeter pulse signals . . . . . . . . . . . . . . . . . . . . . . . . .
Schematic view of legacy and new readout cells in case an electron with 70
GeV ET is injected. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Schematic diagram of the LAr EM calorimeter electronic architecture in
LHC Run-3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Perspective drawing of the HFECs. . . . . . . . . . . . . . . . . . . . . . .
Schematic figure of the LTDB board. . . . . . . . . . . . . . . . . . . . . .
Schematic figure of LTDB optical links . . . . . . . . . . . . . . . . . . .

19
19

25
26
27
28
29

x

List of figures
3.9 Schematic of the CTP and TTC system . . . . . . . . . . . . . . . . . . . .
3.10 Schematic diagram of the LDPS. . . . . . . . . . . . . . . . . . . . . . . .
3.11 LATOME board equipped with optical fibers. . . . . . . . . . . . . . . . .
3.12 Schematic diagram of LATOME firmware. . . . . . . . . . . . . . . . . . .
3.13 Trigger efficiency comparison between the trigger implemented in Run-2
and Run-3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.14 Energy reconstruction of the injected pulse with 50 GeV ET . . . . . . . .
3.15 Front-end installation status in December 2021. . . . . . . . . . . . . . . .
3.16 Current status of ATCA shelves. . . . . . . . . . . . . . . . . . . . . . . .

34
35
37
37

4.1 Three kinds of readout paths for LAr calorimeter. . . . . . . . . . . . . . .
4.2 Back-end data flow of digital trigger. . . . . . . . . . . . . . . . . . . . . .
4.3 Block diagram of the FEB architecture . . . . . . . . . . . . . . . . . . . .
4.4 Schematic figures of SSW scan workflow. . . . . . . . . . . . . . . . . . .
4.5 Pulse pattern of the SSW scan . . . . . . . . . . . . . . . . . . . . . . . .
4.6 Pulse detection algorithm for SSW scan. . . . . . . . . . . . . . . . . . . .
4.7 The example of the FCal Pulse . . . . . . . . . . . . . . . . . . . . . . . .
4.8 Flow diagram of the pulse validation algorithm. . . . . . . . . . . . . . . .
4.9 An example of the validation result for a single LTDB . . . . . . . . . . . .
4.10 Online validation tools. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.11 LTDB validation status on 16/09/2021 and 29/12/2021 . . . . . . . . . . .
4.12 Workflow of BCID calibration. . . . . . . . . . . . . . . . . . . . . . . . .
4.13 Pulses with similar peak sampling points around the peak position. . . . . .
4.14 Comparison of two runs with the same BCID setting. . . . . . . . . . . . .
4.15 RMS distribution of the supercells on the same fiber. . . . . . . . . . . . .
4.16 Comparison of two runs with the same data latency after the optimization. .
4.17 Comparison of two runs with the updated BCID. . . . . . . . . . . . . . .

40
41
42
43
44
45
45
48
49
50
52
54
55
56
56
57
58

5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9

30
31
32
33

Delay pulses with DAC values from 0 to 1500 for a supercell in the 2nd layer. 61
ET / ADC distribution in all regions except for FCal. . . . . . . . . . . . . 62
Distribution of non-dimensional OFC ãi and b̃i . . . . . . . . . . . . . . . . 62
Cut-away view of the first interesting events from the LHC beam splash test. 63
Beam intensity summary for both collision run. . . . . . . . . . . . . . . . 64
Pedestal and RMS value of all supercells . . . . . . . . . . . . . . . . . . . 65
BCID delay between SWROD and main readout with the splash data. . . . 66
Physics pulse in the splash data. . . . . . . . . . . . . . . . . . . . . . . . 67
BCID delay between SWROD and main readout with the collision data. . . 67

List of figures
5.10 Reconstructed ET and τ at different BCID using the calibrated OFC. . . . .
5.11 Reconstructed energy distribution using splash data. . . . . . . . . . . . . .
5.12 Energy comparison between SWROD and main readout. . . . . . . . . . .
5.13 Computed ET using collision data. . . . . . . . . . . . . . . . . . . . . . .
5.14 ET comparison between SWROD and main readout with collision data. . .

xi
68
69
70
71
72

List of tables
3.1
3.2

Granularity of the trigger tower and super cell in the EMB calorimeter. . . .
The numbers of channels in the different region of LTDBs and the numberof LTDBs for each LTDB type. . . . . . . . . . . . . . . . . . . . . . . . .

24

4.1

Problematic channels found by the connectivity scan. . . . . . . . . . . . .

51

5.1

Run information for the data used for analysis. . . . . . . . . . . . . . . . .

64

28

Nomenclature
Acronyms / Abbreviations
ADC Analog-to-Digital Converter
AMC Advanced Mezzanine Card
ATCA Advanced Telecommunications Computing Architecture
ATLAS A Toroidal LHC Apparatus
BCID Bunch Crossing Identification
BSM Beyond Standard Modelc.m.

center-of-mass

CERN European Organization for Nuclear Research
CST

Cathode Strip Chamber

CTP

Central Trigger Processor

EMB Electromagnetic Barrel
EMEC Electromagnetic Endcap
EM

Electromagnetic

ETmiss Missing Transverse Momentum
FCal the Forward Calorimeter
FEB

Front-End Board

FEC

Front-End Cratexvi

Nomenclature

FEX Feature Extractor
HEC Hadronic End-cap Calorimeter
HFEC Half Front-End Crate
HLTHigh Level Trigger

ID

Inner Detector

IP

Interaction Point

IPMC Intelligent platform management controller
L1Calo Level-1 Calorimeter Trigger System
LArC LAr carrier
LAr

Liquid Argon

LATOME LAr Trigger Processing Mezzanine
LB

Luminosity BlockLDPB LAr Digital Processing Blade
LDPS LAr Digital Processing System
LHC Large Hadron Collider
LS2

the second LHC long Shutdown

LSB

Layer Sum Board

LTDB LAr Trigger Digitizer Boards
MDT Monitored Drift TubeMS

Muon spectrometer

ROC Readout Crate
ROD Readout Driver
RPC Resistive Plate Chamber
RTM Rear Transition Module

Nomenclature
SC

Super Cell

SCT

Semiconductor Tracker

SMStandard Model

SPAC Serial Protocol for Atlas Calorimeter
SSW Shaper Switch
SUSY Supersymmetry
TBB Tower Builder Boards
TBM Trigger Busy Module
TDAQ Trigger and Data Acquisition
TGC Thin Gap Chamber
TRT

Transition Radiation Tracker

TTC Timing Trigger and Control
VME Versa Module Eurocard

xviiChapter 1
Introduction
1.1

Theoretical Background

1.1.1

The Standard Model

The Standard Model (SM) of elementary particles describes the fundamental constituentsof matter and their interactions. As shown in Figure 1.1, there are six types of quarks andsix different leptons which are classified into three generations. These particles are spin 1/2
fermions, and all have their own anti-particles. On the other hand, the four-vector bosonson the right side: photon, W/Z, gluon, are the propagator of interactions. And last, a special
scalar Boson, Higgs Boson, which was discovered in 2012 by the ATLAS and CMS at
LHC [1], makes the other particles massive. Generally, the Standard Model is based on thefollowing symmetry group:SU(3)C × SU(2)L ×U(1)Y .

(1.1)

Interactions can be derived from this. SU(3)C describes the strong interaction with chargecolor (C) and corresponds to the local symmetry whose gauging gives rise to quantum
chromodynamics (QCD). The gauge symmetry SU(2)L × U(1)Y describes the electroweak
interaction with the weak isospin (L represents for "left-handed") and the hypercharge (Y)as conserved current. The electroweak interaction is unified from the weak interaction (QCD)
via the spontaneous symmetry breaking.

1.1.2

Beyond the Standard Model

Although the SM theory can successfully explain most of the phenomenon from manydifferent experiments and describe the elementary particles with the Higgs mechanism, it’s

2

Introduction

Fig. 1.1 Elementary Particles in SM. Fermions build the matter and Bosons propagate theinteractions [2]still not the ultimate theory for nature. For example, it cannot include the gravitational
interaction with the other three fundamental interactions. Also, objects that cannot be
observed directly through the SM interactions in our universe are much more than theordinary objects, which can be described in the SM. This is called Dark Matter (DM). Oneof the observational evidence for DM is the Galaxy rotation curves. The relation between
the rotation velocity and the distance from the center of the galaxy is different from whatis expected from Kepler’s Second Law with ordinary matters. Another important questionis the hierarchy problem. SM cannot stabilize the hierarchy between the energy scale thatcharacterizes the electroweak symmetry breaking, MEW ∼ 100 GeV, and the Planck scale(1.22 × 1019 GeV) against large radiative corrections without an unnatural fine-tuning of theparameters of the fundamental theory at the Planck scale.
Hence, Supersymmetry (SUSY) theory is raised as a candidate to explain these unsolved
problems. It’s a generalization of the space-time symmetry that transforms fermions into
bosons and vice versa [3, 4]. The Large Hadron Collider (LHC) at CERN in Switzerland isthen making efforts to find those SUSY particles.

1.2 Research Purpose

1.2

3

Research Purpose

The LHC is built to extend the frontier of particle physics and explore the new physics
beyond the SM. In which, the ATLAS (A Toroidal LHC Apparatus) experiment is the largest
particle detector experiment to probe p-p and A-A collisions. The LHC complex will be
upgraded by 2022. The second long shutdown after the 3-years-operation of Run-2 in 2018
and the upgrade for Run-3 is performed. The center-of-mass energy will also raise from√s ≃ 13 TeV to 13.6 TeV. In order to adapt to this upgrade, the so-called phase-I upgrade
project for the ATLAS Liquid Argon (LAr) Calorimeter is performed, and a new trigger
readout system is installed. The research focuses on the commissioning works on the new
digital trigger readout, such as the front-end electronics validation, correction of mappingof the new readout path, and calibration works. With the calibrated new readout system,
despite the increased beam energy, the trigger performance is more reliable than before andhas better sensitivity for searching the interesting events. One important improvement forthe system is the readout cell. The new readout cells with better granularity provide better
energy resolution and shower shape information to the trigger system and therefore increase
the S/N for the physical analysis.

1.3

Thesis Constitutes

This thesis introduces the LHC-ATLAS experiment in Chapter 2. Chapter 3 discusses LAr
phase-I upgrade project through the introduction of legacy and new electronics. Chapter 4
presents the connectivity check of the new readout path and the validation of LTDBs (LAr
Trigger Digitizer Boards), which are the on-detector electronics for the new trigger system.
Chapter 4 also discusses the timing alignment between the digital trigger readout and themain readout in ATLAS. Chapter 5 shows calibration runs and the analysis for the recent
LHC Pilot run in October 2021. And the last Chapter 6 concludes this thesis and gives
an outlook for the near future, including fixing the remaining firmware problem and theadjustment for the digital trigger system.

Chapter 2
LHC-ATLAS Experiment
2.1

Large Hadron Collider

The LHC at the European Organization for Nuclear Research (CERN) is the largest particle
collider with the highest energy in the world. It has a huge ring with a 27 km circumference
that is about 100 meters underground. The view of the LHC is shown in Figure 2.1. The depth
protects the detector from background noise such as cosmic rays. And the long length allows
the proton to be accelerated to nearly the speed of light under the 8.3 Tesla magnetic field.
The two proton beams are contained in separate pipes throughout most of the circumference
and are brought together into a single pipe at the interaction points (IP). A large number ofprotons in a bunch leads to about 30 or more head-on collisions every 25 ns.
When the two bunches which containing n1 and n2 particles collide head-on with the average collision frequency fcoll , the instantaneous luminosity L is calculated by the following
formula [5, 6]:
L = fcolln1 n2F,
4πσx∗ σy∗(2.1)

where σx∗ and σy∗ characterize the RMS transverse beam sizes in the horizontal and verticaldirections at the IP, and F is a factor of order one that is affected by the geometric effects.
The fcoll value equals the number of bunches per beam times the revolution frequency for thecircular collider. Thus, the number of events Nexp for an interesting process is the product of
the cross-section of the interest σexp and the time integral over the instantaneous luminosity:

Nexp = σexp ×Z

L(t)dt.

(2.2)

6

LHC-ATLAS Experiment

The primary goal of the LHC is to search for the Higgs boson (done in 2012) and new
physics beyond the SM. The energy collision at 7 TeV was achieved in March 2010, andthe first collision at 8 TeV was achieved in April 2012. After the first shut down, the LHC
is upgraded, and the energy is raised to 13 TeV. And then, it started the second run (Run-2)
from 2015 to 2018 with the much higher c.m. energy. After Run-2 is finished, the LS2 isperformed and started the three-year upgrade. This upgrade equips the LHC with betterprotected magnets, more sensitive detectors, and other improvements. For the schedule, themachine will start running experiments again in April 2022.
In Run-2 [7], the LHC successfully collected the collision data with a total integrated
luminosity of 139 fb−1 for physics analysis. After the 3-years LS2 upgrade, the peak
instantaneous luminosity is expected to be the value of 3×1034 cm−2 s−1 , and the c.m. energy√
will be raised from s = 13 TeV to 13.6 TeV. Under such a high luminosity environment,
several proton-proton interactions per bunch crossing occur. About 23 interactions are
typically expected in case of the luminosity of 1034 cm−2 s−1 . It’s rare for more than oneof these interactions to be a "hard scatter" for the target physics, for example, Higgs boson
production. Most of the other interactions are so-called multi-jet productions with low pT(e.g., gluon gluon → gluon gluon). This is referred to as "pile-up".

2.2

ATLAS Experiment"A Toroidal LHC ApparatusS" (ALTAS) is one of the detector for the LHC at CERN. It
investigates a wide range of physics, from the search for new physics beyond the SM to
the precision measurements of SM parameters, including the Higgs boson. The ATLAS
detector has a forward-backward symmetry around the interaction point with a diameter of
25 m, a length of 44 m, and a weight of 7000 tons. It consists of an inner detector (ID), an
electromagnetic and a hadronic calorimeter, and a muon spectrometer (MS). The schematic
view of the ATLAS detector is shown in Figure 2.2. The nominal interaction point defines
the origin of the coordinate system. The beam direction defines the z-axis, the positive
x-axis goes from the interaction point to the center of the LHC, and the positive y-axis points
towards the surface. Therefore the x-y plane is transverse to the beam direction. Additionally,on this plane, the coordinates (r, φ ) are used, where r is the radial distance from the interaction
point and φ is the azimuthal angle around the beam axis. Figure 2.3 shows the illustrationof the coordinate system. Generally, the positive direction of the z-axis is A-side, and thenegative direction is called C-side.

2.2 ATLAS Experiment

7

Fig. 2.1 Overall view of the LHC. There are four main detectors: ATLAS, CMS, ALICE and
LHCb [6].

8

LHC-ATLAS ExperimentFig. 2.2 Schematic view of the ATLAS experiment and its main components [8]The pseudorapidity η for particles coming from the primary vertex is defined as:θη = − ln(tan( )).
2

(2.3)

where θ is the polar angle. Therefore η is 0 for the x-y plane vertex and infinite for the beam
direction. Figure 2.4 shows how it is changed by θ . And this is the approximation of therapidity y in the limit of ultrarelativistic particles (which set the invariant mass to 0):y=

1 E + pzln(),
2 E − pz

(2.4)

where pz is the projection of the particle momentum along the z-axis. Differences in rapidity
are invariant under Lorentz transformations and the density of emitted particles dN
dy as afunction of the rapidity is almost constant.
To help the reconstruction of the collision events, quantities such as transverse momentum
pT = p sin θ and transverse energy ET = E sin θ is defined. Also, to measure the angulardistance of two objects, a quantity ∆R is defined in the transverse plane:q∆R = (∆η)2 + (∆φ )2 .

(2.5)

2.2 ATLAS Experiment

9

Fig. 2.3 The ATLAS coordinate system [9].

2.2.1Inner Detector

The Inner Detector is designed for tracking the primary and secondary vertex, which isabove a given pT threshold within the pseudorapidity |η| < 2.5. Also it provides theelectron identification over |η| < 2.0. Thus the high luminosity demands the ID to have finegranularity. To meet such requirement, materials used for ID is quite limited because of theissue of multiple scattering.
The ID is immersed in a 2 T magnetic field generated by the central solenoid and containedin the cylindrical envelope of length 3.5 m and diameter 1.15 m. Three sub-detectors are
utilized to meet the physics requirement. Figure 2.5 shows the alignment of those subdetectors. The innermost component is the Pixel Detector, followed by the SemiConductor
Tracker (SCT) and the Transition Radiation Tracker (TRT). The Pixel and SCT cover theregion |η| < 2.5, while the TRT takes the responsibility of the track reconstruction up to|η| = 2.0.
Pixel detector The pixel detector provides the charged particle track recognition within
the range of |η| <2.5. It is not only responsible for the track reconstruction and vertices
identification but also measures the track impact parameter, which is defined as the minimum
distance between the track and the primary vertex.
SemiConductor Tracker Four layers of silicon microstrip detectors are placed in concentric cylinders around the beam axis. The end-cap modules are arranged onto nine wheels,
each of which has up to three rings of modules.
Transition Radiation Tracker
The TRT system is a straw tracker which provides useful
information for electron identification by means of measuring the transition radiation [11].
Each straw tube has a diameter of 4 mm and was filled with a gas mixture of 70% Ar, 27%

10

LHC-ATLAS ExperimentFig. 2.4 Pseudorapidity η
CO2 and 3% O2 in Run-2 [12]. The passing charged particles will give rise to the ionization
inside the straws, and the resulting free electrons migrate towards a gold-tungsten wire of 31µm in diameter placed at the center of the tube at the voltage of −1.5 kV.

2.2.2

Calorimeter

The sampling calorimeters are adopted with full φ -symmetry at the ALTAS experiment.
Figure 2.6 shows the overview of the calorimeters. The electromagnetic (EM) is mainly forelectron and photon detection, and the hadronic calorimeter is for hardon detection. These
calorimeters cover the range |η| < 4.9. The fine granularity of these calorimeters satisfies
the physics requirements for jet reconstruction and ETmiss measurements.

LAr electromagnetic calorimeter
The EM calorimeter is designed to trigger on and provide precise measurement of electrons,
photons, jets, and missing transverse energy ETmiss . It uses liquid argon as active material and
1.8 mm lead plates as the absorber. The liquid argon has been chosen for its intrinsic linear
behavior, its response stability, and radiation resistance. The lead absorber induces the EM
shower when an electron or a photon goes through the detector. The shower and the ionizedparticles are therefore measured and used for calculating the energy of the incident particle.
The calorimeter consists of a barrel and two end-caps components, each housed in its own
cryostat. The barrel region cover the range of 0 < |η| < 1.475, and the end-cap region covers

11

2.2 ATLAS ExperimentFig. 2.5 3D vision of the barrel of the Inner Detector [10].
the range of 1.375 < |η| < 3.2. The LAr calorimeter has a four-layer structure (called layer
0, 1, 2, 3), and an accordion geometry is chosen for the absorbers and the electrodes of thecalorimeter. Figure 2.7 shows the three-layer structure (only layer 1, 2, 3) and the accordion
structure with the example of the trigger tower. The front layer (layer 1) is finely segmented
in η to measure the start point of the shower. The middle layer (layer 2) collects the largest
fraction of the shower energy, and the back layer (layer 3) estimates the amount of energy
leaking beyond the middle layer. Additionally, the presampler (layer 0) is implemented in
the range of 0 < |η| < 1.8 to provide a measurement of the energy lost in front of the EM
calorimeters. The resolution of the EM calorimeter can be parameterized with the following
expression [14]:
a
b
σ (E)
=p
⊕
⊕c
E
E(GeV ) E(GeV )

(2.6)

12

LHC-ATLAS ExperimentFig. 2.6 Cut-away view of the calorimeter [13].
where a takes account the stochastic behaviour of the shower development, b accounts forthe electronic and pile-up noise, and c is a constant that quantifies the non-uniform responseof the calorimeter, the aging, and the radiation damage. The pile-up noise means energy
deposits by the pile-up events (low pT multi-jet productions).The intrinsic sampling term a is√typically 10-11% GeV, the noise term b is expected to be 30 MeV-3 GeV in Run-3 pile-up
conditions, and the constant term c is 1-2%.

Hadronic Calorimeter
The hadronic calorimeter consists of the tile calorimeter, the LAr hadronic end-cap calorimeter (HEC), and the LAr forward calorimeter (FCal) [8].
Tile Calorimeter The Tile calorimeter is responsible for the energy reconstruction of thejets and measurement of ETmiss . It covers the range of |η| < 1.7. The central barrel locates in
the region of 0 < |η| < 1.0, and the two extended barrels are in the region of 1 < |η| < 1.7.
Special modules made of steel scintillator sandwiches fill the transition region. Figure 2.8
shows the schematic of the module. Each barrel consists of 64 such modules. The steel gridforms the 1.5mm module to module gap at inner radius and provides the volume for readout
electronic, as well as the flux return for the solenoid field.

2.2 ATLAS Experiment

13

Fig. 2.7 Accordion structure of the calorimeters. Figure also shows the granularity of different
types of layer.
Hadronic End-cap Calorimeter
HEC is a LAr sampling calorimeter with flat-plate
designed copper as the absorber, integrated with the end-cap cryostat and covering the rangeof 1.5 < |η| < 3.2. It consists of a front wheel (HEC1) and a rear wheel (HEC2), and eachwheel is constructed of 32 identical wedge-shaped modules. Figure 2.9 shows the schematic
view of an HEC module. The front wheel consists of 24 copper plates, and meanwhile, therear wheel has 16 copper plates.
Forward Calorimeter
FCal is also integrated into the end-cap cryostat and cover theregion 3.2 < |η| < 4.9. Therefore, the FCal is exposed to high fluxes. It is designed with LAr
gaps to avoid ion build-up problems and provide the highest possible density. FCal provides
a good measurement of the forward jet and helps in reducing the background level in themuon spectrometer. Three modules are performed in FCal. One EM module (FCal1) withcopper absorber to optimize the resolution and the heat removal, and the other two hadronic
modules (FCal2, FCal3) with tungsten absorber to provide containment and minimize thespread of hadronic showers. Figure 2.10 shows the location of the modules.

14

LHC-ATLAS ExperimentFig. 2.8 Geometry of the tile calorimeter module [8].

2.2.3

Muon Spectrometer

The Muon Spectrometer (MS) is designed to provide standalone precise measurements ofmuons up to 3 TeV and in the range of |η| < 2.7 [15]. It forms the outer part of the ATLAS
detector. The capability of reconstructing high-pT up without ID provides the fast muon
triggering in the region |η| < 2.4.
The layout of the spectrometer, as shown in Figure 2.11, is based on the magnetic
deflection of muon tracks inside the detector. Monitored Drift Tube chambers (MDT)and Cathode-Strip Chamber (CSC) provide the precise measurement of the momentum ofmuon track. The trigger system consists of Resistive Plate Chambers (RPC) and Thin Gap
Chambers (TGC) and covers the range |η| < 2.4. The trigger system serves to provide bunchcrossing identification (BCID) with efficiency larger than 99%, well-defined pT thresholds,
and measurement of the unbending direction of the track.
Monitored Drift Tube The barrel MDTs cover the region |η| < 1.05, as well as end-cap
MDTs cover the range of 1.05 < |η| < 2.7. Their size increases with the distance from the2.2 ATLAS Experiment15

Fig. 2.9 Schematic R − φ (left) and R − z (right) views of HEC. Dimensions are in mm [8].
interaction point. They are made of 3 to 8 layers of drift tube filled with a mixture of 93% Ar
and 7% CO2 . The average resolution for a single tube is 80 µm and for a chamber is 35 µm.
Cathode Strip Chamber
The CSC is responsible for the region 2.0 < |η| < 2.7 toimprove the resolution under the high hit rate near the interaction point. Similar to end-cap
MDT, CSC consists of 8 large sectors and 8 small sectors. Trigger time for CSC is less than
30 ns, and resolution is about 60 µm.
Resistive Plate Chamber
The RPCs are the trigger chamber which covers the barrel(|η| < 1.05) region. They are made of parallel resistive plates and a gas-filled gap of 2 µm.
The time resolution for RPC is about 1.5 ns, and position resolution is about 10mm.
Thin Gap Chamber The TGCs are responsible for the end-cap region (1.05 < |η| < 2.4).
Time resolution for TGC is about 4 ns, and position resolution is 2-6 mm.

2.2.4

Data Acquisition and Trigger System

The ATLAS system provides 1.7 billion pp collisions per second for a combined data volumeof 60 million MB per second. Bunches of protons cross 40 million per second (40 MHz),and each bunch contains 1011 protons. At a design beam luminosity of 1034 cm−2 s−1 , there
is about 20 collisions per bunch crossing. Therefore only interesting events will be selectedby a complex trigger system. The system consists of the Level-1 (L1) trigger and the HighLevel Trigger (HLT) to reduce the data flow from 40 MHz to about 1 kHz. When the bunches

16

LHC-ATLAS ExperimentFig. 2.10 The figure show the three FCal modules location in the end-cap cryostat [8].
collide, such a collision produces an "event", and the raw data recorded per event after the
HLT is 1.6 MB, while the physics data is recorded with the 0.1 MB per event [16].

Level-1 trigger
The L1 trigger uses reduced-granularity information from MS and the calorimeters. It
can accept a maximum incoming event rate of 40 MHz and uses hardware processors tounambiguously identify the bunch crossing of interesting events in 2.5 µs since the collisionoccurs. The geometrical location of the trigger objects will then be sent to the next level
trigger with the form of Regions-of-Interest (ROI) at the event rate of 100 kHz. Figure 2.12
shows the workflow of the L1Calo system in Run-3.

High Level Trigger
The HLT is performed by the event filter. It mainly depends on the commercially available
computers and networking hardware. It can handle a maximum rate of 75 to 100 kHz using
the full granularity of the ROI provided by the L1 trigger.

2.3

The LHC Upgrade Project

The phase-I upgrade project, which started after Run 2 ended in 2018, aims to cope with
the increase of luminosity in Run 3 and high-luminosity LHC. The new small wheel (NSW)2.3 The LHC Upgrade Project17

Fig. 2.11 Cut-away view of Muon Spectrometer [8].
is installed to provide much more stringent selection criteria for muons and to handle highbackground and pile-up rates. Meanwhile, for the trigger, the calorimeter introduces the new
readout system to increase the granularity of the readout cells and increase the sensitivity toincrease the ratio of S/N. Therefore, the performance of the L1 trigger system is significantly
improved after the phase-I upgrade. However, due to the coronavirus pandemic starting at
the end of 2019, the phase-I upgrade spent one more year than expected. After the upgradefrom 2019 to 2021, Run-3 will operate for three years and then start the phase-II upgrade
project. During the phase-II upgrade, the LHC will upgrade to the so-called High-Luminosity
LHC, and the luminosity will be increased up to 5 × 1034 cm−2 s−1 . Many systems in the
ALTAS detector will be modified to meet the challenge of the upgrade. For example, theinner detector will be rebuilt, and the muon detector will be modified. Both muon trigger
and electron trigger at the L1 trigger will have fast-tracking information matching tracks toregions of interest.
The new data acquisition system after the phase-I upgrade is shown in Figure 2.13. The
data from the ATLAS detector are first buffered in the Readout Drivers (ROD) and waiting
for judgment from the L1 trigger. The event rate is therefore reduced from 40 MHz to 100
kHz. The events accepted by the L1 trigger are then buffered in the Readout System (ROS)
and waiting for the signal from HLT. With the acceptance of the HLT, the data will be stored

18

LHC-ATLAS ExperimentFig. 2.12 Workflow of the L1Calo system in Run-3. Different colors of paths indicate thedifferent data output [17].
in the permanent storage disk for the physics analysis and exported to the Tier-0 facility at
CERN’s computing center. At last, the schedule of the LHC upgrade and run plan is shownin Figure 2.14.

2.3 The LHC Upgrade Project

19

Fig. 2.13 Work flow of the ALTAS DAQ and trigger system after the phase-I upgrade [18]Fig. 2.14 Schedule for the LHC upgrade plan [19].

Chapter 3
LAr Phase-I Upgrade Project
The Liquid Argon (LAr) phase-I upgrade project is to install the new trigger readout electronics during the LS2 of the LHC from 2018 to 2021. The new readout and trigger systemaims to provide better energy resolution and shower shape information to the Level-1 trigger
system, which is introduced in Section 2.2.4. The better trigger performance will enhancethe discrimination against background events, for example, high pT multi-jet productions
where jets are misidentified as electrons or photons. As a result, this kind of improvement
will increase the S/N ratio and help the analysis for the newly discovered Higgs boson andthe discovery of new particles, which indicate the physics beyond the Standard Model.

3.1

Signal Detection

The EM calorimeters consist of the accordion-shaped copper-kapton electrodes positioned
between lead absorber plates and kept in position by honeycomb spacers while the system isimmersed in LAr. Figure 3.1 gives an example of EMB (EM barrel). The accordion geometry
is to provide the full coverage of the absorbers and active the material in the path of particles.
As long as the energy of the coming photons and electrons meet the threshold, the particles
will be absorbed by the lead layer and give rise to the Bremsstrahlung radiation and the pair
production. This process is repeated until the energy is not enough to do so. Figure 3.2
gives a view of the tracks from the interaction point to the outward Muon Spectrometer,where electrons and photons are stopped at the EM calorimeters. The electrons and positrons
produced by the EM shower ionize the LAr, and the electrons will be attracted to the readout
electrodes by the electric field provided by about 2 kV HV.
The drift time of the electrons depends on the LAr gap width and the drift velocity,and the number of electrons determines the amplitude of the signal. Therefore, the drifted
electrons form the triangular pulse, which is shown in Figure 3.3. After that, the signals are

22

LAr Phase-i Upgrade Project

then converted to the bipolar-shaped pulse obtained by the CR-(RC)2 circuit and sampled at
the LHC bunch crossing frequency of 40 MHz. The integration of the bipolar pulse is 0, andthe amplitude is proportional to the triangular one. With these properties, the energy of theparticle can be therefore calculated with the optimal filtering algorithm (see Chapter 5), anddue to the properties of the waveform, noise can be canceled to some extent.

Fig. 3.1 Accordion structure of the barrel region [20]. The top figure shows the transverseview of the LHC beams. Particles are from left to right.

3.2

New Readout and Trigger System

The LAr readout system consists of front-end electronics (on-detector) and back-end electronics (off-detector). Front-end electronics are mounted directly on the LAr cryostats, andback-end electronics are wired in the USA-15 counting room.

3.2.1

Super Cell

Over 182000 calorimeter cells are equipped as the readout electronics to record energies
in a range from 50 MeV to 3 TeV with the energy resolution expressed in Eq. 2.6. These3.2 New Readout and Trigger System

23

Fig. 3.2 Schematic diagram of the tracks from the splashing particles in ATLAS [21].
elementary cells are also called LAr cells. In Run-1 and Run-2, the energy deposited in the
LAr cells is summed in an area of ∆η × ∆φ = 0.1 × 0.1 as the calorimeter trigger information,
which is called "Trigger Tower." In order to improve the energy resolution, a new scheme
called "Super Cell (SC)" with 10 times finer granularity is introduced in the phase-I upgrade.
Figure 3.4 compares the energy deposit of an electron in the legacy "Trigger Tower" andthe new readout system with "Supercell", and their granularity in the EMB calorimeter are
summarized in Table 3.1.

3.2.2

Front-End Electronics

The readout electronics are shown in Figure 3.5. The front-end system consists of 58
front-end crates (FECs), each hosting two baseplanes that house the following boards.

24

LAr Phase-i Upgrade Project

Fig. 3.3 The triangular pulse from the LAr calorimeter, overlaid with the bipolar-shaped andsampled pulse [22].
Table 3.1 Granularity of the trigger tower and super cell in the EMB calorimeter. nη and nφ
indicate the number of grouped LAr cells in η and φ direction, respectively [22].
layer
0 presampler
1
front
2
middle3back

Elementary Cell

trigger tower∆η × ∆φnη × nφ0.025 × 0.1
0.003125 × 0.1
0.025 × 0.025
0.05 × 0.025

4×1
32 × 1
4×4
2×4

Super Cell∆η × ∆φnη × nφ∆η × ∆φ

0.1 × 0.1

4×1
8×1
1×4
2×4

0.1 × 0.1
0.025 × 0.1
0.025 × 0.1
0.1 × 0.1

Legacy electronics
Front-End Boards (FEB)
Each FEB is a large 10-layer printed PCB that processes
the signals from 128 channels in a specific layer of the calorimeter. They amplify, shape,sample, and digitize the ionization signals and transmit them to the back-end processors.
The digitization is done by the Analog-to-Digital Converters (ADC) hosted on the FEB. The
linear mixers and the Layer Sum Boards (LSB) generate the analog sum signals, which areutilized by the L1 trigger system.
Tower Builder Boards (TBB)
TBBs form the legacy trigger tower signals from thesummed analog signal provided by the LSBs. The trigger tower signals are then transmitted
to the L1 trigger system via analog cables. In the HEC and FCal, Tower Driver Boards areused instead of the TBBs.

25

3.2 New Readout and Trigger System(a) Trigger Towers(b) Super Cell

Fig. 3.4 Schematic view of legacy and new readout cells in case an electron with 70 GeV ET
is injected. Figures show how the electron is seen by the 16 trigger towers (a) and by the 160
super cells (b) [22].
Calibration Boards The calibration boards are an essential part of the commissioning
work during LS2. They control the injecting calibration pulse, which is generated by the
onboard circuit. The calibration pulse is fine-tuned to be close to the bipolar pulse generated
by the LAr ionization signal. However, the calibration pulse is still slightly different from thephysical pulse. Therefore it’s critical to treat the energy calculation carefully with the twodifferent kinds of signals.
Controller Boards
The controller boards receive the configuration and monitoring commands from the SPAC (Serial Protocol for Atlas Calorimeter) masters and the TTC (Timing,
Triggering and Control: timing and control for all the ATLAS detectors) system and distributethe commands to all the other legacy boards in the FEC.

New electronics
Baseplanes
Each FEC is divided into two halves with a baseplane installed in each half,
which is shown in Figure 3.6. The new EMB baseplane has the same slots as the original
baseplane. Space for the new LTDBs comes from the unallocated slot at one end of the FEC.
The EMEC crates house two kinds of baseplane. The EMEC standard crate is similar to the
EMB, and the EMEC special crate has one baseplane for EMEC and another for the HEC tofree the tight slots of the HEC baseplane. The FCal baseplane has two LTDBs for each. In
total, 124 LTDBs are installed on the 114 new baseplane.
Layer Sum Boards The LSB is a plug-in card for the FEB that performs the second levelof summing of the analog signals, while the first level is done by the linear mixer. The new
LSBs change their outputs and provide the signals for supercells in the front and middle
layers and the legacy trigger tower.

26

LAr Phase-i Upgrade Project

Fig. 3.5 Schematic diagram of the LAr EM calorimeter electronic architecture in LHC
Run-3 [14]. The LTDB boards are newly installed during LS2. HEC and FCal electronics
are sightly different.
LAr Trigger Digitizer Boards
To provide the high-granularity and high-quality information to the upgraded trigger processors FEXs (Feature EXtractors), a new kind of on-detector
electronics called LAr Trigger Digitizer Board (LTDB) is installed in the spare slots of the
FECs. Figure 3.7 shows the detail of the LTDB boards. An LTDB board is connected toat most 320 supercell signals ("channels") and transmits the 12-bit flash ADC signals tothe back-end electronics by optical fibers. The total data transfer speed is 21 Tbps. The
124 LTDBs are equipped with about 12k ADC chips (80 chips used per LTDB, the rest arespared). Meanwhile, the LTDBs also send the summed analog legacy signal to the TBBs,
which are used for building the trigger tower. This feature is required to maintain the legacy
trigger system as fully operational to accomplish the transition to the new trigger system in
Run-3. There is one LTDB per baseplane in each EMB, HEC, and EMEC standard crate,
but two for the EMEC special and FCal crates. Therefore the number of LTDBs is different

3.2 New Readout and Trigger System

27

Fig. 3.6 Perspective drawing of a FEC [14]. It is divided into two HFECs (Half FECs) withbaseplanes mounted on the pedestal inside the crate.
for each region. Also, LTDBs are named to be "a single character" + "two-digit numbers" +"one or two characters" with the following rules [23]:• EMB: This type of LTDBs is used for the EMB readout. LTDBs’ names starting with I
are for the EMBA (EMB on the A-side) and H for the EMBC (EMB on the C-side),and the ones ending with L are for left HFECs and R for right HFECs. Examples:
I01L, H02R.
• EMEC Std: This type of LTDBs is used for the EMEC readout. LTDBs’ names startingwith A are for EMECA and C for EMECC, and the ones ending with L are for left
HFECs and the R for right HFECs. Examples: A01L, C02R.
• EMEC Spec 0: The EMEC region that overlaps with the EMB (1.4 < |η| < 1.6) isread by this type of LTDB. LTDBs’ names starting with A are for EMECA and C for
EMECC, and the ones ending with S0 are for right HFECs and the S1 for left HFECs.
Examples: A02S0, C06S1.
• EMEC Spec 1: The EMEC inner wheel and the last trigger tower of the outer wheel(2.4 < |η| < 3.2) are read by this type of LTDB. LTDBs’ names starting with A are
for EMECA and C for EMECC, and the ones ending with S0 are for right HFECs andthe S1 for left HFECs.
• HEC: This type of LTDBs is used for the HEC readout. The names starting with A arefor HECA and C for HECC, and they are all ending with H. Examples: A02H, C06H.

28

LAr Phase-i Upgrade Project• FCAL 0: This type of LTDBs reads out the electromagnetic FCAL. The names startingwith A are for FCALA and C for FCALC, and the ones ending with F0 are for the right
HFEC and the F1 for the left HFEC. Examples: A04F0, C04F0.
• FCAL 1: This type of LTDBs reads out the hadronic FCAL. The names starting with
A are for FCALA and C for FCALC, and the ones ending with F0 are for the right
HFEC and the F1 for the left HFEC. Examples: A04F1, C04F1.

Fig. 3.7 Schematic figure of the LTDB board.
Table 3.2 shows LTDBs set with different types and the detail of connected channels foreach type.
Table 3.2 The numbers of channels in the different region of LTDBs and the number of
LTDBs for each LTDB type.
LTDB Type

Channels

LTDB per region

EMB
EMEC Std
EMEC Spec 0
EMEC Spec 1
HEC
FCal 0
FCal 1

290
312
240
160
192
192
192

64
32
8
8
8
2
2

Total

34048

124

Optical transmitters Each LTDB is set up with 40 fibers for the data links and 10 for thecontrol links. 5848 fibers in total are equipped by the 124 LTDBs. Figure 3.8 shows the data3.2 New Readout and Trigger System

29

Fig. 3.8 Schematic figure of LTDB optical links [14].
and control links of the LTDB. The new Link-on-Chip (LOC) serializer ASIC named LOCx2
on the LTDB board is used to transmit the high-speed serial data, which are the digital signals
from up to 320 ADC channels per LTDB in the format of serial bitstreams [24]. Each LOCx2
can process the data from four ADC chips and transmit them to the back-end via two 5.12Gbps optical links. The GBT serializer-deserializer ASIC GBTx is used for the configuration,
control, and monitoring through five bi-directional control links. The Miniature Transmitter(MTx) and Miniature Transceiver (MTRx) are the optical transmitter and transceiver to fit
between the LTDB PCB and the cooling plate [25].
The ATLAS L1 trigger system requires the transmission latency to be fixed and meet
the time budge of the ATLAS trigger system. Thus LOCx2 are demanded with a maximum
latency of 75 ns [22]. In the commissioning works, the LOCx2 scan is performed to lock thefiber latency and make the data acquisition valid in the digital trigger readout. Otherwise, theADC values in the monitoring data outputted by LATOME will be set to −1 for the fibers
with a bad link.
As a result, the above optical transmitters demand the connectivity check by the test
that involves the entire chain of the LAr trigger and the data acquisition electronics on bothback-end and the front-end, and the detail of such a test will be discussed in Chapter 4.

3.2.3

Back-End Electronics

Legacy electronics
Readout Crates (ROC)
ROCs house the readout driver boards that read the ADC signals
from the FEBs and compute energy and time for each cell signal at the L1 trigger rate. SPAC

30

LAr Phase-i Upgrade Project

Fig. 3.9 Schematic of the CTP and TTC system. The system distributes the timing signals to
all the other system in ATLAS [22]master boards and the Trigger Busy Modules (TBM) are also housed by the ROC. TBM
distributes the TTC signals and receives the busy signals from the RODs.
TTC crates
The ATLAS Timing, Trigger, and Control system (TTC) are used to control
the subsystem like the new LTDBs and the LDPBs. The VME crates receive the TTC
commands from the ATLAS Central Trigger Processor (CTP) system or generate thesecommands and transmit them to the other boards. Figure 3.9 shows the working flow of thetiming control system in ATLAS.

New electronics
LAr Digital Processing System
The new LAr Digital Processing System (LDPS), as shown in Figure 3.10 is installed on
the back-end during the phase-I upgrade project. The LDPS is designed to receive thedigital signals from the LTDBs on the front-end, calculate the transverse energy for eachsupercell and send the results to the L1 calorimeter trigger system (L1Calo) with the LHC
clock frequency of 40 MHz. The whole system consists of LDPB, FELIX, TTC partition,

3.2 New Readout and Trigger System

31

Fig. 3.10 Schematic diagram of the LDPS [22].
Partition Master, PC farm, and Shelf Manager. The detail of the system will be discussed in
Section 4.1.
The new LAr Trigger Digitizer Boards (LDPB) is the most important part of all thephysical elements. The ADC signals of all the 34k supercells are sent from LTDBs to LDPBs
and processed there. The LDPBs compute the supercells’ ET and transmit them to the L1Calo,
which consists of a set of Feature Extractors (FEX). Three Advanced Telecommunications
Computing Architecture (ATCA) shelves are utilized to house the LDPBs and their following
subsystems. Each shelf houses 12 LDPBs and two switchblades.
LAr carrier
The LAr carrier (LArC) is an ATCA standard cut-out blade with a Rear
Transition Module (RTM) to host LATOME boards and transmit data with the ATLAS
Trigger and Data Acquisition (TDAQ) system. The LArC utilizes the ALTAS standard IPMC
to provide all board power and sensor management functions. The RTM is used to connect 5
TDAQ channels via SFP+ modules, and all TDAQ connections are routed through the Xilinx
Virtex-7 FPGA. In total, 30 LArCs corresponding to 30 LDPBs are installed to support
116 LATOMEs, where 28 LArCs host 4 LATOME boards, and two are installed with 4
LATOMEs that are used for receiving signals from FCal.
LAr trigger processing mezzanine The LAr Trigger prOcessing MEzzaine (LATOME)
boards read data from 48 optical fibers, processes the data, and send the results to L1Calowith the 48 output optical fibers. The board fulfills the Advanced Mezzanine Card (AMC)
to provide connectivity. Each LATOME board is connected to input fibers coming from up
to 3 different LTDBs. On the board, the Intel Arria 10 FPGA is equipped to support thehigh-speed computation of energy. Figure 3.11 shows an example of the LATOME. The
black front connector gathers the 48 output fibers together to transmit new FEX system data
at 11.2 Gbps, and the green ones receive data from LTDBs at 5.12 Gbps with 48 input fibers.

32

LAr Phase-i Upgrade Project

Fig. 3.11 LATOME board equipped with optical fibers.
Thus, 5568 input fibers are equipped on the LATOMEs to read out data from the LTDBs.
Firmware on the LATOME boards performs the following four main functions.
• handles the input data from the LTDBs with high-speed links.
supercell• performs the OF (optimal filtering, see later) algorithm to reconstruct ETevery
25 ns and identifies its Bunch Crossing, which corresponds to the timing when its
energy is injected. The latter is called "bunch crossing identification" (BCID).
• outputs results to the L1Calo with high-speed links.
• processes and buffers data to be delivered to the TDAQ readout chain and the monitoring.
LATOME firmware
To accomplish the above requirements, several FPGA firmware
modules, as shown in Figure 3.12, have been developed. Input stage (Istage) block aligns
all 48 input fibers together. Con f igureable remapping (Remap) block reorders input data
supercellaccording to detector topology. User Code block reconstructs ET
and provides somequality bits to the out put summing (OSUM) block. OSUM block groups the data from user
code, calculate the sums over specific η − φ plane, and transmits data to FEX output fibers.
Note that the user code has been developed by the Utokyo group.
Intelligent platform management controller (IPMC) The IPMC supports the hardware
management system for ATCA shelves and LArCs, which provides the ability to manage thepower, cooling, and interconnect needs of intelligent devices.
Feature Extractor
The improved granularity from the supercells will give rise to better isolation, which leads tothe lower energy thresholds preservation. In order to make full use of the improved readout
features, the new feature extraction (FEX) modules are performed in the L1Calo system forthe purpose of object recognition. Three kinds of FEX modules are installed in the phase-I

33

3.2 New Readout and Trigger System

Fig. 3.12 Schematic diagram of LATOME firmware.
missing

upgrade: eFEX for electrons and photons, jFEX for jets and ET, and gFEX for jetsthat ∆R > 1.0. Specifically, the eFEX subsystem employs new cluster-finding algorithms
on the higher granularity data to produce more refined Trigger Objects (TOBs). The jFEX
subsystem uses jet-finding algorithms, and gFEX is designed to identify large-area jets using
detector data with a granularity of 0.2 × 0.2 in the η − φ plane. Figure 3.13 shows the trigger
efficiency comparison between the electron trigger used in Run-2 and the eFEX with different
thresholds using the Z → ee Monte Carlo simulation data. Distinct improvement can be seenfrom the plot. The threshold of 21 GeV is chosen to have the same rate as the Run-2 trigger.

3.2.4

Energy and Timing Reconstruction in Run-3

The reconstruction of energy and timing is done by both the main readout and the digitaltrigger readout at the LAr calorimeter. Both readout systems use the optimal filtering
algorithm for the calculation. The energy computation in the digital trigger readout isperformed on both the user code block shown in Figure 3.12 and the offline monitoring data
analysis.
Energy reconstruction
As shown in Figure 3.14, the amplitude of the bipolar pulse is proportional to the energy.
The proportionality coefficient is derived from the calibration run, and the amplitude of the

34

LAr Phase-i Upgrade Project

Fig. 3.13 Trigger efficiency comparison between the trigger implemented in Run-2 andRun-3 [26].
sampled signal is obtained from the sample points neighbored the peak. Whereas, if only
one sample point is used for the amplitude estimation, electric noise and the pile-up noise
may introduce the fluctuation of the reconstructed energy for about 200 MeV. Four sample
points are utilized to mitigate such fluctuation, and the energy, therefore, can be calculatedby the equation 3.1.

3

ET = ∑ ai (Si − p),

(3.1)

i=0

where ai is the coefficient for each sample point, Si is the ADC value of the sample point,
and the p represents the pedestal value. Notice that the constraint of minimizing the noise israised here to derive the coefficient ai with the method of Lagrange multipliers [27].
Timing reconstruction
The variable τ is introduced to measure the delay of the real signal from the ideal sample
timing. The reconstruction of τ is also done by the optimal filtering algorithm with thefollowing equation:

35

3.2 New Readout and Trigger System

Fig. 3.14 Energy reconstruction of the injected pulse with 50 GeV ET . The interval ofsamples is 25 ns, which is the same as the LHC bunch clock. The sample points used forenergy reconstruction are the ones with the blue circle. The red sample points represent
the corresponding reconstruction result, and the one with the green circle passes the τcriteria [27].

3ET τ = ∑ bi (Si − p),

(3.2)

i=0

where bi is the coefficient for each sample point, and is calibrated together with ai . The
reconstructed τ can be derived from Eq. 3.1 and 3.2. The coefficients ai and bi are called
OFC (optimal filtering coefficients) and details of their calibration are discussed in Chapter 5.
τ criteria
The ET and τ reconstructions are done by both LATOME and offline analysis. And for thereconstructed ET and τ in the LATOME, the computation is done every BC. To choose thecorrect phase among all the BC, a constrain on the timing is performed within the interval of
25 ns. This is called "τ criteria".
−8 ns < τ < 16 ns,
|τ| < 8 ns,

if ET ≥ 10 GeV
if ET < 10 GeV

(3.3)

36

LAr Phase-i Upgrade Project

The tight cut for low energy is to compress the influence from the pile-up and electronicnoise. The outputs of the optimal filter are all set to 0 if the condition is not met, and theonly-passed BC is selected by the above condition.

3.3

Installation Status

The installation of the front-end and back-end electronics needs lots of effort. The following
main steps have been proceeded to install front-end electronics, which was started in April
2019:
1. Extract boards on the FEC from the detector.
2. Exchange an old baseplane of the FEC with a new one, as well as the LSBs.
3. Reinstall legacy boards to the FEC.
4. Install the two LTDBs.
5. Cable the FEC.
6. Replace LVPS cooling hoses.
The installation status of the front-end electronics in December 2021 is shown in Figure 3.15. LTDBs are all installed.
On the other side, about the installation on the back-end, three ATCA shelves and LArCs
are already installed. Figure 3.16 shows all the three ATCA shelves and how the LArC isinserted into the ATCA shelf.
The LAr group, including me, has been proceeding with the commissioning work to test
the newly installed electronics. There was a pilot run in October 2021, and data was takenwith the new digital trigger system.
This thesis reports my contribution to the commissioning work: a connectivity check anda timing alignment in Chapter 4. In addition, this thesis gives results of the analysis for thedata taken by the new readout system in the pilot run in Chapter 5.

3.3 Installation Status

37

Fig. 3.15 Front-end installation status in December 2021 [28]. Installation of phase-I on- andoff-detector electronics are all complete.

Fig. 3.16 Current status of ATCA shelves. Three ATCA shelves have been installed in the
USA-15 counting room (left). A photo of LArC is also shown on the right.

Chapter 4
Connectivity Check and BCID
Calibration
The new digital trigger system will be replaced with the legacy one during the LHC Run-3
experiment so that the commissioning work for the digital trigger readout has to be performed
as much as possible and as fast as possible. As mentioned in Section 3.2.2, during the LS2commissioning period, the pulse signal is sent by the calibration board on the detector. The
calibration pulse sent by the calibration board is then detected by the LAr cells, and the signal
is transmitted to the three kinds of readout paths as shown in Figure 4.1: main readout path,legacy trigger path, new digital trigger path.
For the digital trigger readout, the data path in the front-end is: calibration board (ondetector) → FEB (shape analog signal to bipolar pulse) → LTDB (digitize the infectedbipolar pulse). The data path in back-end is shown in Figure 4.2. The LDPBs receive theinput signals from LTDBs and reconstruct the ET and τ parameters on the LATOME FPGA.
The information is sent to the trigger system, where the trigger decision of the level-1 ismade for each event. Once the level-1 accept (L1A) is issued, data is sent to the Front-End
LInk eXchange (FELIX) system and then to software ROD (SWROD), and finally pulled by
HLT and added to the ATLAS event. SWROD is a new feature introduced by the phase-I
upgrade to replace the combination of the ROD and the ROS. It has already been installed
and implemented to record data in the pilot run in October 2021.
In order to make sure that the new complex data processing system can handle over
34k supercells properly, many efforts on the commission of the digital trigger readout are
made. The commissioning works to which I have largely contributed during LS2 are listed
as follows:40

Connectivity Check and BCID Calibration

Fig. 4.1 Schematic diagrams of the readout system. Three kinds of paths are prepared. From
top to bottom, they are the main readout, legacy trigger tower, and the new digital trigger [22].
• Shaper SWitch (SSW) scan
The SSW scan is performed to validate the digitaltrigger readout connections and verify the supercell mapping. It also checks the pulse
for each supercell.
• Pulse-all runs and BCID calibration.
The pulse-all run is a pulse injection mode
controlled by the calibration board. All supercells will be pulsed with a certain DAC
value, which determines the ET of the calibration pulse. By utilizing signals from thepulse-all run, the stability of the pulse shape is checked, and several quality factors to
the expectation are compared. In this thesis specifically, to align the sampling timingof the pulse, the timing of the pulse (BCID) between the main readout and the digitaltrigger readout will be compared.
• Calibration runs
The calibration runs collect data for pedestal, ramp, and delay
runs and build a database to be used as reference. With the aligned timing and delay
runs, the OFC set with different phases are calculated, and the selection for the best
fitted OFC for ET and τ reconstruction is then made. The ramp runs include the signals
with different DAC values, which can be used for the calculation of the least significant

4.1 Shaper Switch Scan and Connectivity Check41

Fig. 4.2 Data flow of the digital trigger readout system in back-end [14].
bit (LSB). The reconstructed ET and τ can therefore be calculated with the calibrated
LSB and OFC.
• Adjustment with Pilot Run data
LHC pilot run that uses real beam is performedin October 2021. The LAr calorimeter also participated in the operation to test thedigital trigger readout, the main readout, and the legacy readout. The analysis of pilot
run data and the comparison of the digital trigger readout and the main readout is madein the following chapter.
This chapter mainly introduces the first two steps. OFC calibration with calibration runs
and energy reconstruction with Pilot Run data are discussed in Chapter 5.

4.1

Shaper Switch Scan and Connectivity CheckAs discussed in Section 3.2.2, many new electronics like baseplanes and LTDBs are installedin LS2. Therefore, it is checked if each readout path is established as expected after the
installation of the new front-end and back-end electronics. In order to check the supercell
connectivity and internal mapping from LTDB to LATOME, a low-level check on hardware
and the shaper switch scan (SSW) are performed. To apply an SSW scan, the calibration

42

Connectivity Check and BCID Calibration

board sends signals to all supercells, and then the shaper chips controlled by the shaper switch
determine which supercell to read. Figure 4.3 shows the functionality of the shaper chip in
the FEB, and figure 3.3 shows the signal output (bipolar) from the shaper chip. The sampling
positions are separated by 25 ns. By reading raw ADC data from the monitoring output of
the User Code in LATOME (see figure 3.12), pulse shape is checked. Also, according to theshaper switch configuration files, whether the channels are pulsed with the correct supercells
is checked.

Fig. 4.3 Block diagram of the FEB architecture, the shaper chip is controlled by the shaper
switch to form the input signals from LAr Cells [29].

4.1.1

Workflow of Connectivity Check

Specifically, there are mainly two steps to accomplish the whole check procedure. Each step
performs its functionality by changing the pattern of enabled supercells. Figure 4.4 shows
the schematic view of how the shaper switch works. The framework is done by following
procedures.
1. Figure 4.4a shows the first step of the SSW scan. The SSW on the FEB enables all
supercells and sends pulse to the LATOMEs on back-end. The raw ADC data that
includes all channels are read from the monitoring stream and then used for offline
analysis. This pattern of enabled supercells lasts several seconds.
2. Figure 4.4b shows the second step of the SSW scan. After the pulse-all runs, only
one supercell is enabled by the SSW. Therefore, by searching for the only pulsed
channels from the monitoring stream data, the mapping between individual supercell
and channel on LATOME can be specified.
3. After sending pulses to a supercell for 0.5 seconds with the frequency of 40 Hz, theSSW will disable all the supercells for another 0.5 seconds and then enable anothersupercell that hasn’t been pulsed before. Such a pattern lasts until the scan runs overalltarget supercells.

4.1 Shaper Switch Scan and Connectivity Check

43(a) Step 1: Send pulse to all supercells(b) Step 2: Send pulse individually.

Fig. 4.4 Schematic figures of SSW scan workflow.
As for the current configuration, the L1A rate is 40 Hz, which means every second
includes about 40 pulsing events. Every SSW scan is performed for supercells on a single
HFEC. The plot of pulsed events is shown in Figure 4.5. The marked points mean that thepulse signal of the channel is detected in the corresponding events. In Figure, the first 100
events are the pulse-all pattern, which is described in Figure 4.4b. And the discrete dots after
the pulse-all part show how the channels are pulsed with the shifting of SSW. Each LATOME
holds up to 320 channels that connect to supercells, respectively.

4.1.2

Pulse Detection Criteria

The calibration pulse is stored in an array with a length of 32, and the ADC values of thesamples are proportional to the energy. Figure 4.6a shows the legit pulse, and figure 4.6bshows an example of the deformed pulse. In order to exclude deformed pulses and noise, thefollowing criteria are applied to the barrel and end-cap pulse.
1. Peak detection
The first criterion is to check the peak. As shown in Figure 4.6a,
suppose the peak is at the jth sample, and the height of the pulse is defined as the44

Connectivity Check and BCID Calibration

Fig. 4.5 Pulse pattern of the SSW scan for a single LATOME. First is the pulse-all run andthen come with the solely pulsed channel. A dot represents for about 20 continuously pulsed
events.
difference between the peak sample and the first sample. The threshold of the pulse
height is 15 ADC counts. Additional comparison between the ( j − 2)th sample andthe ( j − 1)th sample, as well as the comparison between the ( j + 2)th sample and the( j + 1)th sample are made in order to require the crest structure.
2. Undershoot detection The undershoot structure of the bipolar pulse is also required.
Samples behind the trough (red rectangle in Figure 4.6a) are supposed to be less than( jth sample + 1st sample)/2.
3. Bipolar structure The bipolar structure is performed to check the rough shape of
the pulse. The depth is defined as the difference between the first sample and the nadir
sample. For the pulses from supercells on EMB and EMEC, there is no limitation on
their depth. Only the nadir of the FCal pulse is required to be 4 ADC counts smaller
than the 1st sample. Also, the crest of the signal needs to appear earlier than the trough.
Such requirements meet the assurance of avoiding misjudgment due to bit flipping.
4. Pedestal level
The final condition requires the pulse are all around the pedestal
level, which is calculated by the mean value of the first sample of the first 100 events.
FCal pulse The FCal are specially designed to detect in the high η region. Therefore thepulse shape from FCal is slightly different from the pulse from the barrel and end-cap region.
As shown in Figure 4.7, a sharper crest and larger depth are required to detect the FCal pulse.

45

4.1 Shaper Switch Scan and Connectivity Check(a) Normal Pulse shape and the terminology.

(b) An example of deformed pulse.

Fig. 4.6 Pulse detection algorithm for SSW scan. Pulse from FCal is sightly different.

Fig. 4.7 The example of the FCal pulse.

4.1.3

Scan Validation Algorithm

All channels are classified into several statuses due to the corresponding issues to identify
whether the channels are pulsed as expected in the SSW scan. Figure 4.8 shows the workflow
of the validation algorithm. The blue blocks indicate the different channel statuses. The
problems may be caused by different reasons. The analysis is followed by these conditions:
1. Scan Step 1 and Step 2 in Figure 4.4 and search for channels with −1 samples.
In case the fiber is not locked, the channels on that fiber will output the negativevalue (= −1). Such channels are called "negative channels". The CLKDES scan needs
to be performed to fix such problems.

46

Connectivity Check and BCID Calibration
2. Scan step-1 and step-2 in Figure 4.4 and perform the preselection. The preselection is
to exclude the channels that are not pulsed. The only criterion in this procedure is thatthe pulse height should larger than 15.
3. Scan step-1 and step-2 in Figure 4.4 and perform the exclusion of abnormal channels.
The program runs over the whole scan and counts the number of pulsed events foreach channel. According to the pattern of pulsing supercells in Figure 4.4b, the pulsed
events should not be more than 800 in total for each supercell. Channels that are
pulsed too many times are not expected, and therefore such channels are ignored in thefollowing analysis. Otherwise, the search for solely pulsed channels would be difficult.
This kind of issue is frequently met in the early SSW scan. However, after several
SSW, the system becomes more stable than before, and this issue barely happens again.
4. Scan step-1 in Figure 4.4 and check the pulse shape of all channels. Channels witha reasonable number of pulsed events are checked with the previous 4 criteria. The
pulses that do not pass the selection are recognized as those with deformed shapes andwill also be excluded from the following analysis. The issue may be caused by thewrong ADC configuration or other hardware problems.
5. Scan step-2 in Figure 4.4 and search the pulsed channel. Referring to the figure 4.4b,in step 2 of the scan, each channel should be only pulsed once within 0.5 s. Therefore
if the SSW does not work properly, the target channel may not be pulsed during step 2.
The missed channels are categorized as "only pulsed in pulse-all".
6. The crosstalk events are also checked in step-2. The crosstalk events include several
different pulsed channels, which are not expected. Thus, the "crosstalk channels" areexcluded from the following analysis. This kind of issue rarely occurs since the system
becomes more and more stable.
7. After excluding the above problematic channels, the pulsed channels are checked
for their pulsing duration. Although the expected pulse duration is 0.5 s, the SSW
sometimes delays the switch of the target supercell. In case too much delay is brought tothe scan or the SSW doesn’t work as expected, this selection mechanism is introduced.
The expected number of pulsed events for a good channel is around 20. Therefore if
the number of pulsed events for a particular channel is less than 5 or more than 40, thechannel will be categorized as a "bad channel".
8. The good channels are finally selected after the above steps. And the pulse order ofthe good channels is compared with the order in the list of target supercells. With the4.1 Shaper Switch Scan and Connectivity Check47

comparison between the offline analysis result and the origin configuration file, the listof swapped supercells and ones with correct mapping is therefore obtained.
Figure 4.9 gives a glance at the validation result of the SSW scan for LTDB C12S0. The
channel list is sorted by order of target supercells shown by the index. The "SCNAME,""Channelid," and "SC_ONL_ID" column in the list represents the identification of a supercell
or channel. The left part of the list indicates the pulse injection order of the target supercells.
The right side shows the observed pulsed channel sequence. The hit status means that theobserved pulsed channel is what is expected in the target list. The channels with the status"shifted" are the ones that are not pulsed in the expected order. The arrows show how thesechannels are misordered. What’s more, several problematic channels are also observed in thescan, the "negative channel" 1J_M1, 1J_M2, and the "no pulse channel" 2F_M2.

4.1.4

Validation Status and Auto-processing System

The pulse detection criteria are performed to check if the target channels are validated. Hence,
the result of the SSW scan, which categorizes the channels into several statuses, indicatesthe potential issues on the detector, LTDB, and LATOME. The problematic channels cantherefore be summarized into two types as follows:• Good but swapped channels The stable and good shape pulse signals are observed
from the channels of this type, which means the data acquisition is good as expectedon the chain. However, the mapping for the channel may not be correct. As shown in
Figure 3.5, the calibration chain may be performed with the wrong channel mapping.
In the front-end electronics, the test pulse signal is read out from the motherboards ingroups of 8 channels for the barrel. Then the signals are sent to the FEB and then sent
to LTDBs and the legacy trigger. The connectivity check of FEB is already done by
Run-2. Whereas on the LTDB, the numbering of ADC chips may be mistaken, as wellas the optical fiber mapping from the FEB to LTDB and from the ADC channels to the
LOCx2. What’s more, as the data path on the back-end has shown in Figure 3.10, themapping of optical fibers that transmit data from the LTDB to LDPS and the pinningof LATOME input are also may not correct. Even inside the cryostats, the swappedcabling of supercells is observed from the SSW scan, which is consistent with theobservation in the former report about the swapped channels [30].
• Problematic channels
The problematic channels are more likely as the by-productof the connectivity scan since the pulse shape detection is applied in the algorithm.
They are the channels with a status other than hit and swapped in Figure 4.8. Sometimes

48

Connectivity Check and BCID Calibration

Take SSW scan for a
LTDB

Raw ADC data

Is fiber locked?

Condition 1:

Target SCs list

No

NegativechannelsNo

Not pulsed
channels

No

Ignored channels

No

Deformed pulse
shape

No

Only pulsed inpulse-all

No

CrosstalkchannelsNo

Bad channels

Yes

Height > 15?

Condition 2:

Yes

Pulsing less than 800 times?

Condition 3:

Yes

Passing the pulse detection criteria?

Condition 4 :Yes

Is the channel pulsed in step2?

Condition 5:

Yes
Is this channel the only one
in the event that is pulsed?

Condition 6:

Yes
Does the signal last as
long as expected?

Condition 7:

Yes
Good channels list

Problematicchannels list

Does the pulse order match
to the one in the target list?

Condition 8:
No

Yes
Swappedchannels

Hit channels

Validation result

Fig. 4.8 Flow diagram of the pulse validation algorithm. The blue rounded rectangles indicate
all possible channels statuses.

4.1 Shaper Switch Scan and Connectivity Check49

Fig. 4.9 An example of the validation result for a single LTDB. Several different kinds ofchannel status are observed.
channels with −1 ADC values are observed. They are caused by the unlocked fiber on
LOCx2, which is discussed in the optical transmitter part in Section 3.2.2. Most ofthem can be fixed by the LOCx2 scan. And channels with deformed pulse shapes are
the ones having the wrong ADC configuration of the ADC chips on the LTDB. Mostof them can be fixed by the full ADC calibration.
The validation for all LTDBs and LATOMEs can be pretty heavy since the calibration run
and SSW scan are taken frequently. Therefore an online validation tool has been developed.
Figure 4.10 shows the interface of the tool. With this tool, the LAr calorimeter developers
can easily take calibration runs and SSW scans for any LTDB. And then analyze the data
automatically.
The SSW scans for all LTDBs are done at least once until 29/12/2021, and many problematic channels are reported. Figure 4.11 shows the validation status on 16/09/2021 and29/12/2021. Supercells in the regions with different colors are indicated by:• Blue region – Validated Channels. They are classified into the hit channels in the above
algorithm.

50

Connectivity Check and BCID Calibration

Fig. 4.10 Online validation tools.
• Green region – Channels swapped at somewhere. The issue could occur at both thefront-end and back-end, even in the detector. For now, all found swapped channels
have been fixed.
• Yellow region – Problematic channels. For example, deformed pulse, as shown in
Figure 4.6b, is mainly caused by the incorrect ADC configuration. And sometimes,
samples of certain channels are set to −1, which usually indicates a fiber link issue.
As shown in Figure 4.11a, several swapped supercells are observed from the SSW scanon September 16th. The mapping for these supercells needs to be updated and fixed. And as
shown in Figure 4.11b, all swapped channels and many problematic ones are already fixedin December 2021. Most problematic channels are caused by the unlocked fibers and theincorrect ADC configuration. Actually, the fibers need to be locked again every time after theaccidental power cut of the system. Such an accident may change the system latency sightly,
and the CLKDES scan should be applied to fix the issue. The unlocked fibers bring the dataacquisition problem, and the raw ADC data will be set to −1 in this case. These channels aremarked as the negative channels in the data analysis. Table 4.1 summarizes all the swappedchannels spotted by the SSW scan during the commissioning and their status on 29/12/2021.
According to the table, all swapped channels are fixed. 42 problematic channels in all are left
to be fixed, and 23 out of the 42 channels are listed. The vast majority of these channels havethe problem of setting the wrong ADC configuration. The naming rules of LTDBs can befound in Section 3.2.2.

51

4.1 Shaper Switch Scan and Connectivity Check

Table 4.1 Problematic channels found by the connectivity scan. All swapped channels andpart of problematic channels are listed. The third column gives the number of swappedand problematic channels that were found since the start of the commissioning. The lastcolumn gives the number of remaining swapped and problematic channels on 29/12/2021.
All swapped channels are fixed, and 23 out of 42 problematic channels are listed here.
LTDB typeLTDB# of Swapped/Problematic channels# of unfixed channels

EMBA
EMBA
EMBA
EMBC
EMBC
EMBC
EMECA
EMECC
EMECC
EMECC
EMECC
EMECC
EMECC
EMECC
EMECA
EMECA
EMECA
EMECA
EMECC
EMECC
EMECA
EMECA
EMECA
EMECA
EMECC
EMECC
EMECC
HECA
HECA
HECA
HECA
HECC
HECC
FCALA

I04L
I10R
I12R
H05R
H06R
H07L
A13R
C03R
C05R
C07L
C07R
C08L
C08R
C13R
A02S0
A06S0
A09S0
A12S0
C02S0
C12S0
A02S1
A06S1
A09S1
A12S1
C02S1
C06S1
C12S1
A02H
A06H
A09H
A12H
C02H
C12H
A04F0

2/1
2/0
2/0
262 / 20278 / 12286 / 4
6/0
0/2
0/1
312 / 00 / 200/1
16 / 016 / 0
6/2
6/0
6/0
6/1
6/0
30 / 17
2/0
2/0
2/0
2/0
8/0
6/0
24 / 0
6/1
6/0
6/8
6/0
6/0
24 / 0
6/0

0/1
0/0
0/0
0/0
0/0
0/0
0/0
0/2
0/1
0/0
0 / 13
0/1
0/0
0/0
0/2
0/0
0/0
0/1
0/0
0/1
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/1
0/0
0/0
0/0
0/0
0/0
0/0

Total

34

1348 / 90

0 / 23

(b) Status on 29/12/2021.

Fig. 4.11 LTDB validation status on 16/09/2021 and 29/12/2021. Supercells in the blue region are all validated. The yellow region
represents the problematic supercells, and swapped supercells are shown in green. Swapped supercells in Figure 4.11a and problematicchannels on FCal are fixed in Figure 4.11b.

(a) Status on 16/09/2021.

52
Connectivity Check and BCID Calibration

53

4.2 BCID Calibration

4.2

BCID Calibration

The bipolar pulse signals are sampled with the frequency of 40 MHz, which is the sameas the LHC BC frequency. Therefore the phase of the pulse is usually illustrated by the
BCID, and the phase shift means the variation of BCID value. Such a feature requires thatthe sampling phase of the digital trigger readout should coincide with the sampling phase of
the main readout. Otherwise, phase shifts greater than 25 ns can affect the computation ofreconstructed ET and τ. Figure 3.14 shows how the sampling points are chosen for the ET
computation. In order to have the proper coefficients used for the optimal filter algorithm, thecalibration runs are taken to calibrate the values, which is discussed in Chapter 5. However,the pulse sampling timing must be stabilized before a calibration run can be performed. For
this reason, the BCID calibration was performed immediately after the connectivity check
was completed.
Practically, the BCID of the digital trigger readout is adjusted according to the BCID of
the main readout. A change in BCID brings along a change in the latency of the system. The
latency of the system, which is the time it takes for data to go from one point to another, isdetermined by how the system is designed. Run-3 does not change the circuit of the main
readout from Run-2. Hence, the latency of the main readout has been fixed in Run-2 and will
be used for Run-3. By this feature, the BCID of the digital trigger readout is calibrated withreference to the main readout.

4.2.1

Timing Alignment by Calibrating the Peak Position

The BCID in LATOME should be the same as the one in the main readout to align thetiming between the digital pulse in the main readout and the digital trigger readout. As
shown in Figure 3.14, the peak sampling of the signal matches the third point used for the ET
reconstruction. Therefore, the BCID of the peak sampling is a good reference point for thecalibration. The shift of the pulse between the digital trigger and main readouts can then be
calculated by the following expression.

Offset = PeakBCIDLAT OME − BCIDmain − 2(4.1)

Here, the PeakBCIDLAT OME is the BCID of the peak sampling point in the digital trigger
readout (LATOME), for example, the third sampling point with a blue circle in Figure 3.14.
The BCIDmain is the BCID determined in the main readout, which corresponds to the timingof the signal injection, that is, the first sampling point with a blue circle in Figure 3.14.

54

Connectivity Check and BCID Calibration

Therefore, if the sampling phase is aligned between the digital trigger and the main readouts,
the "Offset" should be 0. Practically, it is difficult to align all channels to 0 offsets because
one fiber reads up to 8 channels so that the BCID can only be adjusted fiber by fiber. Hencethe BCID calibration is done by taking the mean shift of the supercells on each fiber.
Figure 4.12 shows how the BCID calibration for each fiber is performed with an example
supercell. The detailed procedure is given by the following clauses.
1. Take calibration pulse with the pulse-all run and check pulse shape with the digitaltrigger readout.
2. Get the peak position for each supercell. The peak BCID of the example pulse in
Figure 4.12 is 46.
3. Calculate the offset for each supercell by Eq. 4.1, where the BCID of the main readout
is stored in the calibration data. The offset of the example pulse in Figure 4.12 is 0
since the BCID of the main readout is 44. Average the offsets of the supercells foreach fiber.
4. Updated the configuration values, which are for the system latency. The new value isthe old one plus the averaged offset computed in step 3.

Fig. 4.12 Workflow of BCID calibration. The peak BCID in LATOME should be 2 less thanthe BCID in the main readout. A given example on the right side shows that the peak BCID
of LATOME readout is 46. Therefore, BCID of the main readout should be 44.
Basically, this procedure determines all the BCID calibration values. However, sometimes the supercells are not pulsed for some reason, just the same as what we discussed in
Section 4.1.4. In this case, the fiber is calibrated by referring to its neighbor fibers.

4.2 BCID Calibration

4.2.2

55

Instability of the Calibration Value

Besides the problematic channels, a part of good channels has issues in the BCID calibration
mentioned above. The offsets of supercells in some fibers vary from time to time. This
fluctuation is due to noise and the low sampling frequency.
As a matter of fact, the fluctuation of the peak position is observed on many fibers because
two or more sampling points have similar ADC values around the peak position, as shownin Figure 4.13. These 8 supercells are on the same fiber, and the peak position of thesesupercells can be easily changed due to the noise. This kind of instability brings difficulty to
the BCID calibration.

Fig. 4.13 Pulses with similar peak sampling points around the peak position. Letter "M" inthe names of the supercells indicates that the supercells are from the middle layer.
Figure 4.14 shows the calibration result of two similar runs taken on the same day. As
shown in Figure 4.14a, the offsets obtained from the two runs are filled with different colors.
Fibers with bad links or the ADC configuration problems are not included. Regardless of afew outliers (|offset > 2|), most of fibers are already aligned (offset = 0), but several hundredfibers are still shifted by ± 1 BC (±25 ns). Figure 4.14b shows the difference of the offsets
between the two runs, which is supposed to be 0. However, the calibrated BCID varies fromrun to run. In fact, 57 fibers have different BCID calibration values using the calibration
method in Section 4.2.1. Therefore the optimization of the algorithm is needed to stabilize
the calibration result.
Figure 4.15 shows how the RMS of the offset is distributed. Since there are non-zero
RMS values, it indicates that supercells on the same fibers have different offsets. Over 100
fibers have an RMS of 0.5 and the mean offset of ±0.5, which means the offsets of thesupercells on those fibers are half 0 half ±1, for example, offsets of supercells are like "0, 0,

56Connectivity Check and BCID Calibration(a) Offsets from two runs.

(b) Difference of the offsets between two runs.

Fig. 4.14 Comparison of two runs with the same BCID setting. The outliers in the left figure
belong to one LATOME that possesses with deformed calibration pulse. The number of
non-zero offset difference in the right figure is 57.
0, 0, 1, 1, 1, 1". As long as the peak position of some supercells varies due to the noise, theoffset for that fiber also changes.

Fig. 4.15 RMS distribution of the supercells of the offsets of the supercells on the same fiber.

4.2.3

Optimization for Mitigating the Fluctuation

In order to make the result stable, the following optimizations are made.
1. For fibers with mean offset = ±0.5 and RMS = 0.5, the signal delay is set to 0 insteadof ±25 ns. The former version takes the round of the mean offset so that the error of
the operation is larger than 1 BC, which is not acceptable for the calibration.
2. Introduce the method of averaging samples in order to decrease the random error.

57

4.2 BCID Calibration

3. As shown in Figure 4.13. In case the difference between the peak sample and the one
next to the peak is not large enough, the sample phase may change run by run due tothe noise fluctuation. Therefore the former sample is chosen as the peak sample while
the difference is less than height/10. Picking the former sample follows the instructionof minimizing the LATOME firmware latency.
Other optimization methods were attempted but not adopted. For example, the peaksample is given larger weights in many ways to enlarge the difference between peak andsub-peak. But no distinct improvement is observed with these methods.
The result of BCID calibration for the two runs after the above optimizations is shown in
Figure 4.16. The two runs were taken on December 17th with the same latency configuration.
As the application of the 3rd optimization, the number of the fibers shifted by −1 increases.
However, the number of fibers with different calibration values is reduced from 57 to 1.

(a) Offsets from two runs.

(b) Difference of the offsets between two runs.

Fig. 4.16 Comparison of two runs with the same latency after the optimization. These tworuns are more stable, and the problematic LATOME with the outliers in Figure 4.14 is fixed.
As a result, only one fiber is remained to have a different offset in the two runs.
After applying these optimizations, a new run with the stabilized calibrated BCID was
taken on December 29th. The result is shown in Figure 4.17. 4457 out of 4458 fibers are
involved in the figures except for one fiber on the LATOME named with EMECC_8, which
has the link issue. The remaining fibers on the LATOMEs are grounded as the spared ones.
The offsets are stabilized at 0, but several fibers have different BCID. This is because theADC value difference of the peak and sub-peak sample is fluctuating around height/10, whichis acceptable since the peak sampling point is stable within the difference of height/10.

58

Connectivity Check and BCID Calibration(a) Offsets from two runs.

(b) Difference of the offsets between two runs.

Fig. 4.17 Comparison of two runs with the updated BCID. BCID for all fibers is now
calibrated. One fiber has the different BCID calibration values in the runs.

Chapter 5
Calibration Run and Analysis for the
LHC Pilot Run Data
5.1

Test Pulse System and OFC Calibration

5.1.1

Optimal Filtering Coefficient

While the connectivity check and BCID calibration are performed, the efforts on calibrating
the optimal filtering coefficient (OFC) are also made in parallel. OFCs given by the Eq 3.1
and 3.2 can be written as the following expressions [27]:ai =FDAC→MeV · FADC→DAC sin θ × ãi ,bi =FBC→ns · FDAC→MeV · FADC→DAC sin θ × b̃i ,(5.1)

where FDAC→MeV = FDAC→µA · FµA→MeV is a conversion factor that converts DAC value tothe energy, whose value is determined by the two terms on the right side. Here DAC is thediscrete value set by the calibration board and is used for adjusting the input pulse height.
FDAC→µA is defined from the calibration board, and FµA→MeV is a factor that converts thecurrent send to the detector to the equivalent energy for the real EM shower. The conversion
factors for each supercell are derived from the test beam system [31]. FADC→DAC representsa scaling factor of DAC to ADC and is calibrated by the test pulse system. FBC→ns is givenby the time interval of BC, which is 25 ns for the LHC BC. sin θ is the conversion factor
from energy to the transverse energy. ãi and b̃i are the non-dimensional OFC used for the
peak ADC estimation and calibrated by the test pulse system.

60

Calibration Run and Analysis for the LHC Pilot Run Data

Notice that in the user code block of the LATOME firmware shown in Figure 3.12, theET candidate is stored in the buffer with 18 bits, where 1 bit is for the sign and 17 bits are forthe value. Therefore to cover the range of |ET | < 1.6 TeV, the least significant bit (LSB) is
set to 12.5 MeV [32].

5.1.2

Calibration Run

There are two kinds of runs types for the LAr calorimeter. One is the physics run, which
receives the signals of the EM shower from the collisions and then sends them to the FEC.
Another is the calibration run, whose signals are from the calibration boards on the FEC.
There is one calibration board on each HFEC.
The calibration board changes the signal amplitude, pulse timing, and target supercell as
run parameters for different kinds of runs [31]. The amplitude of the signal is controlled by
the 16 bit DAC values. It provides the voltage between 0 and 1 V by the step of 15.26 µV.
The input current is therefore generated from the DAC voltage through a voltage to current
converter, and the current is distributed to the readout channel on the detector motherboard.
The conversion factor of the amplitude of the current is calculated by the following equation:

FDAC→µA =76.296 µV,
Rinj

where Rinj is the injection resistor with the range from 0.5 to 3 kΩ.
Each board is equipped with delay chips to set the delay for the pulse injection timing,
which varies from 0 to 24.95 ns by steps of 1.04 ns and has 128 calibration lines to pulse
between 8 and 32 channels depending on the layer per line [33].
By utilizing these features of the calibration board, three kinds of calibration runs are
performed in the test pulse system: the pedestal run, ramp run, and delay run.
• Pedestal run Pedestal run is taken for obtaining the pedestal level of the supercells.
During the pedestal run, no pulse signal is sent to the supercells. The pedestal, noise,and the autocorrelation of noise are obtained [27].
• Delay run Delay run changes the timing of the injection pulse. Each cell is pulsed N
times (usually N = 100) with a certain DAC value at the delay between the calibration
pulser and the data acquisition system. It divides 1 BC (25 ns) into 24 phases, whichmeans the delay step is 1.04 ns. Hence, unlike the pulse sampled with 25 ns, thedelayed pulses compose a smooth pulse shape to derive the non-dimensional OFC ãi5.1 Test Pulse System and OFC Calibration

61

Fig. 5.1 Delay pulses with DAC values from 0 to 1500 for a supercell in the 2nd layer [31].
and b̃i with the pedestal run. Figure 5.1 shows the pulse shape after overlapping thepulse from the delay run.
• Ramp run
Similar to the delay run, the ramp run is pulsed about 100 times witha set of given DAC values, and the delay set in the ramp run is fixed to 0. With
the non-dimensional OFC ãi and b̃i derived from the delay run, the peak ADC value
is computed, and the curve of ADCpeak to DAC is obtained. The values below thesaturation level are used for the extraction of factor FADC→DAC .

5.1.3

OFC calibration

In Section 5.1.2, FADC→DAC in Eq. 5.1 is obtained from the calibration runs, as well as thenon-dimensional OFC ãi and b̃i . Thus, ET can be calculated from Eq. 5.1 since FDAC→MeV isalready determined from the test beam. Figure 5.2 shows the distribution of ET /ADC for
different η and φ for supercells layer by layer, and each point represents one supercell. This
ET /ADC corresponds to the LSB for each supercell.
Figure 5.3 shows the distribution of non-dimensional OFC ã1 , ã2 , ã3 , ã4 and b̃1 , b̃2 , b̃3 ,b̃4 for the main readout, that is, for LAr cells. Due to the 18 bits constraint, the absolutevalue of non-dimensional OFC are set to less than 70. The OFC of LAr cells is derived fromthe calibration run for the main readout. The dashed lines indicate the range of OFCs, and62

Calibration Run and Analysis for the LHC Pilot Run Data

Fig. 5.2 ET / ADC [MeV] distribution of supercells in all regions except for FCal. The values
are uniformly distributed in the φ direction and vary in the η direction.
none of which exceed the limitation of 70. The non-dimensional OFCs for the supercells arecalibrated in the same way.

(a) OFC ãi(b) OFC b̃iFig. 5.3 Distribution of non-dimensional OFC ãi and b̃i of LAr cells, where the vertical axis
represents the index of i. Dashed line shows the range of the OFCs, and the solid line shows
the mean value attached with the error bar [34].

5.2 Data Analysis with LHC Pilot Run Data

5.2

Data Analysis with LHC Pilot Run Data

5.2.1

LHC Pilot Run

63

The first pilot beams towards Run-3 were circulated in the LHC in October 2021. During thepilot run, the ATLAS LAr calorimeter was fully powered. The LAr digital trigger readout
participates in the data-taking. The corresponding energy reconstruction is also performed toprovide triggers for splashes and collisions with the L1Calo.
Beam 1 that goes from A to C side is triggered on the C side, and beam 2 that goes fromC to A side is triggered on the A side. The first interesting event from the beam splash isshown in Figure 5.4. The particle interactions in the EM and hadronic calorimeters andmuon detectors are displayed. This event is recorded from beam 2 at 15:52 on October
19 th (CEST). ALTAS recorded additional splashes later that evening, and by the requests
from the experiments, another two rounds of splashes were recorded. After the beam splashes
were performed, collisions with stable beams ran for about 40 hours with the center-of-massof 900 GeV.

Fig. 5.4 Cut-away view of the first interesting event from the LHC beam splash test. This
event is recorded at 15:52 on 19/10/2021 (CEST) from run 404473, beam2 [35].
Table 5.1 shows the information for the several runs taken in the pilot run and used foranalysis. The latency between the digital trigger readout and the main readout is not wellaligned since the calibration pulse latency is different from the physics pulse latency. The
adjustment for this timing difference is not applied for both readout paths at the beginningof the pilot run. Therefore the last several runs of splash and collision events are used forthe analysis. At the beginning of each collision run, the pixel detector needs to be calibrated

64

Calibration Run and Analysis for the LHC Pilot Run Data

Table 5.1 Run information for the data used for analysis. Run 405604 is the last collision run.
Run number

Run type

Start time (CEST)

Stop time (CEST)

Hit / Active SCs

405396405495405604

Collision
SplashCollision

28/10/2021 22:33
29/10/2021 18:30
31/10/2021 18:23

29/10/2021 06:34
29/10/2021 22:24
01/11/2021 06:02

15233 / 34019
33970 / 34011
11926 / 32771(a) Run 405396(b) Run 405604

Fig. 5.5 Beam intensity summary for the collision run. Data in the LBs with stable beams(shaded area) are used for physics analysis. As shown in Figure 5.5b, the intensity of beam 1
in the shaded area is stabilized at 0 [36].
first, and due to the limitation of data transfer rate at high L1A, only about 10 samples ofeach channel are taken for the digital trigger readout using SWROD instead of the total 32
samples. After the calibration is done, the SWROD starts to take with the full data recording.
In the last column of Table 5.1, the numbers of hit and active supercells are given. The active
supercells are the ones without the link issue. Among all active supercells, hit supercells are
the ones that receive pulse signals. The peak detection criterion in Section 4.1.2 with theheight of 10 is applied to search the hit supercells. From the numbers of hit supercells, it canbe known that there are more hit supercells in the run 405396. When the height threshold israised from 10 to 20, the number of the hit supercells in the collision runs drops by a factorof 1/3. However, the ones in the splash run barely drop. Therefore, it can be expected thatthe collision data will include more noise than that in the splash data.
Figure 5.5 shows the status of beam1 and beam2 during the two collision runs. The beamenergy is stabilized at 450 GeV. The luminosity block (LB) is a unit of the data taken withspecified time intervals, typically about 1 minute, for several analysis purposes. Obviously,
the intensity of beam1 dropped to nearly 0 during the stable beam flag was raised in the run
405604.

65

5.2 Data Analysis with LHC Pilot Run Data

5.2.2

Peak Sample and BCID Delay

The position of the peak sample is investigated to check the BCID delay of the signals
between the SWROD and the main readout, which can be computed by the Eq. 4.1. 1 BC
offset corresponds to 25 ns BCID delay. The algorithm used for searching the valid pulse
is also similar to the one applied in the SSW scan and BCID calibration (see Section 4.1.2)
except that the part of the collision events is not recorded with the full 32 samples. Therefore,
only the criterion of peak detection with the height threshold of 20 ADC is applied for thepilot run data.
In order to mitigate the impact of noise and check the active supercells, the pedestal level
and RMS for each channel are computed with the luminosity blocks from 360 to 379 in thecollision run 405604. RMS is calculated by the following equation,s
σsupercell =∑i (Si − µped )2nevents(5.2)

where the index i is summed over all the events, and ∑i = nevents . Si represents the ADC value,µped is the mean pedestal ADC value for the supercell. Figure 5.6 shows the distributionof pedestals and RMS for all active supercells in the run. The number of active supercells
is listed in Table 5.1. The uniformity of the statistics for the channels is observed in the
figure. And the RMS for most channels are close to 0 except for the presampler. Especially,
channels on EMB show good statistics and uniformed performance, which is attributed tothe excellent calibration works.

(a) Pedestal distribution(b) RMS distribution

Fig. 5.6 Pedestal and RMS value of all supercells are computed by the LBs from 360 to 379
in run 405604. Both statistics are evenly distributed in the φ range as expected.

66

Calibration Run and Analysis for the LHC Pilot Run Data

Fig. 5.7 BCID delay between SWROD and main readout with the splash data (run 405495).
FCal is not calibrated yet at the time.
Splashes
The BCID calibration is not fully done during the pilot run, and only part of the EMB region
is calibrated within the ±1 BC for the splash run. Therefore the offset between the digitaltrigger readout and the main readout could be significant.
Figure 5.7 shows the BCID delay between the SWROD readout and the main readout frombeam 1. The data sample is from run 405495, the beam splash events, whose information
can be found at table 5.1. It can be seen that the offset from the FCal signal is quite large,
which is consistent with what is observed in Figure 5.7. The BCID delay of most channels in
EMB is adjusted based on the previous run. Therefore, most of them are around 0, which isalso expected. Figure 5.8a shows the physics pulse from EMB in the run. Compared with thecalibration pulse in Figure 4.6a, it can be seen that the undershoot of the calibration pulse
and the physics pulse are different. The pulse from FCal is also shown in Figure 5.8b.
Collisions
Due to the limited time, the BCID in the digital trigger readout is not well-adjusted for thedata-taking in collision events. In addition, the collision run is operated with low energy andlow luminosity. Therefore, the barrel region rarely captures the signal events, and the same
analysis for splash data is hard to perform on the collision data.

67

5.2 Data Analysis with LHC Pilot Run Data(a) EMB physics pulse(b) FCal physics pulse

Fig. 5.8 Physics pulse in the splash data. Both are read from LB 322 (run 405495), which
belongs to the splashes from beam 1.
In the collision runs, over 200 LBs are recorded with 11 samples for each channel, andthe rest of LBs are recorded with full 32 samples. Thus, the pulse detection method in
Section 4.1.2 cannot be completely applied to the collision data. And due to the low beamenergy, most events will be excluded with the tight cut. As a result, only the peak detection
with a height of 20 is performed to mitigate the noise influence.
The result of BCID delay between SWROD and main readout is shown in Figure 5.9.
Two collision runs are analyzed. Due to the low height cut, low luminosity, and low beamenergy, the noise level is much higher than that in the splash events. Both runs indicatethat most channels are not aligned yet, and their BCID delay is around −50 ns. The BCID
calibration performed after the pilot run should fix this issue.

(a) Run 405396(b) Run 405604

Fig. 5.9 BCID delay between SWROD and main readout with the collision data .

68

5.2.3

Calibration Run and Analysis for the LHC Pilot Run Data

Energy and Timing Reconstruction.

It is essential to have the correct ET computation in the digital trigger readout. Therefore theenergy is also computed offline with the pilot run data. The OFC is already calibrated by thecalibration run. However, the analysis results of BCID delay in Section 5.2.2 show that mostchannels are not aligned in the digital trigger readout, which means that the OFCs cannot beapplied to a fixed BCID as it was in Figure 3.14. Therefore, the selection for the proper BCID
should be performed to get the correct reconstructed ET value. As shown in Figure 5.10, theselection is made by finding the BCID that matches the maximum reconstructed ET withthe minimum |τ|. With this method, the energy is computed offline and selected by the τcriteria discussed in Section 3.2.4. The comparison of ET between the selected events fromthe SWROD and the main readout is then made. Notice that only the comparison resultof EMB and EMEC is given. As shown in Figure 5.7, the calibration for FCal is not done
during the pilot run. In addition, the OFCs used for the ET computation are calibrated withthe calibration pulse.

(a) ET selection(b) τ selection

Fig. 5.10 Reconstructed ET and τ at different BCID using the calibrated OFC. The supercell
is from EMBA, and the injected pulse is from run 405495, the beam splash event. The BCID
that matches the maximum ET with the minimum |τ| is selected.

Splashes
The distribution of the computed ET is shown in Figure 5.11. The data is from the SWROD
readout, and the ET is computed supercell by supercell. τ criteria are applied to exclude thenoise. It can be seen that beam 1 was triggered on the C side, and beam 2 was triggered on
the A side. Figure 5.11a and Figure 5.11b each includes 20 events.

5.2 Data Analysis with LHC Pilot Run Data69(a) beam 1(b) beam 2

Fig. 5.11 Reconstructed energy distribution using splash data (run 405495). Beam direction
is shown in the figures. Beam 1 is triggered on the C side, and beam 2 is triggered on the A
side. Each figure includes 20 Events.

70

Calibration Run and Analysis for the LHC Pilot Run Data

With the computed ET from the SWROD, the energy comparison between SWROD
and main readout using splash data is shown in Figure 5.12. Because the BCID calibration
for the digital trigger system was not yet finished at the time, some SCs with larger BCID
offsets also got deviations in the ET computation. Despite that, both figures show the goodconsistency of computed ET value between SWROD and main readout.

(a) beam 1(b) beam 2

Fig. 5.12 Energy comparison between SWROD and main readout for run 405495. Figure 5.12a shows the data from beam 1. Figure 5.12b shows the data from beam 2. The redsolid line is the y = x line drawn as a reference.

Collisions
Run 405604 is used for energy computation. Also, since it was the last run, the LATOME
output switched from ET to ET ID. ET ID records the ET computed in the user code for5.2 Data Analysis with LHC Pilot Run Data

71

supercells passing the τ criteria. The distribution of the reconstructed energy read by
SWROD is shown in Figure 5.13. All LBs recorded by the SWROD are utilized, and theenergy obtained is very low as expected. Moreover, it can be seen from the figure that ET isevenly distributed in η and φ direction.
The comparison result is shown in Figure 5.14. Most events are at low energy and low
occupancy. However, the ET computed from the digital trigger readout is overall sightly larger
than the ET computed from the main readout. One possible reason is that the configuration
used by the digital trigger system was adjusted according to the splash run. Therefore,
although the computed ET from splash run is consistent between digital trigger readout andmain readout, the ET calculation result of collision run may have some deviation.
To summarize, the collision data is not good enough to check the system’s performance.
However, by looking at the splash data, the good connectivity of the supercells can be
confirmed, which means that the SSW scan in Section 4.1 does achieve the significant work.
Moreover, by comparing the analysis results in the two readouts, it turns out that with theadjusted BCID obtained in Section 4.2 and the calibrated OFCs, the signal timing and energy
computation can be finely tuned with the ones in the main readout. These conclusions give a
promise that the digital trigger readout system can achieve the expected performance.

Fig. 5.13 Computed ET using collision data (run 405604). The energy is much less than thatin the splash events.

72

Calibration Run and Analysis for the LHC Pilot Run Data

Fig. 5.14 ET comparison between SWROD and main readout with collision data (run 405604).
The red solid line is the y = x line drawn as a reference.

Chapter 6
Conclusion and Outlook
6.1

Conclusion

The study checked the connectivity of the new digital trigger readout system of the LAr
calorimeter during the LHC-ATLAS phase-I upgrade by using the SSW scan and adjusting
the system latency to make the system have the stable sampling timing. This study also
evaluated the digital trigger readout system with the data from the pilot run in October 2021.
In order to meet the requirement of ATLAS for the L1 trigger with the higher efficiency
in Run-3, a new digital trigger readout system was developed for the LAr readout systemthat provides object recognition information for the trigger system. The new digital trigger
readout system applies over 34000 supercells with 10 times higher granularity than the old
readout cells and uses optical fibers to transmit the digitized data from the front-end to theback-end. The supercells with multi-layer structure cover the region of |η| < 4.9. The signals
read out from the supercells are digitized by the LTDB at the front-end and transmitted to theback-end via optical fibers, where they are calculated by the LATOME board equipped witha high-speed FPGA for energy and timing reconstruction. The computation result is thensent to FEX for object recognition through optical fibers. The commissioning works done in
the thesis are based on the validation of the new system.
Connectivity check is the first work to be done. This work is to check whether the supercells are connected correctly from the detector to the back-end. Without good connectivity,
all the other commissioning works cannot be performed. In this thesis, the connectivity
check of all 34048 supercells was done by the SSW scan, and a large number of swappedsupercells were found and corrected. In addition to the connectivity problems, the SSW scanalso checked pulse shape and pulse pattern to reveal problems such as bad link, bad ADC
configuration, and bad calibration line. By the end of 2021, 99.9% of the supercells havebeen validated to be used for the L1Calo trigger system, and only 42 supercells are left to be

74

Conclusion and Outlook

fixed. As a result, a few places turned out to be different from the design, but we were able
to confirm that they did not affect the trigger performance.
Immediately after the connectivity test comes the fixed system latency, which represents
the time it takes for data to travel from one point to another. For a stable system, its latency
should be well-tuned and fixed. For the ALTAS experiment, the system latency can bechanged by adjusting the BCID, and since the main readout system does not change in Run-3
compared to Run-2, it is used as a reference for adjusting the new digital trigger readout
system. In this study, the pulse-all run is taken to check the BCID of over 4000 fibers on
the LATOMEs. By developing the algorithm, the digital trigger system with a stable BCID
is updated. With the stabilized and calibrated sampling phase, the calibration runs give thecalibrated factors of FADC→MeV and non-dimensional OFCs for each supercell, which areused for the ET and timing computation in the digital trigger readout.
In this research, not only the calibration data but also the LHC pilot run data are used.
The data taken with the LHC pilot run in October 2021 are utilized for the full-powered
new digital trigger readout test. In the pilot run, the detector achieves the purpose of taking
both beam splash and collision data at the center-of-mass of 900 GeV. Thus, the timing
investigation and energy computation are performed with offline analysis in the thesis. The
offline analysis framework uses the raw ADC data read from the digital trigger readout andmain readout to make the comparison of BCID and reconstructed ET between them. Bothresults show that with the adjusted BCID and calibrated OFCs, the digital trigger readout
can be adjusted to achieve the expected performance. This adjustment is actively proceeding
towards the full operation of the digital trigger readout system in Run-3.
In summary, the connection check and BCID calibration for all supercells have been
completed. Although individual supercells still need to be fine-tuned before Run-3, in general,the LAr phase-I upgrade has been successfully completed.

6.2

Outlook on Works towards Run-3

The debugging for the current problematic channels should be soon finished. On the other
hand, although the BCID calibration is stable for most fibers, the fluctuation of ADC fromseveral supercells may change the position of the peak sample. Thus, further checks for the
stable sampling phase after the BCID calibration are required. At the same time, the analysis
for the new OFC set computed with the latest sampling phase needs to be done. As for thepilot run data, analysis for all the runs is not finished yet. A good run list for the collision
data is awaited to be summarized.

6.2 Outlook on Works towards Run-3

75

Besides all the plans above, the improvement on the LATOME firmware is also on theschedule. The current firmware applied on the LATOME in the pilot run and calibration
run is not the version with full functionality activated. The optimization for the timing
consumption of user code block may be accomplished by introducing the application ofHigh-Level Synthesis (HLS).
During the first 6 months after the Run-3 operation started, the new digital trigger system
will still be tested to be stable enough for the data acquisition, and the works mentioned
above should be done before the complete handover of the legacy readout system to thedigital trigger readout system.

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